Aromatic systems nitrations, nitric acid

Recently Smith et alP21 have also reported the novel nitration systems comprising nitric acid, trifluoroacetic anhydride and zeolite HBeta, with or without acetic anhydride for the nitration of deactivated aromatic compounds such as... 

Our earlier nitration system involving nitric acid and acetic anhydride in conjunction with zeolite HP has been adapted to include trifluoroacetic anhydride, which enhances the reactivity considerably and allows the nitration of deactivated aromatics. Optimisation of the process has enabled the most selective double nitration of toluene yet attained, giving 2,4-dinitrotoluene in high yield. 

The nitration of aromatic compounds with nitric acid in an ionic liquid was shown by Earle et al. [96,97]. It was found that triflate and triflimide ionic liquids catalyze nitration reactions with nitric acid. This methodology has the advantage that water is the only by-product (Scheme 5.2-44). This process could also be carried out in phosphonium ionic liquids [46]. Acidic ionic liquids such as [EMIM][HS04] or the Davis-type ionic liquids [41] could also be used but gave lower reaction rates and selectivities [98]. The effect of metal triflates such as Yb(OTf)3 or Cu(OTf)2 dissolved in [N-butyl-N-methylpyrrolidium][triflimide] was investigated by Handy and Egrie [99]. These gave similar yields and selectivities to ionic liquids systems... 

Characteristics of the system as nitrating reagents Wibaut, who introduced the competitive method for determining reactivities (his experiments with toluene, benzene and chlorobenzene were performed under heterogeneous conditions and were not successful), pointed out that solutions of nitric acid in acetic anhydride are useful in making comparisons of reactivities because aromatic compounds are soluble in them. ... 

A further point of preparative significance still requires explanation, however. Highly reactive aromatic compounds, such as phenol, are found to undergo ready nitration even in dilute nitric acid, and at a far more rapid rate than can be explained on the basis of the concentration of N02 that is present in the mixture. This has been shown to be due to the presence of nitrous acid in the system which nitrosates the reactive nucleus via the nitrosonium ion, NO (or other species capable of effecting nitrosation, cf. p. 120) ... 

A novel, mild system for the direct nitration of calixarenes has been developed using potassium nitrate and aluminum chloride at low temperature. The side products of decomposition formed under conventional conditions are not observed in this system, and the p-nitro-calixarenes are isolated in 75-89% yields.17 Such Friedel-Crafts-type nitration using nitryl chloride and aluminum chloride affords a convenient system for aromatic nitration.18 Nitryl chloride was previously prepared either by the oxidation of nitrosyl chloride or by the reaction of chlorosulfonic acid with nitric acid. However, these procedures are inconvenient and dangerous. Recently, a mixture of sodium nitrate and trimethysilyl chloride (TMSC1) has been developed as a convenient method for the in situ generation of nitryl chloride (Eq. 2.6). 

Dinitrogen pentoxide (prepared by the oxidation of N204 with 03) in nitric acid is a potent nitration system. It can be used for nitrating aromatic compounds at lower temperatures than conventional system. It is also convenient for preparing explosives that are unstable in nitrating media containing sulfuric acid (Eq. 2.7).20... 

Attempts of further nitration of dinitro derivative 83 under usual conditions failed. Using 100% nitric acid in fluorosulfonic acid or trifluoromethanesulfonic acid, reagents useful for nitration of deactivated aromatic systems led to the formation of moisture-sensitive nitration products, which undergo further oxidation to give o-quinone-like species 84 and 85. Using the latter conditions, compound 86 can be isolated in 20% yield and converted into the tetraoxo derivative 85 by heating at 220°C (Scheme 4) <1996JOC1898>. 

Replacement of sulfuric acid in "mixed acid" aromatic nitrations by inorganic solids, with accumulation or elimination of produced water results in a fundamental different behaviour of the "HN03 - solid" couple. If water accumulates, nitric acid becomes a selective oxidative coupling agent, whereas when water is eliminated efficiently, nitric acid alone behaves as a strong nitrating agent, with increased paraselectivity as compared to the sulfo-nitric system. 

The reported improvement in yields and selectivities in both mono- and poly-nitration of aromatic compounds using Claycop with acetic anhydride (and if necessary nitric acid) in tetrachloromethane has been investigated.28 The reagent system is found to be modestly catalytic and regioselective in the mononitration of toluene but is neither catalytic nor regioselective in the nitration of 2-nitrotoluene. 

These systems nitrate aromatic compounds by a process of electrophilic substitution, the character of which is now understood in some detail ( 6.1). It should be noted, however, that some of them can cause nitration and various other reactions by less well understood processes. Among such nitrations that of nitration via nitrosation is especially important when the aromatic substrate is a reactive one ( 4.3). In reaction with lithium nitrate in acetic anhydride, or with fuming nitric acid, quinoline gives a small yield of 3-nitroquinoline this untypical orientation (cf. 10.4.246) may be a consequence of nitration following nucleophilic addition.5... 

As regards reactions other than nitration brought about by some of these systems, especially noteworthy are the addition processes undergone by certain indole derivatives when treated with solutions of nitric acid in acetic acid. Products include glycols, nitro-alcohols, and nitro-alcohol acetates.45 Such additions might well be encountered with some polynuclear aromatic compounds, and with such compounds the possibility of nitration by addition-elimination must always be borne in mind. 

We have reported the use of zeolite HP in conjunction with a mixture of acetic anhydride and nitric acid, which currently offers the best combination of yield and para-selectivity for nitration of simple aromatic compounds.11 For example, this system allows quantitative mononitration of toluene with selectivity for the para isomer of around 80 %. For commercial production of 4-nitrotoluene this would require less than half the toluene or nitric acid needed for the conventional mixed acid method and would generate less than one third of the waste. In general, the system offers excellent possibilities for nitration of substrates of moderate activity. However, it is not very successful with deactivated substrates. One aspect of the work reported here is therefore the modification of this approach so that it accommodates deactivated substrates. 

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 presence of carbon dioxide among the products of the reaction of teff-butyldifluoramine with excess nitric acid is a clear indication that C—C bond cleavage occurred. The nitrate and nitrite esters produced in this experiment were mixtures of various alkyl derivatives, and not solely tert-butyl derivatives as in the other cases where nitrate esters were detected. The relative stability of tritvldifluoramine toward oxidative cleavage is fully in accord with known differences between aromatic and aliphatic systems. 

Aqueous sulfuric acid behaves as a strong acid in most systems and forms salts with most metals by a variety of routes, such as reaction of the acid with the metal, neutralization of a metal oxide or hydroxide, or decomposition of metal carbonates. In organic synthesis, it is most well known for its use with nitric acid as a reagent for the nitration of aromatics (equation 24) via the N02 cation. 

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