Synthesis of partially hydrogenated 2,2′-bis(diphenylphosphenyl)-1,1′-binaphthyl (BINAP) ligands and their application to catalytic asymmetric hydrogenation

Abstract

Three pairs of new axially dissymmetric bisphosphane ligands, (R)-(–)- and (S)-(+)-2,2′-bis(dicyclohexylphosphanyl)-1,1′-binaphthyl [(R)-(–)-and (S)-(+)-Cy-BINAP, (R)-(–)-and (S)-(+)2], (R)-(+)- and (S)-(–)-2,2′-bis(diphenylphosphanyl)-5,5′,6, 6′,7,7′,8,8′-octahydro-1,1′-binaphthyl [(R)-(+)- and (S)-(–)-H₈-BINAP, (R)-(+)- and (S)-(–)-3], and (R)-(–)- and (S)-(+)2,2′-bis(dicyclohexylphosphanyl)-5,5′,6,6′,7,7′, 8,8′-octahydro-1,1′-binaphthyl [(R)-(–)- and (S)(+)-Cy-H₈-BINAP, (R)-(–)- and (S)-(+)-4], have been synthesized. The absolute configurations of the isomers 2 were determined by single-crystal X-ray diffraction of the linear 1:1 polymeric complex of (S)-(+)-2,2′-bis(dicyclohexylphosphinoyl)-1,1′-binaphthyl [(S)-(+)-Cy-BINAPO, (S)(+)-6] and (2R, 3R)-(–)-di-O-benzoyltartaric acid [(–)-DBT], and those of the isomers 3 and 4 were established on the basis of CD spectra of the phosphanes and their bisoxides. X-Ray crystallographic studies of two cationic Rh^I complexes, [Rh{(S)-Cy-binap}(cod)]ClO₄[(S)-17] and [Rh{(S)-H₈-binap}(cod)]ClO₄[(S)-18], revealed that complex (S)-17 possesses a dissymmetric structure, while complex (S)-18 has a pseudo-C₂-symmetry and shows a significantly large dihedral angle between the two phenyl rings [80.3(4)°]. The potentiality of ligand 3 for asymmetric catalysis was demonstrated in Ru^II-catalysed stereoselective hydrogenations of methyl 2-(benzamidomethyl)-3-oxobutanoate (21, in up to 92% d.e. and 99% e.e.) and geraniol (22, in 98% optical purity).