Hydrogenation, catalytic, alkene homogenous, mechanism

By contrast with the radical systems, this mechanism is the most common and most studied. It is also historically important for giving the first practical demonstration of catalytic homogeneous hydrogenation of alkenes. 

RhCl(PPhi)i as a homogenous hydrogenation catalyst [44, 45, 52]. The mechanism of this reaction has been the source of controversy for many years. One interpretation of the catalytic cycle is shown in Figure 2.15 this concentrates on a route where hydride coordination occurs first, rather than alkene coordination, and in which dimeric species are unimportant. (Recent NMR study indicates the presence of binuclear dihydrides in low amount in the catalyst system [47].)... 

Group 9 metal complexes can catalyze important homogeneous reactions such as hydrogenation and hydroformylalion of alkenes. Some of them are employed in industry. The mechanisms of the catalytic reactions involve M(I) and M(III) intermediates having 16 and 18 electrons. In some cases, the catalytic intermediates have been isolated, and a mechanistic study of the cataK tic reactions has greatly contributed to the progress of organometallic chemistr. ... 

The mechanism of homogeneous catalysis invoives the same steps as heterogeneous catalysis. An initial tt complex is formed with the reactant. Metal-hydride bonds then react with the complexed alkene to form a C-H bond and a bond between the metal and alkyl group. There can be variation in the timing of formation of the M—H bonds. The metal carbon bond can be broken by either reductive elimination or protonolysis. Note that reductive elimination changes the metal oxidation state, whereas protonolysis does not. The catalytic cycle proceeds by addition of alkene and hydrogen. 

The isolable ethylene complex Ru(TMP) C2H4) is stable to 10 torr at 20°C, but the cyclohexene analogue readily dissociates the coordinated alkene. The competition of alkene binding versus O2 reactivity to give dioxo species is reminiscent of mechanisms in catalytic homogeneous hydrogenation, where hydride and unsaturated routes have been identified the former operates by olefin attack on a metal-hydride, and the latter by attack of H2 on a metal-alkene species. 

In Section 5.9, we saw that alkenes can be converted to alkanes by catalytic hydrogenation by a variety of catalysts, such as palladium and platinum. These are heterogeneous catalysts. We also noted that homogeneous catalytic hydrogenation can be carried out by Wilkinsons catalyst, Ru[(PPh3)3Cl. We now return to that subject to discuss the reaction mechanism. We will find that hydrogenation by Wilkinson s catalyst occurs in a catalytic cycle that is strikingly similar to the catalytic cycles of the reactions we have discussed thus far in this chapter. The transition metal in the Wilkinson catalyst, however, is ruthenium, not palladium. 

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