Electrochemical oxidation, amines

Electropolymerization is also an attractive method for the preparation of modified electrodes. In this case it is necessary that the forming film is conductive or permeable for supporting electrolyte and substrates. Film formation of nonelectroactive polymers can proceed until diffusion of electroactive species to the electrode surface becomes negligible. Thus, a variety of nonconducting thin films have been obtained by electrochemical oxidation of aromatic phenols and amines Some of these polymers have ligand properties and can be made electroactive by subsequent inincorporation of transition metal ions... 

Ionic polysulfides dissolve only in media of high polarity hke water, liquid ammonia, alcohols, nitriles, amines, and similar solvents. In all of these solvents 8 can be reduced electrochemically to polysulfide anions. On the other hand, the electrochemical oxidation of polysulfide anions produces elemental sulfur ... 

Dvorak V, Nemek I, Zyka J (1967) Electrochemical oxidation of some aromatic amines in acetonitrile medium II. benzidine, N,N,N ,N -tetramethylbenzidine, and 1,4-phenylene-diamine derivatives. Microchem J 12 324-349... 

The electrochemical behaviour of silyl-substituted nitrogen compounds is also interesting. The introduction of a silyl group at the carbon adjacent to the nitrogen of carbamates causes a significant decrease in the oxidation potentials, although such effect is much smaller for amines. Preparative electrochemical oxidation of silyl-substituted carbamates in methanol results in smooth and selective cleavage of the C Si bond and introduction of methanol at the a-... 

Itoh et al. (151) employed a tetradentate amine and synthesized the complex [Znn(l)(MeCN)]PF6, where 1 represents the monoanion of 2-methylthio-4-ferf-6-[[bis[2-(2-pyridyl)ethyl]amino]methyl]phenol (see Fig. 31 for 1). This complex was chemically [with (NH4)2[Ce(N03)6]) or electrochemically oxidized yielding the (phenoxyl)zinc species Zn1 (1 )(MeCN )]PF6. It displays phenoxyl K-K tran-... 

A variety of amines have been used as mediators for electrochemical oxidation reactions. In these reactions, the amine is oxidized at an anode surface to form a radical cation. The amine radical cation then oxidizes a second substrate triggering a reaction of synthetic interest. The regenerated amine is then reoxidized at the anode... 

The electrochemical oxidation of amines to imines and nitriles typically utilize a chemical mediator. The use of both Al-oxyl radicals [12, 13] and halogens has been reported for this process [14]. For example, the conversion of benzyl amine (14a) into nitrile (15a) and aldehyde (16a) has been accomplished using the M-oxyl radical of a decahydroquinoline ring skeleton as the mediator (Scheme 5). The use of acetonitrile as the solvent for the reaction generated the nitrile product. The addition of water to the reaction stopped this process by hydrolyzing the imine generated. A high yield of the aldehyde was obtained. In the case of a secondary amine, the aqueous... 

In an another analogous set of reactions, Matsumura and coworkers have shown that electrochemically derived A -acyliminium ions can serve as electrophiles in asymmetric alkylation reactions (Scheme 25) [57]. In this case, the methoxycarbamate of a series of cyclic amines was oxidized in order to generate the AZ-a-methoxy carbamates. 

It is quite often possible to prepare hydroxypyridinone complexes directly by one-pot synthesis from the appropriate hydroxypyranone, amine, and metal salt 90-92). They can also be prepared by reacting complexes such as P-diketonates with hydroxypyridinones (see e.g., Ce, Mo later). Several maltolate complexes, of stoichiometry ML2, ML3, ML4, or MOL2, have been prepared by electrochemical oxidation of the appropriate metal anode, M — a first-row d-block metal (Ti, V, Cr, Mn, Fe, Co, Ni), In, Zr, or Hf, in a solution of maltol in organic solvent mixtures 92). Preparations of, e.g., manganese(III), vanadium(III), or vanadium(V) complexes generally involve oxidation... 

Accordingly, many reactions can be performed on the sidewalls of the CNTs, such as halogenation, hydrogenation, radical, electrophilic and nucleophilic additions, and so on [25, 37, 39, 42-44]. Exhaustively explored examples are the nitrene cycloaddition, the 1,3-dipolar cycloaddition reaction (with azomethinylides), radical additions using diazonium salts or radical addition of aromatic/phenyl primary amines. The aryl diazonium reduction can be performed by electrochemical means by forming a phenyl radical (by the extrusion of N2) that couples to a double bond [44]. Similarly, electrochemical oxidation of aromatic or aliphatic primary amines yields an amine radical that can be added to the double bond on the carbon surface. The direct covalent attachment of functional moieties to the sidewalls strongly enhances the solubility of the nanotubes in solvents and can also be tailored for different... 

The electrochemical oxidation of [Os(bpy)(tpy)(NH3)] " in the presence of secondary amines in aqueous solution (pH 7) leads to [Os(bpy)(tpy)(NNR2)] " which can be reduced to [Os(bpy)(tpy) (NNR2)]. Structural data have been obtained for representative complexes. The redox chemistry of the new compounds has been described. ... 

Indirect electrochemical oxidation of substituted toluenes in methanol affords good yields of side chain substitution products, Tris(2,4-dibromophenyl)amine is... 

The a-aminoalkyl radical intermediates from electrochemical oxidation of amines show a strong tendency to lose a further electron and form an immonium ion. This process shows an anodic polarographic wave at negative electrode poten-... 

For amines having an a-hydrogen atom, electrochemical oxidation leads to the imine as the first detectable intemrediate. In the absence of another nucleophile, this is not usually a useful reaction since the imine is hydrolysed by water present in the solvent leading to a mixture of products [80, 81], Oxidation of ten-butylamine, which has no a-hydrogen atom, leads to loss of a proton flrom the nitrogen atom and the dimerization of nitrogen centred radicals. The product isolated in moderate yields is azo-teri-butane 17 [82], The reaction can be carried out in an... 

Electrochemical oxidation of aliphatic tertiary amines in acetonitrile together with compounds having weakly acidic hydrogens, such as dimethyl malonate, leads to addition of the malonate anion to the immonium cation intermediate. 2,4,6-Collidine is added to combine with protons, which are released during reaction [90],... 

Amines are stable to electrochemical oxidation in acid solution because the nitrogen lone pair is protonated and inaccessible for reaction. This is not the case for N-acetylamines, which are oxidisable at a lead dioxide anode in aqueous sulphuric acid [99]. The primary electron tiansfer step involves the amide function and leads to a radical-cation, which loses a proton from the carbon atom adjacent to nitrogen. Subsequent steps lead to an acylimmonium ion, which is trapped by water. N-acetylated primaiy amines are converted to the corresponding carboxylic acid. 

Electrochemical oxidation of hydroxamic acids in the presence of amines, alcohols or water afforded the corresponding amides, esters or carboxylic acids (equation 19) . ... 

Various oxidations of amines can also generate acyliminium ions. The methods most used in synthetic procedures involve electrochemical oxidation to form z-alkoxy amides and lactams, which then generate acyliminium ions.108 Acyliminium ions are sufficiently electrophilic to react with enolate equivalents such as silyl enol ethers109 and enol esters.110... 

Figure 13.13. Electrochemical oxidation of hindered amines gives nitrenium ions.

Nitrenium ions can be generated from the electrochemical oxidation of amines (Figs. 13.34 and 13.35). Rieker et used a similar procedure to generate... 

Later studies on the ECF of tetraalkyl ammonium salts, known to be very stable towards electrochemical oxidation, showed that these reactions proceeded similarly to tertiary amines, but with increased formation of gaseous cleavage products [183]. 

Poly(propylene amine) dendrimers containing 4, 8, and 64 amidoferrocene peripheral units have also been incorporated in the highly ordered channels of mesoporous silica obtaining a novel type of redox-active materials. One significant feature of these new composite materials is that the ferrocene units of the guest dendrimers are easily accessible to electrochemical oxidation, as revealed by studies carried out in MeCN solutions by using Pt electrodes derivatized with films of such dendrimer-matrix complexes.33... 

As mentioned above, it is difficult to find organic compounds which are suitable as redox catalysts for oxidations. This is the case because organic cation radicals, which are mostly the active forms in indirect electrochemical oxidations, are usually easily attacked by nucleophiles, thus eliminating them from the regenerative cycle. Therefore, the cation radicals must be stabilized towards the reaction with nucleophiles. Nelson et al. demonstrated that the cation radicals of triaryl amines and related compounds are very stable if the para positions of the aryl... 

Technically interesting are the indirect electrochemical oxidations of benzylic alcohols (Table 11, No. 15-18) benzaldehyde dimethylacetals (Table 11, No. 19) and alkyl aromatic compounds (Table 11, No. 20, 21) It could be proven that benzylic alcohols are oxidizable using tris(2,4-dibromophenyl)amine as mediator not only in acetonitrile in a divided cell but also in methanol in an undivided cell... 

Scheme 10. Reactivity pattern for the methoxylation in a-position to oxygen by indirect electrochemical oxidation in MeOH/NaOMe with tris(2,4-dibromophenyl)amine as mediator...

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