Eco-friendly acetylcholine-carboxylate bio-ionic liquids for controllable: N-methylation and N-formylation using ambient CO2 at low temperatures

Article abstract of DOI:10.1039/c8gc03549k

Catalytic fixation of CO₂ to produce valuable fine chemicals is of great significance to develop a green and sustainable circulation of excessive carbon in the environment. Herein, a series of non-toxic, biodegradable and recyclable acetylcholine-carboxylate bio-ionic liquids with different cations and anions were simply synthesized for producing formamides and methylamines using atmospheric CO₂ as a carbon source, and phenylsilane as a hydrogen donor. The selectivity toward products was tuned by altering the reaction temperature under solvent or solvent-free conditions. N-Methylamines (ca. 96% yield) were obtained in acetonitrile at 50 °C, while N-formamides (ca. 99% yield) were attained without a solvent at 30 °C. The established bio-ionic liquid catalytic system found a wide range of applicability in substrates and possessed a high potentiality in scale-up to gram-grade production. The developed catalytic system was fairly stable, which could be easily reused without an apparent loss of reactivity, possibly due to the strong electrostatic interactions between the cation and anion. The combination of experimental and computational results explicitly elucidated the reaction mechanism: PhSiH₃ activated by a bio-IL was favorable for the formation of silyl formate from hydrosilylation of CO₂, followed by a reaction with an amine to give an N-formamide, while an N-methylamine was formed by further hydrosilylation of the N-formamide.

Full text of DOI:10.1039/c8gc03549k

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Eco-friendly acetylcholine-carboxylate bio-ionic
liquids for controllable N-methylation and
N-formylation using ambient CO2 at low
temperatures†
Cite this: DOI: 10.1039/c8gc03549k
Wenfeng Zhao,a Xiaoping Chi,b Hu Li, *a Jian He,a Jingxuan Long,a Yufei Xua and
a
Song Yang
*
Catalytic fixation of CO2 to produce valuable fine chemicals is of great significance to develop a green
and sustainable circulation of excessive carbon in the environment. Herein, a series of non-toxic, bio-
degradable and recyclable acetylcholine-carboxylate bio-ionic liquids with different cations and anions
were simply synthesized for producing formamides and methylamines using atmospheric CO2 as a
carbon source, and phenylsilane as a hydrogen donor. The selectivity toward products was tuned by alter-
ing the reaction temperature under solvent or solvent-free conditions. N-Methylamines (ca. 96% yield)
were obtained in acetonitrile at 50 °C, while N-formamides (ca. 99% yield) were attained without a
solvent at 30 °C. The established bio-ionic liquid catalytic system found a wide range of applicability in
substrates and possessed a high potentiality in scale-up to gram-grade production. The developed cata-
lytic system was fairly stable, which could be easily reused without an apparent loss of reactivity, possibly
due to the strong electrostatic interactions between the cation and anion. The combination of experi-
mental and computational results explicitly elucidated the reaction mechanism: PhSiH3 activated by a
bio-IL was favorable for the formation of silyl formate from hydrosilylation of CO2, followed by a reaction
with an amine to give an N-formamide, while an N-methylamine was formed by further hydrosilylation of
the N-formamide.
Received 13th November 2018,
Accepted 20th December 2018
DOI: 10.1039/c8gc03549k
rsc.li/greenchem
egies involving the formation of C–C, C–O and C–N bonds for
Introduction
chemical fixation of CO2 have been developed.4 Most of the
Carbon dioxide, known as the main reason for the “green- reported CO2 valorization strategies for C–C and C–O bond
house effect”, has stimulated much effort to develop techno- construction, such as the conversion of CO2 into carboxylic
logies and methods for the decrease of the CO2 level in the acids,5 cyclic carbonates6–8 and heteroarene acids,9,10 usually
atmosphere. In view of sustainable development, employing require relatively hash reaction conditions. However, C–N
CO2 as an abundant, nontoxic and cheap C1 building block bond construction, as a promising process for the utilization
resource for the production of value-added chemicals has of CO2, has attracted much attention considering that it can be
recently emerged as a meaningful way to mitigate the green- realized under relatively mild conditions.11 Recently, a series
house effect.1–3 Although the CO2 molecule is fairly stable of formylation and methylation products, which are widely
both in terms of thermodynamics and kinetics, many strat- applied as superior solvents and medicinal chemicals, were
able to be synthesized via reductive functionalization of CO2
with amines.12,13
To obtain N-substituted compounds from CO2 via N-formy-
lation and N-methylation, CO2 should be reduced first.
aState Key Laboratory Breeding Base of Green Pesticide & Agricultural
Bioengineering, Key Laboratory of Green Pesticide & Agricultural Bioengineering,
Ministry of Education, State-Local Joint Laboratory for Comprehensive Utilization of
Although various hydrogen donors such as hydrogen have
been widely used in the C–N bond construction of amines
with CO2, the hydrosilane provided more advantages including
easy handling, low energy dissipation and high selectivity,
showing better application prospects.14–18 In recent years, pre-
cious and transition metal complexes, such as Pd,19 Zn20 and
Cu21 with corresponding ligands, have been developed to acti-
Biomass, Center for Research & Development of Fine Chemicals, Guizhou University,
Guiyang, Guizhou 550025, China. E-mail: jhzx.msm@gmail.com, hli13@gzu.edu.cn;
Fax: (+86)851 88292170; Tel: (+86)851 88292171
bBeijing National Laboratory of Molecular Science, State Key Laboratory of Molecular
Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences;
University of Chinese Academy of Sciences, Beijing 100049, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc03549k
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vate this reaction in the presence of hydrosilanes. As is well able yields (up to 96%) in acetonitrile. On the other hand, the
known, metal complex systems were efficient for C–N bond corresponding formamides were smoothly achieved (up to
construction, but they were toxic for the environment and 99%) under solvent-free conditions. Remarkable electrostatic
needed complicated preparation processes. Many basic metal interactions between the cation and anion in an acetylcholine-
salts such as Cs2CO3 and K2WO4 have also been utilized based IL have been verified by a computational study, illustrat-
in the transformation of CO2 to N-formamide and ing that the IL has good stability. Notably, this readily available
N-methylamine.22,23 However, these catalytic systems suffered acetylcholine-based IL can be reused at least five times with
from relatively rigorous reaction conditions (long time ≥12 h, little loss of activity. A relatively reasonable mechanism was
2 MPa CO2) and the used catalysts could not be reused. On the affirmed and elucidated by using DFT calculations and NMR
other hand, numerous metal-free catalysts such as NHCS,24 study.
TBAF25 and thiazolium carbine26 have been testified to be only
capable of promoting N-formylation, and these catalysts were
unstable which resulted in the difficulty of catalyst recycling as
Experimental
well. In order to enable the recyclability of the catalysts, hetero-
Materials
geneous polymeric materials such as functional ionic poly-
Acetylcholine chloride (98%), chlormequat (98%), ethyltri-
methylammonium iodide (98%), choline hydroxide, formic
acid (98%), acetic acid (99%), propionic acid (99%), valeric
acid (98%), and Amberlyst A26 (OH-form) were purchased
from TCI Inc. (Shanghai). N-Methylaniline (98%),
N-methylformanilide (99%), N,N-dimethylaniline (99%),
mers, zwitterionic covalent organic frameworks, and poly-N-
heterocyclic carbines have been developed for catalysing C–N
construction,27–29 while tedious catalyst preparation processes
with rigorous reaction conditions were often required. In this
regard, ionic liquids (ILs) exhibited unique properties such as
good chemical stability, tuneable active sites and low
vapour pressure, which have been applied in many catalytic
systems.30,31 Although a number of ionic liquids have been
developed for CO2 conversion into valuable chemicals,32–36
only a few studies focused on C–N construction.37,38 Moreover,
the extensive application of those imidazolium-based or
halide-containing ionic liquids was limited by issues of tox-
icity, ecology, and economy.
Choline-based ILs, known as “Bio-ILs” composed wholly
of biomaterials, have superior features of low toxicity,
relatively high stability and fewer negative effects on the
environment.39–42 Not surprisingly, choline-based ILs have
attracted substantial attention in organic synthesis because
they are biodegradable and cheap. These ILs are conjugate
bases of weak acids; therefore these “Bio-ILs” might possess
strong basicity, which could enable efficient catalysis of the
production of formamides and methylamines from CO2 and
amines in the presence of organosilanes. To the best of our
knowledge, the choline-based ILs as catalysts for reductive fix-
ation of CO2 have rarely been reported.
N-methylcyclohexylamine
N-ethylpropylamine (98%), 2-methoxy-N-methylaniline (99%),
N-methylbenzylamine (97%), N-methyl-1-(thiophen-2-yl)
(99%),
N-butylaniline
(99%),
methanamine (95%), N-methyl-4-ethylbenzylamine (97%),
N-benzylaniline (99%), N-methyl-o-toluidine (95%), N-methyl-
p-toluidine (97%), N-ethylaniline (98%), N-benzylmethylamine
(97%), N-cyclohexylaniline (98%), N-allylaniline (97%),
4-chloro-N-methylaniline (97%), 4-methoxy-N-methylaniline
(97%), 4-bromo-N-methylaniline (97%), N-methyl-4-nitroani-
line (97%), diethylamine (99%), N-benzyl-p-anisidine (99%),
N-methyl-4-(4-morpholinyl) benzylamine (97%), naphthalene
(99%), and pyrrolidine (99%) were bought from Aladdin
Industrial Inc. (Shanghai). Phenylsilane (98%), morpholine
(99%), acetonitrile-d3 (99 atom% D), poly(methyl-
hydrosiloxane) (PMHS), n-hexane (99%), ethanol (EtOH, 99%),
acetonitrile (CH3CN, 99%), tetrahydrofuran (THF, 99%), ethyl
acetate (EA, AR), triethoxysilane (98%), diphenylsilane (>98%),
triethylsilane (99%), sodium acetate (99%), and potassium
acetate (99%) were bought from Beijing Innochem Sci. & Tech.
Co. Ltd. Diphenyl (silane-d2, 97 atom% D) was purchased from
Sigma-Aldrich Co. LLC.
Herein, we prepared a series of new acetylcholine-based ILs,
as shown in Scheme 1, which could be used as non-toxic,
cheap and efficient bio-catalysts for producing N-substituted
compounds from CO2 employing a hydrosilane as the reduc-
Catalyst preparation
tant under benign conditions. A variety of amines were able to Acetylcholine-based ionic liquids (ACH-ILs) with carboxylate
be converted into corresponding methylamines in consider- anions with variable carbon-chain lengths, including acetyl-
choline formate (ACH-FA), acetylcholine acetate (ACH-AA),
acetylcholine propionate (ACH-PA), and acetylcholine valerate
(ACH-VA), were synthesized by a two-step ion exchange method
at room temperature (Scheme 1). In a typical procedure, to a
solution of 4 mmol acetylcholine chloride dissolved in 5 mL
methanol in a 25 mL flask, solid Amberlyst A26 (2.5 g) was
added to exchange a chloridion with an hydroxide. After stir-
ring at room temperature for 12 h, the used Amberlyst A26 was
removed by filtration. The ion-exchange process was repeated
by adding fresh Amberlyst A26 (2.5 g) into the filtrate under
Scheme 1 Schematic illustration of the preparation of ionic liquid
catalysts.
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identical reaction conditions to give acetylcholine hydroxide. ACH-AA (6% mmol), 1 bar CO2, 2.0 mL CD3CN-d3 and diphe-
After the removal of Amberlyst A26, acetic acid (4 mmol) was nyl (silane-d2) (0.75 mmol) were successively added, and kept
added into the resulting solution to enable neutralization with stirring at 50 °C for 6 h.
acetylcholine hydroxide. After stirring vigorously for 12 h, the
Scale-up reaction study. 12.5 mmol N-methylaniline (1.35 g),
target IL acetylcholine acetate was obtained by removing 6% mmol ACH-AA (0.3 g), 10 mL CH3CN and 37.5 mmol
methanol and water under reduced pressure. To evaluate the PhSiH3 were added into a 25 mL magnetic Harrington reactor.
effect of cations, other acetate ILs containing different cations, The reaction mixture was stirred at 500 rpm using 1 bar CO2 at
including choline acetate (CH-AA), ethyltrimethylammonium 50 °C for 48 h. This reaction was also conducted with identical
acetate (ETA-AA), and chlormequat acetate (CC-AA), were also feeding using 2.0 MPa CO2 at 50 °C for 20 h. The conversion
synthesized using the same method.
and yield were determined by GC using naphthalene as an
internal standard.
Catalyst characterization
Catalyst recycling study
FT-IR spectra were recorded on a Thermo Fisher Scientific
Nicolet iS50 equipped with a Smart Diamond ATR Accessory
under ambient conditions in KBr disks. Structures of ILs were
The catalyst recyclability of ACH-AA was examined under the
optimal reaction conditions for the synthesis of N,N-dimethyl-
aniline: 0.25 mmol N-methylaniline, 6% mol ACH-AA,
0.75 mmol PhSiH3, 2 mL CH3CN, 1.0 bar CO2, 50 °C and 6 h.
After each cycle of the reaction, 2 mL water was added into the
reaction mixture, and then the aqueous phase was disengaged
by the addition of 2 mL diethyl ether, and repeatedly extracted
for three times. Then, 10 mg activated carbon was added to
the resulting water phase and stirred (200 rpm) at room temp-
erature for 24 h to thoroughly remove siloxane products and
impurities. After the removal of the solid residue by filtration,
the recovered ACH-AA was obtained by removing water under
reduced pressure at 80 °C, which was then directly employed
for the next run.
1
also identified by H NMR and 13C NMR using an Avance III
HD-400 spectrometer at room temperature operating at
400 MHz.
Computational method
All structure optimization and energy calculations were per-
formed using density functional theory (DFT) employing the
B3LYP functional implemented in the Gaussian 09 program
package. Pople’s all-electron basis set 6-311G (d,p) was used
for all atoms.43–45 CC-AA was chosen as a model catalyst for
transient state calculation.
Reaction procedures
Sample analysis
N-Methylation of amines with ambient CO2. The
N-methylation reaction of secondary and primary amines with
CO2 in the presence of a hydrosilane was conducted in a
15 mL reaction tube with an inner joint and a 2 mm glass stop-
cock equipped with a magnetic stirrer. In a typical experiment,
the reaction tube was vacuumized by using a mechanical
pump at first. Then, the amine (0.25 mmol), ACH-AA
(6% mmol), 1 bar CO2, 2.0 mL CH3CN and phenylsilane
(0.75 mmol) were successively added into the tube. This reac-
tion mixture was stirred at 50 °C for 6 h. Upon completion of
the reaction, the conversion and yield were determined by GC
(Agilent 7890B) using naphthalene as an internal standard.
N-Formylation of amines with ambient CO2. In a typical
process to synthesize a formamide from an amine and CO2,
the reaction tube was initially vacuumized by using a mechani-
The concentrations of N-methylaniline, N,N-dimethylaniline
and N-methylformanilide were determined using a GC instru-
ment equipped with an HP-5 chromatographic column
(30 m × 0.320 mm × 0.25 μm) and an FID, using naphthalene
as an internal standard referring to the standard curves (R2 >
0.999) obtained with commercial specimens. Other substrates
and corresponding products were quantified by H NMR spec-
troscopy of the crude reaction mixture using 1,3,5-trimethoxy-
benzene as the internal standard.
1
Results and discussion
Catalyst synthesis and characterization
cal pump. Then, the amine (0.25 mmol), ACH-AA (6% mmol), All acetylcholine-ILs were synthesized mainly from non-toxic
1 bar CO2 and phenylsilane (0.75 mmol) were successively biomass derivatives through a two-step ion-exchange method
added into the reaction tube. The solvent-free reaction mixture under ambient conditions. This newly developed preparation
was stirred at 30 °C for 6 h. After the reaction completed, the approach was benign and much easier to operate, as compared
resulting mixture was diluted with CH2Cl2, and the liquid pro- with previous ones used for the preparation of (DBU)-based
ducts were quantitatively analyzed by GC using naphthalene as ionic liquids (N2 atmosphere and 50 °C),46 phosphonium-
an internal standard.
based ionic liquids (100 °C, 24 h),47 N-methyl-N-propylpyrroli-
Deuterium-labeling study. 1H NMR, 13C NMR, and GC-MS dinium acetate (multi-steps),48 and so on. Importantly, the
spectra were recorded for the reaction mixtures of fairly strong hydrogen-bond interaction between the anion and
N-methylaniline and CO2 after being reacted in CD3CN-d3 with cation of the bio-ILs was observed by computational calcu-
D-labeled silicon: diphenyl (silane-d2). In a typical reaction lations (Fig. S1†), which possibly resulted in better catalyst
process, the reaction tube (15 mL) was vacuumized by using a solubility. It was reported that the catalyst basicity of the
mechanical pump at first, into which the amine (0.25 mmol), hydrogen-bond estimated from the Kamlet–Taft parameters49
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was dominated by the properties of the anion.50 Here, the basi- frequency along with the increased carbon-chain length except
1
city of the ILs mapped with hydrogen-bond basicity was gener- ACH-FA. For example, the NMR H signal of –CH2 of the car-
ally inversely proportional to the acidities of the conjugate acid boxylate anion in ACH-AA gradually moved from 1.7 ppm to
radicals (i.e., the anions of the ILs), typically in the order of 2.01 ppm as compared with that in ACH-VA. Likewise, the 13C
formate (FA) > acetate (AA) > propionate (PA) > valerate (VA).
NMR spectra displayed similar chemical shift rhythmicity for
The FT-IR spectra of various IL catalysts prepared from the carboxylate carbon atoms. In addition, the structures of
different precursors are displayed in Fig. 1. The band at other three acetate ILs containing different cations were also
1760 cm−1 was observed for all ACH-ILs, contained the anhy- identified by 1H and 13C NMR (Fig. S4 and S5†).
dride group, illustrating the existence of acetylcholine. It was
evident that weak absorption was detected for C–O vibrations
of the anhydride in all ACH-ILs, which is consistent with the
effect of strong electrostatic interactions. In addition, the band
at 1470 cm−1 corresponding to the C–N vibrations of ACH in
all prepared ACH-ILs was strong and broad (Fig. 1A). The
bands at 1560 and 1360 cm−1 were the characteristic peaks of
the carboxylate that was derived from the neutralization of an
organic acid with acetylcholine hydroxide. The structures of
other acetate ILs (ETA-AA, CH-AA, and CC-AA) containing
different cations of ethyltrimethylammonium, choline and
chlormequat were also confirmed, respectively (Fig. 1B). The
characteristic peaks of C–Cl (CC-AA) at 711 cm−1 and –OH
(CH-AA) at 3360 cm−1, along with the characteristic peaks of
the carboxylate similar to ACH-ILs, could be observed (Fig. 1B).
To further identify the structures of all as-prepared bio-ILs,
1H and 13C NMR analyses were also conducted. All of the
ACH-IL catalysts with the carboxylate as the anion with vari-
able carbon-chain lengths (Fig. S2 & S3†) were confirmed to be
successfully prepared. The carboxylate hydrogen atom was
found to be sensitive to the concatenation of the positive
charge of the trimethylethanium salt which moved to the high
Catalyst screening for methylation of N-methylaniline with
CO2
To start the catalytic activity investigation, the effect of
different catalysts on the methylation of N-methylaniline with
CO2 was initially investigated. The reactions were accom-
plished at 50 °C within 6 h under ambient CO2 pressure in
CH3CN using phenylsilane (PhSiH3) as the hydrogen-donor in
the presence or absence of a catalyst, and the obtained results
are shown in Table 1. The reaction could not occur without
adding any catalyst or with ACH (Table 1, entries 1 and 2). In
contrast, ACH-AA displayed highly efficient catalytic perform-
ance in the N-methylation of CO2 to N-substituted compound
1b with a yield of up to 96% (entry 3). However, the activity of
ACH ILs bearing formate (FA), propionate (PA), and valerate
(VA) groups were inferior to that with the acetate group (entries
4–6), possibly due to their relatively low permeability and diffu-
sivity of CO2. It was reported that the viscosity of ILs increased
with the increase of the carbon-chain length of carboxylic
acids,51 accordingly increasing the CO2 permeability and diffu-
sivity.52 On the other hand, carboxylic anions can serve as a
strong base because carboxylic acids are weak acids, with basi-
city in the order of HCOO− (FA)
< <
CH3COO− (AA)
CH3CH2COO− (PA) < CH3(CH2)3COO− (VA). However, as the
Table 1 Catalyst screening for selective methylation of N-methylaniline
a
with CO2 in the presence of PhSiH3
Entry
Cat.
Conv. (1a, %)
Yield (1b, %)
Yield (1c, %)
1
2
No
ACH
0
0
0
0
0
0
3
4
5
ACH-AA
ACH-FA
ACH-PA
ACH-VA
ACH-AA
CC-AA
CC-AA
ETA-AA
CH-AA
100
68
99
69
64
100
47
99
94
98
1
96
12
77
42
54
92
34
88
61
57
0
4
28
22
15
6
6
7b
8
0
9b
10
11
12
13
12
11
21
39
1
CH3COOK
CH3COONa
a Reaction conditions: 0.25 mmol 1a, 0.75 mmol PhSiH3, 6 mol% cata-
lyst, 2.0 mL CH3CN, 1 bar CO2, 50 °C and 6 h. Conversion of 1a, and
the yield of 1b and 1c were determined by GC using naphthalene as an
internal standard. b Reaction time is 2 h.
Fig. 1 FT-IR spectra of ACH-based ILs with a carboxylate with variable
alkyl chain lengths (A); FT-IR spectra of CH-AA, CC-AA and ETA-AA (B).
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Table 2 Parameter optimization for the reaction of CO2 with
N-methylaniline using ACH-AA as a catalysta
optimal catalyst used for this system, ACH-AA possessed appro-
priate viscosity and higher nucleophilicity than ACH-PA and
ACH-VA, as well as higher basicity than ACH-FA. For ILs with
different cations, the electron withdrawing order of the substi-
tuent groups in cations is ester (ACH) > chlorine (CC) > ethyl
(ETA) group, which was proportional to the nucleophilicity of
the acetate anion in ILs. Those properties were consistent with
the activity order of the catalysts (entries 3 and 7–10).
Nevertheless, the electron-withdrawing ability of the hydroxyl
group (CH) is higher than that of the ethyl (ETA) group but it
would be able to consume the silane during the reaction
process, resulting in lower efficiency of CH-AA (entry 11) than
that of ETA-AA (entry 10).53 The possible synergistic role of
both anion and cation of ACH-AA could be supported by the
relatively lower activity of two inorganic acetates (entries 12
and 13) employed for the methylation of 1a. It is worth noting
that a much lower conversion of 1a was achieved over
CH3COONa, which could be ascribed to its relatively lower
solubility (46 g/100 g, 20 °C) than CH3COOK (256 g/100 g,
20 °C).54 Based on the above results and discussions, it could
be inferred that both carboxylate anion and the type of qua-
ternary ammonium cation influenced the catalytic activity of
ILs, where the anion as a nucleophilic reagent interacted with
the Si atom on PhSiH3, while the cation collaboratively
adjusted the nucleophilicity of the anion to cooperatively
promote the reaction. In addition, the appropriate viscosity
and superior solubility of ACH-AA also contribute to its pro-
nounced reactivity.
Silane
dosage/ Conv.
Reaction
Yield
Yield
Entry conditions Silane
mmol
(1a, %) (1b, %) (1c, %)
1
2
3
4
5
6
7
8
50 °C, 6 h
RT, 6 h
70 °C, 6 h
50 °C, 2 h
50 °C, 4 h
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
0.75
0.75
0.75
0.75
0.75
0.75
100
94
74
64
89
99
63
58
23
0
96
23
51
54
78
98
54
0
4
0
8
18
98
3
66
10
6
7
1
50 °C, 12 h PhSiH3
50 °C, 6 h
50 °C, 6 h
50 °C, 6 h
50 °C, 6 h
50 °C, 6 h
50 °C, 6 h
50 °C, 6 h
Ph2SiH2 0.75
EtO3SiH 0.75
PMHS
Et3SiH
PhSiH3
PhSiH3
PhSiH3
8
56
16
0
7
10
2
9
0.75
0.75
0.25
0.5
10
11
12
13
26
48
100
1.25
a Reaction conditions: 0.25 mmol 1a, 1 bar CO2, 6 mol% ACH-AA and
2.0 mL CH3CN. Conversion of 1a and the yield of 1b and 1c were deter-
mined by GC using naphthalene as an internal standard.
hydride relative to the substrate were found to be optimum for
N-methylation of 1a (entries 4, 10–13).
Effect of the solvent type on the N-methylation of
N-methylaniline with CO2
Optimizing reaction conditions for methylation of
The reaction activity and selectivity were examined to be also
affected by the type of solvent used (Fig. 2). The reactivity in
different solvents obeyed the following order of DMF ≈
CH3CN > THF > EA > EtOH > n-hexane,57 which was roughly in
N-methylaniline with CO2
The influence of reaction parameters, such as the reaction
temperature, reaction time, hydrosilane type, and catalyst
dosage, on the fixation of CO2 with 1a was further investigated
(Table 2). A considerable yield of 1b with a small quantity of 1c
was obtained using PhSiH3 as the H-donor under relatively
mild reaction conditions of 50 °C and 6 h (Table 2, entry 1). At
room temperature, the dominant product was found to be 1c
(entry 2), while a higher temperature of 70 °C resulted in
unpleasant reaction activity (entry 3), mainly due to the lower
solubility of CO2 at a relatively high reaction temperature.55
The yield of 1b gradually increased on prolonging the reaction
time from 2 to 12 h at 50 °C (entries 1, 4–6). However, only
63% conversion of 1a was attained using Ph2SiH2 (entry 7)
instead of PhSiH3, possibly due to its high steric hindrance.
Moreover, PhSiH3 was testified as the priority of the hydrosi-
lane for this reaction. In contrast, the main product was 1c by
using PMHS and EtO3SiH as hydrogen donors, respectively
(entries 8 and 9). It seemed that these hydrosilanes were inac-
tive and unable to accomplish the reduction process. Even
worse, the reaction did not take place using Et3SiH due to its
poor hydride-donating ability, which was typically activated by
a noble metal such as Pt nanoparticles.56 The results further
indicated the significant effect of the type of hydrosilane. On
the other hand, the reaction activity was unsatisfactory when
Fig. 2 Effect of solvent polarity on catalytic methylation of
N-methylaniline with CO2. Reaction conditions: 0.25 mmol 1a,
0.75 mmol PhSiH3, 6 mol% ACH-AA, 2.0 mL solvent, 1 bar CO2, 50 °C
the dosage of PhSiH3 was insufficient, and 3 equiv. of the and 6 h.
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Table 3 Examples of ACH-AA catalyzing the reaction of CO2 with
amines to yield N-methylaminesa
line with their polarity. The solvent with high polarity was ben-
eficial for N-methylation of an amine with CO2, which could
be attributed to the increased homogenization and nucleo-
philicity of the catalytic system, thus the bio-IL being dissolvable
into the above solvent.58 This result is in agreement with pre-
vious reports, where the amine could be activated by a strong
polar solvent like DMF or DMSO.59 Although EtOH had
polarity comparable to EA, the reaction did not proceed at all,
since PhSiH3 was consumed rapidly by its reaction with an
alcoholic solvent to form siloxane and hydrogen was released
out quickly.60 Under solvent-free conditions, the dominant
product was observed to be 1c at 50 °C (Table S1,† entry 7).
Typically, the solvent plays a promotional role in facilitating
the mass transfer and enhancing the nucleophilicity of the
catalyst. In the absence of a solvent, the use of a viscous IL
would reduce the reaction rate accordingly. This speculation
can be rationalized by tuning the rate of silyl formate for-
mation in solvents with variable polarity, thereby promoting
the generation of 1c other than 1b.38,61,62 Inspired by these
data, we tried to perform reductive functionalization of CO2 at
a lower reaction temperature of 30 °C under solvent-free con-
ditions. Surprisingly, 1c was smoothly formed in 90% yield
after the reaction for 6 h, which supports the speculation that
the reaction rate was reduced in the absence of a solvent at
relatively lower temperatures.
Conversion
(1, %)
Yield
(2, %)
Selectivity
(2, %)
Entry
1
Substrate
99
100
93
85
93
81
87
93
87
2
3
4
5
6
100
99
99
91
8
99
92
53
15
7
100
96
96
8
9
99
98
92
96
93
97
Selective N-methylation of CO2 with various amines
To examine the generality of the developed catalytic system,
selective N-methylation of various aromatic amines was con-
ducted under the optimized reaction conditions discussed
above using 6 mol% ACH-AA, 0.25 mmol substrate and
0.75 mmol PhSiH3 in acetonitrile under 1 bar of CO2 at 50 °C
(Table 3). It was found that most amines could be successfully
transformed into corresponding N-methylated products with
excellent yields after reacting for 6 or 8 h. Among the tested
substrates, the compounds with electron-donating ortho-sub-
stituents (Table 3, entries 1 & 3) showed lower activity than
those with para-substituents (Table 3, entries 2 & 4), which
could be ascribed to the relatively higher steric hindrance
effect of the substituent groups. On the other hand, substrates
bearing electron-donating groups at the para-position (Table 3,
entries 2 & 4) displayed activity superior to those containing
electron-withdrawing groups (Table 3, entries 5 & 6). In
addition, chlorine, nitro, and olefinic species were well toler-
10
11
99
90
80
89
81
99
12b
13b
98
96
87
80
89
83
14b
99
88
89
a Reaction conditions: 0.25 mmol, 1 bar CO2, 6 mol% ACH-AA,
0.75 mmol PhSiH3, 2.0 mL CH3CN, 50 °C and 6 h. b Reaction time is
ated during the reaction process (Table 3, entries 5, 6 & 10). 8 h.
Notably, the carbon-chain length of the substituent groups
attached to the nitrogen of the substrates showed no obvious
effect on activity (Table 3, entries 7–9 & 11). Moderate yields of
N-methylated products were obtained when benzyl aniline under solvent-free conditions in the present study. As shown
with larger steric hindrance (Table 3, entries 12 & 13) and a in Table 4, a variety of secondary aromatic compounds were
heterocyclic amine that is typically inert to react (Table 3, entry examined in this established reaction system, and corres-
14) were used as starting materials.
ponding formylated products were observed with excellent
yields (Table 4, entries 1–9). Due to steric hindrance, bulk sub-
strates afforded relatively low product yields (Table 4, entries 8
Selective N-formylation of various amines
Besides N-methylation discussed above, N-formylation of and 9). Notably, a series of secondary aliphatic amines could
amines with CO2 could also exclusively take place at 30 °C be successfully formylated as well, giving products with
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Table 4 Catalytic results for the reaction of various amines with CO2 to
However, due to the substrate being unable to make sufficient
contact with CO2 under ambient atmosphere (1 atm), the
scale-up reaction system could not afford comparable
efficiency (83% 1a conversion) at 50 °C even after 48 h, and 1c
(75% yield) was found to be the dominant product instead of
1b (7% yield). In this regard, a relatively higher CO2 pressure
would be helpful to increase the selectivity toward 1b.
Gratifyingly, 94% conversion of 1a with 87% yield of 1b was
obtained at 2.0 MPa CO2 pressure at 50 °C in 20 h. These
results show the great potential of the developed catalytic
systems for scale-up upgrading of CO2.
In addition, the IL ACH-AA could be readily separated from
the reaction mixture by extraction with diethyl ether after
adding water (Fig. S6†). The recovered bio-IL (ACH-AA) was
able to be reused at least five times with a slight loss of activity
in the synthesis of 1b under optimal reaction conditions
(Fig. S7†). The minor decline in the catalytic performance of
the IL during consecutive cycles was mostly due to a part of
the IL catalyst being adsorbed by the precipitated silane after
adding water, as demonstrated by FT-IR (Fig. S8†). In addition,
the incorporation of the partial IL into degradation com-
pounds might result in the sealing of active sites to somehow
deactivate the catalyst as well.63 The sealed and adsorbed IL
could be released after adding water and stirring the obtained
reaction mixture for 24 h under ambient conditions, while the
impurities with the catalyst were able to be eliminated with
activated carbon.64 This post-treatment process can signifi-
cantly maintain the IL activity in the consecutive reaction
cycles.
give formamides over ACH-AAa
Conversion
(3, %)
Yield
(4, %)
Selectivity
(4, %)
Entry
1
Substrate
100
93
90
88
91
90
95
92
2
3
4
99
100
90
90
5
6
100
98
90
90
90
92
7
8
90
89
86
81
96
91
9
63
60
95
Mechanism study
To explicitly disclose the reaction mechanism of the bio-IL-cat-
alysed reaction of CO2 with an amine to yield a methylated or
formylated product, a series of control experiments were
conducted. The role of the IL in the reaction was confirmed by
1H NMR analysis, through testing the interaction of the IL
(ACH-AA) with PhSiH3, 1a, and CO2 under the designated reac-
tion conditions. As shown in Fig. S9,† the proton signal inten-
sity of Si–H (4.20 ppm) in PhSiH3 disappeared after mixing
with ACH-AA, implying that this catalyst was capable of activat-
ing the hydrosilane to release the hydride, which could further
promote the production of the key intermediate silyl formate
(Fig. S10†). Subsequently, it was noticed that there was no vari-
ation in the proton signal of the N-CH3 proton (2.72 ppm)
between 1a and its mixture with ACH-AA, indicating that there
was no distinct interaction between them (Fig. S11†).
Moreover, no new proton signal was observed in the mixture of
1a and PhSiH3, demonstrating that 1a cannot be activated by
the hydrosilane (Fig. S12†). In order to affirm the carbon
sources of N-CH3 on 1b originating from CO2 rather than
ACH-AA, a control experiment was performed for methylation
in the absence of CO2 under the optimized reaction con-
10
100
90
90
11
12
98
99
96
90
98
91
13
100
100
100
14
15
100
96
96
93
96
97
a Reaction conditions: 0.25 mmol 3, 1 bar CO2, 6 mol% ACH-AA,
0.75 mmol PhSiH3, solvent-free, 30 °C and 6 h.
superior yields under mild reaction conditions (Table 4,
entries 10–15).
Scaled-up production and catalyst recyclability
From a viewpoint of practical applications, the scaled-up pro- ditions, while no 1a conversion was found (Fig. S13†). The iso-
duction of 1b was fairly necessary. This gram-grade production topic study using diphenylsilane-d2 instead of PhSiH3 as the
experiment was conducted by amplifying 50 times based on H-donor was carried out in deuterated acetonitrile; 1b with
the substrate feed (see the Experimental section for details). 122–125 amu m/z and 1c with 136–137 amu m/z were formed
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in a much higher proportion to the corresponding normal pro- belonging to N-CH3 on 1b, gradually increased with the exten-
ducts with 121 and 135 amu m/z, respectively (Fig. 3 & S14†), sion of the reaction time from 0 to 6 h, and the proton signal
further elucidating that the proton of N-CH3 and N-CHO origi- intensity at 2.77 ppm assigned to N-CH3 of 1a completely dis-
nated from the hydrosilane.
appeared after 4 h. The proton signal intensity at 3.32 ppm
In addition, the reaction of 1a with CO2 and PhSiH3 was and 8.42 ppm, attributed to the proton of the formyl and
conducted in deuterated acetonitrile at 50 °C, and analyzed by methyl groups in 1c, respectively, presented in 2–6 h but it
1H NMR (Fig. 4). A new proton signal intensity at 2.93 ppm, decreased on prolonging the reaction time. It was interesting
that a new signal intensity of 8.18 ppm appeared after 6 h,
which was assigned to the proton of silyl formate formed from
the interaction of residual phenylsilane and CO2 when the sub-
strate 1a was completely consumed. A possible reaction
mechanism for generation of the formylated product from the
amine with CO2 as the carbon source was proposed on the
basis of the above discussions and previous reports
(Scheme 2). The IL catalyst (CC-AA) with a succinct structure
and comparable activity was chosen to further study the reac-
tion mechanism. First, the bio-IL (CC-AA) interacts with
PhSiH3 to form an active intermediate CC-HS, which promotes
the hydrosilylation of CO2 resulting in silyl formate. Second,
the formamide was formed by the formylation of amines with
the formate.
However, there is ambiguity about the mechanism of for-
mation of the methylated product in previous reports.22,39,50,51
The focus was thus concentrated on determining whether the
generation of N-CH3 occurred through direct hydrogenation of
formamide (Path B) or methylation with bis(silyl)acetal(III)
(Path A). From the above NMR experiments (Fig. 3), it was
found that the intensity of 1b gradually increased within
2–6 h, but the intensity of 1c weakened accordingly. Moreover,
no bis(silyl)acetal(III) intensity signal was observed in the reac-
tion. It could be inferred that the reaction pathway for the for-
mation of N-CH3 is Path B. In order to further prove this infer-
ence, DFT calculations were conducted to demonstrate that
Fig. 3 Mass fragmentation analyses (all intensities scaled to 100%) of 1a
(A) and 1c (B) from 1a by using PhSiH3 as the H-donor and diphenyl
(silane-d2) as the D-donor.
Fig. 4 1H NMR spectra for the N-methylation of 1a (0.026 g,
0.25 mmol) with CO2 (1 atm) in CD3CN using PhSiH3 (0.75 mmol) as the
H-donor.
Scheme 2 The possible reaction pathways for producing methylated
and formylated amines using CO2 as the carbon source.
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strated to be efficient for the catalytic transformation of CO2
with amines into high-value N-substituted compounds in the
presence of PhSiH3. The product selectivity toward a forma-
mide or a methylamine could be readily tuned by altering the
reaction temperature together with or without using a solvent.
This bio-IL catalyst exhibited good universality in substrate
scope, and could be recycled without any evident loss of
activity. Furthermore, NMR study and DFT calculations
demonstrated that the bio-IL played a crucial role in both acti-
vating the hydrosilane and reducing the reaction activation
energy in both N-methylation and N-formylation processes of
CO2 upgrading. By comparing the energy profiles of two con-
troversial reaction pathways, a more reasonable catalytic
mechanism was illustrated, where N-formamide was formed
through the reaction of a silyl formate with an amine, followed
by hydrosilylation to give N-methylamine. In addition, a satis-
factory result could be obtained by amplifying the reaction to
gram grade, which provides a promising strategy for large-
scale production of N-substituted compounds from amines
and CO2.
Fig. 5 The reaction energy profiles (in kcal mol−1) for the computation-
ally favorable pathway to synthesize methylation product N,N-dimethyl-
aniline in the absence of a catalyst.
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
Fig. 6 The reaction energy profiles (in kcal mol−1) for the synthesis of a
silyl formate in the presence of a catalyst.
The work is financially supported by the National Natural
Science Foundation of China (21576059 and 21666008), Key
Technologies R&D Program of China (2014BAD23B01), Fok
Ying-Tong Education Foundation (161030), and Guizhou
Science & Technology Foundation ([2018]1037 and [2017]5788).
We also acknowledge the Computer Network Information
Center of the Chinese Academy of Sciences for performing
theoretical calculations.
the formation of the formamide with silyl formate via transient
state TS2 (49.03 kcal mol−1, Fig. 5) is easier than the formation
of bis(silyl)acetal through transient state TS2a (56.18
kcal mol−1, Fig. 5), which further illustrates that the reaction
pathway for production of 1b is Path B in Scheme 2. It is
clearly seen that the activation energy barrier using CC-AA as a
catalyst was reduced by as much as 29.86 kcal mol−1 through
transition state TS3 with a lower energy barrier (ΔG =
22.29 kcal mol−1, Fig. 6), as compared with the catalytic system
without a catalyst (ΔG = 52.15 kcal mol−1, TS1 in Fig. 5) in the
formation of silyl formate, demonstrating the indispensable
role of the bio-IL catalyst in the promotion of the reaction
efficiency. Similarly, the generation of 1b from hydrosilylation
of 1c with the IL catalyst also needed a lower energy barrier
than that without a catalyst, further illustrating that the use of
a catalyst is highly necessary for all these reaction processes
(Fig. S15†).
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