Electrophilic aromatic substitution charge-transfer mechanism

Finally, we ask, if the reactive triads in Schemes 1 and 19 are common to both electrophilic and charge-transfer nitration, why is the nucleophilic pathway (k 2) apparently not pertinent to the electrophilic activation of toluene and anisole One obvious answer is that the electrophilic nitration of these less reactive [class (ii)] arenes proceeds via a different mechanism, in which N02 is directly transferred from V-nitropyridinium ion in a single step, without the intermediacy of the reactive triad, since such an activation process relates to the more conventional view of electrophilic aromatic substitution. However, the concerted mechanism for toluene, anisole, mesitylene, t-butylbenzene, etc., does not readily accommodate the three unique facets that relate charge-transfer directly to electrophilic nitration, viz., the lutidine syndrome, the added N02 effect, and the TFA neutralization (of Py). Accordingly, let us return to Schemes 10 and 19, and inquire into the nature of thermal (adiabatic) electron transfer in (87) vis-a-vis the (vertical) charge-transfer in (62). 

Such an explanation of the selectivity paradox is compatible with the concept of electron transfer mechanisms in an electrophilic aromatic substitution (for a general review of this concept see Refs. [144,145]). The electron transfer occurs at the stages of formation of the intermediate complex XLIV and the d-complex XLV. In 1959 Brown put forward a hypothesis that there should exist a charge transfer stage responsible for the formation of the 7r-complex XLIV in a nitration reaction. A similar view is held by some other authors [146-148], even if the adduct XLIV is not necessarily regarded as a 7r-complex. In the nitration and nitrosation reactions, the electron transfer from an activated aromatic nucleus onto the lower-lying vacant MO s of the ion acceptor is thermodynamically quite advantageous (20-111 kcal/mol), therefore the detailed representation of the reaction scheme of Eq. (5.13) ... 

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