Chiral cooperativity in diastereomeric diphosphite ligands: Effects on the rhodium-catalyzed enantioselective hydroformylation of styrene

Diastereomeric diphosphites (L boolean AND L = 3-8) have been synthesized from enantiomerically pure pentane-2,4-diol and axially chiral 3,3'-bis(trialkylsilyl)-2,2'-bisphenol phosphorochloridites and 3,3'-bis(trialkylsilyl)-2,2'-bisnaphthol phosphorochloridites. These diphosphites have been used;to test the influence of chiral cooperativity in the rhodium-catalyzed asymmetric hydroformylation of styrene. Systematic variation in chirality at both the chiral ligand bridge and the axially chiral biphenyl and binaphthyl substituents revealed a remarkable effect on the selectivity of the hydroformylation catalysts. For the atropisomeric bisnaphthol-based diphosphites, cooperative effects were observed in the asymmetric hydroformylation of styrene. :High enantioselectivities (87%) and,regioselectivities up to 95% for 2-phenylpropanal were found under mild reaction conditions (15-50 degrees C, 20 bar of syn gas CO-H-2 [1:1]) for the ligand derived from (2R,4R)-pentane-2,4-diol and (S)-bisnaphthol. The same high enantiomeric excess was observed for the free-rotating bisphenol-substituted ligands. The highest selectivity was obtained with trimethylsilyl substituents at the ortho position. The solution structures of the active catalysts [HRhL boolean AND L(CO)(2) complexes (L boolean AND L = 3-8)], have been studied by P-31 and H-1 NMR spectroscopy at variable temperature (313-213 K). Spectroscopic data, in combination with the obtained results in catalysis, suggest that diphosphite ligands (L boolean AND L) containing the conformationally flexible axially chiral biphenyl moieties predominantly exist as single-atropisomers in the HRhL boolean AND L(CO)(2) complexes. Comparison of the bisphenol and bisnaphthol substituents suggests that the high enantiomeric excesses obtained with the former are caused by the preferential formation of the most selective diastereomer.