Synthesis of Fluorescent Indazoles by Palladium-Catalyzed Benzannulation of Pyrazoles with Alkynes

Article abstract of DOI:10.1021/acs.orglett.7b00410

The synthesis of indazoles from pyrazoles and internal alkynes is described. Instead of complex benzenoid compounds, readily available pyrazoles were used for the preparation of indazoles by reaction of the C-H bonds of the heterocyclic ring. Oxidative benzannulation was also applied to imidazoles, providing benzimidazoles. This convergent strategy enabled alteration of the photochemical properties of benzo-fused diazoles by varying the substituents at the benzene ring, thus leading to the development of tetraarylindazoles as new fluorophores.

Full text of DOI:10.1021/acs.orglett.7b00410

Letter
pubs.acs.org/OrgLett
Synthesis of Fluorescent Indazoles by Palladium-Catalyzed
Benzannulation of Pyrazoles with Alkynes
Og Soon Kim,† Jin Hyeok Jang,† Hyun Tae Kim,† Su Jin Han,† Gavin Chit Tsui,*,‡
and Jung Min Joo*,†
†Department of Chemistry and Chemistry Institute of Functional Materials, Pusan National University, Busan 46241, Republic of
Korea
‡Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
S
* Supporting Information
ABSTRACT: The synthesis of indazoles from pyrazoles and internal
alkynes is described. Instead of complex benzenoid compounds, readily
available pyrazoles were used for the preparation of indazoles by reaction of
the C−H bonds of the heterocyclic ring. Oxidative benzannulation was also
applied to imidazoles, providing benzimidazoles. This convergent strategy
enabled alteration of the photochemical properties of benzo-fused diazoles
by varying the substituents at the benzene ring, thus leading to the
development of tetraarylindazoles as new fluorophores.
ndazole is one of the most important heterocycles in
medicinal chemistry, leading to the development of many
I
marketed drugs and promising drug candidates having the
indazole nucleus.1 In contrast to the popularity of indazole as a
bioisostere for indole and benzimidazole in drug discovery,
indazole has been utilized much less frequently than other
benzannulated heterocycles in materials chemistry.2 However,
with the availability of new synthetic methods for preparing
electronically and sterically varied indazole chromophores, the
development of new indazole-based functional materials has
been facilitated.3 The same synthetic strategies used for the
identification of biologically active indazole compounds can be
applied to the preparation of fluorescent probes and organic
light emitting diode (OLED) materials, such as functional
group transformation and transition-metal-catalyzed cross-
coupling reactions.4 However, the scope of indazoles that can
be accessed by these routes is dependent on the availability of
indazole reactants, thus leaving a large part of chemical space
conceivable with indazole unexplored.5
Figure 1. (A) Conventional method versus our approach for the
synthesis of indazoles. (B) Pyrazole as a directing group in the
annulation of arenes. (C) Benzannulation of pyrazoles with alkynes for
the synthesis of functionalized indazoles.
Recently, it was found that direct C−H functionalization
reactions are useful for constructing the indazole core via ring
closure (Figure 1A).6,7 In these cyclative approaches, the
pyrazole ring was actually formed by C−H functionalization,
requiring prefunctionalized benzene rings that are not readily
accessible. However, in order to fine-tune the properties of the
indazole core by manipulating the substituents at the benzene
ring, it is desirable to develop a convergent approach that can
generate functionalized benzene rings from readily available
pyrazoles at a later stage of the indazole synthesis. Despite the
remarkable expansion of the C−H functionalization reactions
in the synthesis of heterocycles, there are surprisingly few
reports about the construction of indazoles by manipulating the
C−H bonds of pyrazoles.8 Although it is established that the
C−H bonds of pyrazoles are susceptible to transition-metal-
catalyzed C−H functionalization, simultaneous functionaliza-
tion of two C−H bonds of pyrazoles as a method to rapidly
build fused pyrazoles remains underexplored.9−11
Pyrazoles have also been utilized in C−H functionalization
reactions with alkynes, serving as a directing group.12 When
alkynes were employed for annulation reactions, functionaliza-
tion of the adjacent aryl ring predominated, whereas
benzannulation of the heterocycle itself, which could generate
indazoles, was not favored (Figure 1B).13,14 In contrast, elegant
work by Miura, Satoh, and co-workers showed that
benzannulation of indoles provided carbazoles via a Pd-
Received: February 10, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00410
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Letter
catalyzed 1:2 oxidative coupling with alkynes.15,16 As most C−
H benzannulation methods rely on the use of directing groups,
it is notable that a single catalytic system allowed for the
functionalization of both C−H bonds of the indole ring without
the aid of directing groups.17 However, indole was the only
effective heterocycle, and the method was not extended to
other heterocycles. Previously, we developed Pd-catalyzed C−
H alkenylation of 4-substituted pyrazoles.18 Based on that
study, we hypothesized that the C−H bonds of simple
pyrazoles could be palladated to undergo consecutive
carbometalation reactions with alkynes to ultimately provide
indazoles (Figure 1C). Although the reactivity of the C4 and
C5 positions of pyrazoles are different, we envisioned that
readily available pyrazoles could be a general substrate for the
preparation of indazoles by double C−H functionalization.
To examine the feasibility of this benzannulation strategy, we
first evaluated the catalytic system used for indoles in the
reaction of N-methylpyrazole (Table 1, entry 1).16 Although
Table 2. Pd-Catalyzed and Cu-Mediated Benzannulation of
Pyrazoles
a
a
Table 1. Benzannulation of N-Methylpyrazole
entry
oxidant
Ag2CO3
additive
solvent
yield (%)
b
1
2,6-Me2C6H3CO2H
mesitylene
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
DMA
37
39
39
59
79
72
6
2
3
4
5
6
7
8
9
Ag2CO3
AgOAc
Cu(OAc)2
a
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
O2 (1 atm)
Reaction conditions: pyrazole (0.50 mmol), alkyne (1.5 mmol),
Pd(OAc)2 (0.025 mmol), Cu(OAc)2·H2O (1.5 mmol), 1,4-dioxane
b
(1.5 mL), 120 °C, 16 h. A mixture of inseparable products was
mesitylene
1,4-dioxane
1,4-dioxane
1,4-dioxane
DMA
c
obtained. Performed in the presence of LiOAc (0.50 mmol) and
Me3CCO2H
PhCO2H
pyridine
78
75
75
21
DMA (1.5 mL).
c
10
11
able (entries 5 and 6). This difference is attributed to the ease
of trapping the Pd catalyst by forming a palladacycle between
the nitrogen atom of the pyrazole and the sp2 carbon center of
the tethered N-substituent.13 Diarylalkynes having electroni-
cally varying substituents were smoothly prepared by
decarboxylative coupling of propiolic acid and used for the
benzannulation (entries 7−14).21 The reactions with p-tert-
butyl-, methoxy-, and chloro-substituted diphenyl alkynes were
more efficient in the presence of LiOAc and DMA, which
presumably improved the solubility of the reactants (entries 8,
9, and 12). Not only para- but also meta-substituents were
tolerated in this process (entries 13 and 14). Furthermore, this
approach was applied to dialkylalkynes; coupling with 4-octyne
afforded the corresponding benzannulation product 2j in
moderate yield (entry 15). Unsymmetrical alkynes were also
utilized for benzannulation of the pyrazole, where regioisomers
2k and 2l were the major isolable products (entries 16 and
17).19,22 Electron-deficient alkynes, such as dimethyl acetyle-
nedicarboxylate and ethyl phenylpropiolate, were not suitable
for this process. The use of trifluoromethylated alkynes
consistently generated only the products from the addition of
the acetate to the alkynes and subsequent hydrolysis under
these conditions.23,24 Another limitation was that terminal
alkynes were not tolerated in this process.
a
Reaction conditions: pyrazole (0.50 mmol), diphenylacetylene (1.5
mmol), Pd(OAc)2 (0.025 mmol), additive (0.50 mmol), oxidant (1.5
b
mmol), solvent (1.5 mL), 120 °C. Ag2CO3 (1.0 mmol) and 2,6-
Me2C6H3CO2H (0.25 mmol) were used. Pyridine (0.10 mmol) was
used.
c
the yield was moderate, the formation of indazoles could
successfully be achieved by C−H functionalization of both the
C4 and C5 positions of pyrazoles.19 In addition, Cu(OAc)2·
H2O was superior to Ag2CO3, AgOAc, and Cu(OAc)2 as the
oxidant in 1,4-dioxane (entries 2−5). When the solvent was
switched to DMA, no significant decrease of the yield was
observed (entry 6). However, the combination of copper
acetate monohydrate and mesitylene was not efficient (entry
7). No beneficial effects were observed from carboxylic acids
and pyridine (entries 8−10). In terms of the oxidant, oxygen
could not replace the copper salt for this transformation (entry
11).20
The Pd-catalyzed and Cu-mediated benzannulation reactions
provided a wide variety of indazoles from simple pyrazoles in a
single step (Table 2). The N-alkylpyrazoles, including n-butyl-,
SEM-, and THP-protected pyrazoles and pyrazolyl propanoate,
were transformed to the corresponding indazoles 3, 4, 5, and 6,
respectively (entries 1−4). Although N-benzylpyrazole gave a
mixture of inseparable products, the reactivity of the one-
carbon homologue, phenethyl-substituted pyrazole, was accept-
In order to account for the observed regioselectivity in the
reaction of unsymmetrical alkynes, a series of deuterium
exchange experiments were carried out, indicating that
B
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metalation at the C4 position was favored relative to the C3
and C5 positions (Scheme 1). More interestingly, the presence
This convergent strategy led to the preparation of a library of
electronically diverse π-conjugated indazoles and benzimida-
zoles, facilitating optimization of the electronic properties. In
particular, the solutions of tetraarylindazoles prepared in this
study exhibited notable fluorescence under UV irradiation. The
solid-state fluorescence of the electronically different tetraar-
ylindazoles 2a and 2c−g was measured, where the methoxy
variant 2d emitted deep blue photoluminescence with a
maximum emission peak at 416 nm with a full-width at half-
maximum of 67 nm (Figure 4).22 This result indicates that the
Scheme 1. Deuteration Experiments
of both Pd(OAc)2 and Cu(OAc)2 was crucial for successful
metalation of the pyrazole, indicating that the Cu(II) salt was
not a simple oxidant but possibly served as a Lewis acid to
promote the metalation step.22,25
In an attempt to enhance the regioselectivity, 4-bromopyr-
azoles 9 were prepared to facilitate oxidative addition at the C4
position.26 When 9 were subjected to the catalytic system
comprising Pd(OAc)2 and P(tBu)3H·BF4, the corresponding
indazoles were obtained in good yields (Figure 2). One notable
Figure 4. Solid-state PL emission spectra of tetraarylindazoles.
photochemical properties of tetraarylindazoles can be modu-
lated by the benzene substituents of the indazoles for
application in OLEDs as blue dopants. The development of
blue fluorescent organic molecules with high color purity and
sufficient lifetime has been an important goal for commercial
applications of OLEDs.28 In this context, tetraarylindazoles
offer a new opportunity to develop high-performing blue
fluorescent dopants for OLEDs.
Figure 2. Pd-catalyzed benzannulation of 4-bromopyrazoles.
In conclusion, we developed a new pyrazole annulation
reaction for preparation of indazoles. A catalytic amount of
Pd(OAc)2 along with a stoichiometric oxidant, Cu(OAc)2·H2O,
enabled the construction of indazoles possessing different
substituents on the benzene ring. Alternatively, 4-bromopyr-
azoles could be employed for the formation of the same
products with similar efficiency. Furthermore, this protocol
proved to be general and could be applied to the other diazole,
imidazole, to afford the corresponding benzannulated form,
benzimidazole. Complementary to many cyclization methods
that form heterocyclic rings from functionalized arenes, this
new strategy based on the direct conversion of simple diazoles
is useful for providing benzo-fused heteroarenes having
multiple substituents on the benzene ring. Modification of
the para substituents of the tetraaryl groups around the
indazole core allowed manipulation of the emission wavelength,
demonstrating the potential of tetraarylindazoles as a new
fluorophore.
example was the formation of the N-benzyl derivative 7 that
was not accessible with the Pd(OAc)2 and Cu(OAc)2·H2O
catalytic system. Furthermore, the protocol based on the
Pd(0)/Pd(II) catalytic cycle was slightly more efficient than the
oxidative benzannulation for the synthesis of indazoles 2j, 2k,
and 2l.
Given to the smooth conversion of pyrazoles to the
corresponding benzannulated heterocycles, this method was
applied to the other two-nitrogen-containing 5-membered
heterocycle, namely imidazole (Figure 3). Despite the
considerably low reactivity of the C4 position of imidazoles
compared with the C5 of imidazoles, benzimidazoles 11a−c
were produced directly from the imidazoles through these two
C−H bonds.27
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00410.
Full experimental details and characterization data (PDF)
Crystallographic data for 2a (CIF)
Crystallographic data for 2k (CIF)
Figure 3. Pd-catalyzed and Cu-mediated benzannulation of imidazoles.
C
DOI: 10.1021/acs.orglett.7b00410
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Organic Letters
Letter
Yu, J.-Q. Angew. Chem., Int. Ed. 2016, 55, 10578. (g) Park, Y.; Kim, Y.;
Chang, S. Chem. Rev. 2017, DOI: 10.1021/acs.chemrev.6b00644.
(9) Pd: (a) Goikhman, R.; Jacques, T. L.; Sames, D. J. Am. Chem. Soc.
AUTHOR INFORMATION
■
Corresponding Authors
́
2009, 131, 3042. (b) Rene, O.; Fagnou, K. Adv. Synth. Catal. 2010,
*E-mail: gctsui@cuhk.edu.hk.
*E-mail: jmjoo@pusan.ac.kr.
ORCID
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H.; Tanaka, H. Chem. - Eur. J. 2015, 21, 14370.
Gavin Chit Tsui: 0000-0003-4824-8745
Jung Min Joo: 0000-0003-1645-3322
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(19) Crystallographic data for 2a and 2k in this paper have been
deposited with the Cambridge Crystallographic Data Centre as nos.
CCDC 1531638 and 1531639, respectively. Copies of the data can be
obtained, free of charge, via http://www.ccdc.cam.ac.uk/.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Research
Foundation of Korea (NRF-2014R1A1A1004713 and NRF-
2016R1D1A1B03930762), the Korea Research Institute of
Chemical Technology (Enhancement of Korea Chemical
Bank), and the POSCO Science Fellowship of POSCO TJ
Park Foundation. We thank Prof. Do-Hoon Hwang and Jae-Ho
Jang (Pusan National University) for assistance with analysis of
the photochemical properties of the indazoles and for helpful
discussions. G.C.T. thanks the Chinese University of Hong
Kong for the Direct Grant for Research (Project Code
4053199).
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