Mechanism of N(5)-Ethyl-flavinium Cation Formation Upon Electrochemical Oxidation of N(5)-Ethyl-4a-hydroxyflavin Pseudobase

Abstract

We investigated the oxidation behavior of 5-ethyl-4a-hydroxy-3-methyl-4a,5-dihydrolumiflavin (pseudobase Et-FlOH) in acetonitrile with the aim of determining if the two-electron oxidized Et-FlOH(2+) undergoes a release of hydroxyl cation and the production of 5-ethyl-3methyllumiflavinium cation (Et-Fl(+)). The focus of this work is to investigate the possibility of using Et-FlOH as a catalyst for water oxidation. The cyclic voltammetry demonstrates that Et-FlOH exhibits two one-electron oxidation potentials at +0.95 and +1.4 V versus normal hydrogen electrode (NHE), with the second oxidation potential being irreversible. The production of Et-Fl(+) is observed in the cyclic voltammetry of Et-FlOH and has been previously assigned to the release of OH(+) from the two-electron oxidized Et-FlOH(2+). The results of our study show that this is not the case: (i) we performed bulk electrolysis of the electrolyte solution at +2 V and then added Et-FlOH to the electrolyzed solution. We found that Et-Fl(+) is produced from this solution, even though Et-FlOH itself was not oxidized; (ii) reactions of Et-FlOH with chemical oxidants (ceric ammonium nitrate, nitrosyl tetrafluoroborate, and tetrabutylammonium persulfate) demonstrate that Et-Fl(+) production occurs only in the presence of strong Lewis acids, such as Ce(4+) and NO(+) ions. On the basis of these results, we propose that the production of Et-Fl(+) in the electrochemistry of Et-FlOH occurs because of the shift in the Et-FlOH/Et-Fl(+) acid-base equilibrium in the presence of protons released during anodic oxidation. We identified two sources of protons: (i) oxidation of traces of water present in the acetonitrile releases oxygen and protons and (ii) two-electron oxidized Et-FlOH(2+) releases protons located on the N(5)-alkyl chain. The release of protons from Et-FlOH(2+) was confirmed by cyclic voltammetry of Et-FlOH in the presence of pyridine as a base. The first oxidation peak of Et-FlOH at +0.95 V is reversible in the absence of pyridine. The addition of pyridine leads to the shift of the oxidation potential to a less positive value, which is consistent with a proton-coupled electron transfer (PCET). Furthermore, the anodic current increases, and the cathodic peak becomes irreversible, giving rise to two additional reduction peaks at -0.2 and -1 V. The same reduction peaks were observed in the high scan rate cyclic voltammogram of Et-FlOH in the absence of pyridine, implying that the release of protons indeed occurs from Et-FlOH(2+). To determine which functional group of Et-FlOH(.+) is the source of protons, we performed DFT calculations at the B3LYP/6-311++G** level of theory for a reaction of Et-FlOH(.+) with pyridine and identified two proton sources: (i) the >N-CH(2)- group of the N(5) alkyl chain and (ii) the -OH group in the 4a-position of the radical cation. Because the appearance of new reduction peaks at -0.2 and -1.0 V occurs in the model compound that lacks -OH protons (Et-FlOMe), we conclude that the proton removal occurs predominantly from the >N-CH(2...

Thermolysis and photolysis of 2-ethyl-4-nitro-1(2H)-isoquinolinium hydroperoxide

“…The mechanism of IQ + formation is currently not known, but we postulate that it originates due to the shift in the IQOOH/IQ + equilibrium (IQOOH + H + → IQ + + H 2 O 2 ) due to the release of a proton during the decomposition of IQO radical. We observed similar behavior during the electrochemical oxidation of pseudobases of flavinium ion, which have the pseudobase pKa value of 3.7 . Since the IQOOH/IQ + conversion is expected to occur at the pH = 5.03, the proton removal from IQO .…”

“…This manuscript reports the room‐temperature (RT) thermolysis of one such heterocyclic hydroperoxide, derived from the isoquinolinium ion derivative. In specific, as part of our ongoing interest in organocatalytic water oxidation by iminium ions, the isoquinolinium hydroperoxide (IQOOH) was prepared (Fig. ), and its thermal and photochemical behavior was studied using steady‐state UV/vis and NMR spectroscopy, femtosecond transient absorption (fs TA) laser spectroscopy and density functional theory (DFT) calculations.…”

Novel Fully Organic Water Oxidation Electrocatalysts: A Quest for Simplicity

“…In the mechanism proposed, 9 the EtFl + catalyst adsorbed on the glassy carbon electrode surface is oxidized to a radical dication first, followed by the formation of a pseudo-base upon hydroxylation of the iminium carbon (position C4a) as the key step for oxygen evolution. The contrasting reactivities of EtFl + and Acr + suggest that a highly electrophilic iminium carbon (C4a in EtFl + ) is crucial to the catalytic activity.…”

Flavin Derivatives with Tailored Redox Properties: Synthesis, Characterization, and Electrochemical Behavior

“…The few studies on synthetic flavins, on the other hand, were directed toward understanding substituent effects on redox potentials by theoretical and experimental means. [22,28,43,46] The presents tudy,i n contrast, differentiatesi tself from the extensive prior body of work on natural flavins by focusing on freely-diffusing synthetic flavin molecules, andt heir redox properties, especially from electrocatalysis andp hotocatalysis perspectives. Thus this study focuses on the reductive (i.e.,c athodic) electrochemical behavior of four synthetic flavin derivatives in aqueous media.…”