DOI•
Kinetic studies on the homogeneous hydrogenation of fumaric and maleic acid catalysed by bis(dimethylglyoximato)cobalt(II)
01 Mar 1984-Vol. 93, Iss: 2, pp 73-81
TL;DR: In this paper, the influence of varying concentrations of Co(dmgh)2, NaOH and axial base on the rate of hydrogenation of fumaric and maleic acids has been studied in detail.
Abstract: The influence of varying concentrations of Co(dmgh)2, NaOH and axial base on the rate of hydrogenation of fumaric and maleic acids has been studied in detail. Intramolecular hydrogen bonding in the monoanion of maleic acid and thetrans orientation of carboxylic acid groups in fumaric acid are important factors which account for the difference in the rate of hydrogenation of these substrates. Mono-, di-and trialkyl amines as axial bases modify the activity of the catalyst, dialkylamines conferring the maximum activity and trialkylamines the least. Back-strain on nitrogen atom and solvation energy of the amines are responsible for their different behaviours. A rate law has been proposed and verified
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TL;DR: In this paper, the tris(triphenylphosphine) complexes are exceedingly active catalysts for the rapid and homogeneous hydrogenation, at ca. 1 atmosphere of hydrogen pressure and room temperature, of unsaturated compounds containing isolated olefinic and acetylenic linkages.
Abstract: Tris(triphenylphosphine)chlororhodium(I), RhCl(PPh3)3, has been prepared by the interaction of an excess of triphenylphosphine with rhodium(III) chloride hydrate in ethanol; the corresponding bromide and iodide are also described. The dissociation of the complex in various solvents has been investigated, and its reactions with hydrogen, ethylene, and carbon monoxide and aldehydes studied. Dihydrido- and ethylene complexes have been isolated and studied by nuclear magnetic resonance (n.m.r.) spectroscopy. Approximate values for the formation constants of ethylene and propylene complexes have been obtained; the latter is lower by a factor of over 103. By electron spin resonance spectroscopy, the complex RhCl(PPh3)3 has been shown to contain trace amounts of a paramagnetic species, probably a rhodium(II) complex.In homogeneous solution the tris(triphenylphosphine) complexes are exceedingly active catalysts for the rapid and homogeneous hydrogenation, at ca. 1 atmosphere of hydrogen pressure and room temperature, of unsaturated compounds containing isolated olefinic and acetylenic linkages.The rates of hydrogenation of hept-1-ene, cyclohexene and hex-1-yne have been studied quantitatively and the dependence on factors such as substrate and catalyst concentration, temperature, and pressure determined. The data can be accommodated by a rate expression of the form: Rate =Kp[S][A]//1 +K1p+K2[S] where [S] and [A] are the olefin and catalyst concentrations, respectively, and p is the concentration of hydrogen in solution.From the data for cyclohexene the activation energy for the rate determining step is Ea= 22·9 kcal. mole–1(ΔH‡= 22·3 kcal. mole–1) and the value of ΔS‡= 12·9 e.u.It is shown that the rate of hydrogen–deuterium exchange under selected conditions is quite slow compared with the rates of hydrogenation of olefins and, furthermore, that when H2–D2 mixtures are used in the reactions, alkanes and dideuteroalkanes are the major products. Reductions of maleic and fumaric acids with deuterium shows that cis-addition occurs preferentially. Similarly, in the reduction of hex-2-yne to n-hexane, cis-hex-2-ene is found to be the major olefin intermediate.A mechanism for the hydrogenation is proposed in which the metal complex serves as a template to which a hydrogen molecule and an olefin molecule are briefly co-ordinated before transfer of one to the other takes place. The low kinetic isotope effect (rate H2/rate D2= 0·9) suggests that synchronous breaking of Rh–H bonds and making of C–H bonds takes place in the transition state involving two simultaneous three-centre interactions.
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TL;DR: The most active catalyst for homogeneous hydrogenation of alk-1-enes in benzene or toluene solution is the complex RuClH(PPh3)3.
Abstract: The complex hydridochlorotris(triphenylphosphine)ruthenium(II) as a benzene solvate, RuClH(PPh3)3,C6H6, has been obtained by the interaction of the dichloride, RuCl2(PPh3)3, with molecular hydrogen at ambient temperature pressure in the presence of a base such as triethylamine; other preparative methods are described. The corresponding bromide, RuBrH(PPh3)3,C6H6, has been prepared. From the chloride by interaction with norbornadiene and 2,2′-bipyridyl, the complexes RuClH(C7H8)(PPh3)2 and [RuClH(bipyr)(PPh3)2]2 respectively have been obtained; the complex RuH2(CO)(PPh3)3 has also been prepared.The complex RuClH(PPh3)3 is the most active catalyst yet discovered for the homogeneous hydrogenation of alk-1-enes in benzene or toluene solution. The interaction is highly specific and rates for other types of alkene are slower by a factor of at least 2 × 103. The inherent difficulties of the system however preclude detailed kinetic study and it is shown that slow poisoning of the catalyst occurs under hydrogenation conditions.N.m.r. studies of the hydrido-complex and its deuteride have allowed hydrogen atom exchange studies to be made. Isomerisation of alkenes is studied. The slow exchange between molecular hydrogen and the α-proton of a phenyl group on the phosphine is demonstrated.
169 citations
TL;DR: The present article outlines the most important nonenzymatic reactions of cobalt in vitamin B12 and in model compounds of the cobaloxime type.
Abstract: Studies with vitamin B12 model compounds such as the cobaloximes provide a basis for the understanding of the mode of action of corrinoid coenzymes in enzymatic reactions. They also widen our knowledge of the properties and reactions of organocobalt compounds. The present article outlines the most important nonenzymatic reactions of cobalt in vitamin B12 and in model compounds of the cobaloxime type.
150 citations
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