Synthesis of Carbamates Using Yttria-Zirconia Based Lewis Acid Catalyst

Article abstract of DOI:10.1081/SCC-120026337

A variety of amines react with chloroformates in the presence of catalytic amount of yttria-zirconia based catalyst to afford the corresponding carbamates in excellent yields.

Full text of DOI:10.1081/SCC-120026337

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Synthesis of Carbamates Using Yttria-Zirconia Based
Lewis Acid Catalyst
a
b
b
a
Rajesh K. Pandey , Sharda P. Dagade , Mohan K. Dongare & Pradeep Kumar
a
Division of Organic Chemistry: Technology , National Chemical Laboratory , Pune, India
b
Catalysis Division , National Chemical Laboratory , Pune, India
Published online: 16 Aug 2006.
To cite this article: Rajesh K. Pandey , Sharda P. Dagade , Mohan K. Dongare & Pradeep Kumar (2003) Synthesis of Carbamates
Using Yttria-Zirconia Based Lewis Acid Catalyst, Synthetic Communications: An International Journal for Rapid Communication
of Synthetic Organic Chemistry, 33:23, 4019-4027, DOI: 10.1081/SCC-120026337
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Õ
SYNTHETIC COMMUNICATIONS
Vol. 33, No. 23, pp. 4019–4027, 2003
Synthesis of Carbamates Using Yttria-Zirconia
Based Lewis Acid Catalyst
1
Rajesh K. Pandey, Sharda P. Dagade,
Mohan K. Dongare, and Pradeep Kumar *
2
2
1,
1
Division of Organic Chemistry: Technology and
Catalysis Division, National Chemical Laboratory,
Pune, India
2
ABSTRACT
A variety of amines react with chloroformates in the presence of
catalytic amount of yttria-zirconia based catalyst to afford the
corresponding carbamates in excellent yields.
Key Words: Yttria-zirconia based Lewis acid; Heterogeneous
catalysis; Carbamate; Chemoselectivity.
*
Correspondence: Pradeep Kumar, Division of Organic Chemistry: Technology,
National Chemical Laboratory, Pune–411008, India; Fax: 0091-20-5893614;
E-mail: tripathi@dalton.ncl.res.in.
4
019
DOI: 10.1081/SCC-120026337
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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4
020
Pandey et al.
[
1]
Carbamates are endowed with an array of biological activities.
[
2a]
[2b]
[2c]
antibiotic,
They are known to have pesticide,
other pharmacological properties.They also serve as useful protecting
insecticide,
and
[
3]
groups in organic synthesis, particularly in peptide synthesis. Due to
their high demand, there has been a considerable resurgence of interest in
the synthesis and biological evaluation of carbamates. The carbamate
synthesis is generally performed by the reaction of an amine with chloro-
formate in the presence of bases such as sodium hydroxide, sodium
[
4]
[
3a]
bicarbonate, triethyl amine, or pyridine.
employed to prepare carbamates are the reaction of diethyl carbonate
A number of other methods
[5]
[6]
with amine, from amides, reductive carbonylation of aromatic nitro
[
compounds with Ru (CO) or Ru(CO) (PPh ) and methanol, enzy-
7]
3
12
3
3 2
[
8]
matic oxidative conversion of thio to oxo by Baker’s yeast, reaction of
[
9]
alcohol with trichloroacetyl isocyanate. More recently, the reaction of
amine with chloroformate using stoichiometric amount of activated
zinc and a three-component coupling reaction of amine, CO , and
2
alkyl halide in the presence of cesium carbonate and tetrabutylammonium
iodide have been reported for carbamate synthesis. Although there has
been several examples with regards to carbamate formation, the use of
Lewis acid catalyst to effect the above transformation is rather scarce.
[10]
[11]
[
12]
One of the method reported by Porta and Cenini used aliphatic amine
with diethyl carbonate and catalyzed the transformation with different
Lewis acids. However, this method could not be extended to the synthesis
of N-arylcarbamate.
As part of our research program aimed at developing new catalyst
and its subsequent application for various organic transformations, the
yttria-zirconia based Lewis acid was found to be an extremely efficient
[
13]
catalyst for the Diels-Alder reaction,
transesterification of ß-keto
[14]
[15]
esters and acylation reaction. This prompted us to use this catalyst
for carbamate synthesis and herein we report that a yttria-zirconia based
Lewis acid serves as an excellent catalyst for alkoxycarbonylation of
amines (Sch. 1).
Yttria-zirconia based
Lewis acid catalyst
1
R-NH + ClCOOR
1
R-NH-COOR + HCl
2
MeCN, rt
1
2
3
4
1
R = alkyl, aryl, amino acid; R = methyl, ethyl, benzyl
Scheme 1.

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Yttria-Zirconia Based Lewis Acid Catalyst
RESULTS AND DISCUSSION
4021
Thus, when a variety of amines were treated with chloroformates in the
presence of catalytic amount of the new yttria-zirconia based catalyst, the
corresponding carbamates were obtained in excellent yields. The sub-
strates examined in our studies and the results obtained are summarized
in Table 1. Thus, the present procedure for carbamate synthesis is quite
general as a wide range of structurally varied amines such as open chain,
cyclic, aromatic, heteroaromatic, amino acid underwent reaction
smoothly with chloroformates. The reaction is remarkably fast and leads
to high yields of the products. As summarized in Table 1, aromatic amines
underwent facile carbamation with a variety of chloroformates in excellent
yields. Subsequently, with the introduction of an electron withdrawing
substituent on the aromatic ring (i.e., carbonyl or nitro group), the
amine was rendered less nucleophilic; hence the reactions were sluggish;
however, an excellent yields of product were obtained (Table 1, Entries
11–12). Similarly the reaction of various amines with benzyl chloroformate
was found to be relatively slow and hence a little longer time was required
to complete the reaction; affording excellent yields of the products (Table 1,
Entries 5, 7, 16, 18). It is noteworthy that the reaction is chemoselective in
case of 2-aminophenol as the amine, being more nucleophilic than alcohol,
underwent reaction faster giving the corresponding N-carbamate product
in excellent yield (Table 1, Entry 10). Another notable feature of the
reaction is that even a secondary amine reacted smoothly to afford the
carbamate in high yield (Table 1, Entry 14). Mention must be made here
that amino group in amino acid could be protected easily demonstrating
the practical utility of this protocol particularly in peptide synthesis. Thus
L-phenylalanine ester was smoothly converted to the corresponding
carbamate in excellent yield (Table 1, Entry 15). Similarly in case of
L-tyrosine ester, the amino group is protected selectively in the presence
of hydroxyl group (Table 1, Entries 17, 18). Under the reaction conditions
employed, the ester groups remain unaffected and there is no racemization
of the chiral substrate encompassing amino acid. So this protocol could be
useful and further be extended to the peptidomimetic synthesis.
A time dependent study of carbamation of aniline with methyl
chloroformate in the absence and in the presence of varying concen-
tration of yttria-zirconia catalyst indicated that even small amount of
catalyst (10 wt%) can catalyse and accelarate the reaction, however,
high yields of carbamates and high efficiency of the reaction are found
only using catalyst (20 wt%). In the absence of the catalyst, the limited
carbamation occurred very slowly even when the reaction was continued
for 20 h. The reaction was performed even on large scale in the presence

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4
022
Pandey et al.
Table 1. Preparation of carbamates by yttria-zirconia based Lewis acid catalyst.
a
Entry
Amines
Reaction times (min)
Products
Yields
NH
2
NHCOOMe
1
2
5
5
94
88
1
a
3a
NH
2
NHCOOEt
1
b
3b
3c
NH
2
NHCOOEt
NHCOOMe
3
4
8
7
90
96
1
c
NH
2
2
1
d
3d
3e
NH
NHCOOBn
NHCOOEt
5
15
95
1
d
NH
NH
2
2
6
7
8
7
15
8
90
95
90
H
H
3
3
C
3
H C
1
e
e
3f
NHCOOBn
NHCOOMe
C
3
H C
1
3g
NH
2
1
f
3h
CH
3
CH
3
NH
2
NHCOOEt
9
5
5
94
94
90
90
94
MeO
MeO
1
g
3i
NH
2
NHCOOEt
1
1
1
1
0
1
2
3
OH
NH
OH
NHCOOEt
1
h
3j
2
360
355
15
O
2
N
O N
2
1
i
3k
NH
2
NHCOOEt
COCH3
COCH
3
1
j
3l
N
NH
NH
2
N
3m
NHCOOEt
NCOOEt
1
k
1
1
4
5
10
12
91
93
O
O
3
n
1
l
H
H
NH
2
NHCOOEt
COOCH
3
3
COOCH
3
1
m
3o
3p
H
H
1
1
1
6
7
8
NH2
18
15
20
NHCOOBn
92
92
91
COOCH
COOCH
3
1
m
H
H
NH2
NHCOOEt
COOCH3
COOCH
3
HO
HO
HO
HO
1
n
n
3q
H
H
NH
2
NHCOOBn
COOCH
3
COOCH3
1
3r
a
All the product exhibited physical and spectral (NMR and IR) properties in
accord with the assigned structure.

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Yttria-Zirconia Based Lewis Acid Catalyst
4023
of catalyst and the results could be reproduced. The anhydrous HCl
generated during reaction evolves in the form of gas which has been
experimentally detected by trapping it into the ammonia solution.
Thus, the HCl gas evolved was driven away in order to avoid the
formation of any amine hydrochloride.
The heterogenous catalytic method described here is profitable as it
provides high yields of the product and does not involve the use of
stoichiometric amount of reagent such as base for carbamate synthesis.
Thus, the present protocol could be useful particularly for those
substrates containing base sensitive functionalities. In addition, the
reaction conditions are particularly mild and the work-up procedure is
exceedingly simple and reduced to a mere filtration. Another significant
advantage of the present method lies in the simplicity involved in the
preparation of the catalyst. The recovered catalyst can be reactivated for
ꢀ
reuse by heating it at 500 C in the presence of air. The same catalyst was
recycled for all the reactions without loss of activity and selectivity.
In summary, a facile heterogeneous catalytic method for the
preparation of a variety of carbamates employing a novel yttria-zirconia
based Lewis acid catalyst has been developed. To the best of our
knowledge this is the first report of use of Lewis acid catalyst for carbamate
synthesis from amine using chloroformate. Furthermore, the present
protocol offers mild reaction conditions, selectivity, and short reaction
times. The noteworthy feature of this methodology is that chiral substrate
is resistant to racemization and labile functionalities such as an ester is
compatible with the reaction condition. In addition, the heterogeneous
catalytic method is advantageous in respect of easy separation, higher
and consistent yield, and recyclability of the catalyst. Thus the present
catalytic method should offer a general synthetic method for various
carbamates offering a wide variety of application.
EXPERIMENTAL
Solvents were purified and dried by standard procedures before use
according to reported procedure; petroleum ether of boiling range
ꢀ
6
0–80 C was used. The amines and chloroformates were obtained from
commercial sources and were purified by distillation/recrystallization
before the experiment. Infrared spectra were recorded with ATI
MATT-SON RS-1 FT-IR spectrometer. Proton NMR spectra were
recorded on Bruker AC-200 machine in CDCl with TMS as internal
3
standard. The diffractogram of X-ray powder diffraction pattern
was recorded on a Rigaku diffractometer model D/Max. IIIVC with

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4
024
Pandey et al.
Ni-filtered Cu-Ka radiation. FTIR spectrum of pyridine adsorbed on the
yttrium-based catalyst was recorded on a Nicolet 60 SXB FTIR
spectrometer. TPD profile (ammonia) of the yttrium-based catalyst was
recorded on a Sorbstar apparatus. Determination of specific surface area
was carried out by BET (Brunner-Emmett-Teller) N adsorption using a
2
Omnisorp 100 CX apparatus.
Procedure for the Preparation of Catalyst
The catalyst was prepared by mixing aq. solutions of yttrium nitrate
and zirconyl nitrate in the mole ratio 16:84, to which aqueous ammonia
(
and a precipitate was formed. Washing with deionized water, drying at
28%) was added under vigorous stirring until a pH of 8.5 was achieved
o
10 C for 24 h, treating with sulfuric acid (4 M), drying at 120 C, and
ꢀ
1
subsequent programmed calcination at 500 C for 3 h at a heating rate of
ꢀ
ꢀ
ꢁ1
2
C min resulted in a highly acidic material. The chemical composition
of the final catalyst (determined by XRF technique) was found to
be 82.6 mol% Zr, 15.6 mol% Y, and 1.8 mol% S. The physicochemical
characterization of the catalyst was carried out by titrations, temperature
programmed desorption (TPD), scanning electron microscopy (SEM),
and N adsorption techniques.
2
Typical Experimental Procedure
In a typical experimental procedure, chloroformate (20 mmol) in dry
acetonitrile (15 mL) was added dropwise to a solution of amine (20 mmol)
in dry acetonitrile (25 mL) containing catalyst (20% by weight) with
constant stirring and the mixture was stirred at room temperature for
the indicated length of time (Table 1). The reaction was monitored by
TLC. After completion of reaction, the catalyst was filtered and the solid
was washed thoroughly with ether (100 mL). The combined filtrate was
washed with 10% sodium bicarbonate solution, then water, brine,
and dried over anhydrous sodium sulfate. Evaporation of solvent
gave the crude product which was purified by silica gel chromatography
using petroleum ether:ethyl acetate (97:03) as eluent to give the pure
product.
ꢁ
1
Butylcarbamic acid methyl ester (3a). Yellow oil. IR ꢀmax/cm
1
(
Neat): 3323, 2964, 1710, 1520, 1216. H NMR (200 MHz, CDCl ) ꢁ:
3
0
2
.90 (t, J ¼ 6.5, 3H), 1.25–1.50 (m, 4H), 1.75 (bs, 1H), 3.20 (t, J ¼ 6.5,
H), 3.15 (s, 3H).

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Yttria-Zirconia Based Lewis Acid Catalyst
4025
ꢀ
Cyclohexylcarbamic acid ethyl ester (3b). White solid. M.p.: 60 C. IR
ꢁ1
1
ꢀmax/cm (Neat): 3323, 2962, 1730, 1546, 1235. H NMR (200 MHz,
CDCl ) ꢁ: 1.11 (t, J ¼ 7.5, 3H), 1.15–2.25 (m, 10H), 3.15 (m, 1H), 3.45
3
(
bs, 1H), 4.10 (q, J ¼ 5.6, 2H).
ꢀ
Benzylcarbamic acid ethyl ester (3c). White solid. M.p.: 55 C. IR
ꢁ1
1
ꢀmax/cm (Nujol): 3450, 3018, 1711, 1516, 1216. H NMR (200 MHz,
CDCl ) ꢁ: 1.10 (t, J ¼ 7.5, 3H), 3.90 (q, J ¼ 5.6, 2H), 4.15 (d, J ¼ 5.3,
3
2
H), 7.05–7.50 (m, 5H), 8.50 (bs, 1H).
Phenylcarbamic acid methyl ester (3d). White solid. M.p.: 50 C. IR
ꢀ
ꢁ1
1
ꢀmax/cm (Nujol): 3291, 2950, 1730, 1543, 1216. H NMR (200 MHz,
CDCl ) ꢁ: 3.50 (s, 3H), 6.70 (bs, 1H), 7.10–7.40 (m, 5H).
3
ꢀ
Phenylcarbamic acid benzyl ester (3e). White solid. M.p.: 80 C. IR
ꢁ1
1
ꢀmax/cm (Nujol): 3431, 3019, 1731, 1525, 1214. H NMR (200 MHz,
CDCl ) ꢁ: 5.07 (s, 2H), 6.85–7.38 (m, 10H), 8.31 (bs, 1H).
3
ꢀ
p-Tolylcarbamic acid ethyl ester (3f). White solid. M.p.: 63 C. IR
ꢁ
1
1
(Nujol): 3318, 2981, 3019, 1706, 1529, 1229. H NMR
ꢀmax/cm
(
2
200 MHz, CDCl ) ꢁ: 1.25 (t, J ¼ 7.2, 3H), 2.60 (s, 3H), 4.20 (q, J ¼ 5.6,
3
H), 6.60 (bs, 1H), 7.10 (d, J ¼ 5.45, 2H), 7.25 (d, J ¼ 5.45, 2H).
ꢀ
p-Tolylcarbamic acid benzyl ester (3g). White solid. M.p.: 140 C. IR
ꢁ1
1
ꢀmax/cm (CHCl ): 3431, 3018, 1724, 1526, 1217. H NMR (200 MHz,
3
CDCl ) ꢁ: 2.32 (s, 3H), 5.21 (s, 2H), 6.68 (bs, 1H), 7.1–7.5 (m, 9H).
3
ꢀ
m-Tolylcarbamic acid methyl ester (3h). White solid. M.p.: 50 C. IR
ꢁ
1
1
ꢀmax/cm (CHCl ): 3430, 2922, 1713, 1457, 1226. H NMR (200 MHz,
3
CDCl ) ꢁ: 2.30 (s, 3H), 3.15 (s, 3H), 6.68 (bs, 1H), 7.10–7.80 (m, 4H).
3
(
4-Methoxyphenyl)carbamic acid ethyl ester (3i). White solid. M.p.:
ꢀ
ꢁ1
5–86 C. IR ꢀmax/cm (CHCl ): 3313, 2921, 1697, 1458, 1241. H NMR
3
1
8
(
2
200 MHz, CDCl ) ꢁ: 1.20 (t, J ¼ 7.25, 3H), 3.60 (s, 3H), 4.10 (q, J ¼ 5.9,
3
H), 6.70 (bs, 1H), 6.91 (d, J ¼ 6, 2H), 7.20 (d, J ¼ 6, 2H).
2-Hydroxyphenyl)carbamic acid ethyl ester (3j). White solid. M.p.:
(
0 C. IR ꢀmax/cm (CHCl ): 3224, 2921, 1700, 1456, 1231. H NMR
ꢀ
ꢁ1
1
8
3
(
(
200 MHz, CDCl ) ꢁ: 1.35 (t, J ¼ 7.16, 3H), 4.28 (q, J ¼ 5.9, 2H), 6.80
3
bs, 2H), 6.90–7.25 (m, 4H).
(
ꢀmax/cm (CHCl ): 3221, 2921, 1730, 1458, 1230. H NMR (200 MHz,
4-Nitrophenyl)carbamic acid ethyl ester (3k). Yellow solid. IR
1
ꢁ1
3
CDCl ) ꢁ: 1.34 (t, J ¼ 7.2, 3H), 4.28 (q, J ¼ 5.9, 2H), 7.06 (s, 1H), 7.80
3
(
d, J ¼ 8.9, 2H), 8.20 (d, J ¼ 8.9, 2H).
(
ꢀmax/cm
2-Acetoxyphenyl)carbamic acid ethyl ester (3l). Yellow oil. IR
1
ꢁ1
(CHCl ): 3446, 3018, 1726, 1705, 1582, 1220. H NMR
3
(
(
200 MHz, CDCl ) ꢁ: 1.34 (t, J ¼ 7.25, 3H), 2.64 (s, 3H), 4.28
3
q, J ¼ 5.9, 2H), 6.80 (bs, 1H), 7.10–7.88 (m, 4H).
ꢀ
Pyridin-2-yl-carbamic acid ethyl ester (3m). White solid. M.p.: 100 C.
(CHCl ): 3446, 3220, 3019, 1726, 1705, 1585, 1217.
3
ꢁ
1
IR ꢀmax/cm

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4
026
Pandey et al.
1
H NMR (200 MHz, CDCl ) ꢁ: 1.35 (t, J ¼ 7.25, 3H), 4.25 (q, J ¼ 5.9,
3
2
H), 6.60 (bs, 1H), 7.0 (t, J ¼ 6.6, 1H), 7.75 (t, J ¼ 6.6, 1H), 8.05
(
d, J ¼ 6.8, 1H), 8.5 (d, J ¼ 5.3, 1H).
Morpholine-4-carboxylic acid ethyl ester (3n). Colorless oil. IR
1
ꢁ
1
ꢀmax/cm
(CHCl ): 2927, 1695, 1454, 1103. H NMR (200 MHz,
3
CDCl ) ꢁ: 1.2 (t, J ¼ 7.25, 3H), 3.25 (m, 4H) 3.5 (m, 4H), 4.25
3
(
q, J ¼ 5.9, 2H).
-Ethoxycabonylamino-3-phenylpropionic acid methyl ester (3o).
2
White solid. M.p. 110 C. IR ꢀmax/cm
1
3
7
ꢀ
ꢁ1
(CHCl ): 3381, 2954, 1742,
3
1
708, 1526, 1225. H NMR (200 MHz, CDCl ) ꢁ: 1.25 (t, J ¼ 7.25, 3H),
3
.10 (m, 2H), 3.75 (s, 3H), 4.20 (q, J ¼ 5.9, 2H), 4.74 (s, 1H), 5.20 (m, 1H),
.05–7.50 (m, 5H).
2-Benzyloxycabonylamino-3-phenylpropionic acid methyl ester (3p).
White solid. M.p.: 190 C. IR ꢀmax/cm (CHCl ): 3381, 2954, 1742,
ꢀ
ꢁ1
3
1
708, 1526, 1225. H NMR (200 MHz, CDCl ) ꢁ: 3.10 (m, 2H) 3.75
1
3
(
s, 3H), 4.74 (s, 1H), 5.20 (s, 2H), 5.25 (m, 1H), 7.05–7.50 (m, 10H).
-Ethoxycarbonylamino-3-(4-hydroxyphenyl)propionic acid methyl
2
ester (3q). White solid. IR ꢀmax/cm (CHCl ): 3381, 2954, 1745, 1708,
1
(
(
ꢁ
1
3
1
528, 1220. H NMR (200 MHz, CDCl ) ꢁ: 1.25 (t, J ¼ 7.25, 3H), 3.10
3
m, 2H) 3.75 (s, 3H), 4.20 (q, J ¼ 5.9, 2H), 4.74 (s, 1H), 5.20 (m, 1H), 6.90
s, 1H), 7.01–7.50 (m, 4H).
2-Benzyloxycabonylamino-3-(4-hydroxyphenyl)propionic acid methyl
ester (3r). White solid. M.p.: 200 C. IR ꢀmax/cm (CHCl ): 3380, 2950,
ꢀ
ꢁ1
3
1
740, 1702, 1522, 1220. H NMR (200 MHz, CDCl ) ꢁ: 3.10 (m, 2H) 3.75
s, 3H), 4.74 (m, 1H), 5.22 (s, 2H), 5.25–5.56 (m, 1H), 7.10–7.53 (m, 10H).
1
(
3
ACKNOWLEDGMENTS
RKP thanks CSIR, New Delhi for award of senior research fellowship.
We are grateful to Dr. M. K. Gurjar, Head, Organic Chemistry:
Technology Division for his constant encouragement and support. This
is NCL Communication No. 6647.
REFERENCES
1
2
. (a) The Agrochemicals Handbook, 2nd Ed.; Royal society of
chemistry. 1987; (b) Barthlemy, J.L. Pharm. 1986, 37 (6), 297.
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Received in India June 24, 2003

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