Chiral Cooperativity in Diastereomeric Diphosphite Ligands:  Effects on the Rhodium-Catalyzed Enantioselective Hydroformylation of Styrene

Diastereomeric diphosphites (L∩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 °C, 20 bar of syn gas CO−H2 [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∩L(CO)2 complexes (L∩L = 3−8)], have been studied by 31P and 1H NMR spectroscopy at variable temperature (313−213 K). Spectroscopic data, in combination with the obtained results in catalysis, suggest that diphosphite ligands (L∩L) containing the conformationally flexible axially chiral biphenyl moieties predominantly exist as single atropisomers in the HRhL∩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.