Design, Synthesis, Characterization, and Reactivity of Group III ansa-Metallocenes Containing Bulky Alkyl and Silyl Substituents

Author: Abrams, Michael Benjamin

Year: 1998

Degree: Dissertation (Ph.D.)

Advisor: Bercaw, John E.

Committee Members: Grubbs, Robert H.; Bercaw, John E.; Lewis, Nathan Saul; Myers, Andrew G.

Option: Chemistry

DOI: 10.7907/2v44-pj33

Abstract

The preparations of a series of silyl-bridged bis(cyclopentadiene) complexes containing bulky alkyl and silyl substituents are reported. Incorporation of tertiary alkyl groups are required in order to direct trimethylsilyl groups exclusively adjacent to the silyl linkers. This 1,2,4- substitution is crucial for the directed syntheses of chiral ansa-metallocenes. Transmetallation of Me2Si{2-SiMe3-4-[2-(2-CH3-C10H14)]-C5H2)22- affords exclusively racemo-chiral) Group III metallocenes, whereas transmetallation of Me2Si{3-[2-(2-CH3-C10H14)]- C5H3}22- (lacking the α-TMS groups) proceeds with the opposite enantioselectivity, generating exclusively mesa- (achiral) metallocenes.

Treatment of meso-Me2Si{3-[2-(2-CH3-C10H14)]-C5H3}2MCl-(LiCl)-(THF)2 (meso- AdpMCl-(LiCl)-(THF)2) with allylmagnesium bromide affords mesa-AdpM(ƞ3-C3H5) (M = Sc, Y). Hydrogenolyses of these mesa-metallocenes generate highly reactive 14-electron d0 metal hydride complexes. The methyladamantyl substituents on the Cp rings inhibit formation of bridging hydride dimers, enabling the reactive mesa-AdpMH monomers to react readily with available solvent or metallocene C-H bonds.

Treatment of [rac-Me2Si{2-SiMe3-4-[2-(2-CH3-C10H14)]-C5H2}2Y(μ-Cl)]2 ([ra c-Abp Y(μCl)]2) with LiCH(SiMe3)2 affords the bulky yttrium-alkyl complex rac-AbpYCH(SiMe3)2. This alkyl complex readily polymerizes ethylene, but does not react with cx-olefins. Hydrogenolysis ofrac-AbpYCH(SiMe3)2 generates the homochiral hydride dimer [rac-AbpY(μ-H)]2. The bulk of the methyladamantyl substituents inhibits formation of the heterochiral hydride dimer. Efforts are made to assess the ability of the methyladamantyl groups to inhibit the hydride monomer-dimer equilibria that deactivates previously reported group III Ziegler-Natta catalysts. The hydride dimer [rac-AbpY(μ-H)]2 is an ex-olefin hydrogenation catalyst, but does not oligomerize or polymerize α-olefins. [rac-AbpY(μ-H)]2 also reacts readily with dihydrogen, allene, and 2-butyne; examples of σ-bond metathesis, alkyne insertion, and β-hydride elimination have been observed for [rac-AbpY] complexes.

The fluxional behavior exhibited by a series of d0-metallocene ƞ3-allyl complexes is investigated. Lineshape analysis of NMR studies performed on these complexes indicates activation barriers of ~8-14 kcal/mol for processes involving dissociation of the allyl C=C double bond as the rate determing step. Investigation of Cs-symmetric linked scandocenes and yttrocenes allows elucidation of a second allyl rearrangement mechanism which operates concurrently with olefin dissociation: in-plane rotation rotation of the intact ƞ3-allyl ligand. Allyl rotation is generally fast relative to the rate of olefin dissociation. Ligand, metal, and solvent effects on the observed rates are discussed.

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