Electrophilic addition acceptor radical anions

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

When analyzing plausible mechanisms of the VNS reactions of nitroarenes with a-chlorocarbanions, one should clarify a few key questions how to proceed the addition and subsequent conversion of adducts and how other substituents may affect both of these steps - rate and orientation of the addition, rate of the elimination, etc. It is well known that nitroarenes are active electron acceptors, whereas carbanions are good electron donors thus, these reactants can enter a single-electron transfer (SET) to form anion radicals of nitroarenes and radicals from carbanions [21, 22]. Further coupling of these electrophilic radicals with nucleophilic anion-radical species could give adducts. This SET pathway, alternative to the direct addition, is often favored by authors and the concept is sometimes abused, see [23] and rebuttal [24]. Nevertheless, numerous observations contradict participation of the SET mechanism in the VNS reactions ... 

Formation of the o -adducts of nucleophiles to nitroarenes is a common initial step for aU Sj ArH reactions ONSH, VNS, cine- and fe/e-suhstitution, etc. The key differences between these reactions are pathways of conversion of the o -adducts into final products. Nitroarenes can act not only as electrophilic partners hut also as active electron acceptors. Since carbanions are good electron donors, the reaction between nitroarenes and carbanions can proceed via a simple direct addition or via an SET to produce the corresponding anion radicals of nitroarenes and radicals from the carbanions. Combination of these paramagnetic species can also produce o -adducts. Indeed, in such systems, ESR spectra of the anion radicals of nitroarenes are often observed. On this basis, SET is often postulated as the way of formation of the a-adducts (Scheme 11.20) [36]. 

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