The photosensitised oxidation of amines. Part II. The use of dyes as photosensitisers: evidence that singlet oxygen is not involved

Abstract

The products from the dye-photosensitised oxidation of a number of aliphatic amines are reported. Tertiary amines reacted efficiently to give carbonyl compounds and imines. Secondary and primary amines are relatively unreactive. Only those amines were reactive which were capable of photobleaching the dye in the absence of oxygen. N-Alkylanilines were found to be unreactive. The oxidations sensitised by Rose Bengal were found to increase in efficiency as the concentration of oxygen in the reaction mixture was lowered. These results were interpreted in terms of a mechanism in which the primary photoreaction is abstraction of hydrogen from the amine by the triplet state of the dye. Products are formed by the amino-radicals so formed, reacting with oxygen. In the case of tertiary amines, flash photolysis studies confirmed that the excited state of the dye reacts with the amine in the presence of oxygen. Flash photolysis studies also showed that N-alkylanilines and aniline transfer an electron to the triplet dye and that the radical-ions so produced undergo return electron transfer in preference to proton transfer.