ENANTIOSELECTIVE CONJUGATE ADDITION OF GRIGNARD-REAGENTS TO ENONES CATALYZED BY CHIRAL ZINC(II) COMPLEXES

Various chiral zinc(II) complexes catalyze the asymmetric 1,4-addition of Grignard reagents to α,β-unsaturated ketones with high chemoselectivities (yields of 1,4-adducts, 83-99%), high regioselectivities (1,4/1,2 ratios up to 499) and modest enantioselectivities (ee up to 33%). A study of several factors, i.e. ligand, solvent, counterions, order and rate of additions, temperature, and the nature of Grignard reagents, that influence the regio- and enantioselectivities is given. Based on the addition of isopropylmagnesium halides to 2-cyclohexenone as a model reaction, it was established that the highest enantioselectivities are reached with in situ prepared zinc complexes derived from optically active diamino alcohol ligands using lithium bases in tetrahydrofuran as the solvent. A mechanistic rationalization is given. © 1990, American Chemical Society. All rights reserved.