Palladium-catalyzed C-N bond formation: synthesis of 1-aryl-1H-pyrazoles from β-bromovinyl aldehydes and arylhydrazines

Article abstract of DOI:10.1016/j.tet.2006.04.031

Cyclic and acyclic β-bromovinyl aldehydes are cyclized with an array of arylhydrazines in toluene at 125 °C in the presence of a palladium catalyst and a phosphorus chelating ligand together with NaO^tBu to give 1-aryl-1H-pyrazoles in moderate t

Full text of DOI:10.1016/j.tet.2006.04.031

Tetrahedron 62 (2006) 6388–6391
Palladium-catalyzed C–N bond formation: synthesis
of 1-aryl-1H-pyrazoles from b-bromovinyl
aldehydes and arylhydrazines
b
Chan Sik Choa, and Daksha B. Patel
*
aResearch Institute of Industrial Technology, Kyungpook National University, Daegu 702-701, South Korea
bDepartment of Applied Chemistry, Kyungpook National University, Daegu 702-701, South Korea
Received 10 March 2006; revised 4 April 2006; accepted 7 April 2006
Available online 4 May 2006
Abstract—Cyclic and acyclic b-bromovinyl aldehydes are cyclized with an array of arylhydrazines in toluene at 125 ꢀC in the presence of
a palladium catalyst and a phosphorus chelating ligand together with NaOtBu to give 1-aryl-1H-pyrazoles in moderate to good yields.
Ó 2006 Elsevier Ltd. All rights reserved.
CHO
Br
1. Introduction
PBr3/DMF/CHCl3
[Pd] H2NNH Ar
N
Palladium-catalyzed sp2-carbon–nitrogen bond forming
reaction by the cross-coupling of aryl halides (or triflate)
with primary or secondary amines has been recognized as
an attractive tool in synthetic organic chemistry.1 In connec-
tion with this report, it is known that several N-heterocycles
can be synthesized by palladium-catalyzed coupling protocol
of aryl halides with hydrazones.2–9 Buchwald et al. have
reported on the synthesis of indoles by palladium-catalyzed
coupling of aryl bromides with benzophenone hydrazone
followed by the treatment of ketones under TsOH$H2O.4
Song and Yee have reported a palladium-catalyzed intramo-
lecular amination of N-aryl-N0-(o-bromobenzyl)hydrazines
and N-aryl-N-(o-bromobenzyl)hydrazines leading to 1-aryl-
1H-indazoles and 2-aryl-2H-indazoles, respectively.5,6 It is
also reported by Haddad et al. that indazoles can be synthe-
sized by the palladium-catalyzed coupling of aryl halides
with benzophenone hydrazone and subsequent reaction with
1,3-bifunctional substrates.7,8 Such an intrinsic palladium-
catalyzed C–N bond formation was also applied to the syn-
thesis of 1-aryl-1H-indazoles by us through the coupling
between 2-bromobenzaldehydes and arylhydrazines.9,10 The
indazole protocol led us to extend to thereactionwith b-bromo-
vinyl aldehydes, which are readily prepared from ketones via
the bromo analogue of Vilsmeier reaction (Scheme 1).11
Herein, this report describes a palladium-catalyzed cycliza-
tion of b-bromovinyl aldehydes with arylhydrazines leading
to pyrazoles via an intrinsic C–N bond formation.12,13
O
N
Ar
Scheme 1.
2. Results and discussion
Based on our recent report on palladium-catalyzed synthesis
of 1-aryl-1H-indazoles from 2-bromobenzaldehydes and
arylhydrazines,9 the results of several attempted cyclizations
of 2-bromocyclohex-1-enecarbaldehyde (1a) with phenyl-
hydrazine (2a) are listed in Table 1. Treatment of equimolar
amount of 1a and 2a in toluene under the catalytic system of
Pd(OAc)2 combined with 1,3-bis(diphenylphosphino)pro-
pane (dppp) along with NaOtBu at 100 ꢀC for 24 h afforded
1-phenyl-4,5,6,7-tetrahydro-1H-indazole (3a) in 34% yield
(entry 1). This result indicates that the present reaction,
nevertheless the use of more amount of a palladium catalyst,
is slower than that of 2-bromobenzaldehyde with 2a leading
to 1-phenyl-1H-indazole.9 Higher reaction temperature re-
sulted in an elevated yield of 3a (entry 2). Similar catalytic
activity was observed with PdCl2 combined with dppp (entry
3). Among the catalytic systems of Pd(OAc)2 combined with
mono- and bidentate phosphorus ligands such as PPh3, 1,10-
bis(diphenylphosphino)ferrocene (dppf), and 1,10-bis(di-i-
propylphosphino)ferrocene (dipf) examined dppf revealed
to be the ligand of choice and monodentate PPh3 did not
work at all for the present reaction (entries 4–6). On the other
hand, when the reaction was carried out under a dilute
concentration, 3a was more effectively formed (entry 7).
Keywords: b-Bromovinyl aldehydes; C–N bond formation; Arylhydrazines;
Palladium catalyst; Pyrazoles.
* Corresponding author. Tel.: +82 53 950 7318; fax: +82 53 950 6594;
e-mail: cscho@knu.ac.kr
˚
However, the addition of molecular sieves, 4 A (0.2 g) under
the condition of entry 7 did not give any significant change in
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.04.031

C. S. Cho, D. B. Patel / Tetrahedron 62 (2006) 6388–6391
Table 1. Optimization of conditions for the reaction of 1a with 2a Table 2. Palladium-catalyzed synthesis of 1-aryl-1H-pyrazoles
6389
CHO
Br
b-Bromovinyl
aldehyde 1
Arylhydrazine 2
Product 3
Yield
(%)
cat. [Pd]
H2N NH Ph
N
+
N
Ph
t
NaO Bu
CHO
1a
2a
3a
N
H2N–NH–Ar
N
Entry Ligands
Solvents (mL) Temperature (ꢀC) Yield (%)
Br
Ar
3a
1a
2a Ar¼Ph
79
42
56
68
54
45
56
1
dppp
dppp
dppp
dppf
dipf
Toluene (5)
Toluene (5)
Toluene (5)
Toluene (5)
Toluene (5)
Toluene (5)
Toluene (10)
100
125
125
125
125
125
125
125
125
125
34
59
57
67
60
1
79
77
62
64
2b Ar¼2-MeC6H4
2c Ar¼3-MeC6H4
2d Ar¼4-MeC6H4
2e Ar¼4-MeOC6H4
2f Ar¼2-CF3C6H4
2g Ar¼4-NO2C6H4
3b
3c
3d
3e
3f
2
3a
4
5
6
7
8
9
10
PPh3
dppf
3g
(S)-(ꢁ)-BINAP Toluene (10)
CHO
dppf
dppf
THF (10)
MeCN (10)
N
2a
2a
1b
N
3h 76
Br
Ph
Reaction conditions: 1a (1 mmol), 2a (1 mmol), Pd(OAc)2 (0.05 mmol),
NaOtBu (2 mmol), bidentate ligand (0.075 mmol), monodentate ligand
(0.15 mmol), for 24 h.
CHO
1c
N
a
20
PdCl2 was used in place of Pd(OAc)2.
N
3i
Br
Ph
the yield of 3a (78%). The catalytic system using (S)-(ꢁ)-
BINAP was revealed to be as effective as that using dppp
(entry 8). From the solvents examined THF and MeCN could
be alternatively used, but the yield of 3a was lower than that
when toluene was used (entries 9 an 10). As a result, the best
result in terms of both yield and complete conversion of 1a
was accomplished by the standard set of reaction conditions
shown in entry 7 of Table 1.
CHO
1d
N
2a
2a
3j
N
65
77
Br
Ph
CHO
1e
N
N
3k
Br
Ph
CHO
Having established reaction conditions, various b-bromo-
vinyl aldehydes 1 were subjected to react with various aryl-
hydrazines 2 in order to investigate the reaction scope and
several representative results are summarized in Table 2.
Cyclic b-bromovinyl aldehyde 1a was readily cyclized
with an array of arylhydrazines (2a–g) having electron do-
nating and withdrawing substituents to give the correspond-
ing 1-aryl-1H-pyrazoles (3a–g) in the range of 42–79%
yields. The product yield was not significantly affected by
the electronic nature of the substituent on the aromatic
ring of 2a–g, whereas the position of that had some rele-
vance to the pyrazole yield. With arylhydrazines having
ortho-substituent, the pyrazole yield was generally lower
than that when arylhydrazines having meta- and para-
substituents were used. 2-Bromo-5-methylcyclohex-1-
enecarbaldehyde (1b) reacts similarly with 2a to afford
5-methyl-1-phenyl-4,5,6,7-tetrahydro-1H-indazole (3h) in
76% yield. From the reactions between several cyclic b-
bromovinyl aldehydes (1d and 1e) and 2a, the corresponding
pyrazoles (3j and 3k) were produced in similar yields.
However, lower reaction yield was observed with five-
membered cyclic b-bromovinyl aldehyde 1c. To test the
effect of the position of formyl group and bromide on cyclic
b-bromovinyl aldehydes, 1f and 1g were employed.
However, the cyclization took place similarly irrespective
of the position. Next, we carried out the reaction with trans/
cis mixture (trans/cis¼ca. 1/10 on GLC) of acyclic b-bromo-
vinyl aldehydes (1h–k), which could not be separated by
chromatography.14 Similar treatment of acyclic b-bromo-
vinyl aldehydes (1h and 1i), which have no substituent at
a-position with 2a under the employed conditions afforded
the corresponding 1,5-disubstituted pyrazoles (3n and 3o).
Acyclic b-bromovinyl aldehydes such as 3-bromo-2-
Br
1f
3l
54
2a
2a
N
N
Ph
Br
N
N
CHO
1g
3m 46
CHO
Br
N
Ar
R
N
Ph
Ar
1h Ar=Ph
2a
2a
48
48
3n
1i Ar¼2-naphthyl
3o
CHO
N
N
R
Br
1j R=Ph
Ph
3p
3q
2a
2a
56
40
1k R¼Et
Reaction conditions: 1 (1 mmol), 2 (1 mmol), Pd(OAc)2 (5 mol %), dppf
(7.5 mol %), NaOtBu (2 mmol), toluene (10 mL), 125 ꢀC, for 24 h.
methyl-3-phenylpropenal (1j) and 3-bromo-2-methylpent-
2-enal (1k), which have a substituent at a-position were
also reacted with 2a to give 4-methyl-1,5-dimethyl-1H-
pyrazole (3p) and 5-ethyl-4-methyl-1-phenyl-1H-pyrazole
(3q) in 56 and 40% yields, respectively.
In summary, we have demonstrated that cyclic and acyclic
b-bromovinyl aldehydes are cyclized with an array of aryl-
hydrazines in the presence of a palladium catalyst and
a phosphorus chelating ligand to afford pyrazoles via an

6390
C. S. Cho, D. B. Patel / Tetrahedron 62 (2006) 6388–6391
intrinsic C–N bond protocol. The present reaction is a new
route for the synthesis of pyrazoles from ketones.
23.16, 23.55, 117.45, 122.99, 129.56, 136.43, 137.70,
138.11, 138.47; Anal. Calcd for C14H16N2: C 79.21; H
7.60; N 13.20; found: C 79.08; H 7.81; N 13.06.
3. Experimental
3.1. General
3.2.4. 1-(4-Methoxyphenyl)-4,5,6,7-tetrahydro-1H-inda-
zole (3e). Oil; H NMR (400 MHz, CDCl3) d 1.74–1.86
1
(m, 4H), 2.56–2.59 (m, 2H), 2.64–2.67 (m, 2H), 3.83 (s,
3H), 6.93–6.97 (m, 2H), 7.36–7.40 (m, 2H), 7.42 (s, 1H);
13C NMR (100 MHz, CDCl3) d 21.12, 23.26, 23.50, 23.69,
55.90, 114.54, 117.58, 125.10, 133.79, 138.56, 138.63,
158.73; Anal. Calcd for C14H16N2O: C 73.66; H 7.06; N
12.27; found: C 73.51; H 7.31; N 12.24.
1H and 13C NMR (400 and 100 MHz) spectra were recorded
on a Bruker Avance Digital 400 spectrometer using TMS as
an internal standard. Melting points were determined on
a Thomas–Hoover capillary melting point apparatus and
were uncorrected. The isolation of pure products was carried
out via thin layer (silica gel 60 GF254, Merck) chromato-
graphy. b-Bromovinyl aldehydes 1 were prepared by the
reported methods.11 Commercially available organic and
inorganic compounds were used without further purification
except for toluene, THF, and MeCN, which were distilled by
known methods before use.
3.2.5. 1-(2-Trifluoromethylphenyl)-4,5,6,7-tetrahydro-
1H-indazole (3f). Oil; 1H NMR (400 MHz, CDCl3)
d 1.72–1.81 (m, 4H), 2.33–2.36 (m, 2H), 2.57–2.59 (m,
2H), 7.34 (d, J¼7.5 Hz, 1H), 7.46 (s, 1H), 7.57 (t,
J¼7.5 Hz, 1H), 7.62–7.66 (m, 1H), 7.79–7.81 (m, 1H); 13C
NMR (100 MHz, CDCl3) d 20.92, 21.98, 23.09, 23.36,
116.73, 123.31 (q, J¼272.4 Hz), 127.72 (q, J¼4.8 Hz),
128.68 (q, J¼31.2 Hz), 129.65, 130.56, 132.87, 138.01 (q,
J¼1.9 Hz), 139.01, 140.82; Anal. Calcd for C14H13F3N2:
C 63.15; H 4.92; N 10.52; found: C 63.41; H 5.04; N 10.54.
3.2. Typical procedure for palladium-catalyzed synthe-
sis of pyrazoles from b-bromovinyl aldehydes and
arylhydrazines
A mixture of 2-bromocyclohex-1-enecarbaldehyde (1a)
(0.189 g, 1 mmol), phenylhydrazine (2a) (0.108 g,
1 mmol), Pd(OAc)2 (0.011 g, 0.05 mmol), dppf (0.042 g,
0.075 mmol), and NaOtBu (0.192 g, 2 mmol) in toluene
(10 mL) was placed in a 50 mL pressure vessel. After the
system was flushed with argon, the mixture was stirred at
125 ꢀC for 24 h. The reaction mixture was passed through
a short silica gel column (ethyl acetate) to eliminate inor-
ganic salts. Removal of the solvent left a crude mixture,
which was separated by thin layer chromatography (silica
gel, ethyl acetate/hexane¼1/10) to give 1-phenyl-4,5,6,7-
tetrahydro-1H-indazole (3a) (0.157 g, 79%). All new com-
pounds prepared by the above procedure were characterized
spectroscopically as shown below.
3.2.6. 1-Phenyl-1,4,5,6-tetrahydrocyclopentapyrazole
(3i). Oil; 1H NMR (400 MHz, CDCl3) d 2.57–2.69 (m,
4H), 2.97–3.01 (m, 2H), 7.20–7.24 (m, 1H), 7.38 (s, 1H),
7.39–7.43 (m, 2H), 7.62–7.65 (m, 2H); 13C NMR
(100 MHz, CDCl3) d 23.15, 26.85, 31.37, 119.41, 125.98,
129.64, 129.74, 135.20, 140.88, 148.90; Anal. Calcd for
C12H12N2: C 78.23; H 6.57; N 15.21; found: C 77.91; H
6.81; N 15.06.
3.2.7. 1-Phenyl-1,4,5,6,7,8-hexahydrocycloheptapyrazole
(3j). Solid (hexane/chloroform); mp 90 ꢀC; 1H NMR
(400 MHz, CDCl3) d 1.61–1.72 (m, 4H), 1.82–1.87 (m,
2H), 2.61–2.64 (m, 2H), 2.75–2.78 (m, 2H), 7.33–7.38 (m,
4H), 7.43–7.46 (m, 2H); 13C NMR (100 MHz, CDCl3)
d 26.05, 27.58, 27.63, 28.94, 32.23, 122.37, 125.86,
127.83, 129.30, 140.12, 140.41, 142.54; Anal. Calcd for
C14H16N2: C 79.21; H 7.60; N 13.20; found: C 79.22; H
7.72; N 12.99.
3.2.1. 1-(2-Methylphenyl)-4,5,6,7-tetrahydro-1H-inda-
zole (3b). Oil; H NMR (400 MHz, CDCl3) d 1.75–1.80
1
(m, 4H), 2.08 (s, 3H), 2.35–2.37 (m, 2H), 2.58–2.60
(m, 2H), 7.23–7.34 (m, 4H), 7.44 (s, 1H); 13C NMR
(100 MHz, CDCl3) d 17.44, 20.68, 21.90, 22.88, 23.14,
115.95, 126.29, 127.43, 128.71, 130.93, 135.97, 137.90,
138.74, 139.29; Anal. Calcd for C14H16N2: C 79.21; H
7.60; N 13.20; found: C 79.04; H 7.88; N 13.00.
3.2.8. 1-Phenyl-4,5,6,7,8,9-hexahydro-1H-cyclooctapyra-
zole (3k). Oil; 1H NMR (400 MHz, CDCl3) d 1.46–1.55 (m,
4H), 1.64–1.73 (m, 4H), 2.61–2.64 (m, 2H), 2.72–2.75 (m,
2H), 7.34–7.47 (m, 6H); 13C NMR (100 MHz, CDCl3)
d 22.90, 23.52, 25.42, 25.71, 28.87, 30.44, 119.71, 125.25,
127.54, 128.96, 139.59, 140.18, 140.36; Anal. Calcd for
C15H18N2: C 79.61; H 8.02; N 12.38; found: C 79.40; H
8.16; N 12.22.
3.2.2. 1-(3-Methylphenyl)-4,5,6,7-tetrahydro-1H-inda-
zole (3c). Oil; H NMR (400 MHz, CDCl3) d 1.74–1.82
1
(m, 4H), 2.39 (s, 3H), 2.56–2.59 (m, 2H), 2.70–2.72 (m,
2H), 7.10 (d, J¼7.5 Hz, 1H), 7.24–7.34 (m, 3H), 7.45 (s,
1H); 13C NMR (100 MHz, CDCl3) d 21.14, 21.80, 23.22,
23.57, 24.08, 118.00, 120.33, 124.25, 127.76, 129.10,
138.54, 139.03, 139.49, 140.43; Anal. Calcd for
C14H16N2: C 79.21; H 7.60; N 13.20; found: C 78.98; H
7.85; N 13.13.
3.2.9. 5-Naphthalen-2-yl-1-phenyl-1H-pyrazole (3o). Oil;
1H NMR (400 MHz, CDCl3) d 6.60 (d, J¼1.5 Hz, 1H),
7.22–7.34 (m, 6H), 7.45–7.50 (m, 2H), 7.70–7.80 (m, 5H);
13C NMR (100 MHz, CDCl3) d 108.62, 125.59 (ꢂ2),
126.75, 126.94, 127.05, 127.85, 128.11, 128.34, 128.46,
128.57, 129.36, 133.17, 133.50, 140.59, 140.83, 143.37;
Anal. Calcd for C19H14N2: C 84.42; H 5.22; N 10.36; found:
C 84.54; H 5.33; N 9.91.
3.2.3. 1-(4-Methylphenyl)-4,5,6,7-tetrahydro-1H-inda-
zole (3d). Oil; H NMR (400 MHz, CDCl3) d 1.74–1.82
1
(m, 4H), 2.37 (s, 3H), 2.56–2.59 (m, 2H), 2.67–2.70 (m,
2H), 7.22 (d, J¼8.0 Hz, 2H), 7.34–7.38 (m, 2H), 7.44 (s,
1H); 13C NMR (100 MHz, CDCl3) d 20.74, 20.99, 22.84,
3.2.10. 5-Ethyl-4-methyl-1-phenyl-1H-pyrazole (3q). Oil;
1H NMR (400 MHz, CDCl3) d 1.28 (t, J¼7.5 Hz, 3H), 2.08

C. S. Cho, D. B. Patel / Tetrahedron 62 (2006) 6388–6391
6391
10. For similar palladium-catalyzed synthesis of 1-aryl-1H-inda-
zoles from aryl hydrazones of 2-bromobenzaldehydes and
2-bromoacetophenones: Lebedev, A. Y.; Khartulyari, A. S.;
Voskoboynikov, A. Z. J. Org. Chem. 2005, 70, 596.
11. (a) Coates, R. M.; Senter, P. D.; Baker, W. R. J. Org. Chem.
1982, 47, 3597; (b) Ray, J. K.; Haldar, M. K.; Gupta, S.; Kar,
G. K. Tetrahedron 2000, 56, 909; (c) Zhang, Y.; Herndon,
J. W. Org. Lett. 2003, 5, 2043; (d) Mal, S. K.; Ray, D.; Ray,
J. K. Tetrahedron Lett. 2004, 45, 277; (e) Cho, C. S.;
Patel, D. B.; Shim, S. C. Tetrahedron 2005, 61, 9490; (f)
Ray, D.; Mal, S. K.; Ray, J. K. Synlett 2005, 2135; (g)
Some, S.; Dutta, B.; Ray, J. K. Tetrahedron Lett. 2006, 47,
1221.
(s, 3H), 2.67 (q, J¼7.5 Hz, 2H), 7.17–7.20 (m, 1H), 7.37–
7.41 (m, 2H), 7.60–7.62 (m, 3H); 13C NMR (100 MHz,
CDCl3) d 8.47, 13.47, 20.06, 115.84, 118.40, 125.35,
125.81, 129.27, 140.35, 154.99; Anal. Calcd for
C12H14N2: C 77.38; H 7.58; N 15.04; found: C 77.00; H
7.88; N 14.94.
Acknowledgements
C.S.C. gratefully acknowledges a Research Professor Grant
of Kyungpook National University (2005).
12. For our recent report on a C–N bond formed intermediate or
product during the course of reaction, see: (a) Cho, C. S.;
Lim, H. K.; Shim, S. C.; Kim, T. J.; Choi, H.-J. Chem.
Commun. 1998, 995; (b) Cho, C. S.; Oh, B. H.; Shim, S. C.
Tetrahedron Lett. 1999, 40, 1499; (c) Cho, C. S.; Kim, J. S.;
Oh, B. H.; Kim, T.-J.; Shim, S. C. Tetrahedron 2000, 56,
7747; (d) Cho, C. S.; Kim, J. H.; Shim, S. C. Tetrahedron
Lett. 2000, 41, 1811; (e) Cho, C. S.; Oh, B. H.; Kim, J. S.;
Kim, T.-J.; Shim, S. C. Chem. Commun. 2000, 1885; (f) Cho,
C. S.; Kim, J. H.; Kim, T.-J.; Shim, S. C. Tetrahedron 2001,
57, 3321; (g) Cho, C. S.; Kim, T. K.; Kim, B. T.; Kim, T.-J.;
Shim, S. C. J. Organomet. Chem. 2002, 650, 65; (h) Cho,
C. S.; Kim, J. H.; Choi, H.-J.; Kim, T.-J.; Shim, S. C.
Tetrahedron Lett. 2003, 44, 2975; (i) Cho, C. S. Tetrahedron
Lett. 2005, 46, 1415.
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