Aromatic radical zwitterions

Delocalised radical-zwitterions are formed also from other aromatic tt-systems bearing a positive charge. Tropylium salts 2 show a one-electron reduction wave on polarography in acetonitrile with E>/, = -0.17 V vr. see [26], The zwitterion is more stable in 6 M sulphuric acid where a second one-electron wave is seen at... 

O-H deprotonation of arylalkanol (I), benzoic acid (II) and arylalkanoic acid (III) radical cations eventually leads to radical zwitterions where the electron hole is localized on the aromatic system and the negative charge resides on the oxygen atom originally bonded to the acidic proton (Scheme 65). From the radical zwitterions an oxygen centered radical can be formed by intramolecular electron transfer from the side-chain -0 to the aromatic jr-system, whose chemistry will be discussed below. 

An explanation can be found in terms of the much lower reduction potential of the indole-type radical cations as compared to the phenylalkanoic acid ones, which results in a greater stabilization of the positive charge on the aromatic system thus opposing intramolecular electron transfer. In other words, a later transition state is expected in the decarboxylation of indole-type radical zwitterions as compared to the phenylalkanoic acid ones, with an increased importance of the stability of the carbon centered radical. 

In summary, we emphasize that homoaromatic stabilization appears to be more important for radical cations than for their neutral diamagnetic precursors. There are several reasons why radical cations may assume cyclic conjugated structures with 4n + 17i-electrons, one electron shy of the magic 4n + 2 7i-electrons which achieve aromaticity, when the parent molecules opt for a less delocalized nonaromatic structure. The principal cause might lie in the strength of the carbon-carbon single bond, which generally disfavors biradical or zwitterionic... 

The most important competing process to the bond-formation is the complete electron transfer to form ion-radicals, which occurs where no bond formation is possible, for example, for aromatic donor-acceptor pairs. For vinyl copolymerizable pairs, the bond will form between the components to give a diradical tetramethylene. For the ionic homopolymerization system, on the other hand, it is difficult to distinguish the ion-radicals from zwitterionic tetramethylenes by the kinetic analysis. In this case, the accompanying cycloaddition reaction offers powerful evidence for the zwitterion formation, i.e., the bond-formation. 

There are many other kinds of reactive intermediates, which do not fit into the previous classifications. Some are simply compounds that are unstable for various possible reasons, such as structural strain or an unusual oxidation state, and are discussed in Chapter 7. This book is concerned with the chemistry of carbocations, carbanions, radicals, carbenes, nitrenes (the nitrogen analogs of carbenes), and miscellaneous intermediates such as arynes, ortho-quinone methides, zwitterions and dipoles, anti-aromatic systems, and tetrahedral intermediates. This is not the place to describe in detail the experimental basis on which the involvement of reactive intermediates in specific reactions has been estabhshed but it is appropriate to mention briefly the sort of evidence that has been found useful in this respect. Transition states have no real hfetime, and there are no physical techniques by which they can be directly characterized. Probably one of the most direct ways in which reactive intermediates can be inferred in a particular reaction is by a kinetic study. Trapping the intermediate with an appropriate reagent can also be very valuable, particularly if it can be shown that the same products are produced in the same ratios when the same postulated intermediate is formed from different precursors. 

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