Singlet Oxygen and Electron-Transfer Mechanisms

The 9,lO-dicyanoanthracene-sensitized photooxidation of 2,3-diphenyl1,4-dioxene in CHICN produces ethylene glycol dibenzoate and small amounts of epoxide. Most of the diester is formed from singlet oxygen via the dioxetane, and only a small amount by electron transfer. The epoxide is a primary electron-transfer product. Various mechanistic possibilites for the electron-transfer process are considered. Schaap reported that the 9,lO-dicyanoanthracene (DCA)sensitized photooxidation of 2,3-diphenyl-l,4-dioxene (DPD) in acetonitrile gives ethylene glycol dibenzoate (EGDB) as the only isolable product (Scheme I).' It was assumed that this product is formed by electron transfer, which produces analogous cleavage products from aromatic alkenes with DCA.Z*3 However, singlet oxygen can also be formed in large quantities in this and it has been suggested that singlet oxygen reacts with DPD to give the dioxetane precursor of EGDB? We have reinvestigated the DCA-sensitized photooxidation of DPD to determine whether singlet oxygen, electron transfer, or a combination is responsible for the observed products.