An unexpected catalytic synthesis of novel and known bis(pyrazolyl) methanes by the use of α-aryl-N-phenyl nitrones in aqueous media

Article abstract of DOI:10.3184/174751914X14114871789226

Reaction of -aryl-N-phenyl nitrones with 3.methyl-1-phenyl-2-pyrazoline-5-one in the presence of a catalytic amount of silica tungstic acid in EtOH/H2O afforded, unexpectedly, high yields of bis(3-hydroxy-1-phenylpyrazolyl)-arylmethanes instead of the two possible 1,3-addition products. This is the first report of the synthesis of bis(3-hydroxy-1-phenylpyrazolyl)-arylmethanes via the reaction of nitrones and a pyrazolin-5-one. A possible mechanism for the process is suggested.

Full text of DOI:10.3184/174751914X14114871789226

600 RESEARCH PAPER
VOL. 38 OCTOBER, 600–603
JOURNAL OF CHEMICAL RESEARCH 2014
An unexpected catalytic synthesis of novel and known bis(pyrazolyl)
methanes by the use of α‑aryl‑N‑phenyl nitrones in aqueous media
Khalil Eskandaria*, Bahador Karamia and Saeed Khodabakhshib
aDepartment of Chemistry, Yasouj University, P.O. Box 353, Yasouj, 75918-74831, Iran
bNanotechnology Research Center, Research Institute of Petroleum Industry, Tehran, Iran
Reaction of -aryl-N-phenyl nitrones with 3-methyl-1-phenyl-2-pyrazoline-5-one in the presence of a catalytic amount of silica
tungstic acid in EtOH/H2O afforded, unexpectedly, high yields of bis(3-hydroxy-1-phenylpyrazolyl)-arylmethanes instead of the
two possible 1,3-addition products. This is the first report of the synthesis of bis(3-hydroxy-1-phenylpyrazolyl)-arylmethanes
via the reaction of nitrones and a pyrazolin-5-one. A possible mechanism for the process is suggested.
Keywords: α-aryl-N-phenyl nitrones, 3-methyl-1-phenyl-2-pyrazoline-5-one, bis(3-hydroxy-1-phenylpyrazolyl)-arylmethanes,
silica tungstic acid catalyst
Recently, nitrones as important reactive intermediates have
attracted the attention of chemists because of their applications
in the synthesis of various natural and biologically active
Results and discussion
Following our continuing interest in developing new and green
strategies for organic synthesis, especially new heterocyclic
compounds,21 we turned our attention to the synthesis of new
isoxazole derivatives. For this purpose, we first had to prepare
a-aryl-N-phenyl nitrones 4 via a two-step method involving the
reduction of nitrobenzene (1) by ammonium chloride and zinc
powder to yield phenyl hydroxylamine (2) and then reaction of
2 with aromatic aldehydes 3 (Scheme 1).
The α-aryl-N-phenyl nitrones 4 (Scheme 1), were then
reacted with 3-methyl-1-phenyl-2-pyrazoline-5-one 5 under
reflux in EtOH/H2O. We expected the reaction to produce
the corresponding isoxazole derivatives 7 or isoxazolo-fused
pyrazoles 8 via 1,3-dipolar cycloaddition, but after spectral
characterisation of the products, it was found that instead only
the bis(3-hydroxy-1-phenylpyrazolyl)-arylmethanes 6 were
obtained (Scheme 2).
Those results were obtained without a catalyst and the
yields were only about 50%, we then proceeded to see whether
various acid and base catalysts could improve the yield. We
used as a model the reaction between α-phenyl-N-phenyl
nitrone 4 (G=H) and 3-methyl-1-phenyl-2-pyrazoline-5-one 5
to give product 6a (G=H) and the results are shown in Table 1.
Basic catalysts at 10 mol% (Na2CO3, K2CO3) gave yields in the
range 25–45% (entries 2–4). Acid catalysts such as molybdate
sulfuric acid (MSA), ammonium dihydrogen phosphate (ADP),
FeCl3 or CrCl3 gave yields in the range 50–86% (entries
5–9). The best yield of 92% was obtained with 5 mol% silica
tungstic acid (STA) in EtOH:H2O (2:1) under reflux (entry
11). Other amounts of catalyst were tried, but 5% was optimal.
Gratifyingly the use of STA reduced the reaction time from
4 h to 40 min. It should be mentioned that under all examined
conditions products 7 and 8 were not synthesised and only
product 6 was obtained.
compounds.1 Nitrones possess
a highly polarised C=N
bond and have many advantages due to the similarity with
important functional groups such as imines, hydrazones
and other nitrogen derivatives of aldehydes in reactions with
nucleophiles.2 Nitrones as intermediates have been shown to be
more volatile than others such as ylids3 and can be applied as
highly reactive intermediates to the synthesis of a wide range
of heterocyclic compounds.4–6 Also, biologically these active
intermediates provide a protective action against microvascular
damage.7 Pyrazole derivatives are important diaza-heterocyclic
compounds found in nature and possess extensive biological
activities, for example antianxiety, antipyretic, analgesic,
and anti-inflammatory agents.8–11 Several synthetic routes
to bis(pyrazolyl)methanes have been reported and generally
the methods involve reaction of aromatic aldehydes with
pyrazolones under various conditions such as refluxing in
ethanol in the presence of silica-bonded S-sulfonic acid
(SBSSA) as a Bronsted acid catalyst,12 stirring in water at room
temperature using nano n-propylsulfonated γ-Fe2O3 (NPS-
γ-Fe2O3) as a Lewis acid catalyst,13or stirred in the presence
of 1,3-disulfonic acid imidazolium tetrachloroaluminate as
acidic ionic liquid under solvent-free conditions.14 Recently,
silica tungstic acid (STA) as a highly efficient heterogeneous
solid catalyst has been synthesised, characterised, and
used as an efficient catalyst in organic reactions leading to
benzimidazoles,15 quinoxalines,16 coumarins,17,18 bis-spiro
piperidines,19 and 2-amino-4H-chromenes.20
The synthesis of bis(pyrazolyl)methanes starting from
nitrones has not, however, been reported. We report here a novel
and unexpected route to some new and known bis(pyrazolyl)
methanes based on the reaction of α-aryl-N-phenyl nitrones
with 3-methyl-1-phenyl-2-pyrazoline-5-one using STA as a
highly efficient heterogeneous solid catalyst.
O
H
O
H
EtOH
G
4[H]
NH4Cl, Zn
60 °C
N
Ph NO2
N
Ph
OH
Ph
1
2
4
G
3
Scheme 1
* Correspondent. E-mail: khalileskandari@yahoo.com
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JOURNAL OF CHEMICAL RESEARCH 2014 601
G
H3C
CH3
N
N
Ph
N
N
Ph
OH
HO
6
G
O
N
N
O
Ph
N
STA, 5 mol%
Ph N
N
Ph
H
2O/EtOH
Reflux
O
G
N
Ph
CH3
G
7
4
CH3
5
Ph
N
N
N Ph
HO
O
CH3
8
Scheme 2
To demonstrate the generality of reaction, a wide range of
α-aryl-N-phenyl nitrones were reacted with 3-methyl-1-phenyl-
2-pyrazoline-5-one and the results are summarised in Table 2.
It was observed that the process can tolerate both electron-
withdrawing and electron-donating groups on the benzene
Experimental
Chemicals were purchased from Merck or Aldrich chemical
companies. STA was characterised in accord with our previously
method.15 Phenylhydroxylamine and α-aryl-N-phenyl nitrones were
prepared according to the literature.24,25 Melting points were measured
on an electro thermal KSB1N apparatus. IR spectra were recorded
in the matrix of KBr with a JASCO FTIR-680 plus spectrometer.
1H NMR and 13C NMR spectra were recorded on a FT-NMR Bruker
Avance Ultra Shield Spectrometer at 400.13 and 100.62 MHz in
DMSO-d6 in the presence of TMS as internal standard. TLC was
performed on TLC-Grade silica gel-G/UV 254 nm plates.
1
ring. All novel products were characterised by FTIR, H and
13C NMR spectroscopy and the results for known compounds
compared with authentic samples.12,22,23 The data were in
good agreement with the expected structures. A plausible
mechanism for the STA-catalysed synthesis of bis(pyrazolyl)
methanes 6 is been depicted in Scheme 3. It is assumed that
initially a Knoevenagel-type condensation between 4 and 5
generates Int.1. Subsequent addition of another equivalent of
5 to Int.1 followed by elimination of phenylhydroxylamine and
1,3-proton shift leads to the formation of product 6.
Table 2 Synthesis of bis(pyrazolyl)methanes 6a–m using STA as catalyst
(Scheme 2)a
Product
G
Time/min
Yield/%b
M.p./ºC [lit.]
6a
6b
6c
6d
6e
6f
6g
6h
6i
H
20
20
30
20
15
15
30
15
30
20
30
15
20
92
90
88
90
90
92
88
94
92
90
93
92
86
162–164 (171–172) [22]
190–192 (–)
Table 1 Optimisation of reaction conditions for synthesis of bis(3-
hydroxy-1-phenyl-pyrazolyl)-phenylmethane 6a (G=H) from α-phenyl-
N-phenyl nitrone 4a (G=H) and 3-methyl-1-phenyl-2-pyrazoline-5-one 5
(Scheme 2)
2,4-(OMe)2
3-EtO-4-OH
2,4-Cl2
3-NO2
4-NO2
4-Me
2-Cl
3-Br
4-OMe
212–213 (–)
228–230 (227–229) [12]
161–163 (151–153) [22]
228–230 (229–231) [22]
203–205 (202–204) [22]
238–240 (235–237) [22]
174–176 (173–175) [22]
165–167 (176–177) [23]
228–230 (–)
Yield of
Entry
Catalyst/mol%
Solvent/temp./C
None/80
Time/min
6a
1
2
–
Na2CO3 (10)
Na2CO3 (10)
K2CO3 (10)
MSA (5)
240
240
240
240
180
60
50
45
25
25
70
78
50
65
80
86
92
None/80
3
4
CHCl3/Reflux
THF/Reflux
6j
6k
6l
5
None/80
1-Naphthyl
4-Biphenyl
3-Indolyl
6
7
MSA (5)
ADP (10)
MeOH/Reflux
THF/Reflux
218–220 (–)
242–244 (–)
240
60
60
60
40
6m
8
9
10
11
FeCl3 (10)
CrCl3.6H2O (5)
STA (5)
MeOH/Reflux
EtOH:H2O (2:1)/80
EtOH/Reflux
EtOH:H2O (2:1)/Reflux
aReaction conditions: A solution of α-aryl-N-phenyl nitrone 4 (1 mmol),
3-methyl-1-phenyl-2-pyrazoline-5-one 5 (2 mmol) and STA (5 mol%) in
EtOH/H2O (2:1) (10 mL) was stirred under reflux for an appropriate time.
bIsolated yields.
STA (5)
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602 JOURNAL OF CHEMICAL RESEARCH 2014
H3C
H
G
HO
N
N
O
N
+H+
Ph
Ph
N
H
Ph
N
Ph
O
N
O
Int. 1
4
CH3
5
G
H3C
-H+
N
O
H
N
OH
Ph
HN
Ph
Ph
N
Ph
N
HA
O
HO
N
N
G
A-
CH3
HA : STA
CH3
H3C
CH3
N
N
Ph
N
N
OH
HO
Ph
6
Scheme 3
Synthesis of bis(pyrazolyl)methanes 6a–m
3.90 (q, J=6.9 Hz, 2H, CH2), 2.29 (s, 6H, CH3), 1.25 (t, J=6.9 Hz, 3H,
CH3). 13C NMR (76.46 MHz) δ (ppm): 146.1, 145.1, 142.6, 137.3, 137.0,
133.1, 131.6, 128.9, 125.5, 120.5, 119.7, 115.2, 113.4, 63.9, 32.7, 14.7,
11.6; Anal. calcd for C29H28N4O4: C, 70.15; H, 5.68; N, 11.28; found: C,
70.19; H, 5.57; N, 11.26%.
A solution of an α-aryl-N-phenyl nitrone 4 (1 mmol), 3-methyl-1-
phenyl-2-pyrazoline-5-one 5 (2 mmol) and STA (5 mol%) in EtOH/
H2O (2:1, 10 mL) was stirred under reflux for an appropriate time.
The progress of the reaction was monitored by TLC. After completion,
the reaction mixture was cooled to room temperature and the solvent
was evaporated under reduced pressure. The resulting precipitate was
dried and dissolved in hot EtOH to separate the catalyst. Products 6
were obtained after recrystallisation from boiling EtOH.
bis(5-Hydroxy-3-methyl-1-phenyl-pyrazolyl)-phenylmethane (6a):
Light yellow crystals; IR (KBr) (ῡmax, cm–1): 3424, 3062, 2917, 1598,
1498, 1415, 1284, 1186, 1027, 755, 692; 1H NMR (400.13 MHz) δ
(ppm): 13.96 (s, 1H, OH), 12.39 (s, 1H, OH), 7.71 (d, J=8.4 Hz, 4H,
aromatic CH), 7.45 (t, J=7.2 Hz, 4H, aromatic CH), 7.31–7.24 (m, 6H,
aromatic CH), 7.20–7.17 (m, 1H, aromatic CH), 5.00 (s, 1H, CH), 2.33
(s, 6H, CH3); 13C NMR (100.62 MHz) δ (ppm): 157.6, 146.4, 140.7,
136.9, 128.8, 128.3, 127.1, 126.4, 126.2, 121.3, 105.7, 33.6, 11.5.
bis(5-Hydroxy-3-methyl-1-phenyl-pyrazolyl)-(2,4-dimethoxyphenyl)
methane (6b): Yellow crystals; IR (KBr) (ῡmax, cm–1): 3428, 2996, 2958,
2839, 1613, 1503, 1460, 1406, 1294, 1209, 1122, 1041, 839, 757, 580;
1H NMR (300.13 MHz) δ (ppm): 14.35 (s, 1H, OH), 12.38 (s, 1H, OH),
7.69 (d, J=7.8 Hz, 4H, aromatic CH), 7.39–7.50 (m, 5H, aromatic CH),
7.22 (t, J=6.9 Hz, 2H, aromatic CH), 6.46 (t, J=9.6 Hz, 2H, aromatic
CH), 5.09 (s, 1H, CH), 3.79 (s, 3H, OCH3), 3.69 (s, 3H, OCH3), 2.26 (s,
6H, CH3); 13C NMR (76.46 MHz) δ (ppm): 158.8, 156.7, 146.1, 137.6,
137.4, 137.3, 137.1, 136.7, 133.6, 131.9, 128.8, 125.4, 122.9, 120.5, 104.1,
98.2, 55.4, 55.0, 26.9, 11.6; Anal. calcd for C29H28N4O4: C, 70.15; H,
5.68; N, 11.28; found: C, 70.22; H, 5.62; N, 11.25%.
bis(5-Hydroxy-3-methyl-1-phenyl-pyrazolyl)-(2,4-dichlorophenyl)
methane (6d): Light brown crystals, m.p. 228–229 °C; IR (KBr) (ῡmax
,
cm–1): 3420, 3057, 2920, 1597, 1572, 1500, 1470, 1380, 1298, 1189,
845, 792, 752, 690; 1H NMR (250.13 MHz) δ (ppm): 13.95 (s, 1H, OH),
12.67 (s, 1H, OH), 7.65–7.74 (m, 5H, aromatic CH), 7.53 (d, J=2 Hz,
1H, aromatic CH), 7.36–7.44 (m, 5H, aromatic CH), 7.22 (t, J=7.2 Hz,
2H, aromatic CH), 5.05 (s, 1H, CH), 2.26 (s, 6H, CH3); 13C NMR
(62.89 MHz) δ (ppm): 148.3, 147.1, 146.0, 137.3, 134.9, 128.8, 125.4,
120.5, 119.2, 111.6, 111.5, 104.9, 104.6, 31.7, 11.6.
bis(5-Hydroxy-3-methyl-1-phenyl-pyrazolyl)-(4-nitrophenyl)
methane (6f): Yellow crystals, m.p. 228–230 °C; IR (KBr) (ῡmax, cm–1):
3068, 2920, 1598, 1579, 1524, 1500, 1415, 1348, 785, 753, 691. 1H NMR
(400.13 MHz) δ (ppm): 13.90 (s, 1H, OH), 12.49 (s, 1H, OH), 8.18 (d,
J=8.8 Hz, 2H, aromatic CH), 7.73 (d, J=8.0 Hz, 4H, aromatic CH),
7.54 (d, J=8.8 Hz, 2H, aromatic CH), 7.45 (t, J=8.0 Hz, 4H, aromatic
CH), 7.25 (t, J=8.0 Hz, 2H, aromatic CH), 5.14 (s, 1H, CH), 2.37 (s,
6H, 2CH3); 13C NMR (100.62 MHz) δ (ppm): 150.3, 146.2, 145.8, 137.1,
129.1, 128.9, 128.5, 125.6, 123.3, 120.5, 104.0, 33.1, 11.5.
bis(5-Hydroxy-3-methyl-1-phenyl-pyrazolyl)-(1-naphthyl)methane
(6k): Orangish-white crystals; IR (KBr) (ῡmax, cm–1): 3419, 3062,
2922, 1608, 1542, 1497, 1402, 1370, 1132, 829, 783, 756, 689; 1H NMR
(300.13 MHz) δ (ppm): 13.15 (s, 1H, OH), 12.19 (s, 1H, OH), 8.10–8.00
(m, 1H, aromatic CH), 7.92 (d, J=6.6 Hz, 1H, aromatic CH), 7.80
(d, J=7.2 Hz, 1H, aromatic CH), 7.62–7.75 (m, 5H, aromatic CH),
7.34–7.57 (m, 7H, aromatic CH), 7.15–7.25 (m, 2H, aromatic CH),
5.61 (s, 1H, CH), 2.25 (s, 6H, CH3); 13C NMR (76.46 MHz) δ (ppm):
146.0, 144.1, 140.6, 137.3, 136.7, 133.6, 130.7, 128.8, 128.7, 127.0, 125.9,
125.7, 125.3, 125.2, 123.5, 119.9, 105.6, 30.9, 11.9, 11.8; Anal. calcd for
C31H26N4O2: C, 76.52; H, 5.39; N, 11.51; found: C, 76.57; H, 5.33; N,
11.48%.
bis(5-Hydroxy-3-methyl-1-phenyl-pyrazolyl)-(3-ethoxy-4-
hydroxyphenyl)methane (6c): Dark brown crystals; IR (KBr) (ῡmax
,
cm–1): 3420, 3219, 2985, 2927, 1596, 1498, 1400, 1275, 1214, 1126,
1043, 811, 753, 691. 1H NMR (300.13 MHz) δ (ppm): 13.97 (s, 1H, OH),
12.36 (s, 1H, OH), 8.67 (s, 1H, OH), 7.69 (d, J=8.1 Hz, 4H, aromatic
CH), 7.42 (t, J=7.8 Hz, 4H, aromatic CH), 7.22 (t, J=7.2 Hz, 2H,
aromatic CH), 6.82 (s, 1H, CH), 6.66 (s, 2H, CH) 4.82 (s, 1H, CH),
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3
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bis(5-Hydroxy-3-methyl-1-phenyl-pyrazolyl)-(4-biphenyl)methane
(6l): Orangish white crystals; IR (KBr) (ῡmax, cm–1): 3444, 3026, 2922,
4
5
1
1599, 1580, 1499, 1405, 1294, 818, 751, 692; H NMR (300.13 MHz)
δ (ppm): 14.05 (s, 1H, OH), 12.48 (s, 1H, OH), 7.74 (d, J=7.8 Hz, 4H,
aromatic CH), 7.55–7.62 (m, 4H, aromatic CH), 7.46–7.34 (m, 9H,
aromatic CH), 7.23 (t, J=7.2 Hz, 2H, aromatic CH), 5.01 (s, 1H, CH),
2.35 (s, 6H, 2CH3); 13C NMR (76.46 MHz) δ (ppm): 146.3, 141.5, 140.0,
137.9, 137.4, 137.3, 128.9, 128.8, 127.8, 127.1, 126.5, 125.5, 120.5,
104.9, 104.6, 32.8, 11.6; Anal. calcd for C33H28N4O2: C, 77.32; H, 5.51;
N, 10.93; found: C, 77.38; H, 5.43; N, 10.84%.
bis(5-Hydroxy-3-methyl-1-phenyl-pyrazolyl)-(3-indolyl)methane
(6m): Yellow crystals; m.p. 242–244 °C; IR (KBr) (ῡmax, cm–1): 3470,
3042, 2920, 1618, 1540, 1488, 1405, 1370, 1136, 830, 786, 755, 689;
1H NMR (400.13 MHz) δ (ppm): 12.65 (s, 2H, OH), 9.85 (s, 1H, NH),
8.13–8.11 (m, 2H, aromatic CH), 8.06 (s, 1H, aromatic CH), 8.05–8.01
(m, 2H, aromatic CH), 7.60–7.58 (m, 1H, aromatic CH), 7.44–7.40 (m,
4H, aromatic CH), 7.32–7.29 (m, 4H, aromatic CH), 7.17–7.13 (m, 1H,
aromatic CH), 7.15–7.25 (m, 2H, aromatic CH), 3.49 (s, 1H, CH), 2.39
(s, 6H, 2CH3); 13C NMR (100.62 MHz) δ (ppm): 162.7, 150.8, 138.9,
138.2, 136.9, 136.4, 128.6, 128.1, 123.8, 123.4, 122.0, 118.5, 118.0,
112.8, 112.2, 18.5, 12.9; Anal. calcd for C29H25N5O2: C, 73.25; H, 5.30;
N, 14.73; found: C, 73.31; H, 5.23; N, 14.66%.
6
7
8
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Received 21 August 2014; accepted 5 September 2014
Paper 1402843 doi: 10.3184/174751914X14114871789226
Published online: 17 October 2014
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