Christian A. Sandoval

Metal–ligand bifunctional catalysis for asymmetric hydrogenation

Chiral diphosphine/1,2-diamine–Ru(II) complexes catalyse the rapid, productive and enantioselective hydrogenation of simple ketones. The carbonyl-selective hydrogenation takes place via a non-classical metal–ligand bifunctional mechanism. The reduction of the C=O function occurs in the outer coordination sphere of an 18e trans-RuH2(diphosphine)(diamine) complex without interaction between the unsaturated moiety and the metallic centre. The Ru atom donates a hydride and the NH2 ligand delivers a proton through a pericyclic six-membered transition state, directly giving an alcoholic product without metal alkoxide formation. The enantiofaces of prochiral ketones are differentiated on the chiral molecular surface of the saturated RuH2 species. This asymmetric catalysis manifests the significance of ‘kinetic’ supramolecular chemistry.

Hybrid NH2-Benzimidazole Ligands for Efficient Ru-Catalyzed Asymmetric Hydrogenation of Aryl Ketones

Readily available hybrid NH(2)/benzimidazole ligands (R-bimaH, 1) dramatically influence the outcome of established Ru-based catalysts during asymmetric hydrogenation of aryl ketones. The benzimidazole functionality results in reversal of the typically observed chiral induction and allows for hydrogenation to be uncharacteristically conducted in nonprotic solvents. The developed systems efficiently catalyzed the AH of a number of ketones in up to 99% ee.

Chiral eta(6)-Arene/N-Tosylethylenediamine-Ruthenium(II) Complexes: Solution Behavior and Catalytic Activity for Asymmetric Hydrogenation

Aromatic ketones are enantioseletively hydrogenated in alcohols containing [RuX{(S,S)-Tsdpen}(eta(6)-p-cymene)] (Tsdpen=TsNCH(C(6)H(5))CH(C(6)H(5))NH(2); X=TfO, Cl) as precatalysts. The corresponding Ru hydride (X=H) acts as a reducing species. The solution structures and complete spectral assignment of these complexes have been determined using 2D NMR ((1)H-(1)H DQF-COSY, (1)H-(13)C HMQC, (1)H-(15)N HSQC, and (1)H-(19)F HOESY). Depending on the nature of the solvents and conditions, the precatalysts exist as a covalently bound complex, tight ion pair of [Ru(+)(Tsdpen)(cymene)] and X(-), solvent-separated ion pair, or discrete free ions. Solvent effects on the NH(2) chemical shifts of the Ru complexes and the hydrodynamic radius and volume of the Ru(+) and TfO(-) ions elucidate the process of precatalyst activation for hydrogenation. Most notably, the Ru triflate possessing a high ionizability, substantiated by cyclic voltammetry, exists in alcoholic solvents largely as a solvent-separated ion pair and/or free ions. Accordingly, its diffusion-derived data in CD(3)OD reflect the independent motion of [Ru(+)(Tsdpen)(cymene)] and TfO(-). In CDCl(3), the complex largely retains the covalent structure showing similar diffusion data for the cation and anion. The Ru triflate and chloride show similar but distinct solution behavior in various solvents. Conductivity measurements and catalytic behavior demonstrate that both complexes ionize in CH(3)OH to generate a common [Ru(+)(Tsdpen)(cymene)] and X(-), although the extent is significantly greater for X=TfO(-). The activation of [RuX(Tsdpen)(cymene)] during catalytic hydrogenation in alcoholic solvent occurs by simple ionization to generate [Ru(+)(Tsdpen)(cymene)]. The catalytic activity is thus significantly influenced by the reaction conditions.

Why p-Cymene? Conformational Effect in Asymmetric Hydrogenation of Aromatic Ketones with a η6-Arene/Ruthenium(II) Catalyst

The global reaction route mapping (GRRM) methods conveniently define transition states in asymmetric hydrogenation and transfer hydrogenation of aromatic ketones via the [RuH{(S,S)-TsNCH(C6 H5 )CH(C6 H5 )NH2 }(η(6) -p-cymene)] intermediate. Multiple electrostatic CH/π interactions are the common motif in the preferred diastereometric structures.

A Pd(OAc)2/[mmim]I Catalyst System for Oxidative Carbonylation of Amines to Carbamates, Ureas, and 2-Oxazolidinones

A Pd(OAc)2/[mmim]I ([mmim]I=1-methyl-3-methylimidazolium iodide) catalyst system was applied to the oxidative carbonylation of aliphatic amines, aromatic amines, and amino alcohols to carbamates, ureas, and 2-oxazolidinones under different conditions. The catalytic turnover frequencies (TOF, moles of amines converted per mole of catalyst per hour) were 12417, 17368, and 4114 h−1 for the production of methyl N-phenyl carbamate, N,N′-diphenyl urea, and 2-benzoxazolinone, respectively.