ECEC mechanism

The first electrochemical H2 generation catalyzed by a hetero-nuclear Fe-Ni complex [Ni(L)Fe2(CO)g] (27) [L - = (CH3C6H3S2)2(CH2)3 ] (Fig. 9) with tri-fluoroacetic acid was reported by Schoder and coworkers in 2006 [211]. Based on their electrochemical behavior, spectroscopic data, and DFT calculations of 27, an EECC mechanism was mled out and therefore an ECCE or ECEC mechanism involving the formation of Fe°-H and Ni -H intermediates is likely. In this cycle, six catalytic turnovers were achieved. This value is comparable to those for... 

Radical cations resulting from oxidation of olefins, aromatic compounds, amino groups, and so on, can react by electrophilic addition to a nucleophilic center as shown, for example, in Scheme 1 [2, 3]. The double bond activated by an electron-donating substituent is first oxidized leading to a radical cation that attacks the nucleophilic center. The global reaction is a two-electron process corresponding to an ECEC mechanism. 

Anodic oxidations can produce cationic species that react with nucleophiles. For example, the oxidation of Af-phenylpiperi-dine performed in the presence of cyanide ions affords an o -aminonitrile according to an ECEC mechanism (Scheme 12) [17]. 

The electrooxidation of A,A-diacetyldi-hydroquinoxaline involves a cleavage of the nitrogen-acylcarbon bonds according to an ECEC mechanism (Scheme 92) [139]. 

One postulated reaction mechanism for electrochemical fluorination involves an intermediate nickel fluoride, with nickel in the oxidation stage +III/ + IV, as the active fluorination agent. The induction period in which the nickel fluoride layer is formed at the nickel surface can thus be explained. A radical fluorination mechanism has also been postulated, with oxidation of the fluoride anion to the radical, or as discussed below in the ECEC mechanism.15 The mechanism of this process is still a matter for debate. Reference should be made to a report that does not support the postulates of this section.21 For partial electrochemical fluorination, the ECEC mechanism is postulated as follows. In the first step the starting material is oxidized at the anode (E = electrochemical step). 

The intermediary radical species is formed by an ECEC mechanism, and the first electroreduction takes place at the isoquinolinium moiety. 

The first-formed intermediate (with an impaired electron) in combination with a second Lewis acid molecule has even greater electron affinity, and is reduced at a more positive potential to give a voltammogram that appears to be the result of an irreversible two-electron reduction process. In most cases it is an ECEC process in which each electron transfer (E part of the ECEC mechanism) to the Lewis acid (H30+) is reversible to give a product (H-) that forms a covalent bond with the substrate (H—Eh) (the C part of the mechanism). 

Generation of substituted aryl radical cations in the presence of nucleophiles can lead to products of side-chain substitution (processes such as anodic benzylic substitution of toluenes, which are dealt with in a separate chapter) or to products of addition to the aromatic ring itself. Nuclear addition products in /j /m-substituted systems have been proposed to form in essentially one of two ways, depending on substitution pattern and reaction conditions. Radical cations formed by electrochemical reaction (E) may be trapped by chemical reaction (C) with a nucleophile (or its anion). Repeating this sequence leads to nuclear addition products (LXV), formed by what is referred to as the ECEC mechanism [Eq. (31)] [74]. An analogous pattern may be inferred for or / (9-substituted systems. 

Furan is, as a 7r-electron rich compound, easily oxidized electrolytically. In acetonitrile the oxidation potential of furan is comparable to that of anisole [35]. In methanol the oxidation is an ECEC mechanism in which 2,5-dihydro-2,5-dimethoxyfuran is formed [166] in the Clauson-Kaas reaction ... 

Since the discharge potential of fluoride ions is extremely high (> 2.9V vs. SCE at Pt in acetonitrile [9]), an ECEC mechanism involving initial formation of a cation radical followed by attack of fluoride ions is often suggested ... 

The direct anodic oxidations of phenylacetylene, vinylacetylene and 1,4-diphenylbutadiyne were compared with the oxidation of the respective alkenes in MeCN at a carbon anode. The measured E 1/2 vs SCE were ca 300-600 mV more positive than those of the corresponding alkenes and the oxidation was completely irreversible for phenyl- and vinylacetylenes but only nearly reversible for 1,4-diphenylbutadiyne. The difference in 1/2 for the former two alkynes, determined on several anode materials, was interpreted as involving adsorbed substrates for the first charge transfer of an ECEC mechanism yielding an intermediate... 

The electrochemical reaction-chemical reaction-electrochemical reaction-chemical reaction (ECEC) mechanism is well established for anodic methoxylation reactions of amines, amides, and carbamates. It was confirmed that the anodic methoxylation of fluoroal-Iq l sulfides also proceeds with by way of a typical ECEC mechanism as shown in Scheme 32. ... 

The time windows of experiments were tuned to the half-life of the benzidine rearrangement (t,/2 = 0.06 s in 1.688 M HCIO4. Numerous other mechanisms have been followed by this method, e.g., ECdimEQispi mechanism [90] or ECECE mechanism [91]. 

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