Bromine Azide1


[13973-87-0]  · BrN3  · Bromine Azide  · (MW 121.92)

(used like Iodine Azide in additions to multiple bonds;1-5 for introduction of the azide function;1-5 in synthesis of aziridines,3,6,7 vinyl azides,4a and azirines7)

Alternate Name: azido bromide.

Physical Data: orange-red liquid, mp -45 °C;1b explosive when neat;8 therefore should be prepared in situ in solution. Odor resembles that of the free halogen.

Solubility: sol most organic solvents.

Analysis of Reagent Purity: by titrimetric analysis.1b

Preparative Methods: BrN3 is not available commercially, but can be prepared in situ from NaN3, Br2, and HCl, preferably in CH2Cl2 or ether.2 It can also be prepared from NBS, HN3, and t-BuOH in CHCl33a or from NBS and NaN3 in DME-H2O.3b Typical procedure for addition to alkenes:2 to a well-stirred slurry of 32.5 g (0.5 mol) of NaN3 in 100 mL of CH2Cl2 (or pentane) at 0 °C add 25 mL of 30% aq HCl, followed by 8.0 g (0.1 mol) of Br2. After 30-60 min of further stirring, decant and dry the organic layer. Add 0.1 mol of alkene in MeNO2 (ionic addition) or pentane (radical addition). Warm the mixture to 20 °C and stir for 8-24 h, then wash in turn with water and 5% aq sodium thiosulfate until colorless, and chromatograph (neutral alumina) or crystallize to afford the bromoazide adduct in 35-95% yield.

Handling, Storage, and Precautions: explosive liquid1b (see Iodine Azide); use in a fume hood. After reaction with multiple bonds, the mixture should be washed with sodium bisulfite until colorless, to destroy excess BrN3. Dilute solutions of BrN3 in organic solvents can generally be handled safely.1b Keep away from strong reducing agents.

Additions to Alkenes.

Additions of BrN3 to alkenes are much more influenced by solvent polarity than additions of IN3 and lead to different regioisomeric adducts, depending upon the polarity of the solvent. Ionic additions, proceeding via a three-membered ring bromonium species, are achieved in polar solvents, such as MeNO2-CH2Cl2, or in the presence of acid catalysts and are usually stereospecific for aliphatic alkenes. Radical additions proceed by N3 radical attack on the C=C double bond and are favored in nonpolar solvents such as pentane, in the presence of light or radical initiators, or when the reaction mixture is purged with N2 to remove O2, a radical inhibitor. Thus styrene reacts with BrN3 in MeNO2 to produce the regioisomer opposite to that formed in pentane solution (eq 1).2

Such differences also exist for the behavior of IN3 vs. BrN3. Thus while IN3 addition in MeCN to benzo[b]thiophene 1,1-dioxide leads to the trans-3-azido-2-iodo adduct, BrN3 addition in CH2Cl2 affords the trans-2-azido-3-bromo product.9

In polar solvents the addition of BrN3 to simple alkenes, such as cis- and trans-2-butene, proceeds stereospecifically anti.2 Steroid 2-enes2 and 5-enes3a afford the b-azido-a-bromo diaxial adducts via a-bromonium species, while radical addition to 2-cholestene gives a mixture of regioisomers (eq 2).

On the other hand, BrN3 additions to steroid enones can lead to a-azido-b-bromo steroids or to cis adducts. Thus podocarp-6-enes give 7a-azido-6b-bromo adducts.5b

Enol ethers lead to formation of a-azido ethers as a mixture of stereoisomers.2 However, reaction of sugar 2-enes with BrN3 in the presence of Dibenzoyl Peroxide (radical reaction) led, after treatment with Mercury(II) Acetate, to formation of 2-acetoxy-3-azido sugars.10

Aromatic or other substituents on a double bond, which can stabilize a positive charge well, can lead to a mixture of stereoisomers.2b In polar nucleophilic solvents (MeCN, DMF, HOAc, MeOH), reaction of BrN3 with alkenes leads to solvent participation (eq 3).11

While ionic addition of BrN3 to 3,3,3-triphenylpropene proceeds with phenyl migration, the radical addition to this alkene gives mainly the unrearranged product.12

Addition of BrN3 to allenes at -65 °C leads, like IN3, to a mixture of allylic azides, while addition at rt apparently proceeds via radicals to afford an unstable vicinal diazide.13

Synthesis of Aziridines.

BrN3 addition to alkenes, followed by reduction with LAH or Ph3P-H2O, is a convenient method for the synthesis of aziridines (eq 4).3,14

Synthesis of Vinyl Azides and Azirines.

Bases, like Potassium t-Butoxide, cause stereospecific elimination of HBr from the adducts; the resulting vinyl azides4a provide useful routes for the synthesis of azirines or ketones, or for amination of aromatic compounds (see Iodine Azide).

1. (a) Hassner, A. ACR 1971, 4, 9. (b) Dehnicke, K. Adv. Inorg. Chem. Radiochem. 1983, 26, 169.
2. (a) Hassner, A.; Boerwinkle, F. JACS 1968, 90, 216. (b) Hassner, A.; Boerwinkle, F. P.; Levy, A. B. JACS 1970, 92, 4879.
3. (a) Drefahl, G.; Ponsold, K.; Eichhorn, D. CB 1968, 101, 1633. (b) van Ende, D.; Krief, A. AG(E) 1974, 13, 279.
4. (a) L'abbé, G.; Hassner, A. AG(E) 1971, 10, 98. (b) Hassner, A. MOC 1990, E16a/2, 1243.
5. (a) Carlon, F. E.; Draper, R. W. JCS(P1) 1983, 2793. (b) Cambie, R. C.; Robertson, J. D.; Rutledge, P. S.; Woodgate, P. D. AJC 1982, 35, 863.
6. Lion, C.; Boukou-Poba, J-P.; Saumtally, I. BSB 1987, 96, 711.
7. Hassner, A.; Matthews, G. J.; Fowler, W. F. JACS 1969, 91, 5046.
8. (a) Spencer, D. A. JCS 1925, 127, 216. (b) Coombe, R. D. JCP 1983, 79, 254.
9. Tamura, Y.; Tsunekawa, M.; Bayomi, S. M. M.; Kwon, S.; Ikeda, M. H 1982, 19, 1935.
10. Bovin, N. V.; Zurabyan, S. E.; Khorlin, A. Y. Carbohydr. Res. 1981, 98, 25.
11. Boerwinkle, F.; Hassner, A. TL 1968, 3921.
12. Hassner, A.; Teeter, J. S. JOC 1971, 36, 2176.
13. Hassner, A,; Keogh, J. JOC 1986, 51, 2767.
14. Parish, E. J.; Nes, W. D. SC 1988, 18, 221.

Alfred Hassner

Bar-Ilan University, Ramat-Gan, Israel

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