Chiral memory effects in catalytic hydrogenations with dynamically chiral ligands

The low barrier for interconversion of chiral conformations of the dynamically chiral 2,2′-biphenyl ligand NMe2C6H4C6H4PCy2 is raised upon coordination. The individual enantiomers of the planar chiral arene-tethered complex Ru(η6:η1- NMe2C6H4C6H4PCy2)Cl2 (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(η6:η1-NMe2C6H4C6H4PCy2)((1S,2S)-DPEN)](SbF6)2 (2), which is a catalyst precursor for the hydrogenation of aryl ketones. Two epimers of 2, RAr,S,S and SAr,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 SAr,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 η6-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.

Graphical abstractThe interconversion rate for conformations of dynamically chiral 2,2′-NMe2C6H4C6H4PCy2 decreases upon coordination. Individual diastereomers of planar chiral arene-tethered complex [Ru(η6:η1- NMe2C6H4C6H4PCy2)((1S,2S)-DPEN)](SbF6)2 (2) do not epimerize readily. SAr,S,S-2 is more enantioselective than RAr,S,S-2 in the catalytic hydrogenation of acetophenone. Therefore, the ligand retains a memory of its configuration even though the arene is no longer η6.Figure optionsDownload full-size imageDownload as PowerPoint slide