Nitration electron transfer mechanism

The electron-transfer mechanism for electrophilic aromatic nitration as presented in Scheme 19 is consistent with the CIDNP observation in related systems, in which the life-time of the radical pair [cf. (87)] is of particular concern (Kaptein, 1975 Clemens et al., 1984, 1985 Keumi et al., 1988 Morkovnik, 1988 Olah et al., 1989 Johnston et al., 1991 Ridd, 1991 Rudakov and Lobachev, 1991). As such, other types of experimental evidence for aromatic cation radicals as intermediates in electrophilic aromatic nitration are to be found only when there is significant competition from rate processes on the timescale of r<10 los. For example, the characteristic C-C bond scission of labile cation radicals is observed only during the electrophilic nitration of aromatic donors such as the dianthracenes and bicumene analogues which produce ArH+- with fragmentation rates of kf> 1010s-1 (Kim et al., 1992a,b). 

A review on the nitration of aromatics (using a range of species including N02 and NO3) has appeared. Evidence for electron-transfer mechanisms via radical cations has been reviewed. In addition, another review comparing the reactivity of a range of radicals and radical cations has appeared. While radicals prefer to add to the carbon of CN triple bonds, radical cations were found to prefer addition at the N atom. Ab initio calculations were performed to rationalize this behaviour. 

A. S. Morkovnik, the Oxidation-Reduction Stage in the Nitration Reaction, Russ. Chem. Rev. 57,144 (1988). L. Eberson u. F. Radner, Electron-Transfer Mechanisms in Electrophilic Aromatic Nitration, Acc. Chem. 

The coupled proton-electron transfer mechanism can also be applied to the molybdenum reductases. For nitrate reductase, a scheme such as Reaction 20 is possible. A Mo (IV)-Mo (VI) couple is used to illustrate this, and while such a couple is viable for some nitrate reductases, the Mo(II)-Mo(IV) or the Mo(III)-Mo(V) couple could also be accommodated... 

Benzyltrimethylsilanes, on the other hand, give products of C-Si cleavage, i.e. benzyl nitrate and acetate, on treatment with cerium(IV) ammonium nitrate in AcOH based on ring substituent effects, a one-electron transfer mechanisms seems to be in operation. 

Electron-transfer mechanism in electrophilic nitration of activated heteroaromatic compounds 87ACR53 88UK254. 

The experiments of Rosser, Inami and Wise [57] were the continuation of their w ork on catalytic decomposition of ammonium nitrate [74]. They examined the action of copper chromite. They found that it acted at the early stage of the reaction and its action disappeared after copper diromiie was oxidized by the products of catalytic reaction. Cobalt oxide was found to be an exceptional catalyst it produced NOCl and NO2CI as major products and only a trace quantity of N2O3. Tlic authors came to the conclusion that copper chromite catalysed thermal decomposition of AP according to an electron transfer mechanism (4). 

Treatment of cyclopropyl sulfides bearing a hydroxy group in the side chain with ceric ammonium nitrate (CAN) in methanol gives five- and six-membered cyclic ethers.In this reaction, the formation of cyclic ethers is understood by assuming a single electron transfer mechanism, which involves ring opening of the cation intermediate followed by intramolecular nucleophilic addition. 

Relative rate constants of the nitration were obtained by direct competition between two aromatic hydrocarbons. The rate constants depend markedly on substrate structure, with >10 difference in values between the least reactive (benzene) and most reactive (perylene) compounds studied. The more reactive (and more easily ionised) polycyclic aromatic hydrocarbons interact with NO in solution by an electron transfer mechanism. 

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) ... 

Cyclic and acyclic silyl enol ethers can be nitrated with tetranitromethane to give ct-nitro ketones in 64-96% yield fEqs. 2.42 and 2.43. " The mechanism involves the electron transfer from the silyl enol ether to tetranitromethane. A fast homolydc conphng of the resultant cadon radical of silyl enol ether with NO leads tn ct-nitro ketones. Tetranitromethane is a neutral reagent it is commercially available or readdy prepared. " ... 

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). 

Feng et al. (1986) performed quantum-chemical calculations of aromatic nitration. The resnlts they obtained were in good accordance with the IPs of N02 and benzene and its derivatives. The radical-pair recombination mechanism is favored for nitration whenever the IP of an aromatic molecule is much less than that of N02. According to calculations, nitration of toluene and xylene with N02 most probably proceeds according to ion-radical mechanism. Nitration of nitrobenzene and benzene derivatives with electron-acceptor substituents can proceed through the classical polar mechanism only. As for benzene, both mechanisms (ion-radical and polar) are possible. Substituents that raise the IP of an aromatic molecule to a value higher than that of N02 prevent the formation of this radical pair (one-electron transfer appears to be forbidden). This forces the classical mechanism to take place. It shonld be nnderlined that a solvent plays the decisive role in nitration. 

The title system in AN forms a homogeneous solution. The generation of NO cation takes place. As known, NO is a remarkable, diverse reagent not only for nitrosation and nitration but also for oxidation. Kochi et al. (1973) christened a new general mechanism oxidative aromatic substitution to describe aromatic snbstitntion reactions (Kochi 1990, Bosch and Kochi 1994). This mechanism incorporates ground-state electron transfer before the substitution step (see also Skokov and Wheeler 1999). 

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