Synthesis of pyrazoles by a one-pot tandem cyclization-dehydrogenation approach on Pd/C/K-10 catalyst

Article abstract of DOI:10.1055/s-2007-982545

A novel one-pot synthesis of substituted pyrazoles from chalcones and hydrazines via a tandem cyclization-dehydrogenation approach is described. This process is based on the use of a bifunctional noble-metal/solid-acid catalyst, Pd/C/K-10 montmorillonite

Full text of DOI:10.1055/s-2007-982545

1600
LETTER
Synthesis of Pyrazoles by a One-Pot Tandem Cyclization–Dehydrogenation
Approach on Pd/C/K-10 Catalyst
Synthesis of
P
h
yrazoles ainaz M. Landge, Allison Schmidt, Verona Outerbridge, Béla Török*
Department of Chemistry, University of Massachusetts Boston, 100 Morrissey Blvd., Boston, MA 02125, USA
Fax +1(617)2876030; E-mail: bela.torok@umb.edu
Received 22 March 2007
environmental regulations. Considering the importance of
pyrazoles, the development of sustainable green synthetic
methods is highly desirable.
Abstract: A novel one-pot synthesis of substituted pyrazoles from
chalcones and hydrazines via a tandem cyclization–dehydro-
genation approach is described. This process is based on the use of
a bifunctional noble-metal/solid-acid catalyst, Pd/C/K-10 montmo-
rillonite and microwave irradiation under solvent-free conditions.
The cyclization of chalcones with hydrazines readily takes place on
the strong solid acid while the presence of the metal ensures the
formation of the aromatic product through dehydrogenation. The
reactions are complete in 30 minutes providing good yields and
high selectivities.
Our basic idea is to combine the cyclization and oxidation
reactions into a two steps/one-pot approach by using a
special bifunctional noble-metal/solid-acid catalyst. In
earlier papers we have already described the fundamental
advantages of such catalytic systems. Applications in-
clude the chemoselective C=O hydrogenation of a,b-un-
saturated carbonyl compounds5 and reduction of C=O
bonds to CH2,6 a green alternative to the traditional
Clemmensen reduction.
Key words: chalcones, hydrazines, bifunctional catalysis, micro-
wave heating, pyrazoles
The major thrust of this work is to provide an environmen-
tally benign one-pot approach for the synthesis of pyra-
zoles using the combination of bifunctional (metal/acid)
heterogeneous catalysis and microwave irradiation. The
presence of solid acid will ensure rapid cyclization, while
the metal effectively dehydrogenates the dihydropyra-
zoles to the final product. The schematic depiction of the
process is shown in Scheme 1.
Heteroaromatic compounds possess a wide range of
biological effects and thus attract significant attention.1
Pyrazoles, an important group among heteroaromatics are
particularly known for their wide range of biological
activity.2 They are used as anticancer agents,2a,b show
anti-inflammatory and molluscicidal activity,2c are estro-
gen receptor agonists,2d and are active cyclooxygenase-2
inhibitors.2e Due to the importance of these compounds,
extended efforts were made toward their synthesis. The
several methods available for the synthesis of these het-
eroaromatics include cycloadditions and cyclocondensa-
tions.3 As these methods may result in isomers the interest
turned to regioselective cyclizations.3a,4 Many of these
methods provide excellent results; however, they usually
involve multiple steps. In most regioselective methods the
process involves a cyclization and an oxidation step to
ensure the formation of the heteroaromatic system.3a,4
These oxidation methods require stoichiometric amount
of oxidant, which is not preferable under the tightening of
To test our hypothesis we have chosen the cyclization of
chalcone with phenylhydrazine as a probe reaction. Based
on our experience, we have selected K-10 montmorillo-
nite as a solid-acid catalyst,7 while Pt and Pd were selected
as metals for the dehydrogenation.8 As a first step, several
supported metal catalysts have been studied. The reaction
was carried out using a general procedure that followed
the common practice for solvent-free reactions. All
reactions were carried out in a CEM Discover microwave
reactor at constant temperatures. The reactants and
catalyst were mixed together with the minimum amount
of dichloromethane. Evaporation of the solvent gave a dry
O
R2
H
N
Pd/C/K-10
R3
+
H2N
N
MW, 160 °C
N
R2
R1
R3
R1
R1 = H, F,
R
R
2 = H, OMe, F, Me,
3 = Me, Ph, 3-CF3C6H4, 4-NO2C6H4
Scheme 1 Microwave-assisted one-pot synthesis of pyrazoles from chalcones and hydrazines by bifunctional Pd/C/K-10 catalyst
SYNLETT 2007, No. 10, pp 1600–1604
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Advanced online publication: 07.06.2007
DOI: 10.1055/s-2007-982545; Art ID: S01607ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Pyrazoles
1601
Table 1 Effect of Catalyst on the Microwave-Assisted Synthesis of
Table 2 Effects of Reactant Ratio and Reaction Time on the
Pyrazoles from Chalcone and Phenylhydrazinea
Microwave-Assisted Synthesis of Pyrazoles from Chalcone and
Phenylhydrazine on Pd/C/K-10 Catalysta
Entry Catalyst
Yield
(%)b
Pyrazole Dihydropyrazole
(%)c
89
98
41
90
95
77
99
92
(%)c
11
–
Entry Ratio of Time
Yield
(%)b
Pyrazole Dihydropyrazole
1:2
(min)
(%)c
(%)c
15
11
31
8
1
2
3
4
5
6
7
8
5% Pt/Al2O3
85
35
90
91
25
92
50
92
1
2
3
4
5
6
1:0.8
1:1
20
60
75
81
96
96
96
85
5% Pd/Al2O3
5% Pd/K-10
10% Pt/K-10
5% Pt/C
20
89
59
10
5
1:1.5
1:1.5
1:1.5
1:2
10
69
20
92
30
99
1
K-10
23
–
20
58
42
10% Pd/C
10% Pd/C/K-10
a Conditions: 160 °C, 200 W.
b Determined by GC-MS, based on chalcone.
c Selectivity was determined by GC.
8
a Conditions: 160 °C, 200 W, 20 min, ratio of chalcone:phenyl-
hydrazine = 1:1.
b Determined by GC-MS, based on chalcone.
c Selectivity was determined by GC.
Table 3 Effect of Temperature on the Microwave-Assisted
Synthesis of Pyrazoles from Chalcone and Phenylhydrazine on
Pd/C/K-10 Catalysta
mixture that was transferred to a reaction vial and irradi-
ated for a specified time at the given temperature. The
results are summarized in Table 1.
Entry
Temp
(°C)
Yield
(%)b
Pyrazole
(%)c
Dihydropyrazole
(%)c
1
2
3
4
5
100
120
140
160
180
94
93
89
96
90
25
76
92
92
98
75
33
8
The data show that the reaction is strongly catalyst-depen-
dent. Catalysts with neutral or moderately acidic supports
give high selectivity toward the aromatic product. The
overall yields, however, are low, due to their inability to
efficiently catalyze the cyclization process. It appears that
the K-10 montmorillonite itself is able to provide an aro-
matic product, although with moderate selectivity. The
combination of the metal and K-10 gave the best results.
In terms of the metal, both Pt and Pd are effective in the
dehydrogenation step, although Pd gives higher selectivi-
ties toward pyrazoles. Based on these results, we have se-
lected Pd/C/K-10, a mechanical mixture of 5% Pd/C and
K-10 montmorillonite, for further studies. Another reason
for selecting this catalyst is the commercial availability of
its components. This feature makes the process easily
reproducible compared to tailored Pt or Pd/K-10 catalyst.
8
2
a Conditions: 160 °C, 200 W, 20 min, ratio of chalcone:phenyl-
hydrazine = 1:1.5.
b Determined by GC-MS, based on chalcone.
c Selectivity was determined by GC.
The data indicate that the temperature does not signifi-
cantly affect the cyclization or the conversion of the start-
ing materials. In contrast, the dehydrogenation step shows
strong temperature dependence; below 140 °C, the
aromatization is not satisfactory. The analysis of these
data suggested the use of 160 °C for further reactions.
As a next step, two other parameters, such as the ratio of
reactants and reaction time were examined to optimize our
probe reaction. The results are collected in Table 2.
Based on the above data (Tables 1– 3), we concluded that
the reaction takes place in very high yields and excellent
selectivities under optimized conditions, although adjust-
ment in the reaction time may be necessary when using
other reactants. To widen the scope of the approach, we
have selected several chalcones and phenylhydrazines and
carried out the reaction in order to synthesize a variety of
substituted pyrazoles. The results are summarized in
Table 4.
As the data indicate, the 1:1.5 chalcone-to-phenylhydra-
zine ratio is the most beneficial for the reaction. Excess
phenylhydrazine appears to be necessary for high yields.
Although almost quantitative cyclization occurs at over
50% phenylhydrazine excess, it suppresses the dehydro-
genation process. This is probably due to the hydrogen
donor properties of hydrazines. We have also observed
that longer reaction times favor the formation of aromatic
products. Reaction times over 30 minutes, however, re-
sulted in product decomposition.
The results show that the reactions took place efficiently;
the yields and selectivities are excellent with a few excep-
tions. The substituents show significant effect on neither
the yields nor the selectivities. The minor amount of
byproducts usually includes the intermediate dihydro-
pyrazoles. The other byproduct, a pyrazole isomer, was
After selecting the catalyst, reaction time, and reactant
ratio for the reaction (Scheme 1) we optimized the reac-
tion temperature. The results are tabulated in Table 3.
Synlett 2007, No. 10, 1600–1604 © Thieme Stuttgart · New York

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S. M. Landge et al.
LETTER
Table 4 Microwave-Assisted, One-Pot Synthesis of Pyrazoles from Chalcones and Hydrazines by Pd/C/K-10 Catalyst at 160 °C
O
R2
H
Pd/C/K-10
N
R3
+
H2N
N
MW, 160 °C
N
R2
R1
R3
R1
Entry
R1
H
R2
R3
Time (min)
Yield (%)a
Selectivity (%)b
1
2
3
4
5
6
7
8
9
H
Ph
30
30
20
30
30
30
30
30
30
95
98
98
80
86
98
92
85
96
99
96
H
H
Me
H
H
3-CF3C6H4
4-NO2C6H4
Ph
94
H
H
85
H
4-OMe
4-OMe
4-F
4-F
4-Me
91
H
3-CF3C6H4
Ph
98
4-F
4-F
H
95
3-CF3C6H4
Ph
100
98
a Isolated yields after flash chromatography, based on chalcone.
b For pyrazole, determined by GC-MS.
R2
formed in traces. The first can be explained by the imper-
fect dehydrogenation step, while the second is the result of
unexpected coupling between the chalcones and phenyl-
hydrazine. The combined amount of byproducts usually
does not exceed 1–2%.
R2
R1
R1
– H2O
N
+ (K-10)
O
δ
NH2
HN
R3
HN
R3
δ+
The results indicate that our original idea for designing a
one-pot approach is feasible. The proposed model for the
reaction mechanism involves contribution from both the
solid acid and the metal. The first step is the activation of
chalcone by K-10 montmorillonite (depicted as d+ – a
Lewis acid center). This material is a widely used catalyst
in environmentally friendly synthetic processes.7 Its use is
highly beneficial over liquid acids; it is solid, noncorro-
sive, inexpensive, and recyclable. In addition, it is com-
mercially available and can be used without any
pretreatment. Its strong acidity (Ho = –8) makes a wide
range of reactions possible, while the high surface area
(220–270 m2 g–1) ensures excellent reaction rates. It is an
excellent catalyst for microwave-assisted organic synthe-
sis (MAOS), an area that has attracted significant interest
in recent years.9 The chalcone activation occurs at the
carbonyl group, which results in the formation of a
carbocationic intermediate and continues as a traditional
condensation reaction with the NH2 group of the phenyl-
hydrazine. The catalyst then produces a benzylic cation
and the hydrazone undergoes a cyclization reaction to
form dihydropyrazole. In the last step the metal compo-
nent takes a leading role and dehydrogenates the inter-
mediate to the final aromatic product (Scheme 2).
R2
Pd
– H2
R2
H
H
N
H
N
R3
N
N
R3
R1
R1
Scheme 2 Proposed mechanism for the Pd/C/K-10-catalyzed syn-
thesis of substituted pyrazoles via one-pot electrophilic cyclization–
dehydrogenation tandem reaction
The structure of the final products suggests a step-by-step
cyclization; the concerted mechanism is highly improba-
ble under the extremely polar conditions.
Our efforts on the scale-up of the procedure indicated that
a tenfold increase in reactant amounts did not decrease the
yield (95%), although 10% longer reaction time and 50%
more catalyst was needed. This suggests the realistic
possibility of future larger scale-ups.
Synlett 2007, No. 10, 1600–1604 © Thieme Stuttgart · New York

LETTER
Synthesis of Pyrazoles
1603
3,5-Diphenyl-1-[3-(trifluoromethyl)phenyl]-1H-pyrazole (Ta-
ble 4, Entry 3)
Yellow oil.
1H NMR (300.1 MHz, CDCl3): d = 7.93 (d, 2 H, J = 8.1 Hz, Ar),
7.73 (s, 1 H, Ar), 7.45 (t, 2 H, J = 7.2 Hz, Ar), 7.34–7.39 (m, 7 H,
Ar), 7.26–7.30 (m, 2 H, Ar), 6.84 (s, 1 H, CH).
13C NMR (75.5 MHz, CDCl3): d = 153.3, 144.3, 140.0, 133.1,
130.7, 129.3, 128.77, 128.72, 128.70, 128.2, 127.9, 125.8, 123.7,
121.8, 116.3, 105.9.
In conclusion, the proposed bifunctional solid-acid/noble-
metal (Pd/C/K-10) catalyst is efficient in the described
one-pot cyclization–dehydrogenation reaction. It makes
the synthesis of a wide variety of pyrazoles possible from
commercially available chalcones and hydrazines. The
protocol is assisted by microwave irradiation and the
products are obtained in good to excellent yields and high
selectivities in short reaction times. Solvent-free condi-
tions, solid catalyst, short reaction times and easy product
isolation make this approach an attractive alternative for
19F NMR (282.4 MHz, CFCl3): d = –63.13 (s, 3 F, CF3).
the environmentally benign synthesis of the target com- MS [C22H15F3N2 (364)]: m/z (%) = 364 (100) [M+], 365 (23), 363
(51), 248 (8), 189 (11), 89 (9), 77 (17).
pounds.
1-(4-Nitrophenyl)-3,5-diphenyl-1H-pyrazole (Table 4, Entry
4)4c
Chalcones, hydrazines, K-10 montmorillonite and the metal cata-
lyst were purchased from Aldrich and Engelhard and used without
further purification. The reactions were carried out at constant tem-
peratures in a Discover Benchmate microwave reactor, with contin-
uous stirring. The temperature was measured and controlled by a
Yellow-orange solid, mp 119–121 °C (hexane–CH2Cl2, 80:20).
1H NMR (300.1 MHz, CDCl3): d = 8.19 (d, 2 H, J = 8.7 Hz, Ar),
7.92 (dd, 2 H, J = 7.8, 1.5 Hz, Ar), 7.55 (d, 2 H, J = 9.0 Hz, Ar),
7.30–7.46 (m, 8 H, Ar), 6.86 (s, 1 H, CH).
13C NMR (75.5 MHz, CDCl3): d = 153.3, 145.7, 144.9, 144.8,
132.2, 130.0, 129.0, 128.9, 128.8, 128.7, 128.5, 125.8, 124.46,
124.41, 116.3, 111.6, 107.1.
1
built-in infrared detector. The H NMR, 13C NMR and 19F NMR
spectra were obtained on a 300 MHz Varian NMR spectrometer in
CDCl3. Tetramethylsilane, CFCl3 (for 19F NMR), or the residual
solvent signal were used as reference. The MS identification of the
products has been carried out by an Agilent 6850 GC and 5973 MS
system (70 eV electron-impact ionization) using a 30 m long DB-5
type column (J&W Scientific). The melting points are uncorrected
and were recorded on a MEL-TEMP apparatus.
MS [C21H15N3O2 (341)]: m/z (%) = 341 (100) [M+], 342 (31), 340
(24), 294 (49), 293 (21), 207 (75), 191 (26), 77 (15).
3-(4-Methoxyphenyl)-1,5-diphenyl-1H-pyrazole (Table 4, En-
try 5)4d
General Procedure for the Synthesis of Substituted Pyrazoles
The mixture of chalcone (1.0 mmol) and phenylhydrazine (1.5
mmol) were dissolved in CH2Cl2 (3 mL) in a round-bottomed flask.
Then the mechanically premixed combination of Pd/C (21 mg) and
K-10 (500 mg) was added. After 10 min stirring, the solvent was
evaporated in vacuo. The dry mixture was placed into a reaction vial
and irradiated in the microwave reactor (CEM Discover Bench-
mate, 160 °C). During optimization the reaction was monitored by
TLC and GC-MS. When the reaction was completed, CH2Cl2 was
added to the cold mixture, stirred for 10 min, and the catalyst was
removed by filtration. The products have been purified by flash
chromatography.
Colorless solid, mp 138–140 °C (hexane–CH2Cl2, 80:20).
1H NMR (300.1 MHz, CDCl3): d = 7.92 (d, 2 H, J = 8.4 Hz, Ar),
7.19–7.44 (m, 10 H, Ar), 6.85 (d, 2 H, J = 8.4 Hz, Ar), 6.77 (s, 1 H,
CH), 3.81 (s, 3 H, OCH3).
13C NMR (75.5 MHz, CDCl3): d = 159.5, 153.3, 144.1, 140.1,
133.0, 129.9, 128.8, 128.5, 127.8, 127.3, 125.7, 125.2, 122.9, 113.8,
104.6, 55.2.
MS [C22H18N2O (326)]: m/z (%) = 326 (100) [M+], 327 (27), 325
(32), 311 (18), 180 (9), 152 (8), 77 (37), 51 (14).
3-(4-Methoxyphenyl)-5-phenyl-1-[3-(trifluoromethyl)phenyl]-
1H-pyrazole (Table 4, Entry 6)
Yellow oil.
1H NMR (300.1 MHz, CDCl3): d = 7.85 (d, 2 H, J = 7.8 Hz, Ar),
7.72 (s, 1 H, Ar) 7.26–7.54 (m, 8 H, Ar), 6.98 (d, 2 H, J = 7.8 Hz,
Ar), 6.77 (s, 1 H, CH), 3.86 (s, 3 H, OCH3).
13C NMR (75.5 MHz, CDCl3): d = 159.7, 152.3, 144.4, 140.4,
130.1, 129.2, 128.7, 128.6, 127.8, 127.1, 125.3, 123.6, 123.5, 121.8,
121.7, 116.3, 114.0, 105.5, 55.3.
1,3,5-Triphenylpyrazole (Table 4, Entry 1)4b
Colorless crystals, mp 137–139 °C (hexane–CH2Cl2, 80:20).
1H NMR (300.1 MHz, CDCl3): d = 7.93 (dd, 2 H, J = 7.2, 1.5 Hz,
Ar), 7.27–7.45 (m, 13 H, Ar), 6.82 (s, 1 H, CH).
13C NMR (75.5 MHz, CDCl3): d = 151.9, 144.3, 140.1, 133.0,
130.5, 128.9, 128.7, 128.6, 128.4, 128.2, 127.9, 127.4, 125.8, 125.2,
105.2.
MS [C21H16N2 (296)]: m/z (%) = 296 (100) [M+], 297 (23), 295 (66),
192 (15), 190 (15), 165 (30), 77 (87), 51 (34).
19F NMR (282.4 MHz, CFCl3): d = –63.12 (s, 3 F, CF3).
MS [C23H17F3N2O (394)]: m/z (%) = 394 (100) [M+], 395 (23), 379
1-Methyl-3,5-diphenyl-1H-pyrazole (Table 4, Entry 2)4b
(21), 178 (18), 145 (46), 77 (28), 63 (23).
Colorless needles, mp 58–60 °C (hexane–CH2Cl2, 80:20).
3,5-Bis(4-fluorophenyl)-1-phenyl-1H-pyrazole (Table 4, Entry
1H NMR (300.1 MHz, CDCl3): d = 7.83 (dd, 2 H, J = 8.1, 1.2 Hz,
Ar), 7.38–7.48 (m, 7 H, Ar), 7.24–7.35 (m, 1 H, Ar), 6.61 (d, 1 H,
J = 1.2 Hz, CH), 3.93 (s, 3 H, CH3).
13C NMR (75.5 MHz, CDCl3): d = 150.4, 145.0, 133.3, 130.6,
128.7, 128.6, 128.59, 128.51, 127.5, 125.4, 103.2, 37.5.
MS [C16H14N2 (234)]: m/z (%) = 234 (100) [M+], 235 (17), 233 (26),
189 (18), 103 (13), 102 (13), 77 (32), 51 (15).
7)4e
Colorless solid, mp 92–94 °C (hexane–CH2Cl2, 80:20).
1H NMR (300.1 MHz, CDCl3): d = 7.87 (m, 2 H, Ar), 7.34 (s, 5 H,
Ar), 7.22–7.24 (m, 2 H, Ar), 7.11 (t, 2 H, J = 8.4 Hz, Ar), 7.01 (t, 2
H, J = 8.4 Hz, Ar), 6.73 (s, 1 H, CH).
13C NMR (75.5 MHz, CDCl3): d = 164.3, 161.2, 151.0, 143.4,
139.7, 130.5, 130.4, 128.9, 127.5, 127.4, 127.3, 125.2, 115.7, 115.6,
115.47, 115.40, 104.8.
Synlett 2007, No. 10, 1600–1604 © Thieme Stuttgart · New York

1604
S. M. Landge et al.
LETTER
19F NMR (282.4 MHz, CFCl3): d = –112.68 (m, 1 F, ArF), –114.13
(2) (a) Xiao, L.; Lu, X. Y.; Ruden, D. M. Mini-Rev. Med Chem.
2006, 6, 1137. (b) McDonald, E.; Jones, K.; Brough, P. A.;
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Bioorg. Med. Chem. 2006, 14, 3929. (d) Stauffer, S. R.;
Coletta, C. J.; Tedesco, R.; Nishiguchi, G.; Carlson, K.; Sun,
J.; Katzenellenbogen, B. S.; Katzenellenbogen, J. A. J. Med.
Chem. 2000, 43, 4934. (e) Prasanna, S.; Manivannan, E.;
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(3) (a) Elguero, J. In Comprehensive Heterocyclic Chemistry II,
Vol. 3; Katritzky, A. R.; Rees, C. W.; Scriven, E. F. V., Eds.;
Pergamon Press: Oxford, 1996, 1. (b) Lautens, M.; Roy, A.
Org. Lett. 2000, 2, 555. (c) Wang, X.; Tan, J.; Grozinger, K.
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(4) (a) Huang, Y. R.; Katzenellenbogen, J. A. Org. Lett. 2000, 2,
2833. (b) Katritzky, A. R.; Wang, M.; Zhang, S.; Voronkov,
M. V. J. Org. Chem. 2001, 66, 6787. (c) Kovelesky, A. C.;
Shine, H. J. J. Org. Chem. 1988, 53, 1973. (d) Bishop, B.
C.; Brands, K. M. J.; Gibbs, A. D.; Kennedy, D. J. Synthesis
2004, 43. (e) Mishriky, N.; Asaad, F. M.; Ibrahim, Y. A.;
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(m, 1 F, ArF).
MS [C21H14F2N2 (332)]: m/z (%) = 332 (100) [M+], 333 (22), 331
(57), 225 (8), 210 (13), 208 (8), 183 (17).
3,5-Bis(4-fluorophenyl)-1-[3-(trifluoromethyl)phenyl]-1H-
pyrazole (Table 4, Entry 8)
Yellow oil.
1H NMR (300.1 MHz, CDCl3): d = 7.85–7.90 (m, 2 H, Ar), 7.15 (s,
1 H, Ar), 7.52–7.59 (m, 1 H, Ar), 7.42–7.46 (m, 2 H, Ar), 7.23–7.28
(m, 2 H, Ar), 7.13 (t, 2 H, J = 8.4 Hz, Ar), 7.06 (t, 2 H, J = 8.4 Hz,
Ar), 6.76 (s, 1 H, CH).
13C NMR (75.5 MHz, CDCl3): d = 164.5, 161.1, 151.7, 143.6,
140.1, 130.6, 130.5, 129.4, 128.7, 127.8, 127.5, 127.4, 126.1,
123.96 (q, J = 4.2 Hz), 121.85 (q, J = 4.2 Hz), 116.02, 115.6, 105.7.
19F NMR (282.4 MHz, CFCl3): d = –63.12 (s, 3 F, CF3), –111.85 (m,
1 F, ArF), –113.59 (m, 1 F, ArF).
MS [C22H13F5N2 (400)]: m/z (%) = 400 (100) [M+], 401 (22), 399
(54), 266 (12), 225 (13), 145 (44), 95 (33).
1,5-Diphenyl-3-p-tolyl-1H-pyrazole (Table 4, Entry 9)4d
Colorless solid, mp 126–128 °C (hexane–CH2Cl2, 80:20).
1H NMR (300.1 MHz, CDCl3): d = 7.81 (d, 2 H, J = 8.1 Hz, Ar),
7.21–7.37 (m, 12 H, Ar), 6.79 (s, 1 H, CH), 2.38 (s, 3 H, CH3).
13C NMR (75.5 MHz, CDCl3): d = 151.9, 144.1, 140.1, 137.6,
130.5, 130.1, 129.2, 128.8, 128.6, 128.3, 128.1, 127.2, 125.6, 125.2,
105.0, 21.3.
MS [C22H18N2 (310)]: m/z (%) = 310 (100) [M+], 311 (19), 309 (55),
191 (12), 165 (19), 77 (65), 51 (28).
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Acknowledgment
The financial support provided by the University of Massachusetts
Boston and the American Chemical Society Petroleum Research
Fund is gratefully acknowledged.
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Synlett 2007, No. 10, 1600–1604 © Thieme Stuttgart · New York

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