Introduction
<P>1,2-Dibromoethane (ethylene dibromide) is commonly used as an ‘entrainment reagent’
to chemically activate magnesium in Grignard reagents. It reacts with magnesium to
expose a clean, reactive surface capable of converting otherwise unreactive halides
into Grignard reagents.
[
1]
It has many advantages over other entrainment agents. It reacts with magnesium to
give MgBr
2 and ethylene as byproducts and hence does not introduce a second Grignard reagent
to the system. 1,2-Dibromoethane is also a useful reagent for activating zinc.
[
2]
[
3]
This reagent can be used as a source of electrophilic bromine for bromination of carbanions,
[
4]
and also acts as an alkylating agent with many enolates.
[
5]
1,2-Dibromoethane is a precursor to numerous 1,2-disubstituted ethane derivatives,
for example 1,2-ethanedithiol.
[
6]
In addition, it acts as a sacrificial reductant in the conversion of thiocarbonyl
compounds to carbonyl compounds,
[
7]
and as an excellent oxidizer in domino carbopalladation-cyclization processes.
[
8]
It was used as a scavenger of lead antiknock agents in gasoline and as a soil fumigant
for fumigation of grains and fruits until the early 1980s. It is a useful intermediate
in the synthesis of dyes and pharmaceuticals.</P>
Preparation
<P>1,2-Dibromoethane is commercially available, but can also be prepared by direct
bromination of ethylene or by reacting hydrobromic acid with acetylene. Lesot et al.
have reported that it can be prepared by bromination of 1,2-ethanediol in the presence
of red phosphorus.
[
9]
</P>
Properties
<P>1,2-Dibromoethane is a colorless, heavy (
d = 2.18 g·cm
-3) liquid with a mild, chloroform-like sweaty odor (mp 9-10 °C; bp 131.4 °C). It is
miscible with all common organic solvents and itself a good solvent for resins, gums,
and waxes. In the Cristol procedure for bromination of bridgehead acids, 1,2-dibromoethane
is used as a solvent instead of CCl
4 to avoid the formation of chloride byproduct.
[
10]
Tests on animals indicated that over-exposure to this reagent may cause reproductive
disorders. For humans it can cause damages to liver, kidneys, and lungs.</P>
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(A) In the synthesis of (-)-frullanolide 3, a natural sesquiterpene, treatment of the enolate formed from lactone 1 with 1,2-dibromoethane afforded the α-bromolactone 2. Dehydrobromination of 2 with excess 1,5-diazabicyclo[4.3.0]nonene-5 (DBN) afforded 3 in about 80% yield. This procedure is apparently the first example of the use of
a vic-dihalide for formation of an α-haloketone.
[4]
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(B) Diphosphinidenecyclobutenes (DPCBs) are rigid P2 ligand systems with strong π-accepting
character that provide unique synthetic catalysts. To synthesize DPCBs with electron-withdrawing
substituents, the classical route cannot be applied since a cyano group destabilizes
the phosphallene and activates [3+2] dimerization. In the new approach 2-bromo-1-phosphapropene
was treated subsequently with butyllithium and 1,2-dibromoethane to afford the desired
cyano-substituted DPCB.
[11]
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(C) For synthetic organic chemists, transition-metal-catalyzed tandem or domino reactions
are very attractive because they allow the combination of two or more bond-forming
reactions into one synthetic operation. In the Pd(OAc)2-catalyzed domino carbopalladation-cyclization reaction of internal alkynes with hindered
Grignard reagents, 1,2-dibromoethane acts as an excellent oxidant to give high yields
of polysubstituted indenes, which are structural constituents of metallocene-based
catalysts for olefin polymerizations, of biologically active compounds, and of functional
materials.
[8]
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(D) In the tandem reaction of pyruvaldehyde, tert-butylhydrazone, and CS2, 1,2-dibromoethane acts as an alkylating agent to give the corresponding asymmetric
azo-aliphatic compound.
[12]
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(E) 1,2-Dibromoethane is a useful reagent in the preparation of diphosphanes from
PH compounds. An interesting example is the oxidative coupling of hypersilylphosphanides
5 to bulky P-secondary hypersilyldiphosphanes 6, forming a mixture of meso- and rac-d,l-diastereomers. The hypersilylphosphanide 5 was synthesized by treatment of t-BuOK with hypersilylphosphane 4. Interestingly, here t-BuOK will not cleave the P-Si bond but rather gives compound 5, which is stable up to one week at room temperature.
[13]
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