A.Prakash Rao

Oxygen atom transfer in the epoxidation of cyclohexene catalysed by Ru(III)—EDTA

Abstract The oxidation of cyclohexene to the epoxide is catalysed by Ru(III)—EDTA in a 50% ethanol/water solution at 30 °C in the pH range 1 – 3 (μ = 0.1 M KCl). The observed reaction rates for the oxidation of cyclohexene are first order with respect to Ru(III)–EDTA and cyclohexene, and one-half order with respect to molecular oxygen. A mechanism has been proposed for the oxidation of cyclohexene by molecular oxygen catalysed by Ru(III)–EDTA. The turnover number for the epoxidation of cyclohexene by molecular oxygen catalysed by Ru(III)—EDTA is 200 mol (mol catalyst) −1 min −1 .

Kinetics and mechanism of ruthenium(III)-catalysed oxidation of allyl alcohol by molecular oxygen

Abstract Oxidation of allyl alcohol by molecular oxygen catalysed by ruthenium(III) chloride in the pH range 1.0 – 3.0 is reported. The kinetics of the reaction, in the pH range 1.0 – 2.0, indicate a first-order dependence on ruthenium(III) ion and allyl alcohol concentrations and a zero-order dependence with respect to molecular oxygen concentration. The rate of oxidation was also found to be directly proportional to the hydrogen ion concentration. In the pH range 2.0–3.0, the rate of oxidation of allyl alcohol was found to be first order with respect to ruthenium(III) ion, first order with respect to allyl alcohol, and inversely proportional to hydrogen ion concentration. The rate is one-half order with respect to the concentration of molecular oxygen. Mechanisms for the oxidation of allyl alcohol catalysed by ruthenium(III) have been proposed in the pH ranges 1.0–2.0 and 2.0–3.0, wherein the reactions were shown to proceed through an oxidase route and an atom transfer (oxygenase) route, respectively. Separation of the catalyst from the reaction products was achieved using an ultrafiltration membrane technique, where the membrane was used as a separating agent. The oxidation products were identified by means of electrochemical techniques.

Synthesis, oxygenation and catalytic properties of ruthenium(III) saloph complexes

Abstract Synthesis and dioxygen affinities of ruthenium(III)-Schiff base complexes with various appended axial base ligands, [Ru(Saloph)(B)Cl] (where Saloph = bis(salicylaldehyde)-o-phenylenediiminato, B = Cl− 1, imidazole 2, 2-methylimidazole 3 and pyridine 4) are reported. The stoichiometry of dioxygen complexes formed was determined by oxygen uptake measurements. Oxygenation studies of complexes 1–4 reveal a 1:1 dioxygen complex formation. Complexes 1–4 were also studied for their catalytic activity by employing cyclohexene as a substrate for oxidation at 35°C using water-dioxane solvent medium. Kinetics of the oxidation reaction catalysed by complexes 1–4 indicate a first-order dependence with respect to catalyst, substrate and dioxygen concentrations, respectively. The product of oxidation was analysed and identified by GLC as the epoxide. A mechanism is proposed for the conversion of cyclohexene to the epoxide, and a subsequent rate law derived with the help of which were determined the values of the rate and equilibrium constants.

A novel wacker route based on ruthenium(III) ion for olefin oxidation to ketones

Abstract The oxidation of 1-hexene to the end ketone 2-hexanone by molecular oxygen is catalysed by Ru(III) ion, [RuCl 2 (H 2 O) 4 ] + at 35°C (μ = 0.1 M) in 1:1 (v/v) water-dioxane solution at pH 2.00. The reaction shows a firstorder dependence in catalyst and substrate concentrations and a one-halforder dependence with respect to molecular oxygen concentration. The product of oxidation, 2-hexanone, as identified and analysed by GLC, is formed in 54% yield. A mechanism involving a combination of a peroxometallocycle and the Wacker oxidation route to ketone formation is proposed. Finally, on the basis of kinetics and the mechanism proposed, a rate law for the oxidation reaction was derived and the corresponding rate constants evaluated. The various equilibrium constants involved in the mechanism have been directly obtained by spectrophotometric measurements. The system represents the first simple monometallic and noncorrosive pathway for Wacker oxidation of olefins to ketones, and proceeds with a turnover frequency of 62 mol ketone per mol catalyst per hour at 35°C.

Dioxygen affinities of some ruthenium(III) schiff base complexes

Abstract The synthesis and dioxygen affinities of some ruthenium(III) Schiff base complexes in DMF solution in the presence of different axial bases are reported. The ligands used are bis(salicylaldehyde)ethylenediimine (salen), bis(salicylaldehyde)diethylenetriimine (saldien), bis(picolinaldehyde)- o -phenylenediimine (picoph), bis(picolinaidehyde)ethylenediimine (picen) and bis(picolinaldehyde)diethylenetriimine (picdien). The axial ligands employed are chloride (Cl − ), imidazole (Im) and 2-methylimidazole (2-MeIm). From the oxygenation constants it is found that electron donating substituents on the Schiff bases increase the affinity for dioxygen. Equilibrium dioxygen uptake measurements at 278, 288 and 303 K provide values of Δ H ° and Δ S ° of oxygenation that fall in the range − 6.1 to −13.3 kcal mol − 1 for Δ H ° and − 10 to − 31 cal deg − 1 mol − 1 for Δ S °. The dioxygen adducts of Ru III were characterized by electrochemistry, UV–vis, IR and EPR techniques as Ru IV superoxo complexes.

Kinetic studies on ph dependent oxidase vs. oxygenase mechanism for Ru(III)-EDTA catalyzed oxidation of allyl alcohol by molecular oxygen in aqueous acidic medium

Abstract The kinetics of the oxidation of allyl alcohol by molecular oxygen catalyzed by Ru(III)-EDTA complex is investigated in the pH range 1.0–3.0 at 35 °C (μ = 0.1 M) in aqueous medium. In the pH range studied, the reaction is first order with respect to catalyst and substrate concentrations. An inverse first-order dependence on H + concentration is observed below pH 2.0, while the rate is independent of [H + ] above pH 2.0. The rate of reaction is independent of O 2 concentration at pH 1.5 and first order with respect to O 2 concentration at pH 2.5. Mechanisms for the oxidation of allyl alcohol catalyzed by Ru(III)-EDTA have been proposed in the pH ranges 1.0–2.0 and 2.0–3.0, wherein the reactions are shown to proceed through oxidase and oxygenase routes, respectively. The rate laws for these routes are derived and the corresponding rate constants reported.

Electrochemical oxidation of organic substrates mediated by the high-valent ruthenyl species

Abstract The ruthentium(V)—oxo species [RuV(O)(H2O)3Cl2]+ was generated in situ by the coulometric oxidation of ruthenium(III) aquo ion, [RuIII(H2O)4Cl2]+ 1. The electrochemical oxidation of 1 proceeds by a stepwise electron transfer at + 0.386 V (RuIII/RuIV) and + 0.870 V (RuIV/RuV) vs. SCE at pH 1.85. Coulometric oxidation of 1 to the stable oxo species 2 was performed at an electrode potential of + 0.950 V. Complex 2 was isolated and characterised by UV—Vis, IR and electrochemical studies. The oxo species 2 was then used as an electrocatalyst for the effective oxidation of olefinic substrates. The oxidation products were identified and analysed by GLC and HPLC techniques. A ‘shuttle’ mechanism was proposed for the conversion of Ru(III) 1 to Ru(V) 2 and back to 1 continuously at the electrode surface.

Kinetics of the epoxidation of cyclohexene by molecular oxygen catalyzed by dichlorotetraaquoruthenium(iii) in the presence of the reductants 1-ascorbic acid and ethanol

Epoxidation of cyclohexene by oxygen atom transfer from coordinated dioxygen is catalyzed by dichlorotetraaquoruthenium(III) [RuCl2(H2O)4]+ 1a in a 1:1 (vv) mixture of water-ethanol and water- 1,4-dioxan at 35 °C and μ = 0.1 M KCl. The rate of epoxidation of cyclohexene by molecular oxygen was also investigated using dichlorodiaquoruthenium(III)-ascorbate chelate [RuIIICl2(H2O)2(HA)] 1 as catalyst under identical conditions in 1:1 water-1,4-dioxan mixture, and was found to be more rapid than that of catalyst 1a. These reactions, which act as models for monooxygenases, proceed by the insertion of one of the oxygen atoms of the coordinated dioxygen into the substrate with the formation of the oxidized product, the other oxygen atom being released as OH−. The rate of the epoxidation reaction is first order with respect to the concentration of cyclohexene, catalyst and molecular oxygen in all the cases. Inverse first order and zero order dependences in hydrogen ion concentration were observed in the presence and absence of ascorbic acid, respectively. The dependence in the concentration of reductants [ascorbic acid] and [ethanol] is first and zero order, respectively. A comparative study of the epoxidation of cyclohexene from product analysis, kinetic and mechanistic points of view is reported and discussed in detail.

Kinetics and mechanism of the epoxidation of styrene and substituted styrenes with O2 catalysed by [RuIII(EDTA)(H2O)]-

Epoxidation of styrene, 2-methylstyrene, 3-chlorostyrene and 4-methoxystyrene with molecular oxygen catalysed by [RuIII(EDTA)(H2O)]− (1a) (EDTA = ethylenediaminete-traacetate) was studied as a function of catalyst (1a), substrate and dissolved oxygen concentration in 50% water-dioxane medium. The rate of epoxidation was found to be first order with respect to complex 1a and substrate concentrations and one-half order with respect to dissolved oxygen concentration. At high substrate concentrations a zero-order dependence of rate with respect to substrate concentration was observed in each case. Stoichiometric oxidation of styrene and substituted styrenes by [RuV = O(EDTA)]− (2) was studied, and results are compared with the catalytic epoxidation reactions with molecular oxygen.

Oxygenation reactions of saturated and unsaturated substrates by molecular O2 catalyzed by the versatile catalyst K[Ru(EDTAH)Cl]·2H2O: Rate and activation studies

Abstract Oxygenation of olefinic and saturated substrates, including triphenylphosphine, with molecular oxygen is catalyzed by (ethylene-diaminetetraacetato)aquoruthenate(III) ion, [Ru III (EDTA)(H 2 O)] − 1 , in a mixed solvent medium to yield a variety of products. The wide range of products formed includes oxides, epoxides, alcohols, ketones and aldehydes. The kinetics of these oxidation reactions have been investigated by oxygen absorption methods and product analysis at pH 3.0 (μ = 0.1 M NaClO 4 ) in the temperature range 288–318 K in water—dioxane medium (1:1). In order to eliminate any radical chemical pathway with dioxane:H 2 O, the oxygenation of a few selected substrates, cyclohexane and 1-hexane was conducted in water—DMF (1:1) medium. General mechanisms are proposed for both saturated and unsaturated substrates on the basis of the kinetic data and are discussed in terms of reactivity. In all cases, a μ-peroxoruthenium(IV)—substrate complex was suggested as the active intermediate. The activation parameters corresponding to rate constants at different temperatures were computed. The activation enthalpies are much more favourable for the oxidation of PPh 3 and olefins, as compared to saturated substrates.