Synthesis of 2-azastilbene derivatives with intramolecular charge transfer

Article abstract of DOI:10.1023/A:1020629619982

The condensation reaction of 2-cyanomethyl-5-nitropyridine with aromatic aldehydes has been carried out with the aim of preparing 2-azastilbene derivatives having intramolecular charge transfer. The yield of the condensation products can be increased if the reaction is carried out in the medium used for obtaining the starting 2-cyanomethyl-5-nitropyridine without separating or purifying it. The electronic absorption spectra of the compounds show a charge-transfer band, the energy of which increases and the intensity falls with lowering of the electron-donor properties of the substituent in the 4-position. Introduction of the heteroatom into the acceptor part when changing from the stilbene to the 2-azastilbene system is accompanied by a decrease in the energy and increase in the intensity of the charge-transfer electronic transition.

Full text of DOI:10.1023/A:1020629619982

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Chemistry of Heterocyclic Compounds, Vol. 38, No. 7, 2002
SYNTHESIS OF 2-AZASTILBENE
DERIVATIVES WITH INTRAMOLECULAR
CHARGE TRANSFER
V. Kokars, A. Yanishevskii, and V. Kampars
The condensation reaction of 2-cyanomethyl-5-nitropyridine with aromatic aldehydes has been carried
out with the aim of preparing 2-azastilbene derivatives having intramolecular charge transfer. The
yield of the condensation products can be increased if the reaction is carried out in the medium used for
obtaining the starting 2-cyanomethyl-5-nitropyridine without separating or purifying it. The electronic
absorption spectra of the compounds show a charge-transfer band, the energy of which increases and
the intensity falls with lowering of the electron-donor properties of the substituent in the 4-position.
Introduction of the heteroatom into the acceptor part when changing from the stilbene to the
2-azastilbene system is accompanied by a decrease in the energy and increase in the intensity of the
charge-transfer electronic transition.
Keywords: pyridine derivatives, intramolecular charge transfer.
A comprehensive theoretical and experimental study of non-linear optical effects in molecules and
materials [1-3] has revealed a basic dependence of the change in hyperpolarizibility with the structure and has
led to the synthesis of a series of novel chromophoric fragments and compounds. Despite this significant
progress as a whole, it should be recognized that the range of compounds studied remains rather narrow, despite
the potential of broadening it. The aim of our work is the synthesis of novel compounds with intramolecular
charge transfer involving the use of a heterocyclic acceptor part.
The synthesis of 2-cyanomethyl-5-nitropyridine (8) [4] and its condensation reaction were carried out
according to the scheme given below. This includes the successive introduction of a nitro group at position 5
and halogen at position 2 into the pyridine system with subsequent construction of a carbon chain at position 2
of the 2-chloro-5-nitropyridine obtained. The nitration of 2-aminopyridine gave a mixture of the two isomeric
3-nitro- (2a) and 2-amino-5-nitropyridines (2b) in the ratio 1:4. In contrast to work in [5] in which a mixture of
the isomers 2a and 2b was used to prepare compound 3, we have separated this mixture by the method reported
in [6] and it has enabled us to increase the yield of the 2-hydroxy-5-nitropyridine (3).
The synthesis of compound 6 and its hydrolysis and decarboxylation in the reaction mixture without
separation of the intermediate compound 7 was carried out according to the data in [4]. Compound 8 was
purified using column chromatography. Its spectroscopic parameters are given in Table 1. Compound 8 has a
low melting point and is difficult to crystallize and this lowers the yield of pure material.
__________________________________________________________________________________________
Riga Technical University, Riga LV-1048, Latvia; e-mail: vkokars@ktf.rtu.lv. Translated from Khimiya
Geterotsiklicheskikh Soedinenii, No. 7, pp. 912-917, July, 2002. Original article submitted May 12, 2000.
0009-3122/02/3807-0805$27.00©2002 Plenum Publishing Corporation
805

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NO2
O2N
O2N
+
H2SO4/HNO3
H2SO4/NaNO2
NH2
N
2a
N
NH2
N
OH
N
2b
NH2
1
3
PCl5/POCl3
O2N
O2N
NCCH2COOBu-t
O2N
TsOH/Tol
CN
COOH
5
CN
K2CO3/THF
N
Cl
N
N
t
4
6
CO2Bu-
7
O
H
γ
O2N
β
O2N
R
9a–e
o
α
m
N
o
CN
10a–e
9, 10 a R = NMe2, b R = OMe, c R = Cl, d R = H, e R = CN
N
CH2CN
R
m
8
The condensation reaction of compound 8 with different para-substituted benzaldehydes 9a-e gave the
3-(4-R-phenyl)-2-(5-nitro-2-pyridyl)acrylonitriles 10a-e (see Table 1).
Examination of the solubility of the condensation products showed that compounds 10a-e are poorly
soluble in both polar and non-polar solvents, hence the condensation reaction was carried out without separation
of compound 8 from its reaction medium. This led to better yields of the condensation products (see Table 1).
Compounds 10a-e constitute a donor-acceptor system in which the donor fragment with a changing
electron donor ability is bound to a constant electron-acceptor fragment (the 2-(5-nitropyridine)). In the
electronic absorption spectra of compounds of this type an absorption band due to intramolecular charge
transfer should be observed. The energy of the charge-transfer transition depends on the ionization potential of
the donor fragment, i.e. it should correlate with the σ+-constant for the R substituent.
As might have been expected, intense and non-overlapped long-wavelength absorption bands due to
intramolecular charge transfer are observed only in the experimental spectra of compound 10a (479 nm,
78,400 l/mol·cm) and 10b (379 nm, 20,200 l/mol·cm). Their intensity rapidly falls with the increase in transfer
energy. The long wavelength absorption bands in the spectra of compounds 10c-e are overlapped by absorption
bands at 260 nm and are markedly broadened (half width 7000 cm-1) when compared with the long wavelength
absorption band in the spectrum of 10a (half width 4600 cm-1). Hence, since the electronic spectrum of 10a
shows bands at 280, 300, and 320 nm, it is quite possible that in the case of compounds 10c and 10d the charge-
transfer bands are overlapped also by the absorption band at 320 nm. All of the experimentally determined band
maxima in the spectra of compounds 10c and 10d may be considered, with little error, as characteristic of
charge-transfer bands since their intensity is decreased when compared with that of the corresponding band in
10b. The maximum intensity long-wavelength band (327 nm) of compound 10e is close to that of compound
10b despite the appreciable increase in transition energy. To all appearances, the charge-transfer band in this
case is fully overlapped by the absorption of the other type and the observed band maximum does not
characterize the charge transfer energy.
806

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TABLE 1. 5-Nitro-2-pyridylacetonitrile and Its Condensation Products with Aromatic Aldehydes
Found, %
——————
IR spectrum,
Com-
Empirical
formula
UV spectrum
1
δ
Calculated, %
mp, °C
66
H NMR spectrum, , ppm, J (Hz)
Yield, %
51
, cm-1
νCN
pound
λmax
ε
C
H
, nm
log?
C7H5N3O2
51.26
51.54
3.04
3.09
2252
2204
8
4.00 (2H, s, CH2); 7.61 (1H, d, J = 9, β-H);
8.48 (1H, dd, J = 9, J = 3, γ-H);
9.33 (1H, d, J = 3, α-Н)
3.07 (6H, s, NMe2); 6.67 (2H, d, m-Н );
7.69 (1H, d, J = 8, β-H); 7.96 (2H, d, о-Н);
8.40 (1H, dd, J = 8, J = 3, γ -H);
C14H14N4O2
61.95
62.21
5.13
5.22
225-226
173-174
479
4.3
90
67
10а
8.44 (1H, s, =CH); 9.29 (1H, d, J = 3, α-Н)
3.84 (3H, s, OMe); 6.96 (2H, d, J = 9.5, m-H);
7.81 (1H, d, J = 9, β-H); 8.00 (2H, d, J = 9.5, о-Н);
8.48 (1H, dd, J = 9, J = 3, γ-H); 8.54 (1H, s, =CH);
9.33 (1H, d, J = 3, α-Н)
C15H11N3O3
64.16
64.05
4.02
3.94
379
4.3
2212
10b
C14H8ClN4O2
C14H9N3O2
C15H8N4O2
56.17
56.11
2.65
2.69
182-183
161-62
339
335
334
4.2
4.0
4.3
2216
2232
2228
7.46 (2H, d, J = 9, m-Н); 7.89 (1H, d, J = 9, β-H);
56
24
71
10с
10d
10e
7.96 (2H, d, J = 9, о-Н); 8.53 (1H, dd, J = 9,
J = 3, γ-H); 8.60 (1H, s, =CH); 9.39 (1H, d, J = 3, α-Н)
66.71
66.93
3.53
3.61
7.48 (3H, m, Ph); 7.87 (1H, d, J = 10, β-H);
7.98 (2H, m, Ph); 8.50 (1H, dd, J = 10, J = 2, γ-H);
8.62 (1H, s, =CH); 9.37 (1H, d, J = 2, α-Н)
7.76 (2H, d, m-Н ); 7.95 (1H, d, J = 9, β-H);
8.08 (2H, d, о-Н); 8.58 (1H, dd, J = 9, J = 3, γ-H);
8.67 (1H, s, =CH); 9.40 (1H, d, J = 3, α-Н)
65.03
65.22
2.78
2.92
232-233

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E, cm-1
Fig. 1. Dependence of the energy of the long-wavelength transition (E) for compounds 10a-e
on the substituent σ+-constant.
As seen in Fig. 1, the energy of the long-wavelength transitions of compounds 10a-e do, in fact,
correlate with the substituent σ+-constant and this confirms the proposals presented above. Comparison of the
parameters for compound 10a and the corresponding stilbene (446 nm, 30,500 l/mol·cm) shows that the
introduction of the heteroatom is accompanied by a marked bathochromic shift and increase in band intensity
with charge transfer.
EXPERIMENTAL
IR spectra were obtained on a Specord 75-IR spectrometer using paraffin oil and hexachlorobutadiene.
1
UV Spectra were measured on a Specord UV-vis spectrometer for ethanol solutions. H NMR spectra were
obtained on a Bruker WH-90/DS spectrometer (90 MHz) for solutions in CDCl3 with TMS internal standard.
The purity of the synthesized compounds and monitoring of the reaction course was achieved using TLC on
Silufol UV-254 plates with the system benzene–ethyl acetate (10:1). The spectroscopic parameters for
compounds 8, 10a-e are given in Table 1.
2-Amino-5-nitropyridine (2b) [7]. 2-Aminopyridine (25.3 g, 0.27 mol) (1) was dissolved in conc.
H2SO4 (50 ml) with constant stirring and maintenance of the temperature below 20°C. The mixture was cooled
to 5-10°C and a mixture of conc. H2SO4 (20 ml) and 72% HNO3 (d = 1.42, 20 ml) was added gradually dropwise
with the temperature not exceeding 20°C. The reaction mixture obtained was placed on a water bath
thermostatted to 46-48°C. An exothermic nitramine rearrangement reaction began at 43-45°C. The reaction
mixture was held at this temperature for 12-16 h (until gas bubbling had fully ceased). In the course of the
nitramine rearrangement the reaction mixture changed color from light-brown to dark-claret. The mixture
obtained was poured onto ice and carefully neutralized using ammonia (200 ml). At pH 5.0-5.5 there appeared a
dark-brown precipitate which lightened at pH 5.5-6.0. The filtered precipitate was a mixture of two isomers. For
separation of the isomers [5], the mixture was suspended in 50 parts of water and conc. HCl was added to full
solution of the precipitate. The dark claret solution was then filtered to remove an oily admixture. The obtained
filtrate was taken to pH 4-5 by the addition of 50% NaOH solution and a dark brown precipitate of compound
2b was formed. When the remaining filtrate was taken to pH 9 a light yellow precipitate of the aminopyridine
2a appeared. Yield 20.3 g (80%); mp 188°C.
808