Indirect Electrochemical Generation of Radicals

In contrast to the direct reduction as described above, the indirect electrochemical reduction of perfluoroalkyl halides is a versatile and novel method for generating perfluoroalkyl radicals selectively. Saveant et al. have demonstrated many successful examples. Using terephthalonitrile as a mediator, the indirect reduction of CF3Br in the presence of styrene leads to the dimer of the radical adduct obtained by the attack of CF on styrene. On the other hand, in the presence of butyl vinyl ether, the mediator reacts with the radical adduct obtained by the attack of CF3. on the olefin (Scheme 3.4) [14]. 

Several research groups ha ve been involved in the study of ET reactions from an electrochemically generated aromatic radical anion to alkyl halides in order to describe the dichotomy between ET and polar substitution (SN2). The mechanism for indirect reduction of alkyl halides by aromatic mediators has been described in several papers. For all aliphatic alkyl halides and most benzylic halides the cleavage of the carbon-halogen bond takes place concertedly with the... 

A principally different approach for the indirect electrochemical oxidation of aromatic compounds goes via the formation of hydroxyl radicals from cathodically generated hydrogen peroxide and from reductively formed iron(II) ions. The thus in situ formed Fenton reagent can lead to side-chain as well as nuclear oxidations of aromatic compounds. Side-chain oxidations to form benzaldehydes according to Eqs. (18)—(24) can also be initiated by the redox pairs and Cu instead of... 

Oxidative cleavage by means of electrochemically generated cation-radicals is also possible thus benzyl ethers may be cleaved and carboxylates decarboxylated using cation-radicals of brominated triphenylamines in acetonitrile containing a weak base.34 35 Such as indirect reaction makes it... 

Cation radicals may be generated by direct or indirect electrochemical oxidation of the molecule of interest, and many such oxidations are synthetically useful. However, several other methods are also available, which fall into two broad categories thermally-induced electron transfer (TIET) and photo-induced electron transfer (PIET). 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Indirect electrochemical oxidations using the nitrate ion as redox catalyst proceed via the electro-generated NOj radical. They are useful for the oxidation of secondary alcohols and of alkyl aromatic compounds in the side-chain... 

In an indirect process, the anode merely serves as a convenient source of an oxidant, such as CI2 by oxidation of CP or NOj by oxidation of N03, or an organic radical, such as methoxy radical from methoxide ion, which then attacks the organic substrate in a reaction similar to that taking place when the reagent is generated in any other way. The indirect processes also encompass reactions in which an organic compound is oxidized by an electrochemically generated oxidant, a so-called mediator, the reduced form of which is continuously reoxidized and therefore, in principle, needs only to be present in catalytic amounts. 

Besides direct electroreduction of PTFE on metal cathode, the indirect dehalogenation was carried out by electrochemically generated reactants. A classical reaction employs naphthalene radical cation, which is generated electrochemically in a solution of naphthalene in dimethylformamide + NBU4BF4 or NBU4CIO4 [50,56] (Eq. 4.20). The radicals react with PTFE while the naphthalene is regenerated (4.20a) and the reaction propagates in a catalytic loop ... 

A v ety of reactions are catalyzed by electrochemically generated Ni(0) (62). Electrochemical reduction of Ni(bipy)3Br2 affords a reagent that couples acid chlorides and alkyl or aryl halides to form unsymmetrical ketones (63). Symmetrical ketones are formed from alkyl halides and carbon dioxide (64). Reductive electrochemical carboxylation of terminal alkynes, enynes and diynes can be accomplished with 10% Ni(bipy)3(Bp4)2 in DMF (65-68). Terminal allies lead selectively to a-substituted acrylic acids. Electrocatalytic hydrogenation on hydrogen-active electrodes has been reviewed (69). Radical cyclizations of vinyl, alkyl and aryl radicals can be carried out by indirect electrochemical reduction with a Ni(II) complex as a mediator (70). 

In contrast to the direct process, an indirect process is one in which a foreign molecule or ion serves to shuttle electrons between the electrode and the substrate molecules. An indirect oxidation process may also involve the transfer of a hydrogen atom from a suitable substrate to a radical generated electrochemically. Indirect processes are typically observed for saturated aliphatic hydrocarbons and substrates that are more difficult to oxidize than the solvent-supporting electrolyte system. 

Whenever vinyl monomers are electropolymerized, two situations must be distinguished depending on whether the vinyl monomer is the precursor of the initiating species or not. Either the polymerization is directly initiated by the activated monomer or the initiation is indirect whenever the active species (radical, anion, or cation) is generated by a compound other than the monomer (conducting salt, solvent, or properly selected additives). Besides these electroinitiated polymerization processes, a very recent report also demonstrated that electrochemistry is a valuable tool to mediate atom transfer radical polymerization (ATRP). Indeed, an externally applied electrochemical potential can activate the copper catalyst by a one-electron reduction of an irutially added air-stable cupric species (Cu /ligand) allowing... 

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