Isostructural phosphine-phosphite ligands in rhodium-catalyzed asymmetric hydroformylation

Isostructural phosphine−phosphite ligands 7−10 have been synthesized from the condensation of chiral 3,3′-bis(trialkylsilyl)-2,2′-bisnaphthol phosphorochloridites and phenol-phosphanes 3−6. These ligands were evaluated as chiral inducers in the rhodium-catalyzed asymmetric hydroformylation reaction (AHF) of vinyl acetate and a series of styrene derivatives. The highest enantioselectivity, 78% ee, was observed in the AHF of vinyl acetate using phenylphosphino-based ligand 10. A direct correlation between the enantioselectivity and the Hammett constant σ of the substituent in the substrates was found (4-Me-styrene, 15% ee; styrene, 23% ee; 4-Cl-styrene, 51% ee) under mild reaction conditions for phenylphosphole-based ligand 8. Several rhodium-hydride and rhodium-acyl complexes were prepared and characterized by HP-NMR and HP-IR spectroscopy. Rhodium-hydride complexes of the formula [HRh(L)(CO)2] A with ligands 7, 8, and 10 were found to be highly conformationally fluxional in solution. The reaction of 4-Cl-styrene with rhodium-hydride complexes of ligands 7 and 8 gave the corresponding rhodium-acyl derivatives [(RCO)Rh(CO)2(L)] E. A comparative analysis of the spectroscopic properties of these rhodium-acyl complexes revealed that [(RCO)Rh(CO)2(7)] was more conformationally labile than [(RCO)Rh(CO)2(8)].