Nitrating agent nitronium tetrafluoroborate

Nitronium salts are efficient and powerful nitrating agents. Nitronium tetrafluoroborate is the most commonly used nitronium salt for nitration and is commercially available as a solid or as a solution in sulfolane in which the ions are highly solvated and exist as an ion pair. Nitronium tetrafluoroborate shows poor solubility in most organic solvents and so the more soluble nitronium hexafluorophosphate is sometimes preferred for nitrations. Olah and co-workers - have studied the nitration of a vast array of aromatics with nitronium salts. Solutions of nitronium salts in aprotic organic solvents are useful for the nitration of acid sensitive or readily oxidized substrates. Nitronium tetrafluoroborate has been used for the... 

Salts containing the nitronium ion can be prepared and are reactive nitrating agents. The tetrafluoroborate salt has been used most frequently,8 but the... 

The nitration of olehns, particularly those containing electron donating substituents, by nitronium salts is complicated by side reactions and does not always lead to the expected result. In many instances nitration with nitronium tetrafluoroborate takes place with higher yields in the presence of a-picoline [91,132]. In this case the nitrating agent is apparently not the nitronium salt but 2-methyl-W-nitropyridinium tetrafluoroborate, which is formed rapidly when NO2 BFJ is mixed with a-picoline [73b],... 

Salts containing the nitronium ion can be prepared, and they are reactive nitrating agents. The tetrafluoroborate has been used most frequently," but the trifluoromethanesulfonate can also be prepared readily. Pyridine and quinoline form iV-nitro salts on reaction with N02Bp4. These N-nitro heterocycles in turn can act as nitrating reagents. This is called transfer nitration. ... 

This salt turned out to be remarkably stable and a powerful, convenient nitrating agent for a wide variety of aromatics (and later also aliphatics). Over the years, this chemistry was further developed, and nitronium tetrafluoroborate is still a widely used commercially available nitrating agent. 

Nitronium tetrafluoroborate is a very efficient reagent for the D-nitration of alcohols and glycols containing both primary and secondary hydroxy functionality (Equation 3.7). Yields of nitrate ester are high and frequently quantitative. 0-Nitration with nitronium salts produces a strong acid and so an acid-binding agent should be present for acid sensitive substrates. 

Numerous Lewis acids promote the formation of nitronium ions when in the presence of nitric acid. Nitric acid-boron trifluoride, and the nitric acid-hydrogen fluoride-boron trifluoride reagents described by Olah are practical nitrating agents the latter provides a convenient preparation of nitronium tetrafluoroborate. Olah reports that nitric acid-magic acid (FSOsH-SbFs) is extremely effective for the polynitration of aromatic substrates. 

Nitronium tetrafluoroborate (1) is stable in the absence of moisture, but is hygroscopic with subsequent decomposition and when heated it decomposes at ca. 170°C without melting or subliming.83 It is a powerful and strong nitrating agent.83,89... 

Nitronium tetrafluoroborate See NITRATING AGENTS See other APROTIC SOLVENTS... 

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 isoxazoles (1,2-oxazoles) are nitrated exclusively at position 4. Isoxazole itself is nitrated with difficulty, and the yield of the nitro derivative does not exceed 3.5% [165, 166], With nitronium tetrafluoroborate as nitrating agent it is possible to increase the yield of 4-nitroisoxazole to 35% [167],... 

During the reaction of isoxazole with nitronium tetrafluoroborate 4-nitroisoxazole is isolated with a yield of 35% [167], With nitronium tetrafluoroborate as nitrating agent it is possible to realize nitration in a neutral medium without protonation of the azole ring. In this case it was possible to isolate /V-nitroimidazoles [324] and 1,2,4-triazoles [348], or their formation as intermediate products was inferred [349, 350] (Scheme 36). 

However, it was not possible to obtain l-trityl-2-nitroimidazole by this method as, for example, also during the nitration of the lithium derivative of 1-tritylimida-zole by nitronium tetrafluoroborate [356], This is probably due to the elimination of the triphenylmethyl group during nitration. However, it was possible to obtain l-trityl-2-nitroimidazole by using propyl nitrate as nitrating agent [356], The product... 

Direct N-nitration of secondary amines by nitric acid is possible only for weakly basic amines. The more basic amines can be nitrated under neutral conditions widi reagents such as dinitrogen pentoxide and nitronium tetrafluoroborate, but nitrosamines are significant by-products. The nitrate ester CF3CMe20N02 has been recommended as a nonacidic nitrating agent for secondary amines which avoids the problem of contamination of the products by A(-nitrosamines piperidine and pyrrolidine were nitrated in yields of 75% and 72%, respectively. Amides and imides are efficiendy N-nitrated using ammonium nitrate in trifluoroacetic anhydride. ... 

N-Halogenation and other oxidative reactions of pyridines and related heterocycles have been re-viewed. ° Thus, pyridines and some diazines can be aminated by mesitylenesulfonylhydroxylamine (32) and similar reagents. / -Nitration of 2-picoline by nitronium tetrafluoroborate gives the salt (46) which is itself a nitrating agent. 

The evidence for the existence of the nitronium ion is compelling. Salts such as nitronium perchlorate, N02 C10/, and nitronium tetrafluoroborate, N02 Bp4, have been isolated and successfully used as nitrating agents. A line in the Raman spectrum at 1400 cm that originates from a linear triatomic species, which N02 is, provides spectroscopic confirmation. 

This occnrs readily by reaction of pyridines with nitroninm salts, snch as nitronium tetrafluoroborate Protic nitrating agents snch as nitric acid of course lead exclnsively to iV-protonation. 

A nearly quantitive yield of the stable nitronium salt can be obtained. An undergraduate in an afternoon s work, can prepare up to a pound of the salt which is now also commercially available. To my best knowledge in its two decades of use, there was never any difficulty connected with its stability. Nitronium tetrafluoroborate, and subsequently other complex fluoride salts,such as the hexafluorophosphate,have gained widespread application as most reactive nitrating agents. 

Nitronium tetrafluoroborate and related nitronium salts are extremely active nitrating agents for arcmatics... 

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