Nitronium Tetrafluoroborate1

NO2+BF4-

[13826-86-3]  · BF4NO2  · Nitronium Tetrafluoroborate  · (MW 132.82)

(electrophilic nitrating agent;1 mild oxidant;2 hydride and halide abstracting agent2)

Physical Data: decomposes to NO2F and BF3 at temperatures >180 °C.

Solubility: sol sulfolane, acetonitrile; slightly sol nitromethane. Compared with NO2+BF4-, NO2+PF6- has much higher solubility in organic solvents.

Form Supplied in: colorless crystalline compound; commercially available.

Handling, Storage, and Precautions: due to its hygroscopic nature, NO2+BF4- should be stored and used under anhydrous conditions. Commercial NO2+BF4- often contains varying amounts of NO+BF4-. Removal of the NO+ from NO2+BF4- is difficult. Therefore, pure nitronium tetrafluoroborate should be prepared directly from urea-treated purified HNO3.3

Nitration.

NO2+BF4- is a highly efficient electrophilic nitrating agent for organic compounds.1 A variety of arenes have been nitrated by this reagent (eq 1). The ability of NO2+BF4- to nitrate under anhydrous conditions makes it a reagent of choice for the nitration of aromatics with acid sensitive functional groups, such as nitriles, acid halides, and esters. The nitrations are conveniently carried out in sulfolane solution, although methylene chloride and nitromethane are also used. With severely deactivated arenes such as polyhaloarenes and 1,3-dinitrobenzene, superacids such as Fluorosulfuric Acid and Trifluoromethanesulfonic Acid are used as solvents to achieve satisfactory results.4 The increased reactivity of the nitronium ion in strongly acidic solution results from protosolvation (protonation) of the lone pair electron of NO2+ to form the protonitronium dication.5 Nitration of hexamethylbenzene, nitropentamethylbenzene, bromopentamethylbenzene, and pentamethylbenzoic acid with excess NO2+BF4- leads to formation of 1,2-dinitro-3,4,5,6-tetramethylbenzene.6 NO2+BF4- reacts with methylheptafluoronaphthalene to form addition products.7

Due to its less oxidative nature compared with conventional mixed acids (HNO3/H2SO4), NO2+BF4- is widely applied in the nitration of heterocyclic compounds. Alkylpyridines,8 thiophene,9 and furan,9 as well as polyheteroatom cyclic compounds10-12 such as uracils and pentaazaphenalenes, are nitrated under mild conditions. Even macroheterocyclic compounds like porphyrins and metal phthalocyanines (metal = iron, cobalt, nickel) are nitrated with NO2+BF4-.13

Electrophilic nitration of alkanes is still of little preparative value in organic synthesis. However, in special cases such as adamantane, nitration with NO2+BF4- occurs readily to give 1-nitroadamantane in 66% yield (eq 2).14

The reaction of NO2+BF4- with alkenes generally gives a mixture of addition, addition-elimination, and oligomerization products, with the ratio depending on the nature of the substrates and the reaction conditions.1c However, with alkenes such as allyl esters15 and vinyl halides16 which contain functional groups capable of stabilizing the reaction intermediates, the addition (nitrofluorination) products can be obtained preferentially. When Pyridinium Poly(hydrogen fluoride) is used as additional fluoride source, alkenes react with NO2+BF4- to give b-nitroalkyl fluorides in good yields.17 With acetonitrile18 and acetic anhydride,19 solvoaddition products are also obtained (eq 3).

Nitration of alkenyloxysilanes and enol acetates with NO2+BF4- leads to formation of a-nitro ketones.20 Allylsilanes react with the salt readily to yield nitroalkenes.21 Nitration of trimethylsilylacetylene or alkynylstannanes with NO2+BF4- leads to formation of nitroalkynes.22

Both primary and secondary amines react with NO2+BF4- to form nitro amines (eq 4).1,23 Pyridines are N-nitrated to form N-nitropyridinium tetrafluoroborates, which have been widely used as transfer nitrating agents.24 The advantage of using N-nitropyridinium salts as nitrating agents is that they react under acid-free conditions. Unsubstituted sulfodiimides or their N,N-bis(trimethylsilyl) derivatives react with NO2+BF4- to afford interesting N,N-dinitrosulfodiimides.25

Nitration of amides with NO2+BF4- yields N-nitroamides.26 The nitroamides of benzoic and chloroacetic acid are obtained in satisfactory yields but the yield of nitroacetamide is only 12%. N-alkyl- and N,N-dialkylamides are nitrolyzed to form corresponding nitroamines and N,N-dinitroamines in satisfactory yields.27 N-Alkyl-N-nitroamides react similarly with NO2+BF4- to form dinitroamines in good yields.28

Aliphatic isocyanates react with NO2+BF4- in ethyl acetate or acetonitrile to form nitroamines (after hydrolysis).29 On the other hand, in conjunction with HNO3, NO2+BF4- reacts with isocyanates to yield N,N-dinitroamines.30 Alcohols are nitrated with NO2+BF4- to alkyl nitrates (eq 5).31 In the presence of other nucleophiles, epoxides are ring-opened by NO2+BF4-.32

Oxidation.

NO2+BF4- is a mild oxidant. The oxidative ability of the salt is manifested by the formation of products resulting either from coupling or from oxidation in its reaction with electron rich aromatics.33 The oxidative property of NO2+BF4- has been utilized to regenerate the masked carbonyls from 1,3-dithioacetals, oximes, and dimethylhydrazones.34 NO2+BF4- is also capable of oxidizing ethers to carbonyls,35a or converting them into N-alkylacetamides by trapping the intermediates with acetonitrile.35b

Organic sulfides are oxidized with NO2+BF4- to form sulfoxides (eq 6).36 With additional added NO2+BF4-, the sulfoxides can be further converted to sulfones.36 Phosphines are similarly oxidized to phosphine oxides in quantitative yield.36 More recently, fullerene (C60) has been oxidized to fullerols by NO2+BF4- in the presence of carboxylic acids.37

Hydride and Halide Abstraction.

Like Nitrosonium Tetrafluoroborate, NO2+BF4- is a hydride and halide abstracting agent.38-41 However, it is less efficient in this aspect compared to NO+BF4-.

Related Reagents.

Nitric Acid.


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George A. Olah, G. K. Surya Prakash, Qi Wang & Xing-ya Li

University of Southern California, Los Angeles, CA, USA



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