Divinylcyclopropane-cycloheptadiene conversion

When ring strain is relieved, Cope rearrangements can occur at much lower temperatures and with complete conversion to ring-opened products. A striking example of such a process is the conversion of cw-divinylcyclopropane to 1,4-cycloheptadiene, a reaction which occurs readily at temperatures below —40°C.139... 

A detailed description of the numerous examples of vinylcyclopropropanes used in transition metal mediated organic synthesis is far beyond the scope of this section and can be found in several reviews. Prominent examples are conversions to open-chain products, as well as formation of four-, five-, six- and seven-membered rings via ring expansion or incorporation of other substrates such as carbon monoxide, alkenes or alkynes. Thus divinylcyclopropanes, obtainable via transition metal catalyzed cyclopropanation reactions, undergo a facile thermal Cope rearrangement to form cycloheptadienes. ... 

When the C-3 and C-4 carbons of 1,5-dienes are connected to a cyclopropane ring, the reaction rates are accelerated due to favorable interactions of the diene termini resulting in the reduction of the enthalpy of activation. For example, the conversion of cfr-divinylcyclopropane 61 to 1,4-cycloheptadiene 62 occurs readily at temperatures below -40 °C [37]. 

R = R = H) underwent equilibration with its cis-fuscd isomer and only a trace of the cycloheptadiene was observed presumably steric interactions make the Cope rearrangement very difficult. The results suggest an initial epimeriza-tion of irons- to cis-fused isomers followed by a normal Cope rearrangement. An analysis of the kinetics of conversion of ( — )-trans-divinylcyclopropane into cyclohepta-1,4-diene indicates that the ring-opening is not a concerted pro-cess. 

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