Stereochemistry in carbenoid formation by bromine/lithium and bromine/zinc exchange reactions of 1,1-dibromoalkenes: higher reactivity of the sterically more hindered bromine atom

Abstract

Both lithium and zincate carbenoids (R 1 -(R 2 )C=C(Br)M; M=Li and Zn(Bu) 2 Li) generated by the halogen/metal exchange reaction of 1,1-dibromoalkene 1 with BuLi and (Bu) 3 ZnLi, respectively, are configurationally stable at low temperatures, but in the presence of excess 1, the lithium carbenoids undergo facile isomerization at the carbenoid carbons. Under kinetically controlled conditions, both the Br/Li and Br/Zn exchange reactions take place preferentially at the sterically more hindered bromine atom of 1