13693-18-0Relevant academic research and scientific papers
Electron transfer between protonated and unprotonated phenoxyl radicals
Omura, Kanji
, p. 858 - 867 (2008/09/19)
(Chemical Equation Presented) The reaction of phenoxyl radicals with acids is investigated. 2,4,6-Tri-tert-butylphenoxyl radical (13), a persistent radical, deteriorates in MeOH/PhH in the presence of an acid yielding 4-methoxycyclohexa-2,5-dienone 18a and the parent phenol (14). The reaction is facilitated by a strong acid. Treatment of 2,6-di-tert-butyl-4-methylphenoxyl radical (2), a short-lived radical, generated by dissociation of its dimer, with an acid in MeOH provides 4-methoxycyclohexa-2,5-dienone 4 and the products from disproportionation of 2 including the parent phenol (3). A strong acid in a high concentration favors the formation of 4 while the yield of 3 is always kept high. Oxidation of the parent phenol (33) with PbO2 to generate transient 2,6-di-tert-butylphenoxyl radical (35) in AcOH/H2O containing an added acid provides eventually p-benzoquinone 39 and 4,4′-diphenoquinone 42, the product from dimerization of 35. A strong acid in a high concentration favors the formation of 39. These results suggest that a phenoxyl radical is protonated by an acid and electron transfer takes place from another phenoxyl radical to the protonated phenoxyl radical, thus generating the phenoxyl cation, which can add an oxygen nucleophile, and the phenol (eq 5). The electron transfer is a fast reaction.
The Reaction of the 2,6-Di-tert-butyl-4-methylphenoxy Radical with Phenols
Omura, Kanji
, p. 921 - 927 (2007/10/02)
Dimer 3 dissociates in solution to give the phenoxy radical 4.When solid 3 is dissolved in a solution of phenol 11a, radical 4 dehydrogenates the phenol to afford phenol 5 and phenoxy radical 12.Radical 12 couples with more 4 to afford principally bis(cyclohexadienone) 16a and a minor amount of the dimeric bis(cyclohexadienone) 13.The preferential formation of 16a over 13 is explained as the result of a solvent-cage reaction.Compounds 13 and 16a are isomerized in the presence of silica gel or triethylamine to 4,4'-dihydroxybiphenyl 14 and phenolic dienone 17a, respectively.Compound 13 is relatively stable in hexane at 30 deg C, while 16a slowly dissociates under these conditions to afford parent radicals 4 and 12.If 11a is present, radical 4 either recombines with 12 or dehydrogenates the phenol.Two radicals of 12 generated by the dissociation of 16a and the subsequent dehydrogenation of 11a couple to give 13.Therefore, dissolving 3 in hexane containing excess 11a and keeping the resulting solution at 30 deg C results in the preferential formation of 13.Dissolving 3 in triethylamine containing phenols 11 or 21 similarly affords dienones 17 or 23.The yields of these dienones increase with increasing electron-donating capability of the substituent of the phenol ring.
