New chiral phosphine-phosphite ligands in the enantioselective rhodium-catalyzed hydroformylation of styrene

A series of chiral phosphine-phosphite ligands la-g have been synthesized from monophosphines 2-4, enantiomerically pure propene oxide or styrene oxide, and 3,3',5,5'-tetra(tert-butyl)-2,2'-bisphenol phosphorochloridite or enantiomerically pure 3,3'-bis(trimethylsilyl)-2,2'-binaphthol phosphorochloridites. These phosphine-phosphites have been used in the rhodium-catalyzed asymmetric hydroformylation of styrene; The structures of the active catalysts, [HRh(L-L)(CO)(2)] complexes (L-L = ligands 1a-g), have been studied using high-pressure NMR and IR spectroscopy. The obtained spectroscopic data show that the ligands coordinate in an equatorial-apical fashion to the rhodium center with the phosphine in apical position. Systematic variation in configuration of the stereocenters at both the ligand bridge and the phosphine moiety revealed a remarkable cooperative effect on the selectivity of the hydroformylation reaction. Under mild reaction conditions ee's of 63% and regioselectivities up to 92% toward 2-phenylpropanal were obtained (25-60 degrees C, 20 bar of syn gas CO:H-2 [1:1]) for ligands 1. The absolute configuration of the product is governed by the stereogenic center of the backbone of the ligand. There is a large cooperative effect, however, from the phosphine moiety. Spectroscopic data, in combination with the obtained results in catalysis, suggest that phosphine-phosphite ligands (L-L) containing the conformationally flexible and axially chiral biphenyl moiety exist predominantly as single atropisomers in the HRh(L-L)(CO)(2) complexes. Comparison of the bisphenol and binaphthol substituents suggests that the high enantiomeric excesses obtained with the former are caused by the preferential formation of the most selective diastereomer.