Proton electroreduction

Proton electroreduction catalyzed by metal complexes is different from reduction at a metal electrode. It definitely involves the formation of metal hydride species through protonation of electroreduced, low-oxidation-state metal complexes that function as Bronsted base (Equation (5)). From protonated... 

The influence of varying the electrode material upon proton electroreduction has been the subject of extensive study . This reaction can be regarded as inner-sphere in that it often involves a rate-limiting, proton-transfer step to form an adsorbed hydrogen atom . Although superficially different, the conventional treatment of substrate effects upon such reactions is similar to that presented here. Thus the marked increase in the rate constants seen for Hj evolution as the metal-hydrogen atom bond energy... 

Razavet M, Artero V, Fontecave M (2005) Proton electroreduction catalyzed by cobaloximes functional models for hydrogenases. Inorg Chem 44(13) 4786-4795. doi 10.1021/ic050167z... 

Complex 15 displays an electrochemically irreversible reduction at -1.98 V. We have described the proton electroreduction catalyzed by 15 in the presence of weak proton acid (HOAc) by cyclic voltammogram. The results show that the first reduction event at -1.98 V is significantly sensitive to the acid concentration. That is, the first reduetion event at -1.98 V is electrocatalytically active in terms of proton reduction. Bulk electrolysis shows that the total... 

Wang YX, Balbuena PB. 2004 Roles of proton and electric field in the electroreduction of O2 on Pt(lll) surfaces Results of an ab-initio molecular dynamics study. J Phys Chem B 108 4376-4384. 

It is well accepted7 that the electroreduction of proton to dihydrogen (Equation (1)) at metal surfaces proceeds via the Volmer-Heyrowsky-Tafel reaction mechanism depicted in Equations (2) (4). 

Square planar complexes [M(mru)2]3 (M = Rh, Co, mnt = dithiomalonitrile), generated by electroreduction of [M(mnt)2]2, are protonated to form [M(mnt)2I I]2 ", from which dihydrogen is evolved at a mercury electrode at potentials more negative than those of the reversible [M(mnt)2]2 3 redox couple.44... 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

In the CP-O2 complex, the CP surface is an electron density donor. For example, in the case of PANI, the bond orders in adsorbed 02 molecules decrease by about 30%, and the bond lengths L increase by about 24%. Thus, the adsorbed 02 molecules have a fairly high degree of activation and can readily interact with the protons. Hence, quantum-chemical analysis confirms the mechanism of O2 electroreduction and gives possibility to understand the reasons of PANI catalytic activity. 

A demonstration of the utility of the electroreductive cyclization reaction is provided by the formal total synthesis of the antitumor agent quadrone (16, Scheme 4) [17]. The first stage of the synthesis involved a controlled potential reduction of (9) in the presence of dimethyl malonate as the proton donor. An efficient cyclization ensued, leading to the formation of the y-hydroxy ester (10)... 

Indirect electroreduction of allyl alcohols leading to the corresponding unsaturated hydrocarbons is attained using a mercury electrode in a strongly acidic medium containing an iodide salt [554]. The reaction involves transformation of the alcohol into the iodide, the reaction of the iodide with mercury, the protonation of the... 

The mechanism of the electroreductive cyclization reaction has been studied in some detail [22], The initial thought was that it occurred via the cyclization of the radical anion derived, for example, from 25 in the first reduction step. A moment s reflection, however, reveals that there are many more mechanistically viable pathways, especially when one realizes that the transformation involves five steps - two electron transfers (symbolized below by e and d , the latter corresponding to a homogeneous process), two protonations ( p ), and cyclization ( c ). In principle, these could occur in any order, and with any one of the steps being rate-determining. 

The electroreductive cyclization reaction of 6-heptene-2-one 166, producing CIS-1,2-dimethylcyclopentanol 169, was discovered more than twenty years ago [166]. In agreement with Baldwin s rules, the 5-exo product is obtained in a good yield. Since that time, the mechanism of this remarkable regio- and stereoselective reaction has been elucidated by Kariv-Miller et al. [167-169]. Reversible cyclization of the initially formed ketyl radical anion 167 provides either the cis or the trans distonic radical anion. Subsequent electron transfer and protonation from the kinetically preferred 168 leads to the major cis product 169. The thermodynamically preferred 170 is considered as a source of the trace amounts of the trans by-product 171 (Scheme 32). 

Yamagnchi et al. described an interesting extension of the CD process for CdS ns-ing a parallel electrochemical step [66]. They termed this process electrochemi-cally induced chemical deposition. It is based on electroreduction of protons in so-Intion, which resnlts in an increase in pH locally at the electrode. They nsed thioacetamide as a snlphnr sonrce. In the acid solutions in which the deposition is carried ont (pH between 1.6 and 4.6), no film deposition of CdS occurs (although it does precipitate in the solntion) in the absence of the electrochemical proton re-dnction. In the presence of proton rednction, CdS films were formed. These films... 

An alternative description of the one electron reduced product of NP was earlier associated with the protonation of nitrosyl (20,57a). Further electrochemical studies discarded the onset of this species in the overall electroreduction of NP (57e). The results of... 

In the case of a preceding chemical step such as the electroreduction of proton from the dissociation of a weak acid... 

Other metals, the performances of which as cathodes in the electroreduction of C02 have already been described in detail, were suggested for the development stage. Thus, Delacourt et al. [87] proposed an electrochemical cell, the design of which was taken from proton exchange membrane fuel cell (PEMFC) technology,... 

On the other hand, an electroreductively induced anionic chain reaction system has been found to be highly useful as a synthetic reaction 35). In this reaction system, cathodic reduction of a halide AX forms an anion A- in the first step, A- attacks an electrophile B to yield an intermediate AB, and AB abstracts a proton from AH to regenerate the first anion A-. 

For example, electroreduction of 61 in DMF/E NCIQ containing acetic acid as a proton source gives the hydrogenation product 62a (98-100%) while electroreduction of 61 in C02-saturated media affords carboxylic acid 62b (81-95%)39). Acetylation at the a/pfei-carbon atom is also achieved by the electroreduction of 61 in the presence of a 10-fold excess of acetic anhydride (30-70%)40). 

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