Quantitative photooxidation of 4-hydroxy-3-pyrazolinylcoumarins to pyrazolyl derivatives

Article abstract of DOI:10.1016/j.mencom.2007.11.016

4-Hydroxy-3-pyrazolinylcoumarins undergo quantitative photooxidation to pyrazolyl derivatives under illumination with visible light at room temperature in a carbon tetrachloride solution.

Full text of DOI:10.1016/j.mencom.2007.11.016

Mendeleev
Communications
Mendeleev Commun., 2007, 17, 345–346
Quantitative photooxidation of 4-hydroxy-
3-pyrazolinylcoumarins to pyrazolyl derivatives
Valery F. Traven,*a Ivan V. Ivanov,a Aleksandr S. Pavlov,a
Aleksandr V. Manaev,a Irina V. Voevodinaa and Valery A. Barachevskiib
a D. I. Mendeleev University of Chemical Technology of Russia, 125047 Moscow, Russian Federation.
Fax: +7 495 609 2964; e-mail: traven@muctr.edu.ru
b Photochemistry Center, Russian Academy Sciences, 119421 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2007.11.016
4-Hydroxy-3-pyrazolinylcoumarins undergo quantitative photooxidation to pyrazolyl derivatives under illumination with visible
light at room temperature in a carbon tetrachloride solution.
In a study of the synthesis and structural transformations of
4-hydroxycoumarin derivatives,1–4 we found that 4-hydroxy-
l
k
k
3-pyrazolinylcoumarin 1 was photooxidised to 4-hydroxy-
j
3-pyrazolylcoumarin 2 under very mild conditions (Scheme 1).†
j
h
h
i
We found that compound 1 (dissolved in CCl4) changed its
electronic absorption spectrum under illumination with visible
light for 1 h (Figure 1).
N
g
OH
O
N
OMe
a
b
c
Hf
i
e H
These spectral transformations are not due to the solvato-
chromism of 1 since its solution in CCl4 does not change its
electronic absorption spectrum in the dark for a long time.
These transformations are insensitive to oxygen, and they
cannot be explained by hydroxy(hydrazono)/keto(hydrazino)
tautomerism. Moreover, these transformations are irreversible.
We followed the reaction by measuring 1H NMR spectra and
found that the photooxidation of pyrazoline 1 to pyrazole
O
d
1 enol
l
k
k
j
j
h
h
i
N
g
O
O
HN
O
VIS-light
(CCl4)
OMe
a
1
b
c
derivative 2 occurred. The H NMR spectra of compound 1
Hf
i
e H
1 keto
before and after illumination are compared in Figure 2 (curves
1 and 2, respectively). We also prepared compound 2 by an
independent way via the oxidation of 1 by potassium bichromate.
d
j
†
1H NMR spectra were recorded on a Bruker WP-200-SY spectrometer
i
i
(200 MHz). Electronic absorption spectra were obtained on a PD-303 UV
UV-VIS spectrophotometer. Fluorescence spectra were recorded on a
Shimadzu RF-500 spectrofluorimeter. Chromatomass spectra were obtained
on a PE SCIEX API165 (ELSD UV254) spectrometer; column, Synergi-
2u-Hydro-RP-Mercury, 20×2.0 mm. Phototransformations of 1 were
studied under illumination by visible light (Camelion lamp LH30-4U).
4-Hydroxy-3-(3'-p-anisyl-2'-phenyl-5'-pyrazolinyl)coumarin 1. A mix-
ture of 4-hydroxy-3-(p-methoxycinnamoyl)coumarin (3 g, 0.009 mol),
phenylhydrazine (3.65 ml, 0.037 mol) and 50 ml of acetic acid was
refluxed for 1 h. Then, the reaction mixture was cooled until a preci-
pitate was formed. The precipitate was filtered off, dried and recrystal-
lised from ethanol, 1, yellow crystals, yield 67%, mp 181–183 °C. 1H NMR
(CDCl3) d: 3.60 (m, 1H, He), 3.82 (s, 3H, OMe), 4.24 (m, 1H, Hf), 5.18
(m, 1H, Hg), 6.88 (m, 3H, Hi, Hl), 6.96 (d, 2H, Hj, 3J 7.4 Hz), 7.25 (m,
4H, Hk, Hh), 7.37 (m, 2H, Hb, Hd), 7.60 (m, 1H, Hc), 8.06 (d, 1H, Ha,
3J 7.7 Hz), 14.10 (s, 1H, OH). MS, m/z: 412 (100%). Found (%): C, 72.85;
H, 4.91; N, 6.77. Calc. for C25H20N2O4 (%): C, 72.82; H, 4.85; N, 6.80.
4-Hydroxy-3-(3'-p-anisyl-2'-phenyl-5'-pyrazolyl)coumarin 2. A mix-
ture of 1 (0.5 g, 0.0012 mol), K2Cr2O7 (0.14 g, 0.0005 mol) and 30 ml
of acetic acid was refluxed for 0.5 h. Then, the reaction mixture was
poured into water (50 ml) and ice. The precipitate was filtered off, dried
and recrystallised from ethanol to give 2, yellow crystals, yield 56%,
mp 149–151 °C. 1H NMR ([2H6]DMSO) d: 3.71 (s, 3H, OMe), 6.97 (d,
VIS-light
(EtOH)
h
h
f
f
g
g
N
OH
O
N
O
OMe
a
b
c
e
d
2
Scheme 1
1
The H NMR spectrum of pyrazole 2 is shown in Figure 2
(curve 3).
Pyrazoline 1 also undergoes photooxidation in other solvents.
A change of CCl4 as a solvent for acetonitrile and DMF
decreased the rate of the transformation. The reaction stopped
at all in an ethanol solution. This fact is due to tautomeric
transformations of 1. Figure 3 shows that a change from 100%
CCl4 to 100% DMF affected the equilibrium of tautomeric enol
and keto forms of 1 (Scheme 1). Enol form is stable in nonpolar
solvents (CCl4, CHCl3) since it contains a strong H-bond
between 4-hydroxy and 3-hydrazono functions. Polar solvents
(DMF, DMSO, EtOH) weaken the H-bond; therefore, keto form
3
2H, Hg, J 8.3 Hz), 7.25 (m, 3H, Hf, He), 7.48 (m, 7H, Hb, Hd, Hh, Hi,
1
of 1 becomes significant. This suggestion is proved by H NMR
3
Hj), 7.73 (m, 1H, Hc), 7.96 (d, 1H, Ha, J 8.3 Hz), 13.80 (s, 1H, OH).
spectrum recorded in [2H6]DMSO (signals of OH and NH
functions can be seen).‡
MS, m/z: 410 (75%). Found (%): C, 73.11; H, 4.26; N, 6.77. Calc. for
C25H18N2O4 (%): C, 73.17; H, 4.39; N, 6.83.
– 345 –
© 2007 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2007, 17, 345–346
700
600
500
400
300
200
100
0
1.8
1.2
0.6
7
1
1
300
400
500
600
700
800
7
l/nm
300
400
500
600
l/nm
Figure 4 Fluorescence spectra of 1 before (lem 547 nm) and after (lem
423 nm) illumination with visible light (solvent, acetonitrile).
Figure 1 Electronic absorption spectra of 1 in CCl4 under illumination
with visible light for 1 h: (1) 1 min, (7) 60 min.
by 366 nm light at a low concentration provided its complete
transformation into 2 for several minutes.
He
Different substituents in phenyl rings provide the same
phototransformation of pyrazolinylcoumarin 1 derivatives, even
though they affect much the photooxidation rate. As an example,
p-X-substituents in the 2'-phenyl ring of compound 1 analogues
and their relative photooxidation rates are listed below: NO2,
0.02; H, 1; F, 1.81; Me, 2.51; OMe, 4.51.
3
He
The phototransformation of 1 into 2 is also followed by
definite change of fluorescence spectrum. A shift of fluore-
scence is well seen in Figure 4. A maximum of electron
emission at 547 nm is due to the fluorescence of starting 1 and
a maximum at 423 nm is due to formed pyrazole 2.
2
f
He,H
Hg
1
The phototransformation of 1 into 2 provides a new mild way
for the aromatization of pyrazoline derivatives. The aromatiza-
tion of these derivatives and their analogues, as a rule, requires
strong oxidising reagents, for example, heating with potassium
bichromate in acetic acid or with DDQ in toluene.6,7 Several
procedures of the photooxidation of pyrazoline derivatives have
also been reported.8,9 However, these procedures require the
presence of oxygen and dyes as photosensitizers.
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
d/ppm
Figure 2 1H NMR spectra of compound 1 before and after illumination
1
with visible light (curves 1 and 2, respectively); (3) H NMR spectrum of
compound 2 prepared by the independent oxidation of 1 (solvent, CCl4).
The rate of transformation also depends on the concentration
of solution and the energy of illumination: under visible light, at
a concentration of 10–5 mol dm–3, compound 1 undergoes 100%
phototransformation for 1 h. At concentrations of 10–3 mol dm–3,
the reaction was complete in 8 h: a solution of 1 (0.015 g,
0.036 mmol) in 30 ml of CCl4 was illuminated by visible light
for 8 h; after solvent evaporation, compound 2 was isolated:
0.0148 g (99% yield), mp 148–150 °C. The illumination of 1
References
1 A. V. Manaev, T. A. Chibisova, K. A. Lyssenko, M. Yu. Antipin and
V. F. Traven, Izv. Akad. Nauk, Ser. Khim., 2006, 2012 (Russ. Chem. Bull.,
Int. Ed., 2006, 55, 2091).
2 A. V. Manaev, T. A. Chibisova and V. F. Traven, Izv. Akad. Nauk, Ser.
Khim., 2006, 2245 (Russ. Chem. Bull., Int. Ed., 2006, 55, 2226).
3 V. F. Traven’, O. B. Safronova, L. I. Vorob’eva, T. A. Chibisova and
I. N. Senchenya, Zh. Obshch. Khim., 2000, 70, 847 (Russ. J. Gen.
Chem., 2000, 70, 793).
4 V. F. Traven, A. V. Manaev, O. B. Safronova and T. A. Chibisova, J.
Electron Spectrosc. Relat. Phenom., 2002, 122, 47.
5 V. Vijayabaskar, S. Perumal, S. Selvaraj, A. Lyka, R. Murugan and
M. Balasubramanian, Magn. Reson. Chem., 1999, 37, 133.
6 A. R. Katritzky and A. F. Pozharskii, Handbook of Heterocyclic Chemistry,
2nd edn., Pergamon, Oxford, 2000, p. 427.
0.8
0.7
0.6
5
0.5
4
3
2
0.4
0.3
1
7 N. A. Evans, Aust. J. Chem., 1974, 27, 2267.
8 N. A. Evans, Aust. J. Chem., 1975, 28, 433.
9 M. Mella, M. Fagnoni, G. Viscardi, P. Savarino, F. Elisei and A. Albini,
0.2
0.1
J. Photochem. Photobiol. A: Chem., 1997, 108, 143.
0.0
250 300 350 400 450 500 550 600 650
l/nm
Figure 3 Electronic absorption spectra of 1 in (1) 100% CCl4, (2) 60%
CCl4 + 40% DMF, (3) 40% CCl4 + 60% DMF, (4) 20% CCl4 + 80% DMF
and (5) 100% DMF.
‡
Signals of protons He, Hf and Hg were assigned:5 3.37 (dd, 1H, He),
3.72 (s, 3H, OMe), 4.14 (dd, 1H, Hf), 5.38 (dd, 1H, Hg), 6.65–6.95 (m,
4H, Hh, Hk), 7.00–7.30 (m, 5H, Hi, Hj, Hl), 7.35–7.50 (m, 2H, Hb, Hd),
7.70 (t, 1H, Hc), 8.00 (d, 1H, Ha), 9.57 (s, 0.4H, NH), 13.82 (s, 0.5H,
OH).
Received: 16th March 2007; Com. 07/2893
– 346 –