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.