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 )].