[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, α‐ and β‐keto esters, α‐ and β‐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 β‐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.