Efficient N-arylation catalyzed by a copper(I) pyrazolyl-nicotinic acid system

Article abstract of DOI:10.1016/j.tet.2010.09.102

Catalyst 6-(1H-pyrazol-1-yl)nicotinic acid L-CuCl behaves as a very active promoter of the N-arylation reactions, as it has been demonstrated with varieties of substrates under mild reaction conditions. A Cu(I) complex based on L of precatalyst has been isolated by a hydrothermal method and structurally characterized.

Full text of DOI:10.1016/j.tet.2010.09.102

Tetrahedron 66 (2010) 9141e9144
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Efficient N-arylation catalyzed by a copper(I) pyrazolyl-nicotinic acid system
Hai-Yang Liu , Zhen-Tao Yu ,*, Yong-Jun Yuan a , Tao Yu , Zhi-Gang Zou a
a
a
,
b
,
b
a
a
Eco-Materials and Renewable Energy Research Center (ERERC), National Laboratory of Solid State Microstructures, 22 Hankou Road, Nanjing University, Nanjing 210093, China
Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 June 2010
Received in revised form 27 September 2010
Accepted 29 September 2010
Available online 14 October 2010
Catalyst 6-(1H-pyrazol-1-yl)nicotinic acid L-CuCl behaves as a very active promoter of the N-arylation
reactions, as it has been demonstrated with varieties of substrates under mild reaction conditions. A Cu(I)
complex based on L of precatalyst has been isolated by a hydrothermal method and structurally
characterized.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
N-Arylation
Copper catalysis
Pyrazolyl-nicotinic acid
Cu(I) precatalyst
1. Introduction
Consequently, we have selected a new combinated ligand 6-
(1H-pyrazol-1-yl)nicotinic acid (L), that is, an ideal bidentate che-
N-arylation is an important transformation in organic synthesis,
and the products have high utilities in synthetic chemistry, life
lator containing two coordinating nitrogen atoms generated from
introducing a pyrazolyl group at the 1-position of the nicotinic acid,
to unravel its catalytic activity for CeN bond-forming reaction.
Additionally, the multidentate ligands display externally as simple
added reagents, the copper/ligand catalytic processes have not
been well-determined, especially the structures of Cu(I) pre-
1
e3
sciences, and polymer industries.
Catalysis in this type of re-
action by low-cost and environmental-friendly copper is attracting
considerable attention, due to the potential for the industrial and
practical applications. During the last decade, some remarkable
3
2e34
achievements in copper-catalyzed N-arylation reactions have been
catalysts are rarely investigated.
In this paper, we describe the
made.4
e13
Most of these cases are performed with CuI as the cat-
O and CuCl with
ligand 6-(1H-pyrazol-yl)nicotinic acid and its promising catalytic
application with CuCl in arylation of N-nucleophiles. Concurrently
a dark red Cu(I) complex Cu(L)Cl$CuCl (1) with a 1D double-chain
structure as Cu(I)-precatalyst was synthesized with a good yield
from the ligand (L) with copper chloride dihydrate under a special
hydrothermal reduction condition.
alyst, and other Cu sources, such as inexpensive Cu
low sensitivity to light and air are rather limitly used.
identification to promote efficient and economical N-arylation re-
2
14e18
Ligand
19e21
actions is of stimulating interest in such catalytic system.
Much
effort is still needed in the search for new ligands that can be used
to further improve the efficiency and generality of copper-catalyzed
N-arylation.2
2e26
Indeed, N-heterocycles carboxylic acid ligands
2
. Results and discussion
have been demonstrated as highly effective ancillary ligand for
accelerating Cu(I)-catalyzed CeN bond-forming proceses,27e30
probably because they bond to the metal centers through the car-
boxylate group to make the Cu(I) species more reactive toward an
eventual first oxidative addition or coordination of the substrate
aryl halide.31 Also, pyrazole and its derivatives are found previously
to be quite effective for coupling of a variety of nitrogen-containing
heterocycles with aryl halides.
From the lower cost starting materials, ligand L was easily pre-
pared through the substitution reaction at nitrogen of deproto-
nated pyrazole with methyl 6-chloronicotinate and acidic
3
5
hydrolysis in high yield and on a multigram scale. Treatment of
the ligand L with CuCl in THF gave an organic precipitate. The
resulting product is almost insoluble in common organic solvents
except in DMF and it is thus difficult to determine structural in-
formation and satisfactory analytical data. Fortunately, hydrother-
mal reduction treatment of L and CuCl
the Cu(I) complex 1 based on ligand L by the reduction of CuCl
with 4-hydroproline as a reducing additive agent (Scheme 1).
2
was effective to generate
*
Corresponding author. Fax: þ86 25 83686632; e-mail address: yuzt@nju.edu.cn
Z.-T. Yu).
2
(
0
040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.09.102

9142
H.-Y. Liu et al. / Tetrahedron 66 (2010) 9141e9144
ꢀ
ꢀ
When using CuCl as the starting reagent without the reductant,
single-phase 1 could not be obtained. The successful incorporation
of ligand L inside the structure of 1 was confirmed with elemental
87.69(6) and 97.41(6) , respectively. In the solid-state structure of
1, the electron-withdrawing carboxylic acid groups of ligands are
not coordinated to metal copper atom and extend outboard of the
chains (Fig. 1). The chains in 1 are held together tightly mainly
through electrostatic interactions resulting in its poor solubility
except in DMF (Fig. 2). The result should provide good insight on
solvent choice for improved catalytic applications of 1.
(
C, H, N) analyses, and by comparison of the observed powder XRD
patterns with that generated from single-crystal structural data.
HOOC
HOOC
N
N
N
N
N
Cu
N
Cu
Cl
Cu
HOOC
Cl Cl
Cu
Cl
Cl
Hydrothermal, CuCl
2
Cu
Cu
Cu
N
o
Cl
Cl
1
40 C, 2d
Cl
N
N
Cl Cl
L
N
Cu
N
N
N
N
N
COOH
(84% yield)
COOH
1
Scheme 1. Preparation of Cu(I) complex (1) from ligand 6-(1H-pyrazol-yl)nicotinic
acid (L).
Complex 1 is stable in air or water for several months, and in-
soluble in water and common organic solvents, such as ethanol,
acetone, acetonitrile, and THF but soluble in DMF. It crystallizes in
1
a monoclinic system with space group P2 /c. In the structure of 1,
there are two types of Cu coordination environments. The Cu(1)
site is coordinated by two nitrogen donors of the rigid ligand
fragment and by two Cl atoms to furnish a distorted tetrahedral
Fig. 2. One-dimension chain structure of 1 along the a-axis (color codes: Cu cyan, Cl
green, O red, N blue, C gray).
ꢀ
geometry. The distance between Cu(1) and N(1) is 2.135(5) A,
ꢀ
whereas the distance between Cu(1) and N(3) is 2.000(5) A. Cu(2)
The catalytic activities of complex 1 and the designed ligand L
together with copper(I) were initially evaluated by using the CeN
coupling of bromopyridine with pyrazole as a model reaction. The
N-arylation of nitrogen-containing heterocycles is of great interest
due to the importance of the resulting products with possible
synthetic applications in biochemistry and medical science. The
accelerating effectivity of the ligand L toward N-arylation of pyr-
azole with bromide pyridine in DMF with different bases under
a 10 mol % catalytic copper is given in Table 1. The preliminary
studies revealed that the aryl bromide can be successfully coupled
is trigonal-coordinated by one
Fig. 1). The two different Cu atoms are joined together through the
common -Cl atom to form infinite 1D double-chain [Cu Cl L]
along the c axis. It is interesting to stress that in 1 two Cu(1) atoms
and two Cu(2) atoms bridged by two -Cl atoms and two -Cl
atoms, form a distorted 8-ring [Cu Cl ] and a cyclic [Cu Cl ] core in
2 3
m -Cl atom and two m -Cl atoms
(
m
2
2
2
n
m
2
m
3
4
4
2
2
which the four atoms are nearly located in a plane. The in-
ꢀ
termolecular Cu(2)/Cu(2A) distance of 2.852(17) A in the complex
is much longer than that in Cu metal, showing the existence of
10
36e38
weakly bonding interactions between the d copper atoms.
ꢀ
with pyrazole at 100 C in the presence of catalytic amounts of CuCl
The angles of Cu(1)eCl(1)eCu(2) and Cl(1)eCu(2)eCl(2A) are
and the ligand L (Cu/L¼1:1) to afford a 99% yield of the desired
product 2-(pyrazol-1-yl)pyridine. Noticeably, this work was carried
out on a 5 mmol scale and demonstrated that this reaction has the
potential for gram-quantity production. Both K
3
PO
4
2 3
and Cs CO
Table 1
a
Cu-catalyzed N-arylation of pyrazole
N
N
HN
Cu/L, DMF
100 oC, 24h
Br
+
N
N
N
Entry
Cu source
Ligand
Base
Yieldb [%]
1
2
3
4
5
6
7
8
CuCl
CuCl
1
CuCl
CuCl
CuCl
d
L
Cs
Cs
Cs
2
2
2
CO
CO
CO
3
3
3
<20
92
87
99
91
85
81
74
d
L
L
L
L
K
K
K
K
K
3
3
2
3
3
PO
PO
4
4
2
$3H O
CO
3
Cu(CH
Cu
3 4
CN) PF
6
PO
PO
4
4
2
O
L
a
Reaction conditions: copper compound (0.5 mmol), ligand 6-(1H-pyrazol-yl)
nicotinic acid (L) (0.5 mmol) or 1 (0.5 mmol), pyrazole (7.5 mmol), 2-bromopyridine
(
5 mmol), base (10 mmol), DMF (20 mL).
Isolated yield after silica-gel flash chromatography (average of two runs).
b
Fig. 1. View of the complex 1 (color codes: Cu cyan, Cl green, O red, N blue, C gray).

H.-Y. Liu et al. / Tetrahedron 66 (2010) 9141e9144
9143
gave better results in DMF. When using K
3
PO
4
$3H
2
O as the base,
of potassium salt produced from 4-pyrazolecarboxylic acid and
PO in DMF. The coupling of imidazole with bromobenzene
the yield only decreased a little bit, indicating that this system was
not very sensitive to trace water. The accelerating effect of L is ev-
ident when a similar ligand-free reaction by CuCl returned below
K
3
4
afforded the corresponding N-phenylimidazole in a yield of 91%.
And the arylaton of imidazole with 2-bromopyridine gave a 94%
yield. We also observed that the N-arylation of 1-chloro-4-iodo-
benzene with twofold excess of pyrazole afforded exclusively the
mono N-arylpyrazole product in satisfactory yield. Such reactions
include nitrogen-containing heterocycles were previously per-
formed with CuI as the catalyst. Here, in combination with the
lower cost CuCl, the ligand L exhibited very high catalytic activity
for the N-arylation under mild reaction conditions.
2
0% yield (Table 1, entry 1), and 2-bromopyridine is often known
less reactive. More importantly, the catalyst was proved to be
particularly efficient, as even a 5 mol % catalyst loading furnished
the product in 72% yield. When the discrete complex 1 was used
with Cs CO as a base, a conversion of 87% was achieved (entry 3),
2 3
which is similar as when the copper(I) catalytic species were gen-
erated in situ from ligand L and CuCl (entry 2). The use of other
copper sources, such as Cu(CH
3
CN)
4
PF
6
(entry 7) and Cu
2
O (entry 8)
The products of arylation reactions were multidentate nitro-
gen-containing compounds, which were also used as N-donor
chelating regents in coordination chemistry for transition-metal
together with ligand L resulted also in satisfying isolated yields. The
high activity contributes possibly to the rigid chelating spacer of L,
which might provide better skeletal support to the metal cop-
41,42
catalysis or medicinal chemistry.
The product 2-(pyrazol-1-yl)
3
9,40
per.
It should be pointed out that, the exact nature of catalyt-
pyridine (L1) (in Table 1) turns out to be a complexing agent for
Cu(I). Herein we synthesized a new tetranuclear Cu(I) complex
Cu Cl (L1) (2) from L1 and CuCl . The dark red complex 2 was
4 4 2 2
ically active species of 1 is not clear, because the chain structure of 1
might be broken in polar DMF and the formed activated Cu(I)
species bearing one ligand L probably promote the reaction effi-
ciency as well, which has been reported previously for similar
catalytic behavior of the one-dimensional coordination polymer
obtained with the similar condition for 1. Complex 2 crystallizes
in the triclinic, space group Pꢁ1.
The structure of 2 also contain two different Cu(I) atoms. Cu(1) is
four-coordinated by two nitrogen atoms from bidentate ligand L1,
3
4
[Cu
2
I
2
L]
n
(L¼[NNNN] ligand.
With the optimized procedure in hand, the CuCl/L catalytic
and two
Cu(2) is bidentated by Cl(1) and Cl(2) atoms. Notably, Cu(1) and Cu
(2) are bridged by two -Cl atoms, forming an distorted [Cu Cl
ring structure (Fig. 3). The angles of Cu(1)eCl(1)eCu(2) and Cu(1)e
2
m -Cl atoms, exhibiting a distorted tetrahedral geometry.
system was applied to the selective arylation of imidazole and
pyrazole with aryl bromides in DMF. As depicted in Table 2, in the
m
2
4
4
]
ꢀ
presence of excess of K
3
PO
4
as the base at 100 C, N-arylation
ꢀ
ꢀ
products were afforded in good yields except that only small
amount of the desired product was detected when 4-pyr-
azolecarboxylic acid was employed. The possible reason for low
yield in the case of 4-pyrazolecarboxylic acid is the poor solubility
Cl(2)eCu(2A) are 76.42(7) and 81.25(7) , respectively. Though the
Cu1eCu2 distances are confined to a short separation of 2.786
ꢀ
(14) A, there is no CueCu formal bonding expected. The structure of
complex 2 is different from that of the complex 1, and even unlike
that of dinuclear Cu(I) coordination complex based on other pyri-
dine ligands. The distance of Cu(1) and N(1) atom from pyridine
Table 2
ꢀ
group (2.115(6) A) also has longer than that of Cu(1) and N(3) from
CeN Coupling products catalyzed by CuCl/L system in the N-arylation of imidazole
ꢀ
and pyrazolea
pyrazole group (2.039(6) A) (Fig. 3). Since the interesting structure
of complex 2, we expect its catalytic application for analogic re-
action systems in our further study. The rationality of the structures
of 1 and 2 is confirmed by valence bond theory. According to the
Brown parameters, the valence bond sum of each Cu(I) cation in
the two complexes based on strength analysis is consistent with the
expected formal oxidation states, suggesting that both two struc-
tures are reasonable.
Entry
AreX
Nucleophile
Product
Yieldb [%]
4
3
1
2
3
4
85
91
94
81
5
96
6
7
76
88
4 4
Fig. 3. View of [Cu Cl ] ring structure in 2.
3. Conclusions
In conclusion, we have demonstrated that 6-(1H-pyrazol-1-yl)
8
84c
nicotinic acid L mixed with CuCl can efficiently promote N-arylation
reactions of nitrogen-containing heterocycles with aryl bromides,
and a simple and powerful Cu(I) complex 1 for single-molecular
catalysis was determined by single crystal X-ray diffraction analysis.
We anticipate that the successful synthesis of 1 may assist in the
design and exploitation of stable new precatalysts with useful
applications.
a
Reaction conditions: CuCl (0.5 mmol), ligand 6-(1H-pyrazol-yl)nicotinic acid (L)
(
0.5 mmol), N-nucleophiles (7.5 mmol), aryl halide (5 mmol), K
3
PO
4
(10 mmol),
ꢀ
DMF (20 mL), reaction temperature 100 C, reaction time 24 h.
b
Isolated yield [%] after silica-gel flash chromatography (average of two runs).
c
10 mmol of pyrazole was used.

9144
H.-Y. Liu et al. / Tetrahedron 66 (2010) 9141e9144
4
4
. Experimental section
those previously reported data. See Supplementary data for
detailed characterization data.
.1. General methods
Acknowledgements
All the CeN bond-forming reactions were carried out under
N
2
atmosphere by using standard Schlenk techniques. DMF was
distilled under vacuum over CaH . Unless otherwise indicated, all
chemicals were used as received without purification. NMR
spectra were recorded in CDCl with chemical shifts referenced
to SiMe as internal standard on Bruker AMX500 500 MHz FT
NMR spectrometers. The powder XRD patterns (Cu K radiation)
were recorded on Rigaku RINT2000 diffractometer with
We are grateful to the National Science Foundation of China
grant 20901038) and the National Basic Research Program of China
grant 2007CB613301).
2
(
(
3
4
Supplementary data
a
a
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2010.09.102. These data
include MOL files and InChIKeys of the most important compounds
described in this article.
a graphite monochromator at room temperature. Analyses for C,
H, and N were carried out on a PerkineElmer 240C analyzer.
Mass spectra were obtained on a Finnigan spectrometer in ESI
mode. Diffraction intensity data for single crystals of 1 and 2
were collected at room temperature on a Bruker Smart CCD
References and notes
diffractometer equipped with graphitemonochromated Mo K
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a
ꢀ
l
¼0.71073 A). The structures were solved by the direct
1
2
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2
F
with anisotropic thermal parameters for all non-hydrogen
1
atoms.
Synthesis of complex 1: A mixture of CuCl
.085 g), (0.25 mmol, 0.045 g), 4-hydroproline (0.25 mmol,
.033 g), and H
4
2
2
$2H O (0.5 mmol,
1
0
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O (10 mL) was heated at 140 C in a Teflon-lined
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7
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ganic solvents. Elemental analysis: C 27.86, H 2.01, N 12.11% (calcd:
C 28.01, H 2.05, N 12.25%).
8
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1
2
1
1
1
5
1
1
2
1
1
2
Crystal data for 1 (C
monoclinic, space group P2
9
H
7
N
3
O
2
Cl
2
Cu
2
) (CCDC 772363), M¼387.17,
1
/c, a¼5.007(2), b¼16.861(7), c¼14.454
ꢀ
ꢀ
ꢀ
3
ꢁ3
(
6) A,
b
¼102.50(13) , V¼1191.3(8) A, Z¼4,
r
¼2.153 g cm , 2342
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¼0.0523, wR
2
¼0.1156 (I>2
s
), GOF¼1.031.
Cl Cu
) (CCDC 772364), M¼686.29,
4
Crystal data for 2 (C16
H
14
N
6
4
4
2
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3
ꢁ3
(
16) A,
b
¼74.19(2) , V¼550.3(16) A, Z¼1,
r
¼2.358 g cm , 2117
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2
2
2
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.2. General procedure for the copper-catalyzed N-arylation
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5
of nucleophiles
3
3
A 50 mL Schlenk flask equipped with a magnetic stirring bar was
charged with a copper compound (0.5 mmol), ligand L (0.5 mmol)
or 1 (0.5 mmol; without additional ligand), base (10 mmol), DMF
4
3
3
4. Li, F. W.; Hor, T. S. A. Chem.dEur. J. 2009, 15, 10585e10592.
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(
(
ca. 20 mL), imidazole or pyrazole (7.5 mmol), and aryl halide
5 mmol). After two additional cycles of evacuation/backfill with
36. Liu, X. Y.; Mota, F.; Alemany, P.; Novoa, J. J.; Alvarez, S. Chem. Commun. 1998,
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1149e1150.
N
2
, the flask was sealed under N
2
atmosphere and stirred at 100 C.
3
7. Blake, A. J.; Brooks, N. R.; Champness, N. R.; Hanton, L. R.; Hubberstey, P.;
The reaction duration was not vigorously optimized but generally
carried out for 24 h for convenience. After the resulting mixture
was cooled to room temperature, the precipitate was removed by
filtration. After evaporation, the obtained residue was purified by
flash silica-gel column chromatography to give the N-arylated
product. Yields reported in this paper are of isolated material and
represent an average of two independent runs. Compounds were
€
Schroder, M. Pure Appl. Chem. 1998, 70, 2351e2358.
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