Reactivity of Radical Anions

The electro-synthetic reactions of activated alkenes involve carbon-carbon bond formation, which, after much controversy, is now believed generally to involve radical-anion/radical anion coupling rather than the alternative radical-anion/substrate reaction. The history of this mechanistic debate is well documented168. 

Carbene type radical anions have been often postulated as reactive intermediates. An tetratrimethylsilyl substituted silylene (16) was reduced with several alkali metals in dimethoxyethane to yield a persistent silylene radical anion.169 

The close association between metal ions and p-benzoquinones catalyzes their Diels-Alder reactions with anthracenes. The efficiency of the metal cations correlates with their Lewis acidity171. A mechanism proceeding via radical-anions for a [3,3] sigmatropic rearrangement was established172. 

One may assume that after a long history of annulene radical ions hardly any novel aspects in this field can be established. Nevertheless some news can be 

Reversible dimerization reactions have been described for a number of aromatic radical ions. In aromatic oligomers and polymers with a more extensive delocalization of the charge and the spin such dimerizations are disfavoured. This was shown in a study of several aromatic molecules with extended it systems at different redox stages179. An extensive overview on fluorinated aromatic radical anions has been presented180. 

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As described above, spectroelectrochemical methods are useful in studying the reactivity of radical anions and cations in non-aqueous solutions. Related to this, electrochemiluminescence (ECL), which is often caused by the reaction between radical... 

These observations clearly indicate that (in these cases) solvation and ion pairing effects on the cleavage reactivity of radical anions strongly depend upon the localization of the negative charge. 

Reduction potential as a predictor of reactivity of radical-anions with oxygen... 

The reactivity of pyrene anion radical toward ArS02R may be demonstrated as follows adding some ArS02R to the pyrene solution leads to an increase of the peak current corresponding to reduction of the pyrene, while this step becomes progressively irreversible. It may also be noted that for small amounts of ArS02R, the specific step for the sulphone more or less vanishes. This corresponds to the following scheme ... 

Nowadays, ultramarine-type pigments are produced synthetically. Inside the zeolite structure the highly reactive sulfur radical anions are well protected which explains the stability of the blue color over thousands of years in air. However, the species responsible for the blue color should not be confused with the sulfur radical cations responsible for the blue color of sulfur solutions in fuming sulfuric acid (oleum) and similar oxidizing mixtures... 

The extended Hiickel method has been used in a discussion of properties and reactivity of radicals and biradicals (75). We have found it possible to correlate the basicity constants, pKbh. of radical anions with extended Hiickel data (76). 

A significant amount of the 02 metabolized by the human organism is converted to the highly reactive superoxide radical anion 02. Endogenous overproduction of 02 may cause considerable... 

The observation of radical anions has been confirmed by ESR measurements as illustrated by [Ir4(CO)12], (g = 2.002) 208). Similarly, a toluene solution of Co4(CO)i2 reacts with cobaltocene precipitating a brown compound which is extremely reactive and contains a cobaltocenium cation for each four cobalt atoms of the anion55. With excess cobaltocene (or alkali metals) in THF the reaction proceeds further as shown in Eq. (20),... 

In the light of the success of the Birch conditions for reducing organic compounds it is not surprising that epoxides can be opened by solvated electrons [6-9]. The initially formed radical is then further reduced to give carbanionic species, which do not display the reactivity of radicals. This concept has been extended by Bartmann [10], Cohen et al. [11], Conrow [12], and Yus et al. [13,14] who employed aromatic radical anions as the reduc-... 

Alonso et al. (2005) described anion-radical proton abstraction from prochiral organic acids. If the anion radicals were formed from homochiral predecessors, asymmetric deprotonation can be reached. However, low reactivity of the anion radical is required Slow proton transfer, that is, high activation energy of the reaction discriminates well between diastereoselective transition states. 

There are a number of non-electrochemical techniques that have proven invaluable in combination with electrochemical results in understanding the chemistry and the kinetics. Laser flash photolysis (LFP) is a well-established technique for the study of the transient spectroscopy and kinetics of reactive intermediates. The technique is valuable for the studying of the kinetics of the reactions of radical anions, particularly those that undergo rapid stepwise dissociative processes. The kinetics of fragmentation of radical anions can be determined using this method if (i) the radical anion of interest can be formed in a process initiated by a laser pulse, (ii) it has a characteristic absorption spectrum with a suitable extinction coefficient, and (iii) the rate of decay of the absorption of the radical anion falls within the kinetic window of the LFP technique typically this is in the order of 1 x 10" s to 1 X 10 s . 

Schmittel M,Ghorai MK(2001) Reactivity patterns of radical ions-a unifying picture of radical-anion and radical-cation transformations. In Balzani V (ed) Electron transfer in chemistry, vol 2. Organic molecules. Wiley-VCH, Weinheim, pp 5-54... 

Schmittel M, Ghorai MK (2001) Reactivity patterns of radical ions - a unifying picture of radical-anion and radical-cation transformations. In Balzani V (ed) Electron transfer in chemistry, vol 2. Organic molecules. Wiley-VCH, Weinheim, pp 5-54 Schoneich C, Bonifacic M, Dillinger U, Asmus K-D (1990) Hydrogen abstraction by thiyl radicals from activated C-H-bond of alcohols, ethers and polyunsaturated fatty acids. In Chatgilialoglu C, Asmus K-D (eds) Sulfur-centered reactive intermediates in chemistry and biology. Plenum, New York, pp 367-376... 

Relative reactivities of both anions towards CMe2N02, p-N02C6H4CH2 and p-N02C6H4C Me2 have also been reported with a change of the counterion and the solvent117. The results indicate that the (EtO)2PSion is a better trap than the (EtO)2PO ion for a-nitroalkyl radicals, particularly when ion pairing is important. 

The reactions of aromatic radical cations follow a scheme of reactivity which mirrors that of radical anions. Thus, typical reactions include addition of a charged or neutral nucleophile leading ultimately to a substituted aromatic (or its dihydro derivative, Scheme 29) and addition of a radical to yield a cation, which in turn either deprotonates or adds a nucleophile to yield respectively a substituted aromatic or a dihydro derivative (Scheme 30). The attacking radical (and in several cases also the nucleophile reacting in the second step) usually result from the fragmentation of the radical anion formed in the original electron transfer step. 

Consequent upon the orbitals preferred for the unpaired electron, a high spin population occurs at C-4 in each of 1,52, and 53. These are the anion-radicals found to exhibit the least persistence.35 Relative reactivities of the anion-radicals have been estimated by a cyclic voltammetric method as 1 > 1,3,5-triazine anion-radical > 52 > 53 > 22 > 54, which corresponds with qualitative observations on persistence and accords very closely with the ranking order of maximum spin populations at carbon.119 120 Dimerization of the radicals at positions of high spin population at carbon is proved for 1 and 52, although the mechanism is by no means clear for 52 and other modes of reaction can also occur, e.g., proton abstraction from solvent or adventitious water.30,35,121-127... 

The reactivities of several azanaphthalene anion-radicals, alter polaro-graphic generation, are discussed by van der Meer.122,123 This includes the radicals of quinazoline and cinnoline whose ESR spectra do not appear to have been reported. The stability of carbon-halogen bonds in various azine anion-radicals, including quinoxaline and quinoline, has been discussed, as has the reactivity of quinoline anions toward alkyl halides.92,182 In the latter reaction alkylation occurs at the 1,2- and 2,4-positions. Heteroarylation of a range of electron-rich substrates by azine anion-radicals has been reported.183 No mechanism is implied in the available abstract, but the apparent electrophilicity on the part of the azine anions is surprising. 

Most data were obtained from copolymerization studies. The copolymerization parameter r (see Chap. 5, Sect. 5.2) is the rate constant ratio for the addition of two different monomers to the same active centre. The inverse values of r j determined for the copolymerization of a series of monomers with the monomer M, define the relative reactivities of these monomers with the active centre from the first monomer, M°,. Thus it is possible to order monomers according to their reactivities in radical, anionic, cationic and coordination polymerizations from the tabulated values of copolymerization parameters [101-103]. 

Bimolecular reactions of radical anions are largely restricted to arene aocqRors owing to dteir generally more persistent character. The ambivalence of arene radical anions generally relates to the reactivity towards acids, electrophiles and electron acceptors. 

Since electrode measurements involve low substrate concentrations, reactive impurities have to be held to a very low level. The physical data and purification methods for several organic solvents used in electrode measurements have been summarized (Mann, 1969). But even when careful procedures for solvent and electrolyte purification are employed, residual impurities can have profound effects upon the electrode response. For example, the voltam-metric observation of dications (Hammerich and Parker, 1973, 1976) and dianions (Jensen and Parker, 1974, 1975a) of aromatic hydrocarbons has only been achieved during the last ten years. The stability of radical anions (Peover, 1967) and radical cations (Peover and White, 1967 Phelps et al., 1967 Marcoux et al., 1967) of aromatic compounds was demonstrated by cyclic voltammetry much earlier but the corresponding doubly charged ions were believed to be inherently unstable because of facile reactions with the solvents and supporting electrolytes. However, the effective removal of impurities from the electrolyte solutions extended the life-times of the dianions and dications so that reversible cyclic voltammograms could be observed at ambient temperatures even at very low sweep rates. 

This review article deals with addition and cycloaddition reactions of organic compounds via photoinduced electron transfer. Various reactive species such as exdplex, triplex, radical ion pair and free radical ions are generated via photoinduced electron transfer reactions. These reactive species have their characteristic reactivities and discrimination among these species provides selective photoreactions. The solvent and salt effects and also the effects of electron transfer sensitizers on photoinduced electron transfer reactions can be applied to the selective generation of the reactive species. Examples and mechanistic features of photoaddition and photocycloaddition reactions that proceed via the following steps are given reactions of radical cations with nucleophiles reactions of radical anions with electrophiles reactions of radical cations and radical anions with neutral radicals radical-radical coupling reactions addition and cycloaddition reactions via triplexes three-component addition reactions. 

Reactivity Patterns of Radical Ions—A Unifying Picture of Radical-anion and Radical-cation T ransformations... 

Scheme 5. Illustration of the similar reactivity patterns of radical anions and radical cations in bond-formation reactions.

The above categorization emphasizes the similarities of the reactivity patterns of radical anions and radical cations. On the basis of the bond and fragment electron affinities (Table 1) or ionization potentials (Table 2), we can now devise systems that should undergo, for example, facile bond cleavage. As a prerequisite the a (or cr) orbital representing the scissile bond has to be made a major contributor to the LUMO (or HOMO). 

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