Enantioselective Hydroboration of Styrenes with Markovnikov Selectivity Catalyzed by a Rhodium(I)-NHC Complex

Abstract

Asymmetric catalysis with phosphine free ligands is a rapidly developing field where N-heterocyclic carbenes (NHCs) are making increasingly more impact. Herein, we report the synthesis and characterization of carbene-oxazoline containing proligands that exhibit two chiral centers. The corresponding rhodium­(I) complexes show excellent activity toward the asymmetric hydroboration of styrenes, which occurs with good regio– and enantioselectivity (up to 96% ee). The resulting boronic esters were subsequently oxidized, and the corresponding chiral alcohols were isolated in moderate to good yields (up to 75%). Overall, the hydroboration reaction occurs under ambient conditions (CH2Cl2; 25 °C), is compatible with a variety of functional groups, and provides easy access to chiral secondary alcohols from the corresponding alkenes.