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

Both lithium and zincate carbenoids (R1-(R2)C=C(Br)M; M = Li and Zn(Bu)2Li) generated by the halogen/metal exchange reaction of 1,1-dibromoalkene 1 with BuLi and (Bu)3ZnLi, 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.