[Zinc-Diamine]-Catalyzed Hydrosilylation of Ketones in Methanol. New Developments and Mechanistic Insights

The Zn-promoted direct reduction of various ketones, including alkyl aryl ketones, - and -keto esters, - and -keto amides, into the corresponding alcohols with polymethylhydrosiloxane (PMHS) in protic conditions is reported. The reaction proceeds chemoselectively under smooth conditions, with simple catalyst combinations based on a zinc precursor [ZnEt2, Zn(OMe)2, Zn(OH)2] and a 1,2-diamine ligand, e.g., N,N-dibenzylethylenediamine (dbea). The reaction rates are significantly faster than under aprotic conditions. For -keto esters and amides, the use of an excess of PMHS is required. Moderate enantioselectivities (ee up to 55%) have been obtained using a variety of enantiopure diamine ligands. Two mechanisms are proposed for the new catalytic reaction, on the basis of the synthesis of [(diamine)Zn(alkoxide)2] complexes which are models of the possible reaction intermediates and a study of their activity