Synthetic and Mechanistic Studies of Organoscandium Compounds. Dimerization and Branching of Alkenes Catalyzed by Scandocene Hydrides Citation Bunel, Emilio Enrique (1989) Synthetic and Mechanistic Studies of Organoscandium Compounds. Dimerization and Branching of Alkenes Catalyzed by Scandocene Hydrides. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3HDJ-ST65. https://resolver.caltech.edu/CaltechETD:etd-05242007-075254 Abstract Mixed-ring scandocene alkyl and hydride derivatives of structure Cp*CpSc(CH₃)(PMe₃), Cp*CpSc( η ¹- η ⁵-C₅H₄(Cp*Sc(H), Cp*( η ⁵-1,3,4-C₅Me₃H₂)Sc(CH₃)(PMe₃), and Cp*( η ⁵-1,3,4-C₅Me₃H₂)Sc(H)(PMe₃) were prepared (Cp* = η ⁵-C₅Me₅, Cp = η ⁵-C₅H₅. Dimethylsilicon bridged scandocene alkyl and hydride derivatives Me₂Si( η ⁵-C₅Me₄)₂ScCH(SiMe₃)₂, Me₂Si( η ⁵-C₅Me₄)₂Sc(H)(PMe₃), meso -Me₂Si( η ⁵- t -butylC₅H₃)₂ScCH₂SiMe₃, and ( meso -Me₂Si( η ⁵- t -butylC₅H₃)₂Sc(H))₂ were also prepared. The activation energy for phosphine dissociation in Cp*CpSc(CH₃)(PMe₃) and Cp*( η ⁵-1,3,4-C₅Me₃H₂)Sc(H)(PMe₃) were measured. The crystal structures of Me₂Si( η ⁵-C₅Me₄)₂ScCH(SiMe₃)₂ and Me₂Si( η ⁵-C₅Me₄)₂Sc(H)(PMe₃) were determined. The hydride derivatives, Me₂Si( η ⁵-C₅Me₄)₂Sc(H)(PMe₃) and ( meso -Me₂Si( η ⁵- t -butylC₅H₃)₂Sc(H))₂ catalyze the following carbon-carbon bond forming reactions: (1) The catalytic dimerization of α -olefins to head-to-tail dimers, (2) The catalytic cyclization of α , ω -diolefins to methylenecycloalkanes with ring sizes between 5 and 9 carbon atoms, (3) The catalytic formation of six-membered ring nitrogen and sulfur heterocycles by catalytic cyclization of bisallyl amines and bisallyl sulfides respectively and (4) The catalytic formation of the spiro hydrocarbons 2-methylene spiro[4.4] nonane and 2-methylene dispiro [4.1.4.2] tridecane by catalytic cyclization of 5-methylene-1,8-nonadiene and 5,8-dimethylene-1,11-dodecadiene. ( meso -Me₂Si( η ⁵- t -butylC₅H₃)₂Sc(H))₂ catalyzes the ring opening reaction of methylenecyclopropane to butadiene and methylenecyclobutane to 1,4-pentadiene. Labelling experiments show that intramolecular olefin insertion in ω -alkenyl scandium complexes and β -alkyl elimination in cycloalkylmethyl scandium complexes is reversible when rings containing 3 or 4 carbon atoms are involved. The hydride derivative ( meso -Me₂Si( η ⁵- t -butylC₅H₃)₂Sc(H))₂ also catalyzes the isomerization of 1,4-pentadiene to isoprene and 2-methyl-1,4-pentadiene to 2,3-dimethylbutadiene and the isomerization of 3-methyl-1,4-pentadiene to 1,5-hexadiene, which is then cyclized to methylenecyclopentane. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: Chemistry Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Bercaw, John E. Thesis Committee: Gray, Harry B. (chair) Bercaw, John E. Schaefer, William P. Myers, Andrew G. Defense Date: 1 June 1988 Non-Caltech Author Email: ebunel (AT) anl.gov Record Number: CaltechETD:etd-05242007-075254 Persistent URL: https://resolver.caltech.edu/CaltechETD:etd-05242007-075254 DOI: 10.7907/3HDJ-ST65 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 2018 Collection: CaltechTHESIS Deposited By: Imported from ETD-db Deposited On: 24 May 2007 Last Modified: 18 Dec 2020 02:11 Thesis Files Preview PDF - Final Version See Usage Policy. 3MB Repository Staff Only: item control page

Synthetic and Mechanistic Studies of Organoscandium Compounds. Dimerization and Branching of Alkenes Catalyzed by Scandocene Hydrides. Thesis by Emilio E. Bunel In Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy California Institute of Technology Pasadena, California 1989 (Submitted June 1, 1988) - ii - To all my women : Maria Jose, Paula and Susana - iii - ACKNOWLEDGEMENTS First I would like to thank John Bercaw for his support and advice during the past years. He has been an incredible advisor and a model of what an advisor should be like. I also would like to thank William P. Schaefer, Verner Schomaker, Richard Marsh and Larry Henling for the xray structures reported in Chapter 1. Janet E. Nelson and Pam Shapiro did a great job putting this work into English. To all the members of the Bercaw Group thanks for being friendly with me during my stay at Caltech. Finally, I would like to thank my wife Susana for her support and patience over the past years. - ivABSTRACT Mixed-ring scandocene alkyl and hydride derivatives of structure Cp*CpSc(CH3)(PMe3) , Cp*CpSc ( f'/ 1-,, 5-C5H4)Cp*Sc(H), Cp*(,, 5-1,3,4-C5Me3H2)Sc(CH3)(PMe3) C5Me3H2)Sc(H)(PMe3) were prepared (Cp* scandocene alkyl and hydride and Cp*(,, 5-1,3,4- =,,5-C5Me5, Cp =,, 5-C5H5). Dimethylsilicon bridged Me2Si(,, 5-C5Me4)2ScCH(SiMe3)2, derivatives Me2Si (,, 5-C5Me4)2Sc(H) (PMe3), meso-Me2Si (f'/ 5-t-butylC5H3)2ScCH2SiMe3 and (meso-Me2Si (,, 5-tbutylC5H3)2Sc(H) )2 were also prepared. The activation energy for phosphine dissociation in Cp*CpSc(CH3)(PMes) and Cp*(,., 5-1,3,4-C5Me3H2)Sc(H)(PMe3) were measured. structures of Me2Si(,, 5-C5Me4)2ScCH(SiMe3)2 and The crystal Me2Si(,, 5-C5Me4)2Sc(H)(PMe3) were determined. The hydride derivatives, Me2Si(,, 5-C5Me4)2Sc(H)(PMe3) and (meso-Me2Si(,, 5-t- butylC5H3)2Sc(H))2 catalyze the following carbon-carbon bond forming reactions: (1) The catalytic dimerization of a-olefins to head-to-tail dimers, (2) The catalytic cyclization of a, c.rdiolefins to methylenecycloalkanes with ring sizes between 5 and 9 carbon atoms, (3) The catalytic formation of six-membered ring nitrogen and sulfur heterocycles by catalytic cyclization of bisallyl amines and bisallyl sulfides respectively and (4) The catalytic formation of the spire hydrocarbons 2methylene spiro[4.4) nonane and 2-methylene dispiro [4.1.4.2) tridecane by catalytic cyclization of 5-methylene-1,8-nonadiene and 5,8-dimethylene-1, 11-dodecadiene. (meso-Me2Si('1 5-t-butylC5H3)2Sc(H))2 catalyzes the ring opening reaction of methylenecyclopro pane to butadiene and methylenecyclobut ane to 1,4-pentadiene. Labelling experiments show that intramolecular olefin insertion in c.ralkenyl scandium complexes and {3alkyl elimination in cycloalkylmethyl scandium complexes is reversible when rings containing 3 or 4 carbon atoms are involved. The hydride derivative (meso-Me2Si(,, 5-t-butylC5H3)2Sc(H))2 also catalyzes the isomerization of 1,4-pentadiene to isoprene and 2-methyl-1,4-pentadiene to 2,3dimethylbutadiene and the isomerization of 3-methyl-1,4-pentadiene to 1,5-hexadiene, which is then cyclized to methylenecyclopentane. -v- TABLE OF CONTENTS page Acknowledgement iii Abstract iv Introduction 1 Chapter 1: The Synthesis and Characterization of Scandium Alkyl and Hydride Derivatives. 4 Chapter 2: Carbon-Carbon Bond Formation Catalyzed by Scandocene Hydride Derivatives. The Catalytic Dimerization of a-Olefins and Cyclization of a , w-Diolefins. 47 Chapter 3: Carbon-Carbon Bond Activation via ,8-Alkyl Elimination. Reversible Branching of 1,4-Pentadienes Catalyzed by Scandocene Hydride Derivatives. 73 -1- INTRODUCTION The formation of carbon-carbon bonds and the reverse process, the cleavage of carboncarbon bonds, are crucial steps in the elaboration of hydrocarbons. The search for catalysts to perform both processes is an important field of investigation. A wide variety of organometallic compounds effect the formation of carbon-carbon bonds, but few have been reported to perform carbon-carbon bond breaking reactions.£1] organometallic derivatives of the Some of the most fascinating in this regard are lanthanides and related group 3 transition metals. Permethylmetallocene derivatives of these metals are of current interest because of their simple and interesting reactivity patterns toward small hydrocarbons. These metal complexes,£2] of general structure Cp*2MR(L) (Cp* =,., 5-C5Me5; M =Sc, Y, La, ... Lu; R =H, alkyl, aryl; L =two electron donor), with do or d 0fn metal centers and 14 or 16 electron counts are highly Lewis acidic, properties which greatly influence their ability to promote carbon-carbon bond forming or breaking processes for simple hydrocarbons. These derivatives are active as olefin polymerization catalysts (Equation 1) [3] and olefin hydrogenation catalysts (Equation 2).£4] They also react with the C-H bonds of arenes (Equation 3), olefins (Equation 4) and alkanes (Equation 5).£ 1a] Cp*2LuCH 3 ~ Cp*2Lu-(CH2-CH2)n-CH3 n CH 2=CH 2 + Cp*2M-H + Eq. 1 Eq. 2 H2 M = Lu, Sm, Nd, La Cp*2Sc-CH 3 + Cp*2Sc-CH 3 + Cp*2Sc-CH 3 + CsHs =< 1aCH4 Cp* 2Sc-CsHs + CH 4 Eq. 3 Cp*2S c ] + CH 4 Eq. 4 13 Cp*2Sc- CH 3 + CH 4 Eq. 5 -2The olefin chemistry of the Cp*2M-H derivatives is rather simple. Olefin insertion across the M-H bond and subsequent insertion of a second equivalent of olefin into the resulting M-alkyl bond represent the initiation and propagation steps in the polymerization of ethylene. With these complexes, ethylene is the only olefin which has been successfully polymerized; propylene and higher a-olefins are only oligomerized and the catalyst lifetimes are rather limited.£3] The metalalkyl bond can undergo ,B-hydrogen or ,B-alkyl elimination giving M-H or M-R with the simultaneous extrusion of olefin.£5] Scandium is the smallest member of the group 3 transition metals and the lanthanides. Thus, the steric constraints imposed by the ligands should be most severe with scandium. It is the purpose of the present work to describe the olefin chemistry of modified cyclopentadienyl scandocene derivatives as an extension of the chemistry previously observed in the bis(pentamethylcyclopentadienyl) scandium system.CS] The first chapter of this thesis presents both the synthesis and characterization of mixed ring scandocene hydrides and dimethylsilicon-brid ged cyclopentadienyl scandocene hydrides. It is found that rather modest changes in the ligands around scandium cause major changes in chemical reactivity. For example, as described in the second chapter, a-olefins are catalytically dimerized in a head to tail fashion in contrast with the [Cp*2Sc] system, which reacts only stoichiometrically with a-olefins to yield the alkenyl derivatives. Further applications of the aolefin dimerization are found in the catalytic cyclization of a , w-diolefins to methylenecycloalkanes as well as in the cyclization of bisallyl methyl amine and bisallyl sulfide to 3-methylene N-methyl piperidine and 3-methylene perhydro-thiane. The third chapter describes the carbon-carbon bond activation chemistry of these complexes exemplified in the isomerization of 1,4pentadienes. catalytically 1,4-Pentadiene, converted to 2-methyl-1,4-pentadiene isoprene, and 2,3-dimethylbutadiene 3-methyl-1,4-pentadiene and are methylenecyclopentane respectively. The key steps in these transformations are reversible olefin insertion and ,B-alkyl elimination. -3- REFERENCES 1. (a) Watson, P.L.; Parshall, G.W.Acc. Chem. Res. 1985, 18, 51. (b) Bunel, E.; Burger, B.J.; Bercaw, J.E. J. Am. Chem. Soc. 1988, 110, 976. 2. (a) Thompson, M.E,; Bercaw, J.E. Pure and Appl. Chem. 1984, 58, 1. (b) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P.N.; Schumann, H.; Marks, T.J. J. Am. Chem. Soc. 1985, 107, 8091. (c) Watson, P.L.; Herskovitz, T. Am. Chem. Symp. Series 1983, 212, 459. 3. Watson, P.L J. Am. Chem. Soc. 1982, 104, 337. 4. Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T.J. J. Am. Chem. Soc. 1985, 107,8111. 5. Watson, P.L; Roe, D.C. J. Am. Chem. Soc. 1982, 104, 64 71. 6. Thompson, M.E.; Baxter, S.M.; Bulls, A.R.; Burger, B.J.; Nolan, M.C.; Santarsiero, B.D.; Schaefer, W.P .; Bercaw, J.E. J. Am. Chem. Soc. 1987, 109, 203. -4- CHAPTER 1 THE SYNTHESIS AND CHARACTERIZATION OF SCANDIUM ALKYL AND HYDRIDE DERIVATIVES. -5 INTRODUCTION The chemistry of Cp*2Sc-R (R = alkyl, aryl, H) is rich in stoichiometric reactions, but the catalytic activity towards small molecules such as a-olefins remains restricted essentially to ethylene polymerization and hydrogenation of some olefins.£1 l Unfortunately, Cp*2Sc-R (R= H, alkyl) complexes do not react with a-olefins in the same way they do with ethylene. The reactivity of Cp*2Sc-R with a-olefins differs from that with ethylene in that only stoichiometric reactions (insertion of one molecule of a-olefin) have been observed in the case of scandium. The reason for the low reactivity toward insertion of more than one a-olefin is likely steric in origin. With more bulky olefins than ethylene, reaction at the vinylic C-H bond of the olefin, (a-bond metathesis), is preferred over a second insertion giving scandium vinyl complexes and free alkane.£1 b] This pathway minimizes the steric constraints by orienting the alkyl substituents of the a-olefin away from the Cp* rings. This chapter describes several new compounds designed to help elucidate the importance of steric effects in the bis(cyclopentadienyl)scandium system in relation to the insertion of a-olefins. To accomplish this goal, several Cp*Cp ScR(PMe3) (Cp' = 77 5-CsHs = Cp, 0 77 5-1,3,4-Me3C5H2 ""'Cp 0), Me2SiCp 2ScR (Cp' = 77 5-C5Me4, 77 5-t-butylC5H3) and Me2SiCp°Cp"ScR 0 (Cp' = 77 5-CsMe4, Cp" = derivatives were synthesized. 77 5-t-butylC5H3, (-)-2-methylbutyl-77 5-C5H3, (+)-menthyl-77 5-C5H3) -6RESULTS AND DISCUSSION Synthesis of Mixed Rings Scandocene Derivatives To decrease the steric hindrance at the scandium center in the [Cp*2Sc] system, a cyclopentadienyl ring less sterically demanding has been pentamethylcyclopentadieny l ring. We found that Sc(acac)s (acac replaced for one =acetylacetonate) reacts with Cp*MgCI · THF in toluene giving cleanly Cp*Sc(acac)2 (1) as a yellow crystalline, air stable material, in good yield. Reaction of 1 with aluminum trichloride provides (Cp*ScCl2)n (2), a convenient starting material for the synthesis of mixed-ring scandocenes. 2 is completely insoluble in toluene and is easily isolated from the reaction mixture. The 1H NMR spectrum in THF-da shows only a singlet at 2.07 ppm, assigned to the Cp* ring. Toluene, 25°C Sc(acac)a + Cp*MgCl-THF Cp*Sc(acac) 2 + (acac)MgCI + Cl2 Al(acac) 1 0 Toluene, 25 C Cp*Sc{acac) 2 + 2 AICl 3 1/n(Cp*ScCl2 )n 2 Cp*ScCl 2 + . Cpli Toluene, so0 c . Cp*CpScCI + LiCI 3 C~ = Cp* 4: Cp =Cp 5 : Cp = 1,3,4-Me3 C5 H2 Reaction of 2 with one equivalent of Cp*Li, Cpli or Cp0 Li cleanly yields Cp*2ScCI (3), Cp*CpScCI (4) or Cp*Cp 0ScCl(5) respectively. Cp*2ScCI had been previously reported.£1 b] 4 is sparingly soluble in benzene suggesting a dimeric structure similar to that reported for (Cp2ScCl) 2.C2 l The 1H NMR spectrum (400 MHz) at room temperature shows two broad resonances: one at 2.1 ppm (v112 =3 Hz), assigned to the Cp* ring, and another at 6.4 ppm ( v112 -7- =7.5 Hz), assigned to the Cp ring. When the sample is heated to 80°C, the 1H NMR (400 MHz) spectrum shows sharp singlets at 2.0 and 6.3 ppm. These NMR data suggest that an equilibrium between monomer and dimer exists. Compound 5 is quite similar to 4 in regard to solubility, and is probably a dimer. Low solubility of 4 and 5 in benzene precluded measurement of the molecular weight. Reaction of 4 or 5 with methyllithium in diethyl ether in the presence of PMe3, followed by extraction with petroleum ether cleanly gives Cp*CpSc(CH3)(PMe3) (6) or Cp*Cp0Sc(CH3)(PMe3) (7). Attempts to generate the methyl complexes in the absence of PMe3 gave only intractable materials. Cp*Cp ScCI + CH 3 Li + PMe3 Cp*Cp Sc(CH 3 )(PMe 3 ) + LiCI . 6: Cp = Cp 1 H NMR spectra (25 ° C, 400 MHz, CsDs) of both 6 and 7 show the methyl protons as singlets. This lack of coupling of the hydrogens of the methyl groups to the phosphorus of the PMe3 suggests a rapid equilibrium between phosphine-complexed and uncomplexed forms. In the low temperature 1 H NMR (400 MHz, -80°C, C7 D8) spectrum of 6 the methyl group (-0.6 ppm, JHP =2.9 Hz) appears as a doublet due to splitting by the phosphorus. ' Cp*Cp Sc(CH 3 )(PMe3 ) Cp*Cp Sc-CH 3 + PMe3 loss of coupling of the methyl group to the phosphine occurs at -70°C. At -80° C, the spectrum of 7 shows one singlet at 2.01 ppm assigned to the Cp* ring and three singlets at 1. 73, 1.86, and 2.33 ppm assigned to the three different methyl groups of the Cp0 ring. The hydrogens of the Cp0 ring are coupled to each other (J =2.3 Hz) and one set is coupled to the phosphorus of the phosphine. Upon irradiation of the non-phosphorus coupled signal the other signal collapses to a doublet (JHP = 6 Hz). Coalescence of the two hydrogens of the Cp0 ring occurs at -29 °C, -8 yielding a rate constant for phosphine exchange of k(-2a 0 c) =218 sec-1 and an activation energy * of ~G = 11.5 kcal-moi-1. Hydrogenolysis of alkyl complexes Cp*CpSc(CH3}(PMe3) (6) and Cp*Cp0 Sc(CH3}(PMe3) (7) provides a convenient entry to hydride complexes. Hydrogenolysis of Cp*CpSc(CH3}(PMe3) gives Cp*CpSc-(77 1-77 5-C5H4)Cp*Sc-H (8), rather than the expected product. 2 Cp'*CpSc(CH 3 )(PMe3 } + H2 <:::i7 \ . . -· H--s/\ :!~ Sc .. 8 The 1H NMR ( 400 MHz, CaDa) spectrum shows two different Cp* rings at 1.93 and 2.17 ppm, a singlet from one Cp ring at 5.93 ppm, and four multiplets from the 77 1 , 77 5-C5H4 ring centered at 6.13, 6.26, 6.38, and 6.63 ppm. There is no evidence in the 1H NMR spectrum (400 MHz, C7Da) for the hydride resonance even at -80°C. Excessive broadening of the hydride resonance is attributed to the quadrupole moment of 45 Sc. The formation of product 8 can be explained according to Scheme 1. The reversibility of the last step can be observed in the reaction of 6 with D2; in this case the product isolated is Cp* ( 77 5-C5D5)Sc-(77 1-77 5-C5D4)Cp*Sc-D. Hydrogenolysis of 6 in THF-da (as monitored by 1H NMR) yields Cp*CpSc(H)(THF), analogous to the previously reported Cp*2Sc(H)(THF) complex.£1 b] The 1H NMR spectrum of Cp*CpSc(H)(THF) shows a singlet at 2.0 ppm assignable to the Cp* ring and another singlet at 5.95 ppm assignable to the Cp ring. If the overpressure of H2 is eliminated, Cp*CpSc(H)(THF) cleanly decomposes to 8. similar to 8 was recently reported in the thermolysis of Cp*2Y(H)(THF)J3] A compound -9Hydrogenolysis of 7 gave the expected product, Cp*Cp0Sc(H)(PMe3) (9). The 1 H NMR spectrum (-B0°C, 400 MHz, C7Ds) of 9 shows a similar pattern as for 7. There is a broad peak (1.1 112 = 33.7 Hz) at 4.55 ppm that is assigned to the hydride, broadened due to the quadruple moment of scandium. Coalescence was obtained for the hydrogens of the Cp0 ring at -29°C yielding kc-2s c) 0 =90.6 sec-1 and t.G* =12 kcal-mo1-1• 9 Comparison of the rates of phosphine exchange for 7 and 9 shows that this process is faster for the methyl derivative. This behavior can be explained by steric arguments since the phosphine would presumably have more room to bind to the hydride derivative than to the methyl derivative. Scheme 1 Cp* ' QJ + _.Sc-CH3 Cp* ' .Sc-H QJ' 2 / \ _ H .. Cp* ~······~ _:;:H Sc QJ .. Sc '.Sc-H I :\ QJ Cp* cp ... " H 1- H2 cp• "'---~( ~I Cp*' .. Sc / '\ i H QJ PMe3 -10Synthesis of Scandocene Derivatives with SiMe2 Bridging the Cyclopentadienyl Rings. Rather than altering ring substituents, another way to decrease the steric hindrance at the scandium center is to "tie" the rings together with a dimethylsilyl bridge. This approach was developed first by MarksC4l in an effort to obtain more reactive uranium and thorium derivatives (relative to the analogous Cp*2MR2 , M =Th, U). This linkage produces a smaller ring centroid- metal-ring centroid angle, thus opening the coordination sphere of the metal center. The Me2Si(C5Me4H2)2 ligand is prepared as shown in Scheme 2. The key starting material for the synthesis of this ligand is 1,2,3,4-tetramethylcyclopentadiene. Reduction of the commercially available 2,3,4,5-tetramethyl-2-cyclopentenone with LiAIH 4 provides 2,3,4,5tetramethyl-2-cyclopentenol in excellent yield; dehydration of the alcohol provides the desired 1,2,3,4-tetramethylcyclopentadiene in moderate yield. A seemingly trivial, but important modification was found to be the isolation of the lithium salt of 1,2,3,4-tetramethylcyclopentadiene prior to the reaction with dichlorodimethylsilane. Me2Si(C5Me4H2)2 is isolated as a non-volatile oil which requires no further purification. The dilithium salt Me2Si(C5Me4)2Li2 (10) is prepared via deprotonation of the ligand with n-butyllithium. Similarly, Me2Si(t-butylC5H4)2 is prepared according to the following "one-pot" synthesis (Scheme 3). Treatment of t-butylcyclopentadienyl lithium with dichlorodimethylsilane provides Me2Si(t-butylCpH)2, which is isolated as an oil and converted without further purification to the dilithium salt, Me2Si(t-butylCp)2Li2 (11), by reaction with n-butyllithium. Metallocenes derived from 11 could conceiveably provide a mixture of meso and racemic isomers depending on the relative positions of the t-butyl groups in the metallocene. Curiously, only the mesa compound is formed (vide infra). Treatment of tetramethylcyclopentadienyllithium with dichlorodimethylsilane in a 1: 1 molar ratio provides (C5Me4H)SiMe2CI (12) in high yield. This compound is isolated as an oil - 11 - Scheme 2 0 n-butyl-Li THF ~i 2~; l~-. fV'fq__ ; We Si 14 T:.:utyl-U I P.Ether -12which solidifies at -40°C. From (C5Me4H)SiMe2CI as the starting material, another variety of silicon bridged ligands bearing only one tetramethylcyclopentadienyl ring is obtained (Scheme 4). Treatment of 12 with t-butylC5H4Li, (-)-2-methyl butylC5H4Li or (+)-menthylC5H4Li provides respectively or Me2Si(C5Me4H)((+)-menthylC5H4) which were converted to their respective dilithium salts via deprotonation with n-butyllithium. The (-)-2-methylbutyl and (+)-menthyl groups were used as ancillary ligands to induce the preferential formation of one optically active metallocene. Scheme 3 1. THF 2. 1 ~-.,We Si 11 n-butyl-Li I P. Ether -13- Scheme 4 Me2 SiCl2 ~ + j THF ~--;We Si......_ 13 Cl 14 a : 1. 2. b : 1. 2. c : 1. 2. t-butylCpLi I THF n-butyl-Li I Et2 0 (-)-2-methylbutylCpLi I THF n-butyl-Li I Et2 0 (+)-menthylCpLi I THF n-butyl-Li I Et2 0 15 -14- Scheme 5 ScCl 3 • 3 THF Toluene R...CI ..... ·"' THF sc, ,,,,,.,.- Me2Si ~Cl L1 ........_ THF 16 -:._,Me Me Si ScCl 3 • 3 THF Toluene n 17 '·., Me Me Si~Li ScCl 3 • 3 THF Toluene ~Cl. Me,Si~Cl;;;u::...THF ,THF R R 18 : R = t-butyl 19 : R = (-)-2-methylbutyl 20 : R = (+)-menthyl -15The silicon-bridged metallocenes, 16-20, shown in Scheme 5 are obtained by treating ScCl3 · 3THF with the dilithium salts of the ligands. Metathesis of the bridged scandocene chlorides with bulky lithium alkyls such as Me3SiCH2Li or (Me3Si)2CHLi yields the product without coordinated solvent (i.e. tetrahydrofuran or diethylether). Me2Si(C5Me4)2ScCH(SiMe3)2 (21) is obtained as a bright yellow, highly crystalline material from the reaction of Me2Si(C5Me4)2ScCI · LiCI · (Et20)2 (16) with (Me3Si)2CHLi. The 1H NMR spectrum (400 MHz) at room temperature shows four singlets assigned to the methyl groups of the C5Me4 ring, suggesting that rotation around the scandium-carbon a bond is restricted. The spectrum does not change upon heating the sample to 100°C, which implies a large barrier(~ 20 kcal-mol-1). 21 Treatment of ScCl3·3THF meso-Me2Si(t-butylC5H3)2ScCI (17). with affords The low solubility of 17 in benzene precludes the determination of the molecular weight. The 1H NMR spectrum in CD2Cl2 shows one singlet at 1.22 ppm assigned to the t-butyl groups, two singlets at 0.54 and 0.87 ppm assigned to the methyl groups of the bridge and three triplets at 6.33, 6.54 and 6. 70 ppm assigned to the hydrogens of the equivalent Cp rings. These NMR data suggest that of the two possible isomers, 17a and 17b, only 17a actually forms. In structure 17a, the methyl groups of the silicon bridge are equivalent, whereas in structure 17b they are inequivalent. The NMR spectra for all the [Me2Si(t-butylC5H3)2Sc] derivatives are similar. Metathesis of meso-Me2Si(t-butylC5H3)2ScCI with Me3SiCH2Li cleanly gives meso-Me2Si(t-butylC5H3)2Sc-CH2SiMe3 (22). -16- 17a 17b For all the other scandocene chlorides, treatment with (Me3Si)2CHLi provides the Sc-CH(SiMe3)2 derivatives as shown in Scheme 6. The presence of optically active groups, (-)-2-methylbutyl in 14 and (+)-menthyl in 15 could lead to the preferential formation of one of two possible diastereomers. The 1H NMR spectrum of 23 shows that the SiMe3 groups in the ScCH(SiMe3)2 moiety are inequivalent. There are 4 resonances due to the SiMe3 groups in the case of 24 and 25, two of them per diastereomer. Integration of these resonances provides a simple way to determine the enantiomeric excess. The diastereomers of 24 are formed in equal proportion, but in the case of 25 there is a slight excess of one isomer (8 % ee). Scheme 6 23 : R = t-butyl 24 : R = (-)-2-methylbutyl 25 : R = (+)-menthyl -17The bulky alkyl derivative Me2Si(C5Me4)2ScCH(SiMe3)2 reacts with dihydrogen in petroleum ether to yield (Me2Si(C5Me4)2ScH)x as a yellow precipitate. Unfortunately, (Me2Si(C5Me4)2ScH)x decomposes in tetrahydrofuran or benzene solution, precluding the observation of its NMR spectrum. However, when the hydrogenation is performed in the presence of one equivalent of trimethylphosphine, Me2Si(C5Me4)2Sc(H)(PMe3) (26) may be isolated. The X-ray structure of 26 (vide infra) confirms the structure in the solid state. 26 Reaction of meso-Me2Si(t-butylC5H3)2Sc-CH2SiMe3 with dihydrogen in petroleum ether gives the hydride, meso-(Me2Si(t-butylC5H3)2Sc-H)2 (27) in good yield. Its molecular weight, as determined by ebulliometry, indicates a dimeric structure for 27. The 1H NMR spectrum in toluene-de shows the same pattern as that for the alkyl derivative 22: one singlet for the t-butyl group at 1.21 ppm, two singlets at 0.82 and 0.21 ppm due to the dimethylsilicon bridge, and three triplets centered at 6.27, 6.67 and 7.33 ppm assigned to hydrogens of the cyclopentadienyl ring. The hydride proton is not observed at temperatures between 25°C and -80°C. Extreme broadening due to the bridging of the hydrogen between two quadrupolar 45Sc nuclei may explain this. However, when one equivalent of trimethylphosphine is added, a broad peak centered at 4.68 ppm appears in the 1H NMR spectrum which is assigned to the hydride proton. 2 27 -18Structure Determination of Me2Si(C5Me4)2ScCH(SiMe3)2 In order to define the distortion produced by a bridge in the biscyclopentadienyl scandocene system, the X-ray structure of 21 was determined. Crystals were grown by slowly cooling a saturated solution of 21 in toluene from 100°C to room temperature. The compound crystallizes in space group# 14 (P2 1/c) with a= 17.151 (4) A; b = 9.466(3) A; c = 18.123(4) A; f3 = 90.14 °. A perspective view of compound 14 and the atom labelling scheme are given in Figure 1 and 2 respectively. Selected bond distances and angles are summarized in Table 2. The scandium center adopts a relatively distorted 77 5-coordination of the cyclopentadienyl ring as is reflected in different Sc-Cring distances: C1-Sc = 2.447(2) A, C2-Sc = 2.498(3) A, C3-Sc = 2.609(3) A. This distortion is observed in the difference between the ring centroid-Sc-ring centroid angle (129.9(1) 0 ) and the angle between the normals to the rings (117.2(2) 0 ). The ring centroid-Sc-ring centroid angle (129.9(1) Cp*2ScCHa.C 1 b] 0 ) is contracted 14.7 ° relative to that in the structure of Although Me2Si(C5Me4)2ScCH(SiMe3)2 lies in a general four-fold position in P21/c, two nearly perfect mirror planes pass through Si1 Gaining the two C5Me4 rings) and Sc. One of these planes contains Si2, Si3, C1 O and C20; the other plane is perpendicular to it. The C27 shows essentially sp 2 hybridization, since the angles about this carbon (Sc-C27-Si2, Sc-C27Si3 and Si2-C27-Si3) are all approximately 120°. The peaks in the electron density map indicated that the electron density of the hydrogen on C27 was split equally above and below the plane. This geometry arises from two causes: the three electropositive neighbors of C27 encourage maximum p-character in the bonds to them, and the bulky trimethylsilyl groups, which are forced for steric reasons to be in the plane bisecting the C5Me4 rings, require that the geometry about C27 be nearly planar and thus that the carbon atom adopt sp 2 hybridization. An almost pure p orbital remains to bond the hydrogen atom. Two of the trimethylsilyl groups are in the Sc-Si2-Si3 plane. One, C25, is 3.684(4) A from the scandium atom; the other, C21, is 3.490(4) A away. The hydrogens atoms on C25 are arranged so the closest Sc····H-C25 approach is 3.460(5) A. In contrast on C21 one of the three hydrogen atoms approaches Sc with a Sc····H distance 2.832(7) -19- A. This asymmetry about the pseudo-mirror plane in the molecule is not enough, however, to lock C27 into a single position; it appears to be equally above and below the plane. In comparison with the structure of Me2Si(C5Me4)2NdCH(SiMe3)2 this implies some kind of long range Sc.. ·H· .. C(SiMe3)2 interaction rather than the Nd····CH3""SiMe2CHSiMe3 interaction observed by Marks.£5] In Me2Si(C5Me4)2NdCH(SiMe3)2 the distortion of the hydrogens on the analogous carbon does not minimize any particular Nd····H distance; rather it is 2.862 Aaway from Nd, which suggests a two-electron- bridging alkyl type of bonding. In contrast C21 and C25 in the Sc structure are extremely far from the metal center, 3.490 Aand 3.684 Arespectively. -20- Figure 1: Molecular Structure of Me2Si(C5Me4)2ScCH(SiMe3)2. - 21 - C7 C22 Cl9 Cl4 Cl8 Figure 2: The atomic labeling for Me2Si(C5Me4)2ScCH(SiMe3)2. -22- Formula weight : 502.91 Space group: P21/c(#14) Temperature: 22°C a= 11.151 (6) A b = 9.466(3) A ,8=90.14(1) 0 c = 18.123(7) A v = 2942(3) A3 >.MoKa = o. 71073 A µ = 3.87 cm-1 p = 1.135 g cm·3 Crystal size 0.68x0.39x0.41 mm µrmax = 0.17 -23Table 2: Bond Lengths (A) and Angles ( 0 Atoms Distance Atoms Distance Sc-C27A Sc-C27B Sc-Cp*1 Sc-Cp*2 Si1-C1 Si1-C10 Si1-C11 Si1-C20 Si2-C21 Si2-C22 Si2-C23 Si2-C27A Si2-C27B Si3-C24 Si3-C25 Si3-C26 Si3-C27A Si3-C27b C27A-C27B C1-C2 C1-C5 C2-C3 C2-C6 C3-C4 Sc-C15 2.270(2) 2.272(7) 2.226 2.221 1.877(3) 1.869(3) 1.870(4) 1.870(4) 1.882(4) 1.867(4) 1.875(5) 1.855(7) 1.823(7) 1.885(5) 1.881 (4) 1.873(4) 1.816(7) 1.862(7) 0.452(10) 1.429(4) 1.433(4) 1.412(4) 1.515(4) 1.397(4) 2.486 C3-C7 C4-C5 C4-C8 C5-C9 C11-C12 C11-C15 C12-C13 C12-C16 C13-C14 C13-C17 C14-C15 C14-C18 C15-C19 C-Cring ave Cring -CH3 ave Sc-C1 Sc-C2 Sc-C3 Sc-C4 Sc-C5 Sc-C11 Sc-C12 Sc-C13 Sc-C14 1.505(4) 1.414(4) 1.505(4) 1.510(4) 1.439(4) 1.445(4) 1.410(4) 1.501 (4) 1.404(4) 1.505(4) 1.414(4) 1.492(4) 1.504(4) 1.420(16) 1.505(7) 2.447(2) 2.498(2) 2.609(3) 2.607(3) 2.489(3) 2.460(3) 2.507(3) 2.601 (3) 2.558(3) Atom Atom Angle ) for Me2Si(C5Me5)2ScCH(SiMe3)2 Cp*1 Sc Cp*2 Cp*1 Sc C27A Cp*2 Sc C27B Cp*1 Sc C27B Cp*2 Sc C27B C1 Si1 C11 Cp*1 C1 Si1 Cp*2 C11 Si1 Sc C27A Si2 Sc C27B Si2 Sc C27A Si3 Sc C27B Si3 Si2 C27A Si3 Si2 C27B Si3 angle between normals to Cp*1 and Cp*2 129.9(1) 120.2(2) 109.8(2) 108.((2) 121.2(2) 97.2(1) 161.5(2) 160.0(3) 114.7(3) 116.0(3) 121.8(4) 119.5(3) 120.5(4) 119.7(2) 117.0(2) -24- Table 3: Final Parameters X 104 for Me2Si(C5Me4)2ScCH(SiMe3)2 Atom x y z Ueq Sc Si1 Si2 Si3 C1 C2 C3 C4 7271 (.3) 5928(.4) 8914(.5) 8200(.5) 5957(1) 5864(1) 6208(2) 6523(2) 6379(1) 5367(2) 6133(2) 6854(2) 6564(2) 5780(2) 6915(1) 7639(2) 8245(2) 7923(2) 7111 (2) 7790(2) 9103(2) 8383(2) 6591 (2) 5122(2) 8729(2) 9934(2) 8932(3) 9106(2) 7354(2) 8145(3) 8250(4) 8106(4) 4783(.5) 2736(.8) 69.31 (.9) 6562(.9) 4696(3) 5385(3) 6738(3) 6916(3) 5669(3) 4916(3) 7889(3) 8273(3) 5516(3) 2209(3) 2329(2) 2570(3) 2698(3) 2558(3) 2340(3) 2455(3) 2761 (3) 2436(3) 1963(3) 1777(3) 6309(4) 6424(6) 8909(4) 5913(5) 5682(4) 8491 (4) 6018(8) 6401 (7) 3846(.3) 4519(.5) 3960(.5) 2346(.5) 4368(1) 3671 (2) 3712(2) 4417(2) 4824(1) 3024(2) 3144(2) 4717(2) 5635(2) 5503(2) 4140(2) 4522(2) 4003(2) 3294(2) 3365(2) 5336(2) 4176(2) 2599(2) 2725(2) 4036(2) 4930(2) 3720(2) 4017(3) 1866(3) 1870(2) 2121 (2) 3306(4) 3366(4) 375(1) 474(2) 591 (2) 593(2) 409(6) 449(6) 489(7) 475(7) 449(6) 669(8) 727(9) 718(9) 650(8) 748(9) 432(6) 461 (6) 505(7) 509(7) 476(7) 671 (8) 723(9) 748(9) 670(8) 784(10) 809(10) 1068(14) 1179(15) 1080(13) 868(11) 1066(13) B 4.0(2) B 3.6(1) cs C6 C7 CB C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27A C27B = = -25- Structure Determination of Me2Si(C5Me4)2Sc(H)(PMe3) Crystals of Me2Si(C5Me4)2Sc(H)(PMe3) were grown by slowly cooling a saturated solution of 26 in toluene from 100 ° C to room temperature. The compound crystallizes in space group P2 1/c(#14), with a= 9.052(2) A, b = 28.717(2) A, c = 10.345(1) A, fJ = 115.56(1) 0 • A perspective view of Me2Si(C5Me4)2Sc(H)(PMe3) and the atom labelling scheme are given in Figure 3 and 4 respectively. Selected bond distances and angles are summarized in Tables 5 and 6. The scandium-hydrogen distance, 1.87(3) A, is somewhat longer than the sum of the covalent radius for hydrogen (0.30 A) and the single-bond metallic radius for scandium (1.44 A), but it is in the range of other distances between heavy atoms and hydrogen. The Sc-P bond length, 2. 752(1) A, is 0.21 A longer than the sum of the radii, which suggests that the bond is weaker than a single bond. The ring centroid-Sc-ring centroid angle was found to be 4 ° bigger than that of the Me2Si(C5Me4)2Sc-CH(SiMe3)2. This indicates that the presence of a CH(SiMe3)2 group causes a much bigger distortion in the geometry probably due to the size of the alkyl group. Other distances and angles are normal and are in close agreement with those found for Me2Si (CsMe4)2ScCH (SiMe3)2. -2 6- () 2.752A Figure 3: Mo lec ula r St ruc tur e of Me2Si(C 5Me4)2Sc(H){PMe3). - 27 - ME8 Figure 4: The atomic labeling for Me2Si(C5Me4)2Sc(H)(PMe3). - 28- Table 4: Crystal Data for Me2Si(C5Me4)2Sc(H)(PMe3) Formula : ScPSiC23H40 Formula weight : 420.59 Space group: P21/c(#14) Temperature: 23°C a = 9.052(2) A b = 28.717(2) A ,8=115.56(1) 0 c = 10.345(1) A v = 2426.8(7) A3 µ =4.56 cm-1 Crystal size 0.90x0.14x0.15 mm A.MoKa p =o. 71073 A =1.152 g cm-3 µrmax =0.21 -29- Table 5: Bond Lengths (A) for Me2Si(C5Me5)2Sc(H)(PMe3) Atoms Distance Atoms Distance Sc-HSc Sc-CP1 Sc-CP2 Sc-CP3 Sc-CP4 Sc-CPS Sc-CP6 Sc-CP7 Sc-CPS Sc-CP9 Sc-CP10 Sc-P P-PMe1 P-PMe2 P-PMe3 Si-SiM1 Si-SiM2 Si-CP1 Si-CP6 1.87(3) 2.438(3) 2.471 (3) 2.580(3) 2.602(3) 2.500(3) 2.456(3) 2.523(3) 2.611 (3) 2.588(3) 2.473(3) 2.753(1) 1.828(4) 1.825(5) 1.826(4) 1.850(4) 1.872(4) 1.879(3) 1.879(30) CP1-CP2 CP1-CP5 CP2-CP3 CP3-CP4 CP4-CP5 CP2-Me2 CP3-Me3 CP4-Me4 CP5-Me5 CP6-CP7 CP6-CP10 CP7-CP8 CP8-CP9 CP9-CP10 CP7-Me7 CP80Me8 CP9-Me9 CP10-Me10 1.431 (4) 1.438(4) 1.407(4) 1.406(4) 1.417(4) 1.502(4) 1.508(4) 1.506(4) 1.500(4) 1.440(4) 1.447(4) 1.414(4) 1.409(4) 1.404(4) 1.500(4) 1.496(4) 1.508(4) 1.501 (4) -30- Table S: Bond Angles( 0 ) for Me2Si(C5Me5)2Sc(H)(PMe3) Atom Atom Atom Angle R1 SiM1 CP1 CPS CP1 CPS CP1 Sc Sc Sc PMe2 PMe3 PMe3 CP2 CP5 CP5 CP3 Me2 Me2 CP4 Me3 Me3 CP5 Me4 Me4 CP4 Me5 Me5 CP7 CP10 CP10 CPS Me7 Me7 CP9 Mes Mes CP10 Me9 Me9 CP9 Me10 Me10 Sc Si Si Si Si Si Si R2 SiM2 SiM1 SiM1 SiM2 SiM2 CPS PMe1 PMe2 PMe3 PMe1 PMe1 PMe2 Si Si CP2 CP1 CP1 CP3 CP2 CP2 CP4 CP3 CP3 CP5 CP1 CP1 CP4 Si Si CP7 CPS CPS CPS CP7 CP7 CP9 CPS CP8 CP10 CPS CPS CP9 133.9(0) 101.4(20) 114.7(1) 114.9(1) 114.2(1) 114.7(1) 97.7(1) 113.9(1) 123.3(2) 111.S(1) 102.5(2) 99.3(2) 102.S(2) 125.4(2) 123.S(2) 105.S(2) 109.1 (2) 12S.0(3) 122.S(3) 1OS.3(3) 124.5(3) 12S.S(3) 10S.2(3) 127.1 (3) 123.9(3) 1OS.S(2) 129.1 (3) 121.8(3) 123.9(2) 125.7(2) 105.0(2) 109.1 (2) 128.0(3) 122.1 (3) 10S.3(3) 123.6(3) 127.1(3) 10S.2(3) 126.2(3) 125.2(3) 109.4(2) 127.2(3) 123.3(3) p p p p p p CP1 CP1 CP1 CP2 CP2 CP2 CP3 CP3 CP3 CP4 CP4 CP4 CP5 CP5 CP5 CPS CPS CPS CP7 CP7 CP7 CP8 CPS CPS CP9 CP9 CP9 CP10 CP10 CP10 - 31 - Table 7: Final Parameters X 104 for Me2Si(C5Me4)2Sc(H)(PMe3) Atom x y z Ueq Sc p Si PMe1 PMe2 PMe3 SiM1 SiM2 CP1 CP2 CP3 CP4 CPS Me2 Me3 Me4 Me5 CP6 CP7 CPS CP9 CP10 Me7 Me8 Me9 Me10 1202(.6) 3964(1) -1613(.9) 5497(4) 5203(5) 3533(5) -3328(4) -1969(4) 475(3) 910(3) 2607(3) 3269(3) 1974(3) -175(4) 3544(4) 5055(4) 2287(4) -1504(3) -531 (3) -163(3) -886(3) -1692(3) -174(4) 570(4) -968(4) -2654(4) 1342(.2) 1787 558(.3) 1900(1) 1569(2) 2386(1) 552(1) 1 (1) 607(1) 916(1) 1000(1) 749(1) 510(1) 1104(1) 1268(1) 668(1) 165(1) 1091 (1) 1123(1) 1596(1) 1868(1) 1567(1) 747(1) 1767(1) 2392(1) 1734(1) 2400(.5) 4337(1) 1058(.9) 3665(4) 6142(4) 4662(5) -759(4) 1820(4) 1095(3) 228(3) 910(3) 2204(3) 2339(3) -1230(3) 253(4) 3165(4) 3519(4) 2147(3) 3673(3) 4056(3) 2802(3) 1638(3) 4773(3) 5571 (3) 2732(4) 130(3) 306(1) 521 (2) 377(2) 811(11) 1116(17) 887(12) 638(10) 616(9) 334(6) 354(6) 387(7) 388(7) 363(7) 545(8) 597(8) 611 (10) 568(9) 320(6) 356(6) 382(7) 377(6) 342(6) 551 (8) 615(9) 576(9) 507(8) -32CONCLUSIONS The synthetic chemistry described offers straightforward routes to a broad family of etherand halide-free bis (cyclopentadienyl)scandium complexes. Mixed ring derivatives containing one pentamethylcyclopentadieny l ring are easily obtained from [Cp*ScCl21x· Alkylation in the presence of trimethylphosphine, followed by hydrogenation yields mixed ring hydride derivatives. Dimethylsilicon-bridged bis(cyclopentadienyl)compo unds were prepared as previously described for lanthanides. The use of bulky lithium alkyls such as LiCH2SiMe3 or LiCH(SiMe3)2 affords alkyl derivatives where the undesirable coordination of ether or halide is avoided. Hydrogenolysis readily yields the corresponding hydrides. The synthesis of Me2Si(C5Me4H)(R*C5H4) offered a convenient approach to the synthesis of optically active scandium complexes by introduction of chiral groups (R*) in one cyclopentadienyl ring. -33- Table 8 : 1 H NMR Data.8 Compound Cp *sc(acac)2 (90MHz,C5D5) Cp*ScCl2 (90MHz,THF-da) Cp*CpScCI (400MHz,C7Da,80 ° C) Cp*Cp°CsCI (90MHz,C5D5) Cp *cpSc(CH3)PMe3 (90MHz,C5D5) Cp *cp 0 Sc(CH3)PMe3 (90MHz,C505) Cp *cpSc-('7 1 , '7 5-C5H4)ScH (90MHz,C505) Cp *cp 0 Sc(H)PMe3 (90MHz,C5D5) Me2Si (C5Me4)2Li2 (90MHz,THF-da) Assignment c5(ppm) and J(Hz) '7 5-Cs(CH3)5 CH3COCHCOCH3 CH3COCHCOCH3 2.1 (s) 1.8(s) 5.2(s) '7 5·C5(CH3)5 2.1 (s) '7 5-Cs(CH3)5 '7 5-CsHs 2.0(s) 6.3(s) '7 5-Cs(CH3)5 '7 5-1,3,4-C5(CH3)3H2 '7 5-1,3,4-C5(CH3)3H2 1.9(s) 1.9(s); 1.B(s) 6.2(s) '7 5-Cs(CH3)5 '7 5-CsHs Sc-CH3 P(CH3)3 1.9(s) 6.0(s) -0.6(s) 0.7(d, J=6Hz) '7 5-Cs(CH3)5 '7 5-1,3,4-C5(CH3)3H2 '7 5-1,3,4-C5(CH3)3H2 Sc-CH3 P(CH3)3 2.1 (s) 1.9(s); 2.1 (s) 5.8(s) -0.?(s) 0.83(d, J=4Hz) '7 5-Cs(CH3)5 '7 5-CsHs '7 1 , '7 5·C5H4 2.0(s); 2.1 (s) 5.5(s) 6.0(m); 6.1 (m); 6.4(m); 6.6(m) '7 5-Cs(CH3)5 '7 5-1,3,4-C5(CH3)3H2 '7 5-1,3,4-Cs(CH3)3H2 P(CH3)3 2.1 (s) 1.7(s); 2.3(s) 5.8(s) 0.8(d, J=4.5Hz) CH3Si '7 5·C5(CH3)4 0.4(s) 1.8(s); 2.0(s) -34- Me2Si (t-butylCp)2Li2 (90MHz,THF-da) CH3Si 17 5-t-C4H9-C5H3 11 5-t-C4H9-C5H3 0.3(5) 1.2(5) 5.7-5.S(m) CH3Si 11 5-Cs(CH3)4 (CH3CH2)20 (CH3CH2)20 0.9(5) 1.8(5); 1.9(5) 1.1 (t, J=6.6Hz) 3.6(t, 6.6Hz) CH3Si 11 5-Cs(CH3)4 0.9(5) 1.83(5); 1.84(5); 1.85(5); 1.96(5) 0.1 (5) 1.1(5) Me2Si(C5Me4) 2ScCl-LiCl-(Et2 0)2 (90MHz,THF-da) Me2Si(C5Me4)2ScCH(SiMe3)2 (400MHz,C5D5) CH(Si(CH3)3)2 CH(Si(CH3)3)2 Me2Si (C5Me4)2Sc(H) PMe3) (400MHz,C1Da ,-10°C) Me2Si (t-butylCp)2ScCI (400MHz,CD2Cl2) Me2Si(t-butylC p)2ScCH2SiMe 3 (90MHz,CsDs) Me2Si (t-butylCp)2ScH (90MHz,CsDs) C5Me4H2 (90MHz,CDCl3) t-butylCpli (90MHz,THF-da) P(CH3)3 0.92(5); 1.03(5) 1.85(5); 1.94(5); 2.01 (5); 2.46(5) 0.78(d, J=3.5Hz) CH3Si 17 5-t-C4H9-C5H3 11 5-t-C4H9-C5H3 0.5(5); 0.8(5) 1.2 6.3(m); 6.5(m); 6. 7(m) CH3Si CH2Si(CH3)3 CH2Si(CH3)3 11 5-t-C4H9-C5H3 17 5-t-C4H9-C5H3 0.3(5) 0.5(5); 0.8(m) 1.0(5) 1.1 (5) 5.3(m); 5.8(m); 7.0(m) CH3Si 17 5-t-C4H9-C5H3 17 5-t-C4H9-C5H3 0.2(5); 0.8(5) 1.2(5) 6.3(m); 6.7(m); 7.3(m) C5(CH3)4H2 C5(CH3)4H2 1.8(5); 1.9(5) 2.7(5) t-C4H9-C5H4 t-C4H9-C5H4 1.2(5) 5.4(m) CH3Si 17 5-Cs(CH3)4 -35C5Me4SiMe2CI (90MHz,CsDs) Me2Si (C5Me4) (t-butylCp) Li2 (90MHz,THF-da) Me2Si(C5Me4)((-)-2-methylbuty1Cp)Li2 (500MHz,THF-da) Me2Si(C5Me4)((+)-menthylCp)Li2 (500MHz,THF-da) Me2Si(C5Me4)(t-butylCp)ScCl(THF) (400MHz,CsDs) Cs(CH3)4Si(CH3)2CI C5(CH3)4Si(CH3)2CI 1.7(s); 1.9(s) 0.12(s) CH3Si Cs(CH3)4 t-C4H9-C5H4 t-C4H9-C5H4 0.6(s) 1.9(s); 2.0(s) 1.2(s) 5.8-5.9(m) CH3Si Cs(CH3)4 CH3CH2-CH(CH3)-CH2C5H3 0.4(s) 2.05(s); 1.88(s) 2.46(m); 2.23(m) 5.72(m); 5.71 (m); 5.59(m) CH3Si Cs(CH3)4 C5H3 0.39(s) 2.1 (s); 1.9(s) 5.85(m); 5.77(m); 5.73(m) CH3Si 11 5-Cs(CH3)4 0.74(s); 0.87(s) 1.59(s); 2.03(s); 2.22(s); 2.11 (s) 1.57(s) 5.61 (m); 5.94(m); 6.02(m) 1.13(m) 3.35(m) 11 5-t-C4H9-C5H3 11 5-t-C4H9-C5H3 O-CH2-CH2 O-CH2-CH2 Me2Si(C5Me4)(t-butylCp)ScCH(SiMe3)2 (400MHz,CsDs) CH3Si 11 5-Cs(CH3)4 11 5-t-C4H9-C5H3 17 5-t-C4H9-C5H3 CH (Si (CH3)3)2 CH(Si(CH3)3)2 0.85(s); 0.57(s) 1.52(s); 1.BO(s); 1.91 (s); 2.11(s) 1.0B(s) 5.92(m); 6.17(m); 7.00(m) 0.05(s) -0.25(s); 0.33(s) Me2Si(CsMe4)((-)-2-methylbutylCp)ScCH(SiMe3)2 (500MHz,CsDs) CH(Si(CH3)3)2 1.72(s); 1.73(s); 1.74(s); 1.83(s); 1.85(s); 1.86(s) 2.16(d); 2.43(d); 2. 79(d); 3.01 (d) 5.31 (m); 5.36(m); 5.38(m);6.8 9(m) O.OB(s); 0.09(s); 0.1 B(s); 0.19(s) Me2Si (C5Me4) ((+)-menthylCp)ScCH (SiMe3)2 (500MHz,CsDs) 1.63(s); 1.69(s); -361.771.78(s); 1.92(s); 1.93(s); 2.02(s) 5.58(m); 5.68(m); 5.92(m); 6.15(m); 7.19(m); 7.23(m) 0.02(s); 0.03(s); 0.27(s); 0.29(s) Me2Si (C5Me4) ((+)-menthylCp) ScH (500MHz,CsDs) ScH 8 1.68(s); 1.69(s); 2.08(s); 2.11(s); 2.17(s); 2.22(s); 2.23(s); 2.24(s)(s) 5.69(m); 5.78(m); 6.37(m); 6.54(m); 7.00(m); 7.08(m) 4.25(broad) NMR spectra were taken at ambient temperature, unless otherwise stated. -37EXPERIMENTAL All manipulations were carried out by using either high vacuum line or glove-box techniques.£6] vermiculiteC71 Hydrogen, deuterium, and argon were purified by passing over MnO on and activated 4A molecular sieves. Benzene, toluene, cyclohexane, methylcyclohexane, petroleum ether (bp 30-60 ° C), perdeuterobenzene, and perdeuterotoluene were first vacuum transferred from LiAIH4 or 4A molecular sieves and then from titanocene prior to use[S]. Diethyl ether, tetrahydrofuran and perdeuterotetrahydrofuran were stored over benzophenone ketyl. and Methanol, aluminum trichloride, scandium oxide, dimethyldichlorosilane trimethylphosphine were used as received. Sc(acac)3, [9] LiCH2SiMe3, [ 1O] (Me3Si)2CHLi·0.5 Et20,l 11 1 Cp*MgCl·THF,l 121 ScCl3·3THF,l 131 Cp0 H,l 141 t-butylC 5H5,[ 15] (-)-2-methylbuty1CsHsl 16] and (+)-menthylCsHsl 171 were prepared according to literature procedures. CpLi, Cp0 Li, (-)-2-methylbutylC5H4Li and (+)-menthylC5H4Li were prepared by reacting C5H5, Cp0 H, (-)-2-methylbutylC5H5 and (+)-menthylC5H5 with n-butyllithium in petroleum ether. 1H NMR spectra were recorded on Varian EM 390 (90 MHz), JEOl GX 400 (399.78 MHz), and Brucker WM500 (500.13 MHz) spectrometers. Elemental analyses were performed by the analytical facility of the California Institute of Technology. Molecular weights were determined by using the vapor phase osmometry techniques developed by Signer and described by Clark[18] Cp*Sc(acac)2 (1 ): Sc(acac)3 (7.5 g, 21.9 mmol) and Cp*MgCI · THF (6.0 g, 22.5 mmol) were stirred in toluene (50 ml) at room temperature for three hours. Toluene was removed in vacuo. The subsequent operations were carried out in air. Methanol (30 ml) was added to the solid residue. The mixture was stirred for 10 minutes, filtered, washed with three portions of methanol (30 ml each), and dried. Yield 7.2 g (86.7%). Recrystallization from hot toluene gave 1 as yellow crystals. Anal. Calcd. for C20H29Sc04: C, 63.48; H, 7.72. Found: C, 63.27; H, 7.52. -38[Cp*ScCl 2]x (2): Cp*Sc(acac)2 (2.0 g, 5.3 mmol) and aluminum trichloride (1.4 g, 10.5 mmol) were stirred in toluene (20 ml) at room temperature for three hours. The toluene was heated to boiling, and the precipitate was filtered off and dried. 1.1 g (82. 7%) of white solid 2 were obtained. Anal. Calcd. for C10H15ScCl2: C, 47.83; H, 6.02. Found: C, 47.32; H, 5. 73. Cp*CpScCI (4): Cp*ScCl2 (7.0 g, 27.9 mmol) and Cpli (2.2 g, 30.6 mmol) were heated in toluene (80 ml) to 80°C for four hours. The toluene was removed in vacuo, and the solid residue was soxhlet extracted with benzene over twelve hours. The benzene was removed and petroleum ether (ca. 20 ml) was added. The solid was filtered and washed with petroleum ether. 5.9 g (75.4%) of yellow solid 4 were obtained. Recrystallization from hot toluene gave an analytically pure sample. Anal. Calcd. for C1sH20ScCI: C, 64.18; H, 7.18. Found: C, 64.19; H, 7.11. Cp*Cp 0ScCI (5): Cp*ScCl2 (2.09 g, 8.3 mmol) and Cp 0Li (1.0 g, 8.8 mmol) suspended in toluene (30 ml) were heated to 80 °C for four hours. The solution was filtered, the toluene was removed in vacuo, and petroleum ether (ca. 10 ml) was added. The yellow solid 5 was filtered and dried. Yield 1.75 g (65%). 5 was purified by sublimation at 140 °C and 104 torr. Anal. Calcd. for C10H25ScCI: C, 66.97; H, 8.12. Found: C, 66.60; H, 7.99. Cp*CpSc(CH3){PMe3) (6): Cp*CpScCI (0.315 g, 1.12 mmol), methyl lithium (0.03 g, 1.36 mmol), and PMe3 (0.12 ml, 1.24 mmol) were dissolved in diethyl ether (20 ml). After stirring at room temperature for three hours, the diethyl ether was removed in vacuo and replaced with petroleum ether (ca. 20 ml). The lithium chloride was removed from the solution by filtration, and the solution was cooled overnight (-78°C) to afford 6 (0.22 g, 58.4%) as white crystals. Anal. Calcd. for C19H32ScP: C, 67.84; H, 9.42. Found: C, 67.62; H, 9.42. -39Cp*CpoSc(CH3)(PMe3) (7): The procedure for 6 was followed except Cp*Cp0ScCI (1.00 g, 31 mmol), methyl lithium (0.08 g, 3.6 mmol), and PMe3 (0.35 ml, 3.6 mmol) were dissolved in diethyl ether(20 ml). The product is less soluble in petroleum ether than Cp*CpSc(CH3)(PMe3) and required several extractions to remove it from the lithium chloride. Cooling the solution to -78°C for two hours afforded 7 (0.65 g, 55.5%) as white crystals. Anal. Calcd. for C22H3aScP: C, 69.82; H, 10.12. Found: C, 69. 79; H, 10.11. Cp*CpSc-(71 1-71 5-C5H4)Cp*Sc-H (8): Cp*CpSc(CH3)(PMe3) (0.5 g, 1.48 mmol) dissolved in petroleum ether ( ca. 5 ml) was loaded into a thick-walled glass reaction vessel. The flask was cooled to -196°C, and one atmosphere of dihydrogen was admitted. minutes a white solid began to precipitate. After stirring for thirty Stirring was continued for two hours, and the suspension was transferred to a frit assembly. The white solid 9 was filtered and dried. Yield 0.23 g (63%). Recrystallization from hot methylcyclohexane afforded 9 as white crystals. Anal. Calcd. for C30H40Sc2: C, 73.45; H, 8.21. Found: C, 73.32; H, 8.06. Cp*Cp0Sc(H)(PMe3) (9): Cp*CpSc-(71 1-71 5-C5H4)Cp*Sc-H. The same procedure was used as described for 8 is very soluble in petroleum ether; to obtain crystals a concentrated petroleum ether solution was cooled to -78 °C for twenty four hours. Cp*Cp0Sc(CH3)(PMe3) (0.2 g, 53 mmol) yielded 0.12 g (62%) of 8 as yellow crystals. Anal. Calcd. for C21 HasScP: C, 69.21; H, 9.96. Found: C, 69.31; H, 10.20. C5Me4H2 : LiAIH4 (14.16 g, 723 mmol) and anhydrous diethyl ether (500 ml) were placed in a 2 l three-necked flask equipped with a reflux condenser, a magnetic stirrer and a dropping funnel. 2,3,4,5-Tetramethyl-2-cyclopentenone (100 g, 373 mmol) dissolved in anhydrous diethyl ether (120 ml) was added, dropwise and with stirring, to the reaction flask at a rate which maintained gentle refluxing. After the addition was complete, the reaction solution was refluxed for an additional 30 minutes and cooled to room temperature. The complex was -40hydrolyzed and the excess LiAIH 4 was destroyed by the cautious addition, dropwise and with stirring, of water (35 ml). The resulting reaction mixture was poured into 1200 ml of cold aqueous 10% sulfuric acid. The ether layer was separated, and the residual aqueous phase extracted with three 300 ml portions of ether. The combined ether solutions were concentrated in vacuo to ca. 200 ml, and concentrated sulfuric acid (1 ml) was added (argon atmosphere). The mixture was stirred for three hours at room temperature, and the mixture was washed successively with one 100-ml portion of water and one 100-ml portion of saturated, aqueous NaHC03 and then dried over MgS04. The product was vacuum distilled (bp: 60-70°C/37-40 torr). Yield 38 g (43%). C5Me4HLi: C5Me4H2 (20.0 g, 164 mmol) was dissolved in tetrahydrofuran (100 ml) and cooled to-78°C. n-Butyllithium (1.6M in hexanes,105 ml, 168 mmol) was added via syringe. The reaction mixture was slowly warmed to room temperature and stirred for two hours. The white precipitate was filtered and washed with petroleum ether and dried. Yield 19 g (90.5%). Me2Si(C5Me4)2Li2 (10): C5Me4Hli (15.0 g, 117 mmol) was suspended in tetrahydrofuran (500 ml). Me2SiCl2 (7.0 ml, 58 mmol) was added via syringe against an argon counterflow, and the mixture was refluxed for 60 hours. The tetrahydrofuran was removed in vacuo and replaced with petroleum ether (200 ml). The lithium chloride was removed from the solution by filtration. The petroleum ether was removed in vacuo leaving an oily residue. The oil was redissolved in petroleum ether (200 ml), cooled to -78°C, and n-butyllithium (1.6M in hexanes, 75 ml, 120 mmol) was added. The solution was warmed to room temperature and stirred overnight. The product was filtered, washed with petroleum ether and dried. Yield 15.4 g (89. 7%). t-butylC5H4Li: t-ButylC5H5 (20.0 g, 164 mmol) dissolved in diethyl ether (100 ml) was cooled to -78°C and n-butyllithium (1.6M in hexanes, 105 ml, 168 mmol) was added via syringe against an argon counterflow. The mixture was warmed to 0°C and stirred for one hour. The mixture was warmed to room temperature and stirred for two additional hours. crystalline solid was filtered, washed with diethyl ether, and dried. Yield 16.8 g (80%). The white -41 Me2Si(t-butylC5H3)2Li2 (11): t-ButylC5H4Li (10.0 g, 78 mmol) was dissolved in tetrahydrofuran (100 ml) and cooled to -78°C. Me2SiCl2 (4.7 ml, 38.7 mmol) was added via syringe against an argon counterflow, and the solution was stirred at room temperature for one hour. The tetrahydrofuran was removed in vacuo and replaced with petroleum ether (ca. 80 ml). The lithium chloride was removed from the solution by filtration. A reflux condenser was attached to the reaction flask, and n-butyllithium (1.6M in hexanes, 50 ml, 80 mmol) was added against argon counterflow over 30 minutes due the exothermicity of the reaction. The mixture was refluxed for two hours. The white solid, 13,was filtered, washed with petroleum ether, and dried. Yield 11.6 g (95%). (C5Me4)SiMe2Cl (12): C5Me4Hli (10.0 g, 78.1 mmol) was suspended in tetrahydrofuran (150 ml) and cooled to -78°C. Me2SiCl2 (9.7 ml, 80 mmol) was added via syringe against an argon counterflow. The mixture was warmed to room temperature and stirred for two hours. The tetrahydrofuran was removed in vacuo and replaced with petroleum ether (ca. 1OOml). The lithium chloride was removed from the solution by filtration. The petroleum ether was removed in vacuo, leaving the product as a colorless liquid that solidified upon cooling to -40 ° C. Yield 16 g (95.5%). Me2Si(C5Me4)(t-butyl-C5H3)Li2 (13): t-ButylC5H4Li (3.0 g, 23.4 mmol) dissolved in tetrahydrofuran (40 ml) was added to C5Me4HSiMe2CI (5.0 g, 23.3 mmol) dissolved in tetrahydrofuran (20 ml). The solution was stirred for twelve hours, and the tetrahydrofuran was removed in vacuo and replaced with petroleum ether (c.a. 50ml). The lithium chloride was removed from the solution by filtration. The petroleum ether was removed in vacuo leaving an oily liquid. Diethyl ether (ca. 80 ml) was added, the solution was cooled to -78°C, and n-butyllithium (1.6 M solution in hexanes, 30 ml, 48 mmol ) was added. The solution was warmed to room temperature and stirred for three hours. Diethyl ether was removed in vacuo and replaced with petroleum ether (ca. 100 ml). The white solid, 13, was filtered, washed with fresh petroleum ether, and dried. Yield 7.0 g (92%). -42Me2Si(C5Me4)((-)-2-methylbutylC5H3)Li2 (14): The same procedure was used as described for 13, except that 13.0 g (94.3%) of 14 were isolated from ((-)-2-Methyl butylC5H4)Li (6.0 g, 42.2 mmol) dissolved in 80 ml of tetrahydrofuran and C5Me4HSiMe2CI (9.0 g, 41.9 mmol) dissolved in 40 ml of tetrahydrofuran. Me2Si(C5Me4)((+)-menthylC5H3)Li2 (15): The same procedure was used as described for 13, except that 5.0 g (98%) of 15 were isolated from ((+)-MenthylC5H4)Li (2.93 g, 13.9 mmol) and C5Me4HSiMe2CI (3.0 g, 14 mmol). Me2Si(C5Me4)2ScCI · LICI · (Et20)2 (16): Me2Si(C5Me4)2Li2 (4.0 g, 12.8 mmol) and ScCl3 · 3THF (4. 7 g, 12.8 mmol) suspended in toluene (80 ml) were heated at 80 ° C for three days. The lithium chloride was removed from the solution by filtration. The solution was concentrated to ca. 1O ml, and petroleum ether (50 ml) was added in vacuo. The precipitate was filtered, washed three times with fresh solvent, and dried. Yield 3.5 g (48 %). 16 was isolated as long white needles by recrystallization from diethyl ether cooled to -78°C. Anal. Calcd. for C2aHsoScLiCl202: C, 59.04; H, 8.85. Found: C, 59.20; H, 8.52. meso-Me2Si(t-butylC5H3)2ScCI (17): Me2Si(t-butylC5H3)2Li2 (31.3 g, 100.3 mmol) and ScCl 3 · 3THF (36.9 g, 100.4 mmol) were heated to 80 ° C in toluene (500 ml) for two days. The lithium chloride was removed from the solution by filtration. Toluene was removed in vacuo and replaced with petroleum ether (ca 1OOml). The solid 17 was filtered, washed with fresh petroleum ether, and dried. Yield 18.7 g (49.2 %). Recrystallization from boiling toluene yielded 17 as white crystals. Anal. Calcd. for C20H30SiScCI : C, 63.33; H, 7.98. Found C, 62.85; H, 7.59. Me2Si(C5Me4)(t-butyl-C5H3)ScCl·LiCl·THF2 (18): ScCl3 · 3THF (2.6 g, 7.07 mmol) and Me2Si(C5Me4)(t-butyl-C5H3)Li2 (2.24 g 7.17 mmol) were heated in toluene (50 ml) for 12 hours. The lithium chloride was removed from the solution by filtration. The toluene was removed in -43vacuo and replaced with petroleum ether (50 ml), and the insoluble white product was filtered and dried. Recrystallization from boiling cyclohexane yielded white crystals of 18 (1.3 g, 40.8%). Anal. Calcd. for C2aH45SiScCl202Li: C, 59.46; H, 8.20. Found: C, 58.98; H,7.46. Me2Si(C5Me4)2ScCH(SiMe3)2 (21): Me2Si(C5Me4)2ScCl-LiCl-(Et20)2 (1.16 g, 2 mmol) and (Me3Si)2CHLi · 0.5 Et20 (0.49 g, 2.4 mmol) were dissolved in toluene (20 ml) and stirred at room temperature for twelve hours. The mixture was heated to 100 ° C, filtered, and slowly cooled to room temperature. The bright yellow crystalline product, 21, was filtered and dried. Yield 0.86 g (84%). Anal. Calcd. for C27H49Si3Sc: C, 64.49; H, 9.82. Found: C, 64.56; H, 9.61. meso-Me2Si(t-butylC5H3)2ScCH2SiMe3 (22): meso-Me2Si(t-butylC5H3)2ScCI (18. 7 g, 49.4 mmol) and LiCH2SiMe3 (1.0M in pentane, 50 ml, 50 mmol) were stirred in toluene (150 ml) at room temperature for twelve hours. Toluene was removed in vacuo and replaced with petroleum ether (ca. 80 ml). The lithium chloride was removed from the solution by filtration, and the solution was cooled overnight to -78°C to afford 22 as yellow crystals. Yield 12.3 g (57.8%). Anal. Calcd. for C24H41Si2Sc: C, 66.93; H, 9.60. Found: C, 66.81; H, 9.49. Me2Si(C5Me4)(t-butyl-C5H3)ScCH(SiMe3)2 (23): Me2Si(C5Me4)(t-butyl-C5H3)ScCI · THF (1.0 g, 2.2 mmol) and (Me3Si)2CHLi · 0.5 Et20 (0.45 g, 2.2 mmol) were dissolved in toluene (30 ml). The solution was stirred at room temperature for 5 hours. The toluene was removed in vacuo and replaced with petroleum ether (10 ml). The lithium chloride was removed from the solution by filtration, and the petroleum ether solution was cooled to -78 ° C. afforded 0.69 g (62. 7%) of 23 as yellow crystals. Anal. Calcd. for C27H49$i3Sc: C, 64.49; H, 9.82. Found: C, 64.25; H, 9.42. Cold filtration -44Me2Si(C5Me4)((-)-2-methylbutylC5H3)ScCH(SiMe3)2 (24): methylbutylC5H3)Li2 (1.0 g, 3.2 mmol) and ScCl3 · 3THF (1.2 g, 3.2 mmol) were refluxed for twelve hours in toluene (30 ml). The solution was filtered to remove the lithium chloride. The toluene was removed in vacuo leaving a viscous oil (19). The oil was redissolved in toluene (30 ml), and (Me3Si)2CHLi · 0.5 Et20 (0.65 g, 3.2 mmol) was added. The bright yellow solution was stirred at room temperature for 5 hours. The toluene was removed in vacuo and replaced with petroleum ether (ca. 20ml). The lithium chloride was removed from the solution by filtration. The petroleum ether was removed in vacuo and replaced with diethyl ether. Cooling to -78°C afforded 0.62 g (37% overall yield from 14) of 24 as yellow crystals. Anal. Calcd. for C2aHs1 Si3Sc: C, 65.06; H, 9.94. Found: C, 64.89; H, 9.49. Me2Si(C5Me4){(+)-menthylC5H3)ScCH(SiMe3)2 (25): 15 (3.0 g, 8.2 mmol) and ScCl3 · 3THF (3.0 g, 8.2 mmol) were dissolved in toluene (50 ml). The mixture was heated at 80°C for 5 hours, filtered to remove the lithium chloride, and dried. Me2Si(C5Me4)((+)-MenthylC5H3)ScCI · THF (20) was isolated as a white powder from petroleum ether. Me2Si(C5Me4)((+)-MenthylC5H3)ScCI · THF (1.0 gr, 20mmol) and (Me3Si)2CHLi · 0.5 Et20 (0.4 g, 20 mmol) were stirred for 4 hours in toluene (30 ml). Yellow crystals of 25 were isolated as described in the case of 24. Yield 0.30 g (30 % overall yield from 15). Anal. Calcd. for C33H59Si3Sc: C, 67.75; H, 10.16. Found: C, 67.79; H, 9.66. Me2Si(C5Me4)2Sc(H)(PMe3) (26): The same procedure was used as described for 9. Me2Si(C5Me4)2ScCH(SiMe3)2 (0.35 g, 0.7 mmol) and PMe3 (0.06 g, 0.8 mmol) were dissolved in petroleum ether (ca. 5ml) and after stirring for 12 hours under 4 atmospheres of dihydrogen, 0.23 g (78%) of 26 were isolated. Recrystallization from hot toluene yields 26 as yellow crystals. Anal. Calcd. for C23H40SiScP: C, 65.68; H, 9.59. Found: C, 65.32; H, 9.32. -45meso-Me2Sl(t-butylC5H3)2ScH (27): The same procedure was used as described for 9. meso-Me2Si(t-butylC5H3)2ScCH2SiMe3 (0.5 g, 1.16 mmol) was dissolved in petroleum ether (ca. 1Oml), and after stirring for 12 hours under 4 atmospheres of dihydrogen, 0.24 g (60%) of 27 were isolated. Recrystallization from hot methylcyclohexane yielded 27 as white crystals. Anal. Calcd. for C20H31ScSi: C, 69.73; H, 9.07. Found: C, 69.80; H, 8.98. Me2Si(C5Me4}((+)-menthylC5H3)ScH (28): 25 (0.2 g, 0.34 mmol) was dissolved in petroleum ether (5 ml) and the solution was loaded in a thick-walled glass bomb. One atmosphere of dihydrogen was admitted at -196°C, the system was warmed slowly to room temperature and stirred for 12 hours. The suspension formed was transferred to a frit and the solid product filtered as a yellow powder. Yield 0.085g (58%). Anal. Calcd. for C33H59SiSc: C, 73.19; H, 9.69. Found: C, 71.52; H, 9.17. X-ray Structure Determination of Me2Si(C5Me4)2ScCH(SiMe3)2: Crystals of Me2Si(C5Me4)2ScCH(SiMe3)2 were grown by slowly cooling a saturated solution of 21 in toluene from 100°c to room temperature and sealed in glass capillaries under dinitrogen to prevent decomposition. Photographs of an irregular crystal about 0.68 x 0.39 x 0.41 mm showed monoclinic symmetry. It was centered on a Nonius CAD-4 diffractometer equipped with graphitemonochromated MoKa radiation and cell dimensions plus an orientation matrix were obtained from the setting angles of 25 reflections with 40° < 28 < 47°. Altogether 12,623 reflections were scanned in a ()-28 mode in the quadrants ±h, ±k, I with 3 ° :S 28 :S 50 °, including three check reflections measured every 10,000 seconds of X-ray exposure. The check reflections showed no fluctuations greater than those expected from counting statistics. Backgrounds were measured for each reflection at each end of the scan; an average background as a function of 28 was calculated and used to correct the measured scan counts. Absences in the data of OkO, k =2n+1, and hOI, I = 2n+1 uniquely identify the space group as P21/c, #14. After deleting space group absences and merging equivalent reflections, 5174 reflections remained of which 4662 had F0 2 > -46O and 4049 had F0 2 > 3u(F0 2). The data were corrected for Lorentz and polarization factors but not for absorption; µrmax is 0.17 and the two forms of data averaged together with a goodness of fit of 1.07. Variances of the individual reflections were assigned based on counting statistics plus an additional term, 0.014 12. Variances for the merged reflections were obtained by standard propagation of error plus another term, 0.014 T2. Scattering factors were taken from reference 19. All 5174 measured reflections were used in the solution and refinement of the structure. The coordinates of the scandium atom were obtained from a Patterson map and the remaining non-hydrogen atoms were found with successive structure factor-Fourier calculations. After a few cycles of full-matrix least squares, minimizing 'i.w(F0 2 - Fc2), hydrogens atoms were included at idealized positions based on difference Fourier maps calculated in the planes where they were expected. A C-H distance of 0.95A was assumed. Each hydrogen atom was given an isotropic thermal parameter of 10 % greater than that of the carbon to which it was bonded. The hydrogen parameters were not refined, but they were adjusted 3 times during the refinement. No hydrogen was assigned to C27, the carbon atom directly bonded to scandium, because that atom displayed planar coordination by scandium, Si2 and Si3. The least squares converged (all atoms anisotropic except C1-C5 and C11-C15) with R = 0.068 for all the reflections with F 2 > O; the 0 goodness of fit was 4.21. The thermal ellipsoid of C27 was elongated perpendicu lar to the Sc-Si2-Si3 plane, suggesting that C27 might be disordered, with half occupancy of a site above the plane and half below. A search of a three-dimensional difference map for a hydrogen atom bonded to C27 revealed two small peaks, one above and one below the plane, approximately where they were expected. Refinement continued with new hydrogen positions, all non- hydrogen atoms except C27 anisotropic, C27 split into two half atoms and including a secondary extinction parameter. At convergence, R =0.054 and the goodness of fit was 3.31; for the data with F0 2 > 3u{F0 2), R = 0.045. The secondary extinction parameter refined to 0.72(7)X10-6. X-ray Structure Determinat ion of Me2Si(C5Me4)2Sc{H)(PMe3): Crystals were grown by slowly cooling a saturated solution of 26 in toluene from 100°C to room temperature, and sealed -47in glass capillaries under dinitrogen to prevent decomposition. Unit cell dimensions were obtained from the setting angles of 24 reflections (four members of each six forms) with 28 =37°. The three check reflections showed only small changes in intensity; no corrections were made for decay. The data were corrected for absorption, and the scandium, silicon and phosphorus were obtained from a Paterson map. A subsequent structure factor-Fourier calculation showed the 23 carbon atoms. Refinement was by full matrix least squares, with the methyl hydrogen atoms placed at calculated positions and then included in the refinement. The maximum excursion in the final difference map was +0.41 eA-3. Calculations were done with programs of the CRYM Crystallographic Computing System and Ortep. Scattering factors and corrections for anomalous scattering were taken from the literaturel1 71. The final A-index was 0.065 and the final goodness of fit 1.45. -48- REFERENCES 1. (a) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T.J. J. Am. Chem. Soc. 1985, 107, 8111. (b) Thompson, M.E.; Baxter, S.M.; Bulls, A.R.; Burger, B.J.; Nolan, M.C.; Santarsiero, B.D.; Schaefer, W.P.; Bercaw, J.E. J. Am. Chem. Soc. 1987, 109, 203. (c) Watson, P.L. J. Am. Chem. Soc. 1982, 104, 337. 2. Atwood, J.L.; Smith, KO. J. Chem. Soc., Dalton Trans. 1973, 2487. 3. den Haan, KH.; Teuben,J.H. Chem. Comm. 1986, 682. 4. Frendrick, C.M.; Mintz, E.A.; Schertz, L.D.; Marks, T.J.; Day, V.W. Organometallics 1984, 3, 819. 5. Jeske, G.; Schock, LE.; Swepston, P.N.; Schumann, H.; Marks, T.J. J. Am. Chem. Soc. 1985, 107,8103. 6. Burger, B.J.; Bercaw, J.E. "New Developments in the Synthesis, Manipulation and Characterization of Organometallic Compounds", Wayda, A.L.; Darensbourg, M.Y. ACS Symposium Series, 357, 1987. 7. Brown, T.L.; Dickerhoof, D.W.; Bafus, D.A.; Morgan, G.L. Rev. Sci. lnstrum. 1962, 33, 491. 8. Marvich, R.H.; Brintzinger, H.H. J. Am. Chem. Soc. 1971, 93, 2046. 9. Morgan, G.T.; Moss, H.W. J. Chem. Soc. 1914, 189. 10. Schrock, R.R.; Fellman, J.D. J. Am. Chem. Soc. 1978, 100, 3359. LiCH2SiMe3 was recrystallized from petroleum ether cooled to -78 ° C. 11. Cowley, A.H.; Kemp, R.A. Synth. React. lnorg. Met. Org. Chem. 1981, 11, 591. 12. Fagan, P.J.; Manriquez, J.M.; Matta, E.A.; Seyam, A.M.; Marks, T.J. J. Am. Chem. Soc. 1981, 103, 6650. 13. Manzer, L.E. lnorg. Syn. 1982, 21, 135. 14. Wolczanski, P.T.; Bercaw, J.E. Organometallics 1982, 1, 793. 15. Riemschneider, R.; Reisch, A.; Hovak, H. Monatsh. 1960, 91, 805. 16. Huang, Q.; Ouian, Y. Synthesis, 1987, 911. -4917. Cesarotti, E.; Kagan, H.B.; Goddard, R.; Krueger, C.J. Organomet. Chem. 1978, 162, 297. 18. Clark, E.P. /nd. Eng. Ed., Anal. Ed. 1941, 13, 820. 19. International Tables for X-ray Crystallography, Vol IV, p. 71, p. 49; Birmingham , Kynoch Press, 1974. - 50- CHAPTER2 CARBON-CAR BON BOND FORMATION CATALVZED BYSCANDOC ENE HYDRIDES DERIVATIVES. THE CATALYTIC DIMERIZATION OF a-OLEFINS AND CYCLIZATION OF a,w-DIOLEFIN S. - 51 - INTRODUCTION The polymerization of olefins catalyzed by transition metal complexes constitutes a widely utilized procedure for the construction of carbon-carbon bonds. The polymerization or oligomerization of a-olefins is catalyzed by either soluble or insoluble catalysts, the former being better understood. Most of the work on the soluble catalysts had its origin in the work developed by Ziegler and Natta. The polymerization or oligomerization of a-olefins, promoted by ZieglerNatta type catalysts (e.g. Cp2TiCl2-RnAICl3-n). involves the following steps:C 11 =:/R n~ R '-nM~ '-n~ R '-nM~ R R Initiation R LnM-H Propagation R + polymer Termination LnM-R + polymer The molecular weight distribution depends on the relative rates of the propagation and termination steps. A "Schultz/Flory" type molecular weight distribution is most common. However, two extreme situations can arise. If the rate of propagation is much greater than the rate of termination high molecular weight polymers are formed. If the rate of termination is much faster than the rate of propagation, only dimers are expected. A new generation of Ziegler-Natta type catalysts,[2] those that do not require a co-catalyst, is based on bispentamethylcyclopentadienyl-early transition metal or -lanthanide alkyl or hydride derivatives. The first example reported was Cp*2Lu-CH3J3J This catalyst polymerizes - 52ethylene at a high rate to give high density polyethylene, even in the presence of Lewis bases such as diethylether. The insertion of ethylene into the metal-hydride or metal-alkyl bond provides a new metal-alkyl fragment which is a catalyst itself for the continued polymerization of ethylene. Mechanistic studies have provided evidence for a transition state wherein metal-carbon bond breaking and making occur concertedly.£4 1 From kinetic studies of the insertion of ethylene into the Sc-C bonds in the Cp*2Sc-R system, it was concluded that the rate of insertion is faster when the alkyl group is a long chain (Cp*2Sc-CH3 compared with Cp*2ScCH2CH2CH3) .£4 ] However, despite the ease with which ethylene is polymerized by catalysts of the type Cp*2M-R ( M =lanthanide; R = H, alkyl), higher olefins such as propane, 1-butene, or 1-pentene are oligomerized at best suggesting that the rate of termination is close to the rate of propagation and oligomers are normally obtained. Furthermore, decomposition pathways such as a-bond metathesis severely limit catalyst lifetimes so that only a few turnovers have been observed.£5) This same type of behavior is widely found with Ziegler-Natta catalysts. The majority of the studies in this area have been intended to elucidate the mechanism of carbon-carbon bond formation. Practical applications for the synthesis of polymers and utilization of these compounds as catalysts for other simple transformations have not yet been reported. Organoscandium complexes, based on the bispentamethylcyclopentadienyl scandium system,[6] react with a-olefins via a sigma-bond metathesis pathway, and hence no catalytic activity is expected. This chapter describes the olefin chemistry of the scandium-hydride compounds reported in Chapter 1. Surprisingly, we found that although a-olefins are not polymerized, there is a clean catalytic reaction which promotes almost exclusively the formation of head to tail dimers. Under the same conditions, a, w-diolefins are catalytically converted to methylenecycloalkanes. Remarkably, the presence of nitrogen and sulfur in the a, w-diolefin is tolerated by the catalyst. Further investigation showed that multiple insertions occur for -535-methylene-1,8-nonadiene and 5,8-dimethylene-1, 11-dodecadiene in an intramolecular fashion to produce catalytically spiro hydrocarbons. - 54- RESULTS AND DISCUSSION Catalytic Dimerizatio n of a-olefins In order to evaluate the reactivity Cp*Cp 0 Sc(H)(PMe3), of Cp*CpSc- a-olefins, each was treated with excess propane (ca. 20 equivalents/mo! Sc(H)) under the same conditions (solvent and concentration). The reactions were followed by 1H NMR to monitor the disappearance of a-olefin. The time and the temperature required to complete the conversion of 20 equivalents of propane are shown in Table 1. Table 1: Catalytic Dimerization of Propane Complex Cp*Cp 0Sc(H)(PMe 3) 1 5 Conditions 1 hr, ao 0 c Cp*CpSc(Tt ·Tt )CpCp*Sc(H) 18 hrs, [Me 2 Si(t-butylC 5 H3 ) 2 Sc(H)] 2 1 hr, Me 2 Si (C 5 Me 4'2Sc(H)) PMe3 ) < 5 min, ao0 c ao0 c 2s 0c In each case the product formed from propane was identified by NMR (1H, 13C) and gas chromatogr aphy and correspond s to 2-methyl-1-pentene. Additionally, in the reaction with Me2Si(C5Me4)2Sc(H)(PMe3) a small proportion (ca. 9%) of 2,4-dimethyl-1-heptene was formed. The observation that all of the scandium hydrides shown in Table 1 promote the selective head-to-tail insertion of propene to give only one dimer contrasts with product distributions obtained with other catalysts, for which the dimerization of propene affords a mixture of all four isomers. [7] The formation of only one dimer demonstrates the selectivity of the catalyst; the two -55the two cyclopentad ienyl ligands force olefin insertion to afford the least crowded of the alkyl intermediates. The catalytic cycle which explains the formation of 2-methyl-1-pentene and 2,4dimethyl-1- heptene is shown in Scheme 1. Scheme 1 Sc/'.... ./ 1~ Sc-H -A ) Sc~ ~ -~ i~ Sc~ Unfortunately, Cp*Cp 0 Sc(H)(PMe3} and Cp*CpSc(77 1-77 5-C5H4}Cp*Sc(H) decompose during the reaction, the former to unidentified products and the latter to 1. The low thermal stability of Cp*Cp 0 Sc(H)(PMe3) and Cp*CpSc(77 1-77 5-CsH 4)Cp*Sc(H) discourages their use in promoting catalytic transformations. However, (meso-Me2Si(t-butylC5H3)2ScH)2 (abbreviated (DpScH)2) and Me2Si(C5Me4)2Sc(H)(PMe3) (abbreviated OpSc(H)(PMe3)) show higher thermal stability than the mixed ring scandium hydrides. <::::;;7 \ .....·· H--s/"> \ ~~ ao0c ~/ \ / Sc VSc~ 1 - 56After all the propane is converted to 2-methyl-1-pentene in the OpSc(H)(PMe3) system, OpSc-CH2CH(CH3)-CH2CH2CH3 is the only scandium product as characterized by 1H NMR. Ultimately, OpSc-CH2CH(CH3)-CH2CH2CH3 is converted to OpSc(CH3)(PMe3) in approximately 24 hours at room temperature. Significantly, this reaction corresponds to ,8-methyl elimination, which has been recognized as a possible termination step in the polymerization of olefinsJB] OpSc~ PMe3 I Op Sc \ CH3 + PM ea (DpScH)2 exhibits a quite different behavior toward 2-methyl-1-pentene. When the dimerization of propene is carried out at 80°C, the final scandium compound is the 11"-allyl complex 3 as characterized by 1H NMR. 3 can be independently prepared from the reaction of (DpScH)2 and 2-methyl-1-pentene. A possible mechanism for the formation of 3 is given in Scheme 2. Scheme 2 DpSc-H DpSc~ + 2 DpSc~ + 3 3 is probably formed via a 1,3-hydrogen shift in the sigma-bond metathesis product of 2 with 2-methyl-1-pentene. The same pathway, i.e. sigma-bond metathesis followed by a 1,3-hydrogen shift, will appear in several examples throughout this chapter. One characteristic of -57 (DpScH) 2 is that there is no spectroscopic (1H NMR) evidence of the formation of 2 during the reaction, which is in contrast with the [OpSc] system where derivatives like 2 are always observed. This observation suggests that the stability of DpSc-alkyl complexes is rather low in comparison to that of OpSc-alkyl complexes and that processes such as olefin insertion and {3hydrogen elimination occur more readily in the former. The thermodynamic driving force for {3hydrogen elimination can be attributed to the stability of (DpScH)2 formed. When the dimerization of 20 equivalents of propane was conducted at 25°C, the total disappearance of starting material took approximately 72 hours, and the only scandium compound detected by 1H NMR during the reaction is (DpScH)2. Due to the remarkable stability of (DpScH)2 in the presence of olefins, the dimerization of liquid a-olefins can be carried out in neat olefin without any noticeable decomposition of the catalyst during the reaction. Propene is not the only a-olefin that can be dimerized; 1-butene and 1-pentene produce 2-ethyl-1-hexene and 2-propyl-1-heptene respectively. The conditions of the reactions and the purity of the products are shown in Table 2. The catalytic dimerization of 1-pentene and 1-butene by (DpScH)2 must be carried out at 25 ° C; otherwise substantial isomerization of the double bond into internal positions is observed. The dimerization fails when attempted with bulky olefins such as allyltrimethylsilane or 4,4-dimethyl-1-pentene; in these cases only stoichiometric reactions are observed. Table 2: Catalytic Dimerization of a,co-Diolelins /'-. ~ ~ (DpSc-H]2 OpSc(H)(PMe3 ) ~ ~ 40 equiv. I 1 hr, 80°C 96.4% ; 0% 45 equiv. I 1 hr, 25°C 90.6% ; 8.7% ~ 30 equiv. I 132 hrs, 25°C 99.9% 43 equiv. I 1 hr, 25°C 98% ~ 37 equiv. I 132 hrs, 25°C 99.2% 45 equiv. I 1 hr, 25°C 99.6% - 58Catalytic Cyclization of a,w-diolefins A transformation which is closely related to a-olefin dimerization takes place when {DpScH)2 or OpSc{H)(PMe3) are treated with a, w-diolefins. Both hydrides promote the catalytic cyclization of a, w-diolefins to methylenecycloalkanes with ring sizes between 5 and 9 carbon atoms. The catalytic reaction proceeds analogously to the dimerization of a-olefins, but involves the intramolecular rather than intermolecular insertion of two olefins to produce cycloalkylmethylscandium derivatives, which undergo ,8-hydrogen elimination to produce a methylenecycloalkane. Scheme 3 shows the catalytic cycle for the formation of methylenecycloalkanes from a, w-diolefins. The conditions required in the cyclizations and the purity of the methylenecycloalkanes formed are presented in Table 3. Scheme 3 j Sc-H Sc"-0 A noticeable difference is observed between OpSc{H)(PMe3) and {DpScH) 2 in the formation of C7, Ca, and Cs rings. The cyclization of 1,7-octadiene, 1,8-nonadiene and 1,9-decadiene to methylenecycloheptane, methylenecyclooctane and methylenecyclononane respectively, can only be achieved with {DpScH)2 as catalyst; if OpSc{H){PMe3) is used, a viscous material is formed, perhaps from intermolecular coupling. The cyclization of 1,5-hexadiene to methylenecyclopentane with (DpScH) 2 as the catalyst must be performed at room temperature; otherwise a substantial amount of 1-methyl- - 59cyclopent ene is formed (40% at B0°C). The formation of 1-methyl- cyclopent ene correspon ds to double bond isomerization of the methylene cyclopent ane formed initially. Table 3: Catalytic Cyclization of a,co-Diolef ins [DpSc-H] 2 6 0 6 6 78 equiv.1132 hrs, 25°C 99.2% 82 equiv.11 hr, 25°C 99.6% 97 equiv.11 hr, 80°C 76 equiv./1 hr, 25°C 97.1% 96.8% 20 equiv./ 72 hrs, 25°C 99.3% 16 equiv.I 72 hrs, 25°C 99.9% 11 equiv./ 72 hrs, 25°C 85.3% ~ 9 "° 18 equivl 10 min, 80°C A : 33%; B : 66% 23 equiv./ 10 min, 80°C A :45%; B :54% 19 equiv./ 10 min, 80°C 99.9% 18 equiv.I 10 min, 8orlc 99.5% A cc j•'\ a) Q 18 equiv./ 10 min, 80°C 99.5% - 60When (DpScH)2 is treated with excess 1,5-hexadiene at B0°C a 11"-allyl complex (4) is obtained. The formation of 4 can be explained analogously to the formation of 3, as is shown in Scheme 4. The same type of structure is observed when OpSc(H)(PMe3) is treated with excess methylenecyclopentane. When the reaction of OpSc(H)(PMe3) with methylenecyclopentane, labelled with 13c in the exocyclic carbon, was monitored by 13C NMR, the first intermediate observed is 5 (54 ppm, t, J finally to 7 (56 ppm, t, J =113Hz), which is rapidly converted to 6 (175 ppm, t, J =102Hz), and =142Hz). All other methylenecycloalkanes, from methylenecyclohexane to methylenecyclononane, react with OpSc(H)(PMe3) to give the cycloalkylmethylscandium derivatives which are unreactive under the same conditions to excess methylenecycloalkane. Scheme 4 LSc-H (*) + 6 (*) v LSc~ 5:L=Op 66 (*) v LSc~ 6:L=Op 4: L =Op 7: L =Op Another variation of the catalytic cyclization is the preparation of heterocycles by introduction of heteroatoms into the chain. Surprisingly for an extremely Lewis acidic metal center, where binding of Lewis bases to the metal center is expected, nitrogen and sulfur were tolerated without noticeably decreasing the activity of the catalyst. Treating OpSc(H)(PMe3) with 10 to 15 molar excess of diallyl sulfide and bisallylmethylamine at 80°C gave 3- methyleneperhydro-thiane and 3-methylene-N-methylpiperidine as is shown in Scheme 5. The - 61 steric bulk of the alkyl groups on the nitrogen of bisallylmethylamine likely prohibits binding of the nitrogen to scandium thus allowing catalytic cyclization to 3-methylene-N-methylpiperidine. Scheme 5 )x" Sc-H s() c ------I ---ScV () 8 X = O,S,NMe When nitrogen or sulfur are replaced with oxygen, the intermediate 8 is very stable, and intramolecular insertion of the free double bond across the scandium-carbon bond occurred only slowly even at 120°C. Table 4 shows the conditions for these transformation.s and the purity of the heterocyclic compounds. Attempts to dimerize monoolefinic sulfides or ethers were unsuccessful, and only one insertion to produce compounds of general structure 9 was observed even at 140°C. Sc-H ~x + X = OMe,SMe 9 Table 4: Catalytic Cyclization of a m-Diolelins ' Containing Heteroatoms OpSc(H)(PMe3) JS\ Q I I ,!~ (\ 10 equiv. I 10 min, ao0 c 99.5% 27 equiv. I 10 min, S0°C 93% - 62After examining the cyclization of terminal diolefins we were interested in exploring the reactivity of internal olefins or f3, {3-disubstituted olefins for similar insertions. In order to do this, OpSc(H)(PMe3) was treated with several diolefins in the presence of hydrogen. The set of diolefins, together with the products formed under these conditions is shown in Scheme 6. The same reactions were also performed in the absence of hydrogen, and after 3 hours of stirring at 25°C the reaction mixture was hydrogenated. The volatile materials from the hydrogenation were analyzed by NMR (1H, 13C) and/or gas chromatography. No significant difference was found in the composition of the products obtained by both methods. Scheme 6 ~ ~ H2 OpSc(H)(PMe3 ) OpSc(H)(PMe3 ) ~+ H2 + OpSc(H)(PMe3 ) ~ ~ 0 H2 H2 OpSc(H)(PMe3 ) 45% fl ~ H2 OpSc(H)(PMe3 ) [Olefin]/[OpSc(H)(PMe3 )] = 10 - 63Assuming that internal and ,8-,8-disubstituted olefins may insert into a Sc-alkyl bonds and that addition of Sc-H to a carbon-carbon double bond always occurs in a anti-Markovnikov fashion, the intermediates 10 - 17, shown in Scheme 7, are expected. In structures 10, 12, 14, 15, or 16, a simple pathway to generate a free olefin would be via ,8-hydrogen elimination. However, ,8-hydrogen elimination only happens for 10, and there is no evidence of formation of intermediates 12, 14, 15, and 16, from NMR or gc. Intermediates 11, 13, and 17 do not have ,8hydrogens so there is no pathway to regenerate the Sc-H; hence, no catalytic activity is expected and none is observed. Moreover, ,8-methyl elimination, although possible, does not appear to take place for 11, 13 or 17. From the results of Scheme 6 and the possible structures shown in Scheme 7 it is possible to conclude: 1. Sc-H adds to terminal, internal and ,B,,8-disubstituted olefins. As expected from steric considerations, Sc-H adds to a-olefins faster than ,8, ,8-disubstituted terminal olefins or internal olefins. 2. a-olefins insert into Sc-alkyl bonds faster than ,8,,8-disubstituted terminal olefins do. Internal olefins do not insert into Sc-alkyl bonds. To design a plausible catalytic transformation, a ,8-hydrogen is required in the last step to regenerate the scandium hydride and complete the catalytic cycle. Without this, the reaction can only be stoichiometric. A ,8-hydrogen can be obtained only from insertion of a-terminal or internal olefins into Sc-alkyl bonds. Since internal olefins do not insert into Sc-alkyl bonds, the only useful substrates are those with at least one terminal olefin. As an extension, OpSc{H)(PMe3) and {DpScH)2 were treated with a, w-diolefins which contain exomethylene units along the chain. Clean conversion to spire hydrocarbons is found. Treating {DpScH)2 or OpSc{H)(PMe3) 5,8-dimethylene-1, 11-dodecadiene results with excess in the 5-methylene-1,8-nonadiene catalytic formation of and 2- - 64methylenespir o[4.4]nonane and 2-methylenedispiro[4.1.4.2]tridecane, respectively, as shown in Scheme 8. The catalytic conditions and the purity of the compounds are shown in Table 5. Scheme 7 ~ ScO Sc~ / ~ ~ Sc~ Sc~ Sc~ /· ScV- 10 11 12 Sc 13 Sc ~ ~# . S c 14 15 ~/Sc~ ~Sc~ ,# ~ Sc JI #. ~ scrff 16 17 - 65 This last transformation, quite apart from being important from a synthetic point of view, shows that multiple insertion of olefins into scandium alkyl bonds are possible, but only in an intramolecular fashion. Table 5: Catalytic Formation of Spiro Hydrocarbons l_A_ ) CX) lA_A __) rxP [DpSc-H] 2 OpSc(H)(PMe~ 7 equiv. 10 min, 80°C 90.1% 8 equiv. 10 min, 80°C 92.9% 12 equiv. 24 hrs, 25°C 85.7% 12 equiv. 30 min, 25°C 98.8% - 66- Scheme 8 Sc-H I ~ / - Sc-H Sc Sc Sc-H j ~Sc j - (Sc-H} Sc - 67CONCLUSIONS The introduc tion of a dimethylsilicon bridge between the rings in the bis(pentamethylcyclopentadienyl)scandium system significa ntly increases the reactivity for sterically sensitive processes such as olefin insertion into a scandium-carbon bond. Thus, Me2Si(C5Me5)2Sc(H)(PMe3) and (meso-Me2Si(t-butylC5H3)2ScH)2 catalyze the head to tail dimerization of a-olefins, the catalytic cyclization of a, w-diolefins to methylenecycloal the cascade cyclization of polymeth ylene a, w-diolefins to spiro hydrocarbons. kanes, and Key steps in these catalytic transformations are olefin insertion and P-hydrogen elimination. From the experimental data describe d previously, the following conclusions are drawn: 1. a-Olefins, p,p-disu bstituted terminal olefins, and internal olefins add across the scandium hydride bond. 2. a-Olefins insert into scandium-alkyl bonds. After this insertion occurs, a p-hydrog en is available on a tertiary carbon, and P-hydrogen elimination occurs readily. 3. p, p-Disubstituted olefins insert into scandium alkyl bonds in an intramolecular fashion. Further insertion of a second equivalent of a-olefin or p,p-disubstituted olefin occurs in an intramolecular fashion and this process continue s until a P-hydrogen is available, whereup on p-hydrog en elimination releases olefin. 4. Internal olefins do not insert into scandium-alkyl bonds. 5. Vinylic C-H bond activation is slow relative to olefin insertion and is observed catalytic oligomerization is completed. only after - 68- Table 6 : 1 H, 13C NMR Data (C6 D6 , 400MHz) 1 CH2 II . . . . . c...... Cl"t.! H2Cb I H2Cc' I .,...N, CciH2 CHa H NMR: a(ppm) cNMR : a(ppm) 13 CH 3 : 2.08(s) =CH 2 : 4.74(s), 4.71(s) C 1H 2 ,C3 H2 : 2.19(m), 1.97(m) C2H2 : 1.52(m) CH3 : 46 Ca-dH 2 : 62, 56, 32, 26 =CH2 : 108 c: 145 C8 H2 : 2.9(s) CbH 2, CdH 2 : 2.3(m), 1.9(m) CcH2: 1.6(m) =CH 2 : 4.6(s) Ca-dH2 : 35.3, 35.1, 31, 29 =CH2 : 108.5 c: 145 CH 3 : -0.02(s) C8 H2 : 1.51(s) CbH 2 : 0.5(m) CcH2 : 1.68(m) CdH2 : 2(m) CH 3 : -3 Ca-dH 2 : 39, 27.5, 26, 14 =CH2 : 107 c: 149 CH 2 : 4.98(s), 4.90(s) CH 2 : 30.4, 28.9, 27.8, 26.7, 24.7, 22.5 CH : 44.4, 39.8 =CH2 : 105 c: 154 CH2 : 4.73(s) CH3 : 14.2, 14 CH 2 : 38, 36, 32, 28, 23, 21 =CH 2 : 109 c: 150 =CH 2 : 4.98, 4.95 CH 2 : 52, 48, 40.9, 40.7, 40.5 39, 38, 32, 24.7, 24.6 =CH2 : 106 c, : 152 C2. Ca : 50.8, 50.3 =CH2 : 4.9 CH 2 : 46.6, 38.8, 38.2, 31.9, 25 =CH2 : 105 c, : 152 C 2 : 50.7 CH2 II . . . . . c...... C H H2Cb I 8 H2Cc' I ,....S C"i"t.! 2 CH2 II . . . . . c...... H2Cb Ca~ H Cl ~.-CHa 2 c' / I CdH2 \ CHa CH2 II C~J CHa ~ CH2-C ~Ha c /'. . c 2 /-cf'C H qLJ c0 ;--d,"CH, c0, - 69Table 7 : 1H and 13C NMR Data (CeDe) Compound Assignmen t S(ppm) and Coupling(Hz ) Cs(CH3)4 2.0(s) 'I 1-,, 5C5H4 7.0(m); 5.2(m) Cs(CH3)4 11.B(q, 126) C5(CH3)4 121 (s) 'I 1-,, 5-C5H4 123(d, 161); 122(d, 162) (CH3)2Si 0.94(s) Cs(CH3)4 2.02(s); 1.98(s) Sc-CH3 -0. 77(s) Sc-P(CH3)3 0.66(d, 3.4) (CH3)2Si 4.4(q, 120) C5(CH3)4 15(q, 126); 13(q, 125) C5(CH3)4 126(s); 121.S(s); 101.7(s) Sc-CH3 not located Sc-P(CH3)3 14.2(Jpc=8.6, JcH=128) OpSc(CH3) (P(CH3)3) DpSc-('1 3-CH2CCHCH2CH2CH2) (CH3)2Si 0.72(s); 0.21 (s) C5H3(C(CH3)3) 1.15(s) C5H3(C(CH3)3) 6.63(m); 6.03(m); 5.35(m) CH2CCHCH2CHi~H2 5.28(m) CH2CCHCH2CH2CH2 2.64(s) CH26CHCH2CH2CH2 not located CH2CCHCH2CH2CH2 1.84(m) CH2CCHCH2CHi~H2 not located - 70 (CH3)2Si -2.2(q, 120); -6.6(q, 122) C5H3(C(CH3)3) 32(q, 127) C5H3(C(CH3)3) 32.8(s) C5H3(C(CH3)3) 149(s); 114(s); 117(d, 167), 114(d,174); 110(d,164) CH2CCHCH2CH2CH2 52 CH2CCH CH2CHi ;H2 168(s) CH2CCHCH2CH2CH2 91(d, 151) CH2CCH CH2CH2 6H2 41.6(t, 128); 33(t, 130); 22.6(t, 131) DpSc-{17 3-CH2C{C H2CH2C H3)CH2) (CH3)2Si 0. 73(s); 0.20(s) C5H3(C(CH3)3 1.11(s) C5H3(C(CH3)3 6.7(m); 6.1 (m); 5.6(m) CH2C(CH2CH2CH3)CH2 1.0(t, 7.3) CH2C(CH2CH2CH3)CH2 2.4(s) (CH3)2Si -1.9(q, 120); -6.5(q, 120) C5H3(C(CH3)3) 31.7(q, 128) C5H3(C(CH3)3) 31.9(s) C5H3(C (CH3)3) 148(s); 114(s); 118(d, 170); 114(d,159); 110(d, 172) CH2C(CH2CH2CH3)CH2 66 CH2C(CH2CH2CH3)CH2 172(s) CH2C(CH2CH2CH3)CH2 47(t, 130) CH2C(CH2CH2CH3)CH2 25(t, 127) CH2C(CH2CH2CH3)CH2 14.4(q, 125) - 71 OpSc-('7 3-CH2CCHCH2CH2CH2) (CH3)2Si 0.88(5) C5(CH3)4 2.02(5); 1.75(5) CH2CCHCH2CH2CH2 2.37(5) CH2CCHCH2CH2CH2 2.72(m) CH2CCHCH2CH2~H2 4.9(m) CH2CCHCH2CH2CH2 not located (CH3)2Si 4.8(q, 118) C5(CH3)4 15(q, 121); 13(q, 120) Cs(CH3)4 104(5); 121(5); one not located CH2CCHCH2C Hi:H2 56(t, 141) CH2CCHCH2CH2CH2 170(5) CH26CHCH2CH2CH2 91(d,147) CH26CHCH2CH2CH2 40(t, 127); 32. 7(t,126) CH2CCHCH2CH2CH2 25(t, 127) - 72EXPERIMENTAL Cp*Cp0Sc(H)(PMe3), Cp*CpSc-(77 1-11 5)Cp*Sc(H), (meso-Me2Si(t-butylC5H3)2ScH)2, and Me2Si(CsMe4)2Sc(H)(PMe3) were prepared as described in Chapter 1. (Matheson) were degassed by several freeze-pump-thaw cycles. Propene, 1-butene 1-Pentene, 1,5-hexadiene, 1,6-heptadiene, 1,7-octadiene, 1,8-nonadiene, 1,9-decadiene, allyl ether, allyl sulfide, allyl ethyl ether, allyl methyl sulfide, 2-methyl-1,5-hexadiene, allyl trimethylsilane, 4,4-dimethyl-1-pentene, 3-methyl-1,5-hexadiene (all Aldrich), cis-divinylcyclohexane (Alfa), 1,5-heptadiene, diallyldimethyl silane, diallylmethyl amine (all Pfaltz & Bauer), and 2-methyl-1,5-heptadiene (K & K) were dried over molecular sieves 4 A, and then vacuum transferred prior to use. 5-Methylene-1,8-nonadiene was prepared according to a published procedureJ9] 5,8-Dimethylene-1,11-dodecadiene was prepared via Wittig reaction of dodeca-1,11-dien-5,8-dioneC 10] with two equivalents of CH2=P(C5H5)s in benzene. The standards for NMR and/or gc, methylenecyclopentane, methylenecyclohexane, 2-methyl-1pentene, methylcyclopentane, 1-methyl-1-cyclopentene, 2-methyl-hexane, 2-methyl-heptane, heptane, and 2,5-dimethylhexane (all Aldrich) were used as received. Methylenecycloheptane, methylenecycl ooctane, methylenecyclononane, and 2-ethyl-1-hexene were prepared via Wittig reaction of cycloheptanone, cyclooctanone, cyclononanone, and 3-heptanone (all Aldrich) respectively with CH2=P(C5Hs)s in benzene. The specifically labeled 13C- C1MethylenecyclopentaneC111 and CH2=P(C5H5)3[ 121 were prepared as reported in the literature. NMR spectra were recorded on Varian EM-390 (1H, 90MHz) and JEOL GX 400 (1H:399.78MHz, 13C:100.38MHz) instruments. The gc analyses were carried out in either a Hewlett-Packard 5790-A using a 4 m column packed with 5% 8,8-0xydiprop ionitrile on Chromosorb W or in a Perkin-Elmer 8410 using a RSL-150 (Alltech) column. - 73General Procedure for Olefin Dimerization: The scandium hydride (ca. 0.06-0.07 mmol) was loaded into an NMR tube and benzene-ds (ca. 0.5 g) was added to the tube. A rubber septum was fitted onto the tube and it was sealed with "Parafilm". The substrate was added via syringe. Experiments involving propane and 1-butene were performed in sealed NMR tubes. The reaction was monitored by 1H NMR. Chromatographic analyses were performed either by injecting the solution directly into the gc or, after the volatiles were vacuum transferred (10 4 torr, 25°C to 100°C) using a high vacuum line, by injecting volatile components into the gc. General Procedure for Hydrogenatio n: OpSc(H)(PMe3) (ca. 30 mg, 0.07 mmol) and benzene-d6 (ca. 0.5 g) were loaded into a thick-walled reaction vessel. The benzene was frozen at -80°C, and the diolefin (ca. 5 to 7-fold molar excess) was added via syringe under an argon counterflow. The reaction vessel was evacuated, and one atmosphere of dihydrogen was admitted at -196°C. The mixture was warmed to room temperature and stirred for 30 minutes. The excess hydrogen was released, and the volatile materials were vacuum transferred and analyzed by NMR <1H or 13C) and/or gc. Cp*Sc(17 1-11 5-C5H4)2ScCp* : Cp*CpSc(17 1-11 5-C5H4)Cp*ScH (0.5 g, 1.02 mmol) were dissolved in 5 ml of methylcyclohexane. The solution was heated at 80 ° C for four days. The solution was cooled to room temperature and transferred to a frit-assembly. Methylcyclohexane was removed in vacuo and replaced with petroleum ether (ca. 5 ml). The yellow solid was filtered and dried. Yield 0.21 g (42.1%). Anal. Calcd. for C15H19Sc: C, 73.55; H, 8.22. Found: C, 73. 71; H, 7.88. Me2Si(C5Me4)2Sc(CH3)(PMe3): OpSc(H)(PMe3) (0.3 g, 0.71 mmol) and 2-methyl-1pentene (90 µl, 0.73 mmol) were dissolved in petroleum ether (ca. 5 ml). The solution was stirred at room temperature for 24 hours. The yellow precipitate, Me2Si(C5Me4)2Sc(CH3)(PMe3), was filtered and dried. Yield 0.22 g (71%). Anal. Calcd. for C24H42ScSiP: C, 66.33 H, 9.74. Found: C, 66.07 H, 9.48. - 74Me2Si(C5Me4)2Sc-(77 3-CH2CCHCH2CH2CH2) : OpSc(H)(PMe3) (0.35 g, 0.83 mmol) and methylenecyclopen tane (0.45 ml, 4.3 mmol) were dissolved in petroleum ether (ca. 7 ml). The solution was stirred at room temperature for 24 hours. Petroleum ether and excess methylenecyclopen tane were removed in vacuo and replaced with fresh petroleum ether (ca. 1O ml). The solution was cooled to -78°C for three hours to afford Me2Si(C5Me4)2Sc-(77 3- CH2CCHCH2CH2CH2). Yield 0.28 g (79.2%). Anal. Calcd. for C25H39SiSc: C, 73.54, H, 9.26. Found: C, 73.40, H, 9.66. meso-Me2Si(t-butylC5H3)2Sc-(77 3-CH2CCHCH2CH2CH2) : DpScH (0.30 g, 0.87 mmol) and methylenecyclopen tane (0.30 ml, 2.85 mmol) were dissolved in petroleum ether (ca. 1O ml). The solution was heated to 80 ° C for 5 hours. Petroleum ether and excess methylenecyclopen tane were removed in vacuo and replaced with fresh petroleum ether (ca. 1O ml). The solution was cooled to -78 °C for three hours to afford meso-Me2Si(t-butylC5H3) 2Sc-(17 3CH2CCHCH2CH2CH2). Yield 0.2 g (54 %). Anal. Calcd. for C25H39SiSc: C, 73.54, H, 9.26. Found: C, 73.56, H, 9.53. meso-Me2Si(t-butylC5H3)2Sc-(77 3-CH2C(CH2CH2CH3)CH2) : OpScH (0.35 g, 1.02 mmol) and 2-methyl-1-pentene (0.6 ml, 4.86 mmol) were dissolved in petroleum ether (ca. 10 ml). The solution was heated to 80°C for two hours. Petroleum ether and excess 2-methyl-1-pentene were removed in vacuo and replaced with fresh petroleum ether (ca. 1O ml). The solution was cooled to -78°C overnight to afford meso-Me2Si(t-butylC5H3)2Sc-(77 3-CH2C(CH2CH2CH3)CH2). Yield 0.15 g, (34.6%). Anal Calcd. for C25H41SiSc: C, 73.19, H, 9.68. Found: C, 72.87, H 9.25. - 75- REFERENCES 1. Collman, J.P.; Hegedus, LS.; Norton, J.R.; Finke, R.G. Principles and Applications of Organometallic Chemistry. University Science Books : Mill Valley, California, 1978, p. 578. 2. Watson, P.L.; Parshall, G.W.Acc. Chem. Res. 1985, 18, 51. 3. Watson, P.L.; Herskovitz, T. Am. Chem. Symp. Series 1983, 212, 459. 4. Burger, B.J. Ph.D. Thesis, California Institute of Technology, 1987. 5. Watson, P.L. J. Am. Chem. Soc. 1982, 104, 337. 6. Thompson, M.E.; Baxter, S.M.; Bulls, A.R.; Burger, B.J.; Nolan, M.C.; Santarsiero, B.D.; Schaefer, W.P.; Bercaw, J.E. J. Am. Chem. Soc. 1987, 109, 203. 7. Muthukumaru, P.S.; Ravindranathan, M.; Sivaran, S. Chem Rev. 1986, 86, 353. 8. Watson, P.L.; Roe, D.C.J.Am. Chem. Soc. 1982, 104, 6471. 9. Barluenga, J.; Yus, M.; Concellon, J.M.; Bern ad, P. J. Org. Chem. 1981, 46, 2721. 1O. Ito, Y.; Konoike, T.; Harada, T.; Saegusa, T. J. Am. Chem. Soc. 1977, 99, 1487. 11. Greenwald, R.; Chaykovsky, M. Corey, E.J. J. Org. Chem. 1962, 28, 1128. 12. Maercker, A. Org. React. 1985, 14, 270. - 76- CHAPTER 3 CARBON-C ARBON BOND ACTIVATION VIA ,8-ALKYL ELIMINATION. REVERSIBLE BRANCHIN G OF 1,4-PENTADIENES CATALVZED BY SCANDOCENE HYDRIDE DERIVATIVES. - 77- INTRODUCTION Although impressive advances have recently been made in activating carbon-hydrogen bonds of hydrocarbons,C 11 the activation of carbon-carbon bonds by soluble transition-metal complexes remains one the most prominent challenges in organometallic chemistry. The first example of carbon-carbon bond activation dates back to 1955 when it was reported that H PtCl 2 6 reacts with cyclopropane to give a platinacyclobutane.C21 <J pyridine () ~Y.. CI Pt I 'Cl PY The silver- and rhodium-catalyzed rearrangements of strained hydrocarbons are further examples of the same type of reaction.C1 a] The silver system probably involves an electrophilic attack of Ag+ at the C-C bond, while that for rhodium involves an oxidative addition process. (Rh(CObClb Another approach to C-C bond activation is the generation of "naked" metal atoms both in the gas phase and in matrices which react with alkanes. In the case of Fe+ ions, generated in the gas phase, C-C bond cleavage is the preferred mode of activationJ3] This preference for C-C bond activation correlates with the relative Fe-CH3+ and Fe-W bond strengths of 69 ± 5 kcal·moi-1 and 58 ± 5 kcal·mor1, respectively.[4] Despite the preference for C-C bond cleavage by "naked" metal atoms cations, C-C bond activation by transition metal complexes is slow due to the poor steric accessibility of the metal center. As a result of this steric disadvantage, saturated hydrocarbons interact with transition - 78metal centers through their C-H bonds. This suggests that one route to C-C activation could be via prior C-H bond activation. For example, only C-H activation occurs when the unsaturated fragment [Cp*Rh{PMe3)] is treated with cyclopropane, to yield 1 at -60 °C. 1 rearranges at higher temperatures, via C-C bond activation, to give 2.£5] + C> )> -60°C -60°C--25 °C Cp*Rh-H " PMe3 1 2 A platinum system that does not involve a initial C-H activation has been reported.£6] Thermolysis of 3 yielded 2-methyl-1,4-pentadiene as the only organic product. This reaction proceeds through the formation of 4 via ,B-alkyl elimination, which then undergoes ,8-hydrogen elimination to give 2-methyl-1,4-pentadiene and Pt{PMe3)2{H){CI). PMe3 I Cl-rt~· PMe3 3 Q Cl-rt~ PMe 140°C [ PMe3 ] PMe3 I Cl-Pt-H I + PMe3 4 3 The absence of ,8-hydrogens in 3 guarantees that the only pathway for decomposit ion is via ,8alkyl elimination. ,8-hydrogen elimination is generally much faster providing a well-known route for the decomposit ion of metal-alkyl fragments.£7] In most of the examples described in the literature, the driving force for ,8-alkyl elimination is the relief of strain in the hydrocarbon fragment. However, a few examples where strain is not involved have been reported. One example is the reaction of lr2H2{Me2C0)2L2 {L=PR3) with dimethylcyclopentane in the presence of t-butylethylene to yield 5 as the isolable product.£81 ~lrMeL2 5 - 79In this example, a methyl group from the cyclopentane ring is transferred to Ir. The exact mechanism of the reaction is still unclear, however. In the thermal decomposition of Cp*2LuCH2CHMe2 in which there is no associated ring strain, both ,8-hydrogen and ,8-alkyl elimination occur competitively. The propene resulting from the ,8-methyl elimination is removed from the equilibrium mixture by subsequent reactions. For this reaction, a four centered transition state (6a or 6b) was proposed to explain both migrations. The transition state derived from conformation 6b is more favorable than that derived from 6a due to the orientation of the methyl groups with respect to the Cp* rings. This renders ,8-alkyl elimination an important decomposition pathway for Cp*2LuCH2CHMe2.£9] Cp*2Lu ~CH3 H 6a / Cp* 2Lu-H + Cp•2LuY.H CH3 =< CH3 6b ' CHa Cp*2Lu-CH 3 + J The observation that, in the [Cp*2Lu] system, olefin insertion into metal-hydride or metalalkyl bonds and ,8-hydrogen and/or ,B-alkyl elimination occur competitively opens the possibility for a new variety of catalytic transformations involving C-C bond making and breaking. This chapter describes a scandocene hydride complex which catalyzes the reversible branching and cyclization of 1,4-pentadienes. The key feature responsible for this unusual reactivity is reversible olefin insertion and ,8-alkyl elimination. - 80RES ULTS AND DISCUSSION In the previous chapter it was shown that (meso-Me2Si(t-butylC5H3)2Sc-H)2 ((DpScH)2) promotes the catalytic cyclization of a, w-diolefins to methylenecycloalkanes. The cyclization involves the insertion of one double bond across the Sc-H bond, followed by the intramolecular insertion of the free olefin into the previously formed Sc-alkyl bond to yield a cycloalkylmethyl scandium intermediate which rapidly undergoes ,8-hydrogen elimination to give methylenecycl oalkane and (DpScH)2. Sc-H + n = 0, 1, 2 We imagined that a cycloalkylmethyl scandium derivative, depending on ring strain, could undergo ,8-alkyl elimination competitively with ,8-hydrogen elimination and olefin insertion. Through a combination of ,8-alkyl elimination, olefin insertion and ,8-hydrogen elimination, an equilibrium between a methylenecycloalkane and a ring open diene could be established. The 1 AH 1 (kcal mor ) ~ 26.0 21.9 l 47.9 ~ 25.3 0 0 ~ ·1 AHcycllzatlon (kcal mol ) 3.8 29.1 20.2 -17.3 23.5 - 81 ease of this equilibration process can be judged by comparing the heats of formation for the exocyclic olefins and the corresponding dienes.C 1O] These values indicate that equilibration between the cyclobutylmethyl scandium complex and the 4-pentenyl-scandium complex should be close to thermoneutral. In order to test for this equilibration, labelling experiments were done with methylenecyclopentane and methylenecyclobutane labelled with 13c in the exocyclic position and 13CH2=CH-CH=CH2. As shown in Scheme 1 for the case of methylenecyclobutane, if ,B-hydrogen elimination, ,B-alkyl elimination and olefin insertion are all ocurring in competition, the label should scramble among some of the positions in the diolefin and/or the methylenecycloalkane. Scheme 1 DpSc-H + * ===(> * !J DpSc-H DpSc-H * * + * + • + :> :>· - 82Treating (DpScH)2 with four equivalents of labelled butadiene at 80°C yields DpSc-(77 313CH -CH-CH-CH ) and DpSc-(77 3 -CH2-CH-CH-13CH3) (7) with no scramblin g of the free 2 3 butadiene. When the reaction temperature was increased to 140°C, only decompo sition of 7 was observed, and when the volatiles were analyzed by 13C NMR only 13CH =CH-CH=CH2 was 2 detected. However, when isolated DpSc-(77 3-13 CH~CH-CH-CH3) and DpSc-(77 3-1 3CH2-CH-C H13CH ) were heated 3 result is to 140°C, 13 explained in c scrambling into all four crotyl positions was observed. This Scheme 2, and suggests that an equilibrium between the cycloprop ylmethyls candium complex and the 3-butenylscandium complex can be establishe d at high temperature without excess free butadiene in solution. Multiple insertion of butadiene into 7 is probably the mayor pathway when excess butadiene is used. Scheme 2 DpSc-~ .~ * DpSc- ;z * DpSc-H + ~ DpSc-H + ~ * * DpSc:: J DpSc~ DpSc: :; Dpscr -c>· ~ DpSc-~·~ DpSc-:-~ ~ * * - 83Treatment of (DpScH)2 with labelled methylenecyclopentane at 80°C yields DpSc-(77 313CH2-C-CH-CH2CH2CH2) (8) as described in Chapter 2. Heating 8 with or without labelled methylenecyclopentane does not scramble the label on the allyl fragment of 8 or the excess methylenecyclopentane at temperatures up to 140°C. The only reaction observed, besides the thermal decomposit ion of 8 to unidentified products, is the conversion of methylenecyclopentane to 1-methylcyclopentene. In contrast to the reactions described above, treating (DpScH)2 with four equivalents of labelled methylenecyclobutane yields at 25°C 13CH2=CH-CH2-CH=CH2 and CH2=CH-1 3CH 2 CH=CH2. Under these conditions, DpScH is the only scandium species detected by 1H or 13c NMR. This result shows that under very mild conditions the equilibria shown in Scheme 1 are operating, but the reaction is driven thermodynamically toward the formation of 1,4-pentadiene. When DpScH is treated with methylenecyclobutane or 1,4-pentadiene at 80 ° C, a clean conversion to DpSc-77 3-CH2-CH(CH3)CHCH3 (9) is observed. The same product is obtained if DpScH is treated with isoprene at 80°C. 9 was characterized by 1H and 13C NMR (see Table 1). DpSc-H + DpSc-H DpSc-H + + These results indicate that methylenecyclobutane is being converted to 1,4-pentadiene, and that there is a new species at higher temperatures responsible for the formation of 9. The suggested " intermediate (11) is shown in Scheme 3. The same type of intermediate was proposed in the -84reaction of NiCl2(PButs)2/AICl(CH2CHMe2)2 with 1,4-pentadiene to explain the catalytic formation of isoprene.£ 11 1 Scheme 3 ~ + DpSc-H I v DpSc/'..... 11 l DpSc-H ~ + DpSc~ In order to test the validity of the proposed mechanism, DpScH was treated with labelled methylenecyclobutane. First methylenecyclobutane was ring-opened to 13CH2=CH-CH2-CH=CH2 and CH2=CH-13 CH~CH=CH2, and then heated to 80°C to produce DpSc-(,, 3-13CH2-C(CH3)CH(CH3)), DpSc-(,, 3-CH2-C(1 3CH3)-CH(CH3)) and DpSc-(,, 3-CH2-C(CH3)-CH(13CH3)). As shown in Scheme 4, this label distribution agrees with the mechanism previously depicted in Scheme 3. Scheme 4 * 6 0 1. 2s c 3 days + DpSc-H 2. ao0 c 1hr * DpSc-H DpSc~ -+==~ ~ * * * + v DpScA 11 j * 11 * DpSc-(-• DpSc-H + ·~·--=~ DpSc~. * * * = locations of 13 C label * -85When excess 1,4-pentadiene or methylenecyclobutane are used, they are catalytically converted to isoprene at 140°C. Unfortunately, the thermal instability of 9 at 140°C results in the production of trimethylethylene as well as isoprene. The formation of trimethylethylene probably arises from the reaction of the allyl moiety with the ligand C-H bonds to give an unidentified ''tuck-in" product. Besides isoprene and trimethylethylene, which have the original hydrocarbon skeleton, cis- and trans-piperylene are formed. [DpSc-H], 140°C 5 turnovers 58.3% 4.9% 1.3% 35.5% Cis- and trans-piperylene are formed from 10 via ,8-hydrogen elimination prior to the formation of 11. The formation of isoprene is kinetically controlled as evidenced by the deviation of the product ratio from the expected composition of a thermodynamic mixture of 1,4-pentadiene, cisand trans-piperylene and isoprene. The equilibrium concentrations are estimated as follows at 413 K: 1,4-pentadiene (0.05%), cis-piperylene (40.5%), trans-piperylene (26. 7%), and isoprene (31.8%). The experimental ratios were: 0.03%, 2%, 7.6% and 90.4%.£12] Because of the formation of cis- and trans-piperylene in the isomerization of 1,4-pentadiene to isoprene, we thought it could be possible to convert these compounds to isoprene through the formation of the same intermediate 11. However the reaction of DpScH with cis- or trans-piperylene is only stoichiometric and yields a linear chain 11'-allyl complex DpSc( 173-CH2-CH-CH-CH2CH3) (12). No isomerization to 1,4-pentadiene or conversion to isoprene was observed. This suggests that the addition of Sc-H to the conjugated olefin, which in principle could yield 13-16, only produces 14 and 15. Intermediate 16 is apparently not formed competitively and 9 or isoprene are not formed. The conversion of 13 to 17, namely the insertion of a terminal olefin into a Sc-alkyl bond as was discussed in Chapter 2, is not likely to occur, but if it would occur the symmetric structure of 17 does not allow the formation of a new hydrocarbon - 86skeleton. In the early stages of the reaction the formation of 18 was detected, but 12 was the final product isolated. Scheme 5 ~ / / ~ ~ OpSc DpSc 13 l DpScH + ' ~ DpSc 14 15 JI j ~~~ ~ DpSc 17 18 ' ~ OpSc 16 ~ DpSc 12 The type of rearrangement found for the conversion of 1,4-pentadiene to isoprene was found to be general for other dienes. 2-Methyl-1,4-pentadiene was converted to 2,3-dimethyl-1,3butadiene and 3-methyl-1,4-pentadiene was converted to methylenecyclopentane. Treatment of DpScH with a stoichiomet ric amount of 2-methyl-1,4-pentadiene yields the 11'-allyl complex DpSc(rJ 3-CH2-C(CH3)-C(CH3)2) (19). 19 is independen tly synthesized by treating DpScH with 2,3-dimethylbutadiene at B0°C. Heating DpScH with excess 2-methyl-1,4-pentadiene at B0°C affords 2,3-dimethylbutadiene as well as 19. Scheme 6 shows a proposed mechanism for the formation of 2,3-dimethyl-1,3-butadiene from 2-methyl-1,4-pentadiene. Four possible different intermediates could be formed in the reaction of 2-methyl-1,4pentadiene with DpScH. Olefin insertion yields the cyclopropylmethyl or cyclobutylmethyl scandium derivatives 21-24. Due to the symmetric structure of 21 and 23, there is only one way - 87 in which ,8-alkyl elimination may occur, that which regenerates the original open chain compound. ,8-alkyl elimination from 24 yields 26 which cannot be transformed further because of the absence of ,8-hydrogens. In conclusion, of all four intermediates only 22 has a productive ,8-alkyl elimination pathway to yield 19. 2,3-Dimethyl-1,3-butadiene is obtained catalytically via ,8- hydrogen elimination from 25. Scheme 6 + DpSc-H DpSc~ DpSc~ DpSc~ 11 DpSclu 21 22 DpSc~ DpScv-- 23 24 !! DpSc~ 25 26 DpScH 19 -88The formation of tetramethylethylene probably is due to side reactions involving metallation of the ligand rings. Cis- and trans-2-methyl-1,3-pentadiene are formed from ,8-hydrogen elimination from 20 prior to olefin insertion. [DpSc-H], 80°C + 8 turnovers + 86.5% 2.7% 4.4% 6.4% The reaction of DpScH with 3-methyl-1,4-pentadiene was also investigated. In this case, the stoichiomet ric reaction of DpScH with 3-methyl-1,4-pentadiene yields a ?r-allyl complex DpSc( 11 3-CH2-CH-CH-CH2CH2CH3) (27) which has a different carbon skeleton from the organic product obtained catalytically. DpScH at 80 ° C. 27 is independen tly prepared by treating 1,3-hexadiene with The formation of methylenecyclopentane from 3-methyl-1,4-pentadiene is explained in Scheme 7. Scheme 7 ~ + / DpSc-H ' DpSc~ D~c~ l ~ DpScJJ DpSc:Y 28 29 ' D~c~ 30 / ~ ~ ~ 27 -89Intermediate 29 is symmetric so there is no productive ,8-alkyl elimination pathway; however, 28 has a different pathway for ,8-alkyl elimination. yields 1,5-hexadiene and 1,4-hexadiene. The ,8-hydrogen elimination from 30 former is catalytically cyclized to methylenecyclopentane, while 1,4-hexadiene reacts stoichiometrically yielding 27. The formation of 27 by treating (DpScH) 2 with excess 1,4-hexadiene was monitored by 1H and 13C NMR. (DpScH) 2 catalyzes first the isomerization of 1,4-hexadiene to 2,4-hexadiene and then 2,4-hexadiene is slowly converted to 1,3-hexadiene which finally yields 27. The formation of 27 from the reaction of 2,4-hexadiene with DpScH occurs much more slowly than the isomerization to 2,4-hexadiene. The catalytic conversion of 3-methyl-1,4-pentadiene to methylenecyclopentane is accompanied by the simultaneous formation of 1-methyl-1-cyclopentene and 2,4-hexadiene. The formation of the later was discussed previously, and 1-methylcyclopentene has been shown previously to form when (DpScH)2 is treated with methylenecyclopentane at 80°C. [DpSc-H], 80°C + 6 turnovers 42.9% 22.0o/o 6.0% 29.2% -90CONCLUSIONS The interconversion between a cyclopropylmethyl-scandium complex and a 3-alkenylscandium complex and between a cyclobutylmethyl-scandium complex and a 4-alkenyl-scandium complex has been demonstrat ed by labelling experiments. These observations implicate the reverse of olefin insertion into Sc-alkyl bonds, namely ,8-alkyl elimination, as a relatively facile process in this system. As in many of the examples reported in the literature, the driving force for ,8-alkyl elimination was found to be the ring strain in the cyclopropylmethyl and cyclobutylmethyl scandium derivatives. However, unlike literature precedents, in the scandium system strained cycloalkylmethyl derivatives are formed from open chain compounds . Catalytic reactions were performed with excess 1,4-pentadiene, 2-methyl-1,4-pentadiene and 3-methyl-1,4-pentadiene, although after several turnovers some other pathway leads to a significant amount of saturated products in conjunction with slowing down the catalytic cycle. - 91 Table 1 : 1H and 13C NMR Data Compound Assignment c5(ppm) and Coupling(Hz) (CH3)2Si 0.23(s); 0.70(s) C5H3(C(CH3)3) 1.12(s) C5H3(C(CH3)3) 5.14(m); 5.94(m); 6.78 (m) '7 3-CH2CHCH(CH 3) 2.80(m) '7 3-CH2CHCH (CH3) 7.08(m,11,5.6) '7 3-CH2CHCH (CH3) 4.60(m,5.6, 15) '7 3-CH2CHCH (CH3) 1.30(d,5.6) (CH3)2Si -6(q, 120); -2(q, 119) C5H3(C(CH3)3) 32(q,124) C5H3(C(CH3)3) 33(s) C5H3(C(CH3)3) 110(d,169); 115(d,164); DpSc-( 77 3-CH2CHCH (CH3)) (C5D5, 80 ° C) 118(d,166); 114(s); 184(s) (C5D5, 80 ° C) '7 3-CH2CHCH(CH 3) 57(t, 144) '7 3-CH2CHCH(CH 3) 151 (d,141) '7 3-CH2CHCH(CH 3) 86(d,148) '7 3-CH2CHCH(CH 3) 19(q, 125) (CH3)2Si 0.21 (s); 0. 73(s) C5H3(C(CH3)3) 1.13(s) C5H3(C(CH3)3) 5.43(m); 6.07(m); 6.63(m) '7 3-CH2C(CH3)CH(CH3) 2.86(m) '7 3-CH2C(CH3)CH(CH3) 1.96(s) '7 3-CH2C(CH3)CH (CH3) 5.13(q,6) -92,, 3-CH2C(CH 3)CH(CH3 ) 1.18(d,6) (CH3)2Si -7(q,123); -2(q,121) C5H3(C(CH3)3) 32(q,124) C5H3(C(CH3)3) 33(s) C5H3(C(CH3)3) 117(d,168); 115(d,165); 110(d,163); 114(s); 149(s) (C5D5, 80 ° C) ,, 3-CH2C(CH 3)CH(CH3 ) 60(t, 143) ,, 3-CH2C(CH 3)CH (CH3) 164(s) ,, 3-CH2C(CH 3)CH(CH3 ) 24(q,125) '7 3-CH2C(CH 3)CH(CH3 ) 81(d,139) ,, 3-CH2C(CH 3)CH (CH3) 14(q,125) (CH3)2Si 0.26(s); O. 72(s) C5H3(C(CH3)3) 1.13(s) C5H3(C(CH3)3) 5.22(m); 5.96(m); 6.83(m) ,, 3-CH2CHC HCH2CH2 CH3 2.88(m) '7 3-CH2CHC HCH2CH2 CH3 7.11(m,11.2,15) ,, 3-CH2CHC HCH2CH2 CH3 4.81(m,6.6,15) '7 3-CH2CHC HCH2CH2 CH3 not located ,, 3-CH2CHC HCH2CH2 CH3 not located ,, 3-CH2CHC HCH2CH2 CH3 0.88(t, 7.3) (CH3)2Si -7(q,121); -3(q,120) C5H3(C(CH3)3) 32(q,126) C5H3(C(CH3)3) 33(s) C5H3(C(CH3)3) 110(d,166); 116(d,165); 148(s); 113(s) -9377 3-CH2CHCHCH2CH2CH3 57(t,148) 77 3-CH2CHCHCH2CH2CH3 150(d,144) 77 3-CH2CHCHCH2CH2CH3 92(d,140) 77 3-CH2CHCHCH2CH2CH3 36(t,126) 77 3-CH2CHCHCH2CH2CH3 25(t,129) 77 3-CH2CHCHCH2CH2CH3 14(q,125) (CH3)2Si 0.36(s); 0.78(s) C5H3(C(CH3)3) 1.08(s) C5H3(C(CH3)3) 5.32(m); 5.99(m); 6.90 (m) 77 3-CH2C(CH3)C(CH3)2 2.75(s) 77 3-CH2C(CH3)C(CH3)2 1.84(s); 1.91 (s); 1.95(s) (CH3)2Si -6(q,122); -3(q,120) C5H3(C(CH3)3) 31 (q,126) C5H3(C(CH3)3) 32(s) C5H3(C(CH3)3) 112(d,162); 119(d,165); DpSc-(77 3-CH2C(CH3)C(CH3)2) (CsDs, 80 ° C) 120(d,169); 116(s); 142(s) 77 3-CH2C(CH3)C(CH3)2 58(t,147) 77 3-CH2C(CH3)C(CH3)2 149(s) 77 3-CH2C(CH3)C(CH3)2 106(s) 77 3-CH2C(CH3)C(CH3)2 21(q,128); 24(q, 123); 24 (q, 125) (CH3)2Si 0.26(s); O. 72(s) C5H3(C(CH3)3) 1.13(s) C5H3(C(CH3)3) 5.20(m); 5.95(m); 6.82(m) 77 3-CH2CHCHCH2CH3 2.84(m) DpSc-( 77 3-CH2CHCHCH2CH3) (CsDs, 80 ° C) -94,, 3-CH2CHCHCH 2CH3 7.07(m) ,,3-CH2CHCHCH 2CH3 4.77(m) ,, 3-CH2CHCHCH 2CH3 not located ,, 3-CH2CHCHCH 2CH3 0.82(t,) (CH3)2Si -6.94(q,121); -2.50(q,120) C5H3(C(CH3)3) 31.6(q,121) C5H3(C(CH3)3) 32.B(s) C5H3(C(CH3)3) 148; 115; 11 O; two were not located ,, 3-CH2CHCHCH 2CH3 56.6{d,147) ,, 3-CH2CHCHCH 2CH3 150(d,143) ,, 3-CH2CHCHCH 2CH3 94(d,138) ,, 3-CH2CHCHCH 2CH3 27.1{t,125) ,, 3-CH2CHCHCH 2CH3 16.2(t, 126) -95- EXPERIMENTAL The hydride derivative (meso-Me2Si(t-butylC5H3)2ScH)2 was prepared as described in c (MSD Isotopes) were used as received. Chapter 1. Butadiene (Matheson) and butadiene-1-1 3 1,4-Pentadiene, methylenecyclobutane, methylenecyclopentane, 2-methyl-1,4-pentadiene, 3methyl-1,4-pentadiene, trimethylethylene, 1,4-hexadiene cis-piperylene, (cis & trans), trans-piperylene, isoprene, 2,3-dimethylbutadiene, 1-methyl-1-cyclopentene, 2-methyl-1,3- pentadiene (cis & trans), 1,3-hexadiene (mostly trans), 1,4-hexadiene (cis & trans), 2,4-hexadiene (mixture of isomers) were purchased from Aldrich and stored over 4A molecular sieves. c and methylenecyclopentane-13C were prepared as reported in the Methylenecyclobut ane-13 literatureC13]. Perdeuterobenzene and petroleum ether were vacuum transferred from titanocene prior to use. NMR spectra were recorded on Varian EM-390 (1H, 90MHz) and JEOL GX 400 (1H, 399.78MHz; 13C, 100.38MHz) instruments. The gc analyses were carried out on a Hewlett- Packard 5790-A using a 4m column packed with 5% B,B-Oxydipropionitrile on Chromosorb W. General Procedure for Olefin Rearrangement: (meso-Me2Si(t-butylC5H3)2ScH)2 (ca. 0.06-0.07 mmol) was loaded into an NMR tube, and benzene-d5 (ca. 0.5 g) was added to the tube. The substrate was condensed into the NMR tube and then the tube was sealed. The reaction was monitored by 1H and 13 c NMR. After the reaction was completed, as determined by NMR, the NMR tube was opened and the volatiles were vacuum transferred (10 4 torr, 25°C) in a high vacuum line and then injected to the gc. meso-Me2Si(t-butylC5H3)2Sc-(11 3-CH2CHCHCH3) (7): (DpScH)2 (0.30 g, 0.87 mmol) was dissolved in petroleum ether (ca. 15ml), the solution was cooled to -196°C and butadiene (1.3 mmol) was condensed. The solution was heated to 80 ° C for three hours. Petroleum ether -96and excess butadiene were removed in vacuo and replaced with petroleum ether (ca. 5 ml). The solution was cooled overnight (-78°C) to afford 7 as orange crystals. Yield 0.22 g, (62 %). Anal. Cale. for C24H37ScSi: C, 72.32; H, 9.36. Found C, 71.98; H, 9.24. meso-Me2Si(t-butylC5H3)2Sc-(77 3-CH2C(CH3)CHCH3) (9): The same procedure for 7 was followed except 1,4-pentadiene was substituted for butadiene. 9 was obtained as orange crystals. Yield 0.19 g, (53 %) Anal. Cale. for C25H39ScSi: C, 72. 77; H, 9.53. Found C, 72.30; H, 9.49. meso-Me2Si(t-butylC5H3)2Sc-(77 3-CH2CHCHCH2CH3) (12): The same procedure for 7 was followed except 1,3-pentadiene was substituted for butadiene. 12 was obtained as orange crystals. Yield 0.20 g, (56 %) Anal. Cale. for C25H39ScSi: C, 72.77; H, 9.53. Found C, 72.50; H, 9.27. meso-Me2Si(t-butylC5H3)2Sc-(77 3-CH2C(CH3)C(CH3)2) (19): The same procedure for 7 was followed except 2-methyl-1,4-p entadiene was substituted for butadiene. 19 was obtained as red crystals. Yield 0.22 g, (59 %) Anal. Cale. forC25H41ScSi: C, 73.19; H, 9.87. Found C, 72.56; H, 9.27. meso-Me2Si(t-butylC5H3)2Sc-(77 3-CH2CHCHCH2CH2CH3) (27): The same procedure for 7 was followed except 3-methyl-1,3-p entadiene was substituted for butadiene. 17 was obtained as red crystals. Yield 0.19 g, (51 %) Anal. Cale. for C25H41ScSi: C, 73.19; H, 9.87. Found C, 72.84; H, 9.33. -97- REFERENCES 1. For recent reviews see for example: (a) Crabtree, R.H. Chem Rev. 1985, 85, 245. (b) Green, M.L.H. Pure Appl. Chem. 1985, 57, 1987. (c) Halpern,J. /norg. Chim. Acta 1985, 100, 41. (d) Rothwell, l.P. Polyhedron 1985, 4, 41. 2. (a) Tipper, C.F.H. J. Chem. Soc 1955, 2043. (b) Binns, S.E.; Cragg, R.H.; Gillard, R.D.; Heaton, B.T.; Pilbrow, M.F.J. Chem. SocA 1969, 1227. 3. (a) Burnier, R.C.; Byrd, G.D.; Freiser, B.S. J. Am. Chem. Soc. 1981, 1, 6501. (b) Byrd, G.D.; Burnier, R.C.; Freiser, B.S. J. Am. Chem. Soc. 1982, 104, 3565. (c) Armentro ut,P.B.; Halle, L.F.; Beauchamp, J.L. J. Am. Chem. Soc. 1981, 103, 6501. 4. (a) Halle, L.F.; Armentrout, P.B.; Beauchamp, J.L. Organometallics 1982, 1, 963. (b) Halle, L.F.; Armentrout, P.B.; Beauchamp, J.L. Organometal/ics 1982, 2, 1818. 5. (a) Periana, RA; Bergman, A.G. Organometa!lics 1984, 3, 508. (b) Periana, A.A.; Bergman, A.G. J. Am. Chem. Soc. 1984, 106, 7272. 6. Statler, J.A.; Flood, T.C. Organometallics 1984, 3, 1795. 7. Collman, J.P.; Hegedus , LS.; Norton, J.R.; Finke, A.G. Principles and Applications of Organometallic Chemistry. University Science Books : Mill Valley, California, 1978, p.388. 8. Crabtree, R.H.; Dion, R.P .; Gibboni, D.J.; McGrath, D.V.; Holt,E.M. J. Am. Chem. Soc. 1986, 108, 7222. 9. Watson, P.L.; Roe, D.C. J. Am. Chem. Soc. 1982, 104, 6471. 10. Burger, B.J. Ph.D. Thesis California Institute of Technology, 1987. 11. (a) Miller, A.G.; Pinke, P.A.; Stauffer, R.D.; Golden,H.J.; Baker, D.J. J. Am. Chem. Soc. 1974, 96, 4211. (b) Miller, A.G.; Pinke, P.A. J. Am. Chem. Soc. 1974, 96, 4221. 12. Stull, D.R.; Westrum, E.F., Jr.; Sinke, G.C. The Chemical Thermodynamics of Organic Compounds. Wiley: New York, 1969, p 330-334.