Anodic Conversions

Generally, the anodic conversion of a functional group into another one is a reaction type which has received relatively little attention during recent years. Numerous experiments have been described in the older literature 4 6 7 but the results have seldom been of any preparative value because of the lack of product selectivity. It is probable that a knowledge of half-wave potentials of substrate and product(s) in combination with cpe might improve the selectivity to a large extent in many of these cases. 

Lund 12°) was first in applying cpe in the oxidation of a primary alcohol to an aldehyde (which under constant current conditions would be partly or completely oxidized to the corresponding carboxylic acid) 121 Anisyl alcohol displays two anodic waves in acetonitrile-sodium perchlorate withiTj /2 of 1.22 and 1.64 V vs. Ag/0.1 M Ag Cpe at the plateau of the first wave (1.35 V) in the same medium consumed only 5 % of the theoretically calculated amount of electricity and no carbonyl compound was formed. Addition of a three-fold excess of pyridine (to act as a proton acceptor) gave a 72 % of anisaldehyde  

Cpe oxidation of other alcohols (fluorenol and 2-naphthalenemethanol) were not successful due to severe filming at the electrode. 

Another case of product control through cpe is the oxidation of glycerol at different potentials in H20-K0H 122  

The overwhelming majority of alcohol oxidations (including those of carbohydrates) have been run in SSE s of relatively high water contents, often with strong acid present, under constant current conditions 123 Selective oxidation of an alcohol to an aldehyde cannot be accomplished under such conditions instead the carboxylic acid and its degradation product is formed. The cpe approach in SSE s of low water contents should no doubt pay rich devidends in this area. The same applies to the oxidation of secondary alcohols, in which the acid SSE s previously used seem to promote anodic degradation of the ketone formed. 

---

If AG and AG represent the thermal free enthalpy barriers for cathodic and anodic conversions, respectively, eqns. 3.10 and 3.11 become... 

Hexavalent chromium Electroplating Anodizing Conversion coating... 

In an example of the construction of such a device, thin films of these materials are deposited on OTEs that are separated by a layer of a transparent ionic conductor such as KCF3SO3 in polyethylene oxide).125 The films can be colored simultaneously (giving deep blue) when a sufficient voltage is applied between them such that the WO3 electrode is the cathode and the PB electrode the anode. Conversely, the colored films can be bleached to transparency when the polarity is reversed, returning the ECD to a transparent state. 

Comparison of Anodic Conversions of Alkanes, Alkenes, and Aromatic... 

Anodic conversion of aromatics proceeds in most cases by le-transfer to the anode to form a radical cation (34) (Scheme 9). Oxidation is facilitated by extension of the 7T-system ( 1/2 vs. Ag/Ag+ benzene 2.08 V, pyrene 0.86 V) and by electron donating substituents ( 1/2 vs. Ag/Ag+p-phenylenediamine —0.15 V). Oxidation potentials of polycyclic aromatics and substituted benzenes are collected in Ref [140-142]. 

Comparison of Anodic Conversions of Aikanes, Aikenes, and Aromatic Compounds 1163... 

Scheme 20 Anodic conversion of ketones to a-hydroxy ketals.

Scheme 22 Anodic conversion of aryl alkyl ketones to methyl phenyl acetates.

Scheme 8 Anodic conversion of an amine to an enamine with subsequent anodic bromination.

Scheme 19 Anodic conversion of amides to N-acyliminium ions.

A second, less well-known example is the anodic conversion of toluenes in methanol as solvent to benzaldehyde-dimethoxy acetals (180, 7S7) ... 

O-alkyl or O-acyl bonds are cleaved in the anodic conversion of 2,3,5,6-tetra-methyl-l,4-dimethoxy- or -1,4-diacetoxybenzene to duroquinone 466 ... 

The anodic cyanation reaction allows the direct installation of cyanide without leaving groups. The cyanide acts in the electrochemical conversion similar to fluoride. After oxidation of the organic substrate the nucleophilic cyanide enters the reaction scene forming a less electron rich product which is deactivated for further anodic conversions. Therefore, the electrochemical cyanation reaction has some significance for aromatic substrates [67] (Scheme 12). 

Some simple biphenols equipped with methyl groups, e.g., 3,3, 5,5 -tetramethyl-2,2 -biphenol 38, have attracted attention as important components of highly potent ligand systems [75-86]. Remarkably, the sustainable synthesis of such biphenols is rather challenging despite their simple scaffolds. In particular, methyl-substituted phenols are prone to side reactions. This is especially the case when 2,4-dimethyl-phenol (37) is oxidatively treated. Upon anodic conversion 37 is preferably transformed into polycyclic architectures [87]. Direct electrolysis in basic media provided only traces of the desired biphenol 38 and the dominating components of the product mixture consisted of Pu in meter s ketone 39 and the consecutive pentacyclic spiro derivative 40 [88]. For an efficient electrochemical access to 3,3, 5,5 -tetramethyl-2,2,-biphenol (38) we developed a boron-based template strategy [89, 90]. This methods requires a multi-step protocol but can be conducted on a multi-kilogram scale (Scheme 17). 

Under basic conditions using KOH as electrolyte, ethylene acetals are methoxylated at a Pt anode in good yield, giving the mixed ort/in-esters, as in the anodic conversion of butyraldehyde ethylene acetal to ortho-t iQt (XCIV) [Eq. (58)]. In general, the corresponding dimethyl acetals give lower yields [141]. 

The anodic conversion of tertiary C—H functions (without any specific activation) is possible, yet it needs rather high potentials. One typical example is certainly that of adamantane [42], the oxidation of which can be achieved in acetonitrile. Under these conditions, the corresponding acetamide (via Ritter reaction) was obtained. 

The ideal cell in order to scale up an electrochemical reaction can depend on the reaction, the electroactivity of the substrate to convert, the concentration of the substrate, as well as the current density at the working electrode. The use of a separator is necessary when the electrode can affect the whole process negatively. With anodic oxidations, the reaction at the counter electrode is most frequently the cathodic formation of hydrogen. In these cases, a separator does not seem indispensable a tank cell (kind of Grignard type reactor equipped with cylindrical electrodes) or a capillary-gap cell (piling of bipolar electrodes in a cylinder-shaped vessel connected to an anodes and a cathode located at the top and the bottom of the cell) can be considered as suitable devices for anodic conversions. More generally, the so-called plate-and-frame cells (Fig. 4) are used in a battery. 

A valuable goal appears to be the anodic conversion of substituted toluenes into the corresponding aldehydes. The reaction can be achieved either in methanol [194] (intermediate formation of a ketal) or in aqueous solution in an indirect manner (presence of Mm11 or/and Cem ions as mediators [195]). The indirect oxidation of polyaromatic hydrocarbons (naphthalene, anthracene) into the corresponding quinones could be achieved in the presence of electrogenerated ceric ions. 

Thus, if a positive overpotential is applied, the first exponential will be larger than the second, a positive inet results, and the net reaction is oxidation or anodic. Conversely, a negative overpotential will lead to a negative inet, and the net reaction is reduction or cathodic. 

An interesting extension of the anodic conversion in fluorosulfonic acid is the trapping of the intermediate carbenium ion with CO in an anodic Koch reaction (equation 4). 

---