A highly efficient heterogeneous copper-catalysed cascade reaction of aryl iodides with acetamidine hydrochloride leading to primary arylamines

Article abstract of DOI:10.3184/174751917X14931195075580

A highly efficient heterogeneous copper-catalysed cascade reaction of aryl iodides with acetamidine hydrochloride was achieved in DMF in the presence of 10 mol% of an MCM-41-immobilised L-proline-copper(I) complex (MCM-41-L-proline-CuI) with Cs2CO3 as base, yielding a variety of primary arylamines in good to excellent yields. The new heterogeneous copper complex can be easily prepared from commercially readily available and inexpensive reagents, recovered by a simple filtration of the reaction solution and used at least seven more times without any decrease in activity.

Full text of DOI:10.3184/174751917X14931195075580

JOURNAL OF CHEMICAL RESEARCH 2017
VOL. 41 JUNE, 315–320
RESEARCH PAPER 315
A highly efficient heterogeneous copper-catalysed cascade reaction of aryl
iodides with acetamidine hydrochloride leading to primary arylamines
a
b
a
a
a
Xue Huang , Ruian Xiao , Chongren You , Tao Yan and Mingzhong Cai *
a
Key Laboratory of Functional Small Organic Molecule, Ministry of Education and College of Chemistry & Chemical Engineering, Jiangxi Normal University,
Nanchang 330022, P.R. China
b
912 Lab Jiangxi Bureau of Geology and Mineral Exploration and Development, Yingtan 335001, P.R. China
A highly efficient heterogeneous copper-catalysed cascade reaction of aryl iodides with acetamidine hydrochloride was achieved in
DMF in the presence of 10 mol% of an MCM-41-immobilised L-proline-copper(I) complex (MCM-41-L-proline-CuI) with Cs CO as base,
2
3
yielding a variety of primary arylamines in good to excellent yields. The new heterogeneous copper complex can be easily prepared from
commercially readily available and inexpensive reagents, recovered by a simple filtration of the reaction solution and used at least seven
more times without any decrease in activity.
Keywords: supported copper catalyst, cascade reaction, acetamidine hydrochloride, primary arylamine, heterogeneous catalysis, MCM-41
Primary aromatic amines are important subunits and precursors
of natural products, agrochemicals, dyes, polymers and
ultrahigh surface area, large and defined pore size, big pore
volume and many silanol groups in the inner walls. So far,
1–3
35–39
pharmaceuticals. The traditional methods for the preparation
of arylamines are based on the hydrogenation of nitro
compounds catalysed by copper-, nickel- or platinum-group
some functionalised MCM-41-immobilised palladium,
4
0
41
42
43-45
rhodium, molybdenum, gold and copper
complexes
have been successfully used as potentially green and
sustainable catalysts in organic synthesis. In continuation of
our efforts to develop economical and eco-friendly synthetic
4
5
metals and nucleophilic amination of haloarenes or phenols.
Although transition-metal-catalysed C–N coupling of aryl
halides with ammonia has been used for the synthesis of primary
arylamines, in most cases, high pressure, high temperature
37–39,43–45
pathways for organic transformations,
we report
here the synthesis of the L-proline-functionalised MCM-41-
immobilised copper(I) complex (MCM-41-L-proline-CuI) and
its successful application to the cascade reaction of aryl iodides
with acetamidine hydrochloride leading to primary arylamines.
The new heterogeneous copper catalyst exhibits high catalytic
activity in the reaction and can easily be recovered by a simple
filtration of the reaction solution; its catalytic efficiency remains
unaltered even after recycling eight times.
6
–8
and sealed reaction vessels were required. Therefore, the
procedures might not be operationally simple and safe from the
perspective of application. The palladium-catalysed amination
of aryl halides/triflates has emerged as a powerful tool for
9
–14
the construction of arylamines in the past decades,
but the
use of ammonia as the nitrogen source does not furnish the
corresponding primary anilines due to competition between
the more reactive aniline product and remaining ammonia,
15–18
Results and discussion
which results in the formation of diaryl and triaryl amines.
Various copper-catalysed aminations of aryl halides have also
The new MCM-41-immobilised L-proline-copper(I) complex
(MCM-41-L-proline- CuI) was prepared from commercially
available and inexpensive reagents according to the procedure
summarised in Scheme 1. Firstly, the chloromethyl-
19–28
been developed for the preparation of primary arylamines.
However, the direct utilisation of ammonia as the amino source
of primary arylamines is still ineffective in the absence of
pressure thus far. Recently, proline-derived copper complexes
have been reported to catalyse the aminations of aryl iodides
functionalised MCM-41 material (MCM-41-CH Cl) was
2
obtained by the condensation reaction of MCM-41 with
4-(chloromethyl)phenyltrichlorosilane in toluene at 100 °C for
24 h, followed by treatment with anhydrous ethanol at 80 °C
with NH Cl or amidine hydrochlorides for efficient synthesis of
4
2
9,30
primary arylamines.
Although these copper-catalysed aminations of aryl halides
are highly efficient for the construction of primary arylamines,
about 10 mol% of copper catalysts are usually used to obtain high
yields. Moreover, the majority of reported systems, in particular
the systems that use homogeneous copper catalysts, have been
susceptible to contamination by the cytotoxic copper ions
due to the formation of copper complexes with the arylamine
products, which restricts the application of such systems in
electronics and biomedicine. These problems are of particular
environmental and economic concern in large-scale syntheses
and in industry. To overcome these drawbacks the development
of highly efficient and recyclable heterogeneous catalysts, for
example by immobilisation of catalytically active species onto
a solid support to generate a molecular heterogeneous catalyst,
for 24 h and then silylation with Me SiCl in toluene at room
3
temperature for 24 h. The MCM-41-CH Cl was then treated
2
with N-Boc-trans-4-hydroxy-L-proline in THF in the presence
of NaH, followed by deprotection with TFA in CH Cl to afford
2
2
the L-proline-functionalised MCM-41 material (MCM-41-L-
proline). Finally, the reaction of MCM-41-L-proline with CuI
in acetone at room temperature for 12 h generated the MCM-41-
immobilised L-proline copper(I) iodide complex (MCM-41-L-
proline-CuI) as a light blue powder. The copper content of the
–1
catalyst was found to be 0.83 mmol g according to inductively
coupled plasma-atomic emission spectroscopy (ICP-AES)
measurements.
Small angle X-ray powder diffraction (XRD) patterns of the
parent MCM-41 and the modified material MCM-41-L-proline-
CuI are presented in Fig. 1. The parent MCM-41 is a well-
ordered mesoporous phase, which can be indexed according to
31,32
is essential.
The discovery of mesoporous MCM-41 has provided a new
possible candidate for an ideal solid support for immobilising
homogeneous catalysts and given an enormous stimulus
33
a hexagonal lattice characteristic of MCM-41. For the MCM-
41-L-proline-CuI complex, the (100) reflection of MCM-41
with decreased intensity remained, while the (110) and (200)
33,34
to research in heterogeneous catalysis.
MCM-41 has
*
Correspondent. E-mail: caimzhong@163.com

316 JOURNAL OF CHEMICAL RESEARCH 2017
Scheme 1
(100)
1
2
(110)
(200)
1
2
3
4
5
6
7
2θ (degrees)
Fig. 1 XRD patterns of the parent MCM-41 (1) and MCM-41-L-proline-
CuI (2).
Fig. 2 Energy dispersive spectra of MCM-41-L-proline-CuI.
reflections became weak and diffuse after introduction of the
copper complex, which may be mainly due to contrast matching
between the silicate framework and organic moieties which are
located inside the channels of MCM-41. These results indicate
that the structure of the parent MCM-41 remains intact through
the functionalisation procedure. Energy dispersive X-ray
spectroscopy (EDS) shows the elements present in the material.
EDS analysis of fresh MCM-41-L-proline-CuI complex shows
the presence of Si, O, C, N, I, and Cu elements (Fig. 2).
Table
1 Optimisation of reaction conditions for the synthesis of
a
3
-nitroaniline
b
Entry Base
Solvent
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMAc
NMP
DMSO
DMF
DMF
Temp. (°C)
110
120
130
140
130
130
130
130
130
130
130
130
Time (h)
24
20
20
20
20
20
20
20
20
24
40
12
Yield (%)
1
2
3
4
5
6
7
8
9
Cs CO3
26
79
94
93
87
75
67
81
76
52
77c
2
Cs CO3
2
The MCM-41-immobilised L-proline-copper(I) complex
Cs CO3
2
(
MCM-41-L-proline-CuI) was then used as a catalyst for the
Cs CO3
2
cascade reaction of aryl iodides with acetamidine hydrochloride.
Initially, 3-iodonitrobenzene (1a) was selected as the model
substrate to optimise the reaction conditions and the results
are summarised in Table 1. Firstly, the effect of temperature
was examined by using 10 mol% of MCM-41-L-proline-CuI
as catalyst and Cs CO as base in dimethylformamide (DMF)
K2CO3
Na CO3
2
K3PO4
Cs CO3
2
2
2
2
Cs CO3
2
3
1
0
Cs CO3
under Ar (Table 1, entries 1–4). It is evident that the yield of
the desired product 2a increased with the increase in reaction
temperature (110–130 °C) and that 130 ºC gave the best result
1
1
Cs CO3
d
12
Cs CO3
94
2
(
Table 1, entry 3), further raising the temperature to 140 ºC did
a
Reaction conditions: 3-iodonitrobenzene (1a) (1 mmol), acetamidine hydrochloride (1.2
mmol), base (2 mmol), solvent (3 mL) under Ar.
Isolated yield.
5 mol% MCM-41-L-proline-CuI was used.
20 mol% MCM-41-L-proline-CuI was used.
not improve the yield (Table 1, entry 4). The effects of base
and solvent on the reaction were also investigated. Among
several inorganic bases tested K CO gave a good yield, both
b
c
d
2
3
Na CO and K PO were substantially less effective (Table 1,
2
3
3
4

JOURNAL OF CHEMICAL RESEARCH 2017 317
entries 5–7) and Cs CO proved to be the best base (Table 1,
To determine whether the observed catalysis was due to the
heterogeneous catalyst MCM-41-L-proline-CuI or to a leached
copper species in solution, the amination of 4-iodonitrobenzene
(1b) was carried out until an approximately 50% conversion of
1b was reached. Then the MCM-41-L-proline-CuI was removed
from the reaction mixture by hot filtration and the filtrate was
again stirred at 130 ºC under an Ar atmosphere for 14 h. In this
case, no significant increase in conversion of 1b was observed,
indicating that leached copper species from the MCM-41-
L-proline-CuI (if any) are not responsible for the observed
catalysis. ICP-AES analysis showed that no copper species
could be detected in the filtrate. These results demonstrated that
the heterogeneous MCM-41-L-proline-CuI complex was stable
during the reaction and the amination reaction catalysed by
copper was intrinsically heterogeneous.
2
3
entry 3). When other strong dipolar aprotic solvents such as
dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP)
and dimethylsulfoxide (DMSO) were used, no improvement
of yield was observed and DMF gave the best result (Table 1,
entries 3 and 8–10). Finally, the amount of the copper catalyst
was also screened. Reducing the amount of the catalyst to 5
mol% resulted in a significant decrease in yield and a longer
reaction time was required (Table 1, entry 11). Increasing the
amount of the catalyst could shorten the reaction time, but did
not improve the yield (Table 1, entry 12). Thus, the optimised
reaction conditions for this transformation were taken as MCM-
4
6
41-L-proline-CuI (10 mol%) and acetamidine hydrochloride
(
1.2 equiv.) in DMF with Cs CO (2 equiv.) as base at 130 ºC
2
3
under Ar for 20 h (Table 1, entry 3).
A plausible mechanism for heterogeneous copper-catalysed
formation of primary arylamines is proposed in Scheme 3.
With this promising result in hand, we started to examine the
scope and limitation of this heterogeneous copper-catalysed
cascade reaction between aryl iodides and acetamidine
hydrochloride under the optimised reaction conditions (Scheme
2
Firstly, reaction of acetamidine hydrochloride with Cs CO3
produces free acetamidine and water. Oxidative addition of
2) and the results are summarised in Table 2. The amination
reaction of aryl iodides having strong electron-withdrawing
groups 1a–g with acetamidine hydrochloride proceeded
smoothly under the optimised reaction conditions to afford the
corresponding electron-deficient primary arylamines 2a–g in
good to excellent yields (Table 2, entries 1–7). However, aryl
iodides having weak electron-withdrawing groups 1h–l exhibited
lower reactivity and the amination reaction was carried out with 2
equiv. of acetamidine hydrochloride to obtain good yields of the
desired products 2h–l (Table 2, entries 8–12). The reactivity of
electron-rich aryl iodides was lower than that of electron-deficient
ones. For example, the amination reactions of electron-rich aryl
iodides 1m–r with acetamidine hydrochloride could not proceed
until the temperature was raised to 140 ºC and the use of 2 equiv.
of acetamidine hydrochloride was also required to produce the
corresponding electron-rich primary arylamines 2m–r in good
yields (Table 2, entries 13–18). The amination of sterically
congested aryl iodides 1s–x with acetamidine hydrochloride (1.2
or 2 equiv.) could also proceed effectively at 130 or 140 ºC to give
the corresponding ortho-substituted primary arylamines 2s–x in
good yields (Table 2, entries 19–24). For instance, the reactions
of 2-iodonitrobenzene (1s) and 4-iodo-1,3-dinitrobenzene (1t)
with 1.2 equiv. of acetamidine hydrochloride at 130 ºC gave the
desired 2s and 2t in 82 and 78% yield respectively. The amination
of 2-chloroiodobenzene (1u) and methyl 2-iodobenzoate (1v)
required the use of 2 equiv. of acetamidine hydrochloride and
the amination of electron-rich aryl iodides 1w and 1x needed
not only the use of 2 equiv. of acetamidine hydrochloride, but
also higher temperature (140 ºC). It is noteworthy that bulky
Table
2 Heterogeneous copper-catalysed synthesis of primary
arylamines by the cascade reaction of aryl iodides with acetamidine
a
hydrochloride
b
Temp.
ºC)
Yield
(%)
Entry ArI
Product
(
1
2
3
4
5
6
7
8
9
3-O NC H I (1a)
130
130
130
130
130
130
130
130
130
130
130
130
140
140
140
140
140
140
130
3-O NC H NH (2a)
94
92
90
88
86
2
6
4
2
6
4
2
4-O NC H I (1b)
4-O NC H NH (2b)
2
6
4
2 6 4 2
4-NCC H I (1c)
4-NCC H NH (2c)
6 4 2
6
4
4-MeCOC H I (1d)
4-MeCOC H NH (2d)
6
4
6 4 2
4-PhCOC H I (1e)
4-PhCOC H NH (2e)
6 4 2
6
4
4-MeOCOC H I (1f)
4-MeOCOC H NH (2f) 85
6
4
6 4 2
3-MeCOC H I (1g)
3-MeCOC H NH (2g)
82
78
82
76
77
72
75
76
81
78
73
92
82
6
4
6
4
2
c
c
4-FC H I (1h)
4-FC H NH (2h)
6
4
6 4 2
4-ClC H I (1i)
4-ClC H NH (2i)
6 4 2
6
4
c
10
3-ClC H I (1j)
3-ClC H NH (2j)
6
4
6 4 2
c
11
12
13
14
15
16
17
18
19
4-BrC H I (1k)
4-BrC H NH (2k)
6 4 2
6
4
c
c
c
c
c
3-BrC H I (1l)
3-BrC H NH (2l)
6
4
6 4 2
4-MeC H I (1m)
4-MeC H NH (2m)
6 4 2
6
4
3-MeC H I (1n)
3-MeC H NH (2n)
6
4
6 4 2
4-MeOC H I (1o)
4-MeOC H NH (2o)
6 4 2
6
4
3-MeOC H I (1p)
3-MeOC H NH (2p)
6
4
6 4 2
c
3,4-Me C H I (1q)
3,4-Me C H NH (2q)
2 6 3 2
2
6
3
c
4-H NC H I (1r)
4-H NC H NH (2r)
2
6
4
2 6 4 2
2-O NC H I (1s)
2-O NC H NH (2s)
2 6 4 2
2
6
4
20
2,4-(O N) C H I (1t) 130
2,4-(O N) C H NH (2t) 78
2
2
6
3
2 2 6 3 2
c
21
22
23
24
25
2-ClC H I (1u)
130
2-ClC H NH (2u)
74
6
4
6
4
2
c
c
2-MeOCOC H I (1v) 130
2-MeOCOC H NH (2v) 76
6
4
6
4
2
2-MeOC H I (1w)
140
140
2-MeOC H NH (2w)
73
70
68
81
6
4
6
4
2
c
c
c
1-iodonaphthalene (1y) and a hetero-aryl iodide 1z were good
2-MeC H I (1x)
2-MeC H NH (2x)
6
4
6 4 2
substrates and afforded the expected products 2y and 2z in good
yields (Table 2, entries 25 and 26). A range of functional groups
such as methyl, methoxy, amino, fluoro, chloro, bromo, cyano,
nitro, ester and ketone were tolerated well. The present method
provides a simple, general and practical route to a wide variety of
functionalised primary arylamines.
1-Naphthyl iodide(1y) 140
2-Pyridinyl iodide (1z) 130
1-Naphthylamine (2y)
Pyridin-2-amine (2z)
26
a
Reaction conditions: aryl iodide 1 (1 mmol), acetamidine hydrochloride (1.2 mmol),
Cs CO (2 mmol), MCM-41-L-proline-CuI (0.1 mmol), DMF (3 mL) under Ar for 20 h.
2
3
b
Isolated yield.
c
Acetamidine hydrochloride (2 mmol) and Cs CO (3 mmol) were used.
2
3
Scheme 2

318 JOURNAL OF CHEMICAL RESEARCH 2017
Scheme 3
aryl iodide 1 to the MCM-41-L-proline-Cu(I) complex A
provides an MCM-41-immobilised L- proline–Ar–Cu(III)–I
complex intermediate B, which reacts with free acetamidine
in the presence of Cs CO to form intermediate C. The latter
undergoes a reductive elimination to give intermediate D and
regenerates the MCM-41-L-proline-Cu(I) complex A. Finally,
intermediate D undergoes addition with water in base medium
and subsequent removal of acetamide to afford the target
product 2. During the purification of the product, acetamide
could be isolated as a white solid in support of the proposed
mechanism.
and excellent reusability of the catalyst can be attributed to the
chelating action of the bidentate L-proline ligand on copper and
the mesoporous structure of the MCM-41 support. The result
is important from industrial and environmental points of view.
The high catalytic activity and excellent reusability of the new
MCM-41-L-proline-CuI complex make it a highly attractive
heterogeneous copper catalyst for the parallel solution phase
synthesis of diverse libraries of compounds.
2
3
In conclusion, we have successfully developed a novel,
practical and environmentally friendly method for the synthesis
For the practical application of a heterogeneous transition-
metal catalyst system, its stability and recyclability are
important factors. We next investigated the reusability of the
MCM-41-L-proline-CuI complex by using the amination
reaction of 3-iodonitrobenzene (1.0 mmol) with acetamidine
hydrochloride (1.2 mmol) and Cs CO (2 mmol) in the presence
2
3
of MCM-41-L-proline-CuI (0.1 mmol) in DMF (3 mL) at 130 ºC
under Ar for 20 h. After completion of the reaction, the catalyst
was separated by simple filtration of the reaction solution and
washed with distilled water and EtOH. After being air-dried,
it was reused directly without further purification in the next
run and almost consistent activity was observed for eight
consecutive reactions (Fig 3). In addition, copper leaching in
the supported catalyst was also determined. The copper content
of the catalyst was found by ICP-AES analysis to be 0.82 mmol
–1
g
after eight consecutive reactions and only 1.2% of copper
Fig. 3 Recycling of the MCM-41-L-proline-CuI catalyst.
had been lost from the MCM-41 support. The high stability

JOURNAL OF CHEMICAL RESEARCH 2017 319
of primary arylamines through the amination reaction of
aryl iodides with acetamidine hydrochloride as the ammonia
surrogate in DMF by using an MCM-41-immobilised L-proline-
copper(I) complex (MCM-41-L-proline-CuI) as catalyst. The
reactions were performed under simple and safe experimental
conditions and generated a variety of primary arylamines in
good to excellent yields and were applicable to a range of aryl
iodides. Importantly, this new heterogeneous copper catalyst
can be easily prepared from commercially available and
inexpensive reagents, recovered by a simple filtration and used
at least seven more times without a significant loss of activity,
thus making this procedure economically and environmentally
more acceptable.
copper complex (MCM-41-L-proline-CuI) (1.15 g) as a light blue
powder. The copper content was found to be 0.83 mmol g .
–1
Heterogeneous copper-catalysed synthesis of primary arylamines;
general procedure
A two-necked flask equipped with a magnetic stirring bar was charged
with Cs CO (2 or 3 mmol), MCM-41-L-proline-CuI (0.1 mmol), aryl
2
3
iodide (1.0 mmol), acetamidine hydrochloride (1.2 or 2 mmol) and
DMF (3.0 mL) under Ar. The reaction mixture was stirred at 130 or
140 ºC for 20 h. After being cooled to room temperature, the mixture
was diluted with CH Cl (10 mL) and filtered. The catalyst was
2
2
washed with distilled water (2 × 5 mL) and EtOH (2 × 5 mL) and air-
dried when reused in the next run. The filtrate was concentrated with
the aid of a rotary evaporator and the residue was purified by column
chromatography on silica gel using petroleum ether (30–60 °C)/ethyl
acetate (10:1 to 1:1) as eluent to give the desired product 2. All the
products 2a–z are known compounds.
Experimental
All chemicals were reagent grade and used as purchased. All solvents
were dried and distilled before use. The products were purified by
flash chromatography on silica gel. A mixture of light petroleum ether
Acknowledgements
(
30–60 °C) and ethyl acetate was generally used as eluent. All products
We thank the National Natural Science Foundation of China
(No. 21462021) and the Key Laboratory of Functional Small
Organic Molecule, Ministry of Education (No. KLFS-
KF-201409) for financial support.
were characterised by comparison of their spectra and physical data
with authentic samples. H NMR spectra were recorded on a Bruker
Avance 400 (400 MHz) spectrometer with TMS as an internal standard
in DMSO-d or CDCl as solvent. C NMR spectra were recorded on
1
13
6
3
a Bruker Avance 400 (100 MHz) spectrometer in CDCl as solvent.
Melting points are uncorrected. Copper content was determined with
by ICP-AES using an Atomscan16 instrument (TJA Corporation).
Electronic Supplementary Information
The ESI (characterisation details of the products) is available
through
3
Microanalyses were carried out using
a Yanaco MT-3 CHN
stl.publisher.ingentaconnect.com/content/stl/jcr/supp-data
microelemental analyser. XRD patterns were obtained on a Damx-rA
instrument (Rigaku). EDS was performed using a JSM-6510 scanning
electron microscope. Mesoporous material MCM-41 was prepared
Received 25 January 2017; accepted 2 April 2017
Paper 1704567
https://doi.org/10.3184/174751917X14931195075580
Published online: 17 May 2017
47
according to a literature method.
Synthesis of MCM-41-CH Cl
2
A solution of 4-(chloromethyl)phenyltrichlorosilane (5.0 g) in dry
toluene (20 mL) was added to a suspension of MCM-41 (5.5 g) in dry
toluene (100 mL). The mixture was stirred for 24 h at 100 °C under
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2
3
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3
dried under reduced pressure at 100 °C for 2 h. The dried white solid
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(
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4
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to obtain the hybrid material MCM-41-CH Cl (7.43 g). The chlorine
2
–1
content was found to be 1.65 mmol g by elemental analysis.
7
8
9
Synthesis of MCM-41-L-proline
NaH (0.28 g, 11.6 mmol) was added to a solution of N-Boc-trans-
10
J.P. Wolfe, S. Wagaw and S.L. Buchwald, J. Am. Chem. Soc., 1996, 118,
4
-hydroxy- L-proline (1.27 g, 5.5 mmol) in dry THF (100 mL) and
7215.
the reaction mixture was stirred at room temperature for 2 h under
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2
6621.
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The white solid (1.5 g) was treated with TFA (8 mL) in CH Cl (10 mL)
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