Solution structure and behavior of benzophenone-based achiral bisphosphine ligands in noyori-type Ru(II)-catalysts

We herein report on solution structural studies of Ru-II catalysts (3a, 9) composed of achiral bisphosphine ligands (4, 8) and the enantiopure 1,2-diphenylethylenediamine (DPEN). Complete chiral induction from enantiopure (R,R)-DPEN to achiral bisphosphine ligand 3a was observed in solution, with the complex adopting a single, stable and non-fluxional (even at 70 degrees C) configuration. The coordination of the C=O moiety in 4 to the cationic Ru-II center is considered to be of key importance in providing the higher thermodynamic and kinetic rotation barrier for the flexible bisphosphine ligand in the complex. The obtained enantioselectivity (91% enantiomeric excess) and sense of chiral induction in the hydrogenation of acetophenone were found to be solely dependent on the chirality of the 1,2-diamine. Consistent with the hydrogenation product, the (R,R)-DPEN induces a M-conformation (right-handed) chirality for flexible phosphine ligand 4 in the complex, resulting in a lambda,lambda-configuration about the Run center.