Promoting the catalytic efficiency of a catalyst by a solvothermal method

Article abstract of DOI:10.1039/c3ra40489g

A solvothermal method has been successfully introduced and applied to catalytic organic reactions. This method provides an easy way to promote catalyst efficiency. Several classical coupling reactions and aminations of aryl halides were performed using common catalysts such as commercial Pd/C and CuI using this method.

Full text of DOI:10.1039/c3ra40489g

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Promoting the catalytic efficiency of a catalyst by a
solvothermal method3
Cite this: RSC Advances, 2013, 3, 5819
Long Sun, Peilei He, Biao Xu, Xiaobin Xu and Xun Wang*
Received 25th December 2012,
Accepted 27th February 2013
DOI: 10.1039/c3ra40489g
www.rsc.org/advances
A solvothermal method has been successfully introduced and
applied to catalytic organic reactions. This method provides an
easy way to promote catalyst efficiency. Several classical coupling
reactions and aminations of aryl halides were performed using
common catalysts such as commercial Pd/C and CuI using this
method.
control is usually limited by the boiling point of the solvent, that
is, the optimized reaction temperature and the boiling point of the
solvent do not necessarily correlate. For example, water will reflux
in a flask at 100 uC at atmospheric pressure and cannot rise to 170
uC. To solve this problem, a solvothermal method is considered to
be a good solution. Solvothermal methods have already been
widely used in the synthesis of many inorganic materials, such as
metals, metallic oxides, metallic sulphides and coordination
Palladium-catalysed carbon–carbon (C–C) coupling reactions, such
as the Suzuki–Miyaura, Heck, Sonogashira and Ullmann reactions,
and copper-catalysed carbon–nitrogen (C–N) coupling reactions,
such as the amination of aryl halides, are important and effective
methods of introducing functional groups in organic synthesis.
Besides the traditional methods of conducting the experiments in
a flask over a catalyst with heating, several novel synthetic
16–25
compounds as well as molecular sieves.
However, to the best
of our knowledge, there are no examples of performing different
types of organic coupling reactions under solvothermal condi-
tions. Herein we report examples of several classical organic
reactions, such as Suzuki, Heck and Sonogashira couplings, and
amination of aryl halides, over common, cheap palladium or
copper catalysts under solvothermal conditions.
1
2
methods, such as photocatalysis methods, microwave methods,
3
4
electrocatalysis methods, and supercritical methods, have been
developed and demonstrated satisfactory results. What’s more,
some transition-metal free reactions and ligand-free reactions
Initially, we focused on the Suzuki–Miyaura coupling reaction.
A series of systematic work on Suzuki–Miyaura coupling in
aqueous media has been undertaken since Toshikazu first
5,6
7–9
26,27
have also been reported. Although the previously mentioned
methods are effective, an easier and more efficient method is still
desired. For a catalytic reaction, the most common way to control
the reaction process is by changing factors such as the
temperature, pressure, catalyst or electromagnetic wavelength as
well as the solvent, which may have a conspicuous effect on both
the chemical thermodynamics and dynamics of the reaction.
Catalysts and their mechanisms have been systematically studied
reported his work in 2002.
In some publications, for example by Choudary and co-workers
28
in 2002, the actual reaction temperature didn’t meet the desired
reaction temperature. They used a mixture of dioxane and water as
the solvent and set the temperature to 100 uC. However, the
boiling point of this solvent is only 87.8 uC. Thus what we are most
concerned with in this work is finding a general method that
could solve this problem rather than a specific solvent or system.
As for the solvothermal method, firstly the solvent was taken
into account (Table 1). Solvent selection is more flexible for
solvothermal methods and any solvent that would not hinder the
reaction could be a possible solvent in theory. To give a brief
comparison, the control experiments conducted in a flask are also
listed in Table 1. We can see that in either water or ethanol, an
enhanced yield is achieved under solvothermal conditions
compared to those reactions conducted in a flask. The highest
yields were achieved from the reactions in the mixed solvent,
under both the standard and solvothermal conditions.
10,11
and applied with great success.
To improve the catalytic ability
of metal catalysts, such as palladium, platinum and copper, using
ligands and decreasing the catalyst size to a nanometre scale are
12–15
effective methods.
However, these methods usually cost a lot
of money because of the use of expensive ligands, precursors,
surfactants and take much time because of complex preparation
processes. Of all the factors mentioned above, temperature is the
easiest and most convenient to control. However, temperature
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China.
E-mail: wangxun@mail.tsinghua.edu.cn; Fax: +8610-62792791;
Tel: +8610-62792791
Thus in order to highlight the fact that the solvothermal
method can promote the catalytic efficiency of a catalyst, a single
solvent should be used. Since water is cheap, green and abundant,
3
Electronic supplementary information (ESI) available: General information;
experimental details; NMR and MS spectra. See DOI: 10.1039/c3ra40489g
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Table 1 Commercial Pd/C-catalyzed Suzuki–Miyaura coupling of bromotoluene
Table 2 Commercial Pd/C-catalyzed Suzuki–Miyaura coupling of aryl halides
a
a
and phenylboronic acid: effect of solvent choice on the reaction yield
with phenylboronic acid under hydrothermal conditions
b
b
Entry Solvent
Actual temperature (uC) Yield (%)
Entry
1
Aryl halide
Product
Yield (%)
1
2
3
4
5
6
7
water
ethanol
water : ethanol = 1 : 1 130
water
water
ethanol
water : ethanol = 1 : 1 78.1
130
130
82
78
95
71
58
63
92
92 (64)
c
c
c
c
100
70
2
3
92 (51)
42 (37)
78.4
a
Reaction conditions: bromotoluene (0.5 mmol), phenylboronic acid
CO (1.5 mmol), TBAB (0.12 mmol), solvent (2 mL),
(0.75 mmol), K
2
3
and commercial Pd/C catalyst (0.6 mol%), 130 uC, 2 h, solvothermal
b
c
treatment. Yields were determined by GC. Conducted in a flask.
4
82 (45)
5
6
96 (10)
67 (49)
we chose it as the example solvent for the following experiments.
We investigated the scope of aryl halides that could react with
phenylboronic acid under hydrothermal conditions and per-
formed control experiments. As shown in Table 2, we can see
that for aryl bromides, the substrates examined under optimal
conditions achieved good yields under solvothermal treatment
while the aryl chlorides achieved lower yields. When compared
with the control experiments which were run in flasks, the
hydrothermal method clearly promotes the catalytic efficiency. It
should be noted that the solvothermal method activates the aryl
chlorides by simply increasing the reaction temperature. The
substituted aryl halides with electron-withdrawing and electron-
donating groups reacted smoothly. Aryl halides with a substituent
at different sites on their benzene rings had different activities.
Due to steric hindrance, substituents at the ortho-position would
hinder the coupling process thus giving a lower yield. What’s
more, this method can be expanded to other aromatic
compounds, such as pyridine.
7
8
9
52 (30)
29 (N.R.)
10 (N.R.)
10
26 (N.R.)
65 (N.R.)
61 (N.R.)
76 (N.R.)
11
The palladium catalyst used in the experiments is a common
commercially available Pd/C catalyst and could be easily dispersed
in an aqueous phase, but cannot be easily dispersed in an oil
phase. Therefore the Pd/C catalyst can be easily recycled after
extraction by simple filtration. We recycled the Pd/C catalyst three
12
13
3
times and its catalytic performance is shown in Table S1, ESI. We
a
Reaction conditions: aryl halide (0.5 mmol), phenylboronic acid
CO (1.5 mmol), TBAB (0.12 mmol), water (2 mL),
can see that even the commercial Pd/C catalyst can be reused at
least twice. The gradual decrease in the yield is due to loss of the
Pd/C catalyst during the filtration step when using normal filter
papers.
(0.75 mmol), K
2
3
and commercial Pd/C catalyst (0.6 mol%), 130 uC, 2 h for aryl
bromides and 4 h for aryl chlorides, hydrothermal conditions.
Control experiment conditions: 100 uC in a flask under atmospheric
b
pressure. Yields of the isolated products. Yields of the control
We then tried to extend this method to some other important
reactions. As shown in Table 3, the commercial Pd/C catalyst can
also catalyze Heck and Sonogashira coupling reactions. To
summarise, the solvothermal method is a suitable and generalized
method for improving the yield of C–C coupling reactions.
Furthermore, we were curious about whether the hydrothermal
method could be applied to C–N bond forming reactions and we
chose the amination of aryl halides as the model reaction. Firstly
experiments are in parentheses. N.R. represents no reaction.
3 2
optimal reaction conditions are as follows: NH ?H O as the base,
coordinating agent and reactant, water as the solvent, CuI as the
catalyst and a reaction temperature of 200 uC.
3
the reaction parameters were optimized (see Table S2, ESI ). The
5
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a
Table 3 Different types of coupling reactions performed under solvothermal
Table 4 Amination of aryl halides under hydrothermal conditions
a
conditions
b
Entry Reaction type Reactant 1 Reactant 2 Product
Yield (%)
62
1
Heck
b
Entry
1
Aryl halide
Product
Yield (%)
95 (N.R.)
2
Heck
86
2
3
91 (N.R.)
89 (N.R.)
3
4
Sonogashira
Sonogashira
74
11
4
18 (N.R.)
a
Heck reaction conditions: Reactant 1 (0.55 mmol), reactant 2 (0.5
CO (1.5 mmol), TBAB (0.12 mmol), dioxane (2 mL),
commercial Pd/C catalyst (0.6 mol%), 130 uC, 2 h, solvothermal
conditions. Sonogashira reaction conditions: Reactant 1 (0.55 mmol),
reactant 2 (0.5 mmol), K CO (1.5 mmol), Et N (1 mL), THF (1 mL),
mmol), K
2
3
5
6
88 (N.R.)
90 (N.R.)
2
3
3
commercial Pd/C catalyst (0.6 mol%), CuI (0.4 mmol), 130 uC, 2 h,
b
solvothermal conditions. Yield of isolated product.
7
8
9
56 (N.R.)
74 (N.R.)
69 (N.R.)
71 (N.R.)
Next we investigated the scope of aryl halides that can be
transformed into primary amines under hydrothermal conditions
and also performed control experiments. As shown in Table 4, we
can see that for both aryl bromides and aryl chlorides, most of the
substrates achieved good yields under hydrothermal conditions,
which greatly surpass the yields of the control experiments.
Similar to the coupling reactions mentioned above, the
substituted aryl halides with electron-withdrawing or electron-
donating groups, and those with different functional groups at
different sites, gave satisfactory yields. Despite the steric hin-
drance, the presence of substituents has a more prominent
influence on the activity of the substrates. In addition, the
substrates can be expanded to include other aromatic compounds,
such as pyridine which resulted in a good yield. Compared with
using complex coordinating reagents, organic solvents and other
additives, the hydrothermal system only uses water, ammonia,
cuprous iodide and heat. Ammonia plays multiple important roles
thus greatly simplifying the system. Moreover, the ammonia will
not evaporate because the autoclave is sealed. As is known, both
ammonia and cuprous iodide are cheap and abundant chemical
reagents and water is a green solvent. Therefore this approach
provides a simple way to cheaply synthesize primary amines.
To better understand the mechanism of the hydrothermal
method, we chose 4-bromoanisole as a cross-coupling substrate to
investigate the process. We performed experiments at seven
different temperatures, from 25 uC to 170 uC, bearing in mind that
the boiling point of water is 100 uC, and we prolonged the reaction
time to 8 h. Control experiments below 100 uC were also
conducted in flasks.
1
1
0
1
65 (N.R.)
12
58 (N.R.)
54 (N.R.)
13
a
Reaction conditions: aryl halide (0.5 mmol), NH ?H O (3.0 mmol),
3
2
water (2 mL), and CuI (5 mol%), 200 uC, 2 h, hydrothermal
conditions. Control experiment conditions: 100 uC in a flask under
b
atmospheric pressure. Yields of isolated products. Yields of the
control experiments are in parentheses. N.R. represents no reaction.
reaction in the autoclave (Fig. 1) was lower because there was no
stirring process to mix the reactants together. When the
temperature was raised to 70 uC and 90 uC, the yields of the
reactions in the two different tanks were almost the same. On one
hand, this substrate has high intrinsic activity, and on the other
As shown in Table 5, the yield fluctuates with increasing
temperature. When the temperature is 25 uC, the yield of the
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a
substrate has its own optimal reaction temperature, which may be
higher or lower than the boiling point of the solvent. For those
whose optimal temperature is higher than the boiling point of the
solvent, such as chlorobenzene, the solvothermal method provides
an easy way to improve its reactivity.
Table 5 Effect of temperature on the hydrothermal cross -coupling reaction
T (uC)
Yield (%)
25
20 (40)
70
75 (72)
90
98 (95)
110
81
130
94
150
98
170
87
b
a
Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic
CO (1.5 mmol), TBAB (0.12 mmol), water (2
acid (0.75 mmol), K
2
3
In summary, we have successfully applied a solvothermal
method to organic synthesis. This method is a generalized way to
realize C–C and C–N bond formation and can promote the
catalytic efficiency of catalysts. The advantages of the solvothermal
method are as follows: the experiments are easy to perform; the
choice of solvent is flexible; the reaction system is simple, without
the need for expensive catalysts and complex ligands; the reactions
occur with a common catalyst; the use of a sealed system means
that it is easy to perform oxygen-sensitive reactions and prevent
volatile compounds from evaporating. The good functional group
tolerance under solvothermal conditions indicates that this
method can be extended to some other reactions.
mL), and commercial Pd/C catalyst (0.6 mol%), 8 h, hydrothermal
conditions. Yields were determined by GC, yields of control
experiments are in parenthesis.
b
hand, when the reaction temperature is high, convection in the
autoclave, like stirring in a flask, results in mass transfer. Initially,
we ran the experiment at 25 uC in a flask and it gave a 40% yield of
the expected product. However, there is the same amount of the
self-coupling product of phenylboronic acid, biphenyl, in solution.
At 70 uC, although the yield is still low, which is due to the slow
reaction rate at low temperature, no byproduct is formed under
hydrothermal conditions. When the temperature was increased to
This work was financially supported by NSFC (91127040,
20921001), and the State Key Project of Fundamental Research for
Nanoscience and Nanotechnology (2011CB932402).
90 uC, the yield was almost 100% due to a faster reaction rate.
However, the yield decreased obviously at 110 uC. According to the
GC–MS analysis result, a small amount of biphenyl appeared
again, which indicates that the side reaction could occur at 110 uC.
When the temperature was increased to 130 uC or 150 uC, we
observed that the yield is nearly 100% again. This demonstrates
that the hydrothermal method can restrain side reactions at
suitably high temperatures. We inferred that the C–Br bond of the
substrate was activated, thus decreasing the activation energy of
the cross-coupling reaction, which has the advantage of increasing
the reaction rate of the desired reaction over that of the side
reaction. Furthermore, the yield decreased again when the
temperature was increased to 170 uC. According to the GC–MS
analysis result, the byproducts included not only biphenyl but also
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