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

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.