Dioxygen Transfer from 4a-Hydroperoxyflavin Anion. 2. Oxygen Transfer to the 10 Position of 9-Hydroxyphenanthrene Anions and to 3,5-Di-tert-butylcatechol Anion

Article abstract of DOI:10.1021/ja00533a028

The reaction of the peroxy anion of N⁵-ethyl-4a-hydroperoxy-3-methyllumiflavin (4a-FlEtO2^-) with the anions of 3,5-di-tert-butylcatechol (VIII), 10-ethoxy-9-phenanthrol (Ia), and 10-methyl-9-phenanthrol (Ib) has been investigated.All products may be accountable through the transfer of O2 from the 4a-FlEtO2^- reactant to the phenolate anions with the production of reduced flavin anion (FlEt^-) and a hydroperoxycyclohexadienone.From VIII^- (t-BuOH) there was obtained 3,5-di-tert-butyl-o-quinone (IX) and Ib^- yielded (t-BuOH or CH2CN) 10-hydroxy-10-methyl-9,10-dihydro-9-phenanthrone (IIIb), while Ia^- provided both 9,10-phenanthrenequinone (V) and monoethyl 1,1'-diphenate (IVa) (the ratio of V:IVa being solvent dependent).The mechanisms for the decomposition of intermediate peroxide anions to products are discussed.The conversion of Ia^- to IVa by oxygen transfer from 4a-FlEtO2^- amounts to a catalysis by FlEt^- of the reaction of ³O2 with Ia and serves as a biomimetic reaction of flavoenzyme dioxygenase.The kinetic for the reaction of VIII^- with 4a-FlEtO2^- require the formation of an intermediate.Since the rate constants for the reaction of both VIII^- and 2,6-di-tert-butyl-4-methylphenolate anion with 4a-FlEtO2^- are identical under saturating conditions by these phenolate ions, it is concluded that the intermediate is formed in a unimolecular reaction from 4a-FlEtO2^- (k = 0.36 s^-) as in eq 19.Dissociation of 4a-FlEtO2^- to FlEt^-+ O2 and reaction of phenolate ions with O2 may be discounted since the second-order rate constants for the reaction of phenolate ions with O2 are less than required for the kinetic competency of this process.Dissociation of 4a-FlEtO2^- to yield neutral flavin radical (FlEt^.) + O2^-. followed by reduction of FlEt^. by fenolate ion to provide FlEt^- and phenoxy radical with the coupling of the latter with O2^-. is also improbable.Thus, though the second-order rate constants for 1e^- reduction of FlEt^. by the various phenolate species are sufficiently large to allow the kinetic competency of this step, there exists no evidence that O2^-. can couple with any radical species to provide a hydroperoxide.The oxygen-donating intermediate formed from 4a-FlEtO2^- is suggested to be the 4a,10-dioxetane (XII) or an oxygen molecule more loosely associated with FlEt^-.The equilibrium constant for the formation of such an intermediate may be as small as 10^-5 if the rate of reaction of phenolate ion with this species approaches a diffusion-controlled process.