Details

Autor(en) / Beteiligte

• Hadzovic, Alen

Titel

Outer sphere hydrogenation of ketones catalyzed by ruthenium(II) hydride complexes

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2007

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• The results of mechanistic and kinetic studies on Noyori-type catalytic systems for the hydrogenation of ketones are presented. The systems consist of a chlorohydrido precatalyst RuHCl(P2)(H2N∩N) (P2 = binap, bis(phosphaadamantyl)propane, and R,R-dppdpen or bis-triphenylphosphine; H2N∩N = R,R-dach, tmen, ampy or appH), a strong base and hydrogen gas. The active catalysts are proposed to be the dihydrides trans-Ru(H) 2(P2)(H2N∩N) and the hydrido-amido complexes RuH(P2)(HN∩N). The amido complexes are 5 coordinate species containing a formally Ru=N double bond as confirmed by X-ray structural analysis. Both groups of complexes are the active catalysts for ketone hydrogenation under mild conditions without a base present. In the cases when the diamines do not contain hydrogen atoms on the carbon α to the nitrogen donor, the amido complexes are stable enough to be isolated. Otherwise they are prone to β-hydride elimination giving complex mixtures of products. The trans dihydride geometry is stabilized using rigid chelating diphosphines (binap) and diamines (tmen) that prevent the isomerization to cis-dihydride species. These cis dihydrides are obtained in the systems containing two triphenylphosphine and the chelating nitrogen donors (R,R-dach, ampy). The cis dihydrides are the catalyst precursors only if the two phosphine ligands are also mutually cis. The complexes of cis-dihydride trans phosphine geometry are catalytically inactive. The replacement of an NH2 group on a diamine with a pyridyl group leads to more active (pre)catalysts for the ketone hydrogenation. The substitution of both NH2 groups with pyridyl groups renders the complexes inactive. The kinetic studies revealed that the diamine complexes follow the rate law rate=k[H2][Ru] where k is the rate constant, [H2] and [Ru] are dihydrogen and catalyst concentrations respectively. The slow step of the catalytic cycle is the heterolytic splitting of H 2 molecule over the Ru=N bond of the amido complex. The transfer of a hydride from Ru and a proton from an NH2 group to the C=O group on a substrate is a facile step. For the complexes containing H2N∩py ligands (one NH2 group in diamine replaced with py group), the kinetic study revealed a more complex mechanism in which the product alcohol is involved in an autocatalytic manner.