Anodic oxidation mediators

Electroorganic synthesis, 40 151-167 anodic oxidations mediated by ledox coatings, 40 153-157... 

In the synthesis of an euglobal skeleton, a quinone methide has been generated Y Y in situ by anodic oxidation mediated... 

Cycloaddition Anodically generated phe-noxy cations, o-quinones, and o-quinone methides react with olefins to bicyclic and tricyclic annelated compounds in stereoselective cycloadditions [250-252]. In the synthesis of a Euglobal skeleton, a quinone methide has been generated in situ by anodic oxidation mediated by DDQ. The cycloaddition was promoted by the use of lithium perchlorate... 

An alternative to the direct anodic oxidation of organic contaminants are the methods of indirect oxidation with the aid of oxidizers formed electrochemically in situ. These oxidizers (or mediators) can be obtained in both anodic and cathodic processes. Anodic agents are the salts of hypochloric acid (hypochlorites), the permanganates, the persulfates, and even ozone. 

To undertake oxidation of both cyclic and acyclic hydroxylamines to nitrones, an electrochemical oxidative system has been developed, where WC>42-/WC>52-are used as cathodic redox mediators and Br /Br2 or I—/I2 as anodic redox mediators (129-131). 

Because the direct electrochemical oxidation of NAD(P)H has to take place at an anode potential of + 900 mV vs NHE or more, only rather oxidation-stable substrates can be transformed without loss of selectivity—thus limiting the applicability of this method. The electron transfer between NADH and the anode may be accellerated by the use of a mediator. At the same time, electrode fouling which is often observed in the anodic oxidation of NADH can be prevented. Synthetic applications have been described for the oxidation of 2-hexene-l-ol and 2-butanol to 2-hexenal and 2-butanone catalyzed by yeast alcohol dehydrogenase (YADH) and the alcohol dehydrogenase from Thermoanaerobium brockii (TBADH) repectively with indirect electrochemical... 

In the addition to nonactivated alkenes, where the direct anodic oxidation is less, satisfactorily good yields can be achieved when Mn(OAc)2 is used as mediator (Table 8, entries 6, 7). Sorbic acid precursors have been obtained in larger scale and high current efficiency by a Mn(III)-mediated oxidation of acetic acid/acetic anhydride in the presence of butadiene [112]. 

Scheme 7 Selective anodic oxidation of diols with TEMPO as mediator.

In addition to using amine oxidation products as mediators, anodic oxidation reactions can be used to functionalize amine compounds. These reactions include both - examples that generate imines and nitriles, as well as examples that lead to the addition of nucleophiles to the carbon alpha to the nitrogen. 

Scheme 5 Mediated anodic oxidation of amine to nitrile and carbonyl compound.

The combination of anodic oxidation of benzene using the Ag(I)/Ag(II) mediator with cathodic oxidation of benzene using the Cu(I)/Cu(II) mediator in a single electrolytic cell produces p-benzoquinone selectively in both the anodic and the cathodic chambers [242]. Silver-mediator promoted electrooxidation of hydrocarbon has been attempted [243]. The kinetics of indirect oxidation of catechol and L-dopa with IrCl6 has been studied in polymer-coated glassy carbon [244]. 

Although cyclizations from the direct anodic oxidation of acyclic 1,3-dicarbonyl compounds have not been reported, the analogous mediated reactions have been studied [24]. Snider and McCarthy compared oxidative cyclization reactions using a stoichiometric amount of Mn(OAc)3 with oxidations using a catalytic amount of Mn(OAc)3 that was recycled at an anode surface (Scheme 11). In the best case, the anodic oxidation procedure led to a 59% yield of the desired bridged bicyclic product with the use of only 0.2 equivalents (10% of the theoretical amount needed) of Mn(OAc)3- Evidence that the reaction was initiated by the presence of the mediator was obtained by examining the electrolysis reaction without the added Mn(OAc)3. In this case, none of the cyclized product was obtained. For comparison, the oxidation using... 

Anodic oxidation of formazane 18 [17], 1-arylmethylenesemicarbazide 19 [55], p-nitrobenzaldehyde phenylhydrazone 20 [56], and 2-benzoylpyridine phenylhydrazone 21 [57] afforded the corresponding heterocycles in a very good yield (Scheme 14). The homogeneous oxidation of compounds 18-20 was carried out by indirect electrolysis by the mediators generated in situ [58]. 

Specifically, the mediators (Med ) used were the radical cations of tris-(4-bromophenyl)amine and 2,3-dihydro-2,2-dimethylphenothiazine-6(17/)-one [59], The results of the oxidative cyclizations under the homogeneous oxidation conditions are parallel to those obtained by direct anodic oxidation. 

The indirect anodic oxidation of ketones 42 in ammonia - containing methanol using iodide as a mediator afforded 2,5-dihydro-IH-imidazols 44 via oxidation of the intermediate ketimine 43 to AT-iodo imine followed by elimination of HI to afford the nitrenium ion, which subsequently reacts with ketimine 43 to give the product 44 [73] (Scheme 23). 

Anodic oxidation is used to promote the recycling of palladium(il) in the Wacker process for the conversion terminal alkenes to methyl ketones. Completion of the catalytic cycle requires the oxidation of palladium(O) back to the palla-dium(li) state and this step can be achieved using an organic mediator such as tri(4-bromophenyljamine. The mediator is oxidised at the anode to a radical-cation and... 

Aqueous periodic acid can be used to achieve glycol cleavage, combined with anodic oxidation of the iodate, which is formed, back to periodate [70]. Oxidation of iodate is catalysed at a lead dioxide anode [71] but at the potentials required, aldehydes are oxidised to the corresponding acids. Due to this further reaction, the redox-mediated cleavage of diols to form an aldehyde may be difficult to achieve witli a catalytic amount of periodic acid. Cleavage using a stoichiometric amount of periodic acid, followed by recovery of the iodic acid and then its electochemical oxidation, has been achieved [72]. 

Anodic oxidation can occur either direct by electron-transfer from the substrate to the electrode or indirect via a mediator. The same holds in reverse for the cathode. The mediator, which is applied in catalytic amounts and is continuously regenerated at the electrode can either take up electrons (electron-transfer) or abstract hydrogen from the substrate (atom-transfer). Atom- or electron-transfer can happen homogeneously to a mediator dissolved in the electrolyte or heterogeneously to a mediator bound to the electrode surface. 

Solid/solid redox couples, as defined by metal oxides of metals exhibiting different valencies (e.g. PbOj/PbS04, Mn02/Mn00H, NiOOH/NiO), are used as oxidants in preparative organic syntheses and may be used as heterogeneous mediators for the anodic oxidation of organics, provided ... 

Typical examples for type 1 are the anodic cleavages of two carbon-sulfur bonds in 1,3-dithianes [46] or dithiolanes [47]. This reaction is especially effective if performed under the conditions of indirect electrolysis using triarylamine cation radicals as regenerable oxidative mediators [47] ... 

Indirect anodic oxidation of a-phenylthiomethylsilane in an alcohol using Ni2+/Ni3+-cyclam mediator also provides a-alkoxylated sulfides, although the turnover of the mediator is low (equation 24)29. 

K. Schnatbaum and H. J. Schafer, Electroorganic synthesis 66 Selective anodic oxidation of carbohydrates mediated by TEMPO, Synthesis (1999) 864-872. 

M. Schamann and H. J. Schafer, TEMPO-mediated anodic oxidation of methyl glycosides and 1-methyl and 1-azido disaccharides, Eur. J. Org. Chem. (2003) 351-358. 

---