Bicyclo octa-3,6-dien-2,8-diyl

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. 

Reaction of 71 with lithium in THF or dimethyl ether at —78 °C led to dilithium semibullvalenide 56b d 57a> which according to Goldstein s and Wenzel s NMR investigations exists as the C2h and D2 diastereoisomers of bis(bicyclo[3.3.0]octa-3,7-diene-2,6-diyl)tetralithium C2h-74 and D2-74, respectively56d) (Scheme 4). 

It would appear that the 2,3-dimethylenecyclohexane-l,4-diyl biradical is involved in this reaction, and it was generated independently by flash photolysis of the appropriate azo compound and ketone and found to give 3,4-dimethylene-1,5-hexadiene and bicyclo[4.2.0]octa-l,5-diene in a 1 2 ratio in solution with log = 16.5 - 24500/23RT (Scheme 9.71). ° ... 

Dimethylenebicyclo[2.2.0]hexane gave a 1 2 mixture 3,4-dimethylene-1,5-hexadiene and bicyclo[4.2.0]octa-1,5-diene at 100°C in the gas phase. The reaction appears to proceed via the 2,3-dimethylenecyclohexane-l,4-diyl (Scheme 9.72). In solution, dimers were formed that gave rise to CIDNP emission signals when formed in an NMR probe indicating triplet precursors. 

The reactions appear to be forbidden 2 + 2 retro- and cycloaddition. The latter reaction probably proceeds via a bicyclo[3.3.0]diyl. In the case of tricy-clo[3.3.0.0 ]octa-3,7-diene, isomerization to a semibullvalene occurs at only 0°C indicating either the [3.3.0]diyl is easily accessible or a forbidden 1,3-shift occurs easily. Relief of ring strain probably plays a role in these cases. However, a semibullvalene cannot be formed in the current case, so the biradical probably has no choice but to reclose to the tricyclic system which apparently is more stable than the cyclooctadiene. 

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