Hydrogenation mechanisms homogeneous catalysis

Hydrogenation by homogeneous catalysis has been widely studied over the years both with respect to mechanism and application. Ruthenium, iridium, and rhodium are the most frequently used metals. The best known is the rhodium catalyst RhClfPPh ), that was first discovered by Wilkinson and bears his name (see Jardine et al., 1967 Osborn et al., 1966 James, 1973). 

The mechanism and rate of hydrogen peroxide decomposition depend on many factors, including temperature, pH, presence or absence of a catalyst (7—10), such as metal ions, oxides, and hydroxides etc. Some common metal ions that actively support homogeneous catalysis of the decomposition include ferrous, ferric, cuprous, cupric, chromate, dichromate, molybdate, tungstate, and vanadate. For combinations, such as iron and... 

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

With the recent development of zeolite catalysts that can efficiently transform methanol into synfuels, homogeneous catalysis of reaction (2) has suddenly grown in importance. Unfortunately, aside from the reports of Bradley (6), Bathke and Feder (]), and the work of Pruett (8) at Union Carbide (largely unpublished), very little is known about the homogeneous catalytic hydrogenation of CO to methanol. Two possible mechanisms for methanol formation are suggested by literature discussions of Fischer-Tropsch catalysis (9-10). These are shown in Schemes 1 and 2. 

In the reactions above we have not explicitly touched upon the reactions of dihydrogen and transition metal complexes. Here the reactions that involve the activation of dihydrogen will be summarised, because they are very common in homogeneous catalysis and because a comparison of the various mechanisms involved may be useful. Three reactions are usually distinguished for hydrogen ... 

Keywords Homogeneous Catalysis Quantum Mechanics/Molecular Mechanics Polymerization Hydrogenation Dihydroxylation... 

Cobalt B12 Enzymes Coenzymes Copper Hemocyanin/Tyrosinase Models Heterogeneous Catalysis by Metals Hydride Complexes of the Transition Metals Hydrocyanation by Homogeneous Catalysis Hydrogen Inorganic Chemistry Mechanisms of Reaction of Organometalhc Complexes Nickel OrganometaUic Chemistry Ohgomerization Polymerization by... 

Asymmetric Synthesis by Homogeneous Catalysis Coordination Organometallic Chemistry Principles Hydrobo-ration Catalysis Hydrogenation Isomerization of Alkenes Mechanisms of Reaction of Organometallic Complexes Silicon Organosilicon Chemistry. 

Unlike the hydrogenation catalysts, most iridium catalysts studied for hydroformylation chemistry are not particularly active and are usually much less active than their rhodium counterparts see Carbonylation Processes by Homogeneous Catalysis). However, this lower activity was useful in utihzing iridium complexes to study separate steps in the hydroformylation mechanism. Using iridium complexes, several steps important in the hydroformylation cycle such as alkyl migration to carbon monoxide were studied. Another carbonylation reaction in which iridum catalysis appears to be conunercially viable is in the carbonylation of methanol. ... 

Ruthenium (II) Chloride. The homogeneous catalysis of the hydrogenation of fumaric acid in aqueous solution by ruthenium (II) chloride has been interpreted (36) in terms of the following mechanism in which the heterolytic splitting of H2 by a Ru -olefin complex is the rate-determining step. 

Tris(triphenylphosphine)chlororhodium(I). The homogenous catalysis of the hydrogenation of ethylene and other olefins by RhCl( PPha) a, recently discovered by Wilkinson and co-workers (6J), probably involves a dihydride intermediate. A plausible mechanism for this reaction, involving steps of the type already described, is shown in Reaction 36. 

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