The preparation of a series of new bis(phospholane) ligands is described. A convenient procedure that allows the synthesis of secondary 2,5-dialkylphospholanes and their borane adducts has been developed. Use of the borane adducts to prepare diastereomeric bis-(phospholane) ligands possessing chiral 2,4-pentane backbones also is described. Stereochemical matching and mismatching effects within these ligands are assessed in rhodiumcatalyzed asymmetric hydrogenation reactions. In the hydrogenation of R-keto esters, substantially higher enantioselectivities (up to 78% ee) are observed with catalysts derived from the matched ligand relative to the analogous mismatched ligand and the ligand possessing an achiral 1,3-propane backbone. Two diastereomeric rhodium catalyst precursors have been characterized by X-ray crystallography, and the results have been analyzed in an effort to correlate the hydrogenation data with specific structural features of the catalysts.
2000organo-phosphorus compounds, nonmetal heterocycles organo-phosphorus compounds, nonmetal heterocycles S 0087
-168Bis(phospholane) Ligands Containing Chiral Backbones. Matching and Mismatching Effects in Enantioselective Hydrogenation of α-Keto Esters.-The synthesis of several novel bis(phospholane) ligands containing a chiral 2,4-pentylene backbone, e.g. SBDPP and RBDPP, is described. Their utility is studied in the hydrogenation of α-aminocarboxylic esters like (I) and of α-keto ester (III) in the presence of a [Rh(cod)(ligand)]OTf catalyst. It is shown, that the sense of stereoinduction is determined by the phospholane stereochemistry of the ligand, since reduction products of the same configuration are obtained irrespective of the configuration of the ligand backbone. In contrast, the degree of stereoinduction strongly depends on the nature of ligand backbone. The best results are obtained for ligand SBDPP (75% e.e.), while with diastereomeric ligand (RBDPP) as well as the parent compound (BDPPR) the reduction products are obtained in only low optical purity. -(BURK, MARK J.; PIZZANO, ANTONIO; MARTIN, JOSE A.; LIABLE-SANDS, LOUISE M.; RHEINGOLD, ARNOLD L.; Organometallics 19 (2000) 3, 250-260; Dep. Chem., Duke Univ., Durham, NC 27706, USA; EN)
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