A Practical Synthesis of Rhodium Precatalysts for Enantioselective Hydrogenative Transformations

Abstract

Herein is described a practical method of preparing enantiopure rhodium(I) complexes that can be used as efficient catalysts for the asymmetric hydrogenation of functionalized alkenes, the hydrogenative kinetic resolution of vinyl sulfoxides, and the desymmetrization of meso-dienes. All these rhodium precatalysts incorporate enantiopure phosphine–phosphite (P–OP) ligands as stereochemical directors of the hydrogenative transformations. The synthetic route starts with the ring-opening of an enantiopure Sharpless epoxy ether with a phosphorus nucleophile followed by isolation of the borane-protected phosphino alcohol derivative by crystallization. The subsequent cleavage of this borane complex, the O-phosphorylation of the resulting phosphino alcohol with the corresponding phosphorus electrophiles (chlorophosphite derivatives), and finally the complexation of the in situ generated P–OP ligands with [Rh(nbd)2]BF4, followed by crystallization, rendered the target precatalysts.