The one-electron oxidation of dialkylcyclohexadienes

Abstract

When α-terpinene, γ-terpinene, isoterpinolene, Δ^3,8-p-menthadiene, or α-phellandrene is subjected to one-electron oxidation in fluid solution, either of two e.s.r. spectra can be observed, neither of which resembles the spectra of the monomer radical cations which are formed when the substrates are γ-irradiated in Freon matrices. Treatment of 1,4-dimethylcyclohexa-1,3-diene or 1,4-dimethylcyclohexa-1,4-diene with a variety of oxidising agents in fluid solution gives rise to the same e.s.r. spectrum, which is different from that of the monomer radical cations in a Freon matrix. This spectrum is assigned to the radical cation of the Diels–Alder adduct between 1,4-dimethylcyclohexa-1,3-diene (as the diene) and 2,5-dimethylcyclohexa-1,3-diene (as the dienophile), and this adduct has been isolated and characterised. It is proposed that the monomer radical cations which are first formed lose a proton which catalyses the interconversion of the various possible dimethylcyclohexadienes, and that hole-catalysed cycloaddition then occurs between the pair of isomers which provides the least sterically hindered combination. [graphic omitted]