Electron-Transfer Photooxygenation. 5. Oxidation of Phenyl-Substituted Alkenes Sensitized by Cyanoanthracenes

Article abstract of DOI:10.1021/ja00539a018

The electron-deficient sensitizers 9,10-dicyanoanthracene (DCA) and 9-cyanoanthracene (CNA) sensitize the photooxidation of 1,1-diphenylethylene (1), trans- and cis-stilbene (12 and 14), and tetraphenylethylene (18) in oxygen-saturated polar solvents.The photoproducts in MeCN include the cleavage products Ph2CO and PhCHO, the epoxides, and, in the case of the stilbenes, cis-trans isomerized olefin.Quantum yields for disappearance of the alkenes are 0.8, 0.5, 0.2, and 0.15 for 1, 12, 14, and 18, respectively.No photooxygenation takes place in nonpolar solvents.The reactions in MeCN are accompanied by quenching of the DCA and CNA fluorescence.The measured quenching rate constants closely resemble values calculated for an electron-transfer process and agree, withing experimental error, with those determined from the acceptor concentration dependence of product formation.The photooxygenations are quenched by addition of small amounts of compounds of lower oxidation potential than that of the alkenes, whereas compounds of higher oxidation potential have little effect.All results are consistent with an electron-transfer mechanism involving a donor radical cation and sensitizer radical anion which subsequently reduces oxygen to superoxide.The data are not compatible with a singlet oxygen mechanism.