Halogen-free process for the conversion of carbon dioxide to urethanes by homogeneous catalysis

Article abstract of DOI:10.1039/b106201h

Carbon dioxide is converted to urethanes (carbamic acid derivatives) through reaction with amine and alcohol catalyzed by tin complexes; the addition of acetals as a dehydrating agent under high CO₂ pressure is the key to achieve high yields.

Full text of DOI:10.1039/b106201h

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Halogen-free process for the conversion of carbon dioxide to urethanes
by homogeneous catalysis
Mahmut Abla, Jun-Chul Choi and Toshiyasu Sakakura*
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba,
Ibaraki 305-8565, Japan. E-mail: t-sakakura@aist.go.jp
Received (in Cambridge, UK) 12th July 2001, Accepted 18th September 2001
First published as an Advance Article on the web 5th October 2001
Carbon dioxide is converted to urethanes (carbamic acid
derivatives) through reaction with amine and alcohol
catalyzed by tin complexes; the addition of acetals as a
dehydrating agent under high CO
achieve high yields.
carbon dioxide with amines and alcohols [eqn.(3)]. This
3)
(
2
pressure is the key to
procedure is not only phosgene-free but also completely
halogen-free.
Polyurethane has many important industrial utilizations and the
Dialkyl carbonate synthesis from alcohol and CO
2
is
1
worldwide production still continues to grow. Current com-
catalyzed by metal complexes such as dialkyl(dimethoxy)tin,
mercial processes for producing polyurethane are based on
isocyanate which is synthesized from a primary amine and
phosgene [eqn.(1)]. This procedure has the major drawbacks of
dialkyl(oxo)tin, and zirconium oxide.1
5,16
However, the alcohol
conversion is very poor. Similarly, the straightforward reaction
of an amine, an alcohol, and carbon dioxide in the presence of
dialkyltin compounds produced urethane only in a poor yield
although the selectivity was satisfactory (Table 1, run 1).
The low conversion observed in run 1 is presumably
attributable to thermodynamic limitations and catalyst deactiva-
tion by co-produced water [eqn.(3)]. In order to overcome the
low conversion, we have devised a new reaction system
utilizing acetals as a chemical dehydrating agent. The idea is
schematically represented in Scheme 1; water is trapped by
acetal to regenerate alcohol. The conversion of amine was
clearly improved by this method without much decrease in the
selectivity (Table 1, runs 2 to 4). The larger acetal–amine ratio
further promoted the reaction. For example, the reaction of t-
BuNH2 and EtOH in the presence of two equivalents of
2,2-diethoxypropane led to a high conversion and selectivity,
100 and 84%, respectively (run 5). Since acetals are easily
regenerated from acetone, the net reaction can be regarded as an
(
1)
using highly toxic and corrosive phosgene, and the liberation of
hydrogen chloride as a co-product.
From the standpoint of ‘green chemistry’, significant efforts
have been made to find an alternative to the phosgene process.
A very attractive substitute for phosgene is carbon dioxide
because it is a typical renewable resource.2 In addition,
utilization of carbon dioxide is also very fascinating due to its
environmentally benign nature (nontoxic, noncorrosive, and
nonflammable). In particular, the application of dense carbon
dioxide to organic synthesis has recently attracted much
,3
4
interest. Most of the approaches in this context rely on the
production of the carbamate anion via the reaction of carbon
dioxide and amines, followed by the reaction with electrophiles
urethane synthesis from CO , an amine, and an alcohol. As for
2
–14
eqn.(2)].5 However, all these methods require an equimolar
[
the catalyst, dialkoxy(dialkyl)tin and its precursors such as
dialkyl(oxo)tin and dialkyl(dichloro)tin exhibited high catalytic
1
6
activities. †
(
2)
amount of electrophiles, usually alkyl halides. In this paper, we
wish to report urethane synthesis by the reaction of dense
Scheme 1
Table 1 Urethane synthesis from amine, alcohol, and CO a
2
b
Run
Amine
Alcohol
Catalys
Acetal
None
Conversion (%) Selectivity (%)
1
2
3
4
t-BuNH
t-BuNH
2
EtOH
EtOH
EtOH
MeOH
EtOH
MeOH
Bu
2
Bu
2
Bu
2
SnO
SnO
SnO
16
59
89
74
100
66
89
90
64
88
84
24
2
Me
Me
Me
Me
2
2
2
2
C(OEt)
C(OEt)
C(OMe)
C(OEt)
2
CyNH
2
2
t-BuNH
t-BuNH
t-BuNH
2
2
2
Me
2
SnCl
SnO
SnO
2
2
c
5
Bu
Bu
2
2
d
6
2
None
a
30 MPa, 200 °C, 24 h. b Selectivity = (yield of
Reaction conditions: amine (10 mmol), alcohol (100 mmol), acetal (10 mmol), catalyst (0.2 mmol), CO
2
urethane)/conversion 3 100. tert-Butylamine (5 mmol), ethanol (50 mmol), 2,2-diethoxypropane (10 mmol). d Reaction conditions: amine (10 mmol),
methanol (100 mmol), dimethyl carbonate (10 mmol), catalyst (0.2 mmol), CO 30 MPa, 200 °C, 24 h.
2
c
2238
Chem. Commun., 2001, 2238–2239
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106201h

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added to a mixture of tert-butylamine (10 mmol), a tin complex (0.2 mmol),
methanol (100 mmol), and biphenyl (50 mg, internal standard for GC
analysis) at room temperature. The initial pressure was adjusted to 30 MPa
at 200 °C and the autoclave was heated at that temperature for 24 h. After
cooling, product yield was determined by GC and the products were further
identified using GC-MS by the comparison of retention times and
fragmentation patterns with authentic samples.
6
1
Polyurethanes had a global demand of 5.1 3 10 tons per year in 1990
6
with an estimated demand of 8.5 3 10 tons per year in 2000. See,
Synthetic Polymers, Technology, Properties, Applications, ed. D.
Feldman and A. Barbalata, Chapman and Hall, London, 1996, p. 273.
P. T. Anastas and J. C. Warner, Green Chemistry, Theory and Practice,
Oxford University, Oxford, 1998.
2
3
4
M. Aresta and E. Quaranta, CHEMTECH, 1997, 32.
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Rev., 1995, 95, 259; P. G. Jessop, T. Ikariya and R. Noyori, Chem. Rev.,
2
Fig. 1 Pressure effect on the Bu SnO-catalyzed urethane synthesis from
cyclohexylamine, methanol and 2,2-dimethoxypropane (200 °C, 24 h).
1
999, 99, 475; D. A. Morgensten, R. M. LeLacheur, D. K. Morita, S. L.
Borkowsky, S. Feng, G. H. Brown, L. Luan, M. F. Gross, M. J. Burk and
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ACS Symposium Series 626, American Chemical Society, Washington,
DC, 1996, pp. 132–151; J. A. Darr and M. Poliakoff, Chem. Rev., 1999,
In order to obtain urethane in good yields, dense phase CO
2
under high pressure is necessary as shown in Fig. 1. The major
side reactions in Scheme 1 were (i) imine formation from
acetone and (ii) alkylation of amines by alcohols. The
improvement of the yield under a high pressure is assigned to
the depression of side reactions. Thus, high pressure promotes
carbamic acid formation (step a, Scheme 1). This will prevent
imine formation and amine alkylation resulting in improvement
of the urethane selectivity.
9
9, 495; W. K. Gray, F. R. Smail, M. G. Hitzler, S. K. Ross and M.
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A possible mechanism for urethane formation is the reaction
2
of alcohols with isocyanates formed from CO and primary
amines [eqn.(4)].17 The inactivity of secondary amines is
8
9
(
4)
1
1
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007.
consistent with this mechanism.18 In addition, a trace amount of
urea was detected in the reaction mixture indicating the
participation of isocyanates. On the other hand, the inter-
mediacy of dialkyl carbonate is not very likely because the
addition of dialkyl carbonate to the reaction mixture did not
improve the urethane yield (Table 1, run 6).
1
2 W. D. McGhee, B. L. Parnas, D. P. Riley and J. J. Talley, U.S. Pat.
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2
000, 151.
1
4 W. D. McGhee, D. P. Riley, M. E. Christ and K. M. Christ,
In conclusion, we have succeeded in the catalytic synthesis of
urethane directly from carbon dioxide, amines, and alcohols
through a halogen-free process. This demonstrates the potential
of carbon dioxide as an environmentally benign and easily
available phosgene alternative.
This study was supported by Industrial Technology Research
Grant Program in 2000 from the New Energy and Industrial
Technology Development Organization (NEDO) of Japan.
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18 Carbamic acids are easily formed from CO
1
Notes and references
†
All manipulations were carried out under purified argon. In a stainless
2
and secondary amines. See,
3
steel autoclave (20 cm inner volume), carbon dioxide (liquid, 6.5 MPa) was
F. Fichter and B. Becker, Ber., 1911, 44, 3481.
Chem. Commun., 2001, 2238–2239
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