Convenient Enantioselective Hydrosilylation of Ketones Catalyzed by Zinc-Macrocyclic Oligoamine Complexes

Chiral macrocyclic tetra- and hexamine macrocycles derived from trans-1,2-diaminocyclohexane (DACH) in complexes with diethylzinc efficiently catalyze the asymmetric hydrosilylation of aryl alkyl ketones with enantiomeric excess of the product up to 89%. The cyclic structure of the trianglamine ligand increases the enantioselectivity of the reaction, in comparison to the catalysis with the acyclic N,N'-dibenzyl-DACH ligand. Density functional theory (DFT) computations on the structures of ligand-zinc complexes and on the structures of these complexes with a coordinated acetophenone molecule allow us to rationalize the direction of the asymmetric induction of the hydrosilylation reaction as well as the superiority of the cyclic ligand compared to the acyclic one. This is the first example of asymmetric catalysis for the hydrosilylation reaction of ketones with the use of a readily available, inexpensive, and reusable macrocyclic trianglamine ligand.