Ruthenium-catalyzed c-h silylation of 1-arylpyrazole derivatives and fluoride-mediated carboxylation: Use of two nitrogen atoms of the pyrazole Group

Article abstract of DOI:10.1055/s-0033-1341230

Carboxylation of 1-arylpyrazole derivatives was developed using a ruthenium-catalyzed ortho silylation in conjunction with fluoride-mediated carboxylation with carbon dioxide. The two nitrogen atoms of pyrazole play crucial roles in promoting ortho silylation via the formation of a five-membered ruthenacycle and in accelerating aryl anion formation by lowering the electron density of the aromatic ring. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1341230

LETTER
▌1291
lRettuer thenium-Catalyzed C–H Silylation of 1-Arylpyrazole Derivatives and
Fluoride-Mediated Carboxylation: Use of Two Nitrogen Atoms of the Pyrazole
Group
Ruthenium-Catalyzed C–H Silylation and Fluoride-Mediated Carboxylation
Tsuyoshi Mita,*a Hiroyuki Tanaka,a Kenichi Michigami,a Yoshihiro Sato*a,b
a
Faculty of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan
Fax +81(11)7064982; E-mail: tmita@pharm.hokudai.ac.jp; E-mail: biyo@pharm.hokudai.ac.jp
b
ACT-C, Japan Science and Technology Agency (JST), Sapporo 060-0812, Japan
Received: 27.02.2014; Accepted after revision: 24.03.2014
substituents of aryltrimethylsilanes (TMSAr), such as
Abstract: Carboxylation of 1-arylpyrazole derivatives was devel-
chloro and nitro groups, were necessary in order to pro-
oped using a ruthenium-catalyzed ortho silylation in conjunction
mote the generation of an aryl carbanion prior to carbox-
with fluoride-mediated carboxylation with carbon dioxide. The two
nitrogen atoms of pyrazole play crucial roles in promoting ortho si-
ylation with CO2.8,9 In addition, its carboxylation was
lylation via the formation of a five-membered ruthenacycle and in conducted in toxic HMPA solvent under 50 atm of CO2.
accelerating aryl anion formation by lowering the electron density
of the aromatic ring.
Quite recently, Kondo reported phosphazenium salt cata-
lyzed carboxylation of TMSAr using CsF under 1 atm of
CO2 at 150 °C.10 However, the product yields were gener-
Key words: C–H activation, carbon dioxide, silylation, ruthenium,
pyrazole
ally moderate and electron-rich arenes were still not appli-
cable.
To reconfirm reactivities of fluoride-mediated carboxyl-
ation, we first investigated several aryltriethylsilanes
(TESAr) possessing various substituents at ortho and
Since carbon dioxide (CO2) is abundant, inexpensive, and
relatively nontoxic,1 it should be an ideal carbon source as
a substitute for petroleum hydrocarbons, the supply of
para positions using an excess amount of CsF in DMF at
which is expected to be greatly diminished this century.
100 °C under 1 atm of CO2 atmosphere (Table 1). Product
Although various catalytic transformations using CO2 gas
yields were determined after methyl esterification using
have been reported over the past decade,1 catalytic car-
MeI in the presence of Cs2CO3. As a result, carboxylations
boxylation of C–H bonds is less exploited,2,3 which moti-
vated us to devise a new and general strategy for C–H
carboxylation using CO2. To realize this type of transfor-
of electron-rich arenes and nonsubstituted arene (TESPh)
did not proceed, with starting arylsilanes 2 remaining (Ta-
ble 1, entries 1–4). In contrast, electron-withdrawing sub-
mation, we have originally devised a sequential protocol
stituents such as 4-Cl, 2-Cl, and 2-NO2 groups enhanced
consisting of a transition-metal-catalyzed C–H silylation
the reactivity of the desired carboxylation in this order
and CsF-mediated carboxylation with CO2. Consequent-
(Table 1, entries 5–7). Considering assumed coordination
ly, benzylic C(sp3)–H bonds could be carboxylated via
of a nitrogen atom to the cesium cation, the 2-(2-pyridyl)
group was substituted at the ortho position of the TES
group, but carboxylation did not occur at all (Table 1, en-
benzylsilane intermediates4 and C(sp3)–H bonds adjacent
to a nitrogen atom were also smoothly silylated and then
carboxylated, affording α-amino acid derivatives.5 Si-
try 8). However, to our surprise, the use of 2-(1-pyrrolyl)
lylations of C(sp3)–H bonds in their first step could be ef-
and 2-(1-pyrazolyl)arylsilanes indeed promoted the car-
fectively catalyzed by iridium,4,5 ruthenium,4 and
boxylation with high efficiency, probably because an
rhodium5 complexes. By using this protocol, C(sp3)–H
electron-withdrawing nitrogen of heteroaromatic com-
bond carboxylation, which had not been achieved catalyt-
pounds facilitates the generation of an aryl carbanion (Ta-
ically, has become possible. However, aromatic C(sp2)–Si
ble 1, entries 9 and 10).9
bonds created as a side reaction in ruthenium catalysis re-
Since carboxylation proceeded smoothly using the pyr-
mained intact even in the presence of an excess amount of
azole substitution, we next screened C–H silylation of 1-
arylpyrazole derivatives using HSiEt3. If the pyrazole
group works as a directing group of silylation as well, a
sequential C(sp2)–H bond silylation–carboxylation reac-
tion might be possible. Transition-metal-catalyzed
C(sp2)–H bond silylation of arenes has been intensively
studied, and several transition metals including rutheni-
um,12 rhodium,13 iridium,14 and platinum15 were reported
to catalyze these silylations. Among these examples,
ortho silylation of 1-phenylpyrazole was tested using
Ru3(CO)1212d or iridium–carbene catalyst,14g but it was not
fully investigated. Thus, we first screened potential cata-
lysts for C(sp2)–H bond silylation using 1-(o-tolyl)pyr-
fluoride.4 Therefore, we next focused on how to promote
the carboxylation of arylsilanes prepared from the corre-
sponding arenes via C(sp2)–H bond silylation.
Compared to fluoride-mediated carboxylation of C(sp3)–
Si bonds with CO2,6,7 carboxylation of arene C(sp2)–H
bonds is relatively difficult since carboxylation is greatly
influenced by the electronic property of the aromatic ring:
Effenberger reported that electron-withdrawing ortho
SYNLETT 2014, 25, 1291–1294
Advanced online publication: 28.04.2014
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9
3
6
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5
2
1
4
1
4
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2
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DOI: 10.1055/s-0033-1341230; Art ID: st-2014-u0173-l
© Georg Thieme Verlag Stuttgart · New York

1292
T. Mita et al.
LETTER
and benzylic C(sp3)–H silylated products in a total yield
Table 1 Carboxylation of Arylsilanes with CO2 Using CsF
12a,b,f
of 97% (Table 2, entry 5). RuH2(CO)(PPh3)3
was
1) CsF (4 equiv)
CO2 (1 atm)
found to exhibit the highest performance to yield the prod-
uct in 98% yield (Table 2, entry 6). Furthermore, catalyst
activity was still maintained with lower catalyst loadings
(2 mol% and 1 mol%, Table 2, entries 7 and 8).
SiEt3
CO2Me
+
H
DMF, 100 °C, 16 h
R
R
R
2) Cs2CO3 (1.2 equiv)
MeI (1.2 equiv)
DMF, r.t., 30 min
2
3
1
Table 2 Silylation of 1-(o-Tolyl)pyrazole
Entry Substrate
Yield of 3 Yield of recovered 2 Yield of 1
(%)a
(%)a
(%)a
catalyst (x mol%)
HSiEt3 (5 equiv)
N
N
N
N
1
2
3
4b
5
6
7
2a R = 4-OMe
2b R = 2-OMe
<1
<1
99
96
97
99
96
<1c
<1
<1
<1
<1
<1
<1
<1
7
additive (y equiv)
toluene, 20 h
SiEt3
1j
2j
2c R = 2-NMe2 <1
Entry Catalyst (x mol%)
Additive Method
(y equiv)
Yield
(%)a
2d R = H
<1
4
2e R = 4-Cl
2f R = 2-Cl
2g R = 2-NO2
1
2
3
4
5
6
7
8
[Ir(cod)Cl]2 (5)c
[Ir(cod)OMe]2 (5)c
[Rh(cod)Cl]2 (5)c
RhH(CO)(PPh3)3 (10)
Ru3(CO)12 (5)
–
–
–
–
reflux
reflux
reflux
reflux
87
77
28
<1
64
85
N
NBEd (5) closed (100 °C) 89 (8b)
8
9
<1
99
<1
SiEt3
RuH2(CO)(PPh3)3 (5) NBEd (5) closed (100 °C) 98
RuH2(CO)(PPh3)3 (2) NBEd (5) closed (100 °C) 99
RuH2(CO)(PPh3)3 (1) NBEd (5) closed (100 °C) 98
2h
N
a Yields were determined by 1H NMR analysis using 1,1,2,2-tetra-
chloroethane as an internal standard.
93
92
<1
<1
4
6
SiEt3
b Yield of the bis-silylated product having an SiEt3 group both at ben-
zylic and ortho positions.
2i
c cod = 1,5-cyclooctadiene.
d NBE = norbornene.
N
N
10
SiEt3
Having decided an optimal catalyst for C–H silylation, we
then conducted silylation and carboxylation in the same
flask as a one-pot operation using 2 mol% of
RuH2(CO)(PPh3)3 (Scheme 1). Despite the fact that an
aprotic and polar solvent such as DMF was suitable for
fluoride-mediated carboxylation,7 C–H silylation did not
proceed efficiently in DMF. Therefore, a solvent ex-
change from toluene to DMF was necessary after the C–H
silylation.4 A variety of substituted aryl substrates worked
very well to afford the desired carboxylic acid derivatives
in up to 90% yield without isolation of the silylated inter-
mediates (1j–o).16 In contrast to the precedented C(sp2)–H
bond silylation using Ru3(CO)12,12d nonsubstituted 1-phe-
nylpyrazole 1k reacted at only one ortho position, afford-
ing monocarboxylic acid derivative 3k in 82% yield. In
addition, 2- and 1-naphthyl substrates were well tolerated.
Furthermore, alkenyl substrate 1r also underwent carbox-
ylation to produce the corresponding product in 54% yield
using seven equivalents of norbornene. Other directing
groups such as indazole and 3,5-dimethylpyrazole were
also applicable to this sequential process.
2j
a Yields were determined by 1H NMR analysis using 1,1,2,2-tetra-
chloroethane as an internal standard.
b Determined by GC analysis.
c Xanthone (4%) and o-anisaldehyde (6%) were produced through a
benzyne intermediate.10,11
azole (1j) as a model substrate. According to the iridium-
catalyzed benzylic silylation developed by our group,4
[Ir(cod)Cl]2 was firstly employed under toluene reflux
conditions without using a hydrogen-trapping reagent. As
a result, the target ortho silylation product was obtained in
87% yield, but a small amount of the starting material re-
mained (Table 2, entry 1). The use of [Ir(cod)OMe]2 and
[Rh(cod)Cl]2 did not improve the yield (Table 2, entries 2
and 3).5 The silylation was completely shut down when
using RhH(CO)(PPh3)3 (Table 2, entry 4). In contrast, ru-
thenium catalysts used with norbornene as a hydrogen-
trapping reagent at 100 °C in a closed system exhibited
much higher catalytic activity: the use of 5 mol% of
Ru3(CO)12 led to the generation of both arene C(sp2)–H
Synlett 2014, 25, 1291–1294
© Georg Thieme Verlag Stuttgart · New York

LETTER
Ruthenium-Catalyzed C–H Silylation and Fluoride-Mediated Carboxylation
1293
ation facilitated by the inductive effect of the nitrogen
atoms.
RuH2(CO)(PPh3)3
(2 mol%)
HSiEt3 (5 equiv)
CO2 (1 atm)
CsF (3 equiv)
N
N
N
N
R
R
DMF, 100 °C, 16 h
then Cs2CO3, MeI
NBE (5 equiv)
toluene, 100 °C
20 h
SiEt3
Acknowledgment
1
2
This work was financially supported by Grants-in-Aid for Young
Scientists (B) (No. 24750081) and for Scientific Research (B) (No.
23390001) from JSPS, by a Grant-in-Aid for Scientific Research on
Innovative Areas ‘Molecular Activation Directed toward Straight-
forward Synthesis (No. 25105701)’ from MEXT, and by the Uehara
Memorial Foundation.
solvent exchange
N
N
N
N
N
N
CO2Me
3j: 85%
CO2Me
3k: 82%
CO2Me
3l: 85%
Supporting Information for this article is available online at
N
N
N
N
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
N
N
CO2Me
3m: 71%
(84 h for silylation)
MeO
CO2Me Ph
CO2Me
3o: 89%
References and Notes
3n: 79%
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N
N
N
N
N
N
CO2Me
CO2Me
CO2Me
3q: 78%
3p: 90%
3r: 54%
(using 7 equiv of NBE)
other directing groups
N
N
N
N
CO2Me
CO2Me
3t: 74%
3s: 85%
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Scheme 1 Substrate scope; isolated yields are shown
It is obvious that the two nitrogen atoms of pyrazole play
two important roles in (1) promoting ortho silylation via
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the formation of
a
five-membered ruthenacycle
intermediate12 and (2) promoting aryl anion generation by
an electron-withdrawing pyrazole nitrogen, which leads
to reduction of the electron density of the aromatic ring as
depicted in Figure 1.
electron-withdrawing
nitrogen
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N
N
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H
five-membered
chelation with a metal
Figure 1 Effects of the two nitrogen atoms of the pyrazole group
In summary, we successfully developed carboxylation re-
actions of 1-arylpyrazole derivatives by using ruthenium-
catalyzed ortho silylation followed by fluoride-mediated
carboxylation. The two nitrogen atoms of pyrazole play
crucial roles in promoting ortho silylation via the forma-
tion of a five-membered ruthenacycle as well as carboxyl-
carboxylations of C(sp)–Si bonds, see: Yonemoto-
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Chem. Int. Ed. 2011, 50, 1393. (b) Mita, T.; Sugawara, M.;
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1291–1294

1294
T. Mita et al.
LETTER
(c) Mita, T.; Chen, J.; Sugawara, M.; Sato, Y. Org. Lett.
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(16) General Procedure for the One-Pot Carboxylation
Into a 10 mL sealed tube was placed substrate 1j (47.5 mg,
0.3 mmol, 1.0 equiv), and then the tube was evacuated and
backfilled with argon (3×). To the reaction tube were added
toluene (0.15 mL, 2.0 M), norbornene (141.2 mg, 1.5 mmol,
5.0 equiv), RuH2(CO)(PPh3)3 (5.6 mg, 0.006 mmol, 2
mol%), and HSiEt3 (174.4 mg, 0.24 mL, 1.5 mmol, 5.0
equiv). The system was closed and stirred at 100 °C for 20 h.
The reaction mixture was directly pumped up at r.t. to
remove volatile materials such as toluene, HSiEt3, and
norbornene, followed by the introduction of CO2 (balloon).
To the residue were added DMF (3.0 mL, 0.1 M) and flame-
dried CsF (136.7 mg, 0.9 mmol, 3.0 equiv). The system was
closed again and stirred at 100 °C under 1 atm of CO2 for 16
h. The reaction mixture was then cooled to r.t. and treated
with Cs2CO3 (117.3 mg, 0.36 mmol, 1.2 equiv) and MeI
(51.1 mg, 22.4 μL, 0.36 mmol, 1.2 equiv) followed by
stirring at r.t. for 30 min. H2O was added, and the mixture
was extracted with EtOAc (3 × 10 mL). The combined
organic layer was washed with H2O (1×) and brine (1×) and
then dried over Na2SO4. After removal of the solvent under
reduced pressure, the residue was purified by silica gel
column chromatography (hexane–EtOAc, 7:1) to afford the
corresponding ester 3j (54.9 mg, 253.9 μmol, 85% yield);
white solid; mp 64.3–66.0 °C. IR (Nujol): 2924, 2854, 1731,
1589, 1518, 1301, 1199, 1149, 1114, 747 cm–1. 1H NMR
(500 MHz, CDCl3): δ = 7.74 (d, J = 7.7 Hz, 1 H), 7.71 (d,
J = 1.7 Hz, 1 H), 7.52 (d, J = 2.3 Hz, 1 H), 7.46 (d, J = 8.3
Hz, 1 H), 7.40 (dd, J = 8.3, 7.7 Hz, 1 H), 6.44 (dd, J = 2.3,
1.7 Hz, 1 H), 3.63 (s, 3 H), 2.10 (s, 3 H) ppm. 13C NMR (125
MHz, CDCl3): δ = 166.2, 140.1, 138.8, 136.9, 134.2, 131.3,
129.6, 128.7, 128.2, 106.0, 52.2, 17.3 ppm. HRMS (EI): m/z
calcd for C12H12N2O2 [M]+: 216.0899; found: 216.0895.
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Synlett 2014, 25, 1291–1294
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