Chiral calix-salen cobalt complexes, catalysts for the enantioselective dynamic hydrolytic kinetic resolution of epibromohydrin

Abstract New calix-salen cobalt (III) complexes were synthesized as a mixture and as pure trimer or tetramer complexes. These cyclic complexes were used as catalysts to promote the dynamic hydrolytic kinetic resolution (HKR) of epibromohydrin in order to evaluate the effect of the cyclic structures size on the cooperative bimetallic interactions. Since the obtained catalysts were easily recovered from the reaction mixture by simple filtration, their efficiency was evaluated in recycling procedures. It was found that both cyclic oligomer complexes (trimer and tetramer) and the mixture of calix-salen complexes delivered the expected diol with high enantioselectivity and yield. Tetramer calix-salen cobalt complex proved to be the most active and selective catalyst of the series. In this case, an optimal conformation to allow the formation of bimetallic species activating respectively both the epoxide and water as nucleophile is probably responsible for an efficient dual activation.