[106-93-4]  · C2H4Br2  · 1,2-Dibromoethane  · (MW 187.86)

(used as an entrainment reagent1,2 and for bromination of carbanions5,7,8)

Alternate Name: ethylene dibromide.

Physical Data: bp 131.7 °C; mp 9-10 °C

Solubility: sol most organic solvents.

Form Supplied in: commercially available as a colorless liquid in 99% purity.

Analysis of Reagent Purity: NMR, GC, bp.

Purification: distillation.

Handling, Storage, and Precautions: 1,2-dibromoethane is toxic and carcinogenic. Exposure causes irritation of the skin, mucus membranes, eyes, and upper respiratory tract. Animal tests indicate that overexposure may cause reproductive disorders. Chronic toxic effects include damage to the liver, kidneys, and lungs. Contact with vapor should be avoided. This reagent should only be handled in a fume hood using proper protective equipment.

Entrainment Reagent.

1,2-Dibromoethane is commonly used to chemically activate Magnesium metal for use in forming Grignard reagents. It reacts with magnesium to expose a clean, reactive surface capable of converting otherwise unreactive halides into Grignard reagents.1,2 When used as an entrainment reagent, 1,2-dibromoethane is added slowly so as to allow the halide with poor reactivity adequate time to react with freshly exposed magnesium surfaces. Pearson and Cowan's conversion of hexachlorobenzene to pentachlorobenzoic acid is a good example of this application (eq 1).2 In this instance, 1,2-dibromoethane is added in dropwise fashion over a 48 h period to a refluxing mixture of hexachlorobenzene and magnesium turnings in ether. This reagent is often used in preference to other alkyl halides for entrainment because ethylene and MgBr2 are produced as byproducts rather than a second Grignard reagent.

1,2-Dibromoethane is also a useful reagent for activating Zinc.3,4 For example, upon heating with this reagent in ethanol, zinc powder reacts exothermically to produce activated zinc that will reduce triple bonds to cis-double bonds in a variety of compounds (eq 2).4 The zinc powder can be further activated by addition of a solution of Copper(I) Bromide and Lithium Bromide.

Reagent for Bromination of Carbanions.

This reagent can be used as a source of positive bromine for bromination of a-methyl lactone enolates.5 Ourisson et al used this methodology effectively in the synthesis of a-methylene-g-lactones, including (-)-frullanolide (eq 3). Because 1,2-dibromoethane is unreactive toward the lactone product, it may offer advantages over bromine. However, bromination of enolates by this reagent is not a general reaction. With many enolates, 1,2-dibromoethane acts instead as an alkylating agent.6

1,2-Dibromoethane also reacts with aryl-7 and vinyllithiums8 to produce the corresponding bromides in good yield. For example, Snieckus and Miah have used this reagent to efficiently convert lithiated O-pyridyl carbamates to the bromides (eq 4).

More recently, the reagent 1,2-dibromotetrafluoroethane, which often gives superior yields, has been used in a similar fashion for the bromination of anions.9

1. Pearson, D. E.; Cowan, D.; Beckler, J. D. JOC 1959, 24, 504.
2. Pearson, D. E.; Cowan, D. OSC 1973, 5, 890.
3. Bellassoned, M.; Gaudemar, M.; Borgi, A. E.; Baccar, B. JOM 1985, 280, 165.
4. Aerssens, M. H. P. J.; van der Heiden, R.; Heus, M.; Brandsma, L. SC 1990, 20, 3421.
5. (a) Greene, A. E.; Muller, J.-C.; Ourisson, G. TL 1972, 2489. (b) JOC 1974, 39, 186.
6. (a) Cope, A. C.; Holmes, H. L.; House, H. O. OR 1957, 9, 107. (b) Newman, M. S.; DeVries, V.; Darlak, R. JOC 1966, 31, 2171.
7. Miah, M. A. J.; Snieckus, V. JOC 1985, 50, 5436.
8. Whitesides, G. M.; Casey, C. P.; Krieger, J. K. JACS 1971, 93, 1379.
9. (a) Wang, W.; Snieckus, V. JOC 1992, 57, 424. (b) Paquette, L. A.; Ross, R. J.; Shi, Y.-J. JOC 1990, 55, 1589. (c) Paquette, L. A.; DeRussy, D. T.; Gallucci, J. C. JOC 1989, 54, 2278. (d) Habata, Y.; Akabori, S.; Sato, M. BCJ 1985, 58, 3540.

George D. Maynard

Marion Merrell Dow, Cincinnati, OH, USA

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