Ammonium Tetrafluoroborate


[13826-83-0]  · BF4H4N  · Ammonium Tetrafluoroborate  · (MW 104.84)

(anion exchange reagent for preparation of isolable diazonium salts;1 weak acid with nonnucleophilic counterion; electrolyte useful for synthetic electrochemistry2)

Physical Data: mp 220-230 °C (sublimes); d 1.871 g cm-3 at 15 °C.

Solubility: sol H2O.

Form Supplied in: white solid; widely available.

Purification: crystallization from water (1 mL g-1) between 100 °C and 0 °C.

Handling, Storage, and Precautions: toxic by ingestion and inhalation. Irritates and can burn skin, eyes, and mucous membranes. Avoid extreme heat. Ammonium tetrafluoroborate is incompatible with acids, oxidizers, and reducing agents. Decomposition products include fluorides, NOX, and NH3. Use in a fume hood with safety goggles and chemically resistant gloves and clothing. Use a dust respirator if much dust will be produced during use.

General Comments.

There are hundreds of references for the use of ammonium tetrafluoroborate in the patent literature. Only a few of these references are included here, but the reader should be aware of the wide use of ammonium tetrafluoroborate as a mild acid-base catalyst, proton source, or ammonia source. The lack of nucleophilicity of the fluoroborate spectator counteranion is generally advantageous.

Preparation of Arenediazonium Tetrafluoroborates.

Stable, isolable salts are obtained as filterable precipitates when arenediazonium chloride solutions are treated with NH4BF4. This is in contrast to arenediazonium salts with other counterions, which are not generally isolated. Thus p-bromobenzenediazonium chloride is converted in high yield to p-bromobenzenediazonium tetrafluoroborate (eq 1). Alternatively, the arenediazonium tetrafluoroborate can be prepared from the arylamine in the presence of NH4BF4, whereupon the fluoroborate salt precipitates directly. For example, p-anisidine is diazotized to provide p-methoxybenzenediazonium tetrafluoroborate in nearly quantitative yield.1

Acid Catalyst.

In a production of a polymethylated tetrahydropyrimidine from acetone and ammonia, NH4BF4 serves as a catalyst. The tetrahydropyrimidine is obtained in 82% yield after 5 h at 50 °C.3 Further reaction of the aforementioned tetrahydropyrimidine with acetone occurs with NH4BF4 catalysis at 70 °C, giving the synthetically useful 2,2,6,6-tetramethylpiperidin-4-one.4

Elecrochemical Synthesis.

In an electrolytic oxidation of p-acetoxybenzaldehyde from p-acetoxytoluene, solutions of NH4BF4 in low molecular weight alcohol/carboxylic acid mixtures are effective electrolytes.2 Various metal alkoxides can be prepared by electrochemical oxidation of the metal with R4NBF4/alcohol as the supporting electrolyte.5 Terephthalaldehyde can be produced electrochemically with the use of NH4BF4.6 Polyfluoroalkyl halides can be converted to perfluorocarboxylic acids by electrochemical initiation of ion-radical reaction with oxygen.7

Inversion of Chiral Amines.

Conversion of chiral amines to potassium alkanediazotates, followed by treatment with NH4BF4 in refluxing NH3, leads to inverted amines (eq 2). Percentages of inversion range from 50% to 70%, and yields are modest (18-37%) due to formation of significant amounts of alcohols (configuration predominantly retained) and alkenes.8

Reductive Dehalogenation of Aromatic Iodides.

Treatment of methyl 2-iodobenzoate with excess of Copper(II) Trifluoromethanesulfonate in aq ammonia provides methyl salicylate in 94% yield. The intermediacy of an arylcopper species was demonstrated by isolation of 98% yield of methyl benzoate in the presence of excess NH4BF4 (eq 3).9

Additive in Catalytic Systems.

A zeolite impregnated with NH4BF4 catalyzed a 99.5% conversion of methanol to mixtures of hydrocarbons (30.4% propene, 19.3% butenes, and 25.6% C5+ aliphatics).10 Catalytic hydrogenation with a Ru(acac)3/tri-octylphosphine catalyst in the presence of NH4BF4 results in a selectivity of 100% (79% conversion) for reduction of succinic anhydride to g-butyrolactone.11 In catalytic reductive amination of acetone to produce an N-isopropyl arylamine, NH4BF4 additive imparts very high selectivity (96.8%) to the Fe2O3-promoted copper-chromium catalyst system.12 An AlCl3 catalyst for production of diphenylamine is improved by addition of NH4BF4.13

1. Roe, A. OR 1949, 5, 193.
2. (a) Bellamy, A. J. ACS 1979, B33, 208. (b) Mortier, F.; Sabatier, J. J.; Fr. Patent (CA 1981, 95, 194 457j).
3. Fukuda, Y. Jpn. Patent (CA 1993, 119, 180 817d).
4. Fukuda, Y.; Kokubu, M.; Tokuyama, Y. Jpn. Patent (CA 1991, 119, 117 133n).
5. Shreider, V. A.; Turevskaya, E. P.; Kozlova, N. I.; Turova, N. Y. IZV 1981, 1687 (CA 1981, 95, 177 637a).
6. Jpn. Patent (CA 1985, 103, 78 306u).
7. Ignat'ev, N. V.; Datsenko, S. D.; Yagupol'skii, L. M. Electrokhimiya 1992, 28, 498.
8. (a) Moss, R. A.; Schueler, P. E.; Lee, T. B. K. TL 1973, 2509. (b) Moss, R. A.; JOC 1966, 31, 1082.
9. Cohen, T.; Cristea, I. JOC 1975, 40, 3649.
10. Miale, J.N.; Chang, C. D. U.S. Patent (CA 1983, 100, 92 095u; CA 1984, 102, 187 880b).
11. Wada, K.; Hara, Y.; Sasaki, K. Jpn. Patent (CA 1988, 109, 149 333n).
12. Uhlar, J.; Pasek, J.; Waradzin, W.; Dolezel, P.; Janik, I.; Halomi, M. Czech. Patent (CA 1988, 108, 114 678f).
13. Timokhin, F. A.; Kissin, B. I.; Faeshkina, N. N.; Kurakin, E. N. USSR Patent (CA 1977, 87, 29 690g).

Gregory K. Friestad & Bruce P. Branchaud

University of Oregon, Eugene, OR, USA

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