ABSTRACT

The preparation, structural and chemical properties of molecules having several metals attached to the same carbon atom have fascinated chemists for a long time. Theoretical predictions speculate that some of these molecules may have a planar tetracoordinated carbon atom [ 1 ,2] . Several polymetallated compounds such as CLi4 [3] have been prepared and their properties were carefully investigated. Unfortunately, owing to their high reactivity, very few synthetic applications have been found, and sequential reactions of the four carbon-metal bonds with different electrophiles could not be realized [3]. On the other hand, mixed l , l -bimetallics, in principle, should be able to react in a selective way with two different electrophiles (E1 and E2) as long as the reactivity of the first carbon-metal bond (C-M l ) is substantially different from the reactivity of the second carbon-metal bond [C-M 2; Eq. ( 1 )] . In t 97 t , Gaudemar discovered a very general and practical preparation of mixed l , t -bimetallics of zinc and magnesium by performing the addition of an allylic zinc halide to alkenylmagnesium halides [4-6] . This reaction allows the convenient study of the reactivity of mixed l , t -bimetallics, and it was soon found by Knochel [7-1 4, 1 9] , that the sequential reaction of these organometallic reagents with various electrophiles can be realized. Furthermore, these compounds proved to be excellent oletination reagents, allowing the stereoselective preparation of polyfunctional (E)- and (Z)-disubstituted oletins [ 1 2-14; Eq. (2)] . This chapter describes the preparation and reactivity of mixed magnesium and zinc t , t -bimetallics. The chemistry of some related reagents will also be covered.