New reaction of photoaromatization of aryl- and hetarylpyrazolines

Article abstract of DOI:10.1007/s11172-008-0135-3

Aryl- and hetarylpyrazolines smoothly undergo photoaromatization under irradiation with the visible light (λ > 400 nm) in the presence of carbon tetrachloride. The reaction is accompanied by an increase in the acidity of the medium and a change in the fluorescence. The structure of pyrazoline and solvent considerably affect the photoaromatization rate. The mechanism of the reaction was proposed, which agrees with the experimental data.

Full text of DOI:10.1007/s11172-008-0135-3

Russian Chemical Bulletin, International Edition, Vol. 57, No. 5, pp. 1063—1069, May, 2008
1063
New reaction of photoaromatization of arylꢀ and hetarylpyrazolines
V. F. Traven and I. V. Ivanov
D. I. Mendeleev Russian University for Chemistry and Technology,
9 Miusskaya pl., 125047 Moscow, Russian Federation.
Fax: +7 (495) 609 2964. Eꢀmail: traven@muctr.ru
Arylꢀ and hetarylpyrazolines smoothly undergo photoaromatization under irradiation with
the visible light (λ > 400 nm) in the presence of carbon tetrachloride. The reaction is accompaꢀ
nied by an increase in the acidity of the medium and a change in the fluorescence. The
structure of pyrazoline and solvent considerably affect the photoaromatization rate. The mechaꢀ
nism of the reaction was proposed, which agrees with the experimental data.
Key words: pyrazolines, pyrazoles, coumarins, 2ꢀquinolones, photoaromatization, carbon
tetrachloride, electron transfer.
Many derivatives of ∆1ꢀpyrazolines (1Hꢀ4,5ꢀdihydroꢀ
pyrazoles) are known as efficient dyes.1 1ꢀArylꢀ3ꢀhetarylꢀ
1Hꢀ4,5ꢀdihydropyrazoles are characterized by especially
high fluorescence (the fluorescence quantum yield is
higher than 0.8).2—5 At the same time, many pyrazoline
derivatives possess low light resistance. Therefore, conꢀ
siderable attention in literature is given to investigation of
problems of the effect of the ultraviolet and visible light
Z = O (1), N—Me (2); R = Ph—CH=CH (3), Ph (4)
on the pyrazoline derivatives.6—11 By studying the strucꢀ
Comꢀ
pound
1a
R1
R2
ture and reactivity of 4ꢀhydroxycoumarins and their
heteroanalogs,12—15 we obtained new data on the photoꢀ
chemical transformations of 1ꢀarylꢀ3ꢀhetarylꢀ1Hꢀ4,5ꢀ
dihydropyrazoles.
pꢀMeOC6H4
Ph
1b
1c
1d
1e
»
»
»
»
pꢀMeC6H4
pꢀFC6H4
pꢀNO2C6H4
pꢀMeOC6H4
Results and Discussion
1f
1g
1h
pꢀMeC6H4
pꢀFC6H4
pꢀNO2C6H4
We have previously found that 3ꢀ[5ꢀ(pꢀanisyl)ꢀ1ꢀpheꢀ
nylꢀ4,5ꢀdihydropyrazolꢀ3ꢀyl]ꢀ4ꢀhydroxycoumarin (1а),
being dissolved in ССl4, under the visible light is rapidly
and quantitatively transformed into the corresponding
pyrazole.16 It turned out that the photoaromatization of
compound 1а is not a single example of this kind. We
photooxidized pyrazolines 1—4 containing various subꢀ
stituents in positions 1, 3, and 5 of the pyrazoline ring.
Since phototransformations of organic substances depend
on their ability to absorb the electromagnetic radiation,
the absorption band maxima and molar absorption coeffiꢀ
cients of compounds 1—4 in the electronic absorption
spectra are collected in Table 1.
1i
1k
1l
pꢀMe2NC6H4
pꢀMe2NC6H4
pꢀMeOC6H4
pꢀFC6H4
pꢀMeOC6H4
Benzothiazolꢀ2ꢀyl
2a
2b
2c
pꢀMeOC6H4
Ph
»
»
pꢀFC6H4
pꢀMeOC6H4
2d
2e
pꢀFC6H4
pꢀMeOC6H4
The most part of the studied compounds in a ССl4
solution under the visible light irradiation rapidly change
the electronic absorption spectra. The changes in the abꢀ
sorption spectrum of compound 1а are shown as an exꢀ
ample in Fig. 1. The observed spectral changes are not a
result of the solvatochromic transformations of compound
1a, because its solution in ССl4 does not change the abꢀ
sorption spectrum in dark for a long time.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1044—1050, May, 2008.
1066ꢀ5285/08/5705ꢀ1063 © 2008 Springer Science+Business Media, Inc.

1064
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
Traven et al.
A (arb. units)
Table 1. Absorption band maxima (λ) and molar absorption
coefficients (ελ/L (mol cm)–1) in the electronic absorption
spectra of compounds 1—4 (ССl4 as solvent)
a
7
0.6
0.4
0.2
1
2
Comꢀ
pound
λ/nm
ελ
Comꢀ
pound
λ/nm
ελ
1a
1b
1c
1d
1e
1f
1g
1h
1i
415
425
395
425
410
424
410
438
405
15430
24680
10460
24680
10820
2020
2800
19900
8110
1k
1l
417
398
400
403
399
401
400
390
365
10370
19720
17550
14110
6700
14690
12240
57180
15990
2a
2b
2c
2d
2e
3
1
7
340
390
440
λ/nm
A (arb. units)
4
b
0.55
5
0.45
0.35
0.25
0.15
To determine reasons for the change in the electronic
absorption spectrum of compound 1а upon irradiation,
we carried out the photochemical experiment at high conꢀ
1
5
1
1
centrations and detected the reaction course by the Н
NMR spectra (Fig. 2, Scheme 1). The intensity of the
pyrazoline signals decreases (5.2, 4.3, and 3.4 ppm) with
1
time in the H NMR spectra, and a signal at 7.2 ppm
appears. The resulting 1Н NMR spectrum coincides comꢀ
pletely with the spectrum of pyrazole prepared by the
oxidation of the starting pyrazoline with sodium bichroꢀ
mate. After the solution was concentrated by evaporation,
pyrazole 5 was isolated in a quantitative yield.
340
390
440
λ/nm
Fig. 1. Electronic absorption spectra of compound 1а in carbon
tetrachloride (a) (с = 0.04 mmol L–1) before (1) and after (2—7)
irradiation with λ > 400 nm for 1 min (2) → 25 min (7) and in
other solvents or their mixtures (b): 100% CCl4 (1), 60% CCl4 +
40% DMF (2), 40% CCl4 + 60% DMF (3), 20% CCl4 + 80%
DMF (4), and 100% DMF (5).
The transformations of a series of pyrazolines can be
1
monitored by the Н NMR spectra. In all cases, the corꢀ
responding pyrazole was formed as the major product.
1
Table 2. The photoaromatization of pyrazolines, which
could not be monitored by 1Н NMR, is confirmed by the
hypsochromic shift of the longꢀwavelength absorption
The H NMR signals of the starting compounds and the
signals of the formed pyrazoles, which were used for moniꢀ
toring the photoaromatization process, are presented in
Scheme 1
(keto form)
(enol form)
i. Visible light, CCl4. ii. Visible light, EtOH—CCl4.

Photoaromatization of arylꢀ and hetarylpyrazolines
He
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
1065
ing the absence of considerable side transformations of
both the starting pyrazoline and formed pyrazole.
The conversion values for the starting pyrazolines esꢀ
timated from a decrease in the intensity of their signals in
the electronic absorption spectra are also listed in Table 2.
The data obtained suggest that the new photoaromatization
reaction of arylꢀ and hetarylpyrazolines can also be of
preparative interest, in particular, for the oxidation of
pyrazolines that are unstable when traditional (for exꢀ
ample, inorganic) oxidants are used.
3
He
2
Ha, Hb
Hc
1
We performed a series of measurements to determine
the chemistry of the mentioned transformations of
pyrazolines. The formation of the corresponding pyrazoles
has been observed earlier upon the ultraviolet irradiation
of various 1,3ꢀdiarylꢀ2ꢀpyrazolines in the presence of air
oxygen in benzene, methanol, cyclohexane, or carbon
tetrachloride.7,8 It was found that the reaction was favored
by the presence in a mixture of sensibilizing dyes (for
instance, Bengal pink and methylene blue). The authors
proposed two routes of pyrazoline oxidation with oxygen
(Scheme 2). The first route: during irradiation the dye
from the ground state (D0) is first transformed into the
singlet state (1D) and then into the triplet state (3D). The
8
7
6
5
4
δ
Fig. 2. 1Н NMR spectra of compound 1а before (1) and after
irradiation (2) and of the corresponding pyrazole (3), prepared
by independent oxidation (CCl4 as the solvent).
bands in the electronic absorption spectra. We have shown
by the quantum chemical ZINDO/S method that for all
compounds 1—4 the transition from the pyrazoline to
pyrazole structure should be accompanied by the shift of
the longꢀwavelength absorption to the shortꢀwavelength
spectral region. A distinct isosbestic point is observed in
all cases of photoaromatization, unambiguously indicatꢀ
3
excited dye molecule D transits its energy to an oxygen
1
molecule, thus transforming it into the singlet state О2.
Table 2. Signals in the 1Н NMR spectra of the starting comꢀ
pounds and products used for controlling the photoaromaꢀ
tization process
1
Thus excited О2 molecule interacts with pyrazoline (Q)
to form pyrazole. In this case, the dye acts as an oxygen
sensibilizer, transferring the photoexcitation energy to the
oxygen.
a
Pyraꢀ
zoline
δ (J/Hz)
α
(%)
Pyrazoline
Hb
Pyrazole
irradiation oxidation
Scheme 2
Ha
Hc
1a
1b
1c
1f
4.23 3.54 5.16
(19.0; (19.0; (12.3;
7.46
7.50
7.50
7.35
7.37
7.66
7.46
7.50
7.50
96
98
93
96
97
98
12.3) 8.2)
8.2)
4.22 3.54 5.11
(19.0; (19.0; (11.8;
12.3) 8.2)
8.2)
Products
4.24 3.54 5.07
(19.0; (19.0; (12.3;
12.3) 8.7)
8.7)
b
4.19 3.43 5.41
(19.0; (19.0; (12.3;
—
3
The second route: the photoexcited dye molecule D
12.3) 7.7)
7.7)
interacts directly with a pyrazoline molecule and detaches an
electron from it. Then the formed pyrazoline radical cation
undergoes dehydrogenation involving oxygen. The reacꢀ
tion rates (see Scheme 2) depend, to a great extent, on the
addition of singlet oxygen to the system of inhibitors.9,10
We carried out control experiments to reveal whether
oxygen is the oxidant in the reaction of pyrazoline
photoaromatization found by us. In one of the experiꢀ
ments, argon was preliminarily passed for 1 h through
carbon tetrachloride to remove dissolved oxygen. In anꢀ
other experiment, the solvent was presaturated with air
b
1k
2e
4.20 3.51 4.98
(19.0; (19.0; (11.8;
—
11.8) 9.5)
9.5)
b
4.25 3.79 5.14
(19.0; (19.0; (11.3;
—
11.3) 8.7)
8.7)
a The conversion of the starting pyrazolines (α) was estimated
by a decrease in the intensity of their bands in the electronic
absorption spectra.
b The marked pyrazolines cannot be oxidized by the traditional
method using sodium bichromate.

1066
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
Traven et al.
Scheme 3
oxygen. However, under the both conditions, the reacꢀ
tion rate only somewhat decreased (see Table 1).
composes with the rejection of the chloride ion and forꢀ
mation of the trichloromethyl radical capable of further
hydrogen atom abstracting from the corresponding subꢀ
strate; in turn, the chloride ion can act as a proton accepꢀ
tor17,18 (Scheme 4).
It was earlier found that the photoaromatization of
pyrazolines was possible in the absence of oxygen: the
irradiation of 1,3ꢀdiarylꢀ2ꢀpyrazolines in an argonꢀ
bubbled acetone solution resulted in the formation of the
corresponding pyrazoles.11 The photooxidation also ocꢀ
curs in the presence of benzophenone upon its irradiation
at the absorption maximum. It seems important that
pyrazoline does not absorb in this region. Presumably,
under these conditions, irradiation induced the electron
transfer from pyrazoline to ketone followed by proton
abstraction by the ketyl radical that formed (Scheme 3).
We observed no accelerating influence of ketone addiꢀ
tives on the photoaromatization of coumarinylpyrazoline
1а in carbon tetrachloride: the addition of acetophenone
resulted only in a considerable decrease in the oxidation
rate. An analogous effect was caused by the addition of
nitrobenzene, which is also prone to electron withdrawꢀ
ing. The corresponding data are given in Table 3.
A special role of carbon tetrachloride in the found
reaction of photoaromatization of arylꢀ and hetarylꢀ
pyrazolines should be mentioned: the addition of even
minor amounts of carbon tetrachloride to chloroform or
toluene noticeably accelerated the reaction. It has previꢀ
ously been shown17,18 that a CCl4 molecule can act as an
electron acceptor. When accepting an electron, CCl4
forms an unstable radical anion, which very rapidly deꢀ
Scheme 4
The specific effect of CCl4 has been reported earlier.
When Hantzschґs 1,4ꢀdihydropyridines were dehydrated
in CCl4 or in a CCl4—acetonitrile mixture, they were
irradiated with the light from a 250ꢀW highꢀpressure merꢀ
cury lamp. The photoaromatization of dihydropyridines
to pyridines was found to be accompanied by the formaꢀ
tion of chloroform and an increase in the acidity of the
medium due to proton abstraction (Scheme 5).19
We found that the studied photoaromatization of
pyrazolinones was also accompanied by an increase in the
acidity of the medium: in the corresponding experiment,
a solution of pyrazoline 1а in ethanol with 4% CCl4 was
stored under the light with λ = 400 nm in the presence of
the Thymol blue indicator (Fig. 3). The intensity of the
A (arb. units)
1
IndH+
10
1.6
Table 3. Influence of the solvent and additives on
the photoaromatization rate of compound 1а
1.2
10
0.8
0.4
Solvent
krel
CCl4 (special purity grade)
CCl4, saturated with argon
CCl4, saturated with air
CCl4 + nitrobenzene (4 wt.%)
CCl4 + CCl4 + acetophenone (4 wt.%)
CHCl3
1
0.748
0.794
0.381
0.491
0.022
0.048
0
330
380
430
480
530
λ/nm
Fig. 3. Electronic absorption spectra of compound 3 with the
Thymol blue indicator before (1) and after (2—10) irradiation
with the visible light for 1 s (2) → 6 min (10) in an EtOH—CCl4
mixture.
CHCl3 + CCl4 (4 wt.%)
EtOH, DMF, toluene

Photoaromatization of arylꢀ and hetarylpyrazolines
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
1067
Scheme 5
absorption maximum of the acidic form of the indicator
increased and the intensity of the absorption maximum of
pyrazoline decreased during the irradiation. As shown by
control experiments, the indicator underwent no transꢀ
formations under these conditions (but in the absence of
pyrazoline).
The observed increase in the medium acidity during
pyrazoline photoaromatization in CCl4 does not allow
one, however, to unconditionally apply Scheme 5 to the
interpretation of our results. This is indicated by the reꢀ
sults of studying the influence of the structure of the
pyrazoline derivatives on the rate of their photoaromatization.
The relative rates of photooxidation of pyrazolines 1—4
in carbon tetrachloride (the krel values were calculated
with respect to pyrazoline 1а) and their ionization potenꢀ
tial (I) and electron affinity values (EA) are presented in
Table 4. According to Scheme 5, it could be assumed that
the values obtained for the relative rates would correlate
with the ionization potentials of the substituted pyrazolines,
because they should undergo photoionization and donate
an electron from the ground state to a CCl4 molecule
according to this scheme. However, the correlation coefꢀ
ficient of the logarithms of krel with the ionization potenꢀ
tial values turned out to be unsatisfactory (r = –0.64). We
found that the relative photoaromatization rates unsatisꢀ
factorily correlated with the electron affinity values of the
pyrazolines (correlation coefficient r = –0.77). The ionꢀ
ization potential and electron affinity values were calcuꢀ
lated by several semiempirical methods. The well consisꢀ
tent results were obtained.
Based on the above consideration, we can propose a
scheme for the photoaromatization of the pyrazolines
(Scheme 6).
Unlike Scheme 5, we assume no direct electron transꢀ
fer from the nonꢀexcited pyrazoline molecule to a CCl4
molecule. In the first step, pyrazoline is transformed, most
likely, into the excited state and then an electron is transꢀ
ferred from the excited pyrazoline molecule to CCl4. The
radical anion of CCl4 dissociates rapidly to the trichloroꢀ
methyl radical and chloride ion because of its extreme
instability.17 The formed pyrazoline radical cation abꢀ
stracts a proton and is transformed into pyrazole upon the
interaction with the trichloromethyl radical. An arylꢀ and
hetarylpyrazoline molecule plays, according to this scheme,
a role of the sensibilizer of CCl4 destruction under the
action of irradiation.
There are published results on carbon tetrachloride
photodegradation, which occurs via the electron transfer
to a CCl4 molecule from the fluorescent dye on the titania
surface, which serves as an electron carrier. In this proꢀ
cess, the dye acts as both the sensibilizer and electron
source. The reaction is initiated by the visible light irraꢀ
diation.20,21
Table 4. Relative rate constants for the photooxidation
of compounds 1—4 and their calculated ionization poꢀ
tential and electron affinity values
Comꢀ
pound
krel
I
EA
eV
1a
1b
1c
1d
1e
1f
1g
1h
1i
1
8.266
8.177
8.331
8.312
8.109
8.156
8.012
8.578
8.212
8.011
8.325
8.067
8.142
7.942
8.145
7.973
8.160
8.067
–1.167
–1.16
2.51
1.81
0.01
1.50
1.90
2.73
0.07
0.83
2.3
0.016
2.85
2.56
4.72
2.01
2.91
3.46
0.84
–1.244
–1.385
–1.153
–1.137
–1.052
–1.371
–1.84
–1.099
–1.354
–0.884
–0.957
–0.868
–0.968
–0.883
–0.245
–0.465
1k
1l
2a
2b
2c
2d
2e
3
Some pyrazolines deviate appreciably from the averꢀ
age values of the photoaromatization rate. Reasons for
these deviations should be discussed in more detail. The
lower reactivity of pyrazolines 3 and 4 in the photoaroꢀ
matization reaction is caused, most likely, by the fact that
the longꢀwavelength absorption bands of these pyrazolines
4

1068
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
Traven et al.
Scheme 6
lie in a shorterꢀwavelength spectral region (see Table 2)
and are poorly overlapped with the visible light band.
The question about the absence of pyrazoline photoꢀ
oxidation in ethanol and DMF containing CCl4 additives
should also be discussed in more detail. As follows from
the data in Fig. 1, b, on going from CCl4 to DMF and
ethanol, the hypsochromic shift of the absorption maxiꢀ
mum is observed in the electronic absorption spectra, and
in the Н NMR spectra (Fig. 4) this shift corresponds to
1
the transformation of pyrazolines into the (keto)enamine
tautomeric form (see Scheme 1). It turned out that this
isomerization is accompanied by the almost complete cesꢀ
sation of pyrazoline photodehydrogenation. It could be
assumed that this fact is explained only by the shift of the
absorption of the corresponding substrate to the shortꢀ
wavelength spectral region. However, this assumption is
poorly consistent with the fact that pyrazolines 3 and 4,
absorbing in the same region but incapable of transformꢀ
ing into the keto form, in a mixture of ethanol with carꢀ
bon tetrachloride also undergo photoaromatization with a
noticeable rate (Fig. 5).
2
1
NH
OH
It should be noted in continuation that the found reꢀ
action of pyrazoline photoaromatization can be of preꢀ
parative interest, because it occurs with rather high conꢀ
centrations (up to 10–3—10–2 mol L–1). In addition to the
already mentioned example for the quantitative photoꢀ
oxidation of pyrazoline 1а, we carried out the preparative
photodehydrogenation of pyrazolines 1e and 1k. We failed
to aromatize these pyrazolines under standard conditions
with potassium bichromate in an acidic medium. At the
14
13
12
11
10
9
8
δ
1
Fig. 4. Н NMR spectra of compound 1а in DMSOꢀd6 (1) and
СDCl3 (2).
A
1
1
same time, according to the Н NMR spectral data, they
1.2
undergo photoaromatization by at least 80% under the
conditions found by us.
5
1.0
6
0.8
0.6
0.4
0.2
7
6
Experimental
Electronic absorption spectra were recorded on an APEL
PDꢀ303UV spectrometer in polar (ethanol, DMF) and nonpolar
(CCl4, chloroform) solvents. 1Н NMR spectra were measured
on a Bruker WP 200ꢀSY spectrometer (200 MHz) in chloroformꢀ
d1, DMSOꢀd6, and ССl4 (in the latter case, a sealed capillary
with acetoneꢀd6 was drawn down into a tube with the sample for
lock tuning) using SiMe4 as internal standard. Quantum chemical
calculations by the semiempirical AM1 and ZINDO/S methods
were performed using the HYPERCHEMꢀ7.5 program packages.
1
7
300 320 340 360 380 400 420 440 λ/nm
Fig. 5. Electronic absorption spectra of compound 3 in ethanol
in the presence of CCl4 (4 wt.%) before (1) and after (2—7)
irradiation with the visible light for 1 min (2) → 5 min (7).

Photoaromatization of arylꢀ and hetarylpyrazolines
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
1069
Photoaromatization of pyrazolines was carried out by the
irradiation of solutions with an LH30ꢀ4U lamp in a glass or
quartz cell closed with a glass cap. A solution in carbon
tetrachloride was prepared in a darkened room in a 25ꢀmL
volumetric flask in which 0.3—0.6 mg of the studied sample was
placed. Taking into account a considerable excess of carbon
tetrachloride as the second reactant, the reaction rate constants
were determined by the graphical method from the linear
dependence in the ln(c)—τ coordinates (τ is the irradiation time),
which corresponded to the rate equation of the first order. The
concentration was calculated using the Bouguer—Lambert—Beer
law. The correlation coefficient for ln(c)—τ in each experiment
was not lower than 0.99. The error of rate constant determination
did not exceed 5%.
In the preparative variant of the photoaromatization
experiment, a solution of compound 1a (0.015 g, 0.036 mmol)
in carbon tetrachloride (30 mL) was irradiated for 8 h. After the
solvent was evaporated, pyrazole 5 was isolated in a yield of
0.0148 g (99%), m.p. 148—150 °C (see Ref. 16).
3ꢀStyrylꢀ1,5ꢀdiphenylꢀ4,5ꢀdihydropyrazole (3). A 0.5ꢀL flask
equipped with a mechanical stirrer and a reflux condenser was
loaded with dibenzalacetone (23 g, 0.1 mol), phenylhydrazine
(13 g, 0.12 mol), and acetic acid (300 mL). The mixture was
refluxed for 3 h with stirring and then cooled. A precipitate of
pyrazoline 3 was formed, filtered off, washed with water, and
dried. The resulting precipitate was recrystallized from acetic
acid. The yield was 62%, m.p. 150—152 °C (cf. Ref. 22:
m.p. 152—153 °C).
3. H. Gesten, G. Heinrich, H. Fruehbeis, Ber. Bunsenges, Phys.
Chem., 1977, 81, 810.
4. Z. Raciszewski, J. F. Stephen, J. Am. Chem. Soc., 1969, 91, 4338.
5. O. Neunhoeffer, J. Aldorf, H. Ulrich, Chem. Ber., 1959, 92, 252.
6. L. Schrader, Tetrahedron Lett., 1971, 2977.
7. N. A. Evans, I. H. Leaver, J. Aust. Chem., 1974, 27, 1797.
8. N. A. Evans, D. E. Rivett, J. F. K. Wilshire, J. Aust. Chem.,
1974, 27, 2267.
9. N. A. Evans, J. Aust. Chem., 1975, 28, 433.
10. Wataru Ando, Rikiya Sato, Masataka Yamashita, Takeshi
Akasaka, Hajime Miyazaki, J. Org. Chem., 1983, 48, 542.
11. M. Mella, M. Fagnoni, G. Viscardi, P. Savarino, F. Elisei,
A. Albini, Photochem. Photobiol. A: Chem., 1997, 108, 143.
12. V. F. Traven, A. V. Manaev, O. B. Safronova, T. A. Chibisova,
K. A. Lysenko, M. Yu. Antipin, Zh. Obshch. Khim., 2000,
70, 853 [Russ. J. Gen. Chem., 2000, 70 (Engl. Transl.)].
13. V. F. Traven, A. V. Manaev, O. B. Safronova, T. A. Chibisova,
J. Electron Spectrosopy and Related Phenomena, 2002, 122, 47.
14. A. V. Manaev, T. A. Chibisova. K. A. Lysenko, M. Yu.
Antipin, V. F. Traven, Izv. Akad. Nauk, Ser. Khim., 2006,
2012 [Russ. Chem. Bull., Int. Ed., 2006, 55, 2091].
15. A. V. Manaev, T. A. Chibisova, V. F. Traven, Izv. Akad. Nauk,
Ser. Khim., 2006, 2144 [Russ. Chem. Bull., Int. Ed., 2006,
55, 2226].
16. V. F. Traven, I. V. Ivanov, A. S. Pavlov, A. V. Manaev, I. V.
Voevodina, V. A. Barachevskii, Mendeleev Commun., 2007,
17, 345.
17. J. Bertran, I. Gallardo, M. Moreno, J.ꢀM. Saveґant, J. Am.
Soc., 1992, 114, 9576.
18. M. Arun Prasad, M. V. Sangaranarayanan, Chem. Phys. Lett.,
2004, 390, 261.
19. MeiꢀZhong Jin, Li Yang, LongꢀMin Wu, YouꢀCheng Liu,
ZhongꢀLi Liu, Chem. Commun., 1998, 2451.
1,3,5ꢀTriphenylꢀ4,5ꢀdihydropyrazole (4). Freshly distilled
acetophenone (69 mL, 0.58 mol), benzaldehyde (60 mL, 0.58 mol),
and phenylhydrazine (64 mL, 0.65 mol) were added to a mixture
of ethanol (600 mL) and 10% NaOH (25 mL) at 35—40 °C. The
reaction mixture was heated to boiling and refluxed with stirring
for 40 min. Then the mixture was cooled, filtered off, washed
with small portions of ethanol until drops of the filtrate became
colorless (80 mL), and dried. A snowꢀwhite product that required
no purification was obtained. The yield was 78%, m.p. 137.5 °C
(cf. Ref. 22: m.p. 134—136 °C).
20. W. Choi, M. R. Hoffmann, J. Phys. Chem., 1996, 100, 2161.
21. Y. Cho, W. Choi, ChungꢀHak Lee, T. Hyeon, HoꢀIn Lee,
Environ. Sci. Technol., 2001, 35, 966.
22. B. M. Krasovitskii, L. M. Afanasiadi, Preparativnaya khimiya
organicheskikh lyuminoforov [Preparative Chemistry of Organic
Luminophores], Folio, Kharґkov, 1997, 202 pp. (in Russian).
23. V. F. Traven, A. V. Manaev, I. V. Voevodina, I. N. Okhrimenko,
Izv. Akad. Nauk, Ser. Khim., 2008, 1479 [Russ. Chem. Bull.,
Int. Ed., 2008, 57, No. 7].
The authors are grateful to I. V. Voevodina for kindly
presented samples of pyrazolines 1a—l of the coumarin
series and to I. N. Okhrimenko for samples of pyrazolines
2a—e of the quinolone series. The synthesis and identifiꢀ
cation of these compounds are described in Ref. 23.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 07ꢀ03ꢀ00936).
References
1 A. Wagner, C. W. Schellhanmer, S. Petersen, Angew. Chem.,
Int. En. Engl., 1966, 78, 699.
2. H. Straechle, W. Seitx, H. Gesten, Ber. Bunsenges, Phys.
Chem., 1976, 80, 288.
Received December 26, 2007;
in revised form February 26, 2008