Efficient transfer hydrogenation of ketones catalyzed by the bis(isocyanide)-ruthenium(II) complexes trans, cis,cis-[RuX2(CNR)(2)(dppf)] (X = Cl, Br; dppf = 1,1 '-bis(diphenylphosphino)ferrocene): Isolation of active mono- and dihydride intermediates

摘要

The bis(isocyanide)-ruthenium(II) complexes trans, cis, cis- [RuX2(CNR)(2)(dppf)] (X = Cl, R CH2Ph (2a), Cy (2b), Bu-t (2c), 2,6-C6H3Me2 (2d), (S)-(-)-C(H)MePh (2e); X = Br, R = CH2Ph (3a), Cy (3b), Bu-t (3c), 2,6-C6H3Me2 (3d), (S)-(-)-C(H)MePh (3e)) have been prepared by reaction of the bis(allyl)-ruthenium(II) derivative [Ru(eta(3)-2-C3H4Me)(2)(dPPf)] (1) with the appropriate isocyanide ligand, in CH2Cl2 at room temperature and in the presence of the corresponding hydrogen halide HX. The structure of the compound trans, cis, cis- [RuCl2(CN-2,6-C6H3Me2)(2)(dppf)] (2d) has been confirmed by X-ray crystallography. The catalytic activity of complexes 2a-e and 3a-e in the transfer hydrogenation of acetophenone by propan-2-ol has been studied, and the most active complex, trans, cis, cis- [RuCl2(CNCH2Ph)2(dppf)] (2a), has been also tested as a catalyst in the transfer hydrogenation of a large variety of ketones. The hydride derivatives cis, cis- [RuHCl(CN-2,6-C6H3Me2)(2)(dppf)] (4) and cis,cis,cis- [RuH2(CN-2,6-C6H3Me2)(2)(dppf)] (5) have been isolated and characterized. Although both hydride complexes catalyze the transfer hydrogenation of acetophenone in the absence of base, the reactions proceed ca. 5 times faster with 5 than with 4, pointing out that the real active species are dihydride -ruthenium complexes.