Carbamate synthesis from amines and dialkyl carbonate over inexpensive and clean acidic catalyst-Sulfamic acid

Article abstract of DOI:10.1016/j.cclet.2010.03.012

Sulfamic acid has been proved to be the most efficient and recyclable catalyst in carbamate synthesis from alkylamine and dialkyl carbonate. High selectivity, cost-efficiency and simple product separation were the advantageous features obtained in this process. Sulfamic acid could be reused several times and keep its initial activity in the recycle runs. In addition, sulfamic acid has also exhibited the potential catalytic ability for alkylation of aromatic amines.

Full text of DOI:10.1016/j.cclet.2010.03.012

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 794–797
www.elsevier.com/locate/cclet
Carbamate synthesis from amines and dialkyl carbonate over
inexpensive and clean acidic catalyst—Sulfamic acid
a
a,b
Bo Wang a, , Jing He , Run Cang Sun
*
a Institute of Biomass Chemistry and Technology, College of Material Science and Technology, Beijing Forestry University, Beijing 100083, China
b State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China
Received 6 November 2009
Abstract
Sulfamic acid has been proved to be the most efficient and recyclable catalyst in carbamate synthesis from alkylamine and
dialkyl carbonate. High selectivity, cost-efficiency and simple product separation were the advantageous features obtained in this
process. Sulfamic acid could be reused several times and keep its initial activity in the recycle runs. In addition, sulfamic acid has
also exhibited the potential catalytic ability for alkylation of aromatic amines.
# 2010 Bo Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Sulfamic acid; Acidic catalysis; Carbamate; Alkylamine; Dimethyl carbonate
Carbamates are important intermediates in the synthesis of pharmaceuticals, pesticides, fungicides, herbicides, fine
and commodity chemicals as well as polyurethane [1]. The development of an environmentally friendly process for
synthesis of carbamates has also attracted extensive attention due to worldwide awareness of pollution hazards of
phosgene [2]. Accordingly, much effort has been continuously made for the replacement of the phosgene route for
preparing carbamates, such as, the oxidative and reductive carbonylation of amines with CO in the presence of
transition metal catalysts, even with photocatalysis [3–5]. However, these routes were always suffered from the
hazards in handling of carbon monoxide under high pressure. In recent years, the replacement of CO with carbon
dioxide for the carbamate synthesis which usually promoted with Cs2CO3 [6] and sieves [7], etc., as catalysts, has
received much attention from research workers mainly due to its non-hazardous nature and safety in handling under
ambient pressure. In addition, the carbamate synthesis could be performed through some peculiar methods such as
Hofmann rearrangement from amides [8,9], which however have the limitations such as formation of viscous black tar
or inorganic salts as by-products.
Very recently, the carbamate synthesis from dialkyl carbonates and amines in the presence of a series of catalysts,
such as lead(II) nitrate [10], molecular sieve [11], and ionic liquids [12], etc., has been reported consequently. The
usage of dialkyl carbonates as the alternative to phosgene or chloroformate provided an environmental friendly choice,
because the only by-product of this reaction is alcohol, which could be removed easily. However, these well-known
catalysts were still restrained from low efficiency or high cost [10–12]. Additional blemish was the poor reusability
* Corresponding author.
E-mail address: mzlwb@bjfu.edu.cn (B. Wang).
1001-8417/$ – see front matter # 2010 Bo Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.03.012

B. Wang et al. / Chinese Chemical Letters 21 (2010) 794–797
795
Scheme 1. Synthesis of carbamate from cyclohexylamine and DMC in the presence of sulfamic acid.
when using a homogeneous catalyst in the reaction system. In some cases, the promoters were also deleterious for
human health, and consequently, a tedious post-treatment has to be necessary on account of environmental problem.
Therefore, it appeared that a cheap, safe, effective solid acidic catalyst would be an attractive channel for the
carbamate synthesis from dialkyl carbonates and amines.
In the past few years, research and development on cheap green acidic catalysts received much attention due to the
pollution impact on environment as well as the consideration from the viewpoint of eco-efficiency, and many known
and commercial available solid acidic materials such as montmorillonite K10 [13], SiO2-sopported NaHSO4 and
KHSO4 [14], organic polyaniline salts [15] have been extensively utilized in organic and catalytic reactions to meet the
requirement of cost–effective property. The alternative of acidic materials or systems for traditional acid catalytic
chemistry was targeted for development of highly productive, environmentally safe, recyclable techniques, which can
be promoted to large-scale applications. Recently, utilizations of sulfamic acid (H2NSO3H), in catalytic and organic
reactions as an alternative for conventional acidic materials have been reported due to its unique characters (white
crystalline solid with an outstanding physical property and stability, nonvolatility, nonhygroscopy, nontoxicity)
[16,17]. With the information in hand, we have ever kept our eyes on the sulfamic acid as acidic catalyst for Beckmann
rearrangement, transesterification, etc. [18–20], herein, we took a further step for the investigation of sulfamic acid
catalyzed carbamate synthesis (Scheme 1).
At first, we investigated comparatively the activities of some representatives of cheap acidic catalysts, sulfamic
acid, NaHSO4, KHSO4 and Montmorillonite K10, which are, nonvolatile, noncorrosive, recyclable, and very
popularly studied recently in green catalysis, over the carbamate synthesis from cyclohexylamine and DMC,
respectively, and the results are listed in Table 1. Initially, the reaction between cyclohexylamine and DMC gave rise to
only 24% of N-cyclohexyl methyl carbamate in the absence of catalyst, which pointed out the necessity of catalyst
efficiency (entry 5). From the catalysts screened, these solid catalysts all showed moderate yield except for sulfamic
acid catalyst under the same conditions. The reaction was promoted smoothly by sulfamic acid to produce 85% of
desired product in the presence of 10% catalyst amount (entry 1). With this in hand, it is preferable that the following
experiments would be carried out using sulfamic acid as acidic catalyst for the carbamate synthesis.
Consequently, the popularization of sulfamic acid for carbamates synthesis as catalyst was screened and the results
were summarized in Table 2. With the increasing worldwide interest in green chemistry, the eco-efficiency should be an
importantparametertoanindustrial process.Therefore, thecatalystamountwasaddedtwiceinordertoimprovetheyield
of carbamates (entry 1), associated with the preferable low-cost and reusability features of H2NSO3H. From the data in
Table 2 we can find that when using diethyl carbonate (DEC) to replace DMC as reactant, the yield of carbamate formed
from cyclohexylamine arrived to 88%, indicating the superior reactivity of DMC to DEC (entry 2). Obviously, the
reactions derived from primary amines provided relative higher yields (entries 4 and 6) than secondary amines (entries 5
and 7), which is consistent with their general reactivity order [12]. Furthermore, the synthesis of dialkyl hexamethylene-
1,6-dicarbamate from 1,6-hexamethylenediamine could be also accomplished successfully with 91% of yield.
Table 1
Activities comparison of some cheap acidic catalysts for cyclohexyl carbamate synthesis from cyclohexylamine and DMCa.
Entry
Catalyst
Content (mol.%)
Time (h)
Yield (%)b
1
2
3
4
5
Sulfamic acid
KHSO4
10
10
10
10c
8
8
85
35
58
57
24
NaHSO4
8
Montmorillonite K10
No catalystd
10
8
a
Reaction condition: cyclohexylamine (24 mmol), DMC (120 mmol), 100 8C.
GC yield.
b
c
d
wt%.
Reaction without catalyst.

796
B. Wang et al. / Chinese Chemical Letters 21 (2010) 794–797
Table 2
Synthesis of carbamates from alkylamines with dialkyl carbonates catalyzed by H2NSO3Ha.
.
Entry
Amines
Carbonates
Products
Yield (%)c
1
2
3b
4
5
6
7
8
9
R1 = cyclohexyl, R2 = H
R1 = cyclohexyl, R2 = H
R1 = cyclohexyl, R2 = H
R1 = n-C9H19, R2 = H
R1 = R2 = n-C4H9
DMC
DEC
Methyl cyclohexylcarbamate
Ethyl cyclohexylcarbamate
Methyl cyclohexylcarbamate
Methyl decylcarbamate
95
88
94
94
88
94
87
88
91
DMC
DMC
DMC
DMC
DMC
DMC
DMC
Methyl dibutylcarbamate
R1 = benzyl, R2 = H
Piperidine
Methyl benzylcarbamate
Methyl piperidine-1-carboxylate
Methyl butylcarbamate
R1 = n-C4H9, R2 = H
1,6-Hexamethylenediamine
Dimethyl hexamethylene-1,6-dicarbamate
a
Reaction condition: amine (24 mmol), dialkyl carbonate (120 mmol), sulfamic acid (4.8 mmol), 100 8C, 8 h.
Catalyst was reused in fourth time.
b
c
Isolated yield.
It is worthy noted that all the products listed in Table 2 could be simply separated by the common procedure. Taking
the reaction between cyclohexylamine and DMC for example, DMC and produced methanol could be recycled by
distillation after reaction; and then, sulfamic acid could be precipitated and recycled by filtration through adding the
anti-solvent CH2Cl2 due to the negligible solubility of SA in CH2Cl2; the filtrate was then evaporated to give pure
carbamate. Cyclohexyl carbamate was obtained with 95% of yield by this procedure (entry 1). Because H2NSO3H has
an excellent stability, it could be reused four times without obvious loss of activity in the reaction between
cyclohexylamine and DMC (entry 3).
The reactivities of aromatic amines for the synthesis of carbamate were also evaluated using sulfamic acid as
catalyst under the same conditions. However, the expected results were not obtained because the N-alkylation of
aromatic amines was found to be the predominant reaction as shown in Table 3. Take the reaction between aniline and
DMC for example (entry 1), when 10% aniline was converted, the alkylation products accounted for 95% of whole
Table 3
Sulfamic acid catalyzed reactions of aromatic amines with DMCa.
Entry Substrates
Con. (%)b Selectivity to products (%)c
II
I
III
IV
1
10
87
8
4
8
1
1
2
12
75
16
d
3
5
99
–
–
1
–
–
a
Reaction condition: aromatic amine (24 mmol), DMC (120 mmol), sulfamic acid (4.8 mmol), 100 8C, 8 h, all data were achieved from GC/MS.
b
c
d
Conversion of aromatic amine.
Selectivity to different products.
No product was detected.

B. Wang et al. / Chinese Chemical Letters 21 (2010) 794–797
797
products (N-methylaniline 87% (I) and N,N-dimethylaniline 8% (II)), the carbamate products were obtained on 5%.
As for the reaction of secondary aromatic amine with carbonate, no carbamate product was detected (entry 3). These
results indicated that sulfamic acid performed effective selectivity for the reaction between aliphatic amine with DMC,
but failed for aromatic amines. However, we can also change the mind from a different vision that there maybe
possible that sulfamic acid could serve as a specific catalyst for the alkylation of aromatic amine through varying or
modifying reaction conditions, where the prospect of research and development about sulfamic acid deserved in the
further investigation.
In summary, sulfamic acid has been proved to be an efficient catalyst for the synthesis of carbamates from aliphatic
amines and DMC carbonates. Compared with classical methods, the reaction using sulfamic acid as catalyst has
exhibited simple product isolation procedure, improved yield and exclusive selectivity. The alkyl carbamates could be
successfully separated from sulfamic acid catalyst by the use of dichloromethane as a precipitation solvent. Recovery
and reuse of sulfamic acid are also satisfactory. At the same time, the alkylation of aromatic amine could be also
catalyzed potentially by sulfamic acid, which demonstrated the reactivity-specific and green aspect of sulfamic acid.
Acknowledgments
Financial supports from the Beijing Forestry University Excellent Young Scientist Fund (No. BLYX200904),
National Innovation Experiment Program for University Students (No. 091002234) and Major State Basic Research
Development Program of China (973 Program, No. 2010CB732204) are gratefully acknowledged. We wish to thank
Prof. Run-Hua Lu and Prof. Li-Ming Yang for much useful criticism during the preparation of this paper and for many
interesting and helpful discussions.
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