Tetra hydrogenolysis mechanism

Gault and coworkers [ 149] have observed that the distribution of products obtained by hydrogenolysis and isomerization of methylcyclopentane was the same as those obtained with hexane. They proposed two competing mechanisms a selective mechanism implying an a, a, p, j0-tetra-adsorbed species and a non-selective mechanism implying coordinated olefin and bis-carbene intermediates (Scheme 38). 

To explain the observed rupture of sterically-hindered substituted tertiarysecondary C-C bond on these low dispersed Pt catalysts, a third mechanism was proposed that competes with the dicarbene mechanism.55 This third mechanism involves a metallocyclobutane species as an intermediate. In fact, both McVicker et al.11 and Coq et al.56 have cited the metallocyclobutane mechanism as responsible for the observed hydrogenolysis of 1,2,4-trimethylcyclohexane and 2,2,3,3-tetra-methylbutane, respectively. 

Hydrodemetallation pathways for Ni-etioporphyrin and Ni-tetra(3-methylphenyl)porphyrin are shown in Fig. 20. Both are characterized by a sequential hydrogenation-hydrogenolysis global mechanism, but important differences are apparent. Ware and Wei (1985a) rationalized the differences in porphyrin reactivity on the basis of porphyrin molecular structure. Structural differences on the periphery of the metalloporphyrin, in particular the substituent groups at the /3-pyrrolic and methine bridge... 

Scheme 18.7 Proposed mechanism of hydrogenolysis of tin tetra-alkyl by a Pt/silica surface covered with hydrogen [113],...

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