A Mechanistic Approach to the Reaction of 2,6-Di-<i>tert</i>-butylphenol with an Iodinating Agent in Methanol: Electrophilically Assisted Solvolysis of Intermediary 4-Iodocyclohexa-2,5-dienones
作者:Kanji Omura
DOI:10.1021/jo951455n
日期:1996.1.1
Reactions of the title phenol (1) and of 4-iodophenol 2 with an iodinating agent, I-2 and H2O2, are conducted in MeOH for varying times with varying amounts of I-2, and the results are compared, The reaction of 1 gives 2, 4,4'-biphenol 3, 4,4'-diphenoquinone 4, 4-methoxyphenol 5, and p-benzoquinone 6, exclusively. The yields of the phenolic products (2, 3, and 5) vary with reaction time, but they disappear or almost disappear eventually, to make 4 and 6 the almost exclusive products, The reaction of 2 always gives 4 and 6 alone. In both of the reactions of 1 and of 2, employment of a higher initial It concentration not only completes the formation of 4 and 6 faster but also makes the final proportion of 6 higher. However, the ultimate yield of 6 from the reaction of 1 is significantly higher than that from the reaction of 2, irrespective of the initial I-2 concentration. These results are interpreted as follows. 4-Iodocyclohexa-2,5-dienone 12, the primary product of electrophilic iodination of 1, undergoes solvolysis (methanolysis), which is electrophilically assisted by I-2. The solvolysis of 12 can be so fast as to overwhelm its prototropic rearrangement to give 2. 4-Methoxycyclohexa-2,5-dienone 13, which is the primary product of the methanolysis of 12 and is suggested to be detectable by H-1 NMR spectroscopy, is converted into 6 via 5, Benzoquinone 6 can also arise from 4,4-diiodocyclohexa-2,5-dienone 7, the product of iodination of 2, by an analogous mechanism. The selectivity of the formation of 6 from 7 is low because the competing reaction, homolytic scission of the C-I bond in 7, predominates. The mechanism of the formation of 3 and 4 is also discussed.