Bis(phospholane) ligands containing chiral backbones.: Matching and mismatching effects in enantioselective hydrogenation of α-keto esters

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 rhodium-catalyzed asymmetric hydrogenation reactions. In the hydrogenation of alpha-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.