New method for the synthesis of β-bromostyrenes

Article abstract of DOI:10.1023/A:1012780906226

A new stereoselective method was developed for the synthesis of β-bromostyrenes (E/Z > 5/1) starting from hydrazones of aromatic aldehydes and bromoform in the presence of CuCl.

Full text of DOI:10.1023/A:1012780906226

Russian Chemical Bulletin, International Edition, Vol. 50, No. 8, pp. 1401—1405, August, 2001
1401
New method for the synthesis of β-bromostyrenes
A. V. Shastin,a V. G. Nenajdenko,b« V. N. Korotchenko,b and E. S. Balenkovab
aInstitute of Problems of Chemical Physics, Russian Academy of Sciences,
142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (096) 515 3588. E-mail: shastin@icp.ac.ru
bDepartment of Chemistry, M. V. Lomonosov Moscow State University,
Leninskie Gory, 119899 Moscow, Russian Federation.
Fax: +7 (095) 932 8846. E-mail: nen@acylium.chem.msu.ru
A new stereoselective method was developed for the synthesis of β-bromostyrenes
(E/Z > 5/1) starting from hydrazones of aromatic aldehydes and bromoform in the pres-
ence of CuCl.
Key words: catalysis, copper salts, bromostyrenes, hydrazones, bromoform, olefination.
Scheme 1
β-Bromostyrenes find wide application in the ste-
reospecific synthesis of substituted alkenes1—5 and as
synthetic precursors of the corresponding acetylenes.6,7
Ar
H
Ar
H
N2H5OH
EtOH
Besides, cis- and trans-bromostyrenes are extensively
used in the total synthesis of natural compounds and
antibiotics.1,8
O
N
NH2
Classical methods for the synthesis of bromostyrenes
involve debromocarboxylation of dibromocinnamic ac-
ids,6 various versions of the Hunsdiecker—Borodin re-
action9,10, and the Wittig reaction.11,12 Generally, these
reactions afford mixtures of (E)- and (Z)-bromoalkenes in
ratios from 1 : 1 to 4 : 1. Pure (E)- and (Z)-bromo-
styrenes can be prepared by bromination of cis- and
trans-alkenylsilanes13,14 and alkenylboron compounds15
or by stereospecific reduction of dibromostyrenes.1,16,17
Previously,18 we have reported the redox reactions of
N-unsubstituted hydrazones of aromatic aldehydes with
carbon tetrachloride in the presence of catalytic amounts
of copper(I) chloride. These reactions represent an es-
sentially new method for the transformation of the C=O
bond into the C=C bond (olefination of carbonyl com-
pounds). We have demonstrated that these reactions can
be used for the synthesis of a wide range of aromatic
and heteroaromatic dichloroolefins.19,20 These reactions
have a general character and can involve other
polyhaloalkanes along with carbon tetrachloride. Thus,
we synthesized fluorine-containing olefins by treatment
of hydrazones, which were prepared in situ, with freons
CCl3CF3 or CCl2FCClF2 (the reactions proceeded
stereoselectively to form predominantly the Z isomers)
(Scheme 1).*
CuCl
CCl4
Cl
CCl3CF3
CCl2FCClF2
Ar
Ar
Ar
H
F
Cl
H
H
CF3
Cl
CClF2
Results and Discussion
We expected that the behavior of bromoform in
these reactions would be analogous to that of CCl4 and
freons and hoped that it would allow us to develop a
new procedure for the preparation of the corresponding
bromoalkenes. Actually, the reactions of bromoform
with solutions of N-unsubstituted hydrazones of aro-
matic aldehydes in DMSO in the presence of CuCl and
a base (an aqueous solution of NH4OH) afforded
bromostyrenes as the major reaction products (Table 1).
We performed the reactions with hydrazones of aro-
matic aldehydes containing substituents of various na-
ture (Scheme 2).
Scheme 2
In the present study, we report new results of inves-
tigation on the use of the catalytic system based on
copper(I) chloride for olefination of aromatic aldehydes.
Ar
H
H
Ar
H
CHBr3
N
CuCl
Br
NH2
1a—m
* The results will be published in Tetrahedron (2001).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1334—1337, August, 2001.
1066-5285/01/5008-1401 $25.00 ©ꢀ2001 Plenum Publishing Corporation

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Russ.Chem.Bull., Int.Ed., Vol. 50, No. 8, August, 2001
Shastin et al.
Table 1. Synthesis of β-bromostyrenes
complexes of copper and other metals described in the
literature.21—23 Since the reactions of hydrazones with
bromoform gave rise to analogous products, we believe
that this mechanism can be extended to the reactions of
formation of bromostyrenes.
Compound
Aryl
Yield (%)
E/Z*
1a
1b
1c
1d
1e
1f
4-O2NC6H4
2-O2NC6H4
4-ClC6H4
2-ClC6H4
4-FC6H4
56
19
67
53
52
84
73
57
64
31
29
84
60
20/1
5/1
6/1
8/1
10/1
5/1
10/1
13/1
5/1
E
At the first stage, CuI was oxidized by bromoform,
the latter being reduced to dibromomethane and CuI
being converted into CuII. Earlier,19 we have examined
the effect of various salts of transition metals as
olefination catalysts and demonstrated that copper
monochloride is the reagent of choice because it made it
possible to circumvent noncontrolled heat evolution
and foaming of the reaction mixture, which were ob-
served in the reactions involving CuCl2 as the catalyst.19
At the subsequent stage, CuII reacted with hydrazone to
form carbenoid copper complex 2 (through intermediate
formation of the corresponding diazoalkane). The reac-
tion of complex 2 with CHBr3 afforded the correspond-
ing bromostyrene accompanied by regeneration of CuII
and gave rise to a new catalytic cycle. Simultaneously,
the carbenoid complex reacted with diazoalkane to yield
symmetrical azine (the side reaction).19,24
2-FC6H4
1g
1h
1i
4-BrC6H4
2-BrC6H4
4-MeC6H4
2,6-Cl2C6H3
4-CF3C6H4
2-CF3C6H4
4-CH3OC6H4
1j
1k
1l
9/1
6/1
5/1
1m
* The isomer ratio was determined based on the data
from 1H NMR spectroscopy; the signals were assigned
to particular isomers by comparing the spin-spin cou-
pling constants of the hydrogen atoms at the double bond
(Jtrans ≈ 14 Hz > Jcis ≈ 8 Hz).
The major and side reactions afforded the common
key intermediate (complex 2), both reactions being
catalytic (completion of both cycles resulted in regen-
eration of CuII).
We studied the reaction of 4-methylbenzaldehyde
hydrazone with bromoform by 1H NMR spectroscopy.
It appeared that the reaction mixture consisted of
bromostyrene, azine, CH2Br2, and an unconsumed ex-
cess of bromoform (Fig. 1). Consequently, under the
reaction conditions, hydrazone was converted into
bromostyrene and azine, while bromoform was con-
verted into bromostyrene and CH2Br2.
It was found that the yields of bromoalkenes 1
depend on the nature of the substituent in the aromatic
nucleus and its position. As in the case of dichloro-
styrenes19 and fluorine-containing alkenes, ortho-substi-
tuted alkenes were obtained in lower yields than the
corresponding para-substituted analogs. The formation
of bromostyrenes proceeded stereoselectively to give
predominantly alkenes in a trans configuration.
This reaction is adequately described by the catalytic
cycle, which we have proposed previously.20 The reac-
tions involved the carbenoid copper complex as the key
intermediate (Scheme 3), which is analogous to carbenoid
Scheme 3
RCHN2•Cu0
H+
CHBr3 CH2Br2
RCH=NNH2
N2
Cu2+
Cu+
RCH=Cu, 2
RCHN2
RCH=N—N=CHR•Cu0
H
Br
R
H
CHBr3
RCH=N—N=CHR
CHBr3
+
CH2Br2

New method for the synthesis of β-bromostyrenes
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 8, August, 2001
1403
CHBr3
CHBr3
A
A
A
B
A
A
B
B
B
B
B
B
B
B
B B
B
B
B
B
8.5
8.0
7.5
7.0
6.5
DMSO
B
A
B
CH2Br2
2.6
2.5
2.4
2.3
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0
Fig. 1. 1H NMR spectrum of the reaction mixture formed in the reaction of 4-methylbenzaldehyde hydrazone with bromoform
(A, signals of symm-azine; B, signals of the E and Z isomers of bromostyrene).
solution (300 mL). The reaction products were extracted with
CH2Cl2 (3 ½ 50 mL), the extracts were dried over Na2SO4, and
CH2Cl2 was evaporated. Products 1a—m were isolated by col-
umn chromatography (SiO2, hexane). Attempts to isolate the E
and Z isomers by column chromatography failed.
To summarize, we developed a new stereoselective
method for the synthesis of bromostyrenes. This method
is characterized by simplicity of the reaction procedure
and isolation of the products, involves readily accessible
and inexpensive reagents, requires mild conditions, and
is applicable to a wide range of substrates.
The spectral characteristics of compounds 1a—c and 1f—m
agree with the published data.1,6,25—28
1-[2-Bromovinyl]-4-nitrobenzene (1a):6,25 a 20/1 mixture of
E—Z isomers was obtained. M.p. 136—137 °C. Rf 0.10 (hexane).
1H NMR, δ, E isomer: 7.02 (d, 1 H, =CH(Br), J = 14.2 Hz);
7.17 (d, 1 H, =CH—Ar, J = 14.2 Hz); 7.44 (d, 2 H, Ar,
J = 8.8 Hz); 8.18 (d, 2 H, Ar, J = 8.8 Hz); Z isomer: 6.67 (d,
1 H, =CH(Br), J = 8.2 Hz); 7.69 (d, 2 H, Ar, J = 8.8 Hz); 7.81
(d, 2 H, Ar, J = 8.8 Hz). The remaining signals of the minor
isomer overlap with the signals of the major isomer.
1-[2-Bromovinyl]-2-nitrobenzene (1b):6 a 5/1 mixture of
E—Z isomers was obtained. M.p. 56—57 °C. Rf 0.10 (hexane).
1H NMR, δ, E isomer: 6.78 (d, 1 H, =CH(Br), J = 14.0 Hz);
7.62 (d, 1 H, =CH—Ar, J = 14.0 Hz); 7.45—7.70 (m, 3 H, Ar);
7.97 (dd, 1 H, Ar, J1 = 9.0 Hz, J2 = 0.8 Hz); Z isomer: 6.62 (d,
1 H, =CH(Br), J = 7.9 Hz); 8.10 (d, 1 H, Ar, J = 8.2 Hz). The
remaining signals of the minor isomer overlap with the signals
of the major isomer.
Experimental
The IR spectra were recorded on a UR-20 spectrophoto-
meter in thin layers (liquids) and Nujol mulls (solids). The 1H
and 13C NMR spectra were measured on a Varian VXR-400
spectrometer (400 and 100 MHz, respectively) in CDCl3 with
Me4Si as the internal standard. The course of the reaction was
monitored by TLC on Merck 60F254 plates. Column chroma-
tography was carried out with the use of silica gel (63—200 mesh)
purchased from Merck.
Hydrazones of aromatic aldehydes were prepared from
commercial aldehydes according to a procedure reported pre-
viously.19
Synthesis of bromostyrenes (general procedure). A solution
of NH4OH (1.2 mL) and CuCl (50 mg, 0.5 mmol, 10 mol.%)
was added to a solution of hydrazone (5 mmol) in DMSO
(5 mL). Then a solution of CHBr3 (1.34 mL, 15 mmol) in
DMSO (10 mL) was added dropwise at 0 °C for 10 min,
vigorous gas evolution being observed. The reaction mixture
was stirred for 24 h and then decomposed with a 0.1 M HCl
1-[2-Bromovinyl]-4-chlorobenzene (1c):1,27 a 6/1 mixture
of E—Z isomers was obtained (colorless oil). Rf 0.60 (hexane).
1H NMR, δ, E isomer: 6.76 (d, 1 H, =CH(Br), J = 14.1 Hz);
7.04 (d, 1 H, =CH—Ar, J = 14.1 Hz); 7.22 (d, 2 H, Ar,
J = 8.5 Hz); 7.28 (d, 2 H, Ar, J = 8.5 Hz); Z isomer: 6.44 (d,
1 H, =CH(Br),
J = 8.2 Hz); 7.01 (d, 1 H, =CH—Ar,

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Shastin et al.
J = 8.2 Hz); 7.33 (d, 2 H, Ar, J = 8.4 Hz); 7.62 (d, 2 H, Ar,
J = 8.4 Hz).
6.98 (d, 1 H, =CH—Ar, J = 8.2 Hz); 7.42 (d, 2 H, Ar,
J = 8.2 Hz); 7.57 (d, 2 H, Ar, J = 8.2 Hz).
1-[2-Bromovinyl]-2-chlorobenzene (1d): a 8/1 mixture of
E—Z isomers was obtained (colorless oil). Rf 0.60 (hexane).
IR, ν/cm–1: 1610 (C=C). Found (%): C, 44.34; H, 2.56.
C8H6BrCl. Calculated (%): C, 44.18; H, 2.78. 1H NMR, δ,
E isomer: 6.76 (d, 1 H, =CH(Br), J = 13.9 Hz); 7.15—7.21 (m,
2 H, Ar); 7.31—7.37 (m, 2 H, Ar); 7.43 (d, 1 H, =CH(Ar),
J = 13.9 Hz); Z isomer: 6.56 (d, 1 H, =CH(Br), J = 8.0 Hz);
7.21—7.26 (m, 3 H, Ar, =CH(Ar)); 7.37—7.39 (m, 1 H, Ar);
7.77—7.81 (m, 1 H, Ar). 13C NMR, δ, E isomer: 109.1 (=CBr);
126.7 (Ar); 126.9 (Ar); 129.2 (Ar); 129.3 (CAr—C); 129.8 (Ar);
133.5 (=CH—); 133.9 (CAr—Cl); Z isomer: 109.3 (=CBr);
126.1 (Ar); 129.3 (Ar); 130.1 (Ar); 132.3 (=CH—).
1-[2-Bromovinyl]-4-fluorobenzene (1e): a 10/1 mixture of
E—Z isomers was obtained (colorless oil). Rf 0.65 (hexane). IR,
ν/cm–1: 1600 (C=C). Found (%): C, 47.60; H, 2.83. C8H6BrF.
Calculated (%): C, 47.80; H, 3.01. 1H NMR, δ, E isomer: 6.67
(d, 1 H, =CH(Br), J = 14.1 Hz); 7.04 (d, 1 H, =CH(Ar),
J = 14.1 Hz); 7.00 (dd, 2 H, CH(3), CH(5), J1 = 17.1 Hz,
J2 = 8.6 Hz); 7.24 (dd, 2 H, CH(2), CH(6), J1 = 8.6 Hz,
J2 = 5.3 Hz); Z isomer: 6.39 (d, 1 H, =CH(Br), J = 8.1 Hz);
7.50 (dd, 2 H, Ar, J1 = 9.1 Hz, J2 = 5.3 Hz); 7.65 (dd, 2 H, Ar,
J1 = 8.6 Hz, J2 = 5.3 Hz). The remaining signals of the minor
isomer overlap with the signals of the major isomer. 13C NMR,
δ, E isomer: 106.7 (=CBr); 115.4 (C(3), C(5), J = 21.4 Hz);
128.4 (C(2), C(6), J = 7.5 Hz); 131.0 (CAr—C, J = 3.5 Hz);
136.7 (=CH—); 163.5 (CF, J = 247 Hz); Z isomer: 106.8
(=CBr); 116.1 (C(3), C(5), J = 22.9 Hz); 131.5 (C(2), C(6),
J = 7.6 Hz); 136.5 (=CH—).
1-[2-Bromovinyl]-2-fluorobenzene (1f):6 a 5/1 mixture of
E—Z isomers was obtained (colorless oil). Rf 0.65 (hexane).
1H NMR, δ, E isomer: 6.79 (d, 1 H, =CH(Br), J = 14.2 Hz);
6.87—6.98 (m, 2 H, Ar); 7.05 (d, 1 H, =CH—Ar, J = 14.2 Hz);
7.10—7.19 (m, 2 H, Ar); Z isomer: 6.42 (d, 1 H, =CH(Br),
J = 8.1 Hz); 7.20—7.22 (m, 2 H, Ar); 7.29—7.33 (m, 2 H, Ar).
The remaining signals of the minor isomer overlap with the
signals of the major isomer.
4-Bromobenzene-1-[2-bromovinyl] (1g):6 a 10/1 mixture
of E—Z isomers was obtained (colorless oil). Rf 0.65 (hexane).
1H NMR, δ, E isomer: 6.77 (d, 1 H, =CH(Br), J = 14.2 Hz);
7.03 (d, 1 H, =CH—Ar, J = 14.2 Hz); 7.15 (d, 2 H, Ar,
J = 8.2 Hz); 7.43 (d, 2 H, Ar, J = 8.2 Hz); Z isomer: 6.47 (d,
1 H, =CH(Br), J = 8.2 Hz); 6.98 (d, 1 H, =CH—Ar,
J = 8.2 Hz); 7.48 (d, 2 H, Ar, J = 8.5 Hz); 7.54 (d, 2 H, Ar,
J = 8.5 Hz).
1-[(E)-2-Bromovinyl]-2,6-dichlorobenzene (1j): was ob-
tained as a colorless oil. Rf 0.50 (hexane). IR, ν/cm–1: 1610
(C=C). 1H NMR, δ: 6.96 (d, 1 H, =CH(Br), J = 14.4 Hz); 7.12
(t, 1 H, CH(4), J = 8.0 Hz); 7.17 (d, 1 H, =CH(Ar),
J = 14.4 Hz); 7.31 (d, 2 H, CH(3), CH(5), J = 8.0 Hz).
13C NMR, δ: 115.4 (=CBr); 129.3 (Ar); 129.6 (Ar); 131.3
(=CH—); 133.5 (CAr—C); 134.8 (CAr—Cl). Found (%): C, 38.03;
H, 2.07. C8H5BrCl2. Calculated (%): C, 38.14; H, 2.00.
1-[2-Bromovinyl]-4-trifluoromethylbenzene (1k):25 a 9/1
mixture of E—Z isomers was obtained (colorless oil). Rf 0.70
(hexane). 1H NMR, δ, E isomer: 6.88 (d, 1 H, =CH(Br),
J = 14.2 Hz); 7.13 (d, 1 H, =CH—Ar, J = 14.2 Hz); 7.37 (d,
2 H, Ar, J = 8.5 Hz); 7.55 (d, 2 H, Ar, J = 8.5 Hz); Z isomer:
6.55 (d, 1 H, =CH(Br), J = 8.3 Hz); 7.08 (d, 1 H, =CH—Ar,
J = 8.3 Hz); 7.62 (d, 2 H, Ar, J = 8.5 Hz); 7.74 (d, 2 H, Ar,
J = 8.5 Hz).
1-[2-Bromovinyl]-2-trifluoromethylbenzene (1l):28 a 6/1
mixture of E—Z isomers was obtained (colorless oil). Rf 0.70
(hexane). 1H NMR, δ, E isomer: 6.77 (d, 1 H, =CH(Br),
J = 13.9 Hz); 7.36—7.51 (m, 4 H, Ar, =CH—Ar); 7.63 (d, 1 H,
Ar, J = 7.9 Hz); Z isomer: 6.88 (d, 1 H, =CH(Br), J = 8.2 Hz);
7.13 (d, 1 H, =CH—Ar, J = 8.2 Hz). The remaining signals
of the minor isomer overlap with the signals of the major
isomer.
1-[2-Bromovinyl]-4-methoxybenzene (1m):1,6,25,27 a 5/1
mixture of E—Z isomers was obtained (colorless oil). Rf 0.20
(hexane). 1H NMR, δ, E isomer: 3.78 (s, 3 H, Me); 6.59 (d,
1 H, =CH(Br), J = 13.9 Hz); 6.83 (d, 2 H, Ar, J = 8.5 Hz);
7.02 (d, 1 H, =CH(Ar), J = 13.9 Hz); 7.21 (d, 2 H, Ar,
J = 8.5 Hz); Z isomer: 3.80 (s, 3 H, Me); 6.28 (d, 1 H,
=CH(Br), J = 8.0 Hz); 6.97 (d, 1 H, =CH(Ar), J = 8.0 Hz);
7.49 (d, 2 H, Ar, J = 9.0 Hz); 7.65 (d, 2 H, Ar, J = 9.0 Hz).
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos. 00-03-
32760 and 00-03-32763).
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Received March 16, 2001;
in revised form May 18, 2001