First enantioselective addition of diethylzinc and dimethylzinc to prostereogenic ketones catalysed by camphorsulfonamide-titanium alkoxide derivatives

The reaction of diethylzinc or dimethylzinc with prostereogenic ketones, in the presence of a stoichiometric amount of titanium tetraisopropoxide and a catalytic amount (20%) of camphorsulfonamide derivatives as chiral ligands leads to the formation of the corresponding enantioenriched tert-alcohols with enantiomeric ratios up to 94.5 : 5.5. The best results were obtained when phenones are used as substrates independently of the dialkylzinc reagent. The enantioselectivity shows a linear relationship with the enantiomeric excess of the ligand and seems to be independent of the reaction yield. A tentative catalytic cycle and mechanistic model are proposed for this new reaction. (C) 1998 Elsevier Science Ltd. All rights reserved.