On the Mechanism of Photodehydrogenation of Aryl(hetaryl)pyrazolines in the Presence of Perchloroalkanes

Article abstract of DOI:10.1111/php.12918

New (1,5-diaryl-3-pyrazolinyl)coumarins have been synthesized. The compounds do not undergo keto-enol tautomeric transformations with changes in the solvent. (1,5-Diaryl-3-pyrazolinyl)coumarins provide dehydrogenation reaction under irradiation in the presence of perchloroalkanes and manifest themselves as effective photogenerators of acidity. Several aspects of photodehydrogenation mechanism have been studied. Oxygen is shown not be involved in the reaction. Polar solvents increase rate of the reaction. The measured rate constants of the photodehydrogenation reactions vary in a significant range according to the structure of pyrazoline. The data correlate with ionization potentials of pyrazolines available from DFT quantum chemical calculations. These results are discussed in terms of proposed scheme of mechanism of pyrazolines photodehydrogenation assuming formation of ion-radical and ion intermediates.

Full text of DOI:10.1111/php.12918

DR. VALERY TRAVEN (Orcid ID : 0000-0002-2204-7438)
MR. DMITRIY CHEPTSOV (Orcid ID : 0000-0002-9774-7922)
Article type : Research Article
On the Mechanism of Photodehydrogenation of
Aryl(hetaryl)pyrazolines in the Presence of Perchloroalkanes
Valery F. Traven*, Dmitriy A. Cheptsov, Maria V. Bulanova, Natalya P. Solovjova,
Tatyana A. Chibisova, Sergei M. Dolotov, Ivan V. Ivanov
D. Mendeleev University of Chemical Technology of Russia, Miusskaya Sq., 9,
Moscow, Russian Federation
*Corresponding author e-mail: valerii.traven@gmail.com (Valery F. Traven)
ABSTRACT
New (1,5-diaryl-3-pyrazolinyl)coumarins have been synthesized. In opposite to (1,5-diaryl-3-
pyrazolinyl)-4-hydroxycoumarins these compounds are not able to keto-enol tautomeric
transformations with changes of the solvent. (1,5-Diaryl-3-pyrazolinyl)coumarins provide
dehydrogenation reaction under irradiation in the presence of perchloroalkanes and manifest
themselves as effective photogenerators of acidity. Several aspects of photodehydrogenation
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mechanism have been studied. Oxygen is shown not be involved in the reaction. Polar solvents
increase rate of the reaction. The measured rate constants of the photodehydrogenation reactions
vary in a significant range according to the structure of pyrazoline. The data correlate with ionization
potentials of pyrazolines available from DFT quantum chemical calculations. These results are
discussed in terms of proposed scheme of mechanism of pyrazolines photodehydrogenation
assuming formation of ion-radical and ion intermediates.
INTRODUCTION
Aryl(hetaryl)pyrazolines are well-known fluorescent compounds that used as fluorescent whitening
agents for different fibers (1, 2). Pyrazolines possess definite photosensitivity and undergo
transformations under irradiation (3-8). Dehydrogenation of pyrazolines with formation of the
corresponding pyrazoles turns to be the predominant reaction among these transformations. For
example, direct irradiation of various 1,3-diphenyl-2-pyrazolines in benzene leads to their
dehydrogenation with pyrazoles formation (3). Evans and Leaver have reported on dye-sensitized
photooxidation of 1,3-diphenyl-2-pyrazoline in dichloromethane yielding 1,3-diphenylpyrazole and
β,β-dimethoxypropiophenone (4). 1,3-Diphenyl-2-pyrazolines substituted in the 4- and 5-positions
with phenyl groups undergo also dye-sensitized photooxidation at widely different rates depending
on the solvent and the substitution pattern (6). Two paths of pyrazolines phototransformation have
been discussed by Evans both including oxygen active participation (4). By the first path,
photodehydrogenation of a pyrazoline (Q) may initially involve interaction of the sensitizer triplet
(3D) with oxygen to form singlet molecular oxygen (1O2) that oxydizes then the pyrazoline molecule.
Alternatively, the pyrazoline may react directly with the dye triplet to form dye and pyrazoline
radicals which may react then with themselves or with oxygen to form products as shown in Scheme
1 (4).
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Another way of pyrazoline derivatives photodehydrogenation has recently been found (9). (1,5-
Diaryl-3-pyrazolinyl)-4-hydroxycoumarins were transformed to (1,5-diaryl-3-pyrazolyl)-4-
hydroxycoumarins during irradiation in the presence of CCl4. In these conditions pyrazolines behave
themselves as effective photogenerators of H+-acidity. Thus, photodehydrogenation of pyrazolines in
the presence of CCl4 is able to generate via proton formation highly fluorescent Rhodamine dye
open form from its lactone form, providing a new effective way for optical information recording
(Scheme 2) (9-13).
Depending on the media composition, 4-hydroxycoumarin derivatives have been earlier found to
undergo keto-enol tautomeric transformations. For example, pyrazoline 1 exists in enol-form in
toluene and in keto-form in dimethylformamide (Scheme 3).
However, (1,5-diaryl-3-pyrazolinyl)-4-hydroxycoumarins can effectively operate as acid
photogenerators being only in the enol-form (12, 13). Since acidity photogenerators have
recently received many significant applications in polymeric materials and in the control of
biochemical reactions (14-18), search of new pyrazolines as acidity photogenerators and
study of their phototransformation mechanistic pattern are in an obvious demand.
In this study we have prepared a series of new (1,5-diaryl-3-pyrazolinyl)coumarins, recorded their
electron absorption spectra, evaluated effects of solvent properties and oxygen presence on the
photodehydrogenation reaction in the presence of С2Cl6, considered dependence of the measured
rate constants on the ionization potentials of pyrazolines evaluated with use of the DFT quantum
chemical calculations.
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MATERIALS AND METHODS
Chemicals and instrumentation. The deuterated solvents (CDCl3, DMSO-d6, acetone-d6) for NMR
spectroscopy were obtained from Merck. The lactone form of Rhodamine B was used as precursor of
fluorescent laser dye (high purity grade, Aldrich). Hexachloroethane was used as a halogen-
containing additive (high purity grade, Aldrich). The other chemicals were Aldrich HPLC or spectral
grade and were used without further purification.
1
The NMR spectra were recorded on a «Bruker Avance» spectrometer: H NMR spectra at 500 MHz,
13C NMR spectra at 126 MHz. In some individual cases acetone-d6 was additionally added to the
ampoule for better dissolution. As the internal standard, SiMe4 or a signal of residual protons and
carbon of 13C solvent with respect to SiMe4 (DMSO-d6: δH 2.50 ppm, δC 39.5 ppm; CDCl3: δH 7.27
ppm, δC 77.0 ppm; acetone-d6: δH 2.05 ppm, δC 29.9 ppm). The mass spectra were obtained using the
«Thermo Scientific» mass spectrometer (ISQ LT Single Quadrupole Mass Spectrometer) by electron
impact with ionizing electron energy of 70 eV. High-resolution mass spectra were recorded on the
«Bruker micrOTOF II» instrument using electrospray ionization (ESI).
Electronic absorption spectra were recorded on an APELPD_303UV spectrometer and fluorescent
spectra – on Cary Eclipse (Varian) spectrofluorimeter in toluene and acetone. Photo-irradiation was
carried out using an L 5283 xenon lamp (HAMAMATZU lamp). Analytical thin layer chromatography
was performed on silica gel plates (Merck, Kieselgel 60, 0.25 thickness) with F254 indicator. Column
chromatography was performed on silica gel (Merck, Kieselgel 60, 70-230 mesh).
Relative rates of the pyrazolines photodehydrogenation reactions have been measured by intensity
of pyrazoline absorption band in solvent in the 0.2-0.7 range of optical density. Rate constants of the
first order reactions have been obtained based on the measured data for 5 runs. Concentrations of
the components have been changed as follows: pyrazoline – 12-50 μM, С2С16 – 8-8.5 mM.
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Quantum mechanical calculations were carried out with GAMESS suite of programs (19). Geometry
optimization and calculations of the energy was carried out with Density Functional Theory (DFT)
(20) using the Becke’s hybrid exchange functional and non-local, correlation functional of Lee, Yang
and Parr (B3LYP) (21) and 6-31++G(d,p) basis set. The geometries were optimized in toluene as a
model solvent with use of the Polarizable Continuum Model. The ionization potentials IP1 values of
pyazolines were determined by the Koopmans theorem based on the DFT quantum chemical
calculations.
Please see the Supporting Information files for the 1H NMR, 13С NMR and HSQC spectra for the 3-
Cinnamoylcoumarins, (1,5-Diaryl-3-pyrazolinyl)coumarins, and (1,5-Diaryl-3-pyrazolyl)coumarins
synthesized in this study.
The synthesis of 3-cinnamoylcoumarins 2a-f. 3-Cinnamoylcoumarins 2a-f were prepared according to
(22).
•
•
•
•
3-[(2E)-3-Phenylprop-2-enoyl]coumarin 2a. Yield 39%, mp. 176-178°C (lit. (22) mp. 177-
179°C). MS (ES+) calcd for C18H12O3 m/z: 276.08, found m/z: 276.12.
3-[(2E)-3-[4-(Dimethylamino)phenyl]prop-2-enoyl]coumarin 2b. Yield 24%, mp. 217-219°C
(lit. (23) mp. 217-218°C). MS (ES+) calcd for C20H17NO3 m/z: 319.12, found m/z: 319.17.
3-[(2E)-3-(4-Nitrophenyl)prop-2-enoyl]coumarin 2c. Yield 20%, mp. 273-274.5°C (lit. (24)
mp. 272-274°C). MS (ES+) calcd for C18H11NO5 m/z: 321.06, found m/z: 321.12.
3-[(2E)-3-(3-Hydroxy-4-methoxyphenyl)prop-2-enoyl]coumarin 2d. Yield 21%, mp. 185-
187°C. HRMS (ESI), m/z: 345.0725 (M+Na)+ (calcd for C19H14O5Na+: 345.0733). MS (ES+) calcd
for C19H14O5 m/z: 322.08, found m/z: 322.21. 1H NMR (CDCl3), δ (ppm): 3.95 (s, 3H, OCH3);
5.69 (s, 1H, OH); 6.87 (d, 1H, H(16), 3J=8.3 Hz); 7.19-7.31 (m, 2H, H(13,17)); 7.35 (t, 1H, H(6),
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3J=7.5 Hz); 7.40 (d, 1H, H(8), 3J=8.3 Hz); 7.64-7.80 (m, 4H, H(5,7,10,11)); 8.56 (s, 1H, H(4)). 13С
NMR (CDCl3), δ (ppm): 56.3; 110.9; 114.2; 116.9; 118.9; 122.6; 123.1; 125.1; 125.9; 128.9;
130.2; 134.3; 145.4; 146.2; 147.9; 149.4; 155.5; 159.5; 186.6.
•
•
3-[(2E)-3-(4-Methylphenyl)prop-2-enoyl]coumarin 2e. Yield 49%, mp. 161-163°C (lit. (23)
mp. 161-163°C). MS (ES+) calcd for C19H14O3 m/z: 290.09, found m/z: 290.19.
3-[(2E)-3-(4-Methoxyphenyl)prop-2-enoyl]coumarin 2f. Yield 29%, mp. 164-165°C (lit. (25)
mp. 169°C). MS (ES+) calcd for C19H14O4 m/z: 306.09, found m/z: 306.18.
The synthesis of (1,5-diaryl-3-pyrazolinyl)coumarins 3a-k. The reactions of 3-cinnamoylcoumarins
with phenylhydrazines are described in sufficient detail (26-30). We have used 2 protocols for
preparation of compounds 3a-k (Scheme 4).
In protocol A the corresponding arylhydrazine (3-4-fold excess by moles) and catalytic amount of
glacial acetic acid were added to boiling suspension of 3-cinnamoylcoumarin 2 (2.88 mmol) in 30 ml
of isopropyl alcohol. The mixture was heated at reflux for 1-5 h. The end of reaction was
determined by TLC. The pyrazoline formed was purified by column chromatography or recrystallized
from toluene or ethanol. The protocol B differs in the following. The corresponding HCl-salt of
arylhydrazine (3-4-fold excess by moles) and potassium hydroxide (0.50 g) were added to boiling
suspension of 3-cinnamoylcoumarin 2 (2.59 mmol) in 30 ml of isopropyl alcohol.
•
3-(1,5-Diphenyl-4,5-dihydro-1H-pyrazol-3-yl)coumarin 3a. Protocol А. Yield 46%, mp. 181-
183°C (lit. (28) mp. 180-182°C). MS (ES+) calcd for C24H18N2O2 m/z: 366.14, found m/z:
1
3
366.32. H NMR (CDCl3), δ (ppm): 3.40 (dd, 1H, НA(25), J=7.4 Hz, 2J=18.4 Hz); 4.12 (dd, 1H,
НB(25), 3J=12.7 Hz, 2J=18.4 Hz); 5.35 (dd, 1H, H(18), 3J=7.4 Hz, 3J=12.7 Hz); 6.84 (t, 1H, H(15),
3
3J=7.3 Hz); 7.09-7.53 (m, 12H, H(6,8,7,13,14,16,17,20-24)); 7.60 (dd, 1H, H(5), J=7.6 Hz,
4J=1.2 Hz); 8.42 (s, 1H, H(4)). 13С NMR (CDCl3), δ (ppm): 45.3; 65.0; 113.7 (2C); 116.4; 119.6;
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119.9; 120.9; 124.7; 125.8 (2C); 127.6; 128.2; 129.0 (2C); 129.1 (2C); 131.5; 137.6; 142.1;
143.0; 144.1; 153.6; 159.7.
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3-[5-[4-(Dimethylamino)phenyl]-1-(4-fluorophenyl)-4,5-dihydro-1H-pyrazol-3-yl]coumarin
3b. Protocol A. Yield 54%, mp. 197°C (dec.). HRMS (ESI), m/z: 450.1587 (M+Na)+ (calcd for
C26H22FN3O2Na+: 450.1588). MS (ES+) calcd for C26H22FN3O2 m/z: 427.17, found m/z: 427.25.
1H NMR (CDCl3+acetone-d6), δ (ppm): 3.18 (s, 6H, N(CH3)2); 3.28 (broad. dd, 1H, НA(25)); 4.07
(dd, 1H, НB(25), 3J=12.5 Hz, 2J=18.3 Hz); 5.43 (dd, 1H, H(18), 3J=7.1 Hz, 3J=12.5 Hz); 6.82-6.98
(m, 4H, H(13,14,16,17)); 7.25-7.30 (m, 2H, H(6,8)); 7.45 (d, 2H, H(20,24), 3J=8.3 Hz); 7.50-7.53
(m, 1H, H(7)); 7.64-7.88 (m, 3H, H(5,21,23)); 8.40 (s, 1H, H(4)). 13С NMR (CDCl3+acetone-d6),
δ (ppm): 44.3; 45,3 (2C); 63.4; 114.0 (d, 2C, 3J=8.2 Hz); 114.7 (d, 2C, 2J=21.8 Hz); 115.4; 118.7;
119.6; 121.2; 122.9 (2C); 124.0; 127.1 (2C); 127.8; 131.1; 137.5; 139.7 (d, C, 4J=2.7 Hz); 142.6;
145.3; 152.8; 156.4 (d, C, 1J=238.9 Hz); 158.3.
•
3-[1-(4-Methylphenyl)-5-(4-nitrophenyl)-4,5-dihydro-1H-pyrazol-3-yl]coumarin 3с. Protocol
А. Yield 47%, mp. 189-191.5°C. HRMS (ESI), m/z: 448.1274 (M+Na)+ (calcd for C25H19N3O4Na+:
1
448.1268). MS (ES+) calcd for C25H19N3O4 m/z: 425.14, found m/z: 425.34. H NMR (DMSO-
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2
d6), δ (ppm): 2.17 (s, 3H, CH3); 3.24 (dd, 1H, НA(25), J=6.4 Hz, J=18.0 Hz); 4.03 (dd, 1H,
3
НB(25), 3J=12.8 Hz, 2J=18.0 Hz); 5.70 (dd, 1H, H(18), J=6.4 Hz, 3J=12.8 Hz); 6.93-7.01 (m, 4H,
H(13,14,16,17)); 7.36-7.41 (m, 2H, H(6,8)); 7.54 (d, 2H, H(20,24), 3J=8.3 Hz); 7.60 (t, 1H, H(7),
3J=7.8 Hz); 7.83 (d, 1H, H(5), J=7.6 Hz); 8.21 (d, 2H, H(21,23), J=8.3 Hz); 8.48 (s, 1H, H(4)).
13С NMR (DMSO-d6), δ (ppm): 20.0; 44.1; 62.5; 113.3 (2C); 115.8; 119.1; 119.7; 124.2 (2C);
124.7; 127.3; 128.4; 128.7; 129.4 (2C); 131.9; 138.4; 141.1; 142.8; 146.8 (2C); 149.7; 152.9;
158.1.
3
3
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•
3-[1-(4-Fluorophenyl)-5-(3-hydroxy-4-methoxyphenyl)-4,5-dihydro-1H-pyrazol-3-
yl]coumarin 3d. Protocol А. Yield 45%, mp. 184-186°C. HRMS (ESI), m/z: 453.1217 (M+Na)+
(calcd for C25H19FN2O4Na+: 453.1221). MS (ES+) calcd for C25H19FN2O4 m/z: 430.13, found
1
3
m/z: 430.33. H NMR (CDCl3), δ (ppm): 3.35 (dd, 1H, НA(25), J=7.8 Hz, 2J=18.3 Hz); 3.88 (s,
3H, OCH3); 4.06 (dd, 1H, НB(25), 3J=12.4 Hz, 2J=18.3 Hz); 5.18 (dd, 1H, H(18), 3J=7.8 Hz, 3J=12.4
Hz); 5.63 (broad. s, 1H, OH); 6.76-6.85 (m, 3H, H(20,23,24)); 6.88-7.05 (m, 4H,
3
H(13,14,16,17)); 7.29-7.34 (m, 2H, H(6,8)); 7.50-7.53 (m, 1H, H(7)); 7.58 (d, 1H, H(5), J=7.9
Hz); 8.37 (s, 1H, H(4)). 13С NMR (CDCl3), δ (ppm): 44.8; 55.5; 64.6; 110.6; 111.5; 114.3 (d, 2C,
2
3J=7.3 Hz); 114.9 (d, 2C, J=22.7 Hz); 115.9; 116.8; 119.0; 120.3; 124.2; 127.6; 130.9; 134.7;
137.0; 140.2 (d, C, 4J=1.8 Hz); 142.6; 145.6; 145.7; 153.1; 158.5 (d, C, 1J=205.3 Hz); 159.1.
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3-[1-(4-Methylphenyl)-5-phenyl-4,5-dihydro-1H-pyrazol-3-yl]coumarin 3e. Protocol А. Yield
91%, mp. 208-210°C. HRMS (ESI), m/z: 419.1167 (M+K)+ (calcd for C25H20N2O2K+: 419.1156).
MS (ES+) calcd for C25H20N2O2 m/z: 380.15, found m/z: 380.29. 1H NMR (DMSO-d6), δ (ppm):
2.17 (s, 3H, CH3); 3.20 (dd, 1H, НA(25), 3J=6.4 Hz, 2J=17.7 Hz); 3.98 (dd, 1H, НB(25), 3J=12.5 Hz,
3
3
2J=17.7 Hz); 5.50 (dd, 1H, H(18), J=6.4 Hz, J=12.5 Hz); 6.94-6.99 (m, 4H, H(13,14,16,17));
7.25-7.41 (m, 7H, H(6,8,20-24)); 7.58-7.62 (m, 1H, H(7)); 7.82 (dd, 1H, H(5), 3J=7.6 Hz, 4J=1.5
Hz); 8.46 (s, 1H, H(4)). 13С NMR (DMSO-d6), δ (ppm): 20.2; 44.6; 63.4; 113.6 (2C); 116.0;
119.4; 120.2; 124.9; 125.9 (2C); 127.6; 128.2; 128.8; 129.1; 129.5 (2C); 131.9; 136.3; 138.2;
141.5; 142.4; 142.7; 153.0; 158.3.
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3-[1-(4-Fluorophenyl)-5-phenyl-4,5-dihydro-1H-pyrazol-3-yl]coumarin 3f. Protocol А. Yield
85%, mp. 191,5-193°C. HRMS (ESI), m/z: 423.0938 (M+K)+ (calcd for C24H17FN2O2K+:
1
423.0906). MS (ES+) calcd for C24H17FN2O2 m/z: 384.13, found m/z: 384.27. H NMR (DMSO-
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d6), δ (ppm): 3.22 (dd, 1H, НA(25), J=6.7 Hz, 2J=18.0 Hz); 3.99 (dd, 1H, НB(25), J=12.5 Hz,
2J=18.0 Hz); 5.50 (dd, 1H, H(18), 3J=6.7 Hz, 3J=12.5 Hz); 7.03-7.42 (m, 11H,
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H(6,8,13,14,16,17,20-24)); 7.59-7.62 (m, 1H, H(7)); 7.82 (dd, 1H, H(5), J=7.6 Hz, J=1.2 Hz);
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8.48 (s, 1H, H(4)). 13С NMR (DMSO-d6), δ (ppm): 44.7; 63.5; 114.4 (d, 2С, 3J=7.3 Hz); 115.4 (d,
2С, 2J=21.8 Hz); 115.8; 119.1; 119.8; 124.7; 125.8 (2C); 127.5; 128.7; 129.0 (2C); 131.9; 138.6;
140.4; 141.9; 143.4; 152.9; 156.2 (d, C, 1J=236.2 Hz); 158.1.
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3-[1-(4-Nitrophenyl)-5-phenyl-4,5-dihydro-1H-pyrazol-3-yl]coumarin 3g. Protocol А. Yield
59%, mp. 235-237°C. HRMS (ESI), m/z: 450.0881 (M+K)+ (calcd for C24H17N3O4K+: 450.0851).
MS (ES+) calcd for C24H17N3O4 m/z: 411.12, found m/z: 411.28. 1H NMR (DMSO-d6), δ (ppm):
3.34 (broad. dd, 1H, НA(25)); 4.10 (dd, 1H, НB(25), 3J=12.1 Hz, 2J=18.3 Hz); 5.77 (dd, 1H, H(18),
3
3J=4.6 Hz, J=12.1 Hz); 7.13-7.45 (m, 9H, H(6,8,13,17,20-24)); 7.64-7.68 (m, 1H, H(7)); 7.86
3
4
3
(dd, 1H, H(5), J=7.6 Hz, J=1.5 Hz); 8.09 (d, 2H, H(14,16), J=9.5 Hz); 8.64 (s, 1H, H(4)). 13С
NMR (DMSO-d6), δ (ppm): 44.7; 62.0; 112.2 (2C); 115.8; 118.7; 118.9; 124.7; 125.4 (2C);
125.5 (2C); 127.7; 128.96; 129.02 (2C); 132.5; 138.4; 140.7; 140.8; 147.7; 148.5; 153.1;
157.7.
•
3-[5-(4-Methoxyphenyl)-1-phenyl-4,5-dihydro-1H-pyrazol-3-yl]coumarin 3h. Protocol B.
Yield 77%, mp. 174-176°C (lit. (29) mp. 169-170°C). HRMS (ESI), m/z: 435.1149 (M+K)+ (calcd
for C25H20N2O3K+: 435.1106). MS (ES+) calcd for C25H20N2O3 m/z: 396.15, found m/z: 396.34.
1H NMR (DMSO-d6), δ (ppm): 3.20 (dd, 1H, НA(25), 3J=6.1 Hz, 2J=17.9 Hz); 3.71 (s, 3H, OCH3);
3.95 (dd, 1H, НB(25), 3J=12.4 Hz, 2J=17.9 Hz); 5.48 (dd, 1H, H(18), 3J=6.1 Hz, 3J=12.4 Hz); 6.75
(t, 1H, H(15), 3J=7.3 Hz); 6.88-7.07 (m, 4H, H(13,17,21,23)); 7.15-7.22 (m, 4H,
H(14,16,20,24)); 7.36-7.42 (m, 2H, H(6,8)); 7.59-7.63 (m, 1H, H(7)); 7.83 (dd, 1H, H(5), 3J=7.6
4
Hz, J=1.5 Hz); 8.48 (s, 1H, H(4)). 13С NMR (DMSO-d6), δ (ppm): 44.5; 55.0; 63.0; 113.3 (2C);
114.3 (2C); 115.8; 119.1; 119.2; 119.3; 124.7; 127.0 (2C); 128.7; 128.8 (2C); 131.8; 134.1;
138.3; 143.1; 143.5; 152.9; 158.1; 158.5.
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•
•
•
3-[5-(4-Nitrophenyl)-1-phenyl-4,5-dihydro-1H-pyrazol-3-yl]coumarin 3i. Protocol B. Yield
65%, mp. 197-199°C. HRMS (ESI), m/z: 412.1293 (M+H)+ (calcd for C24H17N3O4H+: 412.1292).
MS (ES+) calcd for C24H17N3O4 m/z: 411.12, found m/z: 411.28. 1H NMR (DMSO-d6), δ (ppm):
3.26 (dd, 1H, НA(25), 3J=6.1 Hz, 2J=18.0 Hz); 4.06 (dd, 1H, НB(25), 3J=12.7 Hz, 2J=18.0 Hz); 5.74
3
3
3
(dd, 1H, H(18), J=6.1 Hz, J=12.7 Hz); 6.79 (t, 1H, H(15), J=7.3 Hz); 7.03-7.21 (m, 4H,
H(13,14,16,17)); 7.37-7.58 (m, 4H, H(6,8,20,24)); 7.60-7.64 (m, 1H, H(7)); 7.84 (dd, 1H, H(5),
3J=7.9 Hz, J=1.5 Hz); 8.21-8.23 (m, 2H, H(21,23)); 8.52 (s, 1H, H(4)). 13С NMR (DMSO-d6), δ
4
(ppm): 44.2; 62.3; 113.2 (2C); 115.9; 119.1; 119.6; 119.7; 124.2 (2C); 124.8; 127.2 (2C);
128.8; 129.0 (2C); 132.0; 138.9; 143.3; 143.5; 146.9; 149.6; 152.9; 158.1.
3-[5-(4-Methylphenyl)-1-phenyl-4,5-dihydro-1H-pyrazol-3-yl]coumarin 3j. Protocol B. Yield
75%, mp. 194-195°C (lit. (28) mp. 193-194°C). MS (ES+) calcd for C25H20N2O2 m/z: 380.15,
1
found m/z: 380.30. H NMR (DMSO-d6), δ (ppm): 2.26 (s, 3H, CH3); 3.20 (dd, 1H, НA(25),
3J=6.3 Hz, 2J=18.0 Hz); 3.97 (dd, 1H, НB(25), 3J=12.5 Hz, 2J=18.0 Hz); 5.49 (dd, 1H, H(18), 3J=6.3
3
Hz, J=12.5 Hz); 6.74-7.06 (m, 3H, H(13,15,17)); 7.13-7.19 (m, 6H, H(14,16,20,21,23,24));
4
7.36-7.42 (m, 2H, H(6,8)); 7.59-7.63 (m, 1H, H(7)); 7.83 (dd, 1H, H(5), 3J=7.6 Hz, J=1.2 Hz);
8.48 (s, 1H, H(4)). 13С NMR (DMSO-d6), δ (ppm): 20.5; 44.5; 62.8; 113.3 (2C); 115.8; 119.1
(2C); 119.9; 124.7; 125.6 (2C); 128.7; 128.8 (2C); 129.5 (2C); 131.8; 136.6; 138.3; 139.2;
143.1; 143.5; 152.9; 158.1.
3-[5-[4-(Dimethylamino)phenyl]-1-phenyl-4,5-dihydro-1H-pyrazol-3-yl]coumarin
3k.
Protocol B. Yield 62%, mp. 217-219°C (lit. (28) mp. 134-136°C). HRMS (ESI), m/z: 410.1865
(M+H)+ (calcd for C26H23N3O2H+: 410.1863). MS (ES+) calcd for C26H23N3O2 m/z: 409.18,
found m/z: 409.34. 1H NMR (DMSO-d6), δ (ppm): 2.84 (s, 6H, N(CH3)2); 3.18 (dd, 1H, НA(25),
2
3
2
3J=6.1 Hz, J=17.7 Hz); 3.92 (dd, 1H, НB(25), J=12.2 Hz, J=17.7 Hz); 5.40 (dd, 1H, H(18),
3
3
3J=6.1 Hz, J=12.2 Hz); 6.65-6.68 (m, 2H, H(21,23)); 6.74 (t, 1H, H(15), J=7.3 Hz); 7.06-7.18
(m, 6H, H(13,14,16,17,20,24)); 7.36-7.42 (m, 2H, H(6,8)); 7.59-7.62 (m, 1H, H(7)); 7.83 (dd,
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4
1H, H(5), 3J=7.6 Hz, J=1.5 Hz); 8.47 (s, 1H, H(4)). 13С NMR (DMSO-d6), δ (ppm): 40.0 (2C);
44.5; 62.8; 112.7 (2C); 113.3 (2C); 115.8; 119.0; 119.2; 120.1; 124.7; 126.5 (2C); 128.7; 128.8
(2C); 129.4; 131.7; 138.1; 143.1; 143.5; 149.7; 152.9; 158.2.
The synthesis of (1,5-diaryl-3-pyrazolyl)coumarins 4a, b. Oxidation of compounds 3 to (1,5-diaryl-3-
pyrazolyl)coumarins 4 was carried out according to (31).
•
3-[1-(4-Methylphenyl)-5-(4-nitrophenyl)-1H-pyrazol-3-yl]coumarin 4a. Yield 46%, mp. 177-
178°C. HRMS (ESI), m/z: 446.1116 (M+Na)+ (calcd for C25H17N3O4Na+: 446.1111). MS (ES+)
calcd for C25H17N3O4 m/z: 423.12, found m/z: 423.29. 1H NMR (CDCl3), δ (ppm): 2.42 (s, 3H,
CH3); 7.24 (narrow m, 4H, H(13,14,16,17)); 7.32 (t, 1H, H(6), 3J=7.6 Hz); 7.40 (d, 1H, H(8),
3
3J=8.2 Hz); 7.46 (d, 2H, H(20,24), J=8.5 Hz); 7.54-7.61 (m, 3H, H(5,7,25)); 8.18 (d, 2H,
3
H(21,23), J=8.5 Hz); 8.61 (s, 1H, H(4)). 13С NMR (CDCl3), δ (ppm): 21.2; 110.1; 116.5; 119.5;
119.8; 123.8 (2C); 124.6; 125.4 (2C); 128.3; 129.3 (2C); 130.0 (2C); 131.6; 136.4; 137.1;
138.0; 138.8; 141.7; 146.1; 147.3; 153.6; 159.8.
•
3-[5-(4-Methoxyphenyl)-1-phenyl-1H-pyrazol-3-yl]coumarin 4b. Yield 52%, mp. 169-171°C.
HRMS (ESI), m/z: 417.1260 (M+Na)+ (calcd for C25H18N2O3Na+: 417.1210). MS (ES+) calcd for
C25H18N2O3 m/z: 394.13, found m/z: 394.27. 1H NMR (CDCl3), δ (ppm): 3.82 (s, 3H, OCH3);
3
6.84-7.23 (m, 4H, H(20,21,23,24)); 7.31 (t, 1H, H(15), J=7.6 Hz); 7.35-7.42 (m, 7H,
H(6,8,13,14,16,17,25)); 7.54 (t, 1H, H(7), 3J=7.6 Hz); 7.60 (d, 1H, H(5), 3J=7.6 Hz); 8.61 (s, 1H,
H(4)). 13С NMR (CDCl3), δ (ppm): 55.3; 108.8; 113.9 (2C); 116.4; 119.6; 120.4; 122.6; 124.5;
125.5 (2C); 127.8; 128.2; 129.1 (2C); 130.1 (2C); 131.3; 137.7; 140.1; 144.1; 145.7; 153.5;
159.7; 159.9.
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RESULTS AND DISCUSSION
(1,5-Diaryl-3-pyrazolinyl)coumarins 3a-k – effective photogenerators of acidity
Solutions of new pyrazolines 3a-k were irradiated with light λ = 380-465 nm in the presence of
hexachloroethane to study their photosensitivity. Gradual decrease of the absorption maximum at
425-470 nm was observed in the electronic absorption spectra of solutions of the pyrazolines 3. This
absorption maximum is assigned to the starting substrate. The appearance of a shorter wavelength
absorption peak at 300-400 nm, is referred to the absorption maximum of the formed corresponding
pyrazole. As example, electronic absorption spectra of the solution of pyrazoline 3h in acetonitrile
under irradiation in the presence of hexachloroethane are shown in Fig. 1.
As in the case of other pyrazolines, phototransformation of (1,5-diaryl-3-pyrazolinyl)coumarins 3 is
followed by acidity generation. Increasing medium acidity during irradiation of pyrazoline 1 in the
presence of C2Cl6 and Rhodamine B lactone form in DMF leads to appearance of intensive absorption
of formed open form of the laser dye at 560 nm that is in accordance with the Scheme 2. Spectral
changes during irradiation of the compound 3h in dimethylformamide are shown in Fig. 2.
Mechanistic pattern of aryl(hetaryl)pyrazolines photodehydrogenation
The possible mechanism of aryl(hetaryl)pyrazolines photodehydrogenation in the presence of
carbon tetrachloride has earlier been suggested (9). However, no experimental data have been given
to support it. We have studied some aspects of the mechanism.
Effect of oxygen. As it has already been mentioned, the dye-sensitized photodehydrogenation of
pyrazolines is much sensitive to the presence of oxygen. We have evaluated effect of oxygen on
photodehydrogenation of pyrazolines in the presence of perchloroalkanes. Comparison of the rate
constants of photodehydrogenation reactions that are carried out in the presence of C2Cl6 in the
open air and under argon can be seen in the Table 1.
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The very definite conclusion comes from the comparison: oxygen does not participate in the
photodehydrogenation of pyrazolines in the presence of perchloroalkane.
The possible mechanism of aryl(hetaryl)pyrazolines photodehydrogenation in the presence of
carbon tetrachloride provides proton generation along the dehydrogenation process (9). Similar
steps should be proposed for the phototransformation of (1,5-diaryl-3-pyrazolinyl)coumarins in the
presence of hexachloroethane as well (Scheme 5).
In the first step, pyrazoline goes under irradiation into the excited state. The excited pyrazoline
behaves itself as an electron donor. The second step – transfer of electron from the excited
pyrazoline molecule to the C2Cl6 molecule seems not to be a rate-determining step in the whole
mechanism, since it goes without cleavage of any covalent bond. The formed pyrazoline cation-
.
radical releases a proton and transforms into pyrazole upon the interaction with the C2Cl5 -radical.
The (1,5-diaryl-3-pyrazolinyl)coumarin molecule plays, according to the Scheme 5, a role of the
sensitizer of C2Cl6 destruction under irradiation. It is known that alkyl and aryl halides undergo
reductive dissociation very easily after accepting an electron either electrochemically or
photochemically. For example, the lifetime of the anion-radical of CCl4 was reported to be extremely
short. This anion-radical dissociates rapidly to the trichloromethyl radical and chloride-ion (32, 33).
According to the Scheme 5, phototransformation of (1,5-diaryl-3-pyrazolinyl)coumarins in the
presence of hexachloroethane does not need any participation of oxygen.
Effect of solvent. We have established the absence of tautomeric transformations of pyrazolines 3a-
k, that have no hydroxy function in the position 4 of coumarin fragment (Fig. 3).
As example, electron absorption spectra of the pyrazoline 3h in toluene – DMF at different
compositions are shown in Fig. 4 (left). Only minor changes in position of the longest-wave band and
in the absorptivity of this pyrazoline can be seen under transition from 100% of toluene to 100% of
DMF. The noted blue shift equal to 15 nm is specific for solvent effects in the electron absorption
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spectra of the push-pull π-electron systems. For comparison, electronic absorption spectra of
pyrazoline 1 in the same solvent compositions are also given in Fig. 4 (right) as illustration of its keto-
enol transitions (12, 13). The similar effect in spectral properties of 3-(pyrazolinyl)-4-
hydroxycoumarins has been noted by D.-Y. Yang and coworkers (34).
As we have found, formation of the ionic intermediates during the photodehydrogenation of
pyrazolines (Scheme 5) is in accordance with polarity of the solvent. To estimate the influence of the
solvent on the photodehydrogenation reaction, the pseudo-first order reaction constants were
calculated. Irradiation of the compounds 3 was carried out in two solvents (toluene and acetone) in
the presence of hexachloroethane. The rate constants of photodehydrogenation reactions are
presented in Table 2.
As one can see from the data of the Table 2, the rate of photodehydrogenation reaction depends on
the polarity of the medium. Thus, the rates of photoconversion (1,5-diaryl-3-pyrazolinyl)coumarins 3
in polar acetone are definitely higher than that in nonpolar toluene. Effect of the solvent polarity on
photodehydrogenation reaction of 3-(pyrazolinyl)coumarins 3f and 3h has been studied in more
detail (Table 3).
All the polar solvents increase the photodehydrogenation rates comparing with nonpolar toluene.
Polar solvents reduce energy of the charged intermediates due to better their solvatation, thus
ultimately decreasing the activation energy of the reaction.
Effect of pyrazoline structure. As solvent dependencies of the reaction rates are in accordance with
the charged intermediates participation in the mechanism (Scheme 5), structural dependencies of
the photodehydrogenation rates of (1,5-diaryl-3-pyrazolinyl)coumarins 3 provide information on the
rate-determining step of the reaction. Due to the electronodonative function of the (1,5-diaryl-3-
pyrazolinyl)coumarin, the introduction of dimethylamino group into the substrate molecule (for
example, compounds 3b and 3k) increases definitely the rate of photodehydrogenation reaction. On
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the other hand, appearance of nitro group both in the 1-phenyl ring (compound 3g) and in the 5-
phenyl ring (compounds 3c and 3i) of the (1,5-diaryl-3-pyrazolinyl)coumarins 3 strongly deactivates
the photodehydrogenation reaction both in toluene and in acetone.
Looking for the rate-determining step in the above photodehydrogenation mechanism shown in the
Scheme 5 we have evaluated the correlation between the reaction rates and ionization potentials of
the substrates. The ionization potentials IP1 values have been determined by Koopmans theorem
(35, 36) based on the DFT quantum chemical calculations.
IP1 = - ε HOMO
The obtained values are listed in the Table 4 and compared with the rates of the dehydrogenation
reactions.
The data follow to the equation given below with the correlation coefficient r = 0.91 (Fig. 5).
ln k1 = -2.73 IP1 + 7.48
One can see definite dependence of the photodehydrogenation rates of (1,5-diaryl-3-
pyrazolinyl)coumarins 3 from their ionization potentials IP1. Nevertheless, electron transfer from the
exited pyrazoline molecule to hexachloroethane that leads to ion-radicals formation cannot be rate-
determining step in the above scheme (Scheme 5) of photodehydrogenation mechanism, since
electron transfer processes from excited states generally occur in pico- to femto-second time scale.
The next step that is proton elimination from the cation radical formed during ionization of the
substrate exited state seems to be reasonable suggestion for the rate-determining step in the whole
scheme of photodehydrogenation mechanism.
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CONCLUSION
New (1,5-diaryl-3-pyrazolinyl)coumarins 3 do not show tautomeric transformations and undergo
photodehydrogenation during irradiation in the presence of hexachloroethane in organic solvents.
The reaction is accompanied by acidity generation. All the polar solvents increase the
photodehydrogenation rates comparing with nonpolar toluene. Polar solvents reduce energy of the
charged intermediates due to better their solvatation, thus ultimately decreasing the activation
energy of the reaction. As solvent dependencies of the reaction rates are in accordance with the
charged intermediates participation in the photodehydrogenation mechanism, structural
dependencies of photodehydrogenation rates of (1,5-diaryl-3-pyrazolinyl)coumarins 3 provide
information on the rate-determining step of the reaction. The proton elimination from cation radical
that is formed during ionization of the substrate exited state seems to be reasonable suggestion for
the rate-determining step in the whole scheme of photodehydrogenation mechanism. This
suggestion is not in disagreement with the correlation between the reaction rates and ionization
potentials of the substrates.
ACKNOWLEDGEMENTS
This work was funded by the Russian Science Foundation, grant no. 17-13-01302 and the Russian
Foundation for Basic Research, grant no. 17-03-00478.
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SUPPORTING INFORMATION
Additional Supporting Information may be found in the online version of this article:
1H NMR, 13С NMR and HSQC spectra for 3-Cinnamoylcoumarins, (1,5-Diaryl-3-
pyrazolinyl)coumarins, and (1,5-Diaryl-3-pyrazolyl)coumarins are available to readers for
consultation [Part 1, figures S1-S3: 3-Cinnamoylcoumarins; Part 2, figures S1-S33:
(1,5-Diaryl-3-pyrazolinyl)coumarins; and Part 3, figures S1-S4: (1,5-Diaryl-3-pyrazolyl)coumarins].
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FIGURE CAPTIONS
Figure 1. The electronic absorption spectra of the pyrazoline 3h in acetonitrile before [1] and after
[2-10] irradiation in the presence of hexachloroethane; absorption of the notorious pyrazole 4b is
shown by dotted line.
Figure 2. Electronic absorption spectra of pyrazoline 3h before [1] and after [2-7] irradiation in
dimethylformamide in the presence of hexachloroethane and the Rhodamine B lactone form.
Figure 3. Pyrazolines 1 and 3h.
Figure 4. Electronic absorption spectra of pyrazolines 3h (left) and 1 (right) in mixtures of toluene –
DMF of different composition: DMF [1], DMF-toluene, 4:1 [2], DMF-toluene, 3:2 [3], DMF-toluene,
2:3 [4], DMF-toluene, 1:4 [5], toluene [6].
Figure 5. Dependence of the photodehydrogenation rates of (1,5-diaryl-3-pyrazolinyl)coumarins 3
from their ionization potentials IP1.
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TABLES
Table 1. The rate constants of photodehydrogenation (k1) of pyrazolines 3f and 3h in the presence of
C2Cl6 in the open air and under argon.
3f,
3h,
Conditions
k1·10-4, s-1
k1·10-4, s-1
Toluene, open air
7.93
7.39
10.92
9.93
Toluene, under argon
Table 2. Spectral and photokinetic characteristics of (1,5-diaryl-3-pyrazolinyl)coumarins 3.
Solvent
Toluene
ε,
Acetone
№
λabs.
nm
,
λfl.
nm
,
k1·10-4,
λabs.
nm
,
ε,
λfl.
nm
,
k1·10-4,
max.
em.
max.
em.
M-1·cm-1
s-1
M-1·сm-1
s-1
3a
3b
3c
3d
3e
3f
454
456
456
452
462
440
448
458
446
456
460
15860
561
14.0
16.8
5.0
442
434
442
438
438
428
446
440
432
440
444
16531
587
24.7
34.8
0
2092
568
569
565
578
560
558
566
553
562
567
2729
16807
18060
16055
22013
33887
14182
16728
17531
18214
581
597
587
599
583
594
589
585
588
593
14875
15846
22667
13278
28030
17360
12472
11343
16578
9.7
18.3
11.4
15.9
0
9.0
7.9
3g
3h
3i
1.6
10.9
3.4
23.6
0
3j
8.9
19.4
6.2
3k
12.5
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Table 3. Rate constants k1·10-4, s-1of photodehydrogenation of the compounds 3f and 3h
in the presence of C2Cl6.
Compound
Toluene
Acetone
Acetonitrile
DMF
3f
7.9
15.9
23.6
23.2
36.8
9.5
3h
10.9
11.7
Table 4. Relative rate constants of photooxidation (1,5-diaryl-3-pyrazolinyl)coumarins 3, their
calculated values of ionization potentials.
Ionization
Compound
k1·10-4, s-1
ln k1
potential IP1, eV
5.33
3a
3b
3c
3d
3e
3f
14.0
16.8
5.0
-6.57
-6.39
-7.60
-6.94
-7.01
-7.14
-8.77
-6.82
-7.99
-7.02
-6.68
5.28
5.55
9.7
5.22
9.0
5.17
7.9
5.28
3g
3h
3i
1.6
5.88
10.9
3.4
5.28
5.66
3j
8.9
5.28
3k
12.5
5.12
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SCHEMES
hv
D0
1D
+
3D
1O2
D0
Q
O2
k(O2)
3D
k(Q)
products
Q
O2
.
.
-
+
D
+ Q
Scheme 1. Two routes of pyrazoline photodehydrogenation with oxygen participation.
R1
R1
N
N
hv
N
N
R2
R2
H+
+
CCl4
R
R
Et2N
O
NEt2
Et2N
O
N+Et2
Et2N
O
N+Et2
H+
O-
O
- H+
O
OH
O
O
Scheme 2. Formation of the Rhodamine B open form due to acid photogenerated by pyrazolines in
the presence of CCl4.
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N
N
OH
O
N
O
O
O
HN
O
O
O
1, hydroxy-form
1, keto-form
Scheme 3. Keto-enol tautomerism of pyrazoline 1.
R3
O
NHNH2
R3
AcOH, i-PrOH, reflux (protocol A)
KOH, i-PrOH, reflux (protocol B)
R1
R2
R1
R2
N
N
O
+
O
O
O
2a-f
3a-k
2a, 3a: R1 = R2 = R3 = H;
2b, 3b: R1 = H, R2 = NMe2, R3 = F;
2c, 3c, 4a: R1 = H, R2 = NO2, R3 = Me;
2d, 3d: R1 = OH, R2 = OMe, R3 = F;
2a, 3e: R1 = R2 = H, R3 = Me;
2a, 3f: R1 = R2 = H, R3 = F;
2a, 3g: R1 = R2 = H, R3 = NO2;
2f, 3h, 4b: R1 = R3 = H, R2 = OMe;
2c, 3i: R1 = R3 = H, R2 = NO2;
2e, 3j: R1 = R3 = H, R2 = Me;
2b, 3k: R1 = R3 = H, R2 = NMe2
K2Cr2O7, AcOH, reflux
R3
R1
R2
N
N
O
O
4a, b
Scheme 4. Synthesis of (1,5-diaryl-3-pyrazolinyl)coumarins 3a-k and pyrazoles 4a, b.
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.
R1
R1
R1
+
*
C2Cl6
N N
N N
N N
hv
.
-
H
R2
H
R2
H
R2
+
+
C2Cl6
R
R
R
H
H
H
H
H
H
- H+
- Cl-
R = Coumarin-3-yl; R1, R2 = Aryl
R1
R1
N N
H
N N
.
.
R2
R2
R
R
C2Cl5
- C2HCl5
H
H
Scheme 5. Presumptive scheme of the pyrazoline photodehydrogenation mechanism in the
presence of hexachloroethane.
This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.