Generation of Radical Cations

One electron transfer from the highest filled MO of a neutral substrate 170 (Eq. (236) ) to the anode yields a radical cation 171 as product. This may be either a transient intermediate or a stable, long-lived species depending on its substituents and the nucleophilicity of the solvent. The reaction paths of radical cations have been expertly and comprehensively reviewed by Adams 2 5 2 9 so that a short summary seems sufficient at this place. Deprotonation and 1 e-oxidation of 171 with a subsequent Sj l reaction of the resulting cation yields side-chain substitution products 172 (path a), see 9.1. Solvolysis of 171 followed by le-oxidation 

The oxidation potentials 170 ——- 777 of a large number of aromatic hydrocarbons, amines, phenols,heterocycles and olefins are tabulated I0,10a 25-48 65,525-528) an(j nee(j not repeated here. Such potentials have been successfully correlated with HMO-parameters 525 530 538) ie in oxidations with the energy of the highest filled MO (HFMO).Adams 25) and Peover 65) have discussed some precaution to which attention should be paid in such correlations, e.g., shifts in potentials due to the irreversibility of the electrode process or due to fast follow-up reactions. 

In Table 17 representative examples of anodically generated radical cations are compiled. 

Pen tame thoxy benzene C ESR, lifetime 10 sec, HMO correlation with 1/2 538) 

10-Dihy dro-5,10-di-methylphenazine C 2 rev. le-oxidations (Cyclic voltammetry, chronopotentiometry), ESR  

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Another mode for catalyzed cycloaddition involves the generation of radical cations from electron-rich alkenes with single-electron oxidants such as tris(4-bromophenyl)amminium hexachloroantimonate (TBAH). An equivalent reaction involves the photosensitized electron transfer (PET) process (see Section 1.3.2.3.). These processes have been recently reviewed,9 and are limited to electron-rich alkenes capable of producing radical cations. Furthermore, some of the cyclobutanes themselves undergo secondary isomerization under the oxidative conditions, e.g. formation of 31-35.10-12... 

Restriction of hydrocarbon chains to small rings is known to significantly enhance their oxidizability, either in the gas phase or in solution (95), so that photooxidative generation of radical cations from small rings should occur readily. Because... 

The oxidation of l-phenyl-2-(4-methylphenyl)diazenes, 4 -substituted with a dimethylamino or methoxy group, with ferric chloride resulted in the generation of radical cation by a single electron transfer. It has been proposed that an electron, in the first step of oxidation, is extracted from the azo and/or from the amino nitrogen atom.68... 

A rich variety of chemical oxidizing reagents have been applied for the generation of radical cations. The principal reagent types include Bronsted and Lewis acids the halogens certain peroxide anions or radical anions numerous metal ions or oxides nitrosonium and dioxygenyl ions stable organic (aminium) radical cations ... 

The most common triplet state electron acceptors are ketones and quinones, whereas aromatic hydrocarbons, often bearing one or more cyano groups, are the most frequently used singlet state electron acceptors. For the generation of radical cations from a given donor it is important that the exothermidty of the electron transfer reaction can be adjusted to fall within an appropriate range, typically... 

Photochemical electron transfer reactions of electron donor-acceptor pairs in polar solvents provide a convenient and effective method for the generation of radical cations which can be trapped by complex metal hydrides. One of the most effective systems is based on irradiation of a solution of substrate, sodium borohydride and 1,4- or 1,3-dicyanobenzene. A range of bi- and poly-cyclic aromatic hydrocarbons has been converted into the dihydro derivatives in this way. An especially important aspect of this route to dihydroaromatic compounds is that it may give access to products which are regioisomeric with the standard Birch reduction products. Thus, o-xylene is converted into the 1,4-dihydro product (229) rather than the normal 3,6-dihydro isomer (228). The m- and p-xylenes are similarly reduced to (230) and (231), respectively. ... 

Scheme 20-4 Generation of radical cations by irradiation of Freon matrices.

As can be seen from the few examples cited above SET processes are now fairly common in organic photochemistry. One of the areas where considerable study has taken place is the process referred to as a photo-NOCAS. Within this framework Albini and coworkers have shown that the products formed from the reaction of 2,3-dimethylbut-2-ene with 1,4-dicyanobenzene are compounds (22)- 25). The reaction was brought about using phenanthrene as the initial light absorber. This technique leads to cleaner reactions than those where the 1,4-dicyanobenzene is irradiated directly. The solvent system used is methanol/ acetonitrile and products (24) and (25) are the result of solvent incorporation. A further example of photo-NOCAS chemistry has been reported by Arnold and coworkers.Typical of the examples studied is the reaction illustrated in Scheme 2. The cyclization of the dienes (26) was also examined. This specific example deals with the generation of radical cations from (/ )-(+)-a-terpineol and (/ )-(+)-limonene with 1,4-dicyanobenzene as the electron accepting sensitizer. In another detailed study on reactions of this type the factors that control the regiochemistry in photo-NOCAS processes have been assessed. ... 

Two mechanistic studies relevant to photocuring processes have appeared. One deals with the efficient photoinduced generation of radical cations in solvents of medium and low polarity ". These cations act as sensitizers of the polymerization of N-methylacridinium hexafluorophosphate. The other is a study of the photochemistry of triarylsulphonium salts . 

Steenken, S., Warren, C. J., Gilbert, B. C., Generation of Radical Cations from Naphthalene and Some Derivatives, Both by Photoionization and Reaction with S04 Formation and Reactions Studied by Laser Flash Photolysis, J. Chem. Soc., Perkin Trans. 1990, 2, 335 342. 

Table 5.3 First vertical ionization potential (IPj) for some molecules/atoms used as matrices for generation of radical cations by ionizing radiation...

The anodic substitution reaction is another electrochemical approach to induce the Sn and Sn reactions. The term anodic addition reaction is used in the literature, since an anodic process is exploited for the initial electrochemical generation of radical-cation intermediates [81,82]. Hence, anodic addition involves... 

Table III. Influence of Reductones on the Generation of Radical Cations in the Induction Period of a Thermally Treated Glucose/ Na-acetyl-L-lysine Mixture...

The idea of employing the ABTS + radical cation for testing in vitro antioxidant abilities of natural and synthetic substances was first suggested by Miller et al. [12]. This provided a basis for the development of two types of assays that may be used for these purposes, namely, (1) decolorization assays involving the preliminary generation of radical cation ABTS, followed by measuring the ability of the samples to suppress the absorbance of this radical cation at a fixed time point and (2) kinetic assays based on the radical cation formation in the presence of potential AOXs. 

Evans, C.H., and J.C. Scaiano Photochemical Generation of Radical Cations from a-Terthienyl and Related Thiophenes Kinetic Behavior and Magnetic Field Effects on Radical-Ion Pairs in Micellar Solution. J. Am. Chem. Soc. 112, 2694-2701 (1990). 

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