Heterogeneous vanadium catalysts for racemization and chemoenzymatic dynamic kinetic resolution of benzylic alcohols

Abstract

Cheap and readily available oxovanadium compounds such as vanadyl sulfate (VOSO4) and vanadium oxide (V2O4) catalyze the racemization of benzylic alcohols in the absence of any additives (e.g., bases or hydrogen mediators) or co-catalysts. The reaction occurs at 80 °C in n-octane as the solvent and does not require anhydrous or oxygen-free conditions. Experiments clearly demonstrated that VOSO4 is a reusable and truly heterogeneous racemization catalyst. Insight into the reaction mechanism was obtained from 18O-labeling experiments, which showed that the formation of carbenium ion intermediates is a key step. The V-catalyzed racemization was combined with a lipase-catalyzed kinetic resolution to give a chemoenzymatic dynamic kinetic resolution of benzylic alcohols. The dynamic kinetic resolution (DKR) proceeds in one pot and does not need additives or specifically designed acyl donors. Under optimized conditions, the DKR process yields the corresponding esters in good chemical yield (>90%) and high optical purity (>99% ee).