Cyclopropyl conjugation

The largest contribution and variety in the family of polyenes is to be found in the group of bicyclic and polycyclic compounds. For this chapter we selected those compounds which represent the most important prototypes of different kinds of interaction, namely cyclopropyl-conjugation, spiroconjugation, hyperconjugation and homoconjugation. 

SCHEME 1. Difference between cyclopropyl conjugation and cyclopropyl homoconjugation... 

Just as the unusual stability and reactivity of benzene are placed into their proper context by comparison with cyclobutadiene and cyclooctatetraene39, the 4 -electron homo-logues of benzene, it is instructive to compare the formally homoantiaromatic bicyclo [3.1.0]hexenyl/cyclohexadieny 1 cation systems with the homocyclopropenium and homo-tropenylium ions (Scheme 14). Such a comparison not only puts in context the properties of the latter two homoaromatic cations, but also reveals a different mode of cyclopropyl conjugation that occurs in the 4 -electron systems. 

TABLE 1. Comparison of experimental and MM2ERW heats of formation A/ff (kcalmoL ) for cyclopropyl conjugated molecules ... 

Molecular structures of the cyclopropylcarbinyl cation salts lOa-c have been determined by X-ray. Both 10a and 10b show a bisected geometry that optimizes cyclopropyl conjugation with the cationic center, whereas in 10c such conjugation could lead to antiaromatic destabilization, and it was suggested that the structure reflected geometrical distortion to avoid interaction of the cyclopropyl group with the allyl cation portion of 10c. 

It has recently been argued " that the existing body of thermochemical data is inadequate to demonstrate quantitatively the effects of cyclopropyl conjugation that has been claimed for many systems. New measurements of the heats of vaporization of cyclopropyl cyanide and cyclopropylbenzene were made, and using these data and others in the literature the stabilities of a series of substituted cyclopropanes were compared to the corresponding 2-substituted propanes by the isodesmic reactions 1 and 2 shown in Scheme 1 ". ... 

The conformational dependence of cyclopropyl conjugation was illustrated in solvolysis of methyl-substituted cyclopropylcumyl chlorides. Thus for solvolysis of 2-aryl-2-propyl chlorides (14) there was an acceleration by a factor of 154 for replacement of the p-H by cyclopropyl, whereas for the 2-(3, 5 -dimethylphenyl)-2-propyl chloride (15) the acceleration was decreased to a factor of 9.0. ... 

Studies of ring-substituted cyclopropyl peracetates did not reveal any significant rate enhancement attributable to cyclopropyl conjugation (equation 28) ". ... 

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