Direct synthesis of carbamate from CO2 using a task-specific ionic liquid catalyst

Article abstract of DOI:10.1039/c7gc02666h

A superbase-derived protic ionic liquid (IL, [DBUH][OAc]) catalyst was used to directly synthesize carbamate from an amine, CO₂, and a silicate ester. This IL catalyst was easily prepared using its precursors, DBU, and acetic acid. Using 10 mol% of the catalyst under a CO₂ pressure of 5 MPa in acetonitrile at 150 °C, carbamate was isolated in up to 96% yield. Specifically, aliphatic and aromatic amines were activated even though aromatic amines exhibited low activities because of their low pK_a values. Other functional groups in amines were barely activated, affording exclusive chemoselectivity for amine activation. Isotope labeling experiments indicated that the proton in the counter cation is crucial in the catalytic cycle to produce water. In addition, a chemical shift corresponding to a mixture of aniline and [DBUH][OAc] was observed in the ¹H NMR spectrum, related to the formation of hydrogen bonds between aniline and basic acetate anions. The experimental results indicated that the designed IL catalysts require a protonated cation and a basic anion.

Full text of DOI:10.1039/c7gc02666h

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Direct Synthesis of Carbamate from CO2 Using a Task-Specific
Ionic Liquid Catalyst
Qiao Zhang,a Hao-Yu Yuan,b Norihisa Fukaya,a Hiroyuki Yasuda,a and Jun-Chul Choi*ab
Received 00th January 20xx,
Accepted 00th January 20xx
A superbase-derived protic ionic liquid (IL, [DBUH][OAc]) catalyst was used to directly synthesize carbamate from an amine,
CO2, and a silicate ester. This IL catalyst was easily prepared using its precursors, DBU, and acetic acid. Using 10 mol% of the
catalyst under a CO2 pressure of 5 MPa in acetonitrile at 150°C, carbamate was isolated in up to 96% yield. Specifically,
aliphatic and aromatic amines were activated even though aromatic amines exhibited low activities because of their low pKa
values. Other functional groups in amines were barely activated, affording exclusive chemoselectivity for amine activation.
Isotope labeling experiments indicated that the proton in the counter cation is crucial in the catalytic cycle to produce water.
In addition, a chemical shift corresponding to a mixture of aniline and [DBUH][OAc] was observed in the 1H NMR spectrum,
related to the formation of hydrogen bonds between aniline and basic acetate anion. The experimental results indicated
that the designed IL catalysts require a protonated cation and a basic anion.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Alzheimer’s disease,8 as well as in polyurethane (PU)
industries.9 Conventionally, PUs are synthesized in two steps:
first, phosgene (COCl2) and amine react to form isocyanate;
Introduction
All over the world, carbon is not only an essential element
in everyday life, but also plays an important role in energy
industries, because common energy resources, for instance,
petroleum, coal, and natural gas, are derived from carbon. The
developmentof modern industries leads to increased emissions
of carbon dioxide (CO2), which is a crucial component in the
global carbon cycle.1 CO2 is the final product obtained from the
oxidation of elemental carbon, hydrocarbons, and
carbohydrates, as well as primarily increasing the earth’s
temperature. Global CO2 emission is 36 billion tons in 2016.2
Efforts have been focused on the development of
methodologies for the recycling and conversion of CO2 into
valuable chemicals.3 As a C1 building block, CO2 is an
inexpensive, non-flammable, and non-toxic gas, which can be
added via the C−H, C−C, C−N, C−O, and C−S bonds to form
alcohols, carboxylic acids, and their derivatives, e.g., urea,
organic carbonates, and thiazoles.4 Some of the methodologies
have been industrialized. For instance, 150 million tons of urea
was produced from CO2 in 2000.5 Nevertheless, in 2016, this
number increased to 180 million tons,6 and in this process, 132
million tons of CO2 was consumed.
second, polyaddition occurs between isocyanates and polyols,
affording PUs. However, phosgene is highly toxic. In an attempt
to replace COCl2, researchers have investigated novel routes to
produce PUs.10 Carbamates are key precursors for synthesizing
non-isocyanate polyurethane (NIPU). Scheme 1 shows the
various C1 synthons for synthesizing carbamates. The carbon
sources can be thought to be COCl2 and its derivatives, organic
carbonates, and CO2.11 Phosgene is toxic (Scheme 1A), which
has been stated previously. Organic carbonate appears to be
sustainable (Scheme 1B); however, its preparation increased
the total number of steps in carbamate synthesis.12 For
example, dimethyl carbonate (DMC) is prepared from MeOH
and CO2, with a dehydrating reagent. The direct use of CO2 as a
C1 building block obviously minimizes the total steps in the
carbamate synthesis. In 2016, over six million tons of
carbamates were produced from CO2, which is predicted to
reach eleven million tons in 2030.6 Various R’ sources (Scheme
1C), e.g., organohalide (R’X),13 alcohol (R’OH),14 and metal
alkoxide,15 are used in the primary routes for directly
synthesizing carbamates from CO2. In the past two decades,
considerable efforts have been developed in this regard;
however, the disadvantages of these sources limit their use for
synthesizing carbamates. For example, a majority of the
organohalides are not natural products, possibly causing
potential environmental issues. Moreover, alcohols appear to
be a greener alternative as compared to organohalides;
however, only aliphatic amines can be activated because
aromatic amines are poor nucleophiles, with low pKa values.16
Meanwhile, carbamates (urethanes, RNHCOOR’) are
valuable chemicals in agricultural chemistry for producing
pesticides,7 medicinal chemistry for synthesizing hepatitis C
virus (HCV) inhibitors and β-/γ-secretase inhibitors for treating
a. National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. Email:
junchul.choi@aist.go.jp ; Tel: (+81) 029-861-9283.
b. Graduate School of Pure and Applied Sciences, University of Tsukuba, 1 -1-1
Tennodai, Tsukuba, Ibaraki 305-8573, Japan.
Electronic Supplementary Information (ESI) available: 1 H NMR, 13C{1H} NMR, and GC−MS
spectra. See DOI: 10.1039/x0xx00000x
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The use of metal alkoxide in industry is not preferred because
of the stoichiometric M(OR)n coupling with amine and CO2.
diazabicyclo[2.2.2]octane
(DABCO),
1-butyl-3-
methylimidazolium (BMim), and pyridine (Py), were selected,
and acetic acid (AcOH), pivalic acid (PivOH), trifluoroacetic acid
(CF3CO2H, TFA), trifluoroethanol (CF3CH2OH, TFE), and HCl as
the precursors for the counter anions. Scheme 2 shows their
structures. Notably, the precursors to the counter cations were
basic, even superbases, e.g., DBU, TBD, and DBN, while the
counter anion precursors were acids. The synthesis of ILs is
straightforward, involving the careful mixing of a base and an
acid in a Schlenk tube under N2.21 Even though heating was not
required for neutralization, an oil bath at 60°C was sufficient to
decrease the viscosity of the reaction mixture. ILs such as
[nBuDABCO][X] (X = Cl and Br) have been synthesized from
DABCO and nBuCl or nBuBr in CH2Cl2.22 [nBuDBU][OAc] was
synthesized from KOAc and [nBuDBU][Br], the latter one
generated from DBU and nBuBr in CH2Cl2.21, 22c [BMim][X] series
compounds (X = Cl, Br, I, and BF3) are commercially available.
[PyH][Cl] was synthesized from pyridine and a 12 mol L−1 HCl
aqueous solution.
Scheme 1
Evolution of C1 synthons for synthesizing carbamates
Ionic liquids (ILs) are molten salts, and majority of these ILs
exhibit a high boiling temperature, excellent thermostability,
solubility in various solvents, and negligible vapour pressures.17
Task-specificionicliquids (TSILs)areILs withdesigned functional
groups,18 which are extensively used for CO2 capture and
utilization (CCU) and catalysts for various transformations to
cycliccarbonates, ureas, and oxazolidinones.18a, 19 Recently, our
group has reported the synthesis of a carbamate from amine,
CO2, and silicate esters using zinc acetate as the catalyst;20
however, an N-donor ligand, such as 1,10-phenanthroline
(phen), is required to promote the reaction. Encouraged by the
success of this synthesis, herein, we report a superbase-derived
protic TSIL served as a catalyst for the synthesis of carbamates.
This protocol is a metal- and halogen-free methodology to
prepare carbamates from both aliphatic and aromatic amines.
The organocatalyst is easily prepared using commercially
available precursors, and both precursors are natural products.
This catalytic system provided a green, sustainable route to
synthesize carbamates, with a yield of up to96%.
Results and Discussion
Catalyst screening and applications.
In order to search an optimum catalyst, a series of ILs were
synthesized. Precursors tothecountercations, forinstance, 1,8-
Scheme 2
Components and synthesis of ionic liquid
diazabicyclo(5.4.0)undec-7-ene
(DBU),
1,5-
diazabicyclo(4.3.0)non-5-ene (DBN), triazabicyclodecene (TBD),
With various ILs in hand, their activities were examined
(Scheme3). Learning from our previous experience, aniline, the
1,1,3,3-tetramethylguanidine
(TMG),
1,4-
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simplest aromatic amine, was selected as the substrate, and 2 and 14, respectively); however, their reactivities considerably
equiv. of tetramethyl orthosilicate (Si(OMe)4, TMOS) was depended on the basicity of the counter cation: using
selected as the silicate ester for synthesizing methyl phenyl [TBDH][OAc] (pKa = 26.0 for TBDH+),27
while was obtained in only 25% using [TMGH][OAc] (pKa = 23.3
1 was obtained in 92%,
1
carbamate (
solvent. Products were analyzed by high performance liquid
chromatography (HPLC); in additionto the major product , 1,3-
diphenylurea ), by-product, was obtained via
1) at 150°C for 24 h with MeCN (3 mL) as the
for TMGH+).28
1
Table 1 Screening of catalystsa
(
2
a
Entry
catalyst
yield of 1
(%)b
yield of 2
(%)b
intertransformation. Table 1 summarizes the screening of the
catalysts. First, DBU-based ILs were examined (entries 1−9).
[DBUH][OAc]
[DBUH][OAc]
[DBUH][OAc]
[DBUH][OPiv]
[DBUH][TFA]
[DBUH][TFE]
[DBUH][im]
96
88
70
95
96
93
92
18
39
87
85
67
92
25
27
Using 10 mol% of [DBUH][OAc] (entry 1),
in 96% and 3%, respectively. However, with decreasing catalyst
loading to 5 mol% (entry 2) and 1 mol% (entry 3), the yields of
decreasedto 88% and 70%, respectively, indicating that catalyst
loading affects the yield of for a reaction time of 24 h. Hence,
1 and 2 were obtained
2c
3d
11
1
1
the catalyst loading is set to 10 mol% in further studies.
Moreover, using other basic counter anions, e.g., OPiv−, TFA−,
[DBUH][Cl]
[nBuDBU][OAc]
28
21
TFE−, and im− (entries 4−7), greater than 90% yields for
observed. On the other hand, with Cl− as the counter anion
(entry 8), was obtained in only 18% yield. These results
1were
10
11e
12
13
14
15
16
17
18
19
20
21
22
23f
24g
25h
26i
27j
DBU
DBU and AcOH
[DBNH][OAc]
[TBDH][OAc]
[TMGH][OAc]
[nBuDABCO][OAc]
[nBuDABCO][Cl]
[nBuDABCO][Br]
[BMim][Cl]
1
implied that the basicity of counter anions strongly affects the
reactivity of ILs, which was in agreement with previous
studies.21, 23 Nonetheless, basicity has been reported to affect
the metal-base-catalyzed CO2 transformation.20, 24 Moreover,
carboxylate- or carbonate-assisted metalation deprotonation
was considered as key process for C−H acꢀvaꢀons.25 O- or N-
donor basic anions can abstract protons from aniline, forming
hydrogen bonds, whilethehydrogen bond is barelyformed with
Cl−. n-Butyl-substituted IL [nBuDBU][OAc] was also a poor
catalyst (Table 1, entry 9), implying that the protonation of DBU
is imperative for catalysis. Note that DBU itself was found as a
promoter for carbamate synthesis in our study (entry 10) and in
previous reports,13c, 26 however, 10 to 100 equiv. DBU was used,
13c, 26 largely beyond catalyst scale. Furthermore, the difficulty
of separation of DBU from product by Liu and coworkers21
pushed us to choose DBU-derived IL as the preferred catalyst.
From our results, counter anion and substitution group of
counter cation of superbase-derived ILs are two keys for
catalyst design. Furthermore, the addition of 1 equiv. DBU and
1 equiv. AcOH (entry 11), precursors of [DBUH][OAc], did not
decrease the activity too much.
14
18
15
<1
12
[BMim][Br]
<1
<1
13
12
92
90
90
88
86
[BMim][I]
[BMim][BF4]
[PyH][Cl]
21
11
[DBUH][OAc]
[DBUH][OAc]
[DBUH][OAc]
[DBUH][OAc]
[DBUH][OAc]
a Reaction conditions: 1 mmol aniline, 2 mmol TMOS, 10 mol% catalyst, 3 mL
b
MeCN, 5 MPa CO2, 24 h, 150°C. Yields were determined by HPLC using
toluene as the internal standard. c 5 mol% of the catalyst was used. d 1 mol% of
the catalyst was used. e 1 mmol DBU and 1 mmol AcOH. f Catalyst was re- used
once. g Catalyst was re-used twice. h Catalyst was re-used three times. i Catalyst
was re-used four times. j Catalyst was re -used five times.
Ammonium-based
catalysts
[nBuDABCO][X]
and
imidazolium -based catalysts [BMim][X] (X =Cl, Br, I, and BF4),
barely catalyzed the formation of carbamate, because of the
low basicity and aprotic cations (entries 15−21). As compared to
[nBuDABCO][Cl] and [nBuDABCO][Br], [nBuDABCO][OAc] slightly
promoted the reaction, probably related to the contribution
from the acetate anion; however, the yield of
1 was not
sufficiently high (27%, entry 15). The same observation was
made with the use of a pyridine-based catalyst [PyH][Cl] (12%
of 1, entry 22). Based on the results in Table 1, the performance
ofamidinium-and guanidinium-based catalysts was better than
that of the ammonium-, imidazolium-, and pyridinium-based
catalysts. The performance of basic anions (e.g., carboxylate,
im−, and TFE−) was better than that of halide and BF4−. Notably,
the same result was observed for entries 1 and 5; however,
[DBUH][OAc] was selected as the first choice rather than
[DBUH][TFA] because of the cost-effectiveness and
Scheme 3
Synthetic route
[DBNH][OAc], which is another amidinium-based catalyst,
afforded in moderate yield (67%, entry 12). In addition,
1
guanidinium-based catalysts [TBDH][OAc] and [TMGH][OAc]
effectively catalyzed the synthesis of carbamates (entries 13
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sustainability of the AcOH precursor as compared to the produce cyclic carbonate from epoxide.31 Notably, PMHS is
CF3CO2H precursor. The [DBUH][OAc] catalyst was reused at obtained from silicon industrial waste,30b and silanols are
least five more times (entries 23 to 27), indicating that the naturally abundant,32 whichcanbeused as sustainablereagents
catalyst exhibits thermal stability and negligible deactivation.
for the transformation of CO2. In this study, silicate esters were
selected as alkoxy donors for synthesizing carbamates, which
were prepared from abundant SiO2 or silicate minerals.33 The
amount of alkoxy groups strongly affects the reactivity of
silicate esters in carbamate synthesis. MenSi(OMe)4 n (n = 0, 1,
−
2, and 3) was selected as the silicate esters, and the molarity of
the methoxy group was maintained constant in every
experiment. With the decrease in the number of methoxy
groups, the yield of 1decreased from 96% to 88%, 37%, and 8%,
respectively (Table 2). This dramatic decrease is related to the
hydrolysis rate of silicate esters.34 With a high amount of alkoxy
groups, the hydrolysis rate of the silicate ester was more rapid,
thereby promoting reactivity. The hydrolysis products of
MenSi(OMe)4-n (n = 0, 1, 2, and 3) were determined by 29Si
nuclear magneticresonance(NMR), shown in Table3 and Figure
2, and gas chromatography–mass spectrometry (GC–MS, see
supporting information).
The
[DBUH][OAc]-catalyzed
carbamate
synthesis
Table 3 29Si NMR analysisa
significantly depended on temperature, owing to the
requirement of the Si−O bond cleavage in TMOS. The bond
dissociation energy (BDE) of the Si−O bond is approximately 130
kcal mol−1, greater than those of the N−H and C−O bonds.29
silicate ester
chemical shift of silicate
ester (ppm)
chemical shift of hydrolysis
product (ppm)
Si(OMe)4
−78.29
−85.78
MeSi(OMe)3
−39.08
−0.76
18.41
−47.91
−10.83
7.41
Figure 1 shows the yield of
120 C, the yield of slowly increased, and only 25% of
obtained after 24 h. Moreover, in the initial step, the reaction
at 150 C was eight times more rapid than that at 120 C, and the
yield of
at 180
results indicated that the yield of
decreases to 77% during 24 h, caused by the chemical
interconversion between and : at a high temperature of
180 C, the reaction appeared to be more rapid, while the
product thermodynamically shifted from to . Based on
temperature-dependent experiments in Figure 1,
temperature of 150 C and a reaction time of 24 h are selected
1
as a function of temperature. At
Me2Si(OMe)2
Me3SiOMe
°
1
1
was
a 29Si NMR (79.5 MHz) spectra were recorded in CD3CN using J. Young NMR tubes
°
°
1
increased to 96% after 24 h. The initial reaction rate under N2.
C was 2.7 times more rapid than that at 150 C. HPLC
is 87% after 4 h, yet the yield
°
°
1
1
2
°
1
2
a
°
as the optimum conditions for the activation of various amines
in further studies.
Table 2 Reactivities of various silicate estersa
silicate ester
1 yield (%)b
hydrolysis product of silicate ester
Si(OMe)4
MeSi(OMe)3
Me2Si(OMe)2
Me3SiOMe
96
88
37
8
(MeO)3SiOSi(OMe)3
Me(MeO)2SiOSi(OMe)2Me
Me2(MeO) SiOSi(OMe) Me2
Me3SiOSiMe3
a Reaction conditions: 1 mmol aniline, 0.1 mmol [DBUH][OAc], 3 mL MeCN,
5
MPa CO2, 24 h, 150°C. Silicate esters: 2 mmol Si(OMe)4, 2.67 mmol
MeSi(OMe)3, 4 mmol Me2Si(OMe)2, and 8 mmol Me3SiOMe. b The yield of 1
was
CO2 was considered as the carbonyl donor in the carbamate
synthesis. As all experiments involving CO2 were performed
using autoclaves, safety issues are mentioned herein even
though CO2 is a non-toxic, inflammable gas. The CO2 gas cylinder
was located in an area with fresh air, and the autoclave with
high-pressure CO2 was tightly sealed and placed in a safe zone.
Figure 3 shows the effect of CO2 pressure on the yield of
determined by 1H NMR analysis.
Meanwhile, the applications of organosilicon compounds
for the conversion of CO2 to valuable products have attracted
considerable
attention.
Hydrosilanes
and
polymethylhydrosiloxane (PMHS) can serve as reductants for
the N-formylation and N-methylation of amines.30 Silanediols
have been used as a hydrogen-bonding donor catalyst to
carbamates. Under a CO2 pressure of 1 MPa,
1 was obtained in
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75% yield. Moreover, a similar yield was obtained despite the
change in the CO2 pressure from 3 MPa to 13 MPa, due to
excellent gas solubility in IL. The negligible difference with
respect to the CO2 effect prompted the selection of a relative
low-density CO2 for carbamate synthesis. High-density
supercritical CO2 (scCO2) was not required, further indicating
that addition of CO2 in carbamate synthesis is not the rate-
determining step.
With the optimized reaction conditions, the substrate scope
was extended to various substituted amines, affording their
corresponding carbamates directly using CO2 (Scheme 4). Using
substituted aniline, the isolated product yield varied from 50%
to 96% during 24 h. Notably, electron-donating groups
promoted carbamate formation (1a
withdrawing groups hindered the reaction (1e
−
1d), whereas electron-
1g). Only the
−
NH2 group served as the activation site, while the other
functional groups, e.g., −Br, −NO2, −CN, C=C, and C≡C, were not
affected, providing excellent chemoselectivity to NH2
activation. In addition to aniline analogs, heterocyclic amines,
aliphatic amines, and cycloalkylamine were activated to yield
Scheme 4
Carbamate synthesis using various substrates
their corresponding carbamates (1h−1j). Notably, several
studies have reported the activation of aliphatic amines rather
Scheme 5
Gram-scale synthesis of 1
than aromatic amines owing to the low reactivity of aromatic
amines. Our research highlights the activation of both aliphatic
andaromaticamines forsynthesizingcarbamates.Usingvarious
silicate esters, the yields of the corresponding carbamate
ranged from 65% to 75% (1k−1m). Compound 1n was the
corresponding carbamate of toluene diisocyanate (2,4-TDI),
which is an important starting material in the PU industry.
In addition, our synthetic protocol was expanded from the
laboratory scale to the gram scale (Scheme 5). The reaction was
carried out in a 20 mL autoclave using 1.12 g aniline (12 mmol)
as the substrate. After 24 h, 1 was obtained in 92% yield.
Although the yield was slightly less than that obtained on the
laboratory scale, the protocol employed herein affording a high
yield of 1served as a potential route for industrial preparation.
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Mechanism study.
and 52% proton (deuterium) exchange of the NH (ND) peaks,
respectively (supporting information).
With the excellent activity of [DBUH][OAc], the mechanism
for the catalytic synthesis of carbamate, especially the crucial
requirements for designing the catalyst, was investigated. First,
therate order with respect to the catalyst was examined (Figure
4). Various amounts of [DBUH][OAc] as the catalyst were used
to synthesize
initial yield of
mol%, 7.5 mol%, and 10 mol% of the catalyst, 1 was obtained in
1
, and the initial rates were obtained from the
for a reaction time of 4 h. Using 2.5 mol%, 5.0
1
yields of6.8%, 15.1%, 21.8%, and 30.8%, respectively. Thelinear
correlation indicated that the reaction is first order with respect
to [DBUH][OAc].
The results from Scheme 4 suggested that the substitution
group of aniline largely influenced the reactivity. Hammett plot
was shown in Figure 5.35 With methoxy, methyl, nitro, and
cyano substituted aniline, a linear relationship and negative
slope was obtained (ρ = −0.6095), indicaꢀng the binding
between electrophilic DBUH+ and nucleophilic amines.
As indicated by the results shown in Table 1, two
requirements are crucial for a highly reactive IL—a protic
counter cation and a basic counter anion—respectively.
[DBUH][OAc]satisfiedboththeserequirements.Toexaminethe
role of the proton in the cation, an isotope labeled catalyst or
substrate was used to trace the proton. Deuterium-labeled IL
[DBUD][OAc] was synthesized from DBU and AcOD. The
stoichiometric reaction between aniline, TMOS, and
Using secondary amines, e.g., N-methylaniline, as the
substrate, the corresponding carbamate was barely formed,
leading us to consider the isocyanate pathway. Isocyanate has
been reported as an intermediate in the synthesis of
[DBUD][OAc] under CO2 afforded product
GC−MS analysis, both -H (m/z = 151) and
observed. Apparently, the deuterium atom in
1
(Figure 6A). From
-D (m/z = 152) were
-D originated
1
1
1
from [DBUD][OAc], indicative of the dehydrogenation of the
catalyst in the transition state. Analogously, the stoichiometric
reaction between aniline-d2 (PhND2), TMOS, and [DBUH][OAc]
20
37
carbamates,14f,
quinazoline-2,4(1H,3H)diones.23c, 23h, 38 TMOS is considered to be
a methoxy donor for . The hydrolysis of TMOS yields MeOH,
ureas,36 benzimidazolones,21,
and
1
under CO2 furnished both 1-D and 1-H from GC−MS analysis
(Figure 6B). This result is consistent with that shown in Figure
6A. Furthermore, 1H NMRspectra of the mixtures revealed 33%
which directly reacts with isocyanate. Previously, our group, as
well as other researchers, has reported good catalyst
performance using basic anions.20-21, 23 -24 The counter anion is
considered to abstract protons from aniline. A hydrogen bond
is formed with OAc−. This interaction was barely observed
between aniline and Cl−. In addition, a high carbamate yield was
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observed with the use of counter anions such as OPiv−, TFA−, in the chemical equilibrium to
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ARTICLE
1
. In MeCN at 150°C, 87% yield of
TFE−, and im−, with DBUH+ (Table 1, entries 4−7). Either oxygen
or nitrogen can serve as the hydrogen-bonding donor. The
hydrogen bonding interaction was observed by 1H NMR (Figure
7). The NH2 group of aniline in CD3CN exhibited a chemical shift
of 4.05 ppm. With the addition of 1 equiv. of [DBUH][OAc], the
peak shifted to 4.46 ppm, and looked broader. The addition of
another equiv. of [DBUH][OAc] (a total of 2 equiv.) caused the
peak to shift to 4.82 ppm. The chemical shift was in good
agreement with the formation of hydrogen bonds between
[DBUH][OAc] and aniline; hence, with increasing amounts of
[DBUH][OAc], an increased chemical shift was observed. Such a
large 1H NMR chemical shift was also observed for a mixture of
aniline and [DBNH][OAc] or [TBDH][OAc] in CD3CN (supporting
information). However, with 1.0 equiv. of [DBUH][Cl], the
chemical shift of the NH2 group in aniline shifted from 4.05 ppm
to 4.28 ppm (Figure 8), which was less than that of
[DBUH][OAc]. This result indicates a weaker interaction
between aniline and [DBUH][Cl].
1
could be generated from
2
and TMOS with the catalyst.
Scheme 6
A plausible mechanism
Scheme 6 shows the proposed mechanism based on the
experimental results. First, the proton in aniline is activated by
the [DBUH][OAc] catalyst (
bonds between [DBUH][OAc] and aniline promotes the
abstraction of proton in aniline (II), yielding AcOH and III
I), and the formation of hydrogen
.
Second, CO2 is inserted between PhNH and DBUH (from III to
IV). Notably, the formation of carbamic acid (PhNHCOOH)
directly from aniline and CO2 is ruled out: the formation of
carbamic acid from aliphatic amine and CO2 is straightforward;
however, this process appears to be difficult using aromatic
amine owing to the low pKa values of aromatic amines.16 Next,
phenyl isocyanate (VI) is formed, affording DBU (V) and H2O.
Isocyanate can be considered as an important intermediate as
the major product
isocyanate.
1 and minor product 2 are generated from
The hydrolysis of TMOS yields siloxane (VII) and MeOH; the
latter is considered the direct OMe donor of , while using
MeOH in place of MeCN as the solvent did not yield , indicating
the interaction between the stronger nucleophile MeOH with
DBUH+, rather than formation of (III). In addition,
can be
generated from isocyanate and aniline; however, with large
amounts ofTMOS, the yield of is negligiblebecauseoftheshift
1
1
2
2
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The oxygen in water generated from (IV) to (VI) is finally the a single channel automated flash chromatography Yamazen
part of either
or hydrolysis product siloxane (VII). Adding 17O AI−580 system using dichloromethane and n-hexane as the
or 18O labeled water could track this oxygen (Figure 9). The eluents. Molecular weights were determined using a Shimadzu
reaction between phenyl isocyanate (VI) and TMOS yielded GCMS−QP2010 Plus GC–MS system. High resolution mass
1
1
and siloxane (VII) without any catalyst, which was reported spectroscopy (HR–MS) spectra were carried out in a Bruker
previously.20 The addition of H217O (Figure 9B) or H218O (Figure MicroTOF II spectrometer.
9C) increased m/z values of
Moreover, we did not observe the increase of m/z values of
suggesting that water was involved in the formation of siloxane
VII), rather than . The methoxy group of is from TMOS via
(VII) determined by GC−MS.
General procedure for the synthesis of [DBUH][OAc]
1,
[DBUH][OAc] was synthesized according to a previously
reported procedure.21, 39 DBU (0.76 g, 5.0 mmol) and AcOH (0.30
g, 5.0 mmol) were added to a 25 mL Schlenk tube. The reaction
(
1
1
Si−O cleavage.
mixture was heated to 60°C, and after 18 h, unreacted materials
were removed in vacuo to yield a colorless oil (1.01 g, 95%). 1H
NMR (400 MHz, D2O): δ (ppm) 3.45 (d, 2H, J = 9.2 Hz), 3.39 (t,
2H, J = 5.8 Hz), 2.49 (d, 2H, J = 10.0 Hz), 1.88 (p, 2H, J = 6.0 Hz),
1.81 (s, 3H), 1.667–1.508 (m, 6H). 13C NMR (100 MHz, D2O): δ
(ppm) 180.6, 165.9, 54.1, 48.2, 37.9, 32.8, 28.4, 25.8, 23.3, 22.8,
18.9. Other ILs were also synthesized according to previously
reported procedures.22, 23i, 40
Conclusions
Using a series oftheprepared TSILs, [DBUH][OAc]was found
to be the optimum catalyst to synthesize carbamate, with
isolated yields of up to 96%, directly from an amine, CO2, and a
silicate ester. Notably, both aliphatic and aromatic amines can
be activated by the developed protocol, broadening the
potential applications in organic synthesis, drug discovery, and
agricultural chemistry, as well as in the PU industry. The key
step in the mechanism involves the activation of the N−H bond
of aniline using the basic counter anion of the IL. In addition,
protic cations are necessary as H2O was formed in the catalytic
cycle. [DBUH][OAc] meets these two requirements, and its
straightforward preparation, thermostability, and reusability
facilitated a greener route to synthesize carbamates. Currently,
the design of other catalysts as well as studies on CO2
transformation are underway in our laboratory.
General procedure for the synthesis of 1
[DBUH][OAc] (21.2 mg, 0.1 mmol), MeCN (3 mL), aniline (93
mg, 1 mmol), and TMOS (304 mg, 2 mmol) were added in a 10
mL autoclave with a stir bar under N2. The autoclave was tightly
sealed and filled with CO2 to 3 MPa. The autoclave was heated
to 150°C, and the pressure was adjusted to 5 MPa. After 24 h,
theautoclave was cooled to ambient temperature, and CO2 was
gently released. Next, toluene (85.3 mg, 0.1 mL) as the internal
standard was added to the mixture. A small amount of the
mixturewas filtered for HPLC analysis. Theisolated materialwas
purified by automated flash chromatography with
dichloromethane and n-hexane as eluents. All of the isolated
Experimental Section
products (1a–
1n) were characterized by 1H NMR, 13C{1H} NMR,
and GC–MS (supporting information), consistent with either
literature values or authenticmaterials.13a, 20, 41
Materials
Unless stated otherwise, all chemicals were purchased from
Sigma-Aldrich, Tokyo Chemical Industry (TCI), or Wako
Chemicals in the best grade, stored under N2, and used without
further purification. Aniline-d2 (PhND2, 99% D) was purchased
from C/D/N Isotopes. H217O (50% 17O) was purchased from
Nukem Isotopes. H218O (97% 18O) was purchased from Sigma-
Aldrich. CO2 was purchased from Showa Tansan (99.99%).
Caution: High-pressure CO2 gas cylinders should be handled
with care and located in an open area with fresh air, although
no accident was encountered.
The catalyst re-use experiments were carried out in a same
way with our reported method.20
Acknowledgements
We thank New Energy and Industrial Technology
Development Organization (NEDO) to support this research
(Grant No. P16010). We thank K. Matsumoto for HR-MS
analysis.
Instruments
Catalytic reactions were carried out in a 10 mLstainless steel
autoclave equipped with a gas-pressure monitor (maximum
pressure of 25 MPa). All the oxygen-free operation was
conducted in either a glove box or a Schlenk line. Reaction
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Page 11 of 11
Green Chemistry
View Article Online
DOI: 10.1039/C7GC02666H
Table of contents:
An easily synthesized ionic liquid was used as an organocatalyst to synthesize carbamates
directly from amine, CO2, and silicate esters.