Imidazolium-Based Ionic Liquids Catalyzed Formylation of Amines Using Carbon Dioxide and Phenylsilane at Room Temperature

Article abstract of DOI:10.1021/acscatal.5b01274

The CO₂-involved synthesis of chemicals is of significance. In this work, we found that 1-alkyl-3-methylimidazolium ionic liquids (ILs) had high efficiency for catalyzing the formylation of amines using CO₂ and phenylsilane at room temperature, producing the corresponding formylated products in excellent yields under the metal-free condition. The ILs acted as bifunctional catalysts, which activated the Si-H bond of phenylsilane to react with CO₂ to form the formoxysilane intermediate and simultaneously activated the amine substrate through the hydrogen bond. Moreover, the imidazolium cation and the anions of the ILs showed an excellent synergistic effect on catalyzing the formylation of amines.

Full text of DOI:10.1021/acscatal.5b01274

Letter
pubs.acs.org/acscatalysis
Imidazolium-Based Ionic Liquids Catalyzed Formylation of Amines
Using Carbon Dioxide and Phenylsilane at Room Temperature
Leiduan Hao, Yanfei Zhao, Bo Yu, Zhenzhen Yang, Hongye Zhang, Buxing Han, Xiang Gao,
and Zhimin Liu*
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid, Interface and Thermodynamics, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
S
* Supporting Information
ABSTRACT: The CO2-involved synthesis of chemicals is of significance. In
this work, we found that 1-alkyl-3-methylimidazolium ionic liquids (ILs) had
high efficiency for catalyzing the formylation of amines using CO2 and
phenylsilane at room temperature, producing the corresponding formylated
products in excellent yields under the metal-free condition. The ILs acted as
bifunctional catalysts, which activated the Si−H bond of phenylsilane to react with CO2 to form the formoxysilane intermediate
and simultaneously activated the amine substrate through the hydrogen bond. Moreover, the imidazolium cation and the anions
of the ILs showed an excellent synergistic effect on catalyzing the formylation of amines.
KEYWORDS: carbon dioxide, ionic liquid, formylation, room temperature, conversion
he metal-free catalytic process can reduce cost and avoid
promising due to their advantages such as easy separation,
Tthe pollution caused by metals and is thus regarded as a recyclability, stability to air and water, and so on.
Formamides are versatile chemicals and important building
blocks, which are generally produced via the formylation of
amines. Using CO2 instead of toxic CO for the N-formylation
reaction is an attractive and green alternative for the production
of formamides.4c,5a,b,8 However, the reported routes generally
suffered from drawbacks such as requiring metal catalysts, inert
gas atmosphere, complicated reaction system, high temperature
and pressure, and difficulty in catalyst separation, among others.
Therefore, exploring simple, recyclable, green catalytic systems
is still highly desirable.
Herein, we carried out the initial work to use ILs as the
catalysts for the N-formylation reactions of different amines
with CO2 and phenylsilane. It was discovered that imidazolium-
based ILs (as listed in Table 1) could catalyze the reactions at
room temperature, producing the formamides in excellent
yields. It was indicated that the ILs served as bifunctional
catalysts, which activated phenylsilane to react with CO2 to
form the formoxysilane intermediate and simultaneously
activated the substrate through hydrogen bond, leading to the
production of formamides. Moreover, it was found that the
cations and anions of the ILs had excellent synergistic effect on
catalyzing the formylation of amines. In addition, the ILs can be
easily recovered from the reaction solution and can be reused
for five times without activity loss.
green process, which has been paid much attention in chemical
synthesis.1 The CO2-involved chemical synthesis has been
widely investigated in the past decades because CO2 is a cheap,
renewable, abundant, and green C1 resource.2 However, due to
the inherent thermodynamic and kinetic stability of CO2, it is
challenging to activate CO2 and achieve its transformation
under mild conditions, especially at room temperature. So far,
much work has focused on exploring efficient catalysts or
catalytic systems for CO2 conversion.3 Metal-based catalysts
have been widely applied in the CO2-involved chemical
synthesis.4 Recently, metal-free catalytic systems have been
reported, showing promising potential for the CO2 trans-
formation.5 For example, N-heterocyclic carbenes (NHCs) can
activate CO2 and catalyze CO2 conversion at room temperature
and atmospheric pressure.5a However, compared with the metal
catalysis, the nonmetal-catalyzed CO2 conversion is still in an
early stage.
Ionic liquids (ILs), composed of organic cations and
organic/inorganic anions, possess unique features such as
high thermal and chemical stability, negligible vapor pressure,
easy separation and tunable properties. Notably, most of the
ILs are nonmetallic salts, which have displayed promising
applications in many areas, especially in catalysis.6 For example,
task-specific ILs have realized the CO2 capture and conversion
under mild and metal-free conditions. Protic IL (e.g.,
[DBUH][TFE]) served as a bifunctional catalyst and achieved
the CO2 conversion at atmospheric pressure and room
temperature in the synthesis of quinazoline-2,4(1H,3H)- diones
from CO2 and 2-aminobenzonitriles.6d ILs are designable via
selecting cations and anions and thus can provide the ILs
specific functions as a result of the cooperative or synergistic
effects between the ions.7 As nonmetal catalysts, ILs are
In our initial experiments, the formylation of N-methylaniline
(1a) with CO2 and phenylsilane as a model reaction was
performed in the presence/absence of 1-alkyl-3-methylimida-
zolium-based ILs, and the results are listed in Table 1. The
Received: April 27, 2015
Revised: July 26, 2015
© XXXX American Chemical Society
4989
DOI: 10.1021/acscatal.5b01274
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Letter
equivalent [BMIm]Cl to substrate as the catalyst, we explored
the scope of the reactive amine substrates, and the results are
listed in Table 2. It was demonstrated that different kinds of
amines were formylated to the corresponding formamides in
moderate to excellent yields at 30 °C within 5 h. For each
secondary amine, sole formylated product was obtained without
any other byproduct (Table 2, entries 1−5 and 7−11). N-
Methylanilines with electron-donating or electron-withdrawing
groups including methyl, methoxy, 4-chloro-, and 4-fluoro-
were well tolerated (Table 2, entries 2−5).
However, when the substituent group was nitro-, a strong
electron-withdrawing group, the substrate was unreactive
(Table 2, entry 6). Dibenzylamine exhibited good reactivity,
producing dibenzylformamide in a yield of 69% (Table 2, entry
7), and cyclic secondary amine and secondary amine with ether
functional group could convert to the corresponding
formamides in excellent yields (Table 2, entries 8 and 9).
Secondary amines with alkyl chains showed lower activity
compared to N-methylanilines and aliphatic primary amines,
and longer alkyl chains seemed to be more favorable for the
formylation reaction (Table 2, entries 10 and 11, 13 and 14). It
is worthwhile to mention that the yield of N,N-dipropylforma-
mide could be increased to 99% after a longer reaction time
(Table 2, entry 10). For most of primary amines, both N−H
bonds of the amines were reactive for formylation, and mono-
and diformylated products were obtained (Table 2, entries 12−
14). Moreover, prolonging reaction time resulted in the
increase in the yield of the diformylated product (Table 2,
entry 14). Different from the formylation of the above primary
amines, the formylation of aniline and p-methylaniline only
yielded monoformylated products, and p-methylaniline showed
higher activity than aniline, affording corresponding product in
a yield of 97% (Table 2, entries 15 and 16).
Table 1. IL-Catalyzed Formylation of 1a with CO2 and
Hydrosilanes
a
b
c
entry
catalyst
hydrosilane [n]
yield [%]
1
2
--
PhSiH3[2]
PhSiH3[2]
PhSiH3[2]
PhSiH3[2]
PhSiH3[2]
PhSiH3[2]
PhSiH3[2]
PhSiH3[2]
PMHS[5]
Ph2SiH2[3]
Et2SiH2[3]
PhSiH3[2]
0
[BMIm]Cl
[BMIm]Cl
[BMIm]Br
[BMIm]NO3
[BMIm]PF6
[BMIm]BF4
[HMIm]Cl
[BMIm]Cl
[BMIm]Cl
[BMIm]Cl
[BMIm]Cl
93
d
e
3
10/25
4
5
94
83
0
6
7
0
8
92
0
9
10
11
5
0
f
12
92
a
b
Reaction conditions: IL (1 mmol), 1a (1 mmol). n refers to mmol of
c
hydrosilane. Determined by GC analysis using dodecane as the
d
e
internal standard. CO2 pressure (1 atm). Reaction time (24 h).
f
Reused for the fifth time.
reaction did not occur without ILs (Table 1, entry 1). To our
delight, 1-butyl-3-methylimidazolium chloride ([BMIm]Cl)
was very effective for this reaction, affording N-methylforma-
nilide in a yield of 93% within 5 h at room temperature (Table
1, entry 2). Moreover, this reaction could proceed even at
atmospheric pressure in spite of providing product in a lower
yield (Table 1, entry 3). The other four ILs with the same
cation (i.e., [BMIm]+) were examined for catalyzing this
reaction (Table 1, entries 4−7). Both [BMIm]Br and
[BMIm]NO3 were also active, whereas [BMIm]PF6 and
[BMIm]BF4 were not effective. These findings indicated that
anions of the ILs had a significant influence on the activity of
the ILs, which may be ascribed to the different interactions
between the ILs and the reactants. For comparison, 1-hexyl-3-
methylimidazolium chloride ([HMIm]Cl) was used, and it
showed the same catalytic activity as [BMIm]Cl (Table 1, entry
8), indicating that the alkyl substituent on the 1-alkyl-3-
methylimidazolium cation had limited effect on the catalytic
activity of the ILs. The effects of different hydrosilanes
demonstrated the priority of phenylsilane (PhSiH3) for this
reaction (Table 1, entries 9−11). For example, the reaction did
not proceed using poly(methylhydrosiloxane) (PMHS) or
diethylsilane (Et2SiH2), and the product yield was only 5% with
diphenylsilane (Ph2SiH2). In addition, the reusability of
[BMIm]Cl was studied as well, and the yield of N-
methylformanilide remained unchanged after [BMIm]Cl was
used for five times (Table 1, entry 12), indicating that the IL
was stable in this catalytic system and it can be reused without
activity loss.
To gain deep insight into the role of the ILs and the reaction
pathway, NMR analysis was performed on [BMIm]Cl and its
mixtures with CO2, phenylsilane and N-methylaniline,
respectively. No new signal or chemical shift was observed in
the 1H and 13C NMR spectra of the mixture of [BMIm]Cl and
CO2, indicating that this IL cannot activate CO2 noticeably.
1
From the H NMR spectra of [BMIm]Cl and its mixture with
1
phenylsilane (Figure 1), it was found that the H signal of a-H
in [BMIm]Cl shifted downfield from 9.31 to 9.63 ppm due to
mixing with phenylsilane; meanwhile, the signal assigning to
Si−H (b) of phenylsilane in the mixture shifted from 4.15 to
4.11 ppm. A chemical shift of Si in phenylsilane was also
observed in the 29Si NMR spectrum of the mixture (Figure S1).
These shifts demonstrated that there was interaction between
[BMIm]Cl and phenylsilane and that this IL activated the Si−H
bond of phenylsilane, which might make it more favorable for
the insertion of CO2. To confirm this, the solution of
phenylsilane, CO2, and [BMIm]Cl was prepared at 1 MPa
and 30 °C, and was examined by NMR analysis after being
stirred for 5 h and CO2 release. In the 13C NMR spectrum of
the mixture, a new signal appeared at δ = 163.0 ppm (Figure 2),
1
and a new one also appeared in the H NMR spectrum at δ =
8.27 ppm (Figure S2). These two signals were ascribed to
formoxysilane, suggesting that the reaction of phenylsilane with
CO2 occurred catalyzed by [BMIm]Cl. This result was in
agreement with those reported employing other catalysts such
Among the tested ILs, [BMIm]Cl, [BMIm]Br, and
[HMIm]Cl showed excellent activity for catalyzing the
formylation of N-methylaniline with CO2 and phenylsilane at
room temperature. Moreover, a catalytic amount of [BMIm]Cl
(10 mol %) could efficiently catalyze the formylation of N-
methylaniline, affording product in a high yield of 91%;
however, it showed low activity for catalyzing some of the other
amines (Supporting Information, Table S1). Therefore, using
as transition metals or NHCs.3a,8d From the H NMR spectra
1
of N-methylaniline and its mixture with [BMIm]Cl, the obvious
shift of the signal ascribing to N−H in amine was observed
(Figure S3), probably due to the hydrogen bond between the
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Letter
Table 2. Formylation of Various Amines Using CO2 and
a
Phenylsilane
1
Figure 1. H NMR spectra of the [BMIm]Cl, phenylsilane and their
mixture ([D6]DMSO, 298 K).
Figure 2. 13C NMR spectra of the formoxysilane intermediate and
[BMIm]Cl (CDCl3, 298 K).
anion of the IL and the amine.7a That is, the IL, [BMIm]Cl,
could activate N-methylaniline. As N-methylaniline was added
into the reaction solution of phenylsilane, CO2, and [BMIm]Cl
at room temperature, excitingly, N-methylformanilide was
obtained. This result indicated that formoxysilane was the key
intermediate in the formylation of N-methylaniline with CO2
and phenylsilane, which further reacted with the amine,
producing the final product. The above findings indicated
that the IL acted as a bifunctional catalyst, which activated
phenylsilane to react with CO2 to form formoxysilane, and
simultaneously activated the amine via hydrogen bond. These
results can also explain the influence of CO2 pressure on the
product yield. At higher pressure (e.g., 1 MPa), more CO2
could dissolve in the reaction solution and react with the IL-
activated phenylsilane to form formoxysilane, thus forming the
product in a higher yield after formoxysilane further reacted
with the amine (Table 1, entry 3).
In a control experiment using N-methylimidazole instead of
[BMIm]Cl as the catalyst, N-methylformanilide was also
obtained, but in a yield of 51% under the same other
conditions. This suggested that the [BMIm]+ cation of the ILs
played a crucial role in catalyzing the N-formylation of amines
with CO2 and phenylsilane. To further explore the reason for
the activity difference of the ILs with the same cation, ILs with
a
Reaction conditions: [BMIm]Cl (1 mmol), substrate (1 mmol),
different anions including Cl−, Br−, NO3 , BF4− and PF6− were
−
b
PhSiH3 (2 mmol). Determined by 1H NMR using an internal
standard of mesitylene. Yield was determined by GC analysis using
dodecane as the internal standard. Reaction time (10 h). PhSiH3 (4
mmol), 10 h.
examined through 1H NMR analysis. As illustrated in Figure S4,
the 1H signal of 2-H in imidazolium ring of the ILs appeared in
a range of 9.31 to 9.08 ppm, following the order: Cl− > Br− >
c
d
e
4991
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ACS Catalysis
Letter
−
NO3− > PF6− > BF4 , which indicated that the properties of ILs
methylimidazolium-based ILs was demonstrated for the first
time. Different kinds of formamides were synthesized in
moderate to excellent yields under low CO2 pressure and room
temperature. It was found that the cations and anions of the ILs
played synergistic role in catalyzing the formylation reactions.
Moreover, the ILs (e.g., [BMIm]Cl) can be reused at least five
times without loss in activity. We believe that these easily
available, commonly used imidazolium-based ILs can find more
applications in the transformation of CO2 under mild
conditions.
were influenced by the interactions of [BMIm]+ cation and
different anions. In the H NMR spectra of phenylsilane with
1
[BMIm]Br and [BMIm]NO3 (Figure S4), different degree of
chemical shifts assigning to H of the imidazolium ring and Si−
H in phenylsilane were observed, whereas in the spectra of
phenylsilane with [BMIm]BF4 and [BMIm]PF6, no chemical
shift was observed. Similar results on the chemical shift of N−H
in N-methylaniline were obtained in the spectra of N-
methylaniline and its mixture with ILs (Figure S3). These
results indicated that ILs with [BMIm]+ cation and Cl−, Br−,
−
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b01274.
NO3 anions were able to activate phenylsilane and the amine
■
substrate, while [BMIm]BF4 and [BMIm]PF6 were ineffective,
which was in accordance with the experimental results in Table
1. On the basis of these results, it can be deduced that the
cations and anions of the ILs generated synergistic effect, thus
leading to different activities on catalyzing the formylation
reaction studied in this work.
S
Experimental procedures and characterization data
(PDF)
In the NHC catalytic system, there were two viewpoints on
the formation of formoxysilane intermediate. Zhang and Ying
reported that the primary activation process involved the
reaction between NHC and CO2, resulting in the formation of
NHC−CO2 adduct, which further reacted with hydrosilane to
form formoxysilane.9 The calculated results of Wang and co-
workers indicated that NHC preferred to activate the Si−H
bond of hydrosilane and transfer a nucleophilic hydride to CO2
to form the intermediate.10 In this work, we found that
[BMIm]Cl activated Si−H bond of phenylsilane to react with
CO2, forming the formoxysilane intermediate. Our experimen-
tal results supported the calculated results by Wang and co-
workers.10
AUTHOR INFORMATION
Corresponding Author
*E-mail: liuzm@iccas.ac.cn.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
(Nos. 21125314, 21321063, 21403252).
■
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