Nitrosonium Tetrafluoroborate1

NO+BF4-

[14635-75-7]  · BF4NO  · Nitrosonium Tetrafluoroborate  · (MW 116.82)

(efficient nitrosating and diazotizing agent;1 mild oxidant;2 hydride- and halide-abstracting agent2)

Physical Data: d 2.185 g cm-3.3

Solubility: sol acetonitrile.

Form Supplied in: colorless crystalline compound; commercially available.

Purification: commercial NO+BF4- frequently contains some NO2+BF4-; NO2+ can be removed from NO+BF4- by washing with dry benzene; sublimation (200-250 °C/0.01 mmHg) can be used if highly pure NO+BF4- is needed.3

Handling, Storage, and Precautions: due to its hygroscopic nature, NO+BF4- should be stored and used under anhyd conditions; use in a fume hood.

Nitrosation.

NO+BF4- is a highly efficient nitrosating agent, and has been used widely for synthetic purposes. Alcohols and secondary amines react readily with NO+BF4- to form alkyl nitrites and nitrosamines, respectively, in high yields (eqs 1 and 2).4 Nitrosation of less reactive N-alkylamides and sulfonamides can also be achieved without difficulty, where other nitrosating agents are not effective.5,6 Reaction of NO+BF4- with pyridine yields N-nitrosopyridinium tetrafluoroborate.7 Aziridines react with NO+BF4- to yield nitrosoaziridines. Extrusion of N2O from the products under thermal decomposition conditions led to formation of carbon-carbon double bonds.8 This reaction has been employed for the synthesis of cis-diazabis-s-homobenzene.8a

Diazotization and Related Reactions.

When primary amines react with NO+BF4-, diazonium tetrafluoroborates are generated.1 This reaction has been extensively studied with arylamines, since it is particularly useful for the isolation of the corresponding diazonium tetrafluoroborates (eq 3).9 Decomposition of these salts produces fluoroarenes.10 Isocyanates also react with NO+BF4- to yield diazonium salts.2a,11 Primary amides can be converted to acids by NO+BF4- via diazotization and subsequent hydrolysis.12

Treatment of alkyl azides with NO+BF4- led to the formation of carbenium ions, which can be intercepted with various nucleophiles such as fluoride, hydroxide, carboxides, and nitriles.13 The reaction using nitriles has been utilized to prepare dihydroisoquinolines and oxazoles. Vinyl azides react with NO+BF4- to form 1,2,5-oxadiazoles and/or 1,2,4-oxadiazoles, depending on the structures of the substrates.14 When 1,2-dialkylvinyl azides are used, 1,2,5-oxadiazoles were obtained as the only heterocyclic compounds in 75-80% yields.

Various substituted hydrazines react with NO+BF4- to form azides.15

Oxidation.

NO+BF4- is a mild oxidant, with NO/NO+ having 1.5 V standard oxidation-reduction potential in acetonitrile.16 It has been widely used as a single electron transfer oxidant. Many arene- and sulfur-containing radical cations have been prepared by treatment of corresponding precursors with NO+BF4- (eq 4).17,18 The easy removal of the side-product NO makes the procedure especially convenient. These radical cations have played a significant role in the development of organic ferromagnets and conductors.

The oxidative property of NO+BF4- has also been synthetically utilized to regenerate carbonyl groups from masked derivatives such as 1,3-dithioacetals, oximes, and dimethylhydrazones.19

The same principle was applied to stereospecific construction of O-glycosidic linkages.20 Treatment of readily available methyl S-glycosides with an equimolar amount of NO+BF4- in dry methylene chloride at temperatures between 0 and 25 °C gives high yields of O-glycosides in the presence of a hydroxylic component (eq 5). The stereospecificity and lack of complications compared with earlier methods make this procedure very attractive.

Noteworthy is the oxidative iodination of aromatic compounds with NO+BF4- as catalyst.21 A wide range of arenes were iodinated with the system I-/O2/NO+BF4-.21b This method is not as effective as I2/Ag+, but much more efficient than the use of other oxidative iodination systems such as I2/CeIV salts and I2/(NH4)2S2O8. The more important feature of this transformation is the suppression of side-chain oxidation of alkylarenes.21c

Electrophilic Addition to Unsaturated Carbon-Carbon Bonds.

As an electrophilic agent, NO+BF4- is able to initiate electrophilic addition to unsaturated carbon-carbon bonds. Reaction of alkenes with NO+BF4- in the presence of acetonitrile yields imidazoles after reduction (eq 6).22a Changing acetonitrile to dimethyl sulfide led to formation of nitrososulfonium tetrafluoroborate (in dimer form).22b The dimer can be depolymerized, tautomerized, and hydrolyzed to give oxosulfonium salt, which can be utilized synthetically. NO+BF4- was reported to dimerize certain alkenes and alkynes.22c,d In the latter case, the true identity of the reagent that initiates the reaction (whether it is NO+ or NO2+) is unclear. NO2+BF4- is known to induce the same reaction.22d

NO+ can also insert into the cyclopropane ring, offering a new route to 2-isoxazolines.23

Hydride and Halide Abstraction.

NO+BF4- is able to abstract a hydride from a tertiary C-H bond, generating a carbocation. The carbocationic intermediate can be trapped by a fluoride or alkyl nitriles, forming alkyl fluorides or N-alkylamides respectively.24 Silanes also react with NO+BF4- to form fluorosilanes.25

In a similar fashion, NO+ can also abstract a halogen atom from alkyl halides. In the presence of alkyl nitriles, N-alkylamides were obtained as a consequence of the Ritter reaction.26

Addition to Allenylsilanes.

Allenylsilanes react with nitrosonium tetrafluoroborate in acetonitrile at -30 °C to form 4-trialkylsilylisoxazole derivatives in good yield.27 This method provides a regiospecific [3 + 2] annulation route to unsymmetrically substituted isoxazoles. The silylisoxazole products can be further elaborated by electrophilic substitution in the same operation (eqs 7 and 8). In these [3 + 2] annulations, NOBF4 is distinctly superior to other nitrosating agents such as Nitrosyl Chloride and Nitrosylsulfuric Acid.


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2. (a) Olah, G. A. Aldrichim. Acta 1979, 12, 43. (b) Olah, G. A. ACR 1980, 13, 330.
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5. (a) Olah, G. A.; Olah, J. A. in Friedel-Crafts and Related Reactions, Olah, G. A., Ed.; Interscience: New York; Vol. 3, p 1267. (b) Romea, P.; Urpi, F.; Vilarrasa, J. JOC 1989, 54, 3209.
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13. (a) Doyle, M. P.; Wierenga, W. JACS 1972, 94, 3896. (b) ibid. 1972, 94, 3901. (c) McGirk, R. H.; White, E. H. JACS 1973, 95, 3804. (d) Doyle, M. P.; Spoelhof, G. D.; Zaleta, M. A. JHC 1975, 12, 263. (e) Doyle, M. P.; Hedstrand, D. M.; Busman, S. C.; Alexander, D. JACS 1975, 97, 5554. (f) Doyle, M. P.; Whitefleet, J. L.; Zaleta, M. A. TL 1975, 4201. (g) Owen, G. R.; Verheyden, J. P. H.; Moffatt, J. G. JOC 1976, 41, 3010. (h) Doyle, M. P.; Whitefleet, J. L.; Bosch, R. JOC 1979, 44, 2923.
14. Thakore, A. N.; Buchshriber, J.; Oehlschlager, A. C. CJC 1973, 51, 2406.
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19. (a) Olah, G. A.; Ho, T. L. S 1976, 610. (b) Olah, G. A.; Narang, S. C.; Salem, G. F.; Gupta, B. G. B. S 1979, 273.
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21. (a) Makhon'kov, D. I.; Cheprakov, A. V.; Rodkin, M. A.; Beletskaya, I. P. ZOR 1988, 24, 241. (b) Radner, F. JOC 1988, 53, 3548. (c) Galli, C. JOC 1987, 56, 3238.
22. (a) Scheinbaum, M. L.; Dines, M. B. TL 1971, 2205. (b) Chow, Y. L.; Iwai, K. JCS(P2) 1980, 931. (c) Lee, G. H.; Lee, J. M. Jeong, W. B.; Kim, K. TL 1988, 29, 4437. (d) Brittelli, D. R.; Boswell, G. A., Jr. JOC 1981, 46, 312.
23. (a) Mizuno, K.; Ichinose, N.; Tamai, T.; Otsuji, Y. JOC 1992, 57, 4669. (b) Ichinose, N.; Mizuno, K.; Otsuji, Y. CL 1989, 457.
24. (a) Olah, G. A.; Shih, J. G.; Singh, B. P.; Gupta, B. G. B. JOC 1983, 48, 3356. (b) Olah, G. A.; Gupta, B. G. B. JOC 1980, 45, 3532.
25. Prakash, G. K. S.; Wang, Q.; Li, X. Y.; Olah, G. A. NJC 1990, 14, 791.
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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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