S. Vancheesan

Mechanism for transfer hydrogenation of ketones to alcohols catalysed by hydridotri-ironundecacarbonylate anion under phase transfer conditions

Abstract A mechanism for the transfer hydrogenation of ketones to secondary alcohols catalysed by HFe3(CO)11−1 (1) in the presence of Phase Transfer Catalysts (PTC) has been proposed using labelled hydrogen, NMR and IR techniques.

Phase transfer-catalysed transfer hydrogenation of ketones using iron carbonyls as catalysts

Transfer hydrogenation of ketones to alcohols using iron carbonyls as catalysts in liquid-liquid phase occurs in the presence of phase transfer catalysts (PTC). Based on the percentage yield of the products, the efficiencies of the phase transfer agent, iron carbonyls and the donors are compared. Benzyltriethylammonium chloride and 18-crown-6 are equally effective and better than tricaprylmethylammonium chloride (Aliquat® 336). Among the three iron carbonyls the order of efficiency is Fe3(CO)12 > Fe2(CO)9 > Fe(CO)5. 1-Phenylethanol is a better donor than isopropanol. The relative ease of reducibility of various ketones increases with increasing reduction potential. The stereochemistry of reduction of 4-t-butylcyclohexanone is discussed.

Reactions of the carbonate radical with aliphatic amines

Abstract Carbonate radicals react with aliphatic amines by a dual mechanism, viz. (i) hydrogen abstraction and (ii) electron transfer. The former is more probable with primary amines. Tertiary amines react via electron transfer. Both mechanisms may operate in secondary amines. Cyclic tertiary amines react with different rates and their relative reactivities are explained on the basis of the concept of Hoffmanns ‘through bond’ interaction.

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.

Hydrogenation of conjugated dienes catalysed by η6-arenetricarbonylchromium complexes

Abstract (η6-Arene)Cr(CO)3 (Arene = substituted benzene derivatives) complexes catalyse the hydrogenation of conjugated dienes at atmospheric pressures of hydrogen. Hydrogenation of methyl sorbate and myrcene were studied at atmospheric pressures of hydrogen and at 393 K using (η6-Arene)Cr(CO)3 complexes with different substituents on the benzene ring. The catalytic activity of different (η6-Arene)Cr(CO)3 complexes are in the following order: tetralin

Reduction of nitrobenzene by dodecacarbonyl tri-iron under triphase conditions

Abstract Reduction of nitrobenzene to aniline by Fe3(CO)12 under triphase conditions has been investigated and the efficiency of the polymer-supported phase transfer agents compared. Reduction occurs under mild conditions and the isolation of the product is easier than in the homogeneous system. The yield of aniline is similar to that obtained in the Fe3(CO)12/Al2O3 system.

Reaction of the carbonate radical with substituted anilines

Rate constants for the reaction of carbonate radical with aniline and some parasubstituted anilines have been determined by the flash photolysis technique. Using σ+ para values the rate constants at pH 8.5 correlate very well with the Hammett equation yielding ρ= − 1. The carbonate radical oxidises aniline giving the anilino radical. The products so formed have been identified through studies under conditions of continuous irradiation.