Dioxygen Transfer from 4a-Hydroperoxyflavin Anion. 4. Dioxygen Transfer to Phenolate Anion as a Means of Aromatic Hydroxylation

Article abstract of DOI:10.1021/ja00372a028

Potassium 2,6-di-tert-butylphenolate (1^-) in the presence of N⁵-ethyl-4a-hydroperoxy-3-methyllumiflavin anion (4a-FlEtOO^-) yields (30 deg C, absolute t-BuOH; anaerobic) 2,6-di-tert-butylbenzoquinone (2), 4,4'-dihydroxy-3,3',5,5'-tetra-tert-butylbiphenyl (4), and 3,3',5,5'-tetra-tert-butyl-4,4'-diphenoquinone (5).The 4a-FlEtOO^- is converted in turn to N⁵-ethyl-3-methyllumiflavin radical (FlEt^.) and 1,5-dihydro-N⁵-ethyl-3-methyllumiflavin anion (FlEt^-).Kinetic and product studies establish the sequence of eq A to be competent.The product 4 is proposed to arise by rearrangement of 3,3',5,5'-tetra-tert-butyl-<1,1'-bi-2,5-cyclohexadiene>-4,4'-dione (3), which is known to be a product of dimerization of 1.In separate experiments, the rearrangement 3 -> 4 has been shown to occur (absolute t-BuOH) spontaneously (k = 1.3 * 10^-4 s^-1) and to be catalyzed by t-BuO^-K⁺ (k = 3.5 * 10³ M^-1 s^-1).The transfer of the peroxy substituent from 4a-FlEtOO^- to 1^-, yielding the quinone 2 and FlEt^-, undoubtedly occurs via the cyclohexadienone peroxide anion (as shown).This is supported by the finding that the rate constant for conversion of 4a-FlEtOO^- to reactive intermediate (X) is the same (0.37 s^-1) as found previously for peroxidation of other ambident nucleophiles by 4a-FlEtOO^- (i.e., 0.37₅ +/- 0.01₆ s^-1).Of possible relevance to the mechanism of dioxygen transfer are the findings that 1^- undergoes 1e^- oxidation by FlEt^. (k = 2.45 * 10⁴ M^-1 s^-1) (products FlEt^- + 1/2 4), a feature shown by other ambident nucleophilic substrates, and that the second-order rate constant for reaction of 1^- with ³O2 (k ca. 0.9 M^-1 s^-1) is too small for this to be of importance in the formation of 2.It is pointed out that, since dioxygen transfer from 4a-FlEtOO^- to 1^- yields reduced flavin (FlEt^-) and a quinone (2) and since FlEt^- reduces quinones to hydroquinones, the oxygen-transfer mechanism could serve as a means of hydroxylation of phenols.