Nitration Electrophilic dimer

The furoxan ring is notably resistant to electrophilic attack and reaction normally takes place at the substituents. Thus aryl groups attached to monocyclic furoxans and the homocyclic ring of benzofuroxans are nitrated and halogenated without disruption of the heterocycle. Reaction with acid is also slow protonation is predicted to occur at N-5 <89KGS1261> and benzofuroxans have pKj, values of ca. 8, similar to those of benzofurazans. Monosubstituted furoxans are, as expected, less stable and can be hydrolyzed to the corresponding carboxylic acid. Treatment of the parent furoxan (3) with concentrated sulfuric acid results in rearrangement to (hydroxyimino)acetonitrile oxide (HON=CHC=N —O ) and subsequent dimerization to bis(hydroxyiminomethyl)furoxan... 

The mechanistic foundations of indole nitration have been well established <72HC 25-2)539>. While the 3-position is the preferred site for electrophilic attack, nitration of indole itself is complicated by oxidation and dimerization. In strongly acidic solution, the 3-position is protonated and reaction takes place at the 5-position under the directing influence of the iminium moiety (Scheme 114). 

As an example, this apply to enols or tautomeric enols such as maleic acid derivatives. While with a chemical reagent (cerium ammonium nitrate) the only process occurring is oxidative dimerization, when aromatic nitriles are used as the photochemical oxidant, selective trapping of the radicals by an electrophilic alkenes or by the nitrile itself occurs. Under these conditions, both the alkylation of alkenes and the oxidative alkylation/dimerization of dienes have been smoothly obtained (see Scheme 8) and side processes such as double alkylation or polymerization often occurring with other methods have been avoided. A three-component (Nucleophile-Olefin Combination, Aromatic Substitution) process is also possible. ... 

Evidence that stereoelectronic effects are important has been obtained by studying the intramolecular deprotonation selectivity in asymmetric polymethylbenzenes, which is the same in photoinduced SET and in activation by inorganic oxidants, or the methyl vs. iso-propyl selectivity in cymenes. When the benzyl radicals are highly stabilized, they undergo dimerization. More interestingly from the synthetic point of view, these may be used for the photoinduced benzylation of electrophilic alkenes both by using molecular sensitizers such as DCN (1,4-dicyanonaphthalene, see Equation 4.11) and by using titanium dioxide powder photocatalysis, as well as for selective oxidation at the benzylic position " " and photoinduced nitration with tetranitromethane. ... 

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