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The conditions for favorable unimolecular reactionpaths developed by Bader2 and one of the authors3 are used, together with a semilocalized molecular orbital model, to determine the allowed routes in the pyrolysis of cyclobutane and cyclohexene. It is found that the coplanar 2s+ 2s decomposition of cyclobutane should proceed more favorably in two steps, with a predominant contribution from the Eu carbon-carbon stretching mode, than in the concerted Big mode. On the other hand, 2s+ 2s concerted decomposition by puckeringin the Blu mode combined with (Aiu+ Eg) twisting is favored. These conclusions are in agreement with the predictions of Wood-ward and Hoffmann. 4 5The favored paths in cyclobutane are essentially determined by the harmonic terms in the potential energy surface and correspond to symmetry coordinates with low forceconstants. In the Diels-Alder retrogression of cyclohexene, however …