Toluenes oxidation using electrochemically

In addition to the synthesis of saccharin, also a number of other side-chain oxidations have been studied leading to aromatic carboxylic acids by indirect electrochemical oxidation using chromic acid as oxidizing agent. They include the oxidation of p-nitrotoluene 2,4-dinitrotoluene toluene, p-xylene, and p-tolualdehyde... 

In early attempts to oxidize hydrocarbons electrochemically, organic solvents and corrosion-resistant electrodes (PbO, C, Pt) were used to overcome low reactant solubility and anode dissolution at extreme potentials, -I-1.8 V and up to 4.5 V (326, 327). The primary anodic reaction was usually oxygen evolution or solvent decomposition. The electrode material, nonetheless, affected the product even at the small attainable yields. Thus, toluene oxidized to traces of aldehydes on PbO2 (333), while on Pt it yielded up to 19% benzaldehyde (326). The catalytic efifect of the anode, however, on rate and selectivity was not realized. 

The three basic steps in the palladium-catalysed Suzuki-Miyaura reaction involve oxidative addition, transmetalation, and reductive elimination. A systematic study of the transmetalation step has found that the major process involves the reaction of a palladium hydroxo complex with boronic acid, path B in Scheme 3, rather than the reaction of a palladium halide complex with trihydroxyborate, path A. A kinetic study using electrochemical techniques of Suzuki—Miyaura reactions in DMF has also emphasized the important function of hydroxide ions. These ions favour reaction by forming the reactive palladium hydroxo complex and also by promoting reductive elimination. However, their role is a compromise as they disfavour reaction by forming of unreactive anionic trihydroxyborate. A method for coupling arylboronic acids with aryl sulfonates or halides has been developed using a nickel-naphthyl complex as a pre-catalyst. It works at room temperature in toluene solvent in the presence of water and potassium carbonate. ... 

Membranes and Osmosis. Membranes based on PEI can be used for the dehydration of organic solvents such as 2-propanol, methyl ethyl ketone, and toluene (451), and for concentrating seawater (452—454). On exposure to ultrasound waves, aqueous PEI salt solutions and brominated poly(2,6-dimethylphenylene oxide) form stable emulsions from which it is possible to cast membranes in which submicrometer capsules of the salt solution ate embedded (455). The rate of release of the salt solution can be altered by surface—active substances. In membranes, PEI can act as a proton source in the generation of a photocurrent (456). The formation of a PEI coating on ion-exchange membranes modifies the transport properties and results in permanent selectivity of the membrane (457). The electrochemical testing of salts (458) is another possible appHcation of PEI. 

Reduction of fullerenes to fullerides — Reversible electrochemical reduction of Ceo in anhydrous dimethylformamide/toluene mixtures at low temperatures leads to the air-sensitive coloured anions Qo" , ( = 1-6). The successive mid-point reduction potentials, 1/2, at -60°C are -0.82, -1.26, -1.82, -2.33, —2.89 and —3.34 V, respectively. Liquid NH3 solutions can also be used. " Ceo is thus a very strong oxidizing agent, its first reduction potential being at least 1 V more positive than those of polycyclic aromatic hydrocarbons. C70 can also be reversibly reduced and various ions up to... 

Electrochemical redox studies of electroactive species solubilized in the water core of reverse microemulsions of water, toluene, cosurfactant, and AOT [28,29] have illustrated a percolation phenomenon in faradaic electron transfer. This phenomenon was observed when the cosurfactant used was acrylamide or other primary amide [28,30]. The oxidation or reduction chemistry appeared to switch on when cosurfactant chemical potential was raised above a certain threshold value. This switching phenomenon was later confirmed to coincide with percolation in electrical conductivity [31], as suggested by earlier work from the group of Francoise Candau [32]. The explanations for this amide-cosurfactant-induced percolation center around increases in interfacial flexibility [32] and increased disorder in surfactant chain packing [33]. These increases in flexibility and disorder appear to lead to increased interdroplet attraction, coalescence, and cluster formation. 

Recently, interesting results have been obtained using the electrochemically generated ruthenium(IV) complex [(trpy)(bpy)RuO] in the oxidation ofp-xylene and p-toluic acid to terephthalic acid and of toluene to benzoic acid . The current yield is practically quantitative. After 100 turnovers about 75% of the redox catalyst can be recovered giving a turnover number of about 400 (Eq. (16)). 

Because of thermodynamic and electrochemical conventions, standard potentials are defined in the direction of reduction, independently of the respective chemical stabilities of the molecules involved. Thus for the oxidation of toluene to its cation radical, E° refers to the reduction of the highly unstable cation radical into the highly stable toluene. To overcome such a priori chemical nonsence, E is frequently designated as the standard oxidation potential of toluene for example. However, such a term should not be accepted according to canonical rules because it formally implies that the cell now operates in a driven mode, that is, is connected to an external power supply [19]. Thus in this chapter we prefer to use the denomination standard reduction potentials, rather than the usual temi standard potential, as a reminder of the E° definition, although such as expression is basically a pleonasm. 

In some cases, the hydrocarbon oxidation in the presence of a reductant does not need a transition metal complex as catalyst. Thus, a cathode of a carbon whisker has been found to be active for the oxidation of toluene into benzaldehyde and benzyl alcohol during the H2-O2 fuel cell reaction [58a]. During the electrolysis of water at room temperature, the epoxidation of hex-1-ene and hydroxylation of benzene to phenol and hydroquinone occurs simultaneously on the anode and the cathode, respectively [58b]. Finally, selective oxidation of terminal isopropyl groups to the corresponding tertiary alcohols have been carried out by an electrochemical method [58c], Using the... 

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