Synthesis of benzoxazole and benzimidazole derivatives via ligand-free copper(I)-Catalyzed cross-coupling reaction of o-halophenols or o-haloanilines with carbodiimides

Article abstract of DOI:10.1002/adsc.201000022

A novel and efficient synthesis of benzoxazole and benzimidazole derivatives via a ligandfree, copper(I)-catalyzed, one-pot cascade process has been developed. A variety of carbodiimides coupled with o-halophenols or o-haloanilines to give the products in moderate to excellent yields under the mild conditions.

Full text of DOI:10.1002/adsc.201000022

COMMUNICATIONS
DOI: 10.1002/adsc.201000022
Synthesis of Benzoxazole and Benzimidazole Derivatives via
Ligand-Free Copper(I)-Catalyzed Cross-Coupling Reaction of
o-Halophenols or o-Haloanilines with Carbodiimides
Guodong Shena and Weiliang Baoa,*
a
Department of Chemistry, Xi Xi Campus, Zhejiang University, Hangzhou, Zhejiang 310028, Peopleꢀs Republic of China
Fax : (+86)-571-8827-3814; e-mail: wlbao@css.zju.edu.cn
Received: January 11, 2010; Published online: April 7, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000022.
Abstract: A novel and efficient synthesis of benzox-
azole and benzimidazole derivatives via a ligand-
free, copper(I)-catalyzed, one-pot cascade process
has been developed. A variety of carbodiimides
coupled with o-halophenols or o-haloanilines to
give the products in moderate to excellent yields
under the mild conditions.
cessfully applied to the assembly of various heterocy-
clic compounds via one-pot cascade strategies.[7] For
example, Buchwald et al. and Ma et al. had developed
useful Cu-catalyzed cascade coupling/condensation
processes to obtain substituted benzimidazole deriva-
tives.[7] Our group is also interested in one-pot synthe-
sis of heterocycles via Cu-catalyzed addition/cycliza-
tion reactions and have provided convenient and effi-
cient approaches to potentially useful heterocyclic
compounds.[8] However, there is no report about the
versatile synthesis of N-(3-phenylbenzo[d]oxazol-
2(3H)-ylidene)anilines and N,1-diphenyl-1H-ben-
Keywords: copper(I) catalysis; cross-coupling; het-
erocycles; ligand-free conditions; one-pot reaction
AHCTUNGTRENNUNG
Benzoxazoles and benzimidazoles are important ing effort, we wish to report a new example for the
classes of molecules and heterocyclic scaffolds in bio- formation of N-(3-phenylbenzo[d]oxazol-2(3H)-yli-
logically active and natural products.[1] They are also
ACHTUNGTRENdNUGN ene)anilines and N,1-diphenyl-1H-benzo[d]imidazol-
found in a variety of medicinally significant com- 2-amines via a one-pot addition/cyclization reaction
pounds and are important targets in drug discovery.[2] catalyzed by a copper(I) salt under ligand-free condi-
For the synthesis of N-aryl-2-aminobenzimidazoles, tions.
there are few efficient synthetic methods including
Initially, o-iodophenol 1a and diphenylcarbodiimide
the coupling of arylamines with N-aryl-2-halobenzimi- 1b were selected as the model substrates. Catalyzed
dazoles and some multistep procedures.[3] To the best by CuI, different ligands, temperatures, bases and sol-
of our knowledge, there is no report about a versatile vents were examined to set up standard reaction con-
synthesis of N-(3-phenylbenzo[d]-oxazol-2(3H)-yli- dition. The reaction was firstly carried out in DMSO
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
could be formed by cycloaddition of 2,3-disubstituted 808C under an N2 atmosphere. Twenty-four hours
oxiranes to carbodiimides.[4] Although these reactions later, the reaction was stopped and the anticipated
usually proceeded well, they are associated with the product 1c was isolated in 50% yield after flash chro-
use of highly toxic and corrosive reagents, high cost matography (Table 1, entry 1). To increase the yield,
metal catalysts and specific ligands. These facts make the reaction temperature was set to 100–1108C and
the methodologies an unsuitable choice for large and 75–76% yield was achieved (Table 1, entries 2 and 3).
industrial scale applications.
Besides Cs2CO3 a number of other bases such as
2
À
In the past decades, copper-mediated sp C X (X= K3PO4, K2CO3 were also surveyed at 1008C, but the
C, N, O, S, etc.) bond formation reactions,[5] especially yield did not improve greatly (Table 1, entries 4 and
the ligand-free reactions,[6] have attracted considera- 5). 1,10-Phenanthroline was used as the ligand and a
ble attention for their efficiency and low cost. Recent- 73% yield was realized (Table 1, entry 6). Without the
ly, these copper-catalyzed reactions have been suc- ligand, a 40% yield was detected (Table 1, entry 7).
Adv. Synth. Catal. 2010, 352, 981 – 986
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 1. Optimization of the reaction conditions.
Entry
Catalyst/Ligand
Base
Solvent
T [oC]
Yield [%][a]
1
2
3
4
5
6
7
8
CuI/N,N-dimethylglycine
CuI/N,N-dimethylglycine
CuI/N,N-dimethylglycine
CuI/N,N-dimethylglycine
CuI/N,N-dimethylglycine
CuI/1,10-phenanthroline
CuI
CuI/N,N-dimethylglycine
CuI/N,N-dimethylglycine
CuI
Cs2CO3
Cs2CO3
Cs2CO3
K3PO4
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
Toluene
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
80
50[b]
75[b]
76[b]
72[b]
57[b]
73[b]
40[c]
24[b]
99[b]
93[c]
100
110
100
100
100
100
100
100
100
100
100
100
100
K2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
9
10
11
12
13
14
[d,e]
–
–
[c,e]
Cu2O
CuBr
–
60[c]
[c,e]
Cu
N
–
[a]
Isolated yield after flash chromatography based on o-iodophenol.
[b]
[c]
[d]
[e]
Reaction conditions: CuI (0.075 mmol), ligand (0.1 mmol), o-iodophenol (0.5 mmol), N,N’-methanediylidenedianiline
(0.8 mmol) and base (1.0 mmol) in solvent (2.0 mL) under an N2 atmosphere.
Reaction conditions: copper salt (0.075 mmol), o-iodophenol (0.5 mmol), N,N’-methanediylidenedianiline (0.8 mmol) and
base (1.0 mmol) in solvent (2.0 mL) under an N2 atmosphere.
Reaction conditions: o-iodophenol (0.5 mmol), N,N’-methanediylidenedianiline (0.8 mmol) and base (1.0 mmol) in solvent
(2.0 mL) under an N2 atmosphere.
No product was detected.
Table 2. CuI-catalyzed one-pot synthesis of N-(3-phenylbenzo[d]oxazol-2(3H)-ylidene)anilines from o-iodophenols a and car-
bodiimides b.[a]
Entry
R
R1
R2
Product
Yield [%][b]
1
2
3
4
5
6
7
8
C6H5
C6H5
H
H
H
H
H
H
4-CH3
4-tBu
4-Cl
4-F
4-tBu
4-Cl
4-Cl
H
1c
2c
3c
4c
5c
6c
7c
8c
93
89
83
77
75
78
92
85
86
85
83
74
75
12
4-CH3-C6H4
4-CF3O-C6H4
4-Cl-C6H4
2-CH3-C6H4
3-CF3-C6H4
C6H5
C6H5
C6H5
C6H5
4-CH3-C6H4
4-Cl-C6H4
2-CH3-C6H4
i-Pr
i-Pr
C6H5
4-CH3-C6H4
4-CF3O-C6H4
4-Cl-C6H4
2-CH3-C6H4
3-CF3-C6H4
C6H5
C6H5
C6H5
C6H5
4-CH3-C6H4
4-Cl-C6H4
2-CH3-C6H4
C6H5
i-Pr
C6H5
9
9c
10
11
12
13
14
15
16
10c
11c
12c
13c
14c
15c
1c
[c]
H
H
–
15[d]
[a]
Reaction conditions: CuI (0.075 mmol), o-iodophenols (0.5 mmol), carbodiimides (0.8 mmol) and Cs2CO3 (1.0 mmol) in
MeCN (2.0 mL) under an N2 atmosphere.
Isolated yield after flash chromatography based on o-iodophenols.
No product was detected.
[b]
[c]
[d]
o-Bromophenol was used.
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Synthesis of Benzoxazole and Benzimidazole Derivatives
Different solvents such as toluene, acetonitrile were reactions of o-iodophenols a with various substituted
also tested (Table 1, entries 8 and 9). To our delight, carbodiimides b were examined (Table 2, entries 1–
with acetonitrile as solvent, the yield rose up to 99%. 13). The substituted carbodiimides bearing both elec-
It seemed that the polar solvents were much more fa- tron-donating groups (p-Me and o-Me) and electron-
vorable to this reaction. Considering that many b- withdrawing groups (p-Cl, p-OCF3 and m-CF3) were
diaza compounds could serve as a ligand in Cu(I)-cat- able to couple with o-iodophenol in moderate to ex-
alyzed coupling reactions,[5a] we tried a “ligand-free” cellent yields (Table 2, entries 1–6). Furthermore, o-
reaction again, because the intermediates were b- iodophenols with different functional groups (p-CH3,
diaza compounds. Only CuI (15 mol%) and Cs2CO3 p-t-Bu, p-Cl, p-F) were also investigated and good re-
(2 equiv.) were added, the reaction was conducted in sults were detected (Table 2, entries 7–10). The yields
MeCN at 1008C, a 93% yield was obtained (Table 1, were not affected much by the steric hindrance of
entry 10). When CuI was not added in the reaction, both substrates (Table 2, entries 2–10). Substituted o-
no product was detected (Table 1, entry 11). Some iodophenols and carbodiimides were reacted and
other copper salts such as Cu2O, CuBr, and CuACTHNUGTRNEUNG(OAc)2 moderate to excellent yields were reached (Table 2,
were also tested (Table 1, entries 12–14), but CuI pro- entries 11–13). o-Bromophenol was unfavorable for
vided the best yield. Obviously the entry 10 repre- the reaction, only a 15% yield was obtained (Table 2,
sents the best conditions.
entry 16). When N-[(isopropylimino)methylene]ani-
With the optimal conditions established, we tried to line was used to couple with o-iodophenol, the corre-
investigate the scope of this methodology. Firstly, the sponding product could be isolated but the yield was
Table 3. CuI-catalyzed one-pot synthesis of benzimidazole derivatives from o-haloanilines d and diphenylcarbodiimide b.[a]
Entry
X
Y
R
Product
Yield [%][b]
1
2
I
I
H
H
H
H
1g
1g
21[c]
81
3
4
I
I
H
H
4-Cl
2g
3g
72
70
4-CH3
5
6
Br
Br
H
H
H
4-CH3
1g
3g
62
64
7
I
CH3
H
3h
63
[a]
Reaction conditions: CuI (0.075 mmol), o-haloanilines (0.5 mmol), carbodiimides (1.5 mmol) and Cs2CO3 (1.0 mmol) in
MeCN (3.5 mL) under N2 atmosphere, every 6 h, 0.5 mmol carbodiimides in 0.5 mL MeCN was added in 10 min via sy-
ringe.
Isolated yield after flash chromatography based on o-haloanilines.
The conditions were the same as for the synthesis of N-(3-phenylbenzo[d]oxazol-2(3H)-ylidene)anilines.
[b]
[c]
Adv. Synth. Catal. 2010, 352, 981 – 986
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asc.wiley-vch.de
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Guodong Shen and Weiliang Bao
COMMUNICATIONS
Scheme 1. Synthesis of benzimidazole derivatives.
low (Table 2, entry 14). If both of the R groups on the tries 3–6). Finally, o-iodo-N-methylaniline was also
carbodiimide were aliphatic we ccould not obtain the tested, but the yield was moderate (Table 3, entry 7).
corresponding product: N,N’-methanediylidenedicy-
clohexanamine (DCC) was examined and no product carbodiimide, etc., to form another heterocycles. For
was detected (Table 2, entry 15). example, 1g was successfully reacted with o-haloaryl-
Product g can further react with isothiocyanate or
Encouraged by the above results, we further inves- carbodiimide (i) and the product j was formed in 40%
tigated the scope and the generality of the method by yield (Scheme 1).[10]
varying the o-iodophenols to o-haloanilines and o-
A plausible mechanism, which accounts for the for-
iodo-N-methylaniline (Table 3, entries 1–7). Firstly, mation of the N-(3-phenylbenzo[d]oxazol-2(3H)-yli-
we applied the above obtained optimal conditions to dene)anilines from the o-iodophenols and carbodi-
the reaction of diphenylcarbodiimide with o-haloani-
ACHTUNGTRENiNUNG mides, is shown in Scheme 2. The nucleophilic O
lines, but only a 21% yield of the N,1-diphenyl-1H- atom of o-iodophenols (a) would attack the carbon
benzo[d]imidazol-2-amine was achieved (Table 3, atom of the NCN unit of carbodiimides b and inter-
entry 1). From the TLC monitoring of the reaction, it mediate d could be formed in the presence of a
was found that 2-iodoaniline was not consumed com- proper base. With a proper copper(I) catalyst, a che-
pletely. Then 3 equiv. N,N’-methanediylidenedianiline lating intermediate f was formed. After an oxidative
were added in MeCN in three portions at 6 h inter- addition and a reductive elimination process, the N
vals. The yield improved greatly (Table 3, entry 2). atom would succeed in performing a cyclization
Several substituted o-iodoanilines and o-bromoani- action and intermediate d might be converted into
À
lines were coupled with N,N’-methanediylidenediani- product c through intramolecular C N coupling. The
line in moderate to excellent yields (Table 3, en- experiment has proved that the functional groups R
Scheme 2. Plausible mechanism for the reaction of o-iodophenols and o-iodo-N-methylaniline.
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Adv. Synth. Catal. 2010, 352, 981 – 986

Synthesis of Benzoxazole and Benzimidazole Derivatives
and R1 are important in the coupling process (Table 2,
entries 14–16). The plausible mechanism for forming
N,1-diphenyl-1H-benzo[d]imidazol-2-amines is the
same as that shown in Scheme 2.
purified by column chromatography on silica gel to give the
pure product (AcOEt/petroleum ether v/v=1/5).
In summary, we have developed a novel, efficient
and concise method to synthesize benzoxazol and
benzimidazole derivatives under a ligand-free cop-
per(I)-catalyzed condition. A simple “one-pot” opera-
tion was conducted, readily available starting materi-
als were employed and relatively mild conditions
were applied. Various benzoxazole and benzimidazole
derivatives, which might be potentially applicable in
the pharmaceutical and biochemical areas, were con-
veniently synthesized in moderate to excellent yields.
The reaction is also a good example for ligand-free
copper (I)-catalyzed one-pot cascade process.
Acknowledgements
This work was financially supported by the Specialized Re-
search Fund for the Doctoral Program Foundation of Higher
Education of China (No. 2060335036).
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Experimental Section
General Procedure for the Synthesis of N-(3-Phenyl-
benzo[d]oxazol-2(3H)-ylidene)anilines
An oven-dried Schlenk tube equipped with a Teflon valve
was charged with a magnetic stir bar, Cs2CO3 (1.0 mmol,
200 mol%), CuI (0.075 mmol, 15 mol%), and 2-halophenol
(0.5 mmol). The tube was evacuated and backfilled with N2
(this procedure was repeated
3 times). Carbodiimide
(0.8 mmol) was added via syringe under a counter flow of
N2. Then under a counter flow of N2, MeCN (2.0 mL) was
added by syringe. The reaction vessel was sealed and the
mixture stirred for 24 h at 1008C. The reaction was moni-
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sumed completely, the resulting suspension was cooled to
room temperature and filtered through a pad of filter paper
with the help of 30 mL of ethyl acetate. After evaporating
the solvent under reduced pressure, the residue was purified
by column chromatography on silica gel to give the pure
product (AcOEt/petroleum ether v/v=1/20).
General Procedure for the Synthesis of
Benzimidazole Derivatives
An oven-dried Schlenk tube equipped with a Teflon valve
was charged with a magnetic stir bar, Cs2CO3 (1.0 mmol,
200 mol%), CuI (0.075 mmol, 15 mol%), and 2-haloaniline
(0.5 mmol). The tube was evacuated and backfilled with N2
(this procedure was repeated 3 times). Under a counter flow
of N2, MeCN (2.0 mL) was added by syringe. Then under a
positive pressure of N2, the mixture was stirred at 1008C
and
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