Halpem mechanism, olefin hydrogenation

Example 8.9. Olefin hydrogenation with Wilkinson s catalyst. Wilkinson s catalyst is a dihydrido-chloro-phosphino complex of rhodium, H2RhClPh3, where Ph is an organic phosphine such as triphenyl phosphine [48-52]. The dominant mechanism of olefin hydrogenation with this catalyst, established chiefly by Halpem [53-55] in detailed studies that included measurements of equilibria in the absence of reactants and of reaction rates of isolated participants, backed by independent NMR studies [56] and ab initio molecular orbital calculations [57], is shown as 8.69 on the facing page (without minor parallel pathways and side reactions). 

Some of the earliest mechanistic studies on homogeneous catalysis in organic solvents were reported by Halpem on the hydrogenation of olefins with Wilkmson s catalyst and on the mechanism of the asymmetric hydrogenation of dehydroamino acids by Knowles rhodium-DIPAMP system. Two important general conclusions were drawn from these stud-igg 19,20,66-69 gg noted in Chapter 14, the species in a catalytic system that accumulate in sufficient concentration to be identified spectroscopically may or may not lie directly on the catalytic cycle. In some cases, these species are connected to the catalytic cycle by equilibria, while in other cases they are unproductive species formed irreversibly. By associating particular complexes with the kinetic behavior of the catalytic reaction, one can assess whether the observed complex contributes positively or negatively to the rate of the catalytic process. 

Figure 4 Potential energy profile of the entire catalytic cycle of the Halpem mechanism of olefin hydrogenation by the Wilkinson catalyst, in kcal mol at the RHF level relative to 13 + C2H4 + H2. Numbers in parenthesis are MP2 energies for the RHF structures, relative to 15...

The mechanism of hydrogenation involving the cationic diphosphine catalysts has been elucidated by J.Halpem and others, and is shown in Scheme 2. An olefin complex 8 is in equilibrium with catalyst 7. Values of... 

Clearly, the deuterium on the p-carbon is derived from the solvent and neither die olefin nor the hydride route mechanisms are operative because they deliver both hydrogens from H2. Ashby and Halpem also measured the equilibrium constants for the following reactions ... 

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