Study of the mechanism of the antioxidant action of ethoxyquin

A study has been carried out of the reaction of 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline (ethoxyquin, I) with alkylperoxyls. In the presence of a relatively low concentration of 1-cyano-1-methylethylperoxyl, I reacts to form dimer IV (8-(6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinolin-1-yl)-6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline) as the main product and quinoline derivative III (2,4-dimethyl-6-ethoxyquinoline), as a side product, i.e., substances formed by the conversion of the aminyl II (6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinolin-1-yl) alone. Smaller amounts of p-quinoneimine VI and o-quinoneimine VIII (2,2,4-trimethyl-2,6-dihydro-6-quinolone and 6-ethoxy-2,2,4-trimethyl-2,8-dihydro-8-quinolone) have been found; these substances are formed by further oxidation of II. In the presence of relatively high concentrations of tert-butylperoxyls, peroxide IX (6-tert-butylperoxy-6-ethoxy-2,2,4-trimethyl-2,6-dihydroquinoline) is formed as the main product. Substance IX thermally decomposes to form VIII, while in the presence of weak acids IX is converted into VI as the main product. Dimer IV is a medium-strength antioxidant which is gradually converted into 8-(6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinolin-1-yl)-2,2,4-trimethyl-6-quinolone (XI) and 8-(6-ethoxy-3-hydroxy-2,2-dimethyl-4-methylene-1,2,3,4-tetrahydroquinolin-1-yl-)-2,2,4-trimethyl-2,6-dihydro-6-quinolone) (XII). The methyl group in XI actively participates in the antioxidation process. When formed, nitroxide V (6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinolin-N-oxyl) acts as an efficient antioxidant which, however, does not participate in the cyclic mechanism that is employed to explain the action of antioxidants of the HALS type. The antioxidant properties were evaluated on the basis of their effect on the course of the autoxidation of squalene.