Solvent-free copper/iron co-catalyzed N-arylation reactions of nitrogen-containing heterocycles with trimethoxysilanes in air

Article abstract of DOI:10.1016/j.tetlet.2007.08.117

A solvent-free copper/iron-catalyzed N-arylation of nitrogen-containing heterocycles with trimethoxysilanes method for the formation of C-N bonds has been developed. In the presence of Cu, FeCl₃, TBAF, and air, a variety of nitrogen-containing heterocycles including imidazoles and triazoles were coupled with aryltrimethoxysilanes and vinyltrimethoxysilane to afford the corresponding products in moderate to excellent yields. It is noteworthy that the reaction is conducted under solvent-free and relatively low Cu/FeCl₃ loadings conditions.

Full text of DOI:10.1016/j.tetlet.2007.08.117

Tetrahedron Letters 48 (2007) 7845–7848
Solvent-free copper/iron co-catalyzed N-arylation reactions
of nitrogen-containing heterocycles with trimethoxysilanes in air
Ren-Jie Song, Chen-Liang Deng, Ye-Xiang Xie and Jin-Heng Li*
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education),
College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China
Received 13 May 2007; revised 30 August 2007; accepted 31 August 2007
Available online 5 September 2007
Abstract—A solvent-free copper/iron-catalyzed N-arylation of nitrogen-containing heterocycles with trimethoxysilanes method for
the formation of C–N bonds has been developed. In the presence of Cu, FeCl , TBAF, and air, a variety of nitrogen-containing
3
heterocycles including imidazoles and triazoles were coupled with aryltrimethoxysilanes and vinyltrimethoxysilane to afford the cor-
responding products in moderate to excellent yields. It is noteworthy that the reaction is conducted under solvent-free and relatively
low Cu/FeCl loadings conditions.
3
Ó 2007 Elsevier Ltd. All rights reserved.
The transition metal-catalyzed N-arylation of nitrogen-
containing heterocycles (e.g., imidazoles and triazoles)
reactions has received considerable attention in recent
years due to their successful uses in the preparation of
N-aryl compounds in pharmaceuticals, crop-protection
taining heterocycles with arylsilanes using a catalytic
amount of catalyst under solvent-free conditions became
attractive. After a series of trials, we found that a cata-
lytic amount of Cu combined with a catalytic amount of
FeCl as co-catalysts could be effective for the N-aryla-
3
1
chemicals, and material sciences. Among these N-aryla-
tions of the nitrogen-containing heterocycles with aryl-
trimethoxysilanes and vinyltrimethoxysilane in the
presence of TBAF and air. Moreover, the reaction was
carried out under solvent-free conditions (Eq. 1).
tion transformations, the nucleophilic aromatic reagents
2
are usually aryl halides and pseudo-aryl halides.
3
Organometals, such as aryllead triacetates, arylboronic
acids, arylbismuths, diaryl iodonium salts, arylstann-
4
5
6
7
8
Cu/FeCl3
anes, and arylsilanes, are also found to be efficient
reaction partners for the N-arylation reaction with the
nitrogen-containing heterocycles. Although arylsilanes
are widely used as the reaction partners for the palla-
dium-catalyzed Hiyama cross-coupling reactions with
aryl halides because they are readily available and/or
Het NH
+
RSi(OR')
3
Het N R
TBAF, 50 oC, Air
ð1Þ
Het NH = imidazoles, triazoles
R = aryl, vinyl
The N-arylation of 1H-benzo[d]imidazole (1a) with tri-
methoxy(phenyl)silane (2a) in the presence of 3 equiv
9
less toxic, only one paper has been reported on the
of TBAFÆ3H O was first studied to seek a suitable cata-
2
N-arylation reactions of the nitrogen-containing hetero-
8
lyst (Table 1). We have recently demonstrated that
cycles with arylsilanes. In the presence of Cu(OAc)
2
TBAF (n-Bu NF) is an effective base for the solvent-free
4
and TBAF, a variety of imidazoles, amines, and amides
were coupled with silanes under air atmosphere in mod-
erate to good yields. However, the method was con-
ducted in harmful organic solvents besides the
requirement of excess stoichiometric amount of
2
u,9i,10
Pd or Cu-catalyzed cross-coupling reactions.
Thus, we expected to perform this reaction under sol-
vent-free conditions using TBAF as base. As expected,
the N-arylation of 1a with silane 2a proceeded success-
fully under solvent-free conditions using Cu/FeCl as
3
Cu(OAc) promoter. The development of a new and effi-
2
the co-catalysts and TBAF as the base in air atmo-
sphere. Treatment of 1a with silane 2a, a stoichiometric
cient method for the N-arylations of the nitrogen-con-
amount of Cu(OAc) and TBAF after 23 h afforded the
2
corresponding product 3 in 52% yield (entry 1). Identical
results were obtained using a stoichiometric amount of
Cu O instead of Cu(OAc) (entry 2). In the presence
Keywords: Copper; FeCl
*
3
; N-Arylation; Imidazole; Trimethoxysilane.
Corresponding author. Tel.: +86 731 8872530; fax: +86 731
872531; e-mail: jhli@hunnu.edu.cn
8
2
2
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.08.117

7846
R.-J. Song et al. / Tetrahedron Letters 48 (2007) 7845–7848
a
Table 1. Screening the optimum conditions
3 mol % providing the best result in view of the reaction
yield and rate (entries 13–17). The reaction temperature
was also tested, and the yield of 3 was reduced to 62% at
room temperature (entries 16 and 18). It is noteworthy
that the reaction gave unsatisfactory results when it
was carried out in argon atmosphere or oxygen atmo-
sphere (entries 19 and 20). The results showed that the
yield was reduced to some extent in the presence of
either 2 equiv or 1 equiv of TBAF (entries 21 and 22).
N
N
N
+
Si(OMe)3
N
H
1a
2a
3
Entry Catalysis (mol %)
Cu(OAc) (100)
Cu O (100)
Time (h) Isolated yield (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
2
23
23
21
21
52
51
58
61
48
51
53
Trace
60
51
60
52
63
71
82
82
2
Cu (100)
Cu (40)
Cu (20)
Cu (20)/Cu(OAc)
Cu (10)/Cu(OAc)
Cu (10)/CuBr
FeCl
FeCl
Cu (40)/FeCl
Cu (50)/FeCl
Cu (30)/FeCl
Cu (20)/FeCl
We then explored the scope of the reaction of the nitro-
gen-containing heterocycles including imidazoles and
24
2
2
(20) 24
11
triazoles, and the results are summarized in Table 2.
(10) 23
23
It was found that 1H-benzo[d]imidazole (1a) could also
undergo the N-arylation with other silanes 2b–e in mod-
erate to good yields under the standard conditions (en-
tries 1–4). For example, substrate 1a was reacted with
2
(10)
3
3
(40)
(20)
23
24
1
1
1
1
1
1
1
1
1
1
2
2
2
3
(40)
(50)
(30)
(20)
23
17
24
15
trimethoxy(p-tolyl)silane (2c) Cu, FeCl , and TBAFÆ3-
3
3
3
3
H O efficiently to afford the corresponding product 4
2
in a 93% yield (entry 2). However, the other silanes 2f
Cu (5)/FeCl
3
(5)
24
24
Cu (3)/FeCl
Cu (1)/FeCl
Cu (3)/FeCl
Cu (3)/FeCl
Cu (3)/FeCl
Cu (3)/FeCl
Cu (3)/FeCl
3
(3)
(1)
(3)
(3)
(3)
(3)
(3)
Table 2. N-Arylations of imidazoles and triazoles with trimethoxysil-
a
3
3
3
3
3
3
28
24
24
24
24
24
81
62
Trace
32
78
anes in the presence of FeCl
, Cu, and TBAF
3
b
c
d
e
f
b
Entry Substrate
Silane
Yield
(%)
N
1
66 (3)
93 (4)
52
(1a)
Si(OEt)
3
(2b)
N
H
a
Reaction conditions: 1a (1.0 mmol), 2a (2.0 mmol), and TBAFÆ3H
2
O
(
3 equiv) in air atmosphere at 50 °C.
2
(1a)
Si(OMe)3
(2c)
Me
b
c
d
e
f
At room-temperature.
In Ar atmosphere.
3
4
(1a)
(1a)
(1a)
MeO
Si(OMe)3 (2d) 80 (5)
In O
TBAFÆ3H
TBAFÆ3H
2
atmosphere.
2
O (1 equiv).
O (2 equiv).
(2e)
3
66 (6)
2
Si(OMe)
c
5
6
(2f)
3
Trace (7)
Trace (3)
Si(OMe)
of a stoichiometric amount of Cu, the yield was en-
hanced slightly to 58% (entry 3). To our delight, the
same yield of 3 was still obtained when a catalytic
amount of Cu was used as catalyst (40 mol % Cu; entry
c
(1a)
SiMe3 (2g)
N
7
(1b)
Si(OMe)3 (2a)
45 (8)
N
H
4
). However, further decreasing amount of Cu reduced
the yield (20 mol % Cu; entry 5). Unfortunately, at-
tempts to the use of some Cu co-catalysts were unsuc-
cessful (entries 6–8). Very recently, the use of iron as a
co-catalyst was found to improve the copper-catalyzed
N-arylations of nitrogen-containing heterocycles with
aryl halides. Consequently, iron as the catalyst for
the reaction of substrate 1a with silane 2a was examined.
We were happy to find that a catalytic amount of FeCl3
could also catalyze the reaction of substrate 1a with si-
lane 2a and TBAF smoothly to provide the target prod-
uct 3 in a moderate yield (entries 9 and 10). Encouraged
by these results, we subsequently evaluated the effect of
8
9
(1b)
(1b)
(2c)
(2e)
58 (9)
41 (10)
N
c
10
11
(1c) (2c)
(2a)
Trace (11)
68 (12)
N
H
2
x
N
N
(1d)
N
H
1
2
13
(1d)
(1d)
(2c)
(2e)
96 (13)
33 (14)
N
N
1
1
1
4
5
6
(1e)
(2a)
(2c)
52 (15)
N
H
the Cu/FeCl co-catalysts on the reaction. The results
3
showed that the amount of both Cu and FeCl played
3
(1e)
57 (16)
a crucial role on the reaction. In the presence of
N
4
0 mol % of Cu and 40 mol % of FeCl , the same yield
3
N
(1f)
(2c)
Trace (17)
of 3 was isolated as that of either Cu or FeCl as the cat-
3
N
H
alyst (entries 4, 9, and 11). Further increasing amount of
a
both Cu and FeCl resulted in decreasing the yield of 3
Reaction conditions: 1 (1 mmol), 2 (2 mmol), Cu (3 mol %), FeCl3
3
to some extent (entry 12). It is interesting to observe that
(3 mol %), and TBAFÆ3H O (3 mmol) in air at 50 °C for 24 h.
Isolated yield.
2
b
c
the reduction of both Cu and FeCl favored the reac-
3
>
95% of the starting substrate was recovered.
tion, and the loadings of both Cu and FeCl3 are

R.-J. Song et al. / Tetrahedron Letters 48 (2007) 7845–7848
7847
air
Cu (0)
Cu(II)
M(X)LY-1
L
HeterocycleN H
HeterocycleN
1
H
18
M(X)LY
M = Cu, Fe
F
TBAF
R
Si(OMe)3
RSi(OMe)3
air
19
2
L
air M(0)
HeterocycleN M(X)LY-1
+
FB(OH)3
20
R
HeterocycleN R
air or disproportionation
3-17
L
M(X-1)
HeterocycleN M(X+1)LY-1
1 R
2
Scheme 1. A working mechanism.
and 2g were not suitable substrates for the reaction with
a (entries 5 and 6). Substrate 1b, another imidazole,
gen-containing heterocycles including imidazoles and
1
triazoles with aryltrimethoxysilanes using Cu/FeCl as
3
provided moderate yields from the N-arylation with
the three silanes 2a, 2c, and 2e, respectively, under the
same conditions (entries 7–9). Unfortunately, no reac-
tion was observed employing the hindered imidazole
the co-catalysts and TBAF as the base under air atmo-
sphere. In the presence of Cu, FeCl , and TBAFÆ3H O,
3
2
a variety of imidazoles and triazoles worked well with
aryltrimethoxysilanes and vinyltrimethoxysilane in
moderate to excellent yields. Compared with the previ-
ously report, several features are established: (1) The
1
c as the substrate (entry 10). To our delight, the stan-
dard conditions were also suitable for the reaction of
triazoles (entries 11–16). In the presence of Cu, FeCl3,
reaction employs relatively low Cu and FeCl catalyst
3
and TBAFÆ3H O, the couplings of triazoles 1d,e with sil-
loadings, and the loadings of both Cu and FeCl affected
2
3
anes 2a, 2c, or 2e, respectively, proceeded smoothly to
produce the target products 12–16 in moderate to excel-
lent yields (entries 11–15). Unfortunately, the reaction
between another triazole 1f and silane 2c gave trace
amount of the desired product under the same condi-
tions (entry 16).
the yields of the reactions. (2) The reaction was per-
formed under solvent-free conditions. (3) The presence
of argon or oxygen disfavored the reaction, but the best
results were obtained under air atmosphere. Further
applications of the present system in organic synthesis
and the studies of the detailed mechanism are underway.
A working mechanism was proposed as outlined in
Scheme 1 based on the previously proposed mecha-
Acknowledgments
3
–8,12
nism.
Trimethoxysilane can readily react with
TABF to afford a pentavalent silicate intermediate
The authors thank the Program for the New Century
Excellent Talents in University (No. NCET-06-0711),
National Natural Science Foundation of China (No.
20572020), Fok Ying Tung Education Foundation
(No. 101012), Key Project of Chinese Ministry of Edu-
cation (No. 206102), Hunan Provincial Natural Science
Foundation of China (No. 05JJ1002), and Specialized
Research Fund for the Doctoral Program of Higher
Education (No. 20060542007) for financial support.
9
,13
1
9.
The pentavalent silicate 19 undergoes transmeta-
lation with intermediate 18, which was obtained from
the coordination of nitrogen atom with catalyst (Cu(II)
and/or Fe(III)), to generate intermediate 20. Finally,
the reductive elimination of intermediate 20 takes place
directly to give the target product 3–17 and a lower-va-
lent species (Cu(0) and/or Fe(II)). Alternatively, oxida-
tion or disproportionation of intermediate 20 can occur
with the aid of oxidant to give the corresponding higher
oxidation-state intermediate 21, which is more efficiently
8
reductive elimination than intermediate 20. In view of
Supplementary data
the mechanism, there are two roles of air in the present
process including (i) oxidation of the Cu(0) and
M(X ꢀ 1) species to the reactive Cu(II) and M(X) species
and (ii) oxidation of intermediate 20 to intermediate 21.
The presence of pure oxygen may induce other side-reac-
tions resulting in a low yield of the desired product. We
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.08.117.
deduce that FeCl can also play as an oxidant to regen-
erate the active catalyst species beside as catalyst itself.
3
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(3 mol %), and TBAFÆ3H
atmosphere at 50 °C for 24 h until complete consumption
of starting material as monitored by TLC. After the
reaction was finished, the mixture was poured into ethyl
acetate, which was washed with brine. The aqueous layer
was extracted with ethyl acetate and the combined organic
2
(2 mmol), Cu (3 mol %), FeCl
O (3 mmol) was stirred in air
3
2
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(
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2 4
layers were dried over anhydrous Na SO and evaporated
under vacuum. The residue was purified by flash column
chromatography (hexane/ethyl acetate or CH Cl /
R.; Reider, P. J. J. Org. Chem. 2005, 70, 10135; (q) Rao,
H.; Fu, H.; Jiang, Y.; Zhao, Y. J. Org. Chem. 2005, 70,
2
2
CH OH) to afford the desired product.
3
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