J. Rajaram

Ruthenium(III)-catalysed oxidation of secondary alcohols by N-methylmorpholine N-oxide (NMO)

Abstract A catalytic amount of RuCl3 in the presence of excess of N-methyl-morpholine N-oxide (NMO) in DMF oxidizes secondary alcohols to ketones. Spectral studies reveal the formation of a Ru(V)-oxo species which is formed in situ on adding N-oxide. The formation of Ru(V) has been established by cyclic voltammetric studies. The mechanism involves the formation of Ru(V)-oxo species in steady state concentrations from Ru(III) and NMO, and this in turn reacts with the substrate in the rate-determining step.

Hydrogenation and transfer hydrogenation of d-fructose catalyzed by dichlorotris (triphenylphosphine) ruthenium (II)

Abstract d -Fructose is hydrogenated to d -glucitol and d -mannitol using RuCl 2 (PPh 3 ) 3 as catalyst at 100°C and atmospheric pressure. Besides hydrogenation, fructose undergoes transfer hydrogenation when propan-2-ol and butan-2-ol are used as solvents. Under an inert atmosphere (nitrogen), only transfer hydrogenation of fructose is observed in these alcohols. The rate of hydrogenation is comparable with transfer hydrogenation under similar reaction conditions. Cyclohexanol, benzyl alcohol, 1-phenylethanol and benzhydrol are also found to be good hydrogen donors for fructose reduction. Both hydrogenation and transfer hydrogenation yield glucitol and mannitol whose ratio is always 1:1. The catalyst is deactivated when hydrogen donors such as 2-methoxyethanol and tetrahydrofurfuryl alcohol are employed. The deactivation is attributed to the formation of an inactive ruthenium carbonyl complex, viz ., RuHCl (CO) (PPh 3 ) 3 . The hydrogen donating ability of these alcohols and their oxidation potentials are compared and the relative degrees of correlation are rationalized.

RuCl2(PPh3)3-catalyzed transfer hydrogenation of d-glucose

Glucose is transfer hydrogenated by propan-2-ol, butan-2-ol, cyclohexanol, benzyl alcohol, 1-phenylethanol, benzhydrol, 2-methoxyethanol and tetrahydrofurfuryl alcohol in the presence of RuCl2(PPh3)3 at 100 °C and atmospheric pressure. Mixed solvent systems such as dimethylacetamide-water and dioxane-water are utilized for this reaction. The major product from glucose is sorbitol, although glucono-1,5-lactone is invariably formed as a side product from a disproportionation reaction. When 2-methoxyethanol and tetrahydrofurfuryl alcohol are used as hydrogen donors, the catalyst undergoes permanent change to a hydridocarbonyl complex, which catalyzes only disproportionation of glucose. Glucose also acts as a good hydrogen donor when hydrogen acceptors such as cyclohexanone are introduced into the reaction system.

Mechanistic investigation of homogeneous hydrogen transfer from 1-phenylethanol to cyclohexanone catalyzed by dichlorotris(triphenylphosphine)- ruthenium(II)

Abstract In the homogeneous hydrogen transfer from 1-phenyl-ethanol(D) to cyclohexanone(A) catalyzed by RuCl 2 (PPh 3 ) 3 (C) in diphenyl ether as solvent at 140 °C the order of addition of donor, acceptor and catalyst have a pronounced effect on the rate of the reaction. An induction period is observed. The experimental observations correspond to the rate expression: Initial rate = where [C] t is the total concentration of catalyst in the system.

Dichlorotris(triphenylphosphine)ruthenium(II) catalyzed dehydrogenation of some natural products using cyclohexanone as acceptor

Abstract Cyclohexanone has been used as an acceptor for the dehydrogenation of some natural products in the presence of RuCl2(PPh3)3. Only menthol undergoes appreciable dehydrogenation when compared to cholesterol and β-citronellol. Menthone, cholest-4-en-3-one and cholest-1,4-diene-3-one and citronellal are the products from these compounds. When carbohydrates are used as donors, in most cases the corresponding lactones are formed. The ratio of acceptor to donor is maintained at 6 in the latter cases to avoid disproportionation of the carbohydrates. Acids, isomer and epimer are formed in certain cases. It is observed that the dehydrogenation of carbohydrates is increased at elevated temperatures. The susceptibility to dehydrogenation of the carbohydrates is in the order sorbitol > mannitol > L-arabinose ≈ D-xylose > D-glucose ≈ sucrose > D-mannose ≈ D-galactose.

The mechanism of disproportionation of D-glucose catalysed by hydridochlorocarbonyltris(triphenyl-phosphine)ruthenium(II) in tetrahydrofurfuryl alcohol

Abstract The disproportionation of D-glucose in tetrahydrofurfuryl alcohol (THFA) catalysed by RuHCl(CO)(PPh3)3is suggested to proceed by hydride transfer from a dissociated catalyst species to the coordinated aldehyde form of glucose. Subsequent steps involve the coordination of the pyranose form of glucose, formation of the metal alkoxide, release of D-glucitol and hydrogen transfer from alkoxide to the metal. The kinetic data are compatible with the rate expression, rate = (kK[G][Ru]0)/([P] + K), where k, K, [G], [Ru]0 and [P] represent the rate constant of the rate-determining step, the equilibrium constant for the dissociation of the catalyst, concentrations of glucose, catalyst and added triphenylphosphine respectively.

Deactivation of RuCl2(PPh3)3 during disproportionation of D-glucose in amide solvents

Abstract The catalyst RuCl2(PPh3)3 is transformed into a catalytically inactive complex RuCl2(CO)(PPh3)2(DMF) during disproportionation of D-glucose in dimethylformamide (DMF). In dimethylacetamide (DMA), the catalyst is converted to RuCl2(CO)(PPh3)2(DMA) and cis-RuCl2(CO)2(PPh3)2. The aldehydo-glucose is shown to be the main source of carbon monoxide.

Spectral evidence for the formation of active intermediates from RuCl3 and RuCl2(PPh3)3 with N-methylmorpholine N-oxide (NMO) and phenyliodosoacetate (PIA) as mild oxidants

Abstract Electronic, EPR and IR spectral evidence are given for the formation of the following active species: (1) ruthenium(VIII) in ruthenium(III)-PIA system, (2) ruthenium(V) oxo species in ruthenium(III)- N - oxide system, and (3) ruthenium(II)-phosphine oxide complex in ruthenium(II) N -oxide system. Cyclic voltammetric studies also suggest the formation of Ru(V) in ruthenium(III)- N -oxide system.

Kinetics and mechanism of RuCl2PPh3)3-catalyzed oxidation of sulfides by N-methylmorpholine N-oxide

Abstract Oxidation of dibutylsulfide, diphenylsulfide, methylphenylsulfide and dibenzylsulfide to the corresponding sulfoxides by N-methylmorpholine N-oxide (NMO) catalyzed by RuCl2(PPh3)3 in DMF solvent is reported. The reaction is first order in both catalyst and N-oxide. The order with respect to the substrate is variable, being zero at higher concentrations and fractional at lower concentrations. The order of reactivity observed for the substrates is as follows: dibutylsulfide ≈ dibenzylsulfide > methylphenylsulfide > diphenylsulfide. RuCl2(PPh3)3 oxidizes olefins to form epoxides at a slower rate. The order of reactivity observed for the two types of substrates parallels their nucleophilicity (sulfides > alkenes). Spectral studies indicate 1:1 complex formation between RuCl2(PPh3)3 and the sulfides. The active oxidant is the Ru(IV)oxo complex formed by the oxidation of Ru(II) by NMO.

Reaction of the phosphate radical with amines — A flash photolysis study

Rate constants for the reaction of phosphate radical with some aromatic and aliphatic amines have been determined by the flash photolysis technique. The products formed under conditions of continuous irradiation have been identified. In the case of an aromatic amine the major product is the azo compound while in the case of an aliphatic amine a carbonyl compound is formed.