Radicals anion radicals

Reactions of aliphatic nitro compounds with nucleophiles have been reviewed442-444. The oxidative reaction of nitronate anions, e.g. 410, with thiocyanate anions to yield thiocyanates 411 proceeds by a radical radical-anion chain mechanism SrnI (equation 133). Analogous replacements by azide, benzenesulphinate and 4-chlorobenzenethiolate have been reported445. 

Normally, the reaction partners in PET reactions are neutral molecules. That is why a donor radical cation—acceptor radical anion pair is obtained by the PET step. These highly reactive intermediates can be used for triggering interesting reactions. Since the PET is not restricted to neutral molecules PET reactions of donor anions and neutral acceptors or neutral donors and acceptor cations resulting in radical—radical anion (cation) pairs are known as well. These reactions are also called charge shift reactions due to the fact that the overall number of charged species is kept constant throughout the PET step. Finally, a PET process of a donor anion and a acceptor cation is possible as well (Scheme 2). 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

Electro-organic chemistry at the cathode is essentially that of radicals, radical-anions, carbanions, and polyanions (which range from dianions to hexa-anions (Scheme 1). The anions may behave as nucleophiles, bases, and as single electron reductants the factors governing the competition between these roles are not yet fully understood. 

If an electron acceptor is available in homogeneous solution, photochemical reaction can be observed. For example, when 2 is excited (X > 350 nm) in anhydrous dimethylsulfoxide (DMSO), methylation occurs, ultimately giving rise to 9,9-dimethyl-fluorene in >80% yield. By analogy with Tolbert s mechanism for photomethylation in DMSO (4), such a process may be initiated by electron transfer to DMSO to form a caged radical-radical anion pair from which subsequent C-S cleavage occurs (eqn 4). 

Excited states of carbanions are extraordinarily good electron donors (238,239). They offer an additional advantage compared with neutral donors in that the redox partners of the radical-radical anion pair formed by donation from a carbanion to a neutral acceptor are not electrostatically attracted. In a transfer between an excited carbanion and a neutral acceptor, eq. 77,... 

A nonchain Sr j 1-like mechanism is suggested in which the site of methylation is dictated by radical anion charge density. Whether radical-radical (nonchain) or radical-radical anion (chain) coupling dominates the reaction depends on the oxidizability of the anion (254-256). 

Solvation of thiolates is similarly low in both protic and dipolar aprotic solvents because of the size and polarisability of the large weakly basic sulfur atom, so is unlikely to contribute appreciably to the observed solvent effect. The intermediate nitro radical anion is stabilised by H-bonding in a manner which retards its dissociation in the SrnI mechanism (upper equation in Scheme 10.35). In contrast, the electron flow in the direct substitution at X (lower equation in Scheme 10.35) is such that solvation by methanol promotes the departure of the nucleofuge. In summary, protic solvation lowers the rate of the radical/radical anion reactions, but increases the rate of the polar abstraction yielding disulfide. 

A light-induced radical radical-anion chain mechanism is suggested by the fact that... 

An elegant photochemical formation of an aryl-carbon bond through a PET mechanism was recently reported in the total synthesis of the potent antimitotic polycycle (-)-diazonamide A. The reaction was initiated by intramolecular electron transfer between the indole chromophore and the adjacent bromoarene (Scheme 2.10). Thus, compound 21 was treated with an aqueous-acetonitrile solution of LiOH and the resulting lithium phenoxide solution was degassed and photolyzed (Rayonet, 300 nm) to yield biaryl 22 (as a single atropodiastereomer) in a good yield. A radical-radical anion pair (23) was formed upon excitation, and... 

It is obvious that an electric current may be used to generate reactive intermediates— radicals, radical anions, radical cations, cations, and anions so that it is logical that such intermediates may be used in some polymerization reactions. Thus, the overall polymerization process may be initiated either by direct electron transfer to or from the monomer itself or by the generation of an active species in the electrolyte. 

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