Chlorobromination of phenylacetylene with the SnCl4-Br 2 system

Full text of DOI:10.1007/s11176-005-0520-7

Russian Journal of General Chemistry, Vol. 75, No. 11, 2005, pp. 1840 1841. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 11, 2005,
pp. 1925 1926.
Original Russian Text Copyright
2005 by Martynov, Amosova, Makhaeva.
LETTERS
TO THE EDITOR
Chlorobromination of Phenylacetylene
with the SnCl4 Br2 System
A. V. Martynov, S. V. Amosova, and N. A. Makhaeva
Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences, Irkutsk, Russia
Received April 25, 2005
2-Bromo-1-chloro-1-phenylethene can be synthe-
sized by chlorination of bromoethynylbenzene with
sodium chloride in dimethyl sulfoxide [1], chlorina-
tion of bromomethyl phenyl ketone with phos-
phorus(V) chloride [1], conjugate halogenation of
phenylacetylene with bromine in the presence of
copper chloride [2], replacement of the bromine atom
in the -position of (1,2-dibromovinyl)benzene by the
action of SbCl5 in CCl4 [2], reaction of phenylacety-
lene with bromine chloride or tetrabutylammonium
dichlorobromate [3], and treatment of phenylacetylene
with Amberlyte A-26 ion exchanger in the dichloro-
bromate form (BrCl2) [4]. 1-Bromo-2-chloro1-phenyl-
ethene was synthesized by bromination 1-chloro-2-
phenylacetylene with KBr in DMSO [1], and (2,2-di-
bromovinyl)benzene was prepared by decomposition
of phenyl 1,2-dibromo-1-chloro-2-phenylethyl ketone
with sodium methoxide in methanol [5] or by con-
densation of benzaldehyde with bromotrichlorome-
thane in the presence of zinc and triphenylphosphine
[1].
phenylacetylene was determined by dehydrohalogena-
tion of the product mixture (I and II), which led to
formation of 1-bromo-2-phenylacetylene C6H5C CBr.
The absence of chloroethynylbenzene among the de-
hydrohalogenation products unambiguously indicates
formation of 2-bromo-1-chloro-1-phenylethene (I).
The addition of bromine at the terminal carbon atom
of phenylacetylene also follows from comparison of
1
the H NMR spectra of the chlorobromination pro-
ducts with the calculated and publiched values for
different regio- and stereoisomers [1, 2, 6]. According
1
to the H NMR data, compound I is formed mainly as
trans isomer.
1H NMR and gas chromatogrphic mass spectro-
metric studies showed that the time of preliminary
contact of bromine with tin(IV) chloride affects the
reaction selectivity and that the temperature affects
the stereochemistry of the process, for E isomer E-I is
a kinetically controlled product which undergoes trans-
formation into thermodynamically more stable isomer
Z-I. In the temperature range from 20 to 0 C, the
ratio E-I:Z-I is (2.6 3):1, while at 20 C it changes to
(1.2 1.4):1. Heating of a mixture containing mainly
the trans isomer (E:Z = 1.4:1) leads to considerable
increase in the fraction of the cis isomer (E:Z = 1:7,
120 C, 1 h).
We have discovered a new and convenient chloro-
brominating system consisting of tin(IV) chloride and
molecular bromine at a ratio of 1:1. Treatment of
phenylacetylene with that system gives mainly the
corresponding chlorobromination product, 2-bromo-1-
chloro-1-phenylethene (I).
The reaction mixtures obtained by preliminarily
heating SnCl4 with Br2 for a short time (5 20 min)
contained 1,2-dibromo-1,2-dichloroethylbenzene (III)
and 2-bromo-1,1-dichloroethylbenzene (IV) (accord-
SnCl4 Br2
C6H5C CH
Z,E-C6H5CCl=CHBr.
CCl4
I
1
ing to the H NMR and GC MS data). Compound III
We succeeded in attaining 87% selectivity in this
reaction, and the yield of vinylbenzene I was 73%.
The reaction is accompanied by formation of 11% of
1,2-dibromo-1-phenylethene II as by-product. Pre-
sumably, the process involves formation of bromine
chloride via exchange of chlorine in SnCl4 for
bromine.
is formed by further chlorobromination of I, while
compound IV is the hydrochlorination product of I. In
the H NMR spectrum of III, the CHClBr proton
gives a singlet at 6.21 ppm. The mass spectrum of
III contains the molecular ion peak [M]+ with m/z
332 (35Cl, 80Br), and peaks from fragment ions
[PhC CBr]+ (m/z 181), [PhCCl=CH]+ (137), and
[PhC CH]+ (102). Ethylbenzene IV showed in the
1
The regioselectivity in the chlorobromination of
1070-3632/05/7511-1840 2005 Pleiades Publishing, Inc.

CHLOROBROMINATION OF PHENYLACETYLENE WITH THE SnCl4 Br2 SYSTEM
1841
1
1H NMR spectrum a singlet at 4.29 ppm from the
CH2Br group, and its mass spectrum contained the
molecular ion peak [M]+ with m/z 252 (35Cl, 80Br)
and fragment ion peak [PhCCl2]+ with m/z 159.
The H NMR spectra were recorded from solutions
in CDCl3 on a Bruker DPX-400 spectrometer
(400 MHz). The GC MS data were acquired on a
Hewlett Packard HP-5971A mass-selective detector
(electron impact, 70 eV).
Although the overall concentration of the Z and E
isomers of I obtained at low temperature is lower than
in the reaction performed at room temperature, the
overall yield in the chlorobromination of phenylace-
tylene (with account taken of compounds III and IV)
is generally constant and is 80 87%.
ACKNOWLEDGMENTS
This study was performed under financial support
by the Presidium of the Russian Academy of Sciences
(project no. 9.3).
Reaction of phenylacetylene with SnCl4 Br2. A
mixture of 4.28 g of bromine and 7.03 g of tin(IV)
chloride in 20 ml of carbon tetrachloride was stirred
for 8 h at 20 C, 2.76 g of phenylacetylene was added,
and the mixture was stirred for 4.5 h. It was then
treated with a saturated solution of Na2S2O3, the or-
ganic phase was separated and dried over CaCl2, and
the solvent was removed to obtain 4.97 g of a mixture
of isomers Z,E-I and Z,E-II, which contained (accord-
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1
1
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 11 2005