Synthesis of 4-aryl-1H-pyrazoles by Suzuki-Miyaura cross coupling reaction between 4-bromo-1H-1-trityl-pyrazole and arylboronic acids

Article abstract of DOI:10.3987/COM-10-11954

A general procedure for the synthesis of 4-aryl-1H-pyrazoles by the Suzuki-Miyaura cross coupling reaction between 4-bromo-1H-1-tritylpyrazole and commercially available arylboronic acids was developed. Using this procedure, a direct synthesis of 4-aryl-1H-pyrazoles possessing functional groups, such as hydroxyl, nitro, and amino groups, on the aryl ring was realized. Those molecules could not be prepared by our previous synthesis of 4-aryl-1H-pyrazoles via the Kumada cross coupling reaction.

Full text of DOI:10.3987/COM-10-11954

HETEROCYCLES, Vol. 81, No. 6, 2010
1509
HETEROCYCLES, Vol. 81, No. 6, 2010, pp. 1509 - 1516. © The Japan Institute of Heterocyclic Chemistry
Received, 26th March, 2010, Accepted, 22nd April, 2010, Published online, 23rd April, 2010
DOI: 10.3987/COM-10-11954
SYNTHESIS OF 4-ARYL-1H-PYRAZOLES BY SUZUKI-MIYAURA
CROSS COUPLING REACTION BETWEEN 4-BROMO-1H-1-TRITYL-
PYRAZOLE AND ARYLBORONIC ACIDS
Hayato Ichikawa,1,2 Miho Nishioka,1 Masao Arimoto,1 and Yoshihide
Usami1*
1Osaka University of Pharmaceutical Sciences, 4-20-1 Nasahara, Takatsuki
569-1094, Osaka, Japan; E-mail: usami@gly.oups.ac.jp
2College of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino,
Chiba 275-8575, Japan (present address)
Abstract – A general procedure for the synthesis of 4-aryl-1H-pyrazoles by the
Suzuki-Miyaura cross coupling reaction between 4-bromo-1H-1-tritylpyrazole
and commercially available arylboronic acids was developed. Using this
procedure, a direct synthesis of 4-aryl-1H-pyrazoles possessing functional groups,
such as hydroxyl, nitro, and amino groups, on the aryl ring was realized. Those
molecules could not be prepared by our previous synthesis of 4-aryl-1H-pyrazoles
via the Kumada cross coupling reaction.
The exploration of new methods for the synthesis of heterocyclic compounds is important work for
synthetic organic chemists as most drugs or bioactive compounds possess a heterocyclic part in their
molecules. Pyrazoles are one of the most important heterocyclic compounds.1,2 There are a lot of
well-known pyrazole-containing drugs, including celecoxib,3,4 rimonobant,5 and 4-methylpyrazole
(fomepizole).6,7 Extensive studies of the synthesis of substituted or functionalized pyrazoles have been
carried out.8,9 However, most of them deal with the construction of a pyrazole ring by the 2+3
cycloaddition of substituted parts.
The discovery of a novel binuclear platinum (II) complex bearing 4-methylpyrazole as the ligand (4mpz),
which exhibits enhanced antitumor activity in cisplatin-resistant L1210/R cell lines,10 has fueled our
interest in the direct functionalization of pyrazoles at C-4. Pardo and co-workers reported the synthesis of

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HETEROCYCLES, Vol. 81, No. 6, 2010
4-aryl-1H-pyrazoles via the Stille coupling reaction, as illustrated in Scheme 1 (eq. 1).11 In that procedure,
toxic tributyltin chloride was required for the preparation of cross-coupling substrates and the chemical
yields were not satisfactory. Another example is the Suzuki coupling (eq. 2) reported by Fu and
co-workers.12 In this case, relatively expensive tricyclopentylpalladium was required as an additive for the
reaction and 1-benzyl-4-phenyl-1H-pyrazole (1a’) was obtained in 64% yield. This approach is closely
related to a series of Harrity’s synthetic studies on functionalized pyrazoles.13-16 Meanwhile, we have
developed a method for the synthesis of 4-arylpyrazoles via the Kumada coupling with only 0.2 mol%
palladium catalyst, as shown in eq. 3.17 This procedure is quite efficient in that
4-phenyl-1H-1-tritylpyrazole (1a) was synthesized in 99% yield, but has a drawback in that it is difficult
to prepare Grignard reagents at the same time. Then, we attempted to perform an alternative procedure
where we synthesized 4-aryl-1H-pyrazoles under milder conditions. Herein we describe a simple method
for the synthesis of 4-aryl-1H-pyrazoles via the Suzuki-Miyaura coupling reaction18 between
4-bromo-1H-1-tritylpyrazole and commercially available arylboronic acids.
1. t-BuLi, THF, Et2O,78oC
Br
Ar
SnBu3
N NTr
Br
2. Bu3SnCl
83%
TrCl, pyr
63%
ArX
eq.111
090%
N NH
N NTr
N NTr
1
B(OH)2
N NBn
Ph
Pd2(dba)3 (1.0%)
PCy3 (2.4%), K3PO4
dioxane-H2O, 100oC, 64%
eq.212
Cl
+
N NBn
1a'
PdCl2(dppf) (0.2%),
THF, reflux
Ar
Br
eq. 317
PhMgBr
+
99%
N NTr
N NTr
1a
Scheme 1. Known methods for direct arylation at C-4 of 1H-pyrazoles
Commercially available pyrazole was converted into 4-bromo-1H-1-tritylpyrazole (3) via 2 using a
known method.11,17 Compound 3 was reacted with several commercially available boronic acids in the
presence of 5 mol% palladium acetate, triphenylphosphine, and potassium carbonate under reflux in THF.
The results of the Suzuki-Miyaura cross coupling reaction are summarized in Table 1. The reaction with
phenylboronic acid resulted in 92% yield of 1a (entry 1), which is slightly lower than that obtained with

HETEROCYCLES, Vol. 81, No. 6, 2010
1511
ArB(OH)2 1.2 eq.,
Pd(OAc)2 5 mol%
PPh3, K2CO3
Br
Br
Ar
TrCl, Et3N
60%
Br2, CH2Cl2
85%
THF, reflux, 12h
(Table 1)
N NTr
N NH
N NTr
N NH
2
3
1a-i
Scheme 2. Synthesis of 4-aryl-1H-pyrazoles via Suzuki-Miyaura coupling
Table 1. Synthesis of 1 via Suzuki coupling between 3 and arylboronic acids
entry
1
reagent
product
1a : Ar = phenyl
yield (%)
92
phenylboronic acid
2
3
4
5
4-hydroxyphenylboronic acid
3-hydroxyphenylboronic acid
2-methylphenylboronic acid
3-nitrophenylboronic acid
3-aminophenylboronic acid
3-aminophenylboronic acid
2-thiopheneboronic acid
2-thiopheneboronic acid
3-thiopheneboronic acid
2-furylboronic acid
1b : Ar = 4-hydroxyphenyl
1c : Ar = 3-hydroxyphenyl
1d : Ar = 2-methylphenyl
1e: Ar = 3-nitrophenyl
1f : Ar = 3-aminophenyl
1f: Ar = 3-aminophenyl
1g : Ar = thiophen-2-yl
1g : Ar = thiophen-2-yl
1h : Ar = thiophen-3-yl
1i: Ar = fur-2-yl
63
53
58
35
41
36
57
99
6
7a
8
9b
10
11
50
47
a. 10 mol% of palladium catalyst were added.
b. 2.4 Equivalent s of boronic acid were added and the reaction time was 8.5 hr.
our previous method10 but better than that obtained with the reverse type of the Suzuki-Miyaura coupling
reaction.9 Using this procedure, molecules that could not be obtained by the previous method were
prepared. The reaction with arylboronic acids bearing a hydroxyl or an amino group proceeded, as seen in
entries 2, 3, 6, and 7. Increasing the amount of the palladium catalyst was not effective, as shown by the
results of entries 6 and 7. Although 4-(3-nitrophenyl)-1H-1tritylpyrazole (1e) could not be synthesized in
our previous work,17 it was synthesized in this study albeit the low yield (entry 5). Other
heteroarylboronic acids coupled with pyrazoles, as shown in entries 8-11. The Suzuki-Miyaura coupling
reaction between B-(1H-1-tritylpyrazol-4-yl)boronic acid (4), which was prepared along Scheme 3, and
2-bromothiophene gave 4-(thiophen-2-yl)-1H-1-tritylpyrazole (1g) as shown in Scheme 4 in lower yield
compared to entries 8, 9 in Table 1 under similar reaction conditions. Protection with trityl group at N-1 is
necessary in this study since the desired coupled product could not been obtained under the same reaction
condition when 2 was used as a coupling partner to phenylboronic acid. This result agreed to Fu’s work12
that the Suzuki-Miyaura coupling reaction between B-(1H-pyrazol-4-yl)boronic acid and chlorobenzene
gave the desired product in lower yield (8%) than the corresponding reaction (64%) shown in eq. 2
(Scheme 1).10

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1) n-BuLi, Et2O, 78oC, 30 min
2) B(OMe)3
B(OH)2
Br
3) 1N HCl aq.
59%
N NTr
N NTr
4
3
Scheme 3. Synthesis of B-(1H-1-tritylpyrazol-4-yl)boronic acid 4
S
N
N
S
Pd(OAc)2 (5 mol%), PPh3
THF / H2O, K2CO3, 14.5 h
N
N
Br
+
4
+
Tr
Tr
5 (52%)
1.2 eq.
1g (31%)
Scheme 4. Alternative Suzuki-Miyaura cross coupling reaction to furnish 1g
In summary, the Suzuki-Miyaura coupling reaction between 4-bromo-1H-1-tritylpyrazole and arylboronic
acids was found to afford various kinds of 4-aryl-1H-pyrazoles. A direct preparation of
4-aryl-1H-pyrazoles with hydroxyl, nitro, or amino groups on the aryl ring resulted in relatively low
yields. Nevertheless, this procedure is quite simple and practical as commercially available arylboronic
acids can be used.
EXPERIMENTAL
IR spectra were obtained with a JEOL FT/IR-680 Plus spectrometer. HRMS was determined with a JEOL
JMS-700 (2) mass spectrometer. NMR spectra were recorded at 27 oC on Varian UNITY INOVA-500 and
Mercury-300 spectrometers in CDCl3 with tetramethylsilane (TMS) as internal standard. Melting points
were determined on a Yanagimoto micromelting point apparatus and are uncorrected. Liquid column
chromatography was conducted over silica gel (Nacalai, silica gel 60, 70230 mesh or 230400 mesh).
Analytical TLC was performed on precoated Merck glass plates (silica gel 60 F254) and compounds were
detected by dipping the plates in an ethanol solution of phosphomolybdic acid, followed by heating. Dry
THF was distilled over sodium benzophenone ketyl under nitrogen atmosphere.
4-Bromo-1H-pyrazole (2)
To a solution of pyrazole (6.8 g, 100 mmol) in CH2Cl2 (120 mL) was added bromine (5.1 mL, 100 mmol)
o
at 0 C with stirring. After stirring overnight, the reaction mixture was treated with NaHSO4 aq. and
extracted with CH2Cl2. The organic layer was washed with brine, dried over MgSO4, and filtered. The
solvent as removed under reduced pressure to give a crude residue, which was purified by
1
recrystallization to afford 2 (14.7 g, 100%). 2: Colorless crystals; H-NMR (300 MHz, CDCl3): δ 6.97

HETEROCYCLES, Vol. 81, No. 6, 2010
1513
(1H, s, pyrazole-H), 7.60 (1H, s, pyrazole-H).
4-Bromo-1H-1-tritylpyrazole (3)
To a solution of 2 (5.1 g, 35 mmol) in dry Et2O (350 mL) with Et3N (5.9 mL, 42 mmol) was added trityl
chloride (12g, 42 mmol) under nitrogen atmosphere. After stirring for 1 hr at rt, the reaction mixture was
quenched by the addition of water. The organic layer was washed with brine, dried over MgSO4, and
filtered. The solvent was removed under reduced pressure to give a crude residue, which was purified by
recrystallization to afford 3 (12 g, 85%). 3: Colorless crystals; 1H-NMR (300 MHz, CDCl3): δ 7.087.36
(15 H, m, Ph-H), 7.37 (1H, s, pyrazole-H), 7.61 (1H, s, pyrazole-H).
Synthesis of 4-aryl-1H-1-tritylpyrazoles (Table 1, Scheme 4)
General procedure: To a solution of 3 (389 mg, 1.0 mmol) in THF (5.0 mL) were added phenylboronic
acid (293 mg, 2.4 mmol), Pd(OAc)2 (225 mg, 0.05 mmol), triphenylphosphine (20 mg, 0.22 mmol), and
K2CO3 (415 mg, 3.0 mmol). The reaction mixture was stirred under reflux overnight. The reaction
mixture was quenched by the addition of dil. HCl aq. and extracted with EtOAc. The organic layer was
washed with brine, dried over MgSO4, and filtered. The solvent was removed under reduced pressure to
give a crude residue, which was purified by column chromatography (hexane:EtOAc = 20:1) to afford 1a
(360 mg, 92%).
4-Phenyl-1H-1-tritylpyrazole9 (1a): 1H-NMR (300 MHz, CDCl3): δ 7.94 (1H, s, pyrazole-H), 7.61 (1H,
s, pyrazole-H), 7.14–7.45 (20H, m, Tr-H, Ph-H).
4-(4-Hydroxyphenyl)-1H-1-tritylpyrazole8 (1b): 1H-NMR (300 MHz, CDCl3): δ 7.84 (1H, d, J = 0.8 Hz,
pyrazole-H), 7.52 (1H, d, J = 0.8 Hz, pyrazole-H), 7.32–7.17 (15H, m, Tr-H), 7.25 (2H, d, J = 8.7 Hz,
13
Ar-H), 6.74 (2H, d, J = 8.7 Hz, Ar-H), 5.04 (1H, brs, OH); C-NMR (75 MHz, CDCl3): δ 153.7, 142.6,
136.5, 129.8, 128.1, 127.4, 126.5, 124.9, 121.0, 115.4, 78.7 (lack of a carbon signal corresponding to the
trityl-quaternary carbon, probably because of overlap with one of the chloroform signals); HRMS m/z
calcd for C28H22N2O (M+) 402.1732, found 402.1731.
4-(3-Hydroxyphenyl)-1H-1-tritylpyrazole (1c): Colorless crystals; mp 220222 °C; IR (KBr) max 3272
1
(OH), 1610 (C=C), 1598 (C=C) cm-1; H-NMR (300 MHz, CDCl3): δ 7.89 (1H, d, J = 0.7 Hz,
pyrazole-H), 7.59 (1H, d, J = 0.7 Hz, pyrazole-H), 7.33–7.28 (9H, m, Tr-H), 7.21–7.16 (6H, m, Tr-H),
7.15 (1H, t, J = 8.8 Hz, Ar-H), 6.98 (1H, dt, J = 7.7, 1.5 Hz, Ar-H), 6.84 (1H, t, J = 2.4 Hz, Ar-H), 6.60
13
(1H, ddd, J = 8.1, 2.6, 0.9 Hz, Ar-H), 5.08 (1H, brs, OH); C-NMR (75 MHz, CDCl3): δ 155.4, 142.5,
136.8, 129.7, 129.6, 128.9, 127.4, 120.9, 117.6, 113.1, 112.1, 78.8 (lack of a carbon signal probably
because of overlap with one of the chloroform signals); HRMS m/z calcd for C28H22N2O (M+) 402.1732,
found 402.1729.

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4-(2-Tolyl)-1H-1-tritylpyrazole (1d): Colorless crystals; mp 100103 °C; IR (KBr) max 1602 (C=C),
1563 (C=C) cm1; 1H-NMR (300 MHz, CDCl3): δ 7.81 (1H, d, J = 0.7 Hz, pyrazole-H), 7.46 (1H, d, J =
0.7 Hz, pyrazole-H), 7.34–7.28 (10H, m, Tr-H, Tol-H), 7.25–7.1 (9H, m, Tr-H, Tol-H), 2.38 (3H, brs,
CH3); 13C-NMR (75 MHz, CDCl3): δ 143.2, 139.2, 135.2, 132.2, 131.3, 130.6, 130.2, 129.0, 127.9, 127.7,
126.7, 125.9, 120.7, 78.6, 21.2; MS (EI) m/z 400 (M+), 243 (100%); HRMS m/z calcd for C29H24N2 (M+)
400.1940, found 400.1934.
4-(3-Nitrophenyl)-1H-1-tritylpyrazole (1e): Colorless crystals; mp 202204 °C; IR (KBr) max 1570
(C=C), 1528 (ArNO2) cm1; 1H-NMR (300 MHz, CDCl3): δ 8.24 (1H, t, J = 2.2 Hz, 2’-H), 8.04 (1H, ddd,
J = 8.0, 2.4, 1.1 Hz, 3’- or 6’-H), 8.00 (1H, d, J = 0.6 Hz, pyrazole-H), 7.75 (1H, br d, J = 8.0 Hz, 3’- or
6’-H), 7.74 (1H, d, J = 0.6 Hz, pyrazole-H), 7.49 (1H, t, J = 8.0 Hz, 2’-H), 7.38–7.32 (10H, m, Tr-H),
7.24–7.14 (5H, m, Tr-H); 13C-NMR (75 MHz, CDCl3): δ 148.7, 142.7, 137.3, 134.5, 131.3, 130.1, 129.7,
129.6, 128.0, 127.9, 120.9, 120.1, 119.6, 79.2; MS (EI) m/z 431 (M+), 243 (100%); HRMS m/z calcd for
C28H21N3O2 (M+) 431.1634, found 431.1638.
4-(3-Aminophenyl)-1H-1-tritylpyrazole (1f): Colorless crystals; mp 155157 °C; IR (KBr) max 3380
1
(N-H), 1617 (C=C), 1588 (C=C) cm1; H-NMR (300 MHz, CDCl3): δ 7.89 (1H, d, J = 0.7 Hz,
pyrazole-H), 7.57 (1H, d, J = 0.7 Hz, pyrazole-H), 7.38–7.31 (9H, m, Tr-H), 7.27–7.19 (6H, m, Tr-H),
7.09 (1H, t, J = 7.7 Hz, Ar-5H), 6.82 (1H,br d, J = 7.7, Hz, Ar-4H), 6.74 (1H,br s, Ar-2H), 6.52 (1H,dd, J
13
= 7.7, 2.4 Hz, Ar-6H), 3.64 (2H, brs, -NH2); C-NMR (75 MHz, CDCl3): δ 146.8, 143.1, 137.3, 133.5,
130.2, 129.7, 129.2, 127.8, 121.7, 116.0, 113.3, 112.1; MS (EI) m/z 401 (M+), 243 (100%); HRMS m/z
calcd for C28H23N3 (M+) 401.1892, found 401.1885.
1
4-(Thiophen-2-yl)-1H-1-tritylpyrazole10 (1g): H-NMR (200 MHz, CDCl3): δ 7.84 (1H, d, J = 0.8 Hz,
pyrazole-H), 7.55 (1H, d, J = 0.8 Hz, pyrazole-H), 7.33–7.28 (9H, m, Tr-H), 7.23–7.15 (6H, m, Tr-H),
7.08 (1H, dd, J = 5.0, 1.3 Hz, thienyl-5H), 6.99 (1H, dd, J = 3.6, 1.3 Hz, thienyl-3H), 6.94 (1H, dd, J =
5.0, 3.6 Hz, thienyl-4H).
4-(Thiophen-3-yl)-1H-1-tritylpyrazole (1h): Colorless crystals; mp 203205 °C; IR (KBr) max 1595
(C=C) cm1; 1H-NMR (300 MHz, CDCl3): δ 7.88 (1H, d, J = 0.7 Hz, pyrazole-H), 7.56 (1H, d, J = 0.7 Hz,
pyrazole-H), 7.36–7.12 (9H, m, Ar-H); 13C-NMR (75 MHz, CDCl3): δ 143.1, 137.5, 130.1, 129.1, 127.9,
127.8, 127.8, 127.2, 126.1, 126.0, 118.2, 78.7; MS (EI) m/z 392 (M+), 243 (100%); HRMS m/z calcd for
C26H20N2S (M+) 392.1347, found 392.1345.
4-(Fur-2-yl)-1H-1-tritylpyrazole (1i): Colorless crystals; mp 114118 °C; IR (KBr) max 1610 (C=C),
1
1598 (C=C) cm1; H-NMR (200 MHz, CDCl3): δ 7.87 (1H, s, pyrazole-H), 7.61 (1H, s, pyrazole-H),
7.38–7.13 (16H, m, Tr-H, furyl-H), 6.40 (1H, dd, J = 3.6, 1.9 Hz, furyl-H), 6.31 (1H, d, J = 3.6 Hz,
13
furyl-H); C-NMR (75 MHz, CDCl3): δ 146.8, 142.9, 140.6, 136.5, 130.1, 127.9, 127.8, 127.7, 127.2,

HETEROCYCLES, Vol. 81, No. 6, 2010
1515
111.2, 103.7 (lack of a carbon signal probably because of overlap with one of the chloroform signals);
MS (EI) m/z 376 (M+), 243 (100%); HRMS m/z calcd for C26H20N2O (M+) 376.1576, found 376.1579.
B-(1H-1-tritylpyrazol-4-yl)boronic acid (4) (Scheme 2)
To a suspension of 3 (19.6 g, 50 mmol) in Et2O (250 mL) was added 1.6 M n-BuLi in hexane (35 mL, 55
o
mmol) at 78 C under nitrogen atmosphere. After stirring for 30min, B(OMe)3 (6.2 mL, 56mmol) was
added. The reaction mixture was treated with 1N HCl aq., then extracted with EtOAc. The organic layer
was separated, dried over MgSO4, filtered and evaporated to give a crude residue, which was purified by
column chromatography (eluent; MeOH:CH2Cl2= 100:1) to afford 4 (10.5g, 59%). 4: Colorless crystals;
mp 8587 °C; IR (KBr) max 3308 (OH), 1597 (C=C), 1536 (C=C) cm1; 1H-NMR (200 MHz, CDCl3): δ
7.92 (1H, s, pyrazole-H), 7.77 (1H, s, pyrazole-H), 7.34–7.28 (9H, m, Tr-H), 7.18–7.12 (6H, m, Tr-H);
13C-NMR (75 MHz, CDCl3): δ 145.1, 142.9, 138.7, 130.1, 130.0, 127.8, 127.6, 78.8; MS (FAB) m/z 310
([MBO2H]+), 309 ([MB(OH)2]+), 243 (100%); HRFABMS m/z calcd for C22H18N2 ([MBO2H]+) 310.
1470, found 310.1475, m/z calcd for C22H17N2 ([MB(OH)2]+) 309.1392. 1470, found 309.1389.
ACKNOWLEDGEMENTS
We are grateful to Ms. M. Fujitake of this University for MS measurements. This work was supported in
part by a Grant-in-Aid for “High-Tech Research Center” Project for Private Universities: matching fund
subsidy from MEXT (Ministry of Education, Culture, Sports, Science, and Technology), 20062009,
Japan.
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