Hydrogenolysis doublet

The first mechanism proposed for the newly discovered hydrogenolysis of cyclopentane was the doublet mechanism (104), in line with the general multiplet theory of Balandin (105). According to this mechanism, cyclopentane is physically adsorbed on two metal atoms, and stands perpendicular to the surface in the vicinity of physically adsorbed hydrogen atoms (Fig. la). This mechanism, however, does not explain why cyclopentanes are hydro-genolyzed on platinum while cyclohexanes and paraffins are not. 

Fig. I. Doublet (a) and sextet-doublet (b) schemes of the hydrogenolysis of cyclopentane on the (II I) face of platinum (707,108. Large circles, Pt atoms, intermediate circles, C atoms small circles, H atoms.

It was replaced later by the sextet-doublet mechanism (106), in which physically adsorbed cyclopentane lies parallel to the surface (Fig. lb) (107, 108). In the transition state, the five carbon atoms of the ring are located over the interstices of the (111) plane of platinum. The distance between two contiguous interstices allows them to accommodate two consecutive carbon atoms, but one C-C bond in cyclopentane is necessarily stretched, and this favors the hydrogenolysis of the ring. In contrast, all the carbon atoms of cyclohexane or paraffins may be brought in contact with the metal surface, which would explain the selectivity of platinum for Cj-ring hydrogenolysis. In the first presentation of the model, the relative inertness of the tertiary-secondary C-C bonds in substituted cyclopentanes was explained by some kind of steric hindrance. 

The sextet-doublet model was adapted for 1-5 dehydrocyclization, the reverse of cyclopentane hydrogenolysis, and it was proposed that physically adsorbed alkane reacts with chemisorbed hydrogen according to a push-pull mechanism (772) (Scheme 55). 

Besides the sextet-doublet model, a slightly modified form of the doublet model was also presented by the Soviet school of catalysis to explain the main features of hydrocarbon hydrogenolysis on ruthenium, osmium, iridium. 

The doublet and the sextet-doublet mechanisms resemble in many respects our Mechanisms A and B discussed previously, involving it-adsorbed cyclopentene and 1,2-dicarbene complexes, respectively. Mechanism A, like the sextet-doublet mechanism, implies a quasi-planar adsorption of the molecule and is in essence nonselective. In Mechanism B, as in the doublet model, the molecule stands perpendicular to the surface and hydrogenolysis is extremely selective. 

In any event, if ring closure via 1,5 di-ff-adsorbed species occurs in the case of medium-sized ring compounds, the reverse step, C-C bond shift rupture in physically adsorbed cyclopentanes, should also take place on account of microscopic reversibility. As a matter of fact, the first mechanisms proposed for hydrogenolysis of cyclopentanes (the doublet and sextet-doublet mechanisms) involved physically adsorbed cyclopentane. 

Fig. 20. The doublet-sextet model of the hydrogenolysis of cyclopentane on platinum. After Kazanskii et al. 145-147).

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