Some Mechanistic and Synthetic Aspects of the Interaction of Lewis Acids with Bis-Cyclopentadienyltitanium(IV) Alkyls and Bis-Cyclopentadienyltitanacyclobutanes

Author: Ott, Kevin Curtis

Year: 1983

Degree: Dissertation (Ph.D.)

Advisor: Bercaw, John E.

Committee Members: Grubbs, Robert H.; Dougherty, Dennis A.; Evans, David A.; Bercaw, John E.

Option: Chemistry

DOI: 10.7907/fxvt-ac87

Abstract

Titanocene dichloride has been shown to react cleanly with two equivalents of AlMe3 to produce the Lewis acid stabilized titanium methylene [chemical formula; see abstract in scanned thesis for details]. Due to the interest in the utility of this complex in Wittig-type chemistry, in the metathesis of olefins, and in the synthesis of titanacyclobutanes, a study of the mechanism of formation of the complex was carried out. The proposed mechanism for formation of 1 involves an intramolecular proton abstraction by an Al-Me bond from a Ti-CH3 group in the intermediate Cp2TiMeCl•AlMe3. The effect of Cp-ring substitution and halide substitution on the reaction rate along with a deuterium isotope effect of 3 and a large negative entropy of activation were consonant with the proposed mechanism.

Specifically labelled titanacyclobutanes were prepared, and the cleavage reaction with AlMe2Cl to yield 1 as studied. It was found that cleavage occurred with a secondary deuterium isotope of 1.2 to 1.6 (depending upon the titanacyclobutane) and exhibited bimolecular kinetics. Surprisingly, the stereochemistry of the titanacyclobutanes was completely scrambled before cleavage to 1 and olefin. A mechanism for this isomerization was proposed to involve a rapid and reversible trans-metallation of a Ti-C bond with AlMe2Cl, producing a 3-aluminapropyltitanocene chloride. Following rapid inversion at the carbon adjacent to aluminum, the racemized 3-aluminapropyltitanocene chloride could cleave to 1 or close to yield isomerized titanacyclobutane.

The β,β-disubstituted titanacyclobutanes proved to be good sources of the Cp2TiCH2 unit, as indicated by their ready formation of bis-µ-CH2-bis-Cp2Ti (2). Some chemistry and photochemistry of 2 are reported. The Cp2TiCH2 moiety is readily trapped with transiton metal or main group metal Lewis acids such as Cp^TiCl3 or Me3SnCl to produce compounds such as [Cp2TiCl]-µ-CH2-[Cp*TiCl2] (3) and Cp2Ti(CH2SnMe3)Cl. Cp2TiCH2 may also be trapped as the methylene phosphine adduct Cp2Ti(=CH2)PEt3 (4).

The compounds 2 and 3 react with CO to yield insertion products which contain bridging ketene ligands which w ere characterized spectroscopically. Compound 4 also reacts with CO to produce the mononuclear ketene Cp2Ti(η2-CH2CO) in low yield.

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