Use of acylhydrazine- and acylhydrazone-type ligands to promote CuI-catalyzed C-N cross-coupling reactions of aryl bromides with N-heterocycles

Article abstract of DOI:10.1002/ejoc.201100112

A series of ten acylhydrazine- and acylhydrazone-type ligands were designed and synthesized. Their electronic and steric properties were easily modified and tuned by varying the substituents in the vicinity of the acylhydrazine and acylhydrazone units. The effect of ligands on the catalytic activity of Ullmann reactions was assessed by using a combination of these ligands with CuI. The catalytic system is very efficient for the C-N coupling reaction of azoles with aryl and heteroaryl bromides. A simple and efficient procedure for copper-catalyzed C-N cross-coupling reactions between aryl bromides and NH-containingheterocycles is illustrated. Readily available acylhydrazine- and acylhydrazone-typeligands promote the reaction.

Full text of DOI:10.1002/ejoc.201100112

FULL PAPER
DOI: 10.1002/ejoc.201100112
Use of Acylhydrazine- and Acylhydrazone-Type Ligands to Promote
CuI-Catalyzed C–N Cross-Coupling Reactions of Aryl Bromides with
N-Heterocycles
Liuyi Li,[a] Lei Zhu,[a,b] Dagui Chen,[a] Xuelei Hu,*[b] and Ruihu Wang*[a]
Keywords: Copper / Cross-coupling / Ligand design / N,O ligands / Heterocycles
A series of ten acylhydrazine- and acylhydrazone-type li-
gands were designed and synthesized. Their electronic and
steric properties were easily modified and tuned by varying
the substituents in the vicinity of the acylhydrazine and
acylhydrazone units. The effect of ligands on the catalytic
activity of Ullmann reactions was assessed by using a combi-
nation of these ligands with CuI. The catalytic system is very
efficient for the C–N coupling reaction of azoles with aryl and
heteroaryl bromides.
Therefore, the flexible design and synthesis of ligands with
an appropriate nature is crucial for the synthesis of N-aryl-
azoles by copper-catalyzed Ullmann reaction.
Introduction
N-Arylazoles are an important class of compounds that
are used throughout material and life science industries.[1–6]
A number of these compounds have been reported to serve
as common building blocks of natural products and medi-
cal agents,[2] precursors of N-heterocyclic carbenes
(NHCs),[3] and ionic liquids,[4] ligands of coordination poly-
mers,[5] and transition-metal catalysis.[6] The most straight-
forward and efficient synthetic methods involve transition-
metal-catalyzed C–N cross-coupling reactions of heterocy-
cles with aryl halides.[7–9] Among them, the copper-cata-
lyzed Ullmann-type coupling reactions are particularly at-
tractive, because they often allow the use of low-cost start-
ing materials and readily available copper complexes.[8–16]
However, the initial protocols required harsh conditions,
such as high temperatures, stoichiometric amounts of the
copper reagents, and strong alkoxide bases.[17] Recent ad-
vances revealed that a judicious combination of certain li-
gands and copper reagents enables the reaction to be ef-
ficiently performed under milder conditions. It has been
generally thought that ligands increase the solubility of cop-
per salts and prevent their aggregation in the reaction pro-
cess, while the ligands also enhance the reactivity by in-
creasing the electronic density on the catalytic species.[11–16]
Various types of ligands have been introduced to pro-
mote copper-catalyzed Ullmann-type N-arylation reac-
tions.[10–16] Most notable ligands are bidentate N,O,[11–13]
N,N,[14,15] or O,O[16] compounds. The high catalytic activity
attained in these systems can most likely be ascribed to the
formation of chelating CuI species. In addition, the activity,
selectivity, and stability of the catalytic system can be modi-
fied and tuned by varying the steric and electronic proper-
ties of the ligands as well as their coordinating ability with
metal ions. For example, Buchwald and co-workers have re-
vealed that the activity and selectivity of various Cu-cata-
lyzed Ullmann reactions can be controlled by changing the
type, number, and position of substituents in the phenan-
throline backbone.[14] Although many ligands have been re-
ported,[9–14] the rapid assembly and flexible modification of
structurally diverse ligand systems by simple synthetic
methods are still important for the development of effective
catalysts for the widespread applications of coupling reac-
tions.
In our previous study, we synthesized a series of func-
tionalized azolium-based ionic liquids[18,19] and energetic
salts.[20] Their physical properties can be easily tuned by
varying the substituents in the heterocycles. This strategy
was also applied to Pd(NHC)-catalyzed cross-coupling re-
actions,[19] in which the catalytic activity and selectivity
were optimized through ligand modification. As a continu-
ation of our study on the design and synthesis of hetero-
cyclic ligands for application in materials science and catal-
ysis, we are interested in copper-catalyzed Ullmann reac-
tions. Among the various bidentate ligands used in the
cross-coupling reactions, chelating N,O compounds, such as
8-hydroxyquinoline[11] and amino acids,[12] are efficient li-
[a] State Key Laboratory of Structural Chemistry, Fujian Institute
of Research on the Structure of Matter, Chinese Academy of
Science,
Fuzhou, Fujian 350002, P. R. of China
Fax: +86-591-83711028
E-mail: ruihu@fjirsm.ac.cn
[b] School of Chemical Engineering and Pharmacy, Wuhan
Institute of Technology,
Wuhan, Hubei 430073, P. R. of China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100112.
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Acylhydrazine/hydrazone Ligands in C–N Cross-Coupling
gands for copper-catalyzed N-arylation of azoles with aryl
halides because of the synergetic effect of the nitrogen and
oxygen coordination atoms. The transformation between
hindered and functionalized azoles and aryl halides was
achieved in good to excellent yields.
It is known that acylhydrazine and acylhydrazone deriva-
tives possess high affinity and selectivity to metal ions,[21–23]
and some insight into their binding and chelating ability
to copper has also been explored.[22,23] A broad variety of
substituents could also be introduced to the backbone units,
which gives an opportunity to flexibly adjust their proper-
ties and coordinating ability to metal ions by varying the
substituents. Although acylhydrazine- and acylhydrazone-
type ligands have been extensively used in coordination
chemistry,[21–23] they are seldom chosen as ligands in cataly-
sis. The success of N,O-ligands in Cu-catalyzed C–N forma-
tion reactions prompted us to examine whether these types
of ligands could be useful for the N-arylation of azoles with
aryl halides, especially with inexpensive aryl and heteroaryl
bromides. Herein, we report a copper-catalyzed Ullamnn
reaction of aryl bromides promoted by acylhydrazine- and
acylhydrazone-type ligands.
Scheme 1. Acylhydrazine- and acylhydrazone-type ligands used in
this work.
Results and Discussion
As mentioned above, ligands play an important role in
the success of Cu-catalyzed N-arylation of aryl halides and
azoles. To test the catalytic performance and select the most
efficient acylhydrazine- or acylhydrazone-type ligand, the
coupling reaction of 4-bromoacetophenone and imidazole
was chosen as a model reaction by using ligands 1–10
(Scheme 1) in the presence of CuI and Cs2CO3 in dimethyl
sulfoxide (DMSO). As shown in Figure 1, N-(4-ace-
tylphenyl)imidazole was obtained in moderate to excellent
yields. The difference in their yields most likely resulted
from steric and electronic variations of the substituents in
the acylhydrazine or acylhydrazone unit. Use of ligands L2,
L6, L7, and L9 resulted in higher conversion of the 4-bro-
moacetophenone for this transformation compared with L1
and L5, which revealed that the benzohydrazide group
helped the corresponding acylhydrazine to be an effective
ligand. Ligand L2 displayed the best activity among the
ligands tested, giving a 94% GC yield. The ligands that
formed two chelating rings with copper(I), such as L3, L8–
10, were less efficient than L2, which formed one chelating
Figure 1. Ligand comparison in Cu-catalyzed N-arylation of imid-
azole with 4-bromoacetophenone. Reagents and conditions: 4-bro-
moacetophenone (1.0 mmol), imidazole (1.2 mmol), Cs2CO3
(2.0 mmol), CuI (0.05 mmol), ligand (0.05 or 0.10 mmol), DMSO
(2 mL), 120 °C, 14 h.
ring with copper(I); this can probably be ascribed to the L5 afforded N-(4-acetylphenyl)imidazole in moderate yields
strong coordinating ability of the former to copper(I), be- (Entries 1–7). Among them, L5 gave higher GC yields than
cause the catalytically active species involves the simulta- L2 in all copper sources except CuI. Among the bases
neous coordination of both nitrogen and oxygen atoms to tested, Cs2CO3 was found to be the most efficient; catalysis
the copper(I) atom.[8,23]
by L2 and L5 using K3PO4 gave 64 and 80% yields, respec-
To determine the most suitable reaction conditions for tively (Entries 8 and 9); however, the use of K2CO3 and
the N-arylation of imidazole, the catalytic system was exam- Et3N were inefficient for the catalysis (Entries 10–12). In-
ined by using different copper sources, bases, solvents, and vestigation of a variety of solvents revealed that the use
catalyst loading; the results are listed in Table 1. It was of DMSO was superior to either N,N-dimethylformamide
found that the copper source is important for effective ca- (DMF) (Entries 13–16) or toluene (Entries 17 and 18). No
talysis; CuI was the most reactive, and the respective combi- desirable product was detected when the coupling reaction
nation of CuBr, CuSO4·5H2O, and Cu with either L2 or was run in pure water (Entry 21) because of decomposition
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X. Hu, R. Wang et al.
FULL PAPER
of the ligand in water;[11f] however, trace amounts of water this catalytic system is ineffective for chlorobenzene; with
had no effect on the reaction. Further studies revealed that this reagent, only trace amounts of N-phenylimidazole were
catalyst loading also had an important effect on the cata- detected by GC analysis, even when the reaction time was
lytic activity of the reaction system. An apparent decrease prolonged to 48 h.
in catalytic activity was observed when the catalyst loading
was decreased from 5mol-% CuI and 10mol-% L2 to 2mol-
% CuI and 4mol-% L2 (Entry 19), whereas no difference
Table 2. Cu/L2-catalyzed N-arylation of azoles with aryl bromides
and heteroaryl bromides.[a]
was observed by doubling the amounts of CuI and L2 (En-
try 20).
Table 1. Cu/L2-catalyzed N-arylation of imidazole with 4-bromo-
acetophenone under different conditions.[a]
Entry
Copper source
(mol-%)
Ligand
(mol-%)
Base
Solvent Yield[b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
CuI (5)
CuBr (5)
L2 (10) Cs2CO3 DMSO
L2 (10) Cs2CO3 DMSO
94%
64%
61%
67%
65%
75%
75%
64%
80%
46%
19%
8%
52%
41%
28%
40%
11%
10%
68%
95%
trace
CuSO4·5H2O (5) L2 (10) Cs2CO3 DMSO
Cu (5)
CuI (5)
CuBr (5)
CuSO4·5 (5)
CuI (5)
CuI (5)
CuI (5)
CuI (5)
CuI (5)
CuI (5)
CuI (5)
CuI (5)
CuI (5)
CuI (5)
CuI (5)
CuI (2)
CuI (10)
CuI (5)
L2 (10) Cs2CO3 DMSO
L5 (10) Cs2CO3 DMSO
L5 (10) Cs2CO3 DMSO
L5 (10) Cs2CO3 DMSO
L2 (10) K3PO4 DMSO
L5 (10) K3PO4 DMSO
L2 (10) K2CO3 DMSO
L5 (10) K2CO3 DMSO
L2 (10)
Et3N DMSO
L2 (10) Cs2CO3 DMF
L5 (10) Cs2CO3 DMF
L2 (10) K3PO4 DMF
L5 (10) K3PO4 DMF
L2 (10) Cs2CO3 toluene
L5 (10) Cs2CO3 toluene
L2 (4) Cs2CO3 DMSO
L2 (20) Cs2CO3 DMSO
L2 (10) Cs2CO3 H2O
[a] Reagents and conditions: 4-bromoacetophenone (1.0 mmol),
imidazole (1.2 mmol), base (2 mmol) in the presence of copper and
ligand in solvent (2 mL) at 120 °C for 14 h. [b] GC yield.
To further explore the catalytic efficiency of this system,
a more versatile and practical method was applied to the
coupling reactions between aryl bromides and azoles by
using 10 mol-% CuI, 20 mol-%Ligand (L2), and 2.0 equiv.
of Cs2CO3 in DMSO at 120 °C (Table 2). The effect of vary-
ing the aryl bromides used in the reaction was initially in-
vestigated by using imidazole as a substrate (Entries 1–7).
The N-arylimidazoles were provided in good yields. In ge-
neral, aryl bromides bearing electron-withdrawing groups,
[a] Reagents and conditions: aryl bromide (1.0 mmol), azole
(1.2 mmol), Cs2CO3 (2 mmol), CuI (0.10 mmol), L2 (0.20 mmol),
DMSO (2 mL), 120 °C. [b] Isolated yield.
The Cu/L2 catalytic system was also applied to the cou-
such as -COCH3, -CN, and -NO2, were more reactive than pling reactions of nitrogen- or sulfur-containing heteroaryl
bromobenzene and aryl bromides bearing electron-donat- halides with imidazole under the standard conditions. Inter-
ing groups, such as -Me and -OCH3, which afforded the estingly, the reactions with 2-bromothiophene (Entry 8), 2-
corresponding N-arylated imidazoles in lower yields, even bromopyridine (Entry 9), and 2,6-dibromopyridine (En-
with an extended reaction time of 48 h. For example, the try 10) with imidazole produced 2-(1H-imidazol-1-yl)pyr-
reaction of p-bromonitrobenzene and imidazole proceeded idine, 2,6-bis(1H-imidazol-1-yl)pyridine, and 2-(1H-imid-
with 91% isolated yield in 1 h (Entry 2), whereas the inac- azol-1-yl)thiophene in 67, 91, and 90% isolated yields,
tive 4-bromoanisole required more than 48 h reaction time respectively. The former two products are potentially good
to give the corresponding product in 60% isolated yield candidates for the preparation of hemilabile[24] and pincer-
(Entry 7). These results are consistent with well-established type NHC compounds,[25] respectively, and their metal
trends observed for the Ullmann reaction. Unfortunately, complexes have shown excellent activities in many organic
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Eur. J. Org. Chem. 2011, 2692–2696

Acylhydrazine/hydrazone Ligands in C–N Cross-Coupling
was purified by column chromatography on silica gel to provide
the desired products.
transformations. This procedure thus provides a valuable
route for the preparation of functionalized NHC precur-
sors.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures for the synthesis of L1–10, 1H and
13C NMR spectroscopic data of L1–10 and the coupled products
for the Ullmann reactions.
The CuI/L2 catalytic system is also applicable to the
Ullmann coupling reactions of other N-heterocycles with 4-
bromoacetophenone and 2-bromopyridine (Entries 11–17).
The heterocycles, such as pyrrole (Entry 11), pyrazole (En-
tries 12 and 15), triazole (Entries 13 and 16), and benzimid-
azole (Entries 14 and 17) were found to be effective nucleo-
philic counterparts for the coupling process, and the respec-
tive N-arylated products were obtained in good to excellent
yields. Clearly, the catalytic system is more suitable for
heteroaryl bromides, because 2-bromopyridine is more reac-
tive than 4-bromoacetophenone; this is very attractive for
the synthesis of coordination ionic liquids and hemilabile
NHCs.
Acknowledgments
This work was financially supported by the 973 Program
(2010CB933501, 2011CBA00502), the National Natural Science
Foundation of China (20971122) and the “One Hundred Talent
Project” from Chinese Academy of Sciences.
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Experimental Section
General: Ligands L1–3,[26a] L4,[26b] L5,[26a] L6,[26c] L7,[26d] L8,[26e]
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and used without further purification. All reactions were carried
out in Schlenk tubes under nitrogen. DMSO, DMF, and toluene
1
were distilled before use. H and 13C NMR spectra were recorded
with
a Bruker Biospin Avance III spectrometer at 400 and
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100 MHz, respectively, by using CDCl3 as a locking solvent except
where otherwise indicated. Chemical shifts are reported in ppm
relative to TMS.
Typical Procedure for the Ullmann Cross-Coupling Reactions: A
Schlenk tube was charged with CuI (19 mg, 0.1 mmol), ligand
(0.2 mmol), azole (1.2 mmol), aryl or heteroaryl halide (1.0 mmol),
Cs2CO3 (652 mg, 2.0 mmol), and DMSO (2 mL) under nitrogen.
The reaction mixture was stirred at 120 °C for the indicated period
of time. After completion of the reaction, the mixture was cooled
to room temperature and directly passed through a plug of Celite.
The Celite was washed with ethyl acetate (3ϫ5 mL), and the com-
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organic layer was dried with MgSO4 and concentrated. The residue
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Received: January 26, 2011
Published Online: March 30, 2011
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