Preparation and reactivity of alkene-zirconocene complexes

Daniele Choueiry, Purdue University

Abstract

Dialkylzirconocenes containing one or more $\beta$-hydrogen atoms undergo thermal decomposition via $\beta$-hydrogen abstraction to give alkanes and alkene-zirconocenes. The relative reactivity of alkyl groups as $\beta$-hydrogen donors was found to correlate with the degree of substitution at the $\beta$-carbon center, their decreasing order of reactivity in most cases being: $\beta$-methyl $>$ $\beta$-methylene $>$ $\beta$-methine. This generalization can, however, be modified by electronic factors. Furthermore, predictive comparison within one category remains difficult. Nonetheless, this trend permits some rationalization and prediction of the course of $\beta$-hydrogen abstraction of dialkylzirconocenes, which is the most convenient and general route to highly reactive alkene-zirconocene complexes. The $\beta$-hydrogen abstraction of (n-Bu)$\sb2$ZrCp$\sb2$ produces $\eta\sp2$-1-butene-ZrCp$\sb2$ which can react with nonconjugated dienes to form the corresponding zirconabicycles which do not incorporate 1-butene. This reaction most likely proceeds via an associative mechanism involving the intermediacy of monocyclic zirconacyclopentanes. In the case of nonconjugated dienes containing substituted vinyl groups, the reaction can lead to either bicyclization or formation of conjugated dienes-zirconocenes via multipositional regioisomerization. The latter process appears to share a considerable portion of the mechanism with the bicyclization. Although the reactions of alkene-zirconocenes can, in some cases, proceed stereospecifically, they can also undergo stereoisomerization leading to highly stereoselective but nonstereospecific processes, which most likely proceed via dipolar zirconate species. These polar processes represent yet another novel reaction pattern that alkene-zirconocenes display. Alkene-zirconocenes were also found to act as catalysts in various $\rm Cl\sb2ZrCp\sb2$-catalyzed reactions, such as cyclomagnesiation of nonconjugated dienes, ethylmagnesiation or conjugated diynes, cyclic carboalumination of alkynes, and cyclization of enyenes.

Degree

Ph.D.

Advisors

Negishi, Purdue University.

Subject Area

Organic chemistry

COinS