SUBSTITUENT-DEPENDENT ELECTRON-TRANSFER-INDUCED PHOTOOXYGENATION OF 1,1-DIARYLETHYLENES

Rates and products of 9,10-dicyanoanthracene-sensitized photooxygenations of 1,1-diarylethylenes (1a-r) in acetonitrile were studied. If at least one of the aryl groups carries an electron-donating substituent at the para (or ortho) position (1a-1), 3,3,6,6-tetraaryl-1,2-dioxanes (2a-1) are generated in high yields (85-100%). Benzophenones (3) are the only other observable products. 1,1-Diphenylethylene (1n) and its m-methoxy (1m), p-chloro (1q,p), and p-nitro (1q,r) derivatives,however, yield mainly benzophenones (3m-r) (>50%) (the p-nitro compounds only in the presence of biphenyl). 1,2-Dioxanes (2m-p), cyclobutanes (4n-p), and alpha-tetralones (5m-o) are obtained as side products. Dioxanes, benzophenones, and alpha-tetralones are products of electron-transfer induced oxygenations involving triplet ground-state molecular oxygen, O-3(2). Singlet molecular oxygen, O2(1DELTA(g)),contributes to the benzophenone formation from strongly electron-donor substituted diarylethylenes. An exception is the most powerful electron-donor substituted diarylethylene 1a, with which O2(1DELTA(g)) undergoes an electron-transfer reaction affording dioxane 2a. Dioxane formation proceeds via free-radical cations 1.+, which enter into a chain reaction with 1, O-3(2), and another molecule of 1 to yield dioxane 2 and a new radical cation 1.+ that maintains the chain reaction. The efficiency of this chain process, however, is found to be several orders of magnitude smaller than expected. To explain this result, a 1,6-biradical.1-1-O2.is proposed to be generated in this chain reaction as the product-determining intermediate that predominantly fragments into O-3(2) and two molecules of 1. Cyclization to dioxane 2 and transformation to benzophenone 3 occur at presumably less than 0.1% from this biradical. The pathways leading to cyclobutanes (4) and alpha-tetralones (5) are also discussed.