STEREOCHEMISTRY IN CARBENOID FORMATION BY BROMINE/LITHIUM AND BROMINE/ZINC EXCHANGE-REACTIONS OF GEM-DIBROMO COMPOUNDS

Stereochemistry in generation of lithium and zincate carbenoids by bromine/metal exchange reactions of gem-dibromo compounds with BuLi and lithium triorganozincates, respectively, has been investigated. Both lithium and zincate carbenoids derived from 1,1-dibromoalkenes are demonstrated to be configurationally stable at low temperatures whereas, in the presence of the unreacted starling dibromoalkenes, the lithium carbenoids, but not the zincate carbenoids, undergo facile isomerization at the carbenoid carbons. Zincate carbenoids derived from 1,1-dibromocyclopropanes undergo isomerization slowly at the carbenoid carbons in the presence of the starting dibromocyclopropanes. The isomerization reactions of the lithium and zincate carbenoids are proved to proceed through a mechanism involving a bromine/metal exchange between the carbenoids and the starting dirbromo compounds. Highly stereoselective formation of the lithium carbenoids is observed in the reaction of 1,1-dibromoalkenes under the thermodynamic conditions. Under kinetically controlled conditions, both the bromine/lithium and bromine/zinc exchange reactions take place preferentially at the sterically more hindered bromine atom of the starting gem-dibromo compounds. The observation is rationalized by an exchange mechanism involving a linear transition state of either an ate complex formation or an S(N)2 reaction at the more hindered bromine atom where strain relief due to the elongation of the carbon-bromine bond is expected.