Nitronium tetrafluoroborate nitration

Uses. Tetramethylene sulfone has high solvent power for aromatics and has been used extensively by Olah and co-workers for Friedel-Crafts type nitrations and for studies of the mechanism of nitronium tetrafluoroborate nitration of alkyl-benzenes and halobenzenes in homogeneous solution. It is a superior solvent for quaternization of tertiary amines with alkyl halides, since it has a high dielectric constant and does not enter into side reactions observed with nitrobenzene and dimethylformamide. For example in the synthesis of the acridizinium salt (3), Bradsher and Parham effected quaternization of (1) with benzyl bromide in tetramethylene sulfone at room temperature in excellent yield. Several other salts analagous to (2) were obtained in good yield and in crystalline form with use of tetramethylene sulfone, whereas with other solvents the products were colored... 

Another aspect of my early research in Budapest was in nitration chemistry, specifically the preparation of nitronium tetrafluoroborate, a stable nitronium salt. 1 was able to prepare the salt in a simple and efficient way from nitric acid, hydrogen fluoride, and boron trifluoride. 

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. 

The selection of solvents for quantitative work is not easy. Nitro-alkanes are sufficiently inert, but nitronium tetrafluoroborate is poorly soluble in them c. 0-3 %). Nitronium salts react rapidly with acetic anhydride, and less rapidly with acetic acid, A, A -dimethylformamide and acetonitrile, although the latter solvent can be used for nitration at low temperatures. Sulpholan was selected as the most suitable solvent ... 

Quantitative eomparisons of aromatic reactivities were made by using the competitive method with solutions of nitronium tetrafluoroborate in sulpholan, and a concentration of aromatic compounds 10 times that of the salt. To achieve this condition considerable proportions of the aromatic compoimds were added to the medium, thus depriving the sulpholan of its role as true solvent thus, in the nitration of the alkyl- and halogeno-benzenes, the description of the experimental method shows that about 50-60 cm of mixed aromatic compounds were dissolved in a total of 130 cm of sulpholan. 

The data of table 4.1, column [a), do not suggest that nitration of the alkylbenzenes with nitronium tetrafluoroborate in sulpholan occurs upon encoimter mesitylene might be so reacting ( 3.3) but comparison with a more inherently reactive substance would be necessary before this possibility could be considered. 

It has been necessary to comment upon these various studies because Olah and his co-workers have suggested that whilst nitrations, like those with nitronium salts, which give a relative rate of reaction of toluene with respect to benzene not much greater than unity involve the nitronium ion as the electrophile, this is not so in other cases. It is important to consider these opinions closely. In the earlier of the two relevant papers it is agreed that since nitrations of toluene with nitronium tetrafluoroborate in sulpholan show no abnormal o -ratio there... 

Without further studies little weight can be given to these ideas. In particular there is the possibility that with acetanilide, as with anisole, nitrosation is of some importance, and further with nitrations in sulphuric acid the effect of protonation of the substrate needs quantitative evaluation. The possibility that the latter factor may be important has been recognised, and it may account for the difference between nitration in sulphuric acid and nitration with nitronium tetrafluoroborate. 

For nitration with nitronium tetrafluoroborate in sulpholan. " See table 4.1. 

The existence of the nitronium ion in sulfuric-nitric acid mixtures was demonstrated both by cryoscopic measurements and by spectroscopy. An increase in the strong acid concentration increases the rate of reaction by shifting the equilibrium of step 1 to the right. Addition of a nitrate salt has the opposite effect by suppressing the preequilibrium dissociation of nitric acid. It is possible to prepare crystalline salts of nitronium ions, such as nitronium tetrafluoroborate. Solutions of these salts in organic solvents rapidly nitrate aromatic compounds. ... 

Nitration of aromatic rings by nitronium tetrafluoroborate is a general method. Fifty-seven arenes, haloarenes, nitroarenes,... 

With ten 7i-electrons delocalized over seven atoms, trithiadiazepine is electron rich and should undergo electrophilic substitution, which indeed it does. Nitronium tetrafluoroborate at — 10 C or copper(II) nitrate gives the mononitro derivative 5, which can be converted into the dinitro compound 6 by the action of an excess of nitronium tetrafluoroborate at 10 C.388... 

Nitration with concentrated nitric acid degrades chlorins which are sensitive to oxidation. Here nitronium tetrafluoroborate in sulfolane is the reagent of choice. Both mononitrooc-... 

The mechanism of the reaction has been interpreted in terms of a 4- or 6-centre transition state (LV) or (LVI) in which nucleophilic assistance of C-Si bond cleavage occurs this would not be possible for nitration by nitronium tetrafluoroborate. 

Nitration of acetylenes with nitryl iodide followed by elimination of HI gives nitroacetylenes, but nitroacetylenes are too thermally unstable to be useful for organic synthesis.77 Recently, nitro-trimethylsilyl-acetylenes are prepared as stable nitroacetylenes by the reaction of bis(trimethylsily)acetylene with nitronium tetrafluoroborate (Eq. 2.39).78... 

A large number of reagents are available for the preparation of nitro PAHs. These include fuming nitric acid in acetic acid (20) or acetic anhydride (13), sodium nitrate in trifluoroacetic acid (21) or trifluoroacetic acid and acetic anhydride (17), dinitrogen tetroxide in carbon tetrachloride (22), sodium nitrate in trimethyl phosphate and phosphorus pentoxide (23), and nitronium tetrafluoroborate in anhydrous acetonitrile (24). Alternative approaches must be used to synthesize nitro PAHs substituted at positions other than the most reactive carbon. For instance,... 

Nitrones, reaction with olefins, 46, 130 Nitronium tetrafluoroborate, from nitric add, boron trifluoride, and hydrogen fluoride, 47, 56 in nitration of aromatic rings, 47, 60... 

The nitration of the 2,7-diamino-77/-[l,2,4]triazolo[3,4-c][l,2,4]triazolium ion 44 with nitronium tetrafluoroborate affords after workup with base the corresponding potassium bis(iV-nitro imides) 45 <1991IZV1239>. 

Olsen and co-workers reported the nitration of secondary nitroalkanes to m-dinitro compounds with nitronium tetrafluoroborate in acetonitrile at 0 °C. Yields are lower compared to the Kaplan-Shechter reaction and significant amounts of pseudonitroles are formed, but this is possibly due to impure reagent. 

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. 

Allylic and homoallylic alcohols are particularly susceptible to oxidation and dehydration. Hiskey and Oxley successfully nitrated both allyl alcohol and l-buten-4-ol with nitronium tetrafluoroborate in diethyl ether at -71 °C. 

Olah and co-workers ° conducted a comprehensive study into the use of N-nitropyridinium salts for nitration. Such salts are easily prepared from the slow addition of the appropriate heterocyclic base to an equimolar suspension of nitronium tetrafluoroborate in acetonitrile. Olah studied the effect on nitration of changing both the structure of the heterocyclic base and the counter ion. Three of these salts (20, 21, 22) illustrated above have been synthesized and used for the 0-nitration of alcohols with success. Transfer nitrations with Al-nitropyridinium salts are particularly useful for the preparation of nitrate esters from acid-sensitive alcohols and polyols because conditions are essentially neutral. 

Olah and Lin reported obtaining pure TNB (2) in 50 % yield from the nitration of m-dinitrobenzene (32) with nitronium tetrafluoroborate in fluorosulfuric acid at 150 °C higher yields of TNB can be obtained by reducing reaction time but the product then contains some unreacted m-dinitrobenzene. A solution of dinitrogen pentoxide in sulfuric acid at 160 °C is also reported to effect the conversion of m-dinitrobenzene to TNB. TNB is rarely prepared via these routes and is best obtained from TNT via an indirect route (Section 4.9). 

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