Sterically hindered N-heterocyclic carbene/palladium(ii) catalyzed Suzuki-Miyaura coupling of nitrobenzenes

Article abstract of DOI:10.1039/c9cc04634h

Palladium-catalyzed denitrative Suzuki coupling of nitroarenes using 2-aryl-5-(2,4,6-triisopropylphenyl)-2,3-imidazolylidene[1,5-a]pyridines as the ligands is described. The key to success is the use of the NHC ligands which show strong donating ability and suitable steric hindrance allowing the successful oxidative addition of Ar-NO₂ bonds. Both aromatic and aliphatic boronic acids are tolerated, and a variety of biphenyls and alkylarenes were obtained in good to excellent yields.

Full text of DOI:10.1039/c9cc04634h

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DOI: 10.1039/C9CC04634H
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Sterically Hindered N-Heterocyclic Carbene/Palladium(II)
Catalyzed Suzuki-Miyaura Coupling of Nitrobenzenes
Kai Chen,a Wei Chen,a Xiaofei Yi,a Wanzhi Chen,*a Miaochang Liu b and Huayue Wu*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Palladium-catalyzed denitrative Suzuki coupling of nitroarenes denitrative coupling reactions of nitroarenes for the C-O, C-N,
using 2-aryl-5-(2,4,6-triisopropylphenyl)-2,3-imidazolylidene[1,5- and C-S bond formation.12
a]pyridines as the ligands is described. The key to success is the use
of the NHC ligands which show strong donating ability and suitable
N
R3
N
steric hindrance allowing the successful oxidative addition of Ar-
NO2 bonds. Both aromatic and aliphatic boronic acids are tolerated,
and a variety of biphenyls and alkylarenes were obtained in good
to excellent yields.
-
PF6
R2
L11
N
Cl-
n
R1
N
iPr
iPr
N
N
Nitroarenes are fundamental feedstocks for pesticide, dye, and
pharmaceutical industries.1 Nitroarenes are readily available
and cheap, and consequently they are ideal electrophilic
coupling partners serving as arylating agents for manufacture of
high-value chemicals.2 However, transition metal-catalyzed
coupling reactions involving denitration coupling of nitroarenes
are quite rarely studied in contrast to those of haloarenes3 and
(pseudo)haloarenes,4 due to the inertness of C-N bond towards
oxidative addition. In 2011, Wu and coworkers reported
rhodium-catalyzed coupling of arylboronic acids with electron-
deficient nitroarenes leading to ethers.5 The same reaction
could also be furnished by nano CuO.6 The coupling of
nitroarenes with phenols instead of arylboronic acids was also
successful leading to aromatic ethers.7 Later, copper and
palladium-catalyzed C-S coupling reactions of nitroarenes to
thioethers were also described.8 Nakao and coworkers recently
developed the first palladium-catalyzed Suzuki-Miyaura9 and
Buchwald-Hartwig10 cross-couplings of nitroarenes. These C-C
and C-N coupling processes involve the challenging oxidative
addition of Ar-NO2 towards Pd(0) species. The successful
activation of Ar-NO2 bond was attributed to the ancillary bulky
biphenyl monophosphine ligands, which are often expensive.11
Most recently, Liu et al summarized the progresses of
Cl-
iPr
L1: R1 = R2 = R3 = Me, n = 0
L2: R1 = R2 = iPr, R3 = H, n = 0
L3: R1 = R2 = Et, R3 = Me, n = 0
L4: R1 = R2 = Et, R3 = H, n = 0
L5: R1 = R2 = Me, R3 = H, n = 0
L6: R1 = R2 = Me, R3 = Br, n = 0
L7: R1 = R2 = R3 = F, n = 0
L8: R1 = R2 = OMe, R3 = H, n = 0
L9: R1 = R2 = H, R3 = Me, n = 0
L10: R1 = R2 = R3 = Me, n = 1
L12
R1
R1
N
N
Cl-
R3
R3
R2
R2
L13: R1 = R2 = R3 = Me
Ph
, R3 = Me
L14: R1 = R2
=
Ph
Figure 1. Ligand precursors HL·X (X = Cl-, PF6 ).
-
N-Heterocyclic carbenes (NHCs) offer several advantages
over bulky phosphines as ligands. The precursors of NHCs are
generally more stable, and they are prepared under more
convenient conditions using readily available aldehydes and
amines. More importantly, NHCs are much stronger σ-donors,
and their steric bulkiness are tunable through variation of the
N-substituents.13 NHCs have proved successful in many metal-
catalyzed transformations, especially in C-C14, 4g, 4h and C-N15
cross-coupling reactions. Thus, we envisioned that reasonably
designed NHCs with appropriate steric hindrance would
promote the palladium-catalyzed denitrative coupling of
nitroarenes. Most likely, a NHC ligand geometrically resembling
the biphenyl phosphine ligands16 would be the choice. As a
continuation of our organometallic chemistry of NHCs,17 herein
we report the Pd-catalyzed Suzuki coupling of various
a. Department of Chemistry, Zhejiang University, 38 Zheda Road, Hangzhou 310027,
China.
b. College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou,
325027, China.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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nitroarenes with aromatic and aliphatic boronic acids using 5- compound was obtained in a poor yield. Obviously, 5-(2,4-6-
View Article Online
(2,4,6-triisopropylphenyl)imidazo[1,5-a]pyridin-2-ium
(Figure 1) as the ligand precursors.
salts triisopropylphenyl) moiety is important DfoOrI:t1h0e.10s3t9a/bCi9liCzCat0i4o6n34oHf
active palladium species.11 The commonly used bulky IMes (L13)
The challenge associated with the coupling of nitroarenes is and IPh* (L14) ligands were also examined, and they showed no
C-N oxidative addition to palladium(0) depending on the or poor efficiency. Finally, we found that when the reaction was
ancillary
ligands.
A
number
of
2-aryl-5-(2,4,6- extended to 48 hours, the yield could be further increased to
were 82%. When the reaction was performed in more polar solvents
triisopropylphenyl)-2,3-imidazolium[1,5-a]pyridines
prepared.18 We commenced our investigation by choosing the such as DCM, DMSO, and H2O, no coupling product was
reaction of p-nitroanisol 1a and phenylboronic acid 2a as the observed. The soluble organic bases such as Et3N and DBU (1,8-
model to screen the role of NHC ligands. The reaction was diazabicyclo[5.4.0]undec-7-ene) are inefficient. After examining
o
initially performed at 130 C at a loading of 5 mol% Pd(acac)2 palladium sources, solvents, bases, additives, and reagent
and 10 mol% imidazolium salt in dioxane using K3PO4·3H2O as stoichiometries, we established that the best conditions were 5
the base (Figure 2). L1 has proved to be a suitable ligand for the mol% Pd(acac)2 and 10 mol% L5 in dioxane using K3PO4·3H2O as
coupling reaction, and the desired product 3aa was obtained in the base. The optimization details were summarized in Tables
55% yield. Without the ligand, Pd(acac)2 itself did not catalyze S1-S7 (supporting information).
the reaction, and only a trace amount of p-methoxylaniline was
observed resulting from nitro reduction. The use of more
Table 1. Scope of nitroarenesa
HL5.Cl
sterically hindered ligands L2, L3, and L4 led to decreased yields.
Other ligands such as L6, L7, and L8 of the same scaffold with N-
phenyl rings bearing methoxy, fluoro, and bromo substituents
gave low conversions. Comparison of L1-5 and L9 illustrates that
two ortho alkyls are needed to reach high conversion, and the
para-methyl plays a negative role. The Pd/L5 combination gave
the optimal yield 66%. We found that addition of 5 mol% TDA
(tris(3,6-dioxaheptyl)amine) can further improve the reaction
yield to 73%. The role of TDA has been observed in palladium-
catalyzed conversion of aryl chlorides to nitroarenes.19
Pd(acac)2,
K3PO4.3H2O, dioxane
R'
R'
R
NO2
B(OH)2
R
TDA, N2, 130 o
C
3
2
1
N
EtO2C
O
CO2Me
O
O
3aa
, 82%
3ab
3ae
, 72%
3ad, 63%
, 60%
3ac
, 42%
O
O
NO2
[Pd], NHC ligand
NO2
3ag
O
3af
MeO
NO2
base, dioxane
additives,130 o
B(OH)2
3ah
3aj
, 81%
, 59%
3ai, 85%
, 63%
, 15%
C
O
O
O
MeO
O
O
1a
2a
3aa
Ph
Ph
b
3ao
, 50%
3an
3as
, 83%
3al
3am
, 61%
, 60% (70% )
O
3ak
, 70%
O
O
N
F3C
F
3aq, 62%b, c
F
3at, 88%
, 96%
3ap
, 49%
3ar
, 95%
N
N
N
O
N
d
3au
, 94%
3aw
3ax
, 95%
, 70% (94% )
3av
, 34%
aReaction conditions: 1 (0.6 mmol, 1.0 eq.), 2 (1.5 eq.),
Pd(acac)2 (5 mol%), K3PO4·3H2O (3.0 eq.), HL5·Cl (10 mol%),
o
b
TDA (5 mol%), dioxane (3.0 mL), 130 C, N2, 48 h. 2 (2.0 eq.).
cCsF (3.0 eq.) in toluene (3 mL). dWithout TDA.
After optimization of the reaction conditions, HL5·Cl was
chosen to explore the scope of nitroarenes (Table 1). A variety
of nitrobenzenes bearing electron-donating substituents such
as alkyl, phenyl, alkoxy, and amino groups were successfully
coupled with phenylboronic acid and 4-methoxyphenylboronic
acid affording the corresponding biphenyl derivatives 3aa-3as
in up to 96% yields. Nitrobenzenes bearing electron-
withdrawing ester, acetyl, and nitro groups at ortho-, meta- and
para-positions could give biphenyls 3ad-3ah in comparable
yields. However, 4-nitrophenylacenone showed low yield, and
we observed that 4-aminoacetophenone was formed in 20%
Figure 2. Screening of ligands. Reaction conditions: 1a (0.3
mmol, 1.0 eq.), 2a (1.5 eq.), Pd(acac)2 (5 mol%), K3PO4·3H2O (3.0
eq.), HL·Cl (10 mol%), TDA, dioxane (1.5 mL), 130 oC, N2, isolated
yields.
The target product 3aa could be obtained in air in 47% yield.
Notably, under the same conditions, L11 with no substituent at
5-position is totally inert, and no desired biphenyl compound
was detected. L12 containing a less bulky 2,6-dimethylphenyl
group at 5-position showed low activity, and the biphenyl
2 | J. Name., 2012, 00, 1-3
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yield due to reduction. In the case of 1,4-dinitrobenzene, the derivatization of biologically active compounds. NVitierwenArdticipleiOnnelinies
reaction gave a complicated mixture and no coupling product a common antihypertensive drug1c, 1d which can be derivatized
DOI: 10.1039/C9CC04634H
was
detected.
The
coupling
reactions
of
4- delivering N1 and N2 in 73% and 30% yields, respectively
methoxyphenylboronic acid with various nitroarenes showed (Scheme 1, eq. B).
the similar trend. Nitroarenes containing methyl, phenyl and
Table 2. Scope of boronic acids[a]
trifluoromethyl substituents were successfully coupled giving 4-
arylanisols 3aj-3ap in good to excellent yields. The sterically
hindered 1,3-dimethyl-2- nitrobenzene could be transformed to
3al in 61% yield. Interestingly, fluorobenzene is well tolerated
and readily coupled, and 4-fluoro-4'-methoxy-1,1'-biphenyl 3ar
was obtained in nearly quantitative yield. It should be
mentioned that Ph-X (X = Cl, Br, and CN) bonds are much more
reactive than Ph-NO2 bond. Thus, the coupling of Cl and Br-
containing nitrobenzenes, and 4-nitrobenzonitrile afforded
nitro-containing coupling products with Ph-NO2 moieties
remained. The results listed in Table 1 illustrate that the yields
do not strictly correlate to the electronic properties of
substituents. We also examined the possibility of
nitroheteroarenes. Remarkably, 3-nitropyridines can be used as
the coupling partner delivering the corresponding
phenylpyridines 3at and 3au in high yields. Similarly, the Suzuki
coupling of 5-nitroquinoline and 5-nitroisoquinoline could also
be coupled in excellent yields. In sharp contrast, the coupling of
4-nitropyridine and 8-nitroquinoline failed.
O
CO2Et
F
3be
, 65%
3ba
, 87%
CF3
3bd
, 76%
3bb
3bc
, 86%
Ph
, 87%
OMe
N
3bf
, 48%
3bg
, 80%
3bj
, 63%
3bh
3bi
, 82%
, 70%
N
S
O
3bk
3bl
, 94%
, 48%
3bm
, 81%
3bn
, 80%
3bo
, 81%
The scope of the coupling reaction with respect to various
aromatic and aliphatic boronic acids was next investigated. The
results for 1-nitronaphthalene and nitrobenzene were
summarized in Table 2. ArB(OH)2 bearing different functional
groups could furnish the reaction giving the corresponding
products 3ba-3bk in 48-87% yields. Electron-donating groups of
phenylboronic acids generally increase yields of coupling
products 3ba-3bc, while electron-deficient phenylboronic acids
showed diminished yields for 3bd-3bf. The coupling of
thiophen-3-ylboronic acid and 2-methoxy-3-nitropyridine
occurred smoothly affording the target product 3bk in
moderate yields. Unfortunately, heteroarylboronic acids such
as furan-2-yl-, thiophen-2-yl-, and pyridin-3-ylboronic acid are
not suitable coupling partners, and no desired products were
obtained. To our delight, unlike the previously reported
Pd/Brettphos catalyst,9 the present Pd/NHC catalyst works well
for primary aliphatic boronic acids. Alkylation of 1-
nitronaphthalene with different chain length alkylboronic acids
produced alkylated naphthalenes 3bl-3bs in good to excellent
yields. Especially, cyclopropyl group, an important building
block in medicine was successfully introduced. The reaction was
found to be clean, and 1-cyclopropylnaphthalene 3bt was
isolated in 93% yield. The product is the key intermediate of
Lesinurad, which is used to treat hyperuricemia associated
gout.20 Similarly, 5-nitroquinoline and 5-nitroisoquinoline were
also coupled with cyclopropylboronic acid furnishing the
corresponding products 3ca and 3cb in 86% and 96% yields,
respectively. However, reactions of secondary isopropyl-,
cyclobutyl-, and cyclohexylboronic acids failed probably
because of ease of β-H elimination.
9
3bs
3bt
, 93%
3br
, 67%
, 59%
3bq
, 93%
3bp
, 95%
CF3
3by
b
b
b
b
3bx
, 87%
9
, 68%
3bu
3bw
, 75%
, 95%
3bv
, 75%
N
N
O
N
F
3cc
b
3bz
3ca
, 86%
, 47%
3cb
, 96%
, 49%
aReaction conditions: 1 (0.6 mmol, 1.0 eq.), 2 (1.5 eq.), Pd(acac)2
(5 mol%), K3PO4·3H2O (3.0 eq.), HL5·Cl (10 mol%), TDA (5
mol%), dioxane (3 ml), 130 oC, N2, 48 h. b 2 (2.0 eq.).
In summary, we have developed an efficient and
chemoselective palladium/NHC-catalyzed Suzuki coupling
reaction
of
nitroarenes
that
uses
5-(2,4,6-
as the
triisopropylphenyl)imidazoylidene[1,5-a]pyridine
ligands. The NHC ligands are in situ generated and their
precursors are readily prepared from common commercial
reagents. The protocol shows low loading of ligands, broad
scope of substrates, and good chemoselectivity. A diversity of
biphenyls and alkylarenes were obtained in good to excellent
yields. Limitation still exists especially for heterocyclic coupling
partners. Further modification of the NHC ligands is needed to
expand the scope of nitroheterocyclic compounds.
To demonstrate the synthetic utility and practicality of the
Pd/NHC catalyzed Suzuki coupling of nitroarenes, we tested the
reaction
of
1-fluro-4-nitrobenzene
and
4-
methoxyphenylboronic acid at gram scale (Scheme 1, eq. a).
Under the optimized conditions, the coupling of 1-fluoro-4-
nitrobenzene (1.0 g, 7.09 mmol) and 4-methoxyphenylboronic
acid (1.61 g, 10.64 mmol) afforded 1-fluoro-4’-methoxybiphenyl
(1.18 g) in 82% yield. Then we turned our attention to the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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NO2
B(OH)2
standard
condition
F
MeO
F
1.0 g
1.61 g
(10.59 mmol)
5
6
7
(7.09 mmol)
1.18 g, 82% yield
O
O
B(OH)2
MeO
OEt
8
(b)
NO2
N
H
N1, 73% yield
standard
condition
O
O
MeO
OEt
9
N
H
O
O
B(OH)2
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38, 1650-1655.
Nitrendipine
MeO
OEt
N
H
N2, 30% yield
Modification of Nitrendipine
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Scheme 1. Gram scale experiment and derivatization of the
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We thank the National Natural Science Foundation (NSF) of
China (21572203 and 21472140) and the NSF of Zhejiang
Province (LZ16B020001) for financial support.
Conflicts of interest
The authors declare no competing financial interest.
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2015, 9, 3423-3434.
4
4 | J. Name., 2012, 00, 1-3
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N
N
B(OH)2
iPr
iPr
NO2
Pd
or
View Article Online
DOI: 10.1039/C9CC04634H
or
R
iPr
R-B(OH)2
R = Alkyl
53 examples
up to 96% yield
Palladium-catalyzed Suzuki coupling reaction of nitroarenes has been developed using 5-(2,4,6-
triisopropylphenyl)-2,3-imidazolylidene[1,5-a]pyridines as the ligands.