Direct anodes

G lv nic Corrosion. Galvanic corrosion is an electrochemical process with four fundamental requirements (/) an anode (magnesium), 2) a cathode (steel, brass, or graphite component), (J) direct anode to cathode electrical contact, and (4) an electrolyte bridge at the anode and cathode interface, eg, salt water bridging the adjacent surfaces of steel and magnesium components. If any one of these is lacking, the process does not occur (133,134). 

The simplest applications involve direct anodic oxidation in aqueous media. 

The electrosynthesis of 4-methoxybenzaldehyde (anisaldehyde) from 4-methoxy-toluene by means of direct anodic oxidation is performed on an industrial scale [69]. Via an intermediate methyl ether derivative, the corresponding diacetal is obtained, which can be hydrolyzed to the target product. The different types of products - ether, diacetal, aldehyde - correspond to three distinct single oxidation steps. 

The direct anodic oxidation of methanol became much more attractive after it was shown that platinum-ruthenium alloys are catalytically much more active in this reaction than pure platinum (pure ruthenium is totally inactive in this reaction). 

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. 

Germain and Commeyras have found that perfluoro sulfonic esters and fluorosulfates are formed in high yields by direct anodic oxidation of R,I in perfluoroalkane sulfonic acids and fluorosulfuric acid (Eqs. 12 and 13) [36]. 

When the surface is completely covered by an oxide film, dissolution becomes independent of the geometric factors such as surface curvature and orientation, which are responsible for the formation and directional growth of pores. Fundamentally, unlike silicon, which does not have an atomic structure identical in different directions, anodic silicon oxides are amorphous in nature and thus have intrinsically identical structure in all orientations. Also, on the oxide covered surface the rate determining step is no longer electrochemical but the chemical dissolution of the oxide.1... 

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]. 

Direct anodic methoxylation of biscarba-mates of a methyl a, tu-diaminocarboxylate... 

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... 

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. 

These processes can occur by a direct electron transfer reaction to (reduction) or from (oxidation) the present organic pollutant, or by a chemical reaction of the pollutant with previously electrogenerated species. The mechanism is generally viewed as a direct anodic oxidation of organic pollutant involving its reduction by direct electron transfer from organic molecule to the electrode to form a radical cation that readily deprotonates, equation (37) ... 

Benzylideneamino)-phenols (49) can be oxidatively cyclized to form 2-phenyloxa-zols (50) (Eq. (13)) by direct anodic oxidation 2, by Pb(OAc)4AgjO " and nickel peroxide The oxidation of 49 proceeded disappointingly in t-butanoT. water at the nickel hydroxide electrode. 50 was isolated only in traces, benzaldehyde was the major product, which indicated that 49 hydrolysed under the reaction conditions. The hydrolysis could effectively be suppressed by electrolysis in an emulsion of water and cyclohexane, where the portion of water was kept low. The temperature was around 70 °C to secure a fast oxidation. With these reaction conditions good yields of 50 were obtained (Table 16). 

Direct Anodic and Cathodic Electrochemical Conversions of Organic Substrates... 

Contrary to the noncatalytic Kolbe synthesis or other anodically initiated radical reactions at Pt anodes, the anodic functionalization and anodic coupling of vinyl compounds by direct anodic oxidation of olefins in alcohols as... 

The direct anodic oxidation of 1,4-butynediol at Pb02 anodes in sulfuric acid yields the desired acetylenic dicarboxylic acid [11] ... 

The mechanism given in equation 47 has been proposed for this reaction. The initially formed cation radical reacts with molecular oxygen to generate an intermediate, which may couple with a neutral cyclic silane to form species A. The intermediate A decomposes to the final product B and its cation radical B+", which could also be generated by direct anodic oxidation of the siloxane B. A further oxygen insertion step could take place via intermediate C+. ... 

Ti(II) tends to form polymers or aggregates upon increasing the Ti(II) concentration or the liquid acidity. Electrochemical, either galvanostatic or potentiostatic, oxidation of Ti metal produces either passive TiCl3 film or volatile TiCl4 which escapes from the liquid. The oxidation of metallic titanium to Ti(II) by direct anodization of Ti metal in this liquid has not yet been described. 

Since the extreme oxidizing power of the oxyl spin centers is successfully employed in waste water treatment, an application of these intermediates seems to be self-contradictory in terms of synthetic use. However, alkoxylation of hydrocarbons is a very important technical field since it allows the installation of functionalities without using the detour via halogenations. The selective introduction of functional groups on a completely nonactivated hydrocarbon has not yet been realized by BDD technology. In contrast, the direct anodic methoxylations of activated carbons exhibiting benzylic or allylic moieties can be performed at BDD anodes. The results obtained with BDD electrodes are quite similar to those when graphite serves as anode [57]. The anodic synthesis of benzaldehyde dimethyl ketals is industrially relevant and performed on the scale of several thousand tons. A detailed study of the anodic methoxylation of 4-tert-butyltoluene (10) at BDD was performed [58]. Usually, the first methoxylation product 11 and the twofold functionalized derivative 12 are found upon electrochemical treatment (Scheme 5). 

Malkowsky IM, Rommel CE, Wedeking K, Frohlich R, Bergander K, Nieger M, Quaiser C, Griesbach U, Putter H, Waldvogel SR (2006) Facile and highly diastereoselective formation of a novel pentacyclic scaffold by direct anodic oxidation of 2,4-dimethylphenol. Eur J Org Chem 241-245... 

For the case shown in Fig. 8, the anodic and cathodic Evans lines intersect at three points. The polarization curve for this situation appears unusual, although it is fairly commonly observed with CRAs. At low potentials, the curve is identical to that shown in Fig. 5. However, just above the active-passive transition, another Ecmi appears followed by a loop and yet a third ECljU before the passive region is observed. The direction (anodic or cathodic) of the applied current density for each region shown in the polarization curve of Fig. 8 is indicated, showing that the loop consists of cathodic current. The origin of the cathodic loop is the... 

For some examples, the outcome of direct anodic methoxylations of activated carbons (benzylic, allylic, etc.) at BDD anodes is similar to graphite anodes (Putter et al. 2003). Further investigation of the reaction mechanism in the case of the methoxylation of /Me/y-butyl toluene (1) - a process of industrial relevance (Bosma et al. 1999) - showed that BDD electrodes favour the occurrence of bibenzylic intermediates 4 which were not observed at graphite electrodes. This difference has been attributed to a mechanism based on the non-catalytic character of BDD electrodes and to the presence of active functionalities on graphite electrodes (Zollinger et al. 2004a). 

Malkowsky, I.M., Griesbach, U., Putter, H. and Waldvogel, S.R. (2006c) Novel template-directed anodic phenol coupling reaction. Chem.-Eur. J. 12, 7482-7488. 

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