Bicyclo octa-2,4-dienes. radical cations

Dai et al. (1990) have reported direct spectroscopic measurements on a similar bishomobenzene radical cation. The system they studied was the bicyclo[3.3.0]octa-2,6-diene-4,8-diyl radical cation [159] generated by radiolytic oxidation of semibullvalene [83] in Freon matrices. The hydrogen... 

Many studies used radiation chemistry to produce the radical and radical cations and anions of various dienes in order to measure their properties. Extensive work was devoted to the radical cation of norbomadiene in order to solve the question whether it is identical with the cation radical of quadricyclane . Desrosiers and Trifunac produced radical cations of 1,4-cyclohexadiene by pulse radiolysis in several solvents and measured by time-resolved fluorescence-detected magnetic resonance the ESR spectra of the cation radical. The cation radical of 1,4-cyclohexadiene was produced by charge transfer from saturated hydrocarbon cations formed by radiolysis of the solvent. In a similar system, the radical cations of 1,3- and 1,4-cyclohexadiene were studied in a zeolite matrix and their isomerization reactions were studied. Dienyl radicals similar to many other kinds of radicals were formed by radiolysis inside an admantane matrix. Korth and coworkers used this method to create cyclooctatrienyl radicals by radiolysis of bicyclo[5.1.0]octa-2,5-diene in admantane-Di6 matrix, or of bromocyclooctatriene in the same matrix. Williams and coworkers irradiated 1,5-hexadiene in CFCI3 matrix to obtain the radical cation which was found to undergo cyclization to the cyclohexene radical cation through the intermediate cyclohexane-1,4-diyl radical cation. 

As an alternative to a radical chain mechanism for this bromination, a cationic mechanism has been proposed for the reaction between 48 and A-bromosuccinimide. It involves attack of bromine at C6 of 48 leading ultimately to the cyclopropylmethyl cation A. This cation is a bromo derivative of tricyclic cyclopropylmethyl cation, which has been shown to be the common intermediate in the solvolysis of esters of tricyclo[3.2.1.0 ]octan-3-ol, endo- and exo-tricyclo[3.2.1.0 ]octan-4-ol and of cxo-bicyclo[3.2.1]oct-2-en-7-ol. It has been shown that under long-lived ion conditions at — 78 C such cations are the most stable species that are formed from bicyclo[3.2.1]oct-2-en-3-ol and from bicyclo[3.2.1]octa-2,6-dienes. In kinetically controlled reactions, which are postulated to proceed via cyclopropylmethyl cations a tendency can be seen towards formation of products retaining the cyclopropane ring. This case is achieved through loss of one of the protons at C4 of A. 

---