Synergistic Indium and Silver Dual Catalysis: A Regioselective [2 + 2 + 1]-Oxidative N-Annulation Approach for the Diverse and Polyfunctionalized N-Arylpyrazoles

Article abstract of DOI:10.1021/acs.orglett.8b02008

Indium(III)/silver(I)-catalyzed [2 + 2 + 1] annulation of arylhydrazine hydrochlorides with β-enamino esters via multicomponent reactions for the construction of diverse and multisubstituted N-arylpyrazoles has been demonstrated. The oxidative cycloaddition proceeds via a cascade triple Michael addition/elimination/air oxidation. This novel protocol provides a rapid and efficient synthetic route to various 3,4-diester-substituted N-arylpyrazoles. The synthesized compounds are further utilized for various synthetic transformations.

Full text of DOI:10.1021/acs.orglett.8b02008

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synergistic Indium and Silver Dual Catalysis: A Regioselective
[2 + 2 + 1]-Oxidative N‑Annulation Approach for the Diverse and
Polyfunctionalized N‑Arylpyrazoles
Raju S. Thombal and Yong Rok Lee*
School of Chemical Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea
S
* Supporting Information
ABSTRACT: Indium(III)/silver(I)-catalyzed [2 + 2 + 1]
annulation of arylhydrazine hydrochlorides with β-enamino
esters via multicomponent reactions for the construction of
diverse and multisubstituted N-arylpyrazoles has been
demonstrated. The oxidative cycloaddition proceeds via a
cascade triple Michael addition/elimination/air oxidation.
This novel protocol provides a rapid and efficient synthetic
route to various 3,4-diester-substituted N-arylpyrazoles. The synthesized compounds are further utilized for various synthetic
transformations.
Scheme 1. Reported [3 + 2] Annulation Strategies for the
Synthesis of Substituted Pyrazoles
yrazoles are among the most significant heteroaromatic
Pcompounds widely found in biologically and pharmaco-
logically active molecules.1 They exhibit a variety of biological
properties, including anti-inflammatory,2 antibacterial,3 analge-
sic,4 antifungal,5 antipyretic,6 antiviral,7 anticancer,8 antidia-
betic,9 antiobesity,10 and plant growth regulating activities,11 as
well as protein kinase,12 Cox-2,13 and HIV-1 reverse
transcriptase inhibitory functions.14 They have been used as
valuable building blocks and structural motifs in the synthesis
of natural products, agrochemicals, dyes, and medicines.15
Moreover, pyrazoles are employed as ligands, cosmetic
colorings, and UV stabilizers.16 To date, a number of
substituted N-arylpyrazoles, including difenamizole (1),
celecoxib (2), rimonabant (3), and fipronil (4), have been
commercialized as pharmaceuticals and insecticides (Figure
1).17
Owing to the importance and usefulness of N-arylpyrazoles,
a number of methods for their preparation have been
developed (Scheme 1). Typical approaches are based on the
[3 + 2]cycloaddition of hydrazines with the corresponding
substrates through condensation or multistep reactions.18−21
Common strategies include the condensation of hydrazines
with 1,3-dicarbonyl compounds (method a);18 hydrazone
formation, followed by Ru- or V-catalyzed intramolecular
aerobic oxidative C−N coupling (method b);19 allylic
hydrazone formation and Pd-catalyzed aminohydroxylation in
air (method c),20 and Rh-catalyzed cyclization of hydrazines
with alkynes (method d).21 Other facile [3 + 2] annulation
approaches to form substituted pyrazoles have also been
demonstrated, including K2CO3-mediated reaction/[1,5]
sigmatropic rearrangement (method e),22 pyrrolidine-catalyzed
Received: June 27, 2018
Figure 1. Selected bioactive molecules containing N-arylpyrazoles.
© XXXX American Chemical Society
A
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reaction of diazoacetates with carbonyl compounds (method
f),23 Co-catalyzed cycloaddition of diazoacetates with diazo-
nium salts through C−N bond formation (method g),24 and
Bu3P-catalyzed desulfonylative cycloaddition of arylazosulfones
with allylic carbonates (method h).25 Recently, formal [4 + 1]
annulation of hydrazones with 2-bromo-1,3-dicarbonyl com-
pounds via visible-light mediated photoredox catalysis26a as
well as the reactions of hydrazine with Michael acceptors under
visible light26b has been demonstrated for the synthesis of
pyrazoles.
prefunctionalization of substrates through multistep procedure.
In this regard, more facile and efficient multicomponent
protocols using relatively lower loading of cost-effective
catalysts are highly desirable. Recently, a very interesting Cu-
mediated pyrazole formation through the multicomponent
reaction of 2,3-allenoates with amines and nitriles has been
described. However, it is a stoichiometric reaction, and 2.0
equiv of Cu(OAc)2 is needed (eq 1, Scheme 2).27 To the best
of our knowledge, there is no report on the synthesis of diverse
and polysubstituted N-arylpyrazoles by oxidative [2 + 2 + 1]
cycloaddition of readily available arylhydrazine hydrochlorides
with β-enamino esters employing a catalytic reaction. Recently,
we demonstrated the construction of azopyrazoles by a Ag-
catalyzed cascade reaction of diazo compounds with
arylhydrazines.28 We also reported the synthesis of multi-
functionalized 2-hydroxybenzophenones through the benzan-
nulation of 3-formylchromones and β-enamino esters.29 As
part of an ongoing study of the reactions of arylhydrazines and
β-enamino esters, this paper describes a novel and efficient In/
Ag-catalyzed oxidative [2 + 2 + 1] cycloaddition of
arylhydrazine hydrochlorides with β-enamino esters for the
construction of various multisubstituted N-arylpyrazoles (eq 2,
Scheme 2).
Scheme 2. Synthetic [2 + 2 + 1] Annulation Strategies for
Substituted Pyrazoles through Multicomponent Reactions
To obtain the optimized conditions, the reaction of
phenylhydrazine hydrochloride (5a) and methyl (E)-3-
(dimethylamino)acrylate (6a) was carried out in an open air
condition using various catalysts and solvents (Table 1). The
initial attempts using 10 mol % of Cu(OTf)2, Y(OTf)3, and
Yb(OTf)3 in dichloroethane at 70 °C for 12 h provided
product 7a in 31, 17, and 27% yields, respectively (entries 1−3,
Table 1). Encouraged by these results, other catalysts were
Although a number of methods have been demonstrated for
the construction of substituted pyrazoles via [3 + 2]
cycloaddition or visible-light-mediated photoredox catalysis,
most of the reported strategies have limited scope due to high
catalyst loading, the use of expensive catalysts, and
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst (mol %)
additives (mol %)
solvent
time (h)
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Cu(OTf)2 (10)
Y(OTf)3 (10)
Yb(OTf)3 (10)
InCl3 (10)
In(OTf)3 (10)
In(OTf)3 (10)
In(OTf)3 (10)
In(OTf)3 (10)
In(OTf)3 (10)
In(OTf)3 (5)
In(OTf)3 (20)
In(OTf)3 (10)
In(OTf)3 (10)
dichloroethane
dichloroethane
dichloroethane
dichloroethane
dichloroethane
dichloroethane
dichloroethane
dichloroethane
dichloroethane
dichloroethane
dichloroethane
dichloroethane
dichloroethane
dichloroethane
toluene
12
12
12
12
12
12
24
24
12
12
12
12
12
24
12
12
24
12
12
31
17
27
43
48
58
52
55
71
38
68
61
63
0
c
AgNO3 (15)
Ag2O (15)
AgSbF6 (15)
AgSbF6 (15)
AgSbF6 (15)
AgSbF6 (10)
AgSbF6 (20)
AgSbF6 (15)
AgSbF6 (15)
AgSbF6 (15)
AgSbF6 (15)
AgSbF6 (15)
AgSbF6 (15)
In(OTf)3 (10)
In(OTf)3 (10)
In(OTf)3 (10)
In(OTf)3 (10)
In(OTf)3 (10)
10
40
21
32
60
EtOH
DMF
dichloroethane
dichloroethane
d
18
19
e
a
b
c
Reaction conditions: 5a (0.5 mmol) and 6a (1.1 mmol) in solvent (5.0 mL) at 70 °C under air. Isolated yield. Reaction was performed under
d
e
O2 atmosphere. Reaction was performed under N2 atmosphere. Reaction condition: phenylhydrazine (0.5 mmol) and 6a (1.1 mmol) in DCE
(5.0 mL) at 70 °C under air.
B
DOI: 10.1021/acs.orglett.8b02008
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Letter
screened. Using 10 mol % InCl3 and In(OTf)3, the yields of 7a
increased to 43% and 48%, respectively (entries 4 and 5).
When the reaction was carried out under O2 atmosphere, the
yield of 7a slightly increased to 58% (entry 6). Interestingly,
additives also provided a higher yield for the product; when
In(OTf)3 (10 mol %) was used as the catalyst with the
addition of AgNO3 (15 mol %) or Ag2O (15 mol %) resulted
in slightly increased yields of 52 and 55%, respectively (entries
7 and 8). The best yield (71%) was achieved when 10 mol %
of In(OTf)3 was used with AgSbF6 (15 mol %) in
dichloroethane at 70 °C for 12 h, forming a dual catalytic
system (entry 9). Decreasing the In(OTf)3 loading to 5 mol %
(entry 10) or increasing it to 20 mol % (entry 11) did not
improve the yield of 7a. In addition, changing the quantity of
the additive failed to improve the yield (entries 12 and 13).
When the additive AgSbF6 (15 mol %) was used in the absence
of the In(OTf)3 catalyst, the desired product was not obtained
(entry 14). On the other hand, when other nonpolar or polar
solvents such as toluene, EtOH, and DMF were used, 7a was
produced in 10, 40, and 21% yields, respectively (entries 15−
17). When the reaction was carried out under nitrogen
atmosphere, the yield of 7a dramatically decreased to 32%
(entry 18). Further reaction of hydrochloride salt-free
phenylhydrazine with 6a for 12 h provided the desired product
7a in slightly lower yield (60%, entry 19). The structure of 7a
their benzene rings, such as 2-ethyl, 4-methyl, 4-isopropyl, 4-
methoxy, 2,4-dimethyl, and 2,5-dimethyl groups, with 6a for
6−12 h provided the desired products 7b−g in 64−78% yields.
Similarly, reactions of 6a with 5h−l bearing the electron-
withdrawing groups of 2-chloro, 2-fluoro, 4-fluoro, 4-cyano,
and 2,4-dichloro on the benzene ring for 12 h afforded
products 7h−k, m in slightly lower yields (48−70%).
However, reaction of 6a with 5m bearing a strong electron-
withdrawing group of 4-nitro on the benzene ring, did not
afford product 7l. Moreover, the reactions of 2-hydrazinopyr-
idine dihydrochloride (heterocyclic hydrazine) or cyclo-
hexylhydrazine hydrochloride (alkyl hydrazine) with (E)-3-
(dimethylamino)acrylate (6a) were unsuccessful. These
reactions provide a rapid synthetic route to diverse 4,5-
disubstituted N-arylpyrazoles bearing various substituents on
the benzene rings.
Next, the scope of this reaction was explored using other β-
enamino esters 6b−e bearing ethoxy, tert-butoxy, benzyloxy,
and prenyloxy groups (Scheme 4). Reactions of ethyl (E)-3-
Scheme 4. Substrate Scope of Different β-Enamino Esters
1
was determined by the analysis of its spectral data. The H
NMR spectrum of 7a showed a characteristic signal of the vinyl
proton on the pyrazole ring at δ 8.35 ppm as a singlet and a
methylene proton signal at δ 4.00 ppm as a singlet. The
structure was further confirmed by X-ray crystallographic
analysis of the structurally related compound 7m.
To explore the generality of the reaction, reactions of
different arylhydrazine hydrochlorides 5b−m with 6a were
next examined (Scheme 3). Treatment of arylhydrazine
hydrochlorides 5b−g bearing electron-donating groups on
Scheme 3. Substrate Scope of Aryl Hydrazine
Hydrochlorides
(dimethylamino)acrylate (6b) with 5a, 5c, and 5e provided
the expected products 8a−c in 66, 68, and 72% yields,
respectively. A combination of tert-butyl (E)-3-(dimethyl-
amino)acrylate (6c) with 5a, 5c, 5e, 5j, or 5n afforded
products 8d−h in 60−74% yields. Moreover, treatment of
benzyl (E)-3-(dimethylamino)acrylate (6d) with 5a, 5c, or 5f
provided products 8i−k in 73, 80, and 74% yields, respectively,
whereas the reaction of 3-methylbut-2-en-1-yl-(E)-3-
(dimethylamino)acrylate (6e) with 5a or 5c afforded products
8l and 8m in 63 and 64% yields, respectively. In addition, the
reaction of β-substituted enamino ester [methyl (E)-3-
(dimethylamino)but-2-enoate] with phenylhydrazine hydro-
chloride (5a) under the standard reaction conditions did not
produce the desired product, probably due to the direct
involvement of α and β carbons in the pyrazole ring.
Having demonstrated the general applicability of this
cycloaddition, we investigated the possibility of using 1-
naphthalenylhydrazine hydrochloride (9), which will lead to
the formation of a new type of N-arylpyrazole derivative
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(Scheme 5). The reaction of 9 with 6a or 6c led to the
formation of 10a and 10b in 60 and 66% yields, respectively.
Scheme 8. Reactivity of Various Michael Acceptors 6g−m
toward Phenylhydrazine Hydrochloride (5a)
Scheme 5. Reactions of 1-Naphthalenylhydrazine
Hydrochloride (9) with 6a or 6c
bearing less bulky groups reacted smoothly with 5a, while
substrates 6g and 6h remained to be unreactive substrates. Due
to the presence of an extra heteroatom, O or N, on enamino
esters 6i and 6j, the lower reactivity was observed compared to
the enamino ester 6l bearing pyrrolidine ring.
On the basis of the above results, a mechanism was
proposed for the formation of 7a, as shown in Scheme 9. In the
Moreover, we explored the reaction of phenylhydrazine
hydrochloride (5a) with (E)-3-(dimethylamino)acrylonitrile
(6f) (Scheme 6). Interestingly, treatment of 5a with 6f led to
Scheme 6. Reactions of Phenylhydrazine Hydrochloride
(5a) with 6f
Scheme 9. Proposed Reaction Mechanism for the
Formation of 7a
unexpected 4-cyano-substituted pyrazole 10c in lower yield
(24%) together with the recovery of starting material 6f (39%).
However, the reaction of (E)-N,N-dimethyl-2-phenylethen-1-
amine with phenylhydrazine hydrochloride (5a) did not
produce any products. This result reveals that the absence of
the electron-withdrawing group deactivates the ability of
enamino substrates to act as Michael acceptors.
To investigate the application of the novel protocol
described herein, the conversion of the prepared compound
7a (for gram-scale synthesis of 7a, see the Supporting
Information (SI)) into new molecules was attempted (Scheme
7). The reaction of 7a with aniline (11a) in the presence of 20
presence of In(OTf)3, methyl (E)-3-(dimethylamino)acrylate
(6a) first forms complex 6a′, which then reacts with
phenylhydrazine hydrochloride (5a) to form the intermediate
14 through a Michael-type addition followed by the
elimination of dimethylamine. Subsequently, further Michael
addition of 14 to another complex 6a′ forms intermediate 15,
which undergoes an intramolecular Michael addition to furnish
16. Then, further elimination of dimethylamine affords
intermediate 17 followed by aerobic oxidation in the presence
of a silver catalyst, leading to the final product 7a. To elucidate
the elimination of amine in the proposed reaction mechanism,
a control experiment was carried out between 5a and 6l under
the standard reaction condition (see the SI). Importantly,
piperidine was detected in the crude reaction mixture by GC−
MS analysis (see Figures S1−S3). The oxidation of
dihydropyrazoles to pyrazoles in the presence of metal catalysis
has been previously reported.30 On the other hand, the
formation of 10c as shown in Scheme 6 might proceed via a
cascade triple Michael addition followed by elimination of
CH3CN and dimethyl amine rather than the oxidation process
(for the mechanism, see the SI). Through GC−MS analysis,
CH3CN was detected in the crude reaction mixture as an
evidence of this reaction pathway (see Figure S4).
Scheme 7. Conversion of the Synthesized Compound 7a
into New Molecules 12a, 12b, and 13
mol % of InCl3 in refluxing toluene for 24 h chemoselectively
provided 12a in 41% yield, and the same reaction with 4-
methoxyaniline (11b) afforded 12b in 53% yield. Also, further
LAH reduction of 7a provided the diol product 13 in 95%
yield.
To understand the reactivity of Michael acceptors toward
phenyl hydrazine hydrochloride (5a), different reaction
partners 6g−m were treated under the standard reaction
conditions (Scheme 8). The results indicate that the esters
In summary, a facile and efficient methodology for the
construction of diverse and multifunctionalized N-arylpyr-
azoles was developed, featuring a one-pot procedure by
In(III)-catalyzed regioselective [2 + 2 + 1] annulation
D
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reactions of arylhydrazine hydrochlorides with β-enamino
esters. Both In(III) and Ag(I) played distinct catalytic roles in
a synergistic manner, producing N-arylpyrazoles with high
regioselectively. This cascade protocol afforded a rapid
synthetic route to highly functionalized pyrazole derivatives,
which can be widely used for the synthesis of dyes, natural
products, and pharmaceuticals. Further transformation of the
synthesized compound was carried out to obtain biologically
relevant amidopyrazole derivatives.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02008.
Detailed experimental procedures, characterization data,
and 1H NMR and 13C NMR spectra of final products; X-
ray data for compound 7m (PDF)
Accession Codes
CCDC 1832571 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: yrlee@yu.ac.kr. Fax: +82-53-810-4631. Tel: +82-53-
810-2529.
ORCID
Raju S. Thombal: 0000-0003-3760-8856
Yong Rok Lee: 0000-0002-4048-8341
Notes
The authors declare no competing financial interest.
́
(22) (a) Perez-Aguilar, M. C.; Valdes, C. Angew. Chem., Int. Ed.
́
2015, 54, 13729. (b) Perez-Aguilar, M. C.; Valdes, C. Angew. Chem.,
ACKNOWLEDGMENTS
■
Int. Ed. 2013, 52, 7219.
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korean
government (MSIT) (2018R1A2B2004432). This research
was supported by the Nano Material Technology Development
Program of the Korean National Research Foundation (NRF)
funded by the Korean Ministry of Education, Science, and
Technology (2012M3A7B4049675).
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