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Dioxygen Transfer from 4a-Hydroperoxyflavin Anion. 3. Oxygen Transfer to the 3-Position of Substituted Indoles
Muto, Shigeaki,Bruice, Thomas C.
, p. 7559 - 7564 (2007/10/02)
The 4a-hydroperoxyflavin anion (4a-FlEtO2(-)) spontaneously decays with a rate constant of 4.2E-2 s-1 (tert-butyl alcohol, 30 deg C).In the presence of the anions of 2,3-dimethylindole (1a-) and 5-methoxy-3-methyl-2-phenylindole (2a-) the pseudo-first-order rate constant (Kobsd) for disappearance of 4a-FlEtO2(-) increase.Plots of 1/kobsd vs. 1/-> and 1/-> are linear, and the limiting rate constants for 4a-FlEtO2(-) disappearance at high values of -> and -> were calculated, from the intercepts, to be 0.33 and 0.37 s-1, respectively.In a previous study the limiting rate constants were 0.36 and 0.37 s-1 when the anions of 2,6-di-tert-butyl-4-methylphenol and 3,5-di-tert-butylcatechol were employed.This limiting rate constant of 0.36 s-1 is assigned as the forward rate for the conversion of 4a-FlEtO2(-) (in an endothermic equilibrium) to a species (X) which, on being trapped by substrate anion, transfers a peroxy moiety to the trapping agent.The yields of the dioxygen-transfer products formed from 1a- and 2a- are 24percent and 41percent, respectively.The singlet oxygen-trapping agents, 2,5-dimethylfuran and tetramethylethylene, do not increase the rate of disappearance of 4a-FlEtO2(-) and, therefore, do not trap X.Species X cannot be solvent separated 1O2 and FlEt(-).Moreover, the rate constants for reaction of triplet oxygen with 1a- and 2a- are 1E3 - 1E4 too small for X to be solvent separated 3O2 and FlEt(-).The possibility of X being solvent separated FlEt* and O2(-*) is considered.Possible identities for X include a complex of an oxygen and a flavin species and a 4a,10a-dioxetane of reduced flavin.
