Dicyanoanthracene sensitized photo-oxygenation of olefins : Electron transfer and singlet oxygen mechanisms

Cyanoaromatic sensitizers, in particular 9,10-dicyanoanthracene (DCA), sensitize the photo-oxygenation of olefins by two distinct mechanisms. In the case of aryl substituted olefins (OL), which react extremely slowly (if at all) with singlet oxygen, the reaction proceeds by way of electron transfer to produce discrete radical ions (DCA^-and OL⁺). In the presence of oxygen, this ionic process results, ultimately, in the cleavage of the olefin to carbonyl compounds along with production of some epoxide and other minor by-products. Aromatic ethers can interfere with this process by reducing the radical cation by electron transfer, resulting in net quenching of the reaction. With simple alkenes the DCA-sensitized reaction takes a different course, producing hydroperoxide products with distributions which are very similar to those obtained with the singlet oxygen ene reaction. Careful study has shown that this reaction does, indeed, proceed by way of singlet oxygen, which is produced by at least two mechanisms : (1) enhanced intersystem crossing, in which ¹DCA is quenched by interaction with the olefin, leading to a low yield of ³DCA, which subsequently reacts with oxygen to produce singlet oxygen; and (2) direct reaction of ¹DCA with oxygen. At limiting high oxygen concentration, this process produces 2 mol of singlet oxygen for each mol of ¹DCA quenched; the mechanism involves energy transfer to produce ³DCA and 1 mol of singlet oxygen ; the ³DCA reacts again with oxygen to produce a second mol of singlet oxygen. The complex kinetic behaviour of simple olefins in the presence of DCA can be satisfactorily rationalized by these mechanisms.