Photoinduced electron-transfer oxidation of olefins with molecular oxygen sensitized by tetrasubstituted dimethoxybenzenes: A non-singlet-oxygen mechanism

alpha -Methylstyrene (1) was photo-oxidized in the presence of a series of alkylated dimethoxybenzenes as sensitizers in an oxygen-saturated MeCN solu tion to afford the cleaved ketone 2. epoxide 3, as well as a small amount o f the ene product 4 in ca. 1 :1 : 0.04 ratio. The relative rate (if convers ion was well-correlated with the fluorescence quantum yield of sensitizers. Thus, a non-singlet-oxygen mechanism is proposed, in which an excited sens itizer is quenched by (ground-state) molecular oxygen to produce a sensitiz er radical cation and a superoxide ion (O-2(-.)), the former of which oxidi zes the substrate, while the latter reacts with the resulting olefin radica l cation (1(..)) to give the major oxidation products. Photodurability of s uch electron-donating sensitizers is dramatically improved by substituting four aromatic H-atoms in 1,4-dimethoxybenzene with Me or fused alkyl groups , which provides us with an environmentally friendly, clean method of photo chemical functionalization with molecular oxygen, alternative to the ene re action via singlet oxygenation.