Aromatic iodination: a new investigation on the nature of the mechanism

Abstract

Following a suggestion by the late Lennart Eberson, we have employed the ICl–HFP (HFP being hexafluoropropan-2-ol) system in iodination reactions, and found unambiguous evidence for the occurrence of an ET-mechanism of halogenation. The evidence is based on the use of ‘intelligent’ substrates, which make it possible to fix the boundaries between the occurrence of an ET-mechanism and of a conventional polar mechanism. In an ‘intelligent’ substrate, in fact, the nature of the product(s) changes significantly depending on the operating mechanism. The ICl–HFP combination is instrumental to the onset of a one-electron oxidation with electron-rich substrates, followed by halogenation. The most prominent example is that of the electron-rich substrate durene (1,2,4,5-tetramethylbenzene, DUR), when compared to mesitylene (1,3,5-trimethylbenzene, MES): with a ‘conventional’ iodination system (i.e., I₂/Ag⁺) and in common solvents, where the polar mechanism holds, durene is less reactive (k_MES/k_DUR = 46 ± 3), but becomes more reactive (k_MES/k_DUR = 0.23) in HFP with ICl, where the ET-mechanism takes over. Other substrates also support the onset of ET-pathways in HFP. Finally, a preliminary survey of a biohalogenation reaction induced by laccase indicates the modest occurrence of a polar process of iodination with a few substrates.