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, RuIIICl3(bpea), 1, and cis-fac-Δ-[RuIICl{(R)-(bpea)}{(S)-(BINAP)}](BF4), cis-fac-Δ-(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-Δ-(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-Δ-(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-Δ-(R)-(S)-2, with conversions higher than 99.9% and ee > 99. Furthermore, cis-fac-Δ-(R)−(S)-2, also catalyzes the selective hydrogenation of β-keto esters, although the reaction rates are lower than those found with the former substrates.