Relationships between Structures and Reactivities in the Oxidation of Benzothiophene and Dibenzothiophene Derivatives

Basic information concerning the factors controlling oxidative desulfurization was investigated using the structures and reactivity of benzothiophene and dibenzothiophene derivatives with data from molecular orbital (MO) calculations and oxidation reactions. Highly condensed aromatic compounds had higher reactivity, agreeing well with the order based on MO calculations. Methyl groups in the 2-position of benzothiophene accelerated the reaction, also agreeing well with the results of MO calculations. On the other hand, methyl groups at the 4- and/or 6-positions of dibenzothiophene had different effects depending on the reaction conditions and the nature of the active species. MO calculation indicates that methyl groups enhance the reactivity of the substrates because of their electron donating nature. However, conversion of the methylated substrates decreased in reactions using bulky oxidants such as aliphatic peracids or peroxides with aryl substituents, suggesting the importance of steric hindrance by the methyl groups.