Radical cations of strained ring hydrocarbons. Cyclopropane derivatives

A series of radical cations has been generated from various cyclopropane derivatives by photoinitiated electron transfer to chloranil. Nuclear spin polarization effects observed during these reactions have been interpreted as evidence for the structure of these intermediates. Tri- and tetraalkylsubstituted cyclopropanes (1, 3, 4) form "trimethylene" radical cations in which the most highly substituted carbon–carbon bond is broken (or weakened). The benzocyclopropanorbomene (12) forms a radical cation in which spin and charge are located in the benzenoid ring; the tertiary cyclopropane protons are coupled by a homohyperconjugative interaction; otherwise, the cyclopropane ring is unaffected. To the radical cation derived from bis-methanoparacyclophane (17) a trimethylene structure is assigned in which the benzylic carbons are pyramidal. Finally, benzonorcaradiene (20) forms a radical cation in which the two secondary–tertiary bonds are lengthened and in which the methylene group bears spin density.