Chiral memory effects in catalytic hydrogenations with dynamically chiral ligands

Abstract

The low barrier for interconversion of chiral conformations of the dynamically chiral 2,2′-biphenyl ligand NMe 2C 6H 4C 6H 4PCy 2 is raised upon coordination. The individual enantiomers of the planar chiral arene-tethered complex Ru(η 6:η 1- NMe 2C 6H 4C 6H 4PCy 2)Cl 2 ( 1), however, do not undergo racemization readily. A second source of chirality, such as a chiral diamine, can be included by conversion of 1 into a dicationic analogue [Ru(η 6:η 1-NMe 2C 6H 4C 6H 4PCy 2)((1 S,2 S)-DPEN)](SbF 6) 2 ( 2), which is a catalyst precursor for the hydrogenation of aryl ketones. Two epimers of 2, R Ar, S, S and S Ar, S, S, are formed when starting from racemic 1; this 1:1 mixture of diastereomers catalyzed the asymmetric hydrogenation of acetophenone. The enantiomerically pure diastereomers were obtained from resolved 1 and used separately to catalyze the reaction. Each diastereomer showed different selectivity, with S Ar, S, S- 2 being the more selective (61% ee for the hydrogenation of acetophenone). Our studies suggest that ruthenium hydride formation is accompanied by a decrease in hapticity of the η 6-arene and probable detachment of the ring from the metal. Nevertheless, the original conformational chirality of the biphenyl ligand appears to be at least partially retained during the catalysis.