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

Following a suggestion by the late Lennart Eberson, we have employed the IC l-HFP (HFP being hexafluoropropan-2-ol) system in iodination reactions, and found unambiguous evidence for the occurrence of an ET-mechanism of haloge nation. The evidence is based on the use of 'intelligent' substrates, which make it possible to fix the boundaries between the occurrence of an ET-mec hanism 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 ons et of a one-electron oxidation with electron-rich substrates, followed by h alogenation. The most prominent example is that of the electron-rich substr ate durene (1,2,4,5-tetramethylbenzene, DUR), when compared to mesitylene ( 1,3,5-trimethylbenzene, MES): with a 'conventional' iodination system (i.e. , I-2/ 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 ICI, where the ET-mechanism takes over. Oth er substrates also support the onset of ET-pathways in HFP. Finally, a prel iminary survey of a biohalogenation reaction induced by laccase indicates t he modest occurrence of a polar process of iodination with a few substrates .