Electrooxidation carboxylates

A substrate containing an amine carboxylate moiety is converted in an electrolyte solution in the presence of a strong acid to a cationic intermediate, an N-acyliminium cation, by electrooxidative reaction. This species is immediately reacted with an allylsilane [66, 67]. By nucleophilic reachon, C-C bond formahon is achieved. 

Similar experiments performed with other aldehydes and primary alcohols indicate that their electrooxidation on platinum produce primarily the corresponding carboxylic acid in addition to C02,... 

We have also investigated the electrooxidation of phenylethanoate, a system where there is no proton-loss pathway from the intermediate carbocation. Tab. 6.14 shows relative product ratios for phenylacetate in similiar conditions to those used for cyclohexane carboxylate, but employing 100 mA cm current density [59,60]. 

Column III shows the effect of ultrasound upon the product ratio with methanol as solvent. As can be seen there is now 53 % bibenzyl, 32 % of methyl ether and 6% of methyl ester (with a total of 5 % of other products) suggesting a slight shift towards the two-electron products, but with an overall diminuition of solvent discharge (approx. 6% ester) and side-reactions (approx. 6%). This result confirms the fact the phenyl acetate electrooxidation favours the one-electron route (to bibenzyl) in a wide range of conditions [61], and is much less sensitive to mechanistic switches by manipulation of parameters (e. g. ultrasound) than is cyclohexane carboxylate electrooxidation [54]. 

We have also examined the use of higher ultrasonic frequencies (500 kHz and 800 kHz) and found the trend in product distribution from carboxylate electrooxidation at platinum electrodes in methanol to be the same as under sonication in the 20 kHz to 40 kHz frequency range. However, we obtained better yields in spite of the usual reduced cell voltage requirements in the presence of ultrasound. There also seemed to be fewer of the numerous low-yield methoxylated species and other side-products. 

The reduction of benzoic acid at a lead cathode in aqueous sulphuric/citric acids yields the two-electron products benzaldehyde and the four-electron product benzyl alcohol rather than one-electron hydrodimer. In all cases studied by the authors they found that ultrasound favoured the process involving the smaller number of electrons per molecule. This is the opposite of the sonoelectrochemical effect seen in carboxylate electrooxidation [57,59,60] where the process involving the greater number of electrons was favoured by ultrasound. 

We have previously considered [8,9] the redox behaviour and film structure of indole-5-carboxylic acid trimer (ICA trimer) films formed by the electrooxidation of indole-5-carboxylic acid (ICA, Fig. 11.2). We have shown that electrooxidation involves the production and deposition of an asymmetric cyclic trimer (Fig. 11.3) onto the electrode surface... 

Primary and secondary alcohols are oxidized to the corresponding carboxylic acids and ketones, respectively (Eqs. 3.41 and 3.42) [4c, 77]. Electrooxidation using a double mediatory system consisting of RUO4/RUO2 and C /Cr redox couples is also effective for oxidation of alcohols (Eq. 3.43) [77e]. 

Not only methyl groups but practically any other alkyl groups attached to aromatic rings are oxidized to carboxyls by sufficiently strong oxidants, such as nitric acid [460, 461, 462, 463, 464, 891], chromic acid and its derivatives [550, 551, 624, 633, 634, 1129, 1130], and potassium permanganate [503, 841, 880, 881, 882, 883, 1131]. Occasionally, such oxidations have been effected by other reagents, such as ozone [68], sulfomonoperacid (Oxone) [205], sodium hypochlorite [696], and nickel dioxide [933], or by electrooxidation [117] (equation 181). 

A. Olefinic compounds Acetylenic compounds Aromatic compounds Carbonyl compounds F/c-Oxygen compounds Nitrogen compounds Sulfur compounds Halogen compounds Other heteroatom compounds Organometallic compounds Stereoselective and Stereospecific Electrooxidation A. Carboxylic acids Acetoxylation Methoxylation Acetamidation... 

Other procedural benefits in the electrooxidation of cyclohexane carboxylate under ultrasound include a drop in overall cell voltage from 8.3 to 7.3 V needed to maintain the same cd for the galvanostatic system. The reaction approached completion in a shorter time-span despite the apparent switch to the two-electron process, suggesting diminution of parasitic processes. 

Overall, ultrasound appears to favor the two-electron mechanism for the reaction, but the greatest effect of sonication upon product distribution was the substantial enhancement of alkene formation. Accordingly it was decided to examine a carboxylate electrooxidation system where there is no proton-loss pathway from the intermediate carbocation, namely using phenylacetate as a substrate. 

The origin of ultrasonic effect upon carboxylate electrooxidation is not straightforward to establish in view of the complex mechanism of the reaction with different kinetic regimes, the loss of carbon dioxide, and also the role of adsorption... 

It may be that ultrasonic enhancement of mass transport sweeps the intermediate radicals that have escaped the electrode back to the electrode surface where they are further oxidized, although this would depend upon radical lifetimes. Direct enhancement of the second electron transfer to give the cation while species remain in first contact with the electrode would be complicated by the decarboxylation step after the first electron transfer, and also by an observation of weak electrochemiluminescence (ECL) from the electrolysis cell in phenylacetate electrooxidation (see Section 5.2) [215]. This is enhanced by ultrasound, as are a number of other ECL systems. This suggests that at least a proportion of the reaction pathway involves benzyl radicals which escape the electrode, although the sonoelectrochemilumines-cence reaction conditions of low carboxylate concentration, low current density, and presence of electrolyte salt are different to those for the preparative electrolyses. 

A further example of an indirect electrooxidation of alkynes is their transformation into 1,2-diones when catalytic amounts of RUO4 are generated anodically from Ru02 in a two-phase system (CCU-saturated aqueous NaCl solution). Overoxidation to carboxylic acids is a minor reaction for disubstituted acetylenes, but terminal triple bonds are cleaved... 

Fig. 12 Possible reaction pathways for the Kolbe electrooxidation of carboxylic acids in an ultrasonically emulsified medium.

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