Radical ions in photochemistry. 6. The photosensitized (electron transfer) ring opening of aryloxiranes

The photosensitized (electron transfer) irradiation of cis- and trans-2,3-diphenyloxirane ( 1 and 2 ) led to cleavage of the oxirane carbon-carbon bond and subsequent formation of the carbonyl ylide. The resulting carbonyl ylides have been trapped with the dipolarophiles, acrylonitrile, maleonitrile, and fumaronitrile. The resulting isomeric tetrahydrofuran derivatives have been characterized. Sensitizers (electron acceptors) which are effective include 1,4-dicyanonaphthalene, 1,4-dicyanobenzene, dimethyl terephthalate, and methyl 4-cyanobenzoate; 1-cyanonaphthalene was not effective. The proposed mechanism involves formation and cleavage of the oxirane radical cation followed by back electron transfer from the sensitizer radical anion to give the carbonyl ylide. Electrochemical and photophysical evidence which supports the proposed mechanism was obtained.