Conjugation with cyclopropyl group

A variety of other radical species (150-152) substituted with cyclopropyl groups were postulated to occur as transition states or intermediates in some radical rearrangements. However, any conjugative stabilization present in these intermediates was insufficient to give them appreciable lifetimes. 

The lack of additional thermochemical data is admittedly frustrating because there are many other documented cyclopropenones [e.g., see W. E. Billups and A. W. Moorehead, in The Chemistry of the Cyclopropyl Group (Ed. Z. Rappoport), Chap. 24, Wiley, Chichester, 1987, p. 1533] which have higher stability and alternative amounts of conjugation with the affixed substituents. 

It has also been shown that surface delocalization can adopt a preferential direction if the cyclopropyl group interacts with a -conjugated system. There are basically two directions of surface delocalization as indicated in Scheme 9 for vinylcyclopropane (20) and divinylcyclopropane 47. In homoaromatic molecules, surface delocalization is perpendicular to the 1,3-bond while in homoantiaromatic molecules it is parallel to the 1,3-bond27-85. 

Because of the additional calibration, the MM2ERW force field leads to heats of formation of conjugated polyenes or conjugated systems containing the cyclopropyl group in close agreement with experimental heats of formation (see T able 1 in Section II.C). With 1,3-butadiene and vinylcyclopropane as reference compounds, none of these molecules possesses any extra stabilization. This, however, is different for the potentially homocon-jugated molecules listed in Table 2 of Section II.C. 

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