RHODIUM-CATALYZED ASYMMETRIC HYDROFORMYLATION WITH DIPHOSPHITE LIGANDS BASED ON SUGAR BACKBONES

Chiral diphosphite ligands (PP) prepared from {(2,2'-biphenyl-1,1'-diy l), (4,4',6,6'-tetra- t-butyl-2,2'biphenyl-1,1'-diyl), '-di-t-butyl-6, 6'-dimethoxy-2,2'-biphenyl-1'-diyl) and di(2-t-butyl, 6-methylphenyl)} phosphorochloridites and sugar backbones {1,2- O-isopropylidene-D-xyl ofuranose, methyl-2,3-O -isopropyiidene-alpha-D mannopyranoside and (m ethyl-3,6-anhydro)-alpha-D-mannopyranosid alpha-D-glucopyranoside and beta-D-galactopyranoside} have been used in the rhodium catalysed asym metric hydroformylation of styrene. Enantioselectivities up to 64% hav e been obtained with stable hydridorhodium diphosphite dicarbonyl cata lysts (HRhPP(CO)(2)). High regioselectivities (up to 97%) to the branc hed aldehyde were found at relatively mild reaction conditions (T = 25 -40 degrees C, 9-45 bar of syngas pressure). The solution structures o f HRhPP(CO)(2) catalysts have been studied by P-31 and H-1 NMR spectro scopy. Bidentate coordination of the diphosphite ligand to the rhodium centre takes place in a bis-equatorial way. A relation between the tr igonal bipyramidal structure and the enantioselectivity of the HRhPP(C O)(2) complex is found. Rigid ligands with unsuitable geometries for b identate coordination probably coordinate as monodentates and give ris e to unstable catalysts and low selectivities during catalysis.