Mild oxidative bromination of alkenes and alkynes with zinc bromide and lead tetraacetate

Article abstract of DOI:10.1081/SCC-200031007

Zinc bromide and lead tetraacetate is a practical and safe source of bromine. The combined reagents are used to brominate a variety of alkenes to vicinal dibromoalkanes. Similarly, alkynes can be converted into dibromoalkenes in high yields. The reagents are also capable of tetrabromination of alkynes.

Full text of DOI:10.1081/SCC-200031007

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Mild Oxidative Bromination
of Alkenes and Alkynes
with Zinc Bromide and Lead
Tetraacetate
Hussni A. Muathen a
a Department of Chemistry, Umm Alqura
University, P. O. Box 7283, Makkah, Saudi Arabia
Available online: 10 Jan 2011
To cite this article: Hussni A. Muathen (2004): Mild Oxidative Bromination
of Alkenes and Alkynes with Zinc Bromide and Lead Tetraacetate, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 34:19, 3545-3552
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 19, pp. 3545–3552, 2004
Mild Oxidative Bromination of Alkenes and
Alkynes with Zinc Bromide and
Lead Tetraacetate
*
Hussni A. Muathen
Department of Chemistry, Umm Alqura University,
Makkah, Saudi Arabia
ABSTRACT
Zinc bromide and lead tetraacetate is a practical and safe source of
bromine. The combined reagents are used to brominate a variety of
alkenes to vicinal dibromoalkanes. Similarly, alkynes can be converted
into dibromoalkenes in high yields. The reagents are also capable of
tetrabromination of alkynes.
Key Words: Bromination; Alkenes; Alkynes; Zinc bromide; Lead
tetraacetate.
The electrophilic bromination of alkenes is one of the more thoroughly
investigated reactions.[1–3] It has been discussed from synthetic and
*
Correspondence: Hussni A. Muathen, Department of Chemistry, Umm Alqura
University, P. O. Box 7283, Makkah, Saudi Arabia; E-mail: h_muathen@hotmail.com.
3545
DOI: 10.1081/SCC-200031007
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

3546
Muathen
mechanistic points of view.[4–8] Using elemental bromine in this type of reac-
tion presents a potential hazard. Accordingly, several alternative approaches
have been developed to avoid direct contact with bromine, including the
use of bromine-pyridine complexes, such as PyBr2 and PyHBr3.[9]
The other approach involves oxidation of metal bromides or hydrobromic
acid to elemental bromine with several oxidizing agents, e.g., hydrogen per-
oxide,[10] sodium bismuthate,[11] sodium perborate,[12] and ceric ammonium
nitrate.[13] The chief drawbacks of this approach are insolubility of the
metal bromides and the oxidizing agents in nonaqueous solvents, long reaction
time, polymerization of the alkene with the reagents, and limited application
to alkenes that are not affected by acids and aqueous media.
We have previously reported that chlorine can be generated from anhy-
drous tin(IV) chloride upon treatment with lead tetraacetate Pb(OAc)4.[14]
Both reagents, which were soluble in dichloromethane, were used to chlori-
nate a wide variety of aromatic compounds. We would like to report the
same technique for bromination of olefins, conjugated dienes, and acetylenes
with zinc bromide and lead tetraacetate as a source of bromine.
Zinc bromide is insoluble in most chlorinated solvents, which are inactive
towards bromonium ion intermediates as a result of olefins attacking the
polarized bromine molecule.[9] Since chloroform is known to form hydrogen
bonds to bromide ions,[15,16] we expected that the electrophilic solvation by
hydrogen bonding to the bromide ion would make ZnBr2 more soluble in
CHCl3 than any other solvents.
When a solution of dry ZnBr2 in CHCl3 was treated at room temperature
with Pb(OAc)4, a rapid formation of bromine was observed. Upon addition of
the olefin, the red color of bromine faded instantaneously. The vicinal dibro-
mide was isolated by filtration of the inorganic materials and evaporation of
the washed and dried filtrate (Table 1). Changing the order of addition, i.e.,
treating a solution of the olefin and ZnBr2 with Pb(OAc)4, gave almost the
same results (Sch. 1).
Bromination of conjugated dienes, e.g., isoprene (I), afforded the
1,4-addition product: 1,4-dibromo-2-methyl-trans-2-butene (II) and the 1,2-
addition product: 3,4-dibromo-3-methyl-1-butene (III) (Sch. 2).
Scheme 1.

Oxidative Bromination of Alkenes and Alkynes
3547
b

3548
Muathen
Scheme 2.
The same methodology was applied to bromination of alkynes. When an
equivalent amount of alkyne was added to the brominating mixture, the
isomeric mixture of the dibromoalkene (IV) and (V) was isolated in high
yield. The reagents were also capable of further bromination. When the
alkyne was treated with a stoichiometric amount of the reagents, the tetra-
bromo derivative (VI) was isolated in good yield (Sch. 3, Table 2).
EXPERIMENTAL
ZnBr2 and Pb(OAc)4 were purchased from Aldrich. ZnBr2 (98%) was
dried in the oven at 2508C for 3 hours. Pb(OAc)4 (95%) was used without
any purification. The H NMR spectra were determined in CDCl3 on a JEOL
200 MHz spectrometer using tetramethylsilane as an internal standard. Gas
chromatography was carried out on a 10-ft  0.25-inch column packed with
an SE-30 on Chromosorb W. The product mixtures were analyzed by GLC.
Scheme 3.

Oxidative Bromination of Alkenes and Alkynes
3549

3550
Muathen
The products were identified by comparison of their NMR spectra and
physical data with those of authentic samples.
General Procedure for Bromination with ZnBr2
and Pb(OAc)4
Method A
To a vigorously stirred solution of dry ZnBr2 (1.13 g, 5 mmol) in CHCl3
(25 mL) was added Pb(OAc)4 (1.11 g, 2.5 mmol). Stirring continued for 5 min
while a solution of the alkene or the alkyne (2 mmol) in CHCl3 (5 mL) was
added all at once. The mixture was stirred for the specified reaction time
and then filtered by suction. The filtrate was washed with aqueous sodium
metabisulfite and water. It was dried (Na2SO4) and evaporated in vacuo to
give the dibromide. The yields were measured by GC and H NMR.
Method B
Lead tetraacetate (1.11 g, 2.5 mmol) was added portionwise over 5 min
to a vigorously stirred solution of the alkene or the alkyne (2 mmol) and dry
ZnBr2 (1.13 g, 5 mmol) in CHCl3 (25 mL). The mixture was stirred for the
specified reaction time and then filtered by suction. The filtrate was worked
up as in Method A.
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Received in Poland March 31, 2004