Carbenoid insertion into organozirconocene complexes

Abstract

The synthesis of zirconacycles and organochlorozirconocene complexes is well established and the potential for their further elaboration is of interest to the organic chemist.  This thesis focuses on elaboration through carbenoid insertion into the carbon zirconium bond.

The regioselectivity of alkyl and alkenyl carbenoid insertion into unsymmetrical zirconacycles has been investigated.  In most cases studied complete regioselectivity was observed.  The origins of regioselectivity were investigated using zirconacycles with distinct substitution patterns and carbenoids with different electronic properties.  Results were used to support a model that explains regioselectivity through steric hindrance and the rate of 1,2-metallate rearrangement of the intermediate ‘ate’ complex.

The insertion of cyclopropyl carbenoids into organochlorozirconocenes results in the synthesis of vinylcyclopropanes and in alkylidenecyclopropanes.  The technique was applied to the synthesis of di-vinylcyclopropanes.  Investigations into the mechanism of these reactions revealed that the initial transmetallation from lithium to zirconium probably occurs with inversion of configuration. The insertion of cyclopropyl carbenoids generated in situ into organochlorozirconocenes was shown to be a viable synthetic technique.

The sequential addition of 1-halo-1-lithioalkenes and acetylides to zirconacyclopentenes and –anes results in highly elaborated bicycle-[3.3.0]-octenes and –octanes.  These are thought to be generated via an unusual 1,3-alkyl shift resulting in a zirconocene-alkenylidene complex.

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