Rhodium catalysed asymmetric hydroformylation with diphosphite ligands based on sugar backbones

Article abstract of DOI:10.1016/0957-4166(95)00068-Z

Chiral disphosphite 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.