Catalytic ability of a cationic Ru(II) monochloro complex for the asymmetric hydrogenation of dimethyl itaconate and enamides

The synthesis of two Ru chloro complexes, Ru^IIICl₃(bpea), 1, and cis-fac–[Ru^IICl{(R)-(bpea)}{(S)-(BINAP)}](BF₄), cis-fac–(R)-(S)-2, (bpea = N,N-bis(2-pyridylmethyl)ethylamine; (S)-BINAP = 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl), is described. Complex 2 is characterized in solution through UV-vis, cyclic voltammetry (CV), and 1D and 2D NMR spectroscopy. X-ray diffraction analysis indicates that in the solid state it possesses the same structure as in solution, as expected for a low-spin d₆ Ru(II)-type complex. The molecular structure of cis-fac–(R)-(S)-2, consists of a nonsymmetric complex, where the Ru metal center has a significantly distorted octahedral-type coordination because of the bulkiness of the (S)-BINAP ligand. cis-fac–(R)-(S)-2 has a remarkable catalytic performance at P = 6.8 atm of H₂ and T = 70 C toward the hydrogenation of prochiral double bonds both from efficiency and from stereoselectivity viewpoints. As an example, prochiral olefins of technological interest such as dimethyl itaconate, methyl 2-acetamidoacrylate or methyl 2-acetamidocinnamate are catalytically hydrogenated by cis-fac–(R)-(S)-2, with conversions higher than 99.9% and ee > 99. Furthermore, cis-fac–(R)-(S)-2, also catalyzes the selective hydrogenation of -keto esters, although the reaction rates are lower than those found with the former substrates.