A Simple Zinc Catalyst for Carbamate Synthesis Directly from CO2

Article abstract of DOI:10.1002/cssc.201601878

Several zinc salts were employed as catalysts for the synthesis of carbamates directly from aromatic amines, CO₂, and silicate esters. Zn(OAc)₂ offered the best performance among the salts tested. The addition of an N-donor ligand such as 1,10-phenanthroline increased the yield. The best catalytic performance of Zn(OAc)₂ can be explained by carboxylate-assisted proton activation. The interaction between the substrate and the catalyst can be observed by chemical shifts in ¹H and ¹⁵N NMR spectra. Isocyanate was a key intermediate, which was generated from amine and CO₂. Silicate ester was finally converted to siloxane, which was determined by ²⁹Si NMR. The commercially available catalyst system could be reused. The yield of isolated carbamate could reach up to 96 % with various substrates, and the catalytic reaction was amine-selective in the presence of other functional groups.

Full text of DOI:10.1002/cssc.201601878

Accepted Article
Title: A Simple Zinc Catalyst for Carbamate Synthesis Directly from
CO2
Authors: Qiao Zhang, Hao-Yu Yuan, Norihisa Fukaya, Hiroyuki
Yasuda, and Jun-Chul Choi
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201601878
Link to VoR: http://dx.doi.org/10.1002/cssc.201601878
A Journal of
www.chemsuschem.org

10.1002/cssc.201601878
ChemSusChem
FULL PAPER
A Simple Zinc Catalyst for Carbamate Synthesis Directly from
CO2
Qiao Zhang,[a] Hao-Yu Yuan,[b] Norihisa Fukaya,[a] Hiroyuki Yasuda,[a] and Jun-Chul Choi*[a,b]
Abstract: Several zinc salts were employed as catalysts for the
synthesis of carbamates directly from aromatic amines, CO2, and
silicate esters. Zn(OAc)2 offered the best performance among the
salts tested. The addition of an N-donor ligand such as 1,10-
phenanthroline increased the yield. The best catalytic performance of
Zn(OAc)2 can be explained by carboxylate-assisted proton activation.
The interaction between the substrate and the catalyst can be
observed by chemical shifts from 1H and 15N NMR. Isocyanate was a
key intermediate which was generated from amine and CO2. Silicate
phosgene would be viewed as
a
significant synthetic
improvement.[9] One strategy involves designing novel routes to
carbamate syntheses via isocyanate obtained from heated
carbamates. Several routes to carbamate synthesis directly from
CO2 are listed in Scheme 1: (A) The application of organic
halides;[10] (B) The addition of the corresponding alcohol;[11] (C)
The addition of acetal, a dehydrating agent;[12] (D) The use of tin
alkoxide.[13] The use of N-tosylhydrazone[14] or diaryliodonium
salt[15] for carbamate synthesis are considered to be alternative
ester finally converted to siloxane, which was determined by 29Si NMR. methods, but only for aliphatic amine activation. Routes B and C
The commercially available catalyst system could be reused. The
isolated carbamate yield could reach up to 96% with various
substrates, and the catalytic reaction was amine-selective in the
presence of other functional groups.
cannot activate aromatic amines, and routes A and D have
negative environmental effects, because most of the organo-
halides are not natural products, and most of the organo-tin
compounds are toxic. These circumstances have encouraged us
to seek novel green approaches to carbamate synthesis directly
from CO2 and amines, with a particular focus on routes to
aromatic amine activation.
Introduction
Carbon dioxide (CO2) is an abundant, non-toxic,
inexpensive gas and a major component of the biogeochemical
cycle, especially the carbon cycle.[1] As a C1 chemical feedstock,
it has played an important role in the organic syntheses of
valuable chemicals.[2] The direct syntheses of urea, methanol,
and organic carbonate from CO2 have been industrialized, and
more than ten million tonnes CO2 are utilized per year.[3] CO2 is a
weak electrophile and can be attacked by nucleophiles, such as
H2O, ROH, and R2NH. On the other hand, a Lewis acidic metal
center may bind to CO2 through a σ bond with either the carbon
or oxygen center or through a π bond to the C=O double bond. [4]
One goal of the conversion of CO2 is the synthesis of
carbamate (urethane).[5] The carbamate group (RNHCOOR’) is a
key motif in medicinal chemistry,[6] insecticide products,[7] and
polyurethane (PU) industries. Twenty million tonnes PU have
been produced per year for use as surface coatings, synthetic
fibers, and adhesives, and 96% of PUs contain aromatic amine
units.[8] Traditionally, industrial poly-condensation processes used
to produce PUs begin from a reaction between an amine and
phosgene, followed by the poly-addition of polyols. Because
phosgene is a highly toxic compound, the replacement of
Scheme 1. Routes to carbamate synthesis directly from amines and CO2.
[a]
[b]
Drs. Q. Zhang, N. Fukaya, H. Yasuda, Prof. Dr. J.-C. Choi
National Institute of Advanced Industrial Science and Technology
(AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-
8565 (Japan)
E-mail: junchul.choi@aist.go.jp
Tel: (+81) 029 861 9283.
H.-Y. Yuan, Prof. Dr. J.-C. Choi
Graduate School of Pure and Applied Sciences, University of
Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573 (Japan)
Zinc is an inexpensive and biologically friendly first-row
transition metal,[16] and less toxic than tin reagents. In the field of
CO2 fixation, Zn(II) compounds are used as carbonic anhydrase
biomimetics,[17] in the catalytic synthesis of cyclic carbonates,[18]
and in catalytic co-polymerization from epoxides.[19] Inspired by
these results, we postulated that the interaction between a Lewis
acidic Zn(II) and CO2 might be designed for carbamate synthesis.
Herein, we describe a simple catalytic synthesis of carbamates
directly from an amine, CO2, and silicate esters. Both aliphatic and
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201601878
ChemSusChem
FULL PAPER
aromatic amines were found to be activated in a process
catalyzed by commercially available Zn(OAc)2 and an N-donor
ligand.
was methyl N-phenylcarbamate (1), and the minor product was
1,3-diphenylurea (2). In the presence of Zn(OAc)2, 60% 1 and 6%
2 were obtained, the best results obtained among the Zn salts
tested (Cl–, Br–, SO42–, and OTf–). Better catalyst performances
have been obtained from more basic anions, observed previously
by us[12b] and others.[20] The addition of
6 mol% 1,10-
phenanthroline (phen) and 2 mol% Zn(OAc)2 increased the total
yield to 87% (entry 11). The yields obtained from other Zn(II) salts
increased with the addition of phen. The addition of 2,2’-bipyridyl
(bpy) provided results similar to those obtained from Zn(OAc)2
(entry 12). Tetra-N-donor ligands, such as N,N’-bis(2-
pyridylmethyl)-1,2-ethanediamine (bispicen), were effective at
binding to Zn(OAc)2 (entry 13), The macrocyclic ligand 1,4,8,11-
tetraazacyclotetradecane (cyclam) provided moderate yields
(entry 14); however, the hexa-heteroatom-donor ligands, such as
Scheme 2. Catalytic synthesis of 1 directly from aniline, CO2, and Si(OMe)4
(TMOS).
N,N’-di(ethylacetate)-N,N’-bis
(2-pyridylmethyl)-1,2-
ethanediamine (debpn)[21] and ethylenediaminetetraacetic acid
(EDTA), produced worse results. The disappointing results arose
from the crowded coordination environment around the Zn center.
The crystal structure of [Zn(debpn)]2+ revealed an octahedral
coordination geometry with four nitrogen donors and two oxygen
donors from the ligand only, comparable to the structures
obtained from Mn, Fe, Co, and Ni complexes with debpn.[21a] The
crowded coordination environment hindered the reactivity of Zn.
Ligands with fewer donating atoms, such as phen or bpy,
increased the reactivity, whereas ligands with more donating
atoms, such as debpn or EDTA, hindered the reactivity (Scheme
3).
Results and Discussion
Table 1. Catalytic synthesis of 1 from aniline, CO2, and TMOS.[a]
entry
catalyst
ligand
yield of 1
yield of 2
(%)[b]
(%)[b]
1
2
ZnO
-
-
22
18
29
1
13
9
ZnCl2
3
ZnBr2
-
16
12
6
4
ZnSO4
-
5
Zn(OTf)2
Zn(OAc)2
ZnCl2
-
35
60
17
17
2
6
-
6
7
phen
phen
phen
phen
phen
bpy
11
11
18
4
8
ZnBr2
9
ZnSO4
10
11
12
13
14
15
16
17[c]
18[d]
Zn(OTf)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
14
84
83
84
60
26
10
82
81
3
5
bispicen
cyclam
debpn
EDTA
phen
phen
5
12
13
6
3
4
[a] Reaction conditions: 1 mmol aniline, 2 mmol TMOS, 0.02 mmol catalyst,
ligand (0.06 mmol for phen and bpy, 0.03 mmol for bispicen, EDTA, and
debpn), 3 mL MeCN, 5 MPa CO2, 150 °C, 24 h. [b] Calculated from the HPLC
results using toluene as an internal standard. [c] Catalyst reused for the
second run. [d] Catalyst reused for the third run.
Scheme 3. Poly-heteroatom donor ligands and their catalytic activities.
Reusability and recyclability play important roles in catalyst
design and applications. These properties are listed among the
twelve principles of green chemistry.[22] Even though we cannot
recycle 0.02 mmol Zn(OAc)2 (3.7 mg), the reusability results for
the Zn(OAc)2/phen catalyst in carbamate synthesis are listed in
Table 1 (entries 17 and 18). In each catalyst reuse experiment, 1
mmol aniline and 2 mmol TMOS were added to the reaction
system in situ as the standard reaction condition. The catalyst
could be reused at least three times without losing its activity.
Initially, we selected aniline as the aromatic amine substrate.
Reactions were carried out in MeCN at 150°C (Scheme 2), and
the results are listed in Table 1. We screened several Zn(II)
compounds as catalysts, including ZnO, ZnCl2, ZnBr2, ZnSO4,
Zn(OTf)2 (OTf = trifluoromethanesulfonate), and Zn(OAc)2. In the
presence of 2 mol% catalyst and 5 MPa CO2, two products were
detected by HPLC after a 24 h reaction time. The major product
This article is protected by copyright. All rights reserved.

10.1002/cssc.201601878
ChemSusChem
FULL PAPER
Compared with the other reaction systems, such as the Pd-
catalyzed direct arylation,[23] the Zn(OAc)2/phen system was both
catalytic and reusable, thereby extending its applications in
industrial processes.
that diluted the components of the reaction system.[25] Figure 2
shows that the best results were obtained at a CO2 pressure of 5
MPa, which provided the highest yield and selectivity.
Recently, our team synthesized silicate esters directly from
abundant SiO2 and alcohol,[26] thereby providing an economical
synthetic approach to the silicate esters. During the synthesis of
1, silicate esters could be thought of as donating alkoxide groups
to the carbamate motif. The reaction mixture prepared in CD3CN
was assessed by 29Si nuclear magnetic resonance spectroscopy
(NMR) in Figure 3: the new peak at –85.7 ppm could be assigned
as siloxane (MeO)3Si–O–Si(OMe)3. This species formed through
the dimerization of HOSi(OMe)3, the product of TMOS hydrolysis.
Even if a catalyst and ligand are employed in a reaction,
high temperatures (> 100°C) are often required for carbamate
synthesis directly from an amine, CO2, and TMOS. Temperature
dependence plots of the synthesis of 1 are shown in Figure 1. At
a temperature of 120°C, the yield of 1 was only 15% after 24 h. A
higher temperature of 180°C resulted in the rapid initial formation
of 1, but the yield of 1 was only 70% after 24 h, 14% lower than
the yield obtained at 150°C, due to the transformation from 1 to 2.
The optimal reaction temperature was determined to be 150°C
over a 24 h reaction time. The yield of 1 could not be increased,
even over a 15 h reaction time at 150°C.
Figure 3. 29Si NMR spectrum (79.5 MHz) of the reaction mixture. Reaction
conditions: 1 mmol aniline, 2 mmol TMOS, 0.02 mmol Zn(OAc)2, 0.06 mmol
phen, 3 mL CD3CN, 5 MPa CO2, 150°C, 24 h.
Figure 1. Temperature dependence plot of the catalytic synthesis of 1. Reaction
conditions: 1 mmol aniline, 2 mmol TMOS, 0.02 mmol Zn(OAc)2, 0.06 mmol
phen, 3 mL MeCN, 5 MPa CO2. The yields of 2 were omitted for clarity.
The amount of TMOS was screened and results were listed
in Table 2. Various amount of TMOS (1–4 equiv.) was added to
the reaction system. The highest yield of 1 was obtained with 2
equiv. TMOS related with aniline. With more TMOS, yields turned
to be lower, probably due to the dilution by liquid TMOS.
Table 2. The effect of the amount of TMOS on the yields.[a]
entry
TMOS amount
(mmol)
yield of 1 (%)[b]
yield of 2 (%)[b]
1
2
3
4
1
2
3
4
54
84
74
79
4
3
1
1
[a] Reaction conditions: 1 mmol aniline, 2 mmol TMOS, 0.02 mmol
Zn(OAc)2, 0.06 mmol phen 3 mL MeCN, 5 MPa CO2, 150 °C, 24 h. [b]
Calculated from HPLC results using toluene as an internal standard.
Figure 2. The effect of the CO2 pressure on the yields of 1 and 2. Reaction
conditions: 1 mmol aniline, 2 mmol TMOS, 0.02 mmol Zn(OAc)2, 0.06 mmol
phen, 3 mL MeCN, 150°C, 24 h.
The scope of catalytic reaction could be expanded to the
use of silicate esters with longer alkyl groups, and various silicate
esters were investigated for carbamate synthesis, including
Si(OMe)4, Si(OEt)4, Si(OnPr)4, and Si(OnBu)4. The results were
listed in Table 3. With 2 equiv. silicate esters for 24 h reaction, the
use of TMOS yielded 84% 1, much higher than TEOS (35% 1),
TPOS, (30% 1), and TBOS (32% 1). The best reactivity of TMOS
can be attributed to the short chains of alkoxides which provide
facile chemical interactions with other components. Note that time
is another factor to influence the yield, which can be learned from
Figure 2 plots the effect of the CO2 pressure on the yields
of 1 and 2. The yield of 1 did not vary under CO2 pressures of 3–
5 MPa; however, the yield decreased as the pressure was
decreased to 1 MPa. Decreased yields of 1 were observed at high
CO2 pressures (10–14 MPa). The low yield of 1 at high CO2
pressures suggested that a high-density CO2 condensate formed
from the gas phase.[24] As with MeCN, CO2 behaved as a solvent
This article is protected by copyright. All rights reserved.

10.1002/cssc.201601878
ChemSusChem
FULL PAPER
the comparison between entries 4 and 5 of the usage of TBOS.
The extension of reaction time from 24 h to 72 h remarkably
increased to yield form 32% to 92%. This result indicates that high
yield carbamate can be obtained by employing silicate esters with
long chains, however, a longer reaction time is required.
activation of aryl amines, heterocyclic, alkyl, and cycloalkyl
amines may be activated using this approach (1j–1l). The
application of various silicate esters were successful, and the
good yields were obtained (1m–1o). It is worth noting that 1p is a
precursor to methylene diphenyl diisocyanate (MDI), which plays
an important role in the PU industry.
Table 3. The effect of various silicate esters on the yield of corresponding
carbamate.[a]
entry
Si source
yield of carbamate (%)[b]
1
2
3
4
5
Si(OMe)4 (TMOS)
Si(OEt)4 (TEOS)
Si(OPr)4 (TPOS)
Si(OBu)4 (TBOS)
Si(OBu)4 (TBOS)[c]
84
35
30
32
92
[a] Reaction conditions: 1 mmol aniline, 2 mmol silicate ester, 0.02 mmol
Zn(OAc)2, 0.06 mmol phen, 3 mL MeCN, 5 MPa CO2, 150 °C, 24 h. [b]
Calculated from 1H NMR results using mesitylene as an internal standard.
[c] 72 h reaction.
Then we studied the relationship between the reactivity and
the number of alkoxides in silicate esters (Table 4). We chose
Si(OMe)4, MeSi(OMe)3, Me2Si(OMe)2, and Me3SiOMe as silicate
ester sources, and kept same amount of methoxy group in each
reaction (2 equiv. Si(OMe)4, 2.67 equiv. MeSi(OMe)3, 4 equiv.
Me2Si(OMe)2, and 8 equiv. Me3SiOMe, separately). The reactivity
of them were Si(OMe)4 (84% 1) > MeSi(OMe)3 (60% 1) >
Me2Si(OMe)2 (40% 1) > Me3SiOMe (17% 1).
Table 4. The effect of the amount of alkoxide in silicate ester on the yields.[a]
entry
Si source
amount
(mmol)
yield of 1
(%)[b]
yield of 2
(%)[b]
1
2
3
4
Si(OMe)4
MeSi(OMe)3
Me2Si(OMe)2
Me3SiOMe
2
2.67
4
84
60
40
17
3
3
5
2
8
[a] Reaction conditions: 1 mmol aniline, 2–8 mmol silicate ester, 0.02 mmol
Zn(OAc)2, 0.06 mmol phen, 3 mL MeCN, 5 MPa CO2, 150 °C, 24 h. [b]
Calculated from HPLC results using toluene as an internal standard.
With the optimized conditions (1 mmol amine, 2 mmol
TMOS, 0.02 mmol Zn(OAc)2, 0.06 mmol phen, 5 MPa CO2, 3 mL
MeCN, 24 h, and 150°C), we next sought to broaden the
applications of carbamate synthesis (Scheme 4 and supporting
information) to functional group–substituted aryl amines (1a–1i),
in the presence of CO2 and TMOS. The isolated yields
approached 96% (1a, para-substituted OMe). In the presence of
an electron-donating group, excellent yields were obtained (1a–
1e), whereas the electron-withdrawn groups provided moderate
yields (1f–1i). It is worth noting that only the amine group was
activated during the carbamate synthesis. Other functional groups,
such as methoxy-, halo-, nitro-, cyano-, and C=C double bonds,
Scheme 4. Isolated yields of carbamates synthesized from different types of
amines. 72 h reactions were granted for synthesizing 1m, 1n, and 1o.
The carbamate synthesis reaction was scaled up to the
gram scale (Scheme 5) from 1.0 g aniline. The yield of 1 was 81%,
comparable to that obtained from the small-scale experiments
(84%). The success of the gram-scale synthesis potentially
expands our protocol beyond laboratory research.
were not activated. This benign reaction provides
a
chemoselective approach to carbamate synthesis because the
particular activating position is an amine group. In addition to the
This article is protected by copyright. All rights reserved.

10.1002/cssc.201601878
ChemSusChem
FULL PAPER
which decreases the transition state energy. Indeed, this concept
can be expanded to basic anion-assisted proton abstraction, such
as carbonate[31] and peroxo[32] counter anions. The O–H
interaction is required in the transition state. Note that Ru and Pd
catalysts for carboxylate-assisted C–H activation of C–C bond
formation reactions were frequently reported.[28] In 1955, Winstein
and Traylor reported the acetolysis of dialkylmercury in acetic
acid,[33] which can be considered as the first example of this
mechanism. Note that both Zn and Hg are Group 12 elements,
however, to our best knowledge, carboxylate assisted proton
activations catalyzed by Zn compounds were rarely studied. A
very recent study described the mechanism of N–H bond
activation by ZnCl2 or Zn(OAc)2 during the formation of 2-
oxazolidinones from propargylic amines and CO2.[34] Another
case stated the activation of H2 by Cu(OAc)2 during N-formylation
of amine with CO2.[35] Both of them used first-row transition metal
acetates, and were consistent with our results.
Scheme 5. Gram-scale synthesis of 1.
In order to have a better understanding the role of
Zn(OAc)2/phen catalyst for carbamate synthesis, we initially
investigated the rate order of Zn(OAc)2, and it was calculated from
the initial rates of reactions catalyzed by various amount of
Zn(OAc)2 (Figure 3). Reaction was quenched at 4 h from the
beginning. With 1 mol%, 2 mol%, 3 mol%, and 4 mol% Zn(OAc)2,
yield of 1 was 10%, 21%, 32%, and 41%, separately. Since the
yield of 1 reflects the initial rate, the linear relationship between
the amount of Zn(OAc)2 and the initial rate reveals the first-order
kinetics respect to Zn(OAc)2. The result of Hammett analysis was
shown in Figure 4. Reaction rate constant can be reflected by the
yield of carbamate of the beginning 3 h. The activation of para-
substituted aniline gave the slope ρ = –0.5545 (R2 = 0.9942) in
Hammett plot. The negative ρ value suggested the electrophilic
property of Zn(II) binding with nitrogen of amines.
Figure 4. Hammett analysis of catalytic carbamate synthesis from para-
substituted anilines. Reaction conditions: 1 mmol para-substituted aniline
(substitution groups were –OMe, –Me, –H, –Cl, and –CN), 0.02 mmol Zn(OAc)2,
0.06 mmol phen, 3 mL MeCN, 5 MPa CO2, 150 °C, 3 h. YX was the yield of
methyl N-(para-X)phenylcarbamate, and YH was the yield of methyl N-
phenylcarbamate. The linear correlation resulted the slope ρ = –0.5545, with the
coefficient of determination R2 = 0.9942. The σp values were obtained from
reported literature.[27]
Figure 3. Linear relationship between the yield of 1 and the amount of Zn(OAc)2.
Reaction conditions: 1 mmol aniline, 3 equiv. phen respect to Zn(OAc)2, 3 mL
MeCN, 5 MPa CO2, 150 °C, 4 h.
Table 5. The influence of carboxylate on the yield.[a]
entry[b]
catalyst
ligand
yield of 1
(%)[c]
yield of 2
(%)[c]
The exceptional performance of Zn(OAc)2 in carbamate
synthesis encouraged us to uncover the role of carboxylate
counter anion. In addition to Zn(OAc)2, Zn(OPiv)2 (OPiv = pivalate,
-OOCtBu) was synthesized as a substitution of Zn(OAc)2, and the
results were listed in Table 5 (entries 3 and 4). From entries 1–4,
we found that both Zn(OAc)2 and Zn(OPiv)2 gave better results
than zinc salts with other counter anions. Nevertheless, we
observed that the adding a carboxylate source such as LiOAc,
with the poor catalyst ZnCl2 and phen, resulted 78% of 1 and 13%
of 2, much higher than ZnCl2 itself with or without phen ligand
(Table 5, entries 5–7). The promoting effect of Zn(OAc)2 and
Zn(OPiv)2 can be explained by carboxylate-assisted proton
activation,[28] sometimes called ambiphilic metal ligand activation
(AMLA),[29] or concerted metalation deprotonation (CMD),[30]
1
2
3
4
5
6
7
Zn(OAc)2
Zn(OAc)2
Zn(OPiv)2
Zn(OPiv)2
ZnCl2
-
60
84
81
89
18
17
78
6
3
phen
-
9
phen
-
3
9
ZnCl2
ZnCl2/LiOAc[d]
phen
phen
11
13
[a] Reaction conditions: 1 mmol aniline, 2 mmol TMOS, 0.02 mmol catalyst,
0.06 mmol phen (for entries 2, 4, 6, and 7), 3 mL MeCN, 5 MPa CO2, 150 °C,
24 h. [b] Entries 1, 2, 5, and 6 have been listed in Table 1. [c] Calculated
from HPLC results using toluene as an internal standard. [d] 0.04 mmol
LiOAc as carboxylate donor.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201601878
ChemSusChem
FULL PAPER
The interaction between aniline and the catalyst can be
can be considered as the carboxylate-assisted proton activation.
It is followed by CO2 insertion (from III to IV). Analogs of IV,
synthesizing from N-donor ligands and CO2, were reported,[36]
however, the activation of primary amine (R–NH2) was rarely
investigated. IV is decomposed to V and VI (isocyanate). The
formation of isocyanate is in agreement with previous
studies.[11a,12] VI is a very important intermediate since both 1 and
2 are generated from VI. 1 was obtained from the reaction of VI
and MeOH which was formed by the hydrolysis of TMOS, while
VI can also react with aniline to yield 2 as a side reaction. An
independent reaction among phenyl isocyanate (Ph–N=C=O),
TMOS, and a proton donor (acetic acid) as a control experiment
yielded 75% 1 within 2 h, indicating that this step is a fast step.
In the presence of excess TMOS, most of 1 can be
generated due to chemical equilibrium. The transformation from 2
to 1 can be proved by a control experiment: the reaction between
1 mmol 2 and 2 mmol TMOS yielded 73% 1 in 24 h at 150 °C
(Scheme 7) with Zn(OAc)2/phen catalyst.
observed by NMR spectra. Comparisons were shown in Figures
5 and 6 of aniline with and without Zn(OAc)2/phen in CD3CN at
room temperature. For the protons of –NH2 group of aniline, there
is no obvious chemical shift with the addition of phen,
nevertheless, a chemical shift from 4.05 ppm to 4.14 ppm was
observed from 1H NMR spectra with the addition of both Zn(OAc)2
and phen (Figure 5). When we used 15N enriched aniline to
perform 15N NMR, the peak was at 55.59 ppm. The peak slightly
shifted to 55.68 ppm with the addition of phen, but shifted to 56.63
ppm after the addition of both Zn(OAc)2 and phen (Figure 6). The
chemical shifts to the downfield from both 1H and 15N NMR
spectra illustrates the interaction between aniline and acetate,
and the formation of hydrogen bonds.
Figure 5. 1H NMR spectra (400 MHz) of aniline (top), aniline with phen (middle),
and aniline with both Zn(OAc)2 and phen (bottom) in CD3CN. The concentration
of aniline was 0.4 mol L-1.
Scheme 6. A proposed mechanism. Ligands are omitted for clarity.
Figure 6. 15N NMR spectra (40.6 MHz) of aniline (top), aniline with phen
(middle), and aniline with both Zn(OAc)2 and phen (bottom) in CD3CN. Aniline
was 15N enriched and the concentration of aniline was 1.0 mol L-1.
Scheme 7. The transformation of 1 mmol 2, 2 mmol TMOS, catalyzed by 2
mol% Zn(OAc)2 and 6 mol% phen yielded 73% 1 after 24 h at 150 °C.
Based on our experiments, a proposed mechanism of Zn(II)
catalyzed carbamate synthesis is shown in Scheme 6. First, N–H
bond in aniline is activated by Zn(OAc)2 (from I to II). This step
This article is protected by copyright. All rights reserved.

10.1002/cssc.201601878
ChemSusChem
FULL PAPER
Conclusions
Acknowledgements
In summary, we successfully used Zn(OAc)2/phen as the
We acknowledge support from the New Energy and
Industrial Technology Development Organization (NEDO).
catalyst for the synthesis of carbamate directly from amines, CO2,
and silicate esters. Notably, aromatic amines could be activated
to yield up to 96% of the corresponding carbamate. The catalyst
could be reused without losing its activity. Other functional groups
remained intact, indicating that the reaction was chemoselective
toward amine activation. This methodology provides a novel route
to carbamate synthesis, especially to aromatic carbamate
synthesis, which is useful in the PU, medicinal chemistry, and
agricultural chemistry industries. The obvious different catalytic
reactivities of Zn salts between carboxylate and non-carboxylate
counter anions indicated that the activation of aniline goes
through the carboxylate-assisted proton activation (Scheme 6, II).
The designing of other novel catalysts for use in this process and
the development of Zn(OAc)2/phen catalytic system in other CO2
transformations are underway.
Keywords: amine • carbamate • CO2 • silicate ester • zinc
[1]
[2]
a) G. A. Olah, G. K. S. Prakash, A. Goeppert, J. Am. Chem. Soc. 2011,
133, 12881-12898; b) M. He, Y. Sun, B. Han, Angew. Chem. Int. Ed.
2013, 52, 9620-9633; Angew. Chem. 2013, 125, 9798-9812.
a) T. Sakakura, J.-C. Choi, H. Yasuda, Chem. Rev. 2007, 107, 2365-
2387; b) D. J. Darensbourg, Inorg. Chem. 2010, 49, 10765-10780; c) M.
Peters, B. Köhler, W. Kuckshinrichs, W. Leitner, P. Markewitz, T. E.
Müller, ChemSusChem 2011, 4, 1216-1240; d) K. Huang, C.-L. Sun, Z.-
J. Shi, Chem. Soc. Rev. 2011, 40, 2435-2452; e) M. Cokoja, C.
Bruckmeier, B. Rieger, W. A. Herrmann, F. E. Kühn, Angew. Chem. Int.
Ed. 2011, 50, 8510-8537; Angew. Chem. 2011, 123, 8662-8690; f) I.
Omae, Coord. Chem. Rev. 2012, 256, 1384-1405; g) Z.-Z. Yang, L.-N.
He, J. Gao, A.-H. Liu, B. Yu, Energy Environ. Sci. 2012, 5, 6602-6639;
h) Y. Tsuji, T. Fujihara, Chem. Commun. 2012, 48, 9956-9964; i) A. M.
Appel, J. E. Bercaw, A. B. Bocarsly, H. Dobbek, D. L. DuBois, M. Dupuis,
J. G. Ferry, E. Fujita, R. Hille, P. J. A. Kenis, C. A. Kerfeld, R. H. Morris,
C. H. F. Peden, A. R. Portis, S. W. Ragsdale, T. B. Rauchfuss, J. N. H.
Reek, L. C. Seefeldt, R. K. Thauer, G. L. Waldrop, Chem. Rev. 2013,
113, 6621-6658; j) C. S. Yeung, V. M. Dong, Top. Catal. 2014, 57, 1342-
1350; k) M. Aresta, A. Dibenedetto, A. Angelini, Chem. Rev. 2014, 114,
1709-1742; l) B. Yu, L.-N. He, ChemSusChem 2015, 8, 52-62; m) D. Yu,
S. P. Teong, Y. Zhang, Coord. Chem. Rev. 2015, 293–294, 279-291; n)
Z. Yang, X. Gao, Z. Liu, Curr. Opin. Green Sustain. Chem. 2016, 1, 13-
17; o) G. Yuan, C. Qi, W. Wu, H. Jiang, Curr. Opin. Green Sustain. Chem.
2017, 3, 22-27.
Experimental Section
Materials. Unless stated otherwise, all of the chemicals were
purchased from Sigma-Aldrich, Tokyo Chemical Industry, and Wako
Chemicals with the best grade and without further purification and stored
under N2. CO2 was purchased from Showa Tansan. 15N enriched aniline
was purchased from Cambridge Isotope Laboratories. Ligands bispicen[37]
and debpn[21] were synthesized through precedent procedures (see
supporting information Schemes S1 and S2). Zn(OPiv)2 was synthesized
through a similar method of the preparation of Pd(OPiv)2.[23] Caution: High
pressure CO2 gas should be handled with care, and CO2 gas cylinder
should be located in an open area with fresh air.
[3]
[4]
a) E. A. Quadrelli, G. Centi, J.-L. Duplan, S. Perathoner, ChemSusChem
2011, 4, 1194-1215; b) N. J. English, M. M. El-Hendawy, D. A. Mooney,
J. M. D. MacElroy, Coord. Chem. Rev. 2014, 269, 85-95; c) A. Otto, T.
Grube, S. Schiebahn, D. Stolten, Energy Environ. Sci. 2015, 8, 3283-
3297.
Instruments. The catalytic reactions were carried out in a 10 mL
stainless steel autoclave with a gas pressure monitor (maximum 25 MPa).
All of the oxygen-free operations were done in a glovebox. Reaction
mixture was heated in Sibata Chemi-300 Synthesizer. 1H, 13C{1H}, 15N, and
29Si NMR spectra were recorded on a 400 MHz Bruker NMR Spectrometer
at room temperature. Product mixture was analyzed by a Shimadzu HPLC
with MeOH/H2O (v : v = 7 : 3) as the mobile phase at 40°C. Carbamate
a) T. Yu, R. Cristiano, R. G. Weiss, Chem. Soc. Rev. 2010, 39, 1435-
1447; b) Q. Liu, L. Wu, R. Jackstell, M. Beller, Nat. Commun. 2015, 6,
5933.
[5]
[6]
[7]
[8]
D. Chaturvedi, Tetrahedron 2012, 68, 15-45.
isolations were performed in
a Yamazen AI-580 Single Channel
A. K. Ghosh, M. Brindisi, J. Med. Chem. 2015, 58, 2895-2940.
R. J. Kuhr, J. Agric. Food Chem. 1970, 18, 1023-1030.
Automated Flash Chromatography System using dichloromethane and n-
hexane as eluents. Molecular weights were determined by a Shimadzu
GCMS-QP2010 Plus Gas Chromatograph Mass Spectrometer. Fourier
transform infrared (FT-IR) spectra were obtained from a Shimadzu FTIR-
8400 Spectrometer.
a) H.-W. Engels, H.-G. Pirkl, R. Albers, R. W. Albach, J. Krause, A.
Hoffmann, H. Casselmann, J. Dormish, Angew. Chem. Int. Ed. 2013, 52,
9422-9441; Angew. Chem. 2013, 125, 9596-9616; b) L. Maisonneuve, O.
Lamarzelle, E. Rix, E. Grau, H. Cramail, Chem. Rev. 2015, 115, 12407-
12439.
General procedure of synthesis of 1. Under N2 protection,
Zn(OAc)2 (3.7 mg, 0.02 mmol), phen (10.8 mg, 0.06 mmol), MeCN (3 mL),
aniline (93 mg, 1 mmol), and TMOS (304 mg, 2 mmol) were added to an
autoclave with a stir-bar. The autoclave was sealed tightly and filled with
CO2 to 3 MPa. The autoclave was heated to 150 °C, and the pressure of
CO2 was adjusted to 5 MPa. After 24 h, the autoclave was cooled down to
room temperature, and CO2 was released slowly. Toluene (85.3 mg, 0.1
mL) as the internal standard was added to the mixture. A small amount of
the mixture was filtered for HPLC analysis. Isolated material was purified
by automated flash chromatography system using dichloromethane and n-
hexane as eluents. All of the isolated products in Scheme 4 were
characterized by 1H NMR, 13C{1H} NMR, GC-MS, and FT-IR. Results are
comparable with authentic materials and literature.[10f,38]
[9]
a) O. Kreye, H. Mutlu, M. A. R. Meier, Green Chem. 2013, 15, 1431-
1455; b) H. Sardon, A. Pascual, D. Mecerreyes, D. Taton; H. Cramail; J.
L. Hedrick, Macromolecules 2015, 48, 3153-3165.
[10] a) W. McGhee, D. Riley, K. Christ, Y. Pan, B. Parnas, J. Org. Chem.
1995, 60, 2820-2830; b) M. Yoshida, N. Hara, S. Okuyama, Chem.
Commun. 2000, 151-152; c) R. N. Salvatore, S. I. Shin, A. S. Nagle, K.
W. Jung, J. Org. Chem. 2001, 66, 1035-1037; d) J. M. Hooker, A. T.
Reibel, S. M. Hill, M. J. Schueller, J. S. Fowler, Angew. Chem. Int. Ed.
2009, 48, 3482-3485; Angew. Chem. 2009, 121, 3534-3537; e) M. Zhang,
X. Zhao, S. Zheng, Chem. Commun. 2014, 50, 4455-4458; f) D. Riemer,
P. Hirapara, S. Das, ChemSusChem 2016, 9, 1916-1920.
[11] a) A. Ion, C. Van Doorslaer, V. Parvulescu, P. Jacobs, D. De Vos, Green
Chem. 2008, 10, 111-116; b) M. Honda, S. Sonehara, H. Yasuda, Y.
Nakagawa, K. Tomishige, Green Chem. 2011, 13, 3406-3413; c) B. Zhao,
S. Yao, H. An, X. Zhao, Y. Wang, J. Chem. Technol. Biotechnol. 2014,
This article is protected by copyright. All rights reserved.

10.1002/cssc.201601878
ChemSusChem
FULL PAPER
89, 1553-1558; d) G. Fan, S. Luo, T. Fang, Q. Wu, G. Song, J. Li, J. Mol.
[23] J. Kuwabara, M. Sakai, Q. Zhang, T. Kanbara, Org. Chem. Front. 2015,
2, 520-525.
Catal. A: Chem. 2015, 404–405, 92-97; e) R. Zhang, L. Guo, C. Chen, J.
Chen, A. Chen, X. Zhao, X. Liu, Y. Xiu, Z. Hou, Catal. Sci. Technol. 2015,
5, 2959-2972.
[24] a) W. Leitner, Acc. Chem. Res. 2002, 35, 746-756; b) A. Olmos, G.
Asensio, P. J. Pérez, ACS Catal. 2016, 6, 4265-4280.
[12] a) M. Abla, J.-C. Choi, T. Sakakura, Chem. Commun. 2001, 2238-2239;
b) M. Abla, J.-C. Choi, T. Sakakura, Green Chem. 2004, 6, 524-525.
[13] a) H.-Y. Yuan, J.-C. Choi, S.-y. Onozawa, N. Fukaya, S. J. Choi, H.
Yasuda, T. Sakakura, J. CO2 Util. 2016, 16, 282-286. b) N. Germain, I.
Müller, M. Hanauer, R. A. Paciello, R. Baumann, O. Trapp, T. Schaub,
ChemSusChem 2016, 9, 1586-1590.
[25] a) T. Sakakura, Y. Saito, M. Okano, J.-C. Choi, T. Sako, J. Org. Chem.
1998, 63, 7095-7096; b) H. Kawanami, Y. Ikushima, Chem. Commun.
2000, 2089-2090; c) S.-i. Fujita, B. M. Bhanage, Y. Ikushima, M. Arai,
Green Chem. 2001, 3, 87-91; d) H. Kawanami, A. Sasaki, K. Matsui, Y.
Ikushima, Chem. Commun. 2003, 896-897; e) C.-X. Miao, J.-Q. Wang,
Y. Wu, Y. Du, L.-N. He, ChemSusChem 2008, 1, 236-241; f) Y. Ren, J.-
J. Shim, ChemCatChem 2013, 5, 1344-1349; g) T. Ema, Y. Miyazaki, J.
Shimonishi, C. Maeda, J.-y. Hasegawa, J. Am. Chem. Soc. 2014, 136,
15270-15279.
[14] a) W. Xiong, C. Qi, H. He, L. Ouyang, M. Zhang, H. Jiang, Angew. Chem.
Int. Ed. 2015, 54, 3084-3087; Angew. Chem. 2015, 127, 3127-3130; b)
J. Y. Hong, U. R. Seo, Y. K. Chung, Org. Chem. Front. 2016, 3, 764-767.
[15] W. Xiong, C. Qi, Y. Peng, T. Guo, M. Zhang, H. Jiang, Chem. Eur. J.
2015, 21, 14314-14318.
[26] N. Fukaya, S. J. Choi, T. Horikoshi, H. Kumai, M. Hasegawa, H. Yasuda,
K. Sato, J.-C. Choi, Chem. Lett. 2016, 45, 828-830.
[16] S. Enthaler, ACS Catal. 2013, 3, 150-158.
[27] C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91, 165-195.
[28] L. Ackermann, Chem. Rev. 2011, 111, 1315-1345.
[29] Y. Boutadla, D. L. Davies, S. A. Macgregor, A. I. Poblador-Bahamonde,
Dalton Trans. 2009, 5820-5831.
[17] V. M. Krishnamurthy, G. K. Kaufman, A. R. Urbach, I. Gitlin, K. L.
Gudiksen, D. B. Weibel, G. M. Whitesides, Chem. Rev. 2008, 108, 946-
1051.
[18] a) J. Sun, S.-i. Fujita, F. Zhao, M. Arai, Green Chem. 2004, 6, 613-616;
b) M. North, R. Pasquale, C. Young, Green Chem. 2010, 12, 1514-1539;
c) S. Klaus, M. W. Lehenmeier, C. E. Anderson, B. Rieger, Coord. Chem.
Rev. 2011, 255, 1460-1479; d) C. Martín, G. Fiorani, A. W. Kleij, ACS
Catal. 2015, 5, 1353-1370; e) M. Liu, F. Wang, L. Shi, L. Liang, J. Sun,
RSC Adv. 2015, 5, 14277-14284; f) W. Desens, C. Kohrt, A.
Spannenberg, T. Werner, Org. Chem. Front. 2016, 3, 156-164; g) R. Ma,
L.-N. He, Y.-B. Zhou, Green Chem. 2016, 18, 226-231.
[30] D. Lapointe, K. Fagnou, Chem. Lett. 2010, 39, 1118-1126.
[31] S. Pascual, P. de Mendoza, A. M. Echavarren, Org. Biomol. Chem. 2007,
5, 2727-2734.
[32] a) J. Cho, R. Sarangi, W. Nam, Acc. Chem. Res. 2012, 45, 1321-1330;
b) Q. Zhang, A. Bell-Taylor, F. M. Bronston, J. D. Gorden, C. R.
Goldsmith, Inorg. Chem. 2017, 56, 773-782.
[33] S. Winstein, T. G. Traylor, J. Am. Chem. Soc. 1955, 77, 3747-3752.
[34] X. Liu, M.-Y. Wang, S.-Y. Wang, Q. Wang, L.-N. He, ChemSusChem
2017, doi: 10.1002/cssc.201601469.
[19] a) G. W. Coates, D. R. Moore, Angew. Chem. Int. Ed. 2004, 43, 6618-
6639; Angew. Chem. 2004, 116, 6784-6806; b) D. J. Darensbourg, Chem.
Rev. 2007, 107, 2388-2410; c) M. I. Childers, J. M. Longo, N. J. Van Zee,
A. M. LaPointe, G. W. Coates, Chem. Rev. 2014, 114, 8129-8152.
[20] a) Z.-Z. Yang, L.-N. He, X.-Y. Dou, S. Chanfreau, Tetrahedron Lett. 2010,
51, 2931-2934; b) Y. N. Shim, J. K. Lee, J. K. Im, D. K. Mukherjee, D. Q.
Nguyen, M. Cheong, H. S. Kim, Phys. Chem. Chem. Phys. 2011, 13,
6197-6204; c) B. Yu, H. Zhang, Y. Zhao, S. Chen, J. Xu, L. Hao, Z. Liu,
ACS Catal. 2013, 3, 2076-2082; d) W. Lu, J. Ma, J. Hu, J. Song, Z. Zhang,
G. Yang, B. Han, Green Chem. 2014, 16, 221-225; e) J. Sun, X. Yao, W.
Cheng, S. Zhang, Green Chem. 2014, 16, 3297-3304; f) Q.-W. Song, W.-
Q. Chen, R. Ma, A. Yu, Q.-Y. Li, Y. Chang, L.-N. He, ChemSusChem
2015, 8, 821-827; g) J. Hu, J. Ma, Z. Zhang, Q. Zhu, H. Zhou, W. Lu, B.
Han, Green Chem. 2015, 17, 1219-1225; h) D. S. Nguyen, J. K. Cho, S.-
H. Shin, D. K. Mishra, Y. J. Kim, ACS Sustainable Chem. Eng. 2016, 4,
451-460; i) Q.-W. Song, Z.-H. Zhou, M.-Y. Wang, K. Zhang, P. Liu, J.-Y.
Xun, L.-N. He, ChemSusChem 2016, 9, 2054-2058; j) S. Zhang, Q. Mei,
H. Liu, H. Liu, Z. Zhang, B. Han, RSC Adv. 2016, 6, 32370-32373.
[21] a) Q. Zhang, J. D. Gorden, R. J. Beyers, C. R. Goldsmith, Inorg. Chem.
2011, 50, 9365-9373; b) Q. Zhang, C. R. Goldsmith, Inorg. Chim. Acta
2013, 406, 301-306.
[35] H. Liu, Q. Mei, Q. Xu, J. Song, H. Liu, B. Han, Green Chem. 2017, 19,
196-201.
[36] a) Y. Yasuhiko, I. Shigeru, K. Akinori, Y. Tadataka, Y. Masahiro, I. Shohei,
Bull. Chem. Soc. Jpn. 1988, 61, 2913-2916; b) Y. Tang, W. S. Kassel, L.
N. Zakharov, A. L. Rheingold, R. A. Kemp, Inorg. Chem. 2005, 44, 359-
364; c) Y. Syuhei, T. Teppei, W. Akira, F. Yasuhiro, O. Tomohiro, J.
Koichiro, M. Hideki, Chem. Lett. 2007, 36, 842-843; d) J. Notni, S.
Schenk, H. Görls, H. Breitzke, E. Anders, Inorg. Chem. 2008, 47, 1382-
1390; e) D. Domide, E. Kaifer, J. Mautz, O. Walter, S. Behrens, H.-J.
Himmel, Eur. J. Inorg. Chem. 2008, 3177-3185; f) D. Domide, C.
Neuhäuser, E. Kaifer, H. Wadepohl, H.-J. Himmel, Eur. J. Inorg. Chem.
2009, 2170-2178.
[37] a) Q. Zhang, J. D. Gorden, C. R. Goldsmith, Inorg. Chem. 2013, 52,
13546-13554.
[38] a) J. Holt, T. Andreassen, J. M. Bakke, A. Fiksdahl, J. Heterocyclic Chem.
2005, 42, 259-264; b) Q. Yang, A. Robertson, H. Alper, Org. Lett. 2008,
10, 5079-5082; c) X. Zhang, H. Jing, G. Zhang, Synth. Commun. 2010,
40, 1614-1624; d) Z. D. Crane, P. J. Nichols, T. Sammakia, P. J. Stengel,
J. Org. Chem. 2011, 76, 277-280; e) A. Yoshimura, M. W. Luedtke, V. V.
Zhdankin, J. Org. Chem. 2012, 77, 2087-2091; f) S. Kumar, S. L. Jain,
Dalton Trans. 2013, 42, 15214-15218.
[22] P. Anastas, N. Eghbali, Chem. Soc. Rev. 2010, 39, 301-312.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201601878
ChemSusChem
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Zinc rocks: A novel method for
synthesizing carbamates directly from
amines and CO2 was developed. A
commercially available zinc compound
was used as the catalyst. N-donor
ligand provided an isolated yield of
96%. It is worth noting that both
aliphatic and aromatic amines could
be activated, thereby broadening the
utility of this reaction in organic
synthesis and the polyurethane
industry.
Qiao Zhang, Hao-Yu Yuan, Norihisa
Fukaya, Hiroyuki Yasuda, and Jun-Chul
Choi*
Page No. – Page No.
A Simple Zinc Catalyst for Carbamate
Synthesis Directly from CO2
Layout 2:
COMMUNICATION
Author(s), Corresponding Author(s)*
((Insert TOC Graphic here))
Page No. – Page No.
Title
Text for Table of Contents
This article is protected by copyright. All rights reserved.