Sextet-doublet mechanism

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 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. 

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). 

In order to decide in each particular case which mechanism of dehydrogenation—the sextet or the doublet—takes place, it is evidently necessary to have criteria for both. In addition to the radioactive procedure... 

The problem of 77-olefin complexes and 7T-allyl complexes 234, 235) is often discussed in the studies of recent years when treating the mechanism of deuterium exchange. Evidently, one should not oppose mechanisms via 77-complexes in favor of the sextet and doublet complexes. It is not impossible that the sextet complex will prove to be close to the 71-complex, etc, just as in dehydrogenation, different mechanisms seemingly contradictory by nature proved to be connected in the scheme (1.28). 

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