Nitrating agents, reactions review

The nitration of active methylene compounds generally proceeds via the reaction of carbanionic intermediates with an electrophilic nitrating agent such as alkyl nitrate (alkyl nitrate nitration). Details of this process are well documented in the reviews.38 The alkyl nitrate nitration method has been used extensively for the preparation of arylnitromethanes. The toluene derivatives, which have electron-withdrawing groups are nitrated with alkyl nitrates in the presence of KNH2 in liquid ammonia (Eqs. 2.19 and 2.20).39... 

The reaction of alkenes with nitrogen oxides and other nitrating agents have been extensively discussed by Olah, Topchiev, and in numerous reviews." ... 

Abstract Synthesis methods of various C- and /V-nitroderivativcs of five-membered azoles - pyrazoles, imidazoles, 1,2,3-triazoles, 1,2,4-triazoles, oxazoles, oxadiazoles, isoxazoles, thiazoles, thiadiazoles, isothiazoles, selenazoles and tetrazoles - are summarized and critically discussed. The special attention focuses on the nitration reaction of azoles with nitric acid or sulfuric-nitric acid mixture, one of the main synthetic routes to nitroazoles. The nitration reactions with such nitrating agents as acetylnitrate, nitric acid/trifluoroacetic anhydride, nitrogen dioxide, nitrogen tetrox-ide, nitronium tetrafluoroborate, V-nitropicolinium tetrafluoroborate are reported. General information on the theory of electrophilic nitration of aromatic compounds is included in the chapter covering synthetic methods. The kinetics and mechanisms of nitration of five-membered azoles are considered. The nitroazole preparation from different cyclic systems or from aminoazoles or based on heterocyclization is the subject of wide speculation. The particular section is devoted to the chemistry of extraordinary class of nitroazoles - polynitroazoles. Vicarious nucleophilic substitution (VNS) reaction in nitroazoles is reviewed in detail. 

The aromatic electrophilic substitution is usually represented by replacing a hydrogen atom with an electrophilic agent. However it was recently shown that the electrophilic attack is not restricted to aromatic carbon bonded with hydrogen. This is the case in rpso-nitration where the attack occurs at a substituted aromatic ring carbon. The ipso-reaction was placed in focus by Perrin and Skinner [23] in 1971 and two excellent review papers appeared on the subject by Myhre [24] and by Hahn and associates [25]. 

Nitrating medium, that is, nitric acid or mixtures of nitric acid with sulphuric acid or other agents promoting the formation of nitroxonium cation, is obviously a strongly oxidizing medium, hence a number of side reactions — mainly oxidation reactions — take place during nitration. This was described in Vol. I (pp. 75, 437) and more recently reviewed in a number of excellent papers [84-86]. 

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