Cyclobutane, hydrogenolysis

The model has been extended to cyclobutane hydrogenolysis and to cycloheptane reactions. Cyclobutane is assumed to undergo, like cyclopentane, planar adsorption on the surface (114). The stretching, however, is more pronounced than in the case of the C5 rings and propagates to all four C-C bonds of the molecule. These can then be broken easily, and that accounts for the absence of selectivity in the hydrogenolysis of alkylcy-clobutanes (114,115). 

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... 

The initial scission of cyclobutane ring of available pagodane derivative 37 is achieved as a result of homolytic bromination. The tribromide 38, thus formed, underwent facile bromine elimination-fragmentation (and hydrogenolysis of the C-Br bond at the a-methoxycarbonyl center) under the action of metals to form diene 39. Diimide reduction of 39 affected only one double bond and gave product 40, which served as a common precursor for both 35 and 36. The... 

Scheme 1 shows the formal possibilities for hydrogenolysis of one and two bonds in bicyclobutane. Since in no case were products containing a cyclopropane ring obtained and since under the conditions employed cyclopropanes are usually inert to hydrogenation, modes A and C remain only formal possibilities. Mode B can be easily identified since it is the only one which leads to a cyclobutane product. However, in cases where a suitable substitution pattern is absent, it is practically impossible to distinguish between modes D and E. Thus the following reaction (equation 133) can comply with both the... 

The hydrogenolysis of cyclobutanes very much resembles the hydrogenolysis of cyclopentanes, except for the reaction temperature, which is substantially lower (by about 80°) and for the smaller selectivity (18,19). 

Step (c), the reverse of Step (a), may be investigated separately by studying the hydrogenolysis of cyclopentanes and cyclobutanes. In these reactions, the effect of methyl substituents allows one to distinguish between several reaction mechanisms. 

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