Hydrogenolysis mechanism, cyclopropanes

Fig. 20.4 A cyclopropane hydrogenolysis mechanism based on the Horiuti-Polanyi alkene hydrogenation mechanism.

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

One of the most extensively used addition reactions of cyclobutanes is hydrogenolysis.36 With regard to the mechanistic aspect, evidence has been provided that hydrogenolysis of cyclobutane is structure sensitive to the particle size of the platinum on alumina catalysts.37 Moreover, a kinetic study has also revealed that the mechanism for the hydrogenolysis of cyclobutanes is likely to be different from that for cyclopropanes.37... 

If it is assumed that the adsorbed carbonium ion has a tetrahedral configuration, it is evident that the cis-adsorbed species is sterically more favorable than the trans species. Thus, hydride ion transfer to the adsorbed charged species from the catalyst would lead to predominant cis-product formation. In contrast to the Weidlich proposal, these mechanisms call for a 1,4-addition of hydrogen in acidic media. The concept of a hydride ion transfer from the catalyst is not unique to these mechanistic proposals. Hydride ions have been proposed to take part in the catalytic hydrogenolysis of certain substituted cyclopropanes... 

In this paperthe relative stabilities of various small-ring propellanes are discussed in terms of enthalpies of hydrogenolysis of the conjoining bond and dissociation energies of this bond in the various substrates. This is perhaps the place to state that the mechanism of addition of bromine, in the dark, to the conjoining bond of several [m.n.l]propellanes, has been discussed in generaF. It is concluded that thermally initiated low temperature radical chain addition to the cyclopropane rings is involved. 

It is clear, then, that the prediction of the product stereochemistry from a cyclopropane hydrogenolysis is not straightforward. In some cases such as the cleavage of the phenyl bicycloheptane, 16 (Eqn. 20.8), the product stereochemistry was that expected if the simple mechanism depicted in Fig. 20.4 were operational. In other cases it appeared that an olefmic intermediate may be formed and that the product resulted from the hydrogenation of that intermediate. There are also some instances in which neither explanation seems to apply. 

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