AROMATIC IODINATION - EVIDENCE OF REACTION INTERMEDIATE AND OF THE SIGMA-COMPLEX CHARACTER OF THE TRANSITION-STATE

The reactivity of four different procedures of aromatic iodination is compared under the same experimental conditions, and their selectivity toward two substrates in competition, i.e., mesitylene (1,3,5-trimethylbenzene, MES) and durene (1,2,4,5-tetramethylbenzene, DUR), is evaluated. Two of these procedures, namely, S2O8(2-)/I2 and Ce(IV)/I2, present strong oxidizing capacity. Since the same MES/DUR relative reactivity is obtained from the four procedures, it becomes possible to state that a common reactive intermediate, most likely the I+ ion, is generated. The use of the MES/DUR mechanistic probe allows one to describe the reactivity picture of the iodination reaction as one of electrophilic substitution at the aromatic nucleous, with a transition state properly represented in terms of a sigma-complex. The radical cation of durene also forms when the iodination is carried out by means of oxidizing agents, but it is solely responsible for the formation of side-chain substitution products and is not involved in the nuclear substitution process.