Phenol anodic oxidation

It is well-known that the anodic trifluoroacetoxylation of benzene derivatives is a useful method for the preparation of phenol derivatives (Eq. 30). Schafer et al. have successfully achieved CH-functionalization of various hydrocarbons by anodic oxidation in 0.05 M Bu4NPF6/CH2Cl2 containing 20% TFA and 4% (CF3C0)20 as shown in Eqs 31 and 32 [75]. 

Anodic oxidation of phenols gave the corresponding poly(1,4-phenyleneoxide)s by selecting the electrolysis conditions to prevent passivation of the electrode. 

Diarylamides with arenes activated by electron-donating substituents can be converted to azacycles by anodic oxidation through phenolic oxidative coupling reactions that can be a key step in the synthesis of alkaloids (Schemes 16 and 17). According to the nature of substituents and the experimental conditions, either spiro compounds [22] or non-spiro compounds [23, 24] were obtained. 

Derivatives of dihydrofurans are produced in limited yields (20-37%) after the anodic oxidations of phenols (Scheme 59) [81]. 

An alternative route to phenolate-like EGBs is through the cathodic reduction of quinonemethides, (36), [82, 83]. The advantage of these PBs is that they are reduced at modest potentials, which allow EGB formation to take place in situ, and they are ultimately converted into phenols that are easily reoxidized to (36) either by air or by anodic oxidation (60-70% yield) [82]. The radical anion (36a) is expected to have basicity similar to that of (35) , whereas the pK of the conjugate acid of the dianion formed by further reduction can be assumed close to that of triphenylmethane, 30.6. 

A closely related reaction having the phenol protected with a trimethylsilyl group was also examined (Scheme 31) [45]. Unlike the earlier examples, the cyclization reaction resulting from this substrate did not require the presence of a mild acid. The anodic oxidation in methanol solvent with no acetic acid led to a 73% yield of the tricyclic product. In a nearly identical reaction, an anodic oxidation of the trimethylsilyl-protected substrate in the presence of 2,6-lutidine led to the cyclized product in a 60% yield. The use of the silyl group expanded the utility of the anodic C-C bond-forming reaction being studied by allowing for the use of neutral and basic conditions. Hence, it would appear that the cyclization reactions are compatible with the presence of both base and acid sensitive functionality. 

Using a different dimerization method, namely phenolic oxidation, chiral substrates react in a more stereoselective manner than under reductive conditions. The choice of oxidizing reagent may drastically affect the stereochemical outcome of the reaction. Thus, when potassium hexacyanoferrate(III) is used (17 )-l,2,3,4-tetrahydro-6-methoxy-l,2-dimethyl-7-isoquino-linol couples to give a mixture of atropisomers 3 in 38 % yield and with a d.r. (M)I(P) of 45 553,4. Only one single atropisomer, namely (A/)-3, is formed, in a 66% yield by anodic oxidation, which is attributed to electrode surface effects3. 

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). 

Electron-poor arenes can be converted into phenols by anodic oxidation [40]. A mixture of ortho-, meta- and para-substituted phenols is obtained in 18-89 % yields (Eq. (8) TFA is trifluoroacetic acid). Aryl trifluoroacetate is formed first and then hydrolyzed to phenol in a separate step. The oxidation potential of phenyl trifluoroacetate is higher than that of phenol, which explains the absence of overoxidation. 

A great variety of substituted radicals for dimerization can be generated by anodic oxidation of anionic species r5"Me5+, e.g., sodium salts of 1,3-dicarbonyl compounds, aliphatic nitro compounds, phenols, oximes, alkynes, thio-lates or organometallics (Eq. (157) ). 

Smith de Sucre V, Watkinson AP. Anodic oxidation of phenol for waste water treatment. Can J Chem Eng 1981 59 52-59. 

Comninellis C, Nerini A. Anodic oxidation of phenol in the presence of NaCl for wastewater treatment. J Appl Electrochem 1995 25 23-28. 

The current density has a dramatic influence on the yield of 52 and reveals that more than one electrode reaction is involved in the sequence. When the current density is in the range of 2.8. 7 mA/cm2, 52 is directly obtained in about 30% yield. The rationale for the formation of 52 starts with the direct or indirect generation of phenoxyl radicals at the BDD anode. Since the used conditions will provide concentrations of oxyl spin centers, which exclude a recombination, the transformation has to follow a different mechanistic course. The anodic treatment will cause an Umpolung effect because the electron rich phenol is oxidized [110-113]. Such phenoxyl species are still electrophilic despite the liberation of a proton [114-119]. The electrophilic attack occurs at the most electron rich position of the reaction partner which provides the observed connectivity in 52 (Scheme 21). 

Yamamura S, Nishiyama S (2002) Anodic oxidation of phenols towards the synthesis of bioactive natural products. Synlett 533-543... 

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

As is seen (Table 1) anode oxidation leads to increase of relative concentration of phenolic and carbonyl groups and to decrease of carboxyle and carbonate groups concentration. After thermal treatment concentration of phenol and carbonate groups decreased while relative concentration of carbonyl and carboxyl groups increased. 

Many natural products display structural motifs biosynthetically derived from ortho-quinol precursors, and some even feature ortho-quinol moieties in their final structural arrangement [1, 6]. Asatone (7) and related neolignans can be put forward as classic examples of complex natural products derived from cyclodimerization of oxidatively activated simple phenol precursors (Figure 5) biomimetic syntheses of 7 have accordingly been accomplished by anodic oxidation (Section 15.2.1) and by Pelter oxidation (Section 15.2.2) of the naturally occurring phenol 9 [34, 36]. 

Figure 1.7 shows both the experimental and predicted values (continuous line) of both the ICE and COD evolution with the specific electrical charge passed during the anodic oxidation of different classes of organic compounds (acetic acid, isopropanol, phenol, 4-chlorophenol, 2-naphtol). This figure demonstrates that the... 

ComnineUis, Ch. and Pulgarin, C. (1991) Anodic oxidation of phenol for wastewater treatment. J. Appl. Electrochem. 21, 703-708. 

Awad, Y. M. and Abuzaid, N. S. (2000) Influence of residence time on the anodic oxidation of phenol. Sep. Purif. Technol. 18, 227-236. 

Under conditions of high current density and low phenol concentration, the complete combustion to C02 was obtained. In this case, due to the high local concentration of OH radicals, the anodic oxidation was a fast reaction under diffusion control, so that the instantaneous current efficiency decreased during the electrolysis as phenol was oxidised. 

As general trend, it can be observed that for most of the studied phenolic compounds the anodic oxidation at BDD was very fast and, if the applied current density was sufficiently high, the kinetics of the process was under mass-transfer control and it was not appreciably dependent on the nature of the substituting groups in the aromatic ring, as it was reported at more usual electrode materials, like platinum (Torres et al. 2003). 

---