Rhodium catalysed asymmetric hydroformylation with diphosphite ligands based on sugar backbones

Chiral diphosphite ligands (PP) prepared from {(2,2′-biphenyl-1,1′-diyl), (4,4′,6,6′-tetra-t-butyl-2,2′-biphenyl-1,1′-diyl), 4,4′-di-t-butyl-6,6′-dimethoxy-2,2′-biphenyl-1,1′-diyl) and di(2-t-butyl, 6-methylphenyl)} phosphorochloridites and sugar backbones {1,2-O-isopropylidene-D-xylofuranose, methyl-2,3-O-isopropylidene-α-D-mannopyranoside and (methyl-3,6-anhydro)-α-D-mannopyranoside, α-D-glucopyranoside and β-D-galactopyranoside} have been used in the rhodium catalysed asymmetric hydroformylation of styrene. Enantioselectivities up to 64% have been obtained with stable hydridorhodium diphosphite dicarbonyl catalysts (HRhPP(CO)2). High regioselectivities (up to 97%) to the branched aldehyde were found at relatively mild reaction conditions (T = 25-40°C, 9-45 bar of syngas pressure). The solution structures of HRhPP(CO)2 catalysts have been studied by 31P and 1H NMR spectroscopy. Bidentate coordination of the diphosphite ligand to the rhodium centre takes place in a bis-equatorial way. A relation between the trigonal bipyramidal structure and the enantioselectivity of the HRhPP(CO)2 complex is found. Rigid ligands with unsuitable geometries for bidentate coordination probably coordinate as monodentates and give rise to unstable catalysts and low selectivities during catalysis.