Mercury(II) Oxide-Bromine1


[21908-53-2]  · HgO  · Mercury(II) Oxide-Bromine  · (MW 216.59) (Br2)

[7726-95-6]  · Br2  · Mercury(II) Oxide-Bromine  · (MW 159.80)

(reagent for the generation of isolable Br2O,2 which reacts with alcohols and carboxylic acids to give the corresponding alkyl hypobromites, ROBr,3 and acyl hypobromites, RCO2Br,2c respectively)

Physical Data: Br2: mp -7.25 °C; bp 59.5 °C; d 3.187 g cm-3 (0 °C), 7.59 g dm-3 (gas).

Solubility: Br2: sol water ethanol, ether, chloroform, CS2.

Handling, Storage, and Precautions: since releasing the used reagent into the environment could be hazardous, one must follow the appropriate regulations. Use in a fume hood.

Reagent for Preparation of Alkyl Hypobromites.3

Alkyl hypobromites can be prepared by the reaction of alcohols with this reagent in a manner parallel to the preparation of hypoiodites by the reaction of alcohols with Mercury(II) Oxide-Iodine (eqs 1 and 2).

The reactive species generated from the combination of Mercury(II) Oxide and Bromine is Br2O, and, in contrast to I2O from HgO and I2, it can be isolated.2 These alkyl hypobromite intermediates dissociate to alkoxyl and bromine radicals upon heating or irradiation with Pyrex-filtered light, as in the case of hypoiodites, and thus can be used for remote functionalization as well as b-scission in a manner similar to that of the hypoiodites. Thus treatment of cedrol with mercury(II) oxide-bromine, followed by irradiation, gives the corresponding cyclic ether (eq 3).3a The mechanism is parallel to that for the formation of cyclic ethers from alkyl hypoiodites (see Mercury(II) Oxide-Iodine). An example of the b-scission of an alkoxyl radical generated from alkyl hypobromite is illustrated in eq 4.3b

Reagent for the Modified Hunsdiecker Reaction.1

When the silver salt of a carboxylic acid is treated with bromine, the carboxy group is lost as CO2 and alkyl or aryl bromides are produced. This reaction has been known as the Hunsdiecker reaction.4 Cristol found that treatment of excess red mercury(II) oxide in a refluxing solution of an aliphatic carboxylic acid in CCl4 with approximately 1 equiv of Br2 in the dark gave good yields of the corresponding alkyl bromide (eq 5)1 (see also Mercury(II) Oxide-Iodine). Subsequently, a number of investigators have reported that the Cristol method is simpler than the original Hunsdiecker method in that it does not require the preparation of the dry silver salt.5

The reaction involves the formation of RCO2Br intermediates, followed by the removal of CO2 via a radical intermediate. Thus cyclopropanecarboxylic acid is converted to bromocyclopropane in 41-46% yield by the treatment of red HgO and bromine in 1,1,2,2-tetrachloroethane (eq 6).6 6-Bromo-4-t-butylhexanoic acid is converted to 1,5-dibromo-3-t-butylpentane (eq 7).7

Bromination of Alkanes.

The combination of bromine and mercury(II) oxide is a reagent for the radical bromination of alkanes. The reagent, which involves Br2O as the brominating species, is significantly more reactive than Bromine or N-Bromosuccinimide and is applicable to a substitution of the primary and secondary C-H bonds (eq 8).8

1. Cristol, S. J.; Firth, W. C., Jr. JOC 1961, 26, 280.
2. (a) Brenschede, W.; Schumacher, H. J. Z. Phys. Chem., Abt. B 1935, 29, 356. (b) Brenschede, W.; Schumacher, H. J. Z. Anorg. Allg. Chem. 1936, 226, 370. (c) Jennings, P. W.; Ziebarth, T. D. JOC 1969, 34, 3216.
3. (a) Brun, P.; Waegell, B. T 1976, 32, 1137. (b) Bensadoun, N.; Brun, P.; Casanova, J.; Waegell, B. JCR(S) 1981, 236. (c) Bensadoun, N.; Brun, P.; Casanova, J.; Waegell, B. JCR(M) 1981, 2601.
4. Hunsdiecker, H.; Hunsdiecker, C. CB 1942, 75, 291. (b) Johnson, R. G.; Ingram, R. K. CRV 1956, 56, 219. (c) Wilson, C. V. OR 1957, 9, 332.
5. (a) Baker, F. W.; Holtz, H. D.; Stock, L. M. JOC 1963, 28, 514. (b) Cristol, S. J.; Gaston, L. K.; Tiedeman, T. JOC 1964, 29, 1279. (c) Holtz, H. D.; Stock, L. M. JACS 86, 5183. (d) Davis, J. A.; Herynk, J.; Carroll, S.; Bunds, J.; Johnson, D. JOC 1965, 30, 415.
6. (a) Meek, J. S.; Osuga, D. T. OS 1963, 43, 9. (b) Meek, J. S.; Osuga, D. T. OSC 1973, 5, 126.
7. Sakurai, H.; Murakami, M. OPP 1973, 5, 1.
8. Bunce, N. J. CJC 1977, 55, 383.

Hiroshi Suginome

Hokkaido University, Sapporo, Japan

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