Chemoselective synthesis of substituted pyrazoles through AgOTf-catalyzed cascade propargylic substitution-cyclization-aromatization

Article abstract of DOI:10.1039/c2ob27016a

A cascade AgOTf-catalyzed chemoselective approach to 3,5/1,3-disubstitued pyrazoles from propargylic alcohols and para-tolylsulfonohydrazide has been developed. Good chemoselectivity is observed depending on the different substituents in the alkyne moiety of the propargylic alcohols, generating two different kinds of products through different aromatization mechanisms. The pyrazolo[5,1-a]isoquinoline skeleton can also be effectively constructed by this method through a cascade bicyclization process.

Full text of DOI:10.1039/c2ob27016a

Organic &
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Chemoselective synthesis of substituted pyrazoles
through AgOTf-catalyzed cascade propargylic
Cite this: Org. Biomol. Chem., 2013, 11,
294
substitution–cyclization–aromatization†
Su-Xia Xu, Lu Hao, Tao Wang, Zong-Cang Ding and Zhuang-Ping Zhan*
A cascade AgOTf-catalyzed chemoselective approach to 3,5/1,3-disubstitued pyrazoles from propargylic
alcohols and para-tolylsulfonohydrazide has been developed. Good chemoselectivity is observed depend-
ing on the different substituents in the alkyne moiety of the propargylic alcohols, generating two
different kinds of products through different aromatization mechanisms. The pyrazolo[5,1-a]isoquinoline
skeleton can also be effectively constructed by this method through a cascade bicyclization process.
Received 4th September 2012,
Accepted 6th November 2012
DOI: 10.1039/c2ob27016a
www.rsc.org/obc
Over the past decade, transition-metal-catalyzed cascade
reactions of propargylic alcohols have received considerable
Introduction
Pyrazoles and their derivatives have been recognized as impor- attention, since these reactions can directly construct compli-
tant frameworks in pharmaceutical science,1 and certain pyra- cated molecules from readily accessible starting materials
zole derivatives possessing selective COX-2 inhibition, under mild conditions.3 Recently, our group has developed
antiobesity activity and selective human NHE-1 inhibition have effective methods for the construction of O- and N-containing
been developed into clinical drugs (Fig. 1).1a–c Owing to the heterocycles through consecutive transition metal-catalyzed
attractive medicinal properties of pyrazoles, various propargylic substitution reactions and cyclization reactions,
approaches have been developed for their preparation.2 Con- both of which were catalyzed by a single catalyst.4 It seems
ventional approaches involve either the construction of two plausible that this strategy can be extended to prepare pyra-
C–N bonds by condensation of hydrazines with 1,3-dicarbonyl zoles from propargylic alcohols and para-tolylsulfonohydra-
compounds or their 1,3-dielectrophilic equivalents, or the gen- zide. Herein, we wish to report an efficient cascade reaction for
eration of one C–N bond and one C–C bond by intermolecular the synthesis of pyrazoles from propargylic alcohols and para-
cycloaddition reaction.2a However, these methods have some tolylsulfonohydrazide. Notably, good chemical selectivity is
limitations including multi-step procedures, harsh conditions observed in the reaction depending on the different substitu-
and poor selectivity. Accordingly, new approaches allowing for ents in the alkyne moiety of the propargylic alcohols, generat-
efficient assembly of different pyrazole skeletons with diverse ing two different kinds of products through different
substitution patterns are in high demand.
aromatization mechanisms.
Results and discussion
Initially, on the basis of our previous success in FeCl3-cata-
lyzed propargylic substitution,5 we treated the propargylic
alcohol 1a with 2 using 10 mol% FeCl3 as the catalyst in aceto-
nitrile at reflux. However, no amount of the desired product 4a
was observed under these conditions (Table 1, entry 1). Then,
we changed the catalyst to AgOTf, which had been successfully
used in our syntheses of pyrrolo[1,2-a]indoles.4a To our
delight, the target product 4a was furnished in 42% yield
(Table 1, entry 2). Next solvent screening showed that the reac-
tion performed in 1,2-dichloroethane (DCE in abbreviation) at
reflux afforded the product in 90% yield (Table 1, entry 8).
Consequently, catalyst screening demonstrated that AgOTf was
Fig. 1 Selected pyrazole-containing drugs.
Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen
University, Xiamen 361005, Fujian, P. R. China. E-mail: zpzhan@xmu.edu.cn;
Fax: +86 (0)592 2180318; Tel: +86 (0)592-2180318
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c2ob27016a
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Table 1 Screening of catalysts and solventsa
Table 2 Cascade synthesis of 3,5-disubstituted 1H-pyrazolesa
Entry
Catalyst
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
FeCl3
CH3CN
CH3CN
Toluene
CH3NO2
PhCl
DMF
1,4-Dioxane
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
12
2
15
2
2
18
2
1
2
4
3
10
3
2
12
24
24
12
0
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgBF4
AgOAc
InCl3
In(OTf)3
Zn(OTf)2
Cu(OTf)2
BiCl3
42
79
50
50
Trace
45
90
72
55
40
25
30
65
Trace
0
9
10
11c
12
13
14
15d
16d
17d
18d
TFA
p-TSA
TfOH
0
0
a Reaction conditions: 1a (1.0 mmol), 2 (1.2 mmol), and catalyst
(10 mol%) in solvent (5 mL) at reflux for an appropriate time (Tf =
triflyl; TFA = trifluoroacetic acid; p-TSA = para-toluenesulfonic acid;
DMF = N,N-dimethylformamide; DCE = 1,2-dichloroethane). b Isolated
yields based on 1a. c About 50% of starting material 1a was recovered.
d The majority of the starting material 1a was recovered.
still the preferential catalyst (Table 1, entries 9–17). The result
of the investigation into the effect of counterions of silver salts
was very interesting. It was found when adopting AgBF4 and
AgOAc as the catalysts the desired product 4a could be
obtained, but longer reaction time was required and lower
yields were given (Table 1, entries 9 and 10). Such results indi-
cated that the nature of the counterions markedly affected the
catalytic activity of the silver(I): the trifluoromethanesulfonate
anion might be the best counterion to facilitate this cascade
reaction.
It is worth mentioning that some groups including Hinter-
mann,6a Hartwig,6b and Mukaiyama6c have reported that the
“hidden Brønsted acid” produced from the reaction of AgOTf
and DCE at reflux might be responsible for the observed acid-
like catalytic activity of transition-metal salts. However, as can
be seen from the control experiments in Table 1, the reactions
performed under Brønsted acid catalysis gave no desired
product (entries 16–18). On the other hand, the reactions
carried out under the catalysis of different silver salts indeed
afforded the pyrazole in moderate yields (entries 9 and 10).
Thus, this cascade reaction is very likely catalyzed by the metal
salt rather than Brønsted acid.
a Reaction conditions: 1 (1.0 mmol), 2 (1.2 mmol), and AgOTf (10 mol
%) in DCE (5 mL) at reflux for an appropriate time. Isolated yields
based on 1. b The starting material was recovered. c The substitution
and cyclization were performed in DCE and PhCl respectively in a one-
pot mode.
range of functional groups including F, Cl, Br, MeO, CN and
COOMe could be well tolerated, leading to the products in
moderate to good yields. Electron-donating functional groups
increased the reaction rate and gave good yields (Table 2, 4b
and 4c), whereas electron-withdrawing substituents slowed the
transformation down and provided moderate yields (Table 2,
4d–4g). Notably, the reaction of propargylic alcohol with an
α-furanyl group proceeded well, producing the target product
in a moderate yield (Table 2, 4h).
Very interestingly, the next examination into different sub-
stituted hydrazines showed that only p-tolylsulfonohydrazide
worked in the reaction. Reactions with other substituted hydra-
zines under the standard conditions did not give the pyrazole
products (Table 3, entries 2–6). We reasoned that the catalytic
activity of AgOTf was suppressed by stronger alkalinity of those
substituted hydrazines.
Next, the substrate scope was investigated under the opti-
mized conditions (Table 2). We were pleased to find that the
reaction of aryl-substituted secondary propargylic alcohols pro-
ceeded very well, offering an easy access to 3,5-disubstituted
1H-pyrazoles with various substituents. In addition, a wide
Encouraged by these results, we next examined the reactiv-
ity of propargylic alcohols bearing alkyl groups on the alkyne
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Table 3 Screening of different substituted hydrazinesa
Scheme 1 Cascade reactions of TMS-substituted propargylic alcohols with
para-tolylsulfonohydride.
Recoveredb 1a
(%)
Entry Hydrazine
Yieldb 4a (%)
1
2
3
4
TsNHNH2
AcNHNH2
t-BocNHNH2
2,4-Dinitrophenyl-
hydrazine
90
0
0
Complex
mixture
0
98
98
0
5
6
PhNHNH2
PhCONHNH2
0
0
97
92
a Reaction conditions: 1a (1.0 mmol), hydrazine (1.2 mmol), and
AgOTf (10 mol%) in DCE (5 mL) at reflux for one hour. b Isolated yields
based on 1a.
Scheme 2 Isolation of the substitution intermediate and AgOTf-catalyzed
cyclization.
Table 4 Cascade synthesis of 1,3-disubstituted pyrazolesa
Scheme 3 Proposed mechanism for the AgOTf-catalyzed cascade synthesis of
3,5- and 1,3-disubstituted pyrazoles.
products were produced (Scheme 1). It was proposed that the
trimethylsilyl group had been removed from a certain portion
of propargylic alcohols prior to the propargylic substitution,
leading to the corresponding propargylic alcohols with term-
inal alkyne, which would experience the same reaction
pathway as shown in Table 2.
a Reaction conditions: 1 (1.0 mmol), 2 (1.2 mmol), and AgOTf (10 mol
%) in DCE (5 mL) at reflux for an appropriate time. Isolated yields
based on 1.
To provide preliminary insight into the reaction mechanism
for the AgOTf-catalyzed synthesis of pyrazoles, the isolation of
the propargylic substitution intermediate was attempted.
Indeed, when propargylic alcohol 1h was subjected to the
AgOTf-catalyzed conditions, the propargylic substitution
product 3h was isolated in 74% yield. However, to complete
the consecutive AgOTf-catalyzed cyclization, the substitution
product 3h had to be treated at reflux in chlorobenzene
instead of 1,2-dichloroethane (Scheme 2).
As a working hypothesis, we proposed the following plaus-
ible mechanism for the AgOTf-catalyzed cascade synthesis of
pyrazoles (Scheme 3). First, the propargylic alcohol 1 reacts
under silver catalysis to yield the propargylic cation intermedi-
ate 6. Next, propargylic substitution occurs by the regioselec-
tive attack of the terminal nitrogen atom (–NH2) of 2, resulting
in the substitution product 3. Then, through a Ag(I)-mediated
triple-bond activation, compound 3 experiences a 5-endo-dig
cyclization to generate the intermediate 8. After proton
exchange with the metal in 9, the aromatization of compound
moiety. Moderate to good yields were obtained (Table 2, 4i–
4n). However, the presence of a strong electron-withdrawing
group on the propargylic alcohol had an apparent adverse
impact on the conversion (Table 2, 4l and 4m). This result
could be attributed to the instability of the propargylic cation
intermediate formed in the reaction process. Also, we tested
the reactivity of alkyl-substituted secondary propargylic
alcohol, but it was found no expected product was furnished
(Table 2, 4o).
To further define the generality of this reaction, attention
was turned to the reaction of propargylic alcohols with term-
inal alkyne. Interestingly, we found that the tosyl group was
retained in these cases, generating the N-tosyl pyrazoles in
moderate yields (Table 4). It seemed that the unsaturation
degree might come from the oxidation by the oxygen in the
air, rather than the elimination of para-tolylsulfinic acid.
On the other hand, the exploration into the reaction of pro-
pargylic alcohols with trimethylsilyl group showed that mixed
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decoupling. HRMS data were obtained via Ultra-high Resolu-
tion Hybrid Qh-Fourier Transform Mass Spectrometer
(En Apex ultra 7.0 FT-MS).
General procedure and representative characterization data
To a flame-dried flask (10 mL) equipped with a magnetic stir-
ring bar, the propargylic alcohol 1 (1.0 mmol, 1.0 equiv.), 2
(1.2 mmol, 1.2 equiv.), and 1,2-dichloroethane (5 mL) were
successively added. The mixture was stirred until 2 was dis-
Scheme 4 Synthesis of pyrazolo[5,1-a]isoquinoline through AgOTf-catalyzed
cascade bicyclization process.
10 can be completed in two ways: one is the elimination of solved. Subsequently, the catalyst AgOTf (10 mol%, 0.1 mmol)
para-tolylsulfinic acid, and the other is the oxidation of 10 by was added. Then, the mixture was heated to reflux for an
the oxygen in the air. The former route leads to the 3,5-disub- appropriate time, and monitored periodically by thin-layer
stituted 1H-pyrazoles 4, while the latter route affords the tosyl- chromatography. Upon completion of the reaction, the solvent
retained products 5 (1,3-disubstituted pyrazoles).
was removed under reduced pressure, and the residue was pur-
Finally, as a further demonstration, we sought to extend the ified by column chromatography on silica gel to afford the
synthetic utility of the AgOTf-catalyzed cascade sequence. products.
Thus, the N-fused heterocyclic skeleton pyrazolo[5,1-a]isoqui-
3,5-DIPHENYL-1H-PYRAZOLE (4A). Prepared according to the
noline was successfully constructed in 65% yield under the general procedure and the reaction was performed for one
standard reaction conditions through a cascade bicyclization hour. The product was purified by column chromatography on
process (Scheme 4), providing an efficient method to access silica gel (v/v petroleum ether–AcOEt = 15 : 1) to afford a white
this kind of heterocycles.
solid in 90% yield (m.p. 190–192 °C); Rf = 0.21 (TLC, v/v pet-
roleum ether–AcOEt = 7 : 1); 1H-NMR (400 MHz, DMSO): δ 7.17
(s, 1H), 7.31–7.35 (m, 2H), 7.43–7.46 (m, 4H), 7.84 (m, 4H),
13.39 (brs, 1H); 13C-NMR (100 MHz, DMSO): δ 99.6, 125.2,
128.8; IR (film): 3417, 3101, 1610 1494 cm−1; ESI-MS: calc. for
Conclusions
In summary, we have developed an efficient AgOTf-catalyzed C15H12N2 [M + H]+: m/z = 221.1; found: 221.3.
cascade synthesis of pyrazoles from readily available pro-
3-PHENYL-1-TOSYL-1H-PYRAZOLE (5A). Prepared according to the
pargylic alcohols and para-tolylsulfonohydrazide. This reaction general procedure and the reaction was performed for
proceeds through a propargylic substitution–cyclization–aro- 2.5 hours. The product was purified by column chromato-
matization cascade sequence, and a broad range of functional graphy on silica gel (v/v petroleum ether–AcOEt = 15 : 1) to
groups are well tolerated. Depending on the different substitu- afford a white solid in 60% yield (m.p. 139–140 °C); Rf = 0.20
ents in the alkyne moiety of the propargylic alcohols, two (TLC, v/v petroleum ether–AcOEt = 5 : 1); 1H-NMR (400 MHz,
different kinds of products are afforded through different aro- CDCl3): δ 2.41 (s, 3H), 6.73–6.83 (m, 2H), 7.28–7.34 (m, 6H),
matization mechanisms. The heterocyclic skeleton pyrazolo 7.58 (d, 1H, J = 8.4 Hz), 7.86 (d, 2H, J = 8.4 Hz); 13C-NMR
[5,1-a]isoquinoline is also effectively constructed by this (100 MHz, CDCl3): δ 21.6, 124.3, 127.0, 127.8, 128.9, 129.1,
methodology.
129.7, 135.3, 135.5, 139.9, 144.3, 149.8; IR (film): 3180, 1607,
1597 cm−1; ESI-MS: calc. for C16H14N2O2S [M + Na + H]+: m/z
(%) = 322.1; found: 322.5.
N′-(1-(FURAN-2-YL)-3-PHENYLPROP-2-YNYL)-4-METHYLBENZENE-SULFONO-
HYDRAZIDE (3H). Prepared according to the general procedure
and the reaction was performed for 2 hours. The product was
Experimental section
General information
Thin layer chromatography (TLC) was performed on Huanghai purified by column chromatography on silica gel (v/v pet-
pre-coated glass-backed TLC plates and visualized by UV lamp roleum ether–AcOEt = 15 : 1) to afford a white solid in 74%
(254 nm). Column chromatography on silica gel yield (m.p. 197–199 °C); Rf = 0.22 (TLC, v/v petroleum ether–
1
(300–400 mesh) was carried out using Technical Grade AcOEt = 7 : 1); H-NMR (400 MHz, CDCl3): δ 2.41 (s, 3H), 4.03
60–90 °C petroleum ether (distilled prior to use) and Analytical (d, 1H, J = 8.8 Hz), 4.88 (d, 1H, J = 8.8 Hz), 6.17 (d, 1H, J = 2.0
Grade EtOAc (without further purification). Concentration Hz), 6.32 (dd, 1H, J = 3.2, 2.0 Hz), 6.39 (d, 1H, J = 3.2 Hz),
under reduced pressure was performed by rotary evaporation. 7.28–7.36 (m, 4H), 7.42–7.44 (m, 3H), 7.85 (d, 2H, J = 8.4 Hz);
Purified compounds were further concentrated under high 13C-NMR (100 MHz, CDCl3): δ 21.6, 50.8, 83.4, 86.1, 109.3,
vacuum (3–5 mmHg). Yields refer to chromatographically puri- 110.4, 121.2, 128.2, 128.3, 128.8, 129.5, 131.9, 135.3, 143.2,
fied compounds. 1H and 13C spectra were recorded on a 144.0, 149.4; IR (film): 3256, 2232 (weak), 1597, 1493 cm−1
;
Bruker AV-400 spectrometer. Chemical shifts were reported in HRMS(ESI): calc. for C20H18N2O3S [M + Na]+: m/z = 389.0936;
ppm. 1H-NMR spectra were referenced to TMS in CDCl3 found: 389.0935.
(0 ppm) or d6-DMSO (2.50 ppm), and 13C-NMR spectra were
3-PHENYL-5-(TRIMETHYLSILYL)-1H-PYRAZOLE (4P). Prepared accord-
referenced to CDCl3 (77.0 ppm) or d6-DMSO (39.5 ppm). All ing to the general procedure and the reaction was performed
13C-NMR spectra were measured with complete proton in nitromethane for 3 hours. The product was purified by
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column chromatography on silica gel (v/v petroleum ether– 2 For reviews and recent examples, see: (a) S. Fustero,
AcOEt = 15 : 1) to afford 4p (a yellow oil; TLC: Rf = 0.32, v/v pet-
roleum ether–AcOEt = 7 : 1) and 5a in 40% and 15% yields
respectively. H-NMR (400 MHz, CDCl3): δ 0.34 (s, 9H), 6.74 (s,
1H), 7.26–7.33 (m, 1H), 7.38–7.42 (m, 2H), 7.83–7.85 (m, 2H);
13C-NMR (100 MHz, CDCl3): δ −1.2, 109.8, 125.9, 127.6, 128.6,
133.3, 152.0; IR (film): 3164, 3007, 1607 1513 cm−1; HRMS
(ESI): calc. for C12H16N2Si [M + H]+: m/z (%) = 217.1161; found:
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2-PHENYLPYRAZOLO[5,1-A]ISOQUINOLINE (4R). Prepared according
to the general procedure and the reaction was performed for
2 hours. The product was purified by column chromatography
on silica gel (v/v petroleum ether–AcOEt = 15 : 1) to afford a
white solid in 65% yield (m.p. 115–116 °C); Rf = 0.25 (TLC, v/v
petroleum ether–AcOEt = 7 : 1); 1H-NMR (400 MHz, CDCl3): δ
7.99 (d, 1H, J = 7.6 Hz), 7.29 (d, 1H, J = 0.8 Hz), 7.46–7.60 (m,
2H), 7.52–7.60 (m, 2H), 7.72 (dd, 1H, J = 7.6, 1.2 Hz), 8.00–8.02
(m, 2H), 8.13 (dt, 1H, J = 7.6, 0.8 Hz), 8.27 (dd, 1H, J = 8.0, 0.8
Hz); 13C-NMR (100 MHz, CDCl3): δ 94.8, 112.2, 123.8, 124.6,
126.5, 127.4, 127.8, 128.0, 128.5, 128.9, 129.0, 133.3, 140.0,
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Acknowledgements
We thank the National Natural Science Foundation of China
(Nos. 21072159 and 21272190), the 973 Projects (No.
2011CB935901), Xiamen Science and Technology Bureau (No.
3502Z20103006) and the NFFTBS (No. J1030415) for financial
support of this project.
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