ACTIVATION AND TRANSFER OF OXYGEN-XVI. SPONTANEOUS ELECTRON TRANSFER IN FLAVINIUM SALT SOLUTIONS. A NEW PRINCIPLE FOR THE DEVELOPMENT OF OXYGEN ACTIVATION MODELS

Article abstract of DOI:10.1016/S0040-4020(01)91587-X

Blue coloured 10,10^a-ring opened intermediates 5^a,5^b arising in the autoxidation of a dihydroflavin model (Scheme 1) are also formed on proper treatment of some N¹-alkylflavinium salts 7 (Scheme 2).The conditions giving optimal 10,10^a-ring opening have been determined (Figs. 1-2).The flavinium salts (RFl_ox⁺, A^-) show spontaneous electron transfer in the dark, producing a flavosemiquinone (RFl^.) and a counter-radical, probably a radical cation (RFl-A^+.) derived from a flavin adduct (pathway b, Scheme 3).The formation of a neutral, unstable 1-RFl^. appeared from the spontaneous N¹⁰-dealkylation (pathway d₂) competing with the O2-activation (pathway c₁).A generation of CO2 may occur wich indicates the formation of an unstable acyloxy radical (A^.) by a decomposition of RFl-A^+. (pathway d₃).Cl3CCOO^- can even be catalytically decomposed by RFl_ox⁺.This proves that RFl_ox⁺ is recycled from the RFl^.-state also (pathway d₁) for wich O2 is not a prerequisite.On the other hand, Cl3CCOO^- is "repaired" under conditions giving a 10,10^a-ring opening (Fig 3).The preservation of the acid anion and the results of the O2-balance are consistent with the conclusion that the 10,10^a-ring opening is coupled with or followed by an electron transfer from a peroxy radical to RFl-A^+. giving a generation of O2 (Scheme 4).A 10,10^a-ring opened hydroperoxide 5^a (XH=OOH; Scheme 1) is proposed to be the result of a similar one-electron transfer reaction (A=OOH; Scheme 4).