Electroreduction direct

Shukla A, Neergat M, Parthasarathi B, Jayaram V, Hegde MS. 2001. An XPS study on binary and ternary alloys of transition metals with platinized carbon and its bearing upon oxygen electroreduction in direct methanol fuel cells. J Electroanal Chem 504 111-119. 

Anson FC, Shi C, Steiger B. 1997. Novel multinuclear catalysts for the electroreduction of dioxygen directly to water. Acc Chem Res 30 437. 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

In contrast to Co-porphyrin complexes, the direct four-electron reduction of 02 has been only very rarely claimed to be catalyzed by a cobalt phthalocyanine 404 407 In particular cofadal binuclear Co-Pc complexes immobilized on pyrolytic graphite catalyze only the two-electron electroreduction of 02 to H202.408,409 However, recent work has established that an electropolymerized Co-Pc derivative provides a stable four-electron reduction pathway over a wide pH range 410... 

Electrochemical methods are available for the direct dehalogenation of organic halides to a limited extent fluorides and monochlorides are generally not reducible [1], In the presence of transition-metal complexes as mediators (Med), however, the electrolysis of halocarbons (RX) can be performed more effectively and selectively under various conditions [155-158]. Mediated electroreduction is most efficient when the electron transfer step E° (Med/Med -) is more negative than E° (RX/RX -) [157] (cf. Section 18.4.1). 

Direct dyes, 9 171-172, 226, 242 azo, 9 401 411 Direct dyestuffs, 9 223 Directed evolution, 10 264 Direct Electroreduction of Oxides (DERO), 23 831... 

Although the electroreduction of nonac-tivated alcohols requires appropriate activation of hydroxyl groups as mentioned above, activated ones such as allylic and benzylic alcohols can be reduced by direct electroreduction (Scheme 3) [1, 7-9]. 

Because of the highly negative reduction potentials ( —3.0 V vs. SCE) [32], the electroreduction of esters of aliphatic carboxylic acids to primary alcohols by direct electron transfer from the cathode is very difficult and the electrochemical Birch-type reduction of aliphatic esters in MeNH2 or liquid NH3 has not been reported until recently (Scheme 15) [33, 34]. This reaction is not a reduction by direct electron transfer from the cathode to the C=0 bonds of the ester but the reduction by a solvated electron. 

A new electrolysis system comprising two metal redox couples, Bi(0)/Bi(III) and A1(0)/A1(III), has been shown to be effective for electroreductive Barbier-type allylation of imines [533]. The electrode surface structure has been correlated with the activity towards the electroreduction of hydrogen peroxide for Bi monolayers on Au(III) [578], Electroreductive cycliza-tion of the 4-(phenylsulfonylthio)azetidin-2-one derivative (502) as well as the allenecarboxylate (505) leading to the corresponding cycKzed compounds (504) and (506) has been achieved with the aid of bimetallic metal salt/metal redox systems, for example, BiCh/Sn and BiCh /Zn (Scheme 175) [579]. The electrolysis of (502) is carried out in a DMF-BiCh/Py-(Sn/Sn) system in an undivided cell by changing the current direction every 30 s, giving the product (504)in 67% yield. 

Ni-salen is a good alternative to nickel-cyclam to generate Ni intermediates. Indeed the cyclisation of 6-phenyl-l-halo-pent-5-yne occurs very efficiently in the presence of 10% of catalyst, as compared to the direct electroreduction of the reagent (Table 7, entry 8) [78]. 

The catalyst is not necessary either for the electrocarboxylation of aryl halides or various benzylic compounds when conducted in undivided cells and in the presence of a sacrificial anode of aluminum [105] or magnesium [8,106], Nevertheless both methods, i.e., catalysis and sacrificial anode, can be eventually associated in order to perform the electrocarboxylation of organic halides having functional groups which are not compatible with a direct electroreductive process. 

Work by Ono et al. [66] has been specifically directed at ultrasonic control of product-selectivity in electroreductions. Using a lead cathode, in dilute methanolic sulphuric acid, at a constant current of 20 mA cm , Ono electroreduced benzaldehyde under stirred, unstirred and ultrasonic conditions (Fig. 6.17). In an unstirred system, benzyl alcohol (two-electron process) was the major product, while mechanical stirring reversed the position in favour of the hydrodimer (one-electron product). Ultrasonic irradiation from a cleaning bath (100 W, 36 kHz) so strongly favoured the hydrodimer that the alcohol was barely evident (Tab. 6.16). 

Hydroxymethylpyridines are reduced directly to alkylpyridines in the presence of samarium diiodide <99TL8823>. The electroreduction of pyridinedicarboxylic acid derivatives to 1 -and 1,4-dihydropyridines has been achieved in good yield <99TL8587>. 

These results indicate that zinc ions formed by oxidation of the anode do not play a part or only have side effects in the direct electroreductive carbon—carbon bond formation carried out with a zinc anode and a nickel catalyst. In these reactions, a nickel organometallic is involved. 

So the product, R, of the electrochemical reduction reacts in the solution with an electroinactive oxidizer, Ox, to regenerate O, etc. If Ox is present in large excess, the chemical reaction is pseudo-first-order in R and O. For thermodynamic reasons, Rc can only proceed if the standard potential of the redox couple Ox/Red is more positive than that of O/R. Then, for Ox to be electroinactive, it is required that its electroreduction proceeds irreversibly, in a potential range far negative to the faradaic region of the 0/R reaction. Thus, Ox being stable for reasons of the slow kinetics of its direct reduction, it can be said that, in the presence of O, it is being catalytically reduced. 

Little s group has reported electroreductive cyclizations involving ,/l-unsaturated esters (equation 34)96. Based upon CV measurements, the conjugated ester rather than the aldehyde or ketone is suggested to be the electron acceptor. Although direct cyclization of... 

As in the case of Ru(bpy)3 +, luminescence was not observed at potentials more positive than the fluorophore Reduction wave where only the small amount of direct reduction of S2O8 appears (Figure 2c) (12). The similarity between the intensity-potential profile and the reduction voltammogram indicates that the mechanism which leads to luminescence is intimately associated with the electroreduction of the fluorophore. 

The OER is a multi-electron reaction which may include a number of elementary steps and involve different reaction intermediates. There are several pathways for 02 electroreduction (1) a direct four-electron reduction to HzO (in low pH media) or to OH- (in high pH media) (2) a two-electron pathway involving reduction... 

The direct electroreductive conversion of cephalosporin p-nitrobenzyl esters into their corresponding acids has been carried out in a DMF-10% H2S04—(Hg) system... 

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