Asymmetric Hydroformylation of Olefins with Rh Catalysts Modified with Chiral Phosphine−Phosphite Ligands

Abstract

Rhodium complexes stabilized by modularly designed chiral phosphine−phosphite ligands (P−OP) have been tested in the asymmetric hydroformylation of styrene, vinyl naphthalenes, and allyl cyanide. Based on single-crystal X-ray diffraction analysis and NMR studies, restricted aryl rotation has been found to characterize ligands 1e and 1f. The outcome of the rhodium-catalyzed hydroformylation reactions is highly dependent on the nature of the two coordinating functions of the phosphine−phosphite and of the ligand backbone as well. Among the ligands studied, those with an oxyphenylene backbone and PAr2 ends gave the best results, outperforming those with P-stereogenic phosphine groups. The 1-naphthyl-substituted catalyst brought about the hydroformylation of styrene with a 71% ee, while the xylyl catalyst afforded the best results in the hydroformylation of allyl cyanide, yielding an iso/n ratio of 13 and 53% ee in the branched isomer. Several hydrido(carbonyl) species of the formula RhH(CO)2(P−OP) have been generated by reacting Rh(acac)(CO)2/P−OP with syngas. In situ high-pressure NMR experiments showed the phosphine group to occupy an apical position of the trigonal bipyramidal coordination geometry, which allows an aryl−aryl interaction between the phosphine substituents and the substrate during the hydroformylation of vinyl arenes. In line with this finding, a remarkable enantioselectivity of 89% ee was obtained with the naphthyl catalyst and 1-vinyl naphthalene as substrate.