Synthesis of chiral 4,4′-disubstituted 1,1′-spirobiindane-7,7′-diols and related phosphoramidites:: the substituent effect of SIPHOS ligands in Rh-catalyzed asymmetric hydrogenation

Three chiral 4,4'-substituted 1,1'-spirobiindane-7,7'-diols and related monodentate spiro phosphoramidite ligands have been readily synthesized from enantiomerically pure 1, 1'-spirobiindane-7,7'-diol. Excellent enantioselectivities were obtained with these new ligands in the rhodium-catalyzed asymmetric hydrogenation of dehydroamino acid derivatives and enamides. Comparing SIPHOS, ligands 4,4'-dibromo-SIPHOS and 4,4'-diphenyl-SIPHOS gave similarly high enantioselectivities although the rates in hydrogenations of enamides are somewhat slower. Methoxy substituents at the 4,4'-position of ligands slightly reduced enantioselectivities of hydrogenation reactions. (C) 2003 Elsevier Ltd. All rights reserved.