Sequential SNAr Reaction/Suzuki-Miyaura Coupling/C-H Direct Arylations Approach for the Rapid Synthesis of Tetraaryl-Substituted Pyrazoles

Article abstract of DOI:10.1002/asia.201500362

A rapid synthesis of 1,3,4,5-tetraaryl-substituted pyrazoles has been achieved through a sequence of S_NAr reaction/Suzuki-Miyaura coupling/Pd-catalyzed direct arylations that used 3-iodo-1H-pyrazole as a scaffold. Pyrazoles with four different aryl groups were synthesized in a straightforward manner with no extra synthetic steps, such as protection/deprotection or the introduction of activating/directing groups, using readily available substrates and reagents. The developed synthetic approach enabled the structurally diverse synthesis of multiaryl-substituted pyrazoles without using a glovebox technique.

Full text of DOI:10.1002/asia.201500362

DOI: 10.1002/asia.201500362
Communication
Cross-Coupling Reactions
Sequential SNAr Reaction/Suzuki–Miyaura Coupling/CÀH Direct
Arylations Approach for the Rapid Synthesis of Tetraaryl-
Substituted Pyrazoles
Taiki Morita,[a] Daisuke Kobayashi,[a] Keisuke Matsumura,[a] Kohei Johmoto,[b]
Hidehiro Uekusa,[b] Shinichiro Fuse,*[a] and Takashi Takahashi[c]
Sequential cross-coupling approaches that are based on
Abstract: A rapid synthesis of 1,3,4,5-tetraaryl-substituted
pyrazoles has been achieved through a sequence of SNAr
reaction/Suzuki–Miyaura coupling/Pd-catalyzed direct ary-
lations that used 3-iodo-1H-pyrazole as a scaffold. Pyra-
zoles with four different aryl groups were synthesized in
a straightforward manner with no extra synthetic steps,
such as protection/deprotection or the introduction of ac-
tivating/directing groups, using readily available sub-
strates and reagents. The developed synthetic approach
enabled the structurally diverse synthesis of multiaryl-sub-
stituted pyrazoles without using a glovebox technique.
a readily available heteroaromatic scaffold are powerful meth-
ods for the syntheses of multiaryl-substituted heteroaromatics.
Schmitt and co-workers demonstrated the synthesis of pen-
taaryl-substituted pyridines by five Suzuki–Miyaura cross-cou-
pling reactions starting from readily available 2-chloro-3-hy-
droxypyridine.[7] Knochel and co-workers reported the regiose-
lective functionalization of an oxazole scaffold to obtain triaryl-
substituted oxazoles using TMP bases complexed by LiCl
(TMP=2,2,6,6-tetramethylpiperidyl).[8] Itami and co-workers re-
ported the synthesis of tetraaryl-substituted thiophenes start-
ing from 3-methoxy thiophene through sequential CÀH direct
arylations and a subsequent Suzuki–Miyaura coupling.[9] The re-
search groups of Fagnou,[10] Itami,[11] and Murai[12] achieved the
sophisticated synthesis of fully aryl substituted thiazoles by se-
quential CÀH direct arylations.
Multiaryl-substituted pyrazoles are an important class of com-
pounds, because they are frequently used in pharmaceutical
drugs,[1] agricultural chemicals,[2] and as ligands for transition-
metal catalysts.[3] In addition, they are attractive elements for
fluorescent materials.[4] The most conventional approach[5] to
the synthesis of multiaryl-substituted pyrazoles—condensation
of 1,3-diketone or a,b-unsaturated carbonyl compounds with
substituted hydrazines—often suffers from insufficient regiose-
lectivity.[6] Although many synthetic approaches for multiaryl-
substituted pyrazoles have been reported, the development of
a more divergent, regioselective, protection/deprotection-free,
short synthetic route remains important.
With respect to multiaryl-substituted pyrazole synthesis,
McLaughlin et al. reported the construction of 3,4,5-triaryl pyra-
zoles through three Suzuki–Miyaura coupling reactions based
on the rearrangement of a THP protecting group (THP=tetra-
hydropyran).[13] Sames and co-workers demonstrated the syn-
thesis of 3,4,5-triaryl pyrazoles through a Suzuki–Miyaura cou-
pling and the following two C5 direct arylations using the
“SEM switch” method {SEM=2-(trimethylsilyl)ethoxymethyl}.[14]
To date, however, a synthesis of pyrazoles containing four dif-
ferent aryl groups by four sequential cross-couplings based on
a readily available pyrazole scaffold has not been demonstrat-
ed.
We have reported diversity-oriented syntheses for drug dis-
covery and materials development based on Pd-catalyzed
cross-coupling reactions.[15] Herein, we wish to report a regiose-
lective, protection/deprotection-free, sequential SNAr reaction/
Suzuki–Miyaura coupling/CÀH direct arylations approach to
the synthesis of 1,3,4,5-tetraaryl-substituted pyrazoles that uses
readily available, 3-iodo-1H-pyrazole as a scaffold.
[a] T. Morita, D. Kobayashi, K. Matsumura, Dr. S. Fuse+
Department of Applied Chemistry
Tokyo Institute of Technology
2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552 (Japan)
[b] Dr. K. Johmoto, Dr. H. Uekusa
Department of Chemistry and Materials Science
Tokyo Institute of Technology
2-12-1, Ookayama, Meguro-ku, Tokyo 152-8551 (Japan)
We planned to install four aryl groups, Ar1-Ar4, via a SNAr re-
action at the N1-position, Suzuki–Miyaura coupling[16] at the
C3-position, CÀH direct arylation at the C5-position,[14,17] and
CÀH direct arylation at the C4-position[18,19] using 3-iodo-1H-
pyrazole as the scaffold (Scheme 1). Sames and co-workers[14]
and Gorelsky[20] demonstrated a trend in the reactivity of three
CÀH bonds of pyrazole against CÀH direct arylation based on
a concerted metalation and deprotonation (CMD) mechanism
(C5>C4@C3). We theorized that the use of a Suzuki–Miyaura
[c] Prof. Dr. T. Takahashi
Yokohama College of Pharmacy
601, Matano-cho, Totsuka-ku, Yokohama, Kanagawa, 245-0066 (Japan)
[+] Present address:
Chemical Resources Laboratory
Tokyo Institute of Technology
4259, Nagatsuta-cho, Midori-ku, Yokohama 226-8503 (Japan)
E-mail: sfuse@res.titech.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500362.
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Scheme 1. Synthetic plan for tetraaryl-substituted pyrazoles through a se-
quential cross-coupling approach based on a 3-iodo-1H-pyrazole scaffold.
Scheme 3. SNAr reaction at the N1-position of the pyrazole scaffold. DMA:
N,N-dimethyl acetamide.
coupling could accomplish the high-yielding installation of an
Ar2 group at the C3-position. The key to success was in regio-
selectively activating the C5-position without activating the
C4-position as well as to install Ar4 group in a high yield at the
sterically hindered C4-position. In the present study, only com-
mercially available reagents that could be treated without glo-
vebox techniques were examined for the rapid combinatorial
synthesis of a pyrazole library.
The pyrazole scaffold was prepared in accordance with a pre-
viously reported procedure,[21] as shown in Scheme 2. The in-
Scheme 2. Synthesis of the pyrazole scaffold. THF: tetrahydrofuran, Bu:
butyl, TFA: trifluoroacetic acid.
troduction of a sulfonamide group at the N1-position, iodina-
tion at the C5-position, and the following acidic removal of the
sulfonamide group afforded the desired pyrazole scaffold as
a mixture of tautomers 3 and 4. The desired scaffold was readi-
ly prepared in a gram scale (6.9 g).
Scheme 4. Suzuki–Miyaura coupling at the C3-position of 1-aryl-3-iodopyra-
zole 5. *Reaction conditions: 10 mol% [Pd2(dba)3], 24 mol% PCy3·HBF4,
K3PO4, 1,4-dioxane/H2O 1008C, 18 h. Cy: cyclohexyl.
A SNAr reaction at the N1-position was examined, as shown
in Scheme 3. Ar1 groups containing electron-withdrawing ni-
trile and nitro groups were introduced in excellent yields. As
previously mentioned, the iodo pyrazole scaffold was a mixture
of tautomers, but products 5a and 5b were obtained as single
regioisomers. This result was consistent with previous re-
ports.[17f,22]
useful for the Suzuki–Miyaura coupling of iodo pyrazole 5.
Electron-donating, electron-withdrawing, sterically hindered,
and heteroatom-containing aryl groups as well as a simple al-
kenes were introduced in good-to-excellent yields (67%-
quant.). With respect to the synthesis of 6d, our conditions af-
forded the desired product in a low yield, whereas Fu and co-
workers conditions[25] afforded the desired product in a good
yield (81%). Multi-substituted pyrazoles containing either an
amino group or a carboxamide group at their C3-position are
often found in bioactive compounds.[26,27] Therefore, the Buch-
Suzuki–Miyaura coupling at the C3-position was investigat-
ed, as shown in Scheme 4. We recently reported the combina-
tion of tris(dibenzylideneacetone)dipalladium [Pd2(dba)3],[23]
P(tBu)3·HBF4,[24] and Na2CO3 in THF/H2O (1:1)[15d] for a Suzuki–
Miyaura coupling of bromo-thiophenes. This combination was
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Scheme 5. Pd-catalyzed amination and carbonylative amidation at the C3-
position of 5a. Xantphos: 4,5-bis(diphenylphosphino)-9,9-dimethylxan-
thene.[30]
wald–Hartwig amination[28] and carbonylative amidation[29]
were examined, as shown in Scheme 5. As expected, the de-
sired compounds 6h and 6i were obtained in good yields.
Although the CÀH direct arylations of pyrazoles at the C5-
position have been reported,[17] CÀH direct arylations at the
C5-position of 1,3-diaryl pyrazoles have not. The reaction con-
ditions were optimized using 6b. The combinations of phos-
phine ligands (PPh3, PCy3·HBF4, P(tBu)3·HBF4, di(1-adamantyl)-n-
butylphosphine (cataCXium A),[31] 2-(di-tert-butylphosphino)bi-
phenyl (JohnPhos)[32]) and solvents (DMA, 1,4-dioxane) were ex-
amined. The combination of cataCXium A and DMA caused an
undesired double arylation both at the C4- and C5-positions
(see the Supporting Information), whereas the combination of
PCy3·HBF4 and 1,4-dioxane afforded the desired coupling prod-
uct 7a in the highest yield (50%, BRSM 75%) and high regiose-
lectivity (double arylated product/7a=1:30). The substrate
scope of the optimized reaction conditions was examined, as
shown in Scheme 6. Electron-donating, electron-withdrawing,
sterically hindered, and heteroatom-containing Ar3 groups
were introduced in satisfactory-to-good yields (57–75%).
The introduction of an Ar4 group at the sterically hindered
C4-position of 1,3,5-triaryl pyrazole was examined, as shown in
Scheme 7. Santelli and co-workers reported the direct arylation
of 1,3,5-trimethyl pyrazole at the C4-position.[18b] In our case,
however, the C4-position was sterically more hindered by two
adjacent C3- and C5-aryl groups. In the examination of CÀH
direct arylations at the C5-position, we observed a substantial
degree of over-arylation at the C4-position when using the
combination of cataCXium A ligand in a DMA solvent, as previ-
ously described. Therefore, we employed this combination for
CÀH direct arylation at the C4-position, as shown in Scheme 7.
As a result, electron-donating, electron-withdrawing, and het-
eroatom-containing aryl groups were introduced in satisfactory
to good yields (39–84%).
Scheme 6. CÀH direct arylation at the C5-position of 1,3-diaryl-substituted
pyrazole 6. *1: Reaction conditions: 20 mol% Pd(OAc)2, 30 mol% PCy3·HBF4,
and 60 mol% PivOH. *2: The ratio of substrate 6 to ArBr was 1.5:1. The yield
was calculated based on the Ar-Br. BRSM: based on recovered starting mate-
rial; PivOH: pivalic acid.
shown in Figure 1. All four aryl groups were introduced at the
desired positions. They were distorted from coplanarity by
36.78(9)8 (4-CNPh), 47.15(6)8 (4-MePh), 63.34(7)8 (4-NO2Ph), and
22.13(14)8 (4-CF3Ph) with respect to the central pyrazole ring,
which was probably due to the steric repulsion among the aryl
rings.
In summary, we have successfully demonstrated a regioselec-
tive, protection/deprotection-free, divergent synthesis of tet-
raaryl-substituted pyrazole based on a 3-iodo-1H pyrazole scaf-
fold (4). This is the first example of the synthesis of pyrazoles
with four different aryl groups through four sequential cross-
coupling reactions. Our developed approach employed readily
available substrates and reagents, and a glovebox technique
was not required. The 3-iodo-1H pyrazole scaffold enabled the
Thus, we developed a sequential coupling approach for the
synthesis of tetraaryl-substituted pyrazoles. The developed ap-
proach afforded the desired tetraaryl-substituted pyrazoles in
good yields (4-steps; 22–58% yields based on 4).
The structure of tetraaryl-substituted pyrazole 8a was unam-
biguously determined by X-ray crystallographic analysis as
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installation of alkene, amine, and carboxamide groups at the
C3-position. The developed approach would be a valuable aid
as a rapid lead generation in drug discovery and materials de-
velopment based on multiaryl-substituted pyrazoles.
Keywords: C-H activation · cross-coupling · heterocycles ·
palladium · synthetic methods
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Manuscript received: April 7, 2015
Accepted article published: May 28, 2015
Final article published: June 19, 2015
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