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

Citation

Ott, Kevin Curtis (1983) Some Mechanistic and Synthetic Aspects of the Interaction of Lewis Acids with Bis-Cyclopentadienyltitanium(IV) Alkyls and Bis-Cyclopentadienyltitanacyclobutanes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/fxvt-ac87. https://resolver.caltech.edu/CaltechTHESIS:11012019-155844929

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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