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 (kMES/kDUR = 46 ± 3), but becomes more reactive (kMES/kDUR = 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.
根据已故的伦纳特·埃伯森的建议,我们在
碘化反应中使用了ICl–
HFP(
HFP为
氟化
异丙醇)体系,发现了卤化反应中存在电子转移(ET)机制的明确证据。这一证据基于“智能”底物的使用,使得能够界定ET机制与传统极性机制之间的界限。事实上,在“智能”底物中,产物的性质会根据反应机制的不同而显著变化。ICl–
HFP组合对于富电子底物的单电子氧化的启动至关重要,随后发生卤化。最显著的例子是富电子底物二
甲苯(1,2,4,5-四甲基苯,DUR)与美克苯(1,3,5-三甲基苯,
MES)的比较:在一个“传统”的
碘化体系(即I2/Ag+)和常见溶剂中,极性机制主导,二
甲苯的反应性较低(k
MES/kDUR = 46 ± 3),但在
HFP与ICl条件下,变得更加活泼(k
MES/kDUR = 0.23),此时ET机制占主导地位。其他底物也支持在
HFP中ET路径的启动。最后,对通过
漆酶诱导的
生物卤化反应的初步调查表明,在少数底物中,极性
碘化过程的发生相对较少。