The syntheses of carbamates from reactions of primary and secondary aliphatic amines with dimethyl carbonate in ionic liquids

Article abstract of DOI:10.1016/S0040-4039(02)01943-3

At 170°C and using ionic liquids as solvent and catalyst, primary and secondary aliphatic amines could react with dimethyl carbonate to give alkyl carbamates with good yield. Due to its insolubility, the desired carbamate solid could be recovered by simpl

Full text of DOI:10.1016/S0040-4039(02)01943-3

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TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 8145–8147
The syntheses of carbamates from reactions of primary and
secondary aliphatic amines with dimethyl carbonate
in ionic liquids
Tianlong Sima, Shu Guo, Feng Shi and Youquan Deng*
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, China
Received 5 June 2002; revised 4 September 2002; accepted 6 September 2002
Abstract—At 170°C and using ionic liquids as solvent and catalyst, primary and secondary aliphatic amines could react with
dimethyl carbonate to give alkyl carbamates with good yield. Due to its insolubility, the desired carbamate solid could be
recovered by simple filtration from the two-layer mixture of dimethyl carbonate and ionic liquid after reaction and the formation
of N-methylated and deammoniated products were restrained. © 2002 Published by Elsevier Science Ltd.
Carbamates are important intermediates in the synthe-
sis of pharmaceuticals, pesticides, herbicides and other
fine and commodity chemicals.1,2 Furthermore, the
development of an environmentally friendly process for
synthesis of isocyanates, which are usually obtained by
phosgenation of amines, through thermal decomposi-
tion of carbamates has also attracted extensive
attention.3–6
reported with excellent performance.14–17 Based on the
extensive studies on ionic liquids in our laboratory,18–20
they were firstly used in the carbonylation of aliphatic
amines with dimethyl carbonate. High yield was
achieved and the formation of deammoniated, N-
methylated byproducts was restrained (Scheme 1).
The following ionic liquids were used in the reactions:
BMImCl
(BMIm=1-butyl-3-methyl);
BMImBF4;
Now, many alternative routes, such as catalytic reduc-
tive carbonylation of nitro compounds, oxidative car-
bonylation of amines, reaction of amines with carbon
dioxide and alkyl halides, etc., with low environmental
impact have been studied.7–10 A phosgene and halogen
free process through reaction of amines and dimethyl
carbonate was put forward in the last few years. No
high selectivity could be achieved if no solvents and
catalysts were added even if this reaction could easily
occur. As reported previously, Pb and alumina based
catalysts were effective for this reaction.11,12 More
recently, Selva et al. reported that good results could be
obtained in the presence of scCO2, but its pressure was
relatively high.13
BMImPF6; BMImHSO4; BuPyBF4 (BuPy=1-butyl pyr-
idine); EMImBF4 (EMIm=1-ethyl-3-methyl imida-
zole); HMImBF4 (HMIm=1-cetyl-3-methyl imidazole);
and BeMImBF4 (BeMIm=1-benzyl-3-methyl imida-
zole). All ionic liquids were synthesized according to
the previous paper with slight modifications.21 The
purity of prepared ionic liquids was characterized with
FTIR (IFS 120HR, Bruker), and satisfactory results
were obtained.
In order to avoid the reaction between amines and
carbon dioxide in air, all reactions were conducted in a
90 ml autoclave with a glass tube inside equipped with
magnetic stirring. In each reaction, 2 ml of ionic liquid,
2 ml of amines and 5 ml of dimethyl carbonate were
introduced in turn without any additional organic sol-
Room temperature ionic liquids, a new kind of green
solvent and catalyst with unique chemical and physical
properties, have attracted growing interest, and many
catalytic reactions proceeded in ionic liquids were
* Corresponding author. Fax: +86-931-8277088; e-mail: ydeng@
ns.lzb.ac.cn
Scheme 1.
0040-4039/02/$ - see front matter © 2002 Published by Elsevier Science Ltd.
PII: S0040-4039(02)01943-3

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8146
T. Sima et al. / Tetrahedron Letters 43 (2002) 8145–8147
vent and catalyst. The reaction was proceeded at 170°C
for 1 h. If the conversion and selectivity were high
enough, large amounts of solid product would be
formed in dimethyl carbonate layer (a two-layer mix-
ture of dimethyl carbonate and ionic liquid formed
after reaction), and the carbamate products could be
more efficiently recovered by simple filtration after
removal of residual dimethyl carbonate and methanol
with distillation and then addition of water to ‘precipi-
tate’ the products due to their almost complete insolu-
bility in water. However, in order to achieve accurate
data of the reaction, a moderate amount of acetone was
added into the resulted two-layer mixture to obtain a
homogeneous solution after the reaction. Qualitative
and quantitative analyses were conducted with a HP
6890/5973 GC MS and all results were given by the
chemstation directly.
employed as solvent and catalyst, and selectivity of
82.7% to N-cyclohexyl carbamate was obtained. Only
0.46% of cyclohexene, and 2.9% of N,N-dimethyl cyclo-
hexyl amine were found while the amount of N-cyclo-
hexyl-N-methyl carbamate was 14.2%. Several other
ionic liquids with different anions were used to investi-
gate the effect of anions on the reactions (entries 2–5).
Similar results were obtained in comparison with
BMImCl results, and the selectivities to N-cyclohexyl
carbamate were 78.5 and 80.6% when BMImBF4 and
BMImPF6 were employed. Unexpectedly, only 46.9%
of N-cyclohexyl carbamate was given, even no cyclo-
hexene could be detected, and 38.5% of selectivity to
N-cyclohexyl-N-methyl carbamate was produced if
BMImHSO4 was tested. This result indicates that the
−
HSO4 containing BMImHSO4 might have some detri-
mental effect on the reaction between amine and
dimethyl carbonate.
In the reusability experiment of the ionic liquids, the
used ionic liquid was reused after it was easily sepa-
rated from the resulted water containing percolate by
vacuum purification at 100°C and ca. 5 mmHg.
Ionic liquids with different cations were also employed
in the reactions and exhibited quite different results
(entries 6–10). The experimental results suggested that
the chain length of substituted group of imidazole had
a very strong impact on the catalytic activity of ionic
liquids, and 84.7% of selectivity to N-cyclohexyl carba-
mate was achieved in EMImBF4, which was higher
than that using BMImBF4 as solvent and catalyst. With
increasing carbon number of the substituents, more
N-methylated product was produced and its selectivity
reached 60.8% in HMImBF4 while only 23.3% of selec-
tivity to desired product. This implies that ionic liquids
could be suitable reaction media and catalysts for the
formation of dimethylated compounds from the reac-
tions between amines and dimethyl carbonate under
proper reaction conditions. With BeMImBF4 ionic liq-
Firstly, cyclohexyl amine as substrate was selected to
test the catalytic activities of different kinds of ionic
liquids because it is relatively difficult to react with
dimethyl carbonate and the diversity of its products. As
is shown in Table 1, cyclohexyl amine could easily react
with dimethyl carbonate but poor selectivity (33.4%)
could be obtained in the absence of ionic liquids (entry
1). The main byproducts were cyclohexene (16.8%),
N,N-dimethyl cyclohexyl amine (39.6%) and N-cyclo-
hexyl-N-methyl carbamate (10.2%). The formation of
cyclohexene and N,N-dimethyl cyclohexyl amine could
be, however, evidently restrained when BMImCl was
Table 1. Results of carbonylation of amines with dimethyl carbonate

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T. Sima et al. / Tetrahedron Letters 43 (2002) 8145–8147
8147
uid, high selectivity to N-cyclohexyl carbamate was
given, and no N-methylated product could be observed.
In addition, EMImBF4 was also selected to investigate
the reusability of ionic liquid as solvent and catalyst,
85.3% of selectivity was maintained after it was reused
the fifth time. This result showed that the reusability of
the ionic liquid as solvent and catalyst for the reactions
of aliphatic amines with dimethyl carbonate was
possible.
carbonate to yield the desired carbamate products with-
out any additional solvents and catalysts. The N-
methylated products could be produced if appropriate
ionic liquids and reaction conditions were chosen,
which was unexpectedly found in this work for the first
time. Due to its lower solubility in dimethyl carbonate
and the ionic liquid saturated with dimethyl carbonate
and almost complete insolubility in water, the desired
carbamate solid could be efficiently recovered through
simple treatment with distillation, water addition and
filtration after reaction, and the ionic liquid could be
reused without losing activity. Further investigation
about the reaction mechanism and optimization of the
ionic liquid as reaction medium with catalytic function
is now underway.
It is worth noting that no mono-N-methylated product
was found in the above reaction processes. The reason
might be attributed to easy transformation of mono-N-
methylated product to dimethylated or carbonylated
products.
The ionic liquid as solvent and catalyst also showed
high activity for the reactions of unbranched alkyl
substituted and alkyl disubstituted amines with
dimethyl carbonate. Much better results were achieved
if n-butyl amine, n-hexyl amine, dibutyl amine and
N-benzyl-N-methyl amine were used as substrates in
the presence of BMImCl ionic liquid (entries 11–14),
and the selectivities to corresponding carbamates were
higher than 99%.
References
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As indicated above, if the conversion and selectivity
were high enough, large amounts of solid product
would be formed due to lower solubility of carbamates
in dimethyl carbonate and ionic liquids. For entry 6, ca.
80% of isolated yield (desired carbamate+N-cyclohexyl-
N-methyl carbamate) could be achieved by direct filtra-
tion after reaction. However, the carbamate products
could be more efficiently recovered by simple filtration
after removal of residual dimethyl carbonate and
methanol with distillation and then addition of water to
‘precipitate’ the products due to their almost complete
insolubility in water, and 94.7 and 97.3% of isolated
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examined using BMImCl as solvent and catalyst
(entries 15–16). At 120°C, only 23.7% of conversion
was obtained when cyclohexyl amine was used as sub-
strate, although its selectivity was further increased to
95%. On the contrary, high conversion and selectivity
(87.4% and >99%) was still maintained for n-butyl
amine as substrate. These results suggested that
unbranched alkyl substituted amines could react with
dimethyl carbonate for desired products at lower tem-
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In summary, ionic liquids were effective solvent and
catalyst to mediate the clean carbonylation reactions of
primary and secondary aliphatic amines with dimethyl