In search of new approaches to asymmetric conjugate addition:: Screening studies on the use of [Zn(bpy*)X(R)] reagents and α,β-unsaturated amide Michael acceptors

Conjugate additions of [ Zn( bpy*) Cl( Et)] ( bpy* = 4,4 '-di-tert-butyl-2,2 '-bipyridine) to cyclohex-2-en-1-one are promoted by ZnMe2 in 88% ee but in moderate yield under Cu-I phosphoramidite catalysis. In the absence of ZnMe2 the [ Zn( bpy*) Cl( Et)] is inactive indicating a Schlenk-type equilibrium. Other derivatives of [ Zn( bpy*) Cl( R)] ( R = Bu, 4-methylbenzyl), prepared in situ from [ ZnCl( R)] and the bipyridine give low yields due to competing chloride abstraction. C-13 NMR studies indicate facile organo-ligand exchange between [ Zn( bpy*)( Et)(2)] and [ Zn( bpy*) Cl-2] complexes. In the presence of the bipyridine, [ ZnBr( allyl)] disproportionates into [ Zn( bpy*) Br-2] and [ Zn( bpy*)( allyl)(2)] species. In separate studies, simple ( E)-MeCH=CHCONMeR ( R = Me, OMe) alpha,beta-unsaturated amides undergo asymmetric 1,4-addition of EtMgBr in 75-99% yield and 48-79% ee in the presence of the diphosphines JosiPhos or MeDuPhos and copper( I) sources.