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.

The synthesis of P-stereogenic MOP analogues and their use in rhodium catalysed asymmetric addition

“…Complexation of MOP with [Rh 2 Cl 2 (CO) 4 ] resulted in multiple species again being observed in the 31 (It is noteworthy that on a separate run of this complex, the broad peak at 47e48 ppm was observed).…”

“…The few cases where such hybrid ligands were studied have proved inconclusive. Thus, for example, in the case of asymmetric hydroformylation, Pizzano and co-workers found that their PeOP ligands which combined both axial and P-stereogenicity were less selective than their ligands without P-stereogenicity, suggesting that the chiralities were not co-operative [4,5].Conversely, Buchwald and co-workers have reported significantly improved ees when their P-stereogenic binaphthyl substituted monophosphines were tested in palladium catalysed asymmetric vinylation and arylation reactions [6,7]. Previously our group have reported the novel synthesis of P-stereogenic MOP analogues ( Fig.…”

Features and Application in Asymmetric Catalysis of Chiral Phosphine–Phosphite Ligands

“…Thus, M–P distances are typically longer for phosphines. For instance, for Pd(Cl) 2 (P‐OP) complexes, distances between 2.227 and 2.272 Å have been reported for the phosphine group (Table , entries 1–5), while the corresponding distances for the phosphite fragment are in the range between 2.187 and 2.214 Å 31–33. This trend is also observed in the case of [Rh(diolefin)(P‐OP)]BF 4 complexes (Table , entries 8–14), with ranges of 2.265–2.410 and 2.182–2.220 Å for the phosphine and phosphite fragments, respectively 34–38.…”

Asymmetric Hydroformylation

“…This catalytic system provided Scheme 32 Rh-catalyzed asymmetric hydroformylation with the polystyrene supported ligand (R,S)-BINAPHOS (72) good conversions (up to 83%), excellent regioselectivities (up to 90%), and ees up to 93% (Scheme 31).…”