Oxazolidin-2-one as efficient ligand for the copper-catalyzed N- Arylation of pyrrole, imidazole and indole

Article abstract of DOI:10.2174/157017810791112496

Oxazolidin-2-one was found to be a facile ligand for the N-arylation of pyrrole, indole, and imidazole with aryl and heteroaryl iodides, bromides, and chlorides by applying Cul as catalyst. The easy preparation, commercial availability, lower molecular weight, and broad substrate applicability, as well as substituent compatibility of this catalysis system render oxazolidin-2-one great advantages over the Cu-catalyzed methods that have already been utilized in a number of applications.

Full text of DOI:10.2174/157017810791112496

2
12
Letters in Organic Chemistry, 2010, 7, 212-218
Oxazolidin-2-one as Efficient Ligand for the Copper-Catalyzed N-
Arylation of Pyrrole, Imidazole and Indole
Hengchang Ma*, Shang Wu, Qiangsheng Sun and Ziqiang Lei
Key Laboratory of Polymer Materials of Gansu Province, Key Laboratory of Eco-Environment-Related Polymer
Materials Ministry of Education, College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou 730070, People’s Republic of China
Received January 04, 2010: Revised January 12, 2010: Accepted January 19, 2010
Abstract: Oxazolidin-2-one was found to be a facile ligand for the N-arylation of pyrrole, indole, and imidazole with aryl
and heteroaryl iodides, bromides, and chlorides by applying CuI as catalyst. The easy preparation, commercial
availability, lower molecular weight, and broad substrate applicability, as well as substituent compatibility of this catalysis
system render oxazolidin-2-one great advantages over the Cu-catalyzed methods that have already been utilized in a
number of applications.
Keywords: Copper, coupling, catalysis, ligands, arylations.
1
. INTRODUCTION
mentioned above, the applications of these Ullmann methods
might be limited in certain degree by the high expense,
unavailability, or specificity of the ligands. It should be
noted that the successful arylation methods for each of major
classes of nitrogren heterocycles such as imidazoles,
pyrroles, indoles, etc. have been scarcely reported [4, 12-14].
Therefore, more efficient, versatile, and facile ligands for
these coupling reactions under relatively milder conditions
are still in demand.
Nitrogen-containing heterocycles, such as N-arylpyrroles,
N-arylindoles, and N-arylimidazoles, are an important class
of privileged core structure that are ubiquitous in numerous
natural products and biologically active pharmaceuticals [1].
Traditionally, the copper-catalyzed Ullmann-Goldberg
coupling is the general method for introduction of amine
functionality using aromatic halides [2]. However, this
protocol necessitates the use of high temperature and often
requires stoichiometric amount of copper regents, which, on
scale, leads to problems of waste disposal, handlely
inconvenience, and also narrow substrates scope. Thus,
numerous efforts have been devoted to the development of
the methods for the preparation of N-arylazole unit. Despite
the great progress of Pd-catalyzed C-N formation by using
some sterically hindered phosphine ligands [3], the high cost
and air sensitivity of catalysis systems commonly limit their
application to large- and industrial-scale formation of these
bonds. In recent years, the use of ligands to facilitate the
copper-catalyzed N-arylation of nucleophiles with aryl
halides is one of the hottest branches of research due to their
economic attractiveness and excellent efficiency. Indeed,
some efficient ligands have been disclosed in these coupling
reactions, including diamines [4], amino acids [5], diimines
Recently, our research group disclosed the copper-
catalyzed N-arylation of nucleophiles with aryl halides
facilitated by ligands of N-hydroxyimides [15]. In the
ongoing research program, we found that oxazolidin-2-one
could be applied as more efficient ligand in the amidation of
aryl halides and cyclization of ortho-halobenzanilides [16].
Herein, we describe our preliminary investigations on the
use of oxazolidin-2-one as ligand for the copper-catalyzed N-
arylation of nitrogen-containing heterocycles.
2
. RESULTS AND DISCUSSION
In our initial screening experiments, a set of oxazolidin-
-one-based derivatives was screened for the comparison
2
models of pyrrole 1a with iodobenzene 2a and imidazole 1b
with 4-bromoanisole 2b. As shown in Fig. (1), the skeletal
divergence of ligands in this transformation presented
significant different results. Ligands L3 and L4 were not able
to be applied as promoters in our catalysis system. The
sterically more hindered ligands L6, L7, and L8 afforded 3a
and 3n in lower yields. Ligand L5 was evidently less
favorable than ligands L1 and L2. The best results were
obtained only when the ligands contained O or S atom
located in the lactam ring. Presumably, L1 and L2 acted as
tridentate O, N, S-donor ligands that might be sterically
more effective in stabilizing or solublizing the copper
complex [17]. In our cases, ligand L1 was recommended due
to the easily preparation, lower molecular weight, and more
efficient catalysis behavior. It was observed clearly that base
[
6], aminoarenethiolate [7], phosphine ligands [8], 2-
aminopyrimidine-4, 6-diol [9a], 1,10-Phenanthroline [9b],
hydroxyquinoline [10], (S)-pyrrolidinylmethylimidazoles
[
11], 4, 7-dimethoxy-1, 10-phenanthroline [12], 2-
oxocyclohexanecarboxylate [13], and benzotriazole [14].
While many significant results have been achieved for the
copper-catalyzed N-arylation of a variety of nitrogen-
containing heterocycles through the use of those ligands
*
Address correspondence to this author at the Key Laboratory of Polymer
Materials of Gansu Province, Key Laboratory of Eco-Environment-Related
Polymer Materials Ministry of Education, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou 730070,
People’s Republic of China; Fax: +86 931 7970359;
E-mail: mahczju@hotmail.com
1
570-1786/10 $55.00+.00
© 2010 Bentham Science Publishers Ltd.

CuI Catalyzed C-N Coupling Reaction
Letters in Organic Chemistry, 2010, Vol. 7, No. 3
213
N
N
N
OCH
3
NH
I
N
NH
H CO
3
Br
1
a
2a
3a
1b
2b
3n
O
S
S
H
N
H
N
NH2
H
HN
NH
HN
NH
HN
NH
O
N
O
S
O
S
O
S
OH
O
O
O
O
S
S
L1
L2
L3
L4
L5
L6
L7
L8
Fig. (1). Effect of various ligands on the aryl amidation reaction.The reaction of 1a + 2a was performed with 5 mol % of CuI, 10 mol % of
ligand, 1.0 equiv of 1a and 2a, and 1.5 equiv of CH ONa in DMSO at 80 °C for 10 h. The reaction of 1b + 2b was performed with 10 mol %
of CuI, 20 mol % of ligand, 1.0 equiv of 1b and 2b, and 1.5 equiv of CH
3
3
ONa in DMSO at 120 °C for 12 h.
a
Table 1. Copper Oxazolidin-2-One-Catalyzed N-Arylation of N-H Heterocycles
CuI, Ligand
+
ArX
Het-NH
Het-N-Ar
CH ONa , DMSO
3
1
a~1f
2a~2p
3a~3z
C H
2
5
N
N
N
N
NH
NH
N
NH
N
N
H
N
H
H
1
a
1b
1c
1d
1e
1f
o
b
Entry
ArX
Product
( C/h)
Yield
N
1
I
80/10
110/12
80/8
93
2
2
2
a
3a
N
Cl
Cl
Br
2
3
81
86
c
3b
3b
N
Cl
Cl
I
d
NH2
H N
2
4
N
80/10
78
I
2
e
3c

2
14 Letters in Organic Chemistry, 2010, Vol. 7, No. 3
Ma et al.
(
Table 1). Contd…..
o
b
Entry
ArX
Product
( C/h)
Yield
N
NH2
H N
I
5
2
80/10
80/14
95
92
2
f
3d
6
N
I
2
2
g
3e
N
N
7
8
9
80/12
88
78
N
Br
h
3f
N
N
N
120/20
N
Cl
N
2
2
i
3g
N
NO2
O
N
Cl
120/24
120/24
80/10
32
35
91
78
85
67
2
j
3h
F3C
Cl
N
CF3
1
1
1
1
1
0
1
2
3
4
2
k
3i
N
N
2a
2c
2h
3
j
N
N
Cl
120/24
80/15
3
k
N
N
N
3
l
N
N
N
2i
120/16
N
3
m
N
N
OCH3
H CO
Br
1
1
5
6
3
120/12
120/24
83
45
2
b
3n
N
N
CF3
2k
3
o

CuI Catalyzed C-N Coupling Reaction
Letters in Organic Chemistry, 2010, Vol. 7, No. 3
215
(
Table 1). Contd…..
Entry
o
b
ArX
2h
Product
C H
( C/h)
Yield
2
5
N
1
1
7
8
N
80/12
80/18
81
N
3
p
N
2c
85
Cl
3
q
N
1
2
9
0
80/16
80/13
94
90
I
Br
Br
2
I
3r
N
2a
3
s
N
2
1
2m
120/24
62
3
t
N
2
2
120/24
52
H C
Br
3
CH3
2
n
3u
N
2
2
3
4
2c
120/24
120/12
78
79
Cl
3
q
S
S
N
Br
N
2
o
3v

2
16 Letters in Organic Chemistry, 2010, Vol. 7, No. 3
Ma et al.
(
Table 1). Contd…..
o
b
Entry
ArX
2h
Product
( C/h)
Yield
N
N
N
2
2
5
6
80/12
80/18
94
87
3
w
N
N
2a
3
x
N
N
2
2
7
8
80/12
92
I
H CO
OCH3
3
2
p
3y
N
N
N
N
2h
80/12
84
3
z
a
3
Reaction conditions: ArX (1.5 mmol), Het-NH (1.5 mmol), CH ONa (1.5 equiv), DMSO (4 mL); X = I, with CuI (5 mol %)/L1 (10 mol %); X = Br or Cl, with CuI (10 mol %)/L1
(
20 mol %).
b
Isolated yield.
plays a very important role in the transformation. The
common used bases K CO , K PO were found to be
ineffective, amine bases such as DBU or N(CH CH gave
inferior results. However, in the presence of stronger organic
base of CH ONa, model reactions could sharply ascend
coupling efficiency. As documented that more polar solvents
can accelerate the Ullmann-type reactions [4], thus, DMF,
DMSO, toluene were investigated, and DMSO demonstrated
more effectively. Copper salts applied as an alternative
catalyst precursor were tested; the readily available copper
salts such as CuI, CuBr, CuCl afforded a satisfactory result
in the arylation of 1a with 2a, however, for the less reactive
aryl halides, bromo- and chloro-substituted aromatic
compounds, CuI was turned out to give the best results.
previously in the copper-catalyzed N-arylation reactions,
certain groups in the ortho position of aryl halides prefer to
coordinate metal ions and hasten the Ullmann-type reaction
[9]. In our cases, we also found that the accelerating nitrogen
atom in 2-bromopyridine possessed the ability to promote
the coupling reactions (entries 7, 13, 17, 25, and 28, Table
1). Encouraged by these results, the less active 2-
chloropyrazine (2i) was then treated with pyrrole 1a and
imidazole 1b, and gave the coupling products in 78 % and 67
% yields (entries 8, and 14, Table 1). Our catalysis system
provided complete bias to favor more active halogens
amidated compounds when halide bearing both of chloric
and bromo, as well as iodic groups (entries 2, 3，12，18
and 19, Table 1). Generally, aryl bromides and chlorides
react more slowly than aryl iodides and typically require 10
mol % of CuI and 20 mol % of ligand and heating at higher
temperature 110 °C for longer time (up to one day). With
respect to the reactivity for N-H heterocycles, pyrrole and
imidazole are more active partners than others.
2
3
3
4
2
3 3
)
3
From Table 1, we can discern that the coupling reactions
not only tolerate a wide scope of functional groups, but also
have broad substrate availability, especially, imidazole, 2-
ethylimidazole and benzoimidazole which are known less
reactive parterners in C-N cross-coupling reaction, displayed
great efficiency. Interestingly, substrates 2e and 2f reacted
with N-H heterocycles to give corresponding products in
excellent yields (78 %, 95 %, entries 4, 5, Table 1). Since
Based on the previous documents [5, 15], we have
formulated a possible mechanism as described in Scheme 1.
Oxazolidin-2-one acted as tridentate O, O, N -donor ligand
chelated with CuI to generate species A. The subsequent
oxidative addition of aryl halides led to heterocycles reacted
with B readily to afford complex C, which undergoes
reductive elimination to provide the desired product and
finished a catalysis cycle.
2
free NH group substituted iodides are of possibility to self-
polymerizing in Pd-catalyzed C-N formation methods.
Therefore, this result implicated that the formation of
different C-N bonds could be accomplished simply by
choosing Pd- or Cu-based catalysis system. As reported

CuI Catalyzed C-N Coupling Reaction
Letters in Organic Chemistry, 2010, Vol. 7, No. 3
217
O
4. CONCLUSIONS
In summary, we have disclosed that oxazolidin-2-one
acted as a facile ligand for the CuI-catalyzed arylation of N-
H heterocycles. The practical benefits of this arylation
methodology were not only contributed to its easy
preparation and commercial availability as well as lower
molecular weight, but also broad substrate applicability,
substituent compatibility. In short, we believe that the
present catalyst system could provide a good complement to
the Pd- or Cu-catalyzed methods that have already been
utilized in a number of applications, and is of great
importance to the research and development in the
pharmaceutical industry.
HN
O
=
CuI
O
ArNu
ArX
HN
O
reductive elimination
HN
oxidation addition
O
A
O
O
HN
O
X
NuH
base
Nu Ar
X
Ar
C
B
Scheme 1. The possible mechanism.
ACKNOWLEDGEMENTS
We are grateful for the financial support of the National
Natural Science Foundation of China (No.20376071, No.
3
3
. EXPERIMENTAL
2
0674063 and 20774074) and Young Teacher Research
.1. General Information
Foundation of Northwest Normal University (NWNU-
LKQN-08-8).
All NMR spectra are recorded on Bruker Avance Digital
1
13
(
400 MHz for H NMR, 100 MHz for
spectrometers; chemical shifts are expressed in ꢀ units
relative to TMS signal as internal reference in CDCl . Mass
spectra are recorded on a HP5989B mass spectrometers.
C NMR)
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