Theoretical study of the iodination of methoxybenzene by iodine monochloride

Abstract

The pathway of the iodination of methoxybenzene by electrophilic iodine monochloride (ICl) was studied by using density functional theory (DFT) calculations at the B3LYP/6–311G* and MP2/6–311G*//B3LYP/6–311G* levels. The iodination occurs in the position para to the methoxy group, and proceeds in two steps. Initially, a π-complex forms between ICl and aromatic ring of methoxybenzene. With a barrier of 60.81 kcal mol−1 (1 kcal = 4.184 kJ), the π-complex can be activated to an intermediate σ-complex with energy 42.02 kcal mol−1 higher than that of the π-complex. The σ-complex then transforms easily (barrier 3.56 kcal mol−1) into the final products, 1-iodo-4-methoxybenzene and HCl. The total iodination is slightly exothermic. Accompanying to the ICl bond breaking and CI bond formation, a hydrogen atom migrates first to iodine and then to chlorine. According to NBO charge, Wiberg bond index and molecular orbital analysis, both charge separation and charge transfer occur during the iodination. Solvent effects were examined with the IEFPCM method and the B3LYP/6–311G* level. The results imply that polar solvents should play a key role in lowering the energy barrier, and favor the ion-pair route of the iodination reaction. Copyright © 2005 John Wiley & Sons, Ltd.