Origins of enantioselectivity in the chiral diphosphine-ligated CuH-catalyzed asymmetric hydrosilylation of ketones

Abstract

Computational investigations on the asymmetric hydrosilylation of acetophenone over ligated CuH catalysts were performed with the DFT method. The calculations predict that the catalytic reaction involves two steps: (1) CuH addition to the carbonyl groupvia a four-membered transition state (TS) with the formation of copper-alkoxide intermediates; (2) regeneration of the ligated CuH catalyst by an external SiH₄ through a metathesis process to yield the corresponding silyl ether. The calculations in the chiral diphosphine-ligated CuH systems suggest that the metathesis process is the rate-determining step (RDS). The CuH addition step is vital for the distribution of the racemic products and therefore represents the stereo-controlling step (SCT). In this step, the greater steric hindrance between the aromatic rings of the ligands and the substrate is identified as the major factor for enantioselectivity. The corresponding TS in the face-to-face mode, suffering less steric hindrance, is more stable than its analogue in the edge-to-face mode. The enantioselectivities are calculated to be related not only to the P–Cu–P bite angles in the stereo-controlling TSs, but also to the substituents at the P-aryl rings of the chiral ligands. In short, a larger P–Cu–P bite angle and suitably modified P-aryl rings together are necessary to achieve excellent ee values.