Seven-membered trans-cycloalkene

If the reaction was carried out in liquid S02 at —60 °C the thiirenium ion (59) was formed, which in CDC13 at 20 °C yielded (56). This represents the first seven-membered trans-cycloalkene, which is isolable at room temperature. In refluxing chloroform (56) isomerized to the corresponding cis-cycloalkene the activation parameters of this trans-cis-isomerization were determined 86). 

Heteronuclear addition reactions to alkynes usually produce the corresponding trans-alkene derivatives. However, in the case of strained cyclic alkynes, these would be even more heavily strained than the starting alkyne. Thus, in most reactions, cis-products are observed. The initial addition of the electrophile may, however, still proceed in an anh -manner, as was shown by Krebs et al. [1 b] for the reaction of a cycloheptyne derivative with trichloromethylsulfenyl chloride (Scheme 8-21). This reaction produced the first seven-membered rm s-cycloalkene derivative to be isolable at room temperature. 

Calculate energy differences between cis and trans-cycloheptene and cis and trans-cyclooctene. Which is the more stable isomer for each compound Is there a significant (>.008 au or 5 kcal/mol) increase in the energy difference between cis and trans isomers in going from the seven to eight-membered cycloalkene Are your results consistent with the fact that trans-cyclooctene is an isolable, stable compound while trans-cycloheptene is not ... 

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