Heterocyclic addition reactions during low temperature autoxidation

Low temperature air oxidation affects many aspects of the fuels business: weathering, storage stability, hardening, and fouling. Addition reactions taking place during the autoxidation of different heterocyclic compound classes were studied at 130 °C. This investigation reports on the conversion, product selectivity, nature of addition products, and plausible mechanisms that would explain the observations subsequent to the oxidation of indole, 2,3-dihydroindole, quinoline, benzofuran, 2,3-dihydrobenzofuran, and thianaphthene. Of these, indole is known to be prone to oxidative addition. Among the five-membered heterocyclic compounds, the propensity to form oxidative addition products increased in the order: S 蠐 O < N. Addition took place mainly through C-C bond formation. In comparison to hydrocarbons, it was surprising to find that indole and 2,3-dihydroindole were less prone to oxidative addition than the five-membered naphtheno-aromatic hydrocarbon analogues indene and indan. On the basis of the work, some implications for low temperature oxidation processes, and free radical processes in general, are discussed.