Synthesis and properties of trans-2-[2-(2-hetaryl)vinyl]benzothiazoles

Article abstract of DOI:10.1134/S1070428012030141

Electrophilic substitution reactions (formylation and acylation) in the series of 2-[2-(2-furyl)vinyl]- and 2-[2-(2-thienyl)vinyl]benzothiazoles leads to the corresponding derivatives at the α-position of the furan or thiophene ring. The presence of a vin

Full text of DOI:10.1134/S1070428012030141

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2012, Vol. 48, No. 3, pp. 423–425. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © E.V. Drobysheva, A.A. Aleksandrov, M.M. El’chaninov, 2012, published in Zhurnal Organicheskoi Khimii, 2012, Vol. 48, No. 3,
pp. 426–428.
Synthesis and Properties
of trans-2-[2-(2-Hetaryl)vinyl]benzothiazoles
E. V. Drobysheva, A. A. Aleksandrov, and M. M. El’chaninov
South Russian State Technical University (Novocherkassk Polytechnic Institute),
ul. Prosveshcheniya 132, Novocherkassk, 346428 Russia
e-mail: aaanet1@yandex.ru
Received June 1, 2011
Abstract—Electrophilic substitution reactions (formylation and acylation) in the series of 2-[2-(2-furyl)vinyl]-
and 2-[2-(2-thienyl)vinyl]benzothiazoles leads to the corresponding derivatives at the α-position of the furan or
thiophene ring. The presence of a vinylene bridge weakens deshielding effect of the benzothiazole fragment on
π-excessive heterocycles, so that such compounds react at a higher rate and under milder conditions as
compared to hetarylbenzothiazoles having no vinylene bridge.
DOI: 10.1134/S1070428012030141
Introduction of substituents containing an extended
π-system into benzazole molecules favors increase in
quantum yield. Examples are benzothiazolyl stilbenes
containing benzene and pyridine rings, which are used
as optical bleaching agents. We previously studied [1]
the electronic structure and spectral parameters of car-
bonyl derivatives of fused 2-hetarylbenzimidazoles. In
the present work we examined methods of synthesis of
2-[2-(2-hetaryl)vinyl]benzothiazoles I and II contain-
ing a furan or thiophene ring and some their electro-
philic substitution reactions (formylation, acylation)
leading to potential luminophores.
Compounds I and II were synthesized previously in
22 and 65% yield, respectively, by reaction of 2-meth-
ylbenzothiazole with the corresponding aldehydes in
the presence of fused zinc chloride [2]. However, we
obtained better results by condensation of o-aminoben-
zenethiol with the corresponding hetarylacryloyl
chlorides in 1-methylpyrrolidin-2-one. The yields of I
and II were 56 and 78%, respectively. Compounds I
and II were then brought into reactions with electro-
philic reagents, hexamethylenetetramine in polyphos-
phoric acid (PPA), acetic anhydride and Mg(ClO4)2,
and benzoic acid in PPA.
As shown in [3], the presence of a bridge connect-
ing two heteroaromatic systems, e.g., of a CH=CH
bridging moiety in 2-[2-(2-furyl)vinyl]benzimidazole,
sharply weakens the effect of the imidazole ring on the
reactivity of furan or other hetaryl fragment. This was
confirmed by quantum-chemical calculation of the
total (σ + π)-electron charges in the furan ring of
2-(2-hetarylvinyl)-substituted benzimidazoles and
their analogs having no vinylene bridging group.
A similar pattern was observed for 2-(2-hetarylvinyl)-
1
benzothiazoles I and II. Analysis of the H NMR data
revealed a clearly defined correlation between the
downfield shift of signals from 3-H of the hetaryl ring
(which is determined mainly by the inductive effect)
and deshielding effect of the benzothiazole fragment in
2-(2-furyl)- and 2-(2-thienyl)benzothiazoles (δ 7.20
and 7.51 ppm, respectively) [4]; the corresponding
signals appeared in the spectra of compounds I and II
Scheme 1.
O
(CH2)6N4–PPA
or Ac2O–Mg(ClO4)2
or PhCOOH–PPA
4
R
5'
N
N
X
X
5
2'
4'
S
S
6
3'
7
I, II
IIIa–IIIc, IVa–IVc
I, III, X = O; II, IV, X = S; R = H (a), Me (b), Ph (c).
423

DROBYSHEVA et al.
424
at δ 6.59 and 7.25 ppm (weaker deshielding). There-
fore, we expected enhancement of the relative reactiv-
ity of the hetaryl ring in compounds I and II toward
medium-strength electrophiles.
The elemental compositions were determined on
a Perkin Elmer 2400 analyzer. The melting points were
measured in capillaries using a PTP melting point
apparatus.
2-(2-Furyl)benzothiazole is known to undergo
formylation according to Vilsmeier–Haack [2]. Like-
wise, compound I smoothly reacted under mild condi-
tions with the DMF–POCl3 complex to afford 86% of
5-formyl derivative IIIa. Compound II failed to react
with DMF–POCl3; therefore, we followed a different
procedure which was successfully used previously to
obtain formyl-substituted hetarylbenzimidazoles. The
reaction of II with hexamethylenetetramine in poly-
phosphoric acid gave the corresponding aldehyde IVa
in 67% yield (Scheme 1).
2-[2-(2-Furyl)vinyl]-1,3-benzothiazole (I).
2-(2-Furyl)prop-2-enoyl chloride, 1.57 g (10 mmol),
was added to a solution of 1.25 g (10 mmol) of
o-aminobenzenethiol in 10 ml of 1-methylpyrrolidin-2-
one. The mixture was heated for 1 h under reflux,
cooled, and poured into 50 ml of cold water. The pre-
cipitate was filtered off, thoroughly washed with cold
water, and recrystallized from ethanol. Yield 1.27 g
(56%), mp 89– 90°C; published data [7]: mp 92–93°C.
1H NMR spectrum, δ, ppm: 6.48–6.50 m (1H, 4′-H),
6.59 d (1H, 3′-H, J = 3.4 Hz), 7.26 d (1H, 2′-CH, J =
16.0 Hz), 7.36 t (1H, 6-H, J = 8.0 Hz), 7.37 d (1H,
2-CH, J = 16.0 Hz), 7.47 t (1H, 5-H, J = 8.0 Hz),
7.50 d (1H, 5′-H, J = 1.6 Hz), 7.85 d (1H, 7-H, J =
8.0 Hz), 7.98 d (1H, 4-H, J = 8.0 Hz).
Unlike analogous benzimidazoles, compounds I
and II were acetylated according to Dorofeenko [5],
i.e., by heating in acetic anhydride containing a cata-
lytic amount of magnesium perchlorate. Methyl
ketones IIIb and IVb were obtained in 66 and 43%
yield, respectively. We tried different methods for the
benzoylation of compounds I and II, including many
modifications of the Friedel–Crafts reaction; however,
most of them gave no positive results. While searching
for most appropriate reaction conditions and taking
into account known data on successful benzoylation of
analogous benzimidazoles, we resorted to the proce-
dure proposed by Gardner [6] for the acylation of phe-
nols and phenyl ethers. According to this procedure,
compounds I and II were treated with benzoic acid in
PPA at a relatively low temperature (80–100°C; het-
arylbenzothiazoles having no vinyl group were sub-
jected to benzoylation at 170–180°C [4]). As a result,
phenyl ketones IIIc and IVc were synthesized in 77
and 63% yield, respectively.
2-[2-(2-Thienyl)vinyl]-1,3-benzothiazole (II) was
synthesized in a similar way from 1.73 g (10 mmol) of
2-(2-thienyl)prop-2-enoyl chloride. Yield 1.90 g
(78%), mp 116–117°C (from ethanol); published data
[7]: mp 117°C. 1H NMR spectrum, δ, ppm: 7.07 t (1H,
4′-H, J = 5.1 Hz), 7.21 d (1H, 2′-CH, J = 15.9 Hz),
7.25 d (1H, 3′-H, J = 3.3 Hz), 7.35 d (1H, 5′-H, J =
5.8 Hz), 7.36 t (1H, 6-H, J = 7.5 Hz), 7.47 t (1H, 5-H,
J = 7.7 Hz), 7.67 d (1H, 2-CH, J = 15.9 Hz), 7.85 d
(1H, 7-H, J = 8.2 Hz), 7.98 d (1H, 4′-H, J = 8.2 Hz).
5-[2-(1,3-Benzothiazol-2-yl)vinyl]furan-2-carbal-
dehyde (IIIa). Compound I, 1.14 g (5 mmol), was
dissolved in 4.38 g (60 mmol) of N,N-dimethylform-
amide. The solution was cooled to 0°C, 4.61 g
(30 mmol) of phosphoryl chloride was added dropwise
at such a rate that the temperature did not exceed 10°C,
and the mixture was stirred for 10 min at 0°C and for
2 h at 80°C, poured into 50 ml of water, and carefully
neutralized with concentrated aqueous ammonia to
pH 7–8. The product was extracted into chloroform
(2×25 ml) and was subjected to column chromatog-
raphy (20 × 3 cm; 50 g Al2O3; eluent methylene
chloride). Yield 1.10 g (86%), mp 138–139°C (from
Solutions of compounds IIIa–IIIc and IVa–IVc in
ethanol and benzene showed strong fluorescence upon
UV irradiation; therefore, these compounds were pre-
sumed to possess photoluminescent properties.
EXPERIMENTAL
The IR spectra were recorded on a Specord 75 IR
spectrometer from samples dispersed in mineral oil.
The 1H NMR spectra were measured on a Varian Unity
300 instrument at 300 MHz from solutions in CDCl3
using tetramethylsilane as internal reference. The prog-
ress of reactions was monitored by TLC on aluminum
oxide of activity grade II according to Brockmann
using methylene chloride or chloroform as eluent;
spots were developed by treatment with iodine vapor.
1
ethanol). IR spectrum: ν 1680 cm–1 (C=O). H NMR
spectrum, δ, ppm: 6.74 d (1H, 3′-H, J = 3.6 Hz), 7.28 d
(1H, 4′-H, J = 3.6 Hz), 7.38 d (1H, 2-CH, J = 15.8 Hz),
7.40 t (1H, 6-H, J = 8.0 Hz), 7.50 t (1H, 5-H, J =
8.0 Hz), 7.56 d (1H, 2′-H, J = 15.8 Hz), 7.87 d (1H,
7-H, J = 7.8 Hz), 8.04 d (1H, 4-H, J = 8.1 Hz), 9.67 s
(1H, CHO). Found, %: C 66.14; H 3.79; N 5.25.
C14H9NO2S. Calculated, %: C 65.87; H 3.55; N 5.49.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 48 No. 3 2012

SYNTHESIS AND PROPERTIES OF trans-2-[2-(2-HETARYL)VINYL]BENZOTHIAZOLES
425
5-[2-(1,3-Benzothiazol-2-yl)vinyl]thiophene-2-
carbaldehyde (IVa). A mixture of 1.22 g (5 mmol) of
compound II and 2.8 g (20 mmol) of hexamethylene-
tetramine in 20 g of PPA was heated for 2 h at 60–
80°C. The mixture was then diluted with 50 ml of
water and carefully neutralized with a solution of am-
monia. The precipitate was filtered off and recrystal-
lized. Yield 0.91 g (67%), mp 148–149°C (from
of compound I in 20 g of PPA, and the mixture was
stirred for 2 h at 80–100°C. The mixture was decom-
posed with 50 ml of water and neutralized with a con-
centrated solution of ammonia. The product was
extracted into chloroform and isolated by chromatog-
raphy on aluminum oxide using methylene chloride as
eluent. Yield 1.27 g (77%), mp 108–109°C (from
1
ethanol). IR spectrum: ν 1670 cm–1 (C=O). H NMR
1
ethanol). IR spectrum: ν 1660 cm–1 (C=O). H NMR
spectrum, δ, ppm: 6.70 d (1H, 3′-H, J = 3.7 Hz), 7.21 d
(1H, 4′-H, J = 3.7 Hz), 7.36 d (1H, 2-CH, J = 15.9 Hz),
7.38 t (1H, 6-H, J = 8.0 Hz), 7.48 t (1H, 5-H, J =
8.0 Hz), 7.50 d (1H, 2′-CH, J = 15.9 Hz), 7.56 t (3H,
m-H, p-H, J = 7.7 Hz), 7.85 d (1H, 7-H, J = 8.0 Hz),
8.02 d (1H, 4-H, J = 8.0 Hz), 8.07 d (2H, o-H, J =
7.8 Hz). Found, %: C 72.77; H 4.23; N 4.47.
C20H13NO2S. Calculated, %: C 72.49; H 3.95; N 4.23.
spectrum, δ, ppm: 7.31 d (1H, 3′-H, J = 3.9 Hz), 7.38 d
(1H, 2′-CH, J = 15.8 Hz), 7.39 t (1H, 6-H, J = 7.8 Hz),
7.49 t (1H, 5-H, J = 7.8 Hz), 7.64 d (1H, 2-CH, J =
15.8 Hz), 7.70 d (1H, 4′-H, J = 3.9 Hz), 7.87 d (1H,
7-H, J = 7.8 Hz), 8.00 d (1H, 4-H, J = 7.8 Hz), 9.89 s
(1H, CHO). Found, %: C 62.17; H 3.12; N 4.92.
C14H9NOS2. Calculated, %: C 61.97; H 3.34; N 5.16.
1-{5-[2-(1,3-Benzothiazol-2-yl)vinyl]furan-2-yl}-
ethanone (IIIb). Compound I, 1.14 g (5 mmol), was
dissolved in 15 ml of acetic anhydride, 0.01 g
(0.04 mmol) of Mg(ClO4)2 was added, and the mixture
was heated for 0.5 h under reflux. The mixture was
then poured into 50 ml of cold water, and the product
was isolated as described above for IIIa. Yield 0.89 g
(66%), mp 138–139°C. IR spectrum: ν 1640 cm–1
{5-[2-(1,3-Benzothiazol-2-yl)vinyl]thiophen-2-
yl}phenylmethanone (IVc) was synthesized in a simi-
lar way from 1.22 g (5 mmol) of compound II. Yield
1.09 g (63%), mp 150–151°C (from ethanol). IR spec-
1
trum: ν 1660 cm–1 (C=O). H NMR spectrum, δ, ppm:
7.21 d (1H, 3′-H, J = 4.5 Hz), 7.31 d (1H, 2′-CH, J =
16.0 Hz), 7.38 t (1H, 6-H, J = 7.8 Hz), 7.45 t (1H, 5-H,
J = 7.8 Hz), 7.54 t (3H, m-H, p-H, J = 7.8 Hz), 7.59 d
(1H, 2-CH, J = 16.0 Hz), 7.61 d (1H, 4′-H, J = 4.5 Hz),
7.83 d (1H, 7-H, J = 7.8 Hz), 7.98 d (1H, 4-H, J =
7.8 Hz), 8.10 d (2H, o-H, J = 8.0 Hz). Found, %:
C 68.83; H 4.08; N 3.76. C20H13NOS2. Calculated, %:
C 69.14; H 3.77; N 4.03.
1
(C=O). H NMR spectrum, δ, ppm: 2.54 s (3H, CH3),
6.69 d (1H, 3′-H, J = 3.6 Hz), 7.23 d (1H, 4′-H, J =
3.6 Hz), 7.38 d (1H, 2-CH, J = 15.8 Hz), 7.39 t (1H,
6-H, J = 7.8 Hz), 7.50 t (1H, 5-H, J = 8.0 Hz), 7.52 d
(1H, 2′-CH, J = 15.8 Hz), 7.88 d (1H, 7-H, J = 8.0 Hz),
8.02 d (1H, 4-H, J = 8.0 Hz). Found, %: C 67.17;
H 3.88; N 5.43. C15H11NO2S. Calculated, %: C 66.90;
H 4.12; N 5.20.
REFERENCES
1-{5-[2-(1,3-Benzothiazol-2-yl)vinyl]thiophen-2-
yl}ethanone (IVb) was synthesized in a similar way
from 1.22 g (5 mmol) of compound II. Yield 0.61 g
(43%), mp 168–169°C (from ethanol). IR spectrum:
ν 1660 cm–1 (C=O). 1H NMR spectrum, δ, ppm: 2.55 s
(3H, CH3), 7.23 d (1H, 3-H, J = 4.2 Hz), 7.32 d (1H,
2′-CH, J = 15.9 Hz), 7.38 t (1H, 6-H, J = 7.8 Hz),
7.47 t (1H, 5-H, J = 7.8 Hz), 7.60 d (1H, 2-CH, J =
15.9 Hz), 7.60 d (1H, 4′-H, J = 4.2 Hz), 7.85 d (1H,
7-H, J = 7.8 Hz), 7.99 d (1H, 4-H, J = 7.8 Hz). Found,
%: C 62.81; H 4.15; N 5.23. C15H11NOS2. Calculated,
%: C 63.13; H 3.89; N 4.91.
1. Roshal’, A.D., Luk’yanov, B.S., and El’chaninov, M.M.,
Zh. Fiz. Khim., 2003, vol. 77, p. 1899.
2. Kornilov, M.Yu., Ruban, E.M., Fedchuk, V.N., Starin-
skaya, E.V., and Buznik, M.V., Zh. Org. Khim., 1973,
vol. 9, p. 2577.
3. El’chaninov, M.M., Doctoral (Chem.) Dissertation,
Rostov-on-Don, 2006.
4. Mel’nikova, E.B., El’chaninov, M.M., Milov, A.A., and
Luk’yanov, B.S., Khim. Geterotsikl. Soedin., 2008,
p. 1331.
5. Dorofeenko, G.N., Zh. Obshch. Khim., 1961, vol. 31,
p. 994.
6. Gardner, P.D., J. Am .Chem. Soc., 1954, vol. 76, p. 4550.
{5-[2-(1,3-Benzothiazol-2-yl)vinyl]furan-2-yl}-
phenylmethanone (IIIc). Benzoic acid, 2.44 g
(20 mmol), was added to a solution of 1.14 g (5 mmol)
7. Ried, W. and Hinsching, S., Justus Liebigs Ann. Chem.,
1956, vol. 600, p. 47.
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