Chlorination of trans-1,2-diarylethenes in chloroform and acetic acid

Article abstract of DOI:10.1023/A:1012359707597

When chlorinated in chloroform, (E)-stilbenes bearing electron-donor substituents in the para position of the benzene ring give threo-1,2-diaryl-1,2-dichloroethanes, while meta-substituted (E)-stilbenes, predominantly erythro isomers, irrespective of the

Full text of DOI:10.1023/A:1012359707597

Russian Journal of General Chemistry, Vol. 71, No. 2, 2001, pp. 261 264. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 2, 2001,
pp. 290 293.
Original Russian Text Copyright
2001 by Britsun, Serguchev, Ganushchak.
Chlorination of trans-1,2-Diarylethenes
in Chloroform and Acetic Acid
V. N. Britsun, Yu. A. Serguchev, and N. I. Ganushchak
Institute of Organic Chemistry, Academy of Sciences of the Ukraine, Kiev, Ukraine
Franko Lviv National University, Lviv, Ukraine
Received November 30, 1999
Abstract When chlorinated in chloroform, (E)-stilbenes bearing electron-donor substituents in the para
position of the benzene ring give threo-1,2-diaryl-1,2-dichloroethanes, while meta-substituted (E)-stilbenes,
predominantly erythro isomers, irrespective of the nature of the substituent. The reactions in acetic acid result
in preferential formation of threo-1,2-dichloro- and threo-1-acetoxy-1,2-diaryl-2-chloroethanes.
Earlier we showed that the chlorination of (E)-
stilbene in the presence of pyridine N-oxide yields,
along with chloroalkoxypyridinium chloride formed
by conjugate addition, 1,2-diphenyl-1,2-dichloro-
ethane [1, 2]. It was also found in that works that the
chlorination products are mostly threo isomers,
whereas the reactions of (E)-stilbene with benzene-
selenenyl chloride [3] and sulfur dichloride [4] result
in preferential formation of erythro-1,2-diphenyl-1,2-
dichloroethane, while the reactions with phosphorus
pentachloride [5] and dichloroiodobenzene [6] yield
mixtures of the erythro and threo forms in a ratio
depending on reaction conditions.
For detailed study of the products and stereochemis-
try of the reaction, we performed chlorination of stil-
bene, p-methoxystilbene, p-nitrostilbene, p-chlorostil-
bene, m-(trifluoromethyl)stilbene, m-methylstilbene,
p,p -dimethoxystilbene, and p,p -dichlorostilbene in
chloroform and acetic acid. It was found that the re-
actions in chloroform give 1,2-diaryl-1,2-dichloro-
ethanes in the threo (II) and erythro (III) forms. The
reactions in acetic acid afford, along with dichlorides
II and III, 1-acetoxy-1,2-diaryl-2-chloroethanes in
the threo (IV) and erythro (V) forms.
H
H
H
H
H
Ar
H
H
Ar
Ar
Cl
Cl
Ar
Ar
H
Ar
Ar
OAc Ar
Cl
Ar
H
+
Cl + CH COOH
2 3
Cl
Ar
Cl
H
Cl
OAc
Ia Ih
IIa IIh
IIIa IIIh
IVa IVh
Va Vh
Ar = Ar = C H (a); Ar = C H OCH -p, Ar = C H (b); Ar = C H NO -p, Ar = C H (c); Ar = C H Cl-p, Ar = C H
6 5
6
5
6
4
3
6
5
6
4
2
6
5
6
4
(
d); Ar = C H CF -m, Ar = C H (e); Ar = C H CH -m, Ar = C H (f); Ar = Ar = C H OCH -p (g); Ar = Ar =
6
4
3
6
5
6
4
3
6
5
6
4
3
C H Cl-p (h).
6
4
The threo- and erythro-1,2-diaryl-1,2-dichloro-
signals of 1-acetoxy-1,2-diaryl-2-chloroethanes IV
and V, near 6.2 ppm. Therefore, threo- and erythro-
1,2-dichlorides II, III and threo- and erythro-1-
chloro-2-acetates IV and V were identified according
to [3, 7, 8], taking into account that characteristic
signals of the threo form are downfield from those of
the erythro form [ , ppm: CHCl: 5.14 5.34 (II) and
ethanes II, III and threo- and erythro-1-acetoxy-1,2-
diaryl-2-chloroethanes IV, V were identified and the
1
isomeric ratios were determined by H NMR spec-
troscopy by the chemical shifts of the CHCl and CH
OCOCH protons, which were earlier used for identi-
3
fication of chlorination products of stilbene and
cynnamic esters [3, 7, 8]. The CHCl proton signals of
5.08 5.26 (III); CHOCOCH : 6.06 6.20 (IV) and
3
1
5
,2-diaryl-1,2-dichloroethanes II and III are near
6.03 6.14 (V)]. The characteristic signals of the threo
form have lower coupling constants compared with
.2 ppm, while the CHOCOCH methine proton
3
1
070-3632/01/7102-0261$25.00 2001 MAIK Nauka/Interperiodica

262
BRITSUN et al.
Table 1. Chlorination of trans-1,2-diarylethenes Ia Ih in
chloroform
those of the erythro form both for 1,2-diaryl-1,2-di-
chloroethanes and for 1-acetoxy-1,2-diaryl-2-chloro-
ethanes [J, Hz: 1,2-diaryl-1,2-dichloroethanes: 5.7 6.9
(II) and 9.2 12.3 (III); 1-acetoxy-1,2-diaryl-2-chloro-
ethanes: 6.3 7.8 (V) and 7.4 9.8 (V)]. The results of
the chlorination of 1,2-diarylethenes I in chloroform
and the characteristic H NMR signals of the chlrina-
tion products are listed in Table 1. As seen from
Table 1, in chloroform, stilbene, p-methoxystilbene,
trans-
,2-Di- Chlorination II: III
(CHCl), ppm
1
aryl-
ethene
products
ratio
II
5.23
III
1
Ia
Ib
Ic
Id
Ie
If
IIa, IIIa
IIb, IIIb
IIc, IIIc
IId, IIId
IIe, IIIe
IIf, IIIf
IIg, IIIg
IIh, IIIh
66:33
60:40
38:62
69:31
32:68
33:67
50:50
65:35
5.21
5.20, 5.22 5.15, 5.19 p-chlorostilbene, and p,p -dichlorostilbene yield more
5.31, 5.34 5.22, 5.26 threo-1,2-diaryl-1,2-dichloroethanes IIa, IIb, IId, and
5.20, 5.22 5.16, 5.19 IIh (60 69%) and less erythro isomers IIIa, IIIb,
5.26, 5.28 5.19, 5.23 IIId, and IIIh (31 40%). p-Nitrostilbene and 1,2-di-
5.22, 5.24 5.17, 5.20 arylethenes with electron-donor and electron-acceptor
Ig
Ih
5.14
5.18
5.08
5.12
substituents in the meta position [m-(trifluoromethyl)-
stilbene and m-methylstilbene] give more erythro
isomers IIIc, IIIe, IIIf (62 68%) and less threo
isomers IIc, IIe, IIf (32 38%).
The chlorination of p,p -dimethoxystilbene in
chloroform gives a 1:1 mixture of threo and erythro
isomers IIh and IIIh.
Table 2. Chlorination of trans-1,2-diarylethenes Ia Ih in
acetic acid
Tables 2 4 list the results of the chlorination of
Ratio
substituted stilbenes I in acetic acid, as well as the
trans-
1
characteristic H NMR signals and coupling constants
1
aryl-
ethene
,2-Di-
Chlorination
products
of chlorination products IV and V. As follows of the
tabulated data, the major products of the chlorination
of trans-1,2-diarylethenes in acetic acid are 1,2-diaryl-
II+ III:
IV+ V
II: III
IV: V
1
,2-dichloroethanes II and III (65 81%), while the
products of conjugate addition, 1-acetoxy-1,2-diaryl-
-chloroethanes IV and V, are formed in smaller
amounts (19 35%). With p-methoxystilbene and p,p -
dimethoxystilbenes, no 1-acetoxy-1,2-diaryl-2-chloro-
ethanes IV and V were found. It should be noted that
the chlorination of methyl p-methoxycynnamate in
acetic acid, too, involved no conjugate addition [8].
Ia
Ib
IIa, IIIa, IVa, Va 75: 25
IIb, IIIb, IVb, Vb
78: 22
59: 41
65: 35
77: 23
47: 53
46: 54
62: 38
80: 20
60: 40
2
a
Ic
Id
Ie
If
IIc, IIIc, IVc, Vc 74: 26
IId, IIId, IVd, Vd 76: 24
IIe, IIIe, IVe, Ve 65: 35
IIf, IIIf, IVf, Vf 81: 19
IIg, IIIg, IVg, Vg
70: 30
53: 47
71: 29
80: 29
a
Ig
Ih
IIh, IIIh, IVh, Vh 77: 23
55: 45
These results can be interpreted in terms of Shi-
lov’s trimolecular donor acceptor principle (D
a
With p-methoxystilbene (Ib) and p,p -dimethoxystilbene (Ig),
M
A principle) [9]. According to this principle,
no 1-acetoxy-2-chloro-1,2-diarylethanes are formed.
reaction of a donor (D), in our case acetate ion, with
Table 3. 1H NMR spectra of 1-acetoxy-1,2-diaryl-2-chloroethanes IV and V in CDCl ( , ppm)
3
trans-
,2-Diaryl-
ethene
IV
V
1
CHOCOCH3
CHCl
OCOCH3
CHOCOCH3
CHCl
OCOCH3
Ia
Ic
Id
Ie
If
6.15, 6.17
6.18, 6.20
6.12, 6.14
6.14, 6.16
6.12, 6.15
6.08, 6.10
5.11, 5.13
5.19, 5.21
5.10, 5.12
5.16, 5.18
5.10, 5.13
5.06, 5.08
2.14
2.16
2.14
2.15
2.13
2.15
6.11, 6.14
6.10, 6.14
6.05, 6.08
6.08, 6.11
6.08, 6.12
6.03, 6.06
5.09, 5.12
5.14, 5.18
5.08, 5.11
5.10, 5.13
5.03, 5.07
5.02, 5.05
1.93
2.01
1.96
1.97
2.05
1.99
Ih
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 2 2001

CHLORINATION OF trans-1,2-DIARYLETHENES
263
a
complex (M
A), where M is stilbene and A is
complex.
Table 4. Spin spin coupling constants (J, Hz) for charac-
teristic proton signals of 1,2-diphenyl-1,2-dichloroethanes
II, III and 1-acetoxy-1,2-diphenyl-2-chloroethanes (IV, V)
chlorine, should favor ionization of the
Therewith, the stroger ionized the complex, the less
the nucleophilic species will affect the reaction energy
and products. Apparently, in the chlorination of stil-
benes with such strong electron-donor substituents as
trans-1,2-Diaryl-
II
III
IV
V
ethene
methoxy groups, the
complex transforms into a
cationic intermediate the limiting reaction stage; this
process does not involve acetate ions of the solvent,
and the reaction product is formed via reaction of the
carbenium ion with an internal chloride ion. By
contrast, in the chlorination of less reactive stilbenes,
ionization of the complex in the limiting stage in-
volves solvent molecules; as a result, according to the
trimolecular mechanism, the products are 1-acetoxy-
Iaa
Ib
7.0
9.0
b
6.3
6.9
5.7
6.9
5.7
12.3
11.7
10.2
9.2
Ic
Id
Ie
If
6.3
6.3
6.6
7.8
7.4
7.8
8.1
9.8
10.3
a,b
Ig
Ih
a
7.2
9.3
1
,2-diaryl-2-chloroethanes.
a
b
The signal is not split.
ethanes are not formed.
1-Acetoxy-1,2-diphenyl-2-chloro-
1,2-Diaryl-1,2-dichloroethanes from para-substi-
tuted stilbenes are formed predominantly as threo
isomers IIb IId, IIg, IIh (59 80%), and from meta-
substituted stilbenes give mixtures of threo (IIe, IIf)
and erythro (IIIe, IIIf) isomers in roughly equal
amounts. 1-Acetoxy-1,2-diaryl-2-chloroethanes in all
cases are predominanly formed as threo isomers IV
Table 5. Melting points and elemental analyses of erythro-
1,2-diaryl-1,2-dichloroethanes IIIa IIIh
Comp.
no.
mp,
C
Found
Cl, %
Calculated
Cl, %
Formula
(
53 80%).
IIIa
IIIb
IIIc
IIId
IIIe
IIIf
189
190
208
178
125
112
190
193
28.73 C H Cl
2
28.29
25.27
23.99
37.43
22.26
26.79
22.83
44.38
1
4
12
It should be noted that the chlorination of unsym-
25.11 C H Cl O
15 14 2
metrical stilbenes in acetic acid can give rise to two
regioisomers, 2-aryl-1-acetoxy-1-phenyl-2-chloro-
ethanes A and 1-aryl-1-acetoxy-2-phenyl-2-chloro-
ethanes B.
24.15 C H Cl NO
1
4
11
2
2
37.81 C H Cl
1
4 11
3
22.14 C H Cl F
1
5 11 2 3
26.28 C H Cl
1
5
14
2
IIIg
IIIh
22.41 C H Cl O
16 16 2 2
ArCHClCH(OCOCH )C H ArCH(OCOCH )CHClC H
44.62 C H Cl4
14 10
3
6
5
3
6
5
A
B
1
However, as follows from the H NMR spectra, the
reaction occurs regioselectively and gives a single
isomer. Regioisomeric assessment was performed on
as example of conjugate chlorination of 1-phenyl-2-
6.10 ppm) and respectively erythro form Vd (6.05,
6.08 ppm) and erythro form Vh (6.03, 6.06 ppm),
providing indirect evidence to show that IVd and Vd
1
have a p-ClC H CHOCOCH fragment. Thus, the H
6
4
3
(
p-chlorophenyl)ethene (Id), using the chemical shifts
NMR spectral evidence points to regioisomer B, p-
ClC H CH(OCOCH )CHClC H (a Markovnikov
of the CHOCOCH and CHCl groups of stilbenes IVa
3
6
4
3
6
5
and Va and p,p -dichlorostilbenes IVh and Vh
product). Therefore, there are strong grounds to
beleive that unsymmetrical stilbenes are chlorinated
by Markovnikov’s rule. This proposal is confirmed
by the results of the chlorination of cynnamoyl
chloride and methyl cynnamate in the presence of
pyridine N-oxide [2] and of the chlorination of sub-
stituted cynnamic esters in acetic acid [7, 8], where,
too, a single Markovnikov isomer is formed.
(
Table 3). We compared the chemical shifts of the
CHOCOCH and CHCl groups of the threo and
3
erythro isomers of the regioisomer to be identified
(
IVd, Vd) with the corresponding chemical shifts of
IVa, IVh, Va and Vh. The most close to each other
proved to be the CHCl chemical shifts for threo form
IVd (5.10, 5.12 ppm) and threo form IVa (5.11, 5.13
ppm) and respectively erythro form Vd (5.08,
5
.11 ppm) and erythro form Va (5.09, 5.12 ppm),
EXPERIMENTAL
implying that compounds IVd and Vd have a
C H CHCl fragment. A slightly worse correlation is
1
The H NMR spectra in CDCl were measured on a
6
5
3
between the CHOCOCH chemical shifts of threo
form IVd (6.12, 6.14 ppm) and threo form IVh (6.08,
Varian VXR-300 spectrometer at 300 MHz, internal
reference TMS.
3
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 2 2001

264
BRITSUN et al.
trans-1,2-Diarylethenes were obtained by chlorina-
Khim., 1999, vol. 35, no. 5, pp. 811 814.
tion of arylethenes with aryldiazonium chlorides by
the Meerwein reaction, followed by dehydrochlorina-
tion of 1,2-diaryl-1-chloroethanes [10].
3
4
5
. Kamigata, N., Satoh, T., and Yoshida, M., Bull. Chem.
Soc. Jpn., 1988, vol. 61, no. 2, pp. 449 454.
. Barton, T.J. and Zika, R.G., Tetrahedron Lett., 1970,
no. 14, pp. 1193 1196.
1
,2-Diaryl-1,2-dichloroethanes
IIa IIh
and
IIIa IIIh. Chlorine, 2.5 mmol, was bubbled through
a stirred solution of 2 mmol of trans-1,2-diarylethene
in 10 ml of chloroform over the course of 15 20 min
at 20 25 C. The mixture was stirred for 1 h, and the
solvent was removed at room temperature. erythro-
. Timokhin, B.V., Dmitriev, V.K., Sergienko, L.M.,
Ratovskii, G.V., and Ivanova, N.A., Zh. Obshch.
Khim., 1983, vol. 19, no. 4, pp. 832 837.
6. Heublein, G. and Stadermann, D., Z. Chem., 1966,
1
,2-Diaryl-1,2-dichloroethanes were isolated by
vol. 6, no. 4, pp. 147 148.
double recrystallization from ethanol. The melting
points and elemental analyses of compounds IIIa
IIIh are listed in Table 5.
7
8
9
. Cabaleiro, M.C. and Johnson, M.D., J. Chem. Soc. B,
967, no. 6, pp. 565 570.
. Johnson, M.D. and Trachtenberg, E.N., J. Chem. Soc. B,
968, no. 9, pp. 1018 1022.
1
1
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 2 2001