DIRECT AND DYE-SENSITIZED AQUEOUS PHOTOOXIDATION OF 3-INDOLEACETIC ACID, METHYL-3-INDOLEACETATE AND 1-METHYL-3-INDOLEACETIC ACID .2. QUANTUM YIELDS AND MECHANISM

Mechanisms accounting for the types and yields of the products obtained in direct and dye-sensitized photo-oxidation of the compounds 3-indoleacetic acid (1a), methyl-3-indoleacetate (1b) and 1-methyl-3-indoleacetic acid (1c) in aqueous buffer (pH 5 or pH 8) containing 10% methanol are discussed. In the direct irradiation at pH 5 of acid 1a and its methyl ester 1b, the first detectable products are the corresponding 3-indolemethanols 6a and 6b, their methyl ethers 7a and 7b, and the dimeric compounds 14a and 14b, in aerobic and anaerobic conditions. The same primary products are detected in dye-sensitized photo-oxidation at pH 5. These results support the primary formation of radicals I and II, and of the peroxy forms III and IV, from which thermal reactions can explain all the products found. In both direct and dye-sensitized photo-oxidation at pH 8, only products from the oxidative cleavage of the indolic 2,3-double bond are observed. In dye-sensitized reactions, the products found are in accordance with the existence of competing type I (through radicals) and type II (through singlet oxygen) photosensitized oxygenation processes, their relative proportions depending on the pH of the medium. In direct irradiation, the same intermediates seem to be formed. Quantum yield values of 1a disappearance also depend on the experimental conditions, the highest value found being 28.5% for the direct irradiation at pH 5. © 1990.