Ruthenium amino carboxylate complexes as asymmetric hydrogen transfer catalysts

Abstract

The synthesis and characterization of optically active amino carboxylate complexes of formula [(η⁶-arene)Ru(Aa)Cl] (arene = C₆H₆, C₆Me₆, Aa = amino carboxylate) as well as those of the related trimers [{(η⁶-arene)Ru(Aa)}₃][BF₄]₃ are reported. Trimerization takes place with chiral self-recognition: only diastereomers equally configured at the metal, R_RuR_RuR_Ru or S_RuS_RuS_Ru, are detected. The crystal structures of the complexes [(η⁶-C₆H₆)Ru(Pip)Cl] and [{(η⁶-C₆Me₆)Ru(Pro)}₃][BF₄]₃ have been determined by X-ray diffraction methods. Both types of complexes catalyse the hydrogen transfer reaction from 2-propanol to ketones with moderate enantioselectivity (up to 68% ee). The enantiodifferentiation achieved can be accounted for by assuming that Noyori's bifunctional mechanism is operating.