Studies on the Reactivity of Titanocene Alkylidenes and Titanocene Alkyl Chlorides

Citation

Clawson, Leigh Ellen (1988) Studies on the Reactivity of Titanocene Alkylidenes and Titanocene Alkyl Chlorides. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/a114-qy61. https://resolver.caltech.edu/CaltechTHESIS:01172013-151800515

Abstract

The scope and limitations of the reaction of titanocene methylidene with ketones were investigated. Easily enolizable ketones are methylenated quantitatively and isolated in high yield. This reagent also demonstrates chemoselectivity for ketones over esters and regioselectivity for less hindered ketones. Enolates are produced by proton transfer to the methylidene from very sterically (but not particularly acidic) ketones.

A simple one-pot method for the production of a geminal dimethylated center from a ketone and two equivalents of titanocene methylidene has been developed. This is a very sterically restricted process that is complementary to similar reactions using other organometallic reagents.

The heterocyclic compounds, benzothiazole and benzoxazole, react with titanocene methylidene, presumably to form aza-metallacycles, which then undergo a β-elimination to form novel metallacycles. This reaction is not general for other compounds with an X-C=N linkage (X=O, S).

Fundamental processes involved in Ziegler-Natta polymerization have been investigated in a homogeneous Ti/Al-based model system. Lewis acid catalyzed olefin insertion was studied with titanocene alkenyl chlorides. This reaction is irreversible. Deuterium-labelling experiments provide support for the direct insertion mechanism. The reverse process, β-alkyl elimination, was also examined with titanocene methylcyclobutyl chloride. This process is also irreversible with no competing β-hydrogen elimination.

NMR studies were performed to probe the nature of the interaction between titanocene alkenyl chlorides and alkyl aluminum chloride cocatalysts in the intramolecular olefin insertion. This system is a model for the active site in Ziegler-Natta polymerization. A Lewis acid concentration dependence on the reaction is observed. A possible explanation for this concentration dependence is presented.

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