Aromatic cation radicals, electrophilic reactions

THIS CHAPTER IS CONCERNED WITH A REACTION of aromatic and hetero-cyclic cation radicals about which only little is so far known their ability to react with neutral radicals. The reaction is expressed simply for the coupling of an aromatic cation radical (ArH +) with a radical (R-) in equation 1. This simple equation, presently only poorly documented, is nevertheless part of current thinking in two reactions of wide scope electrophilic aromatic substitution and reactions of cation radicals with nucleophiles. The product of equation 1 is a a complex, (ArHR)+, which is structurally the same as that... 

Tetraphenylethylene cyclizes anodically to 9,10-diphenylphenanthrene analogously to its photooxidative cyclization. The attempted anodic cyclization of cis- or frans-stilbene to phenanthrene however failed due to electrophilic reaction of the intermediate radical cation with the solvent 37S Primary aromatic amines are oxidized to radical cations which, depending on the pH of the electrolyte couple to aminodiphenylamines (C-N coupling (84) in Eq. (172) ), yield benzidines (85) at low pH (C-C coupling) or dimerize to hydrazobenzene (86) (N-N coupling) which is subsequently oxidized to azobenzene (Eq. (172) ) 2 5,376,377)... 

Electrophilic aromatic substitution is the mechanism suggested in both cases the initial step being attack of the cation radical on the aromatic compound (ArH) (reaction 101), followed by deprotonation of the intermediate (reaction 102) and further oxidation of the radical to the resulting arsonium or stibonium ion (reaction 103). 

We have already seen that aromatics can be aminated with amine cation radicals. Organosulfur cation radicals react with aromatics carrying an electron-donating group. The reaction appears to be electrophilic in nature (nitrobenzene does not react) and follows the stoichiometry shown in (166). The reactions are second order in... 

The reactivity of the diselenide dication (51) towards various nucleophiles and in redox reactions is investigated. The diselenide dication undergoes electrophilic substitution reactions with activated aromatic compounds to give para-substituted selenonium salts (57), but oxidizes benzenethiol into diphenyl disulfide <90CL393>, both of which are analogous chemical behaviors to those of the disulfide dication (36). Thus, the Se-dication can both function as an electrophile and as an oxidant. The oxidation potential of the aromatic compound it reacts with determines the mode of reaction. Indeed, the reaction with phenothiazine, whose oxidation potential is known to be lower than that of the Se-dication (51), generates a cation radical detected by the color of the reaction mixture and the UV-visible absorptions at 436 and 515 nm. 

The previous sections leave no doubts that aromatic compounds, react with positively charged electrophiles to form a-complexes-arenium ions. But are they the primary intermediates It is not by accident that the problem of preliminary formation of radical cations has arisen. Its statement is an attempts to explain the orientational peculiarities of electrophilic aromatic substitution of hydrogen. The widespread view that the orientation in the reactions of aromatic compounds with electrophiles is dictated by the relative stabilities of the cr-complexes explains but a part of the accumulated material. In the first place this refers to the meta- and para-orienting effects of electron-releasing substituents in benzene in terms of the QCT -approach and to that of the relative reactivity of various aromatic substrates... 

It is difficult to decide whether the above approaches generally applicable to the reactions of aromatic compotmds with electrophiles. It seems desirable to compare it further with the intermediate formation of radical cations of aromatic substrates (see Sect. IV.6) whose conversion into cr-complex is controlled by the spin density distribution and by the relative stability of [Pg.210]

The reaction of pyridine (and picolines and lutidines) with cation radicals is one of the earliest to be established and documented. In fact, it is referred to as the pyridination reaction. Reaction occurs at the pyridine s nitrogen atom. That is, electrophilic aromatic substitution (see below) does not occur, the pyridine ring being too unreactive for this. The pyridine behaves as a nucleophile and leads, for example, with DPA " " to 14, and with 10to 15. The reaction with DPA " has been particularly well studied, and has been shown to be a half-regeneration reaction (28). 

Cation radicals react as electrophiles with aromatics. The scope of this type of substitution is not too well explored, though. Amination of aromatics with dialkylaminium radicals has been known for some time ( 2). In recent years the arylation of organosulfur cation radicals was discovered in our own laboratories (eq. 27) (33, 34). The reaction is limited in that the... 

Mechanistically there is ample evidence that the Balz-Schiemann reaction is heterolytic. This is shown by arylation trapping experiments. The added arene substrates are found to be arylated in isomer ratios which are typical for an electrophilic aromatic substitution by the aryl cation and not for a homolytic substitution by the aryl radical (Makarova et al., 1958). Swain and Rogers (1975) showed that the reaction takes place in the ion pair with the tetrafluoroborate, and not, as one might imagine, with a fluoride ion originating from the dissociation of the tetrafluoroborate into boron trifluoride and fluoride ions. This is demonstrated by the insensitivity of the ratio of products ArF/ArCl in methylene chloride solution at 25 °C to excess BF3 concentration. 

Both the catalytic and electrochemical polymerization reactions appear to result from electron donation from the aromatic species to the surface producing a radical cation that leads to electrophilic addition. 

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

Radical cations resulting from oxidation of olefins, aromatic compounds, amino groups, and so on, can react by electrophilic addition to a nucleophilic center as shown, for example, in Scheme 1 [2, 3]. The double bond activated by an electron-donating substituent is first oxidized leading to a radical cation that attacks the nucleophilic center. The global reaction is a two-electron process corresponding to an ECEC mechanism. 

An additional important feature of this class of polymers lies in the fact that their polymerisation and doping processes may be driven by a single electrochemical operation which, starting from the monomer, first forms the polymeric chain and then induces its oxidation and deposition in the doped form as a conductive film on a suitable substrate. The polymerisation reaction may be basically described as an electrophilic substitution which retains the aromatic structure and proceeds via a radical cation intermediate ... 

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