Anodic oxidation, aromatic compounds

Yoshida has studied anodic oxidations in methanol containing cyanide to elucidate the electrode processes themselves.288 He finds that, under controlled potential ( 1.2 V), 2,5-dimethylfuran gives a methoxynitrile as well as a dimethoxy compound (Scheme 57). Cyanide competes for the primary cation radical but not for the secondary cations so that the product always contains at least one methoxy group. On a platinum electrode the cis-trans ratio in the methoxynitrile fraction is affected by the substrate concentration and by the addition of aromatic substances suggesting that adsorption on the electrode helps determine the stereochemistry. On a vitreous carbon electrode, which does not strongly adsorb aromatic species, the ratio always approaches the equilibrium value. 

On the other hand, the anodic oxidation of 1,3,5-cycloheptatrienes is one of the most powerful key tools for the preparation of a variety of non-benzenoid aromatic compounds such as tropylium salts, tropones, tropolones, 2H -cyclohcpta h furan-2-oncs and azulenes14. 

For /8-substituted 7t-systems, silyl substitution causes the destabilization of the 7r-orbital (HOMO) [3,4]. The increase of the HOMO level is attributed to the interaction between the C-Si a orbital and the n orbital of olefins or aromatic systems (a-n interaction) as shown in Fig. 3 [7]. The C-Si a orbital is higher in energy than the C-C and C-H a orbitals and the energy match of the C-Si orbital with the neighboring n orbital is better than that of the C-C or C-H bond. Therefore, considerable interaction between the C-Si orbital and the n orbital is attained to cause the increase of the HOMO level. Since the electrochemical oxidation proceeds by the initial electron-transfer from the HOMO of the molecule, the increase in the HOMO level facilitates the electron transfer. Thus, the introduction of a silyl substituents at the -position results in the decrease of the oxidation potentials of the 7r-system. On the basis of this j -efleet, anodic oxidation reactions of allylsilanes, benzylsilanes, and related compounds have been developed (Sect. 3.3). 

A special problem can be the passivation of the electrode surface by insulating layers, for example, formation of oxides on metals at a too high anodic potential or precipitation of polymers in aprotic solvents from olefinic or aromatic compounds by anodic oxidation. As a result, the effective surface and the activity of the... 

This chapter deals with anodic oxidation of saturated hydrocarbons, olefins, and aromatic compounds. Substituted hydrocarbons are included, when the substituents strongly influence the reactivity. Anodic functional group interconversions (FGI) of the substituents are covered in Chapters 6, 8-10 and 15. 

The anodic chlorination in some cases allows one to achieve better regioselec-tivities than chemical alternatives (p/o ratio of chlorotoluene in chlorination of toluene anodic 2.2, chemical alternative 0.5-1.0) [215]. Anodic oxidation of iodine in trimethyl orthoformate afforded a positive iodine species, which led to a more selective aromatic iodination than known methods ]216]. Aryliodination is achieved in good yield, when an aryhodide is oxidized in HOAc, 25% AC2O, 5% H2SO4 in the presence of an arene ]217, 218]. Alkyl nitroaromatic compounds, nitroaromatic ketones, and nitroanihnes are prepared in good yields and regioselectivity by addition of the corresponding nucleophile to a nitroarene and subsequent anodic oxidation of the a-complex (Table 13, number 11) ]219, 220]. 

Anodic side chain substitution is a competing reaction to nuclear substitution of aromatic compounds. In side chain substitution, the first formed acidic radical cation is deprotonated at the a-carbon atom of an alkyl group to form a radical. This is further oxidized to a benzyl cation, which reacts with a nucleophile (Scheme 9, path d). The factors that influence the ratio of nuclear to side chain substitution have been described in 5.4.1. 

The conversion of aromatic compounds comprises coupling, nuclear and ben-zylic substitution, and in some cases, addition. Homo- and in a more limited scope, heterocoupling is achieved for unsubstituted and substituted aromatic compounds in direct or indirect anodic processes. Chemically, there is a limited variety of expensive oxidation reagents available, but a large scope of transition... 

The anode acetoxylation of aromatic compounds in solutions of acetic acid carrying alkali or tetraalkylammonium acetates takes the same route. As shown (Eberson 1967, Eberson and Jonsson 1981), the process starts with one-electron oxidation at the anode and then passes through the same stages as in oxidation with cobalt trifluoroacetate. The reaction takes place at potentials sufficient to oxidize the substrate but not sufficient to convert acetate ion into acetoxy radical. 

Reactions between aromatic hydrocarbon radicabcations and cyanide ions, with few exceptions, give low yields of nuclear substitution products [76], In some cases, better results have been obtained by anodic oxidation of the aromatic compound in an emulsion of aqueous sodium cyanide and dichloromethane with tetra-butylammonium hydrogen sulphate as a phase transfer agent [77, 78]. Methoxy-benzenes give exceptionally good yields from reactions in acetonitrile containing tetraethylammonium cyanide, sometimes with displacement of methoxide [79, 80]... 

The possibility to functionalize aromatic compounds by electrochemical methods is of great interest to chemical industry. Therefore, considerable efforts were made to develop the electrochemical oxidation of benzene to p-benzoquinone to the industrial scale thus forming a basis for a new hydroquinone process. The electrochemical oxidation of benzene in aqueous emulsions containing sulfuric acid using divided cells and Pb02 anodes formsp-benzoquinone. The product can then be reduced cathodically to yield hydroquinone in a paired synthesis. 

However, this is not always the case. As an example, electrochemical cyanation 5 5-6 of an aromatic compound can be carried out by anodic oxidation in methanol-sodium cyanide (Eq. (16) ). The current yield (the yield of cyanation product based on the amount of... 

This mode of anodic addition involves oxidation of the substrate RH, an olefin or aromatic compound, to a radical cation, in contrast to the oxidations of the reagent Nu , depicted in the preceding sections a) and b). The adduct is formed in a EC ECj j- sequence (Eq. (119)) ... 

Applied research on indirect electrolytic methods has experienced a spectacular evolution in the last decades. More efficient, faster, and/or cheaper processes than classical anodic oxidation are now operative or in pilot trials. Redox mediators such as Ag(I)/Ag(II) and Co(II)/Co(III), and on-site production of strong oxidants like ozone at the anode and EFR at the cathode, are notable examples. These methods have been specially focused on aromatic compounds, probably because they are easily oxidiz-able, through hydroxyderivatives and quinones, to aliphatic acids. A further photochemical treatment of these products in the presence of Fe(III) allows their mineralization. 

Fuchigami, Marken and coworkers also reported self-supported anodic methoxy-lation and acetoxylation of several aromatic compounds using a simple thin-layer flow cell reactor (interelectrode gap of 80 pm) (Scheme 4.41) [56]. The current efficiency (CE) of this process was 10% at best because of oxidation of methanol at flow rates lower than 0.03 ml/min. Even though CE increased at a faster flow rate (0.5 ml/min), the yield decreased sharply. The importance of selecting an appropriate choice of electrode material also was noted. 

---