Molecular-design of a new phenolic antioxidant - Effect of substituent on o-position

Oils, petroleum products, plastics etc. are degraded by autoxidation, and t he inhibition of such oxidations is an important problem. Phenols, used for such purposes, are known to show activities depending on their molecular s tructures, especially substituents on phenolic nuclei; the relationship bet ween activity and substituent was established in the 1960's. Consequently, new phenols have not been developed since. The authors have some doubts abo ut the relationship, and molecular-designed phenols having an olefinic grou p on o-position (o-hydroxycinnamates) as new antioxidants. Surprisingly, th ey were more highly active than phenols developed in the past. The high act ivities were found to be derived from (1) the enhancement of peroxyl radica l-trapping rate due to the high stability of the phenoxyl radical, (2) the inhibition of coupling of phenoxyl radicals by localization of the electron of the phenoxyl radical, and (3) the trapping of peroxyl radicals. The eff ect of other substituents, in addition, was examined as well, and more acti ve phenols were synthesized by introducing two substituents on o- and P-pos itions of the hydroxy group. The reason, why o-hydroxycinnamates exhibit su ch high activities, was discussed by a semi-empirical molecular orbital met hod, and a possible action mechanism was proposed.