pH-Dependent Transfer Hydrogenation of Ketones with HCOONa as a Hydrogen Donor Promoted by (η6-C6Me6)Ru Complexes
摘要:
The paper reports on the development of a new class of water-soluble organometallic catalysts for pH-dependent transfer hydrogenation. An organometallic aqua complex [(eta(6)-C6Me6)Ru-II(bpy)(H2O)](2+) (1, bpy = 2,2'-bipyridine) acts as a catalyst precursor for pH-dependent transfer hydrogenation of water-soluble and -insoluble ketones with HCOONa as a hydrogen donor in water and in biphasic media. Irrespective of the solubility of the ketones toward water, the rate of the transfer hydrogenation shows a sharp maximum around pH 4.0 (in the case of biphasic media, the pH value of the aqueous phase is adopted). In the absence of the reducible ketones, as a function of pH, complex 1 reacts with HCOONa to provide a formato complex [(eta(6)-C6Me6)Ru-II(bpy)(HCOO)](+) (2) as an intermediate of beta-hydrogen elimination and a hydrido complex [(eta(6)-C6Me6)Ru-II(bpy)H](+) (3) as the catalyst for the transfer hydrogenation. The structures of 1(PF6)(2), 2(HCOO).HCOOH, and [(eta(6)-C6Me6)Ru-II(H2O)(3)]SO4.3H(2)O {4(SO4).3H(2)O}, the starting material for the synthesis of 1, were unequivocally determined by X-ray analysis.