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 f rom monophosphines 2-4, enantiomerically pure propene oxide or styrene oxid e, and 3,3',5,5'-tetra(tert-butyl)-2,2'-bisphenol phosphorochloridite or en antiomerically pure 3,3'-bis(trimethylsilyl)-2,2'-binaphthol phosphorochlor idites. These phosphine-phosphites have been used in the rhodium-catalyzed asymmetric hydroformylation of styrene; The structures of the active cataly sts, [HRh(L-L)(CO)(2)] complexes (L-L = ligands 1a-g), have been studied us ing high-pressure NMR and IR spectroscopy. The obtained spectroscopic data show that the ligands coordinate in an equatorial-apical fashion to the rho dium center with the phosphine in apical position. Systematic variation in configuration of the stereocenters at both the ligand bridge and the phosph ine moiety revealed a remarkable cooperative effect on the selectivity of t he hydroformylation reaction. Under mild reaction conditions ee's of 63% an d 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 con figuration of the product is governed by the stereogenic center of the back bone of the ligand. There is a large cooperative effect, however, from the phosphine moiety. Spectroscopic data, in combination with the obtained resu lts in catalysis, suggest that phosphine-phosphite ligands (L-L) containing the conformationally flexible and axially chiral biphenyl moiety exist pre dominantly as single atropisomers in the HRh(L-L)(CO)(2) complexes. Compari son of the bisphenol and binaphthol substituents suggests that the high ena ntiomeric excesses obtained with the former are caused by the preferential formation of the most selective diastereomer.