Homogeneous hydrogenation of olefins

Solvent-i-CPha aRhQ (Ph3P)2RhCKSolvent)z-i-Ph3P 

The rhodium species in solution is either (Ph3P)2RhCl, or, more likely, a solvated spedes (Ph3P)2RhCl(solyent)i. The complex reacts with mole- 

Kinetic measurements under the conditions of the hydrogenation reactions indicate the equilibria (1) and (2)  

It is suggested that the catalytic reduction of the olefin proceeds by the prior formation of the hydride complex followed by the addition to it of the olefin, which is then reduced, rather than vice versa. This postulate is consistent with the observation that ethylene is reduced by the hydride complex, 10.2, while the ethylene complex, 10.3, is not readily reduced by hydrogen. The lower stability of the propene relative to the ethylene complex explains the ability of the system to act as a hydrogenation catalyst in the former but not in the latter case. When solutions of the rhodium complex are treated with a mixture of hydrogen and propene, the dihydride will be formed preferentially, whereas ethylene would form a more stable complex and prevent more than slow formation of the dihydride. 

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Chiral ligands for homogeneous hydrogenation of olefins and ketones... 

J. A. Osborne, F. J. Jardine, J. F. Young, G. Wilkinson, The Preparation and Properties of Tris(triphenylphosphi-ne)halogenorhodium(I) and some Reactions thereof including Catalytic Homogeneous Hydrogenation of Olefins and Acetylenes and their Derivatives, J. Client Soc A. 1966,1711-1732. 

Figure 9.1 General scheme for the homogeneous hydrogenation of olefins.

Despite the work presented, no definitive mechanism for the homogeneous hydrogenation of olefins with triarylphosphinerhodium(I) halide catalysts is postulated here. However the experimental observations of inhibition by excess hydrogen and possibly excess olefin are inconsistent with the previously postulated mechanisms (2, 3) involving monometallic active species. The analysis presented suggests that a mechanism involving bimetallic intermediates with the metal sites in reasonable proximity can account for all presently available experimental observations. 

Table I. Homogeneous Hydrogenation of Olefins with HD and Cationic Rhodium Complexes0...

This mechanism is remarkably similar to that proposed by Halpern, Harrod, and James (42) for the homogeneous hydrogenation of olefins at certain ruthenium complexes. 

Organometallic chemistry is interesting in part because it has applications to catalytic processes. Since the discovery of C-H bond activation and the homogeneous hydrogenation of olefins, the importance of organometallic complexes has been undisputed. Many experimental smdies of organometallic catalysis have focused on catalyst and substrate structure, kinetics of transformations, mechanisms, thermochemical properties, turnover, selectivity, etc., and a massive quantity of experimental data has been accumulated. Molecular mechanics can be used to compile and analyze these data in order to direct the design of novel catalytic systems. 

Olefin hydrogenation has been known since 1966, when Wilkinson and cowork-ers reported the homogeneous hydrogenation of olefins by rhodium catalysts (see... 

While the homogeneous hydrogenation of olefins catalyzed by group VIII metal complexes is today a rather commonplace reaction and the CsMes-M complexes oflFer mainly rather higher rates by comparison with the Wilkinson catalyst, very few homogeneous catalysts so far reported are active for the hydrogenation of arenes. 

Asymmetric hydrogenation is the archetype of homogeneous catalysis [26]. Wilkinson and Osborn reported that [RhCl(PPh3)3] catalyzes the homogeneous hydrogenation of olefins [27]. This hydrogenation catalyst was rapidly adapted to an enantioselective... 

After these references of general applicability it will be convenient to discuss the large number of specific references to homogeneous hydrogenation of olefins and of other unsaturated molecules in order of the central metal atom in Periodic Table order. 

Osborn, J. A. Jardine, R H. Young, J. R Wilkinson, G. The preparation and properties of tris(triphenylphosphine)halogeno-rhodium(I) and some reactions thereof including catalytic homogeneous hydrogenation of olefins and acetylenes and their derivatives. /. Chem. Soc. A 1966,1711-1732. 

E. Samuel, J. Organomet. Chem., 1980, 198, C6S. Photocatalytic homogeneous hydrogenation of olefins with alkyltitanocene compounds. 

C. Homogeneous hydrogenation of olefins and acetylenes The variety of transition metal complexes which will catalyse the hydrogenation of unsaturated hydrocarbons under homogeneous conditions is exemplified in Table 30. Only two of these reactions, which have been studied in some detail, are discussed here. 

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