On the Structures of the Intermediates from Reversible Coupling between Hindered Phenoxy Radicals

Article abstract of DOI:10.1016/0040-4020(95)00332-3

Hindered phenoxy radicals 1 and 2 are found to undergo reversible, C-C rather than C-O cross-coupling, and give bis(cyclohexadienone)s 14 and 17.These primary products are not isolable but are recovered as phenolic cyclohexadienones 15 and 18, respectively, after treatment with Et3N or as biphenols 16 and 12, respectively, after treatment with TFA.The other products obtained after treating the reaction mixture with Et3N or TFA are phenol 5 and 4,4'-diphenoquinone 13 alone.Dienones 14 and 17 are interconvertible with each other via dissociation into the parent radicals, and 14 appears to be thermodynamically more stable than 17.Phenoxy radical 1 and other, less hindered 2,6-dialkylphenoxy radicals 24 also form intermediates of reversible cross-coupling.Treatment with TFA of the mixtures containing the intermediates provides 2,4'-biphenols 25 preferentially.