An easy base-assisted synthesis of unsymmetrical carbonates from alcohols with dimethyl carbonate

Article abstract of DOI:10.1007/s00706-013-1123-3

A simple and convenient base-mediated synthesis of asymmetrical carbonates from alcohols with dimethyl carbonate is described. The reaction is remarkably influenced by the strength of the base employed and potassium t-butoxide was found to be best promote

Full text of DOI:10.1007/s00706-013-1123-3

Monatsh Chem
DOI 10.1007/s00706-013-1123-3
ORIGINAL PAPER
An easy base-assisted synthesis of unsymmetrical carbonates
from alcohols with dimethyl carbonate
•
Subodh Kumar Suman L. Jain
Received: 8 May 2013 / Accepted: 13 November 2013
Ó Springer-Verlag Wien 2014
Abstract A simple and convenient base-mediated syn-
thesis of asymmetrical carbonates from alcohols with
dimethyl carbonate is described. The reaction is remark-
ably influenced by the strength of the base employed and
potassium t-butoxide was found to be best promoter for this
reaction. Almost in all cases, the reaction is selective and
afforded the corresponding unsymmetrical methyl carbon-
ates in excellent yields (80–90 %).
temperature. Another common method of their synthesis
comprises the reaction of halo compounds with alcohols
and phenols. Most symmetrical and unsymmetrical car-
bonates are formed by these two conventional approaches
[4, 5]. However, both methods are associated with the
limitations of using of toxic and hazardous materials and
producing copious amounts of undesirable chloride salts.
From the standpoint of green chemistry, the development
of reaction systems which do not require hazardous
reagents or volatile organic solvents is of tremendous
interest [6]. In the last two decades, dimethyl carbonate
(DMC) has gained prominence as a green reagent in base-
catalyzed methylation or methoxy-carbonylation of ani-
lines, phenols, active methylene compounds, and
carboxylic acids. DMC is considered to be green as it is
nontoxic and produces only CO2 and MeOH that can easily
be removed by distillation [7–9]. Owing to these aspects,
the reaction of alcohols with DMC to give organic car-
bonates is considered to be an eco-friendly
‘‘carbonylation’’ route. In this context, a number of cata-
lysts namely ZnCl2/Et3N [10], p-CH3C6H4SO3H [11],
YbCl3 [12], and ytterbium(III) triflate [13] have been used.
However, in most cases, symmetrical carbonates were
formed predominantly with a poor yield of the unsym-
metrical carbonates at high temperature. Furthermore the
higher cost of the catalysts and slow reaction rates make
these methods less advantageous. Recently, a liquid-phase
synthesis of alkyl carbonates by coupling of an alcohol,
CO2, and alkyl halide in the presence of Cs2CO3 at ambient
temperature [14] and a solid-phase reaction involving an
alcohol or amine, ligated to a resin through a CO2 linker, in
the presence of Cs2CO3 and tetrabutylammonium iodide
(TBAI) [15] to produce the corresponding carbonates or
carbamates were reported. The use of more than stoichi-
ometric amounts of base, longer reaction times, and
Keywords Transesterification Á Dimethyl carbonate Á
Organic carbonates Á Base Á Alcohol
Introduction
Alkyl organic carbonates have been recognized as extre-
mely valuable synthetic intermediates in the chemical
industry [1]. Owing to their higher stability than esters,
organic carbonates have been widely used as protecting
groups during the synthesis of complex molecules of bio-
logical importance. In addition, they also have numerous
applications as monomers for organic glasses, as solvents
in the manufacture of lithium batteries, and as intermedi-
ates in the synthesis of bioactive chemicals [2, 3]. The
conventional synthesis of organic carbonates involves the
reaction of hydroxyl compounds (aliphatic, aromatic)
with excess pyridine and phosgene at or below room
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-013-1123-3) contains supplementary
material, which is available to authorized users.
S. Kumar Á S. L. Jain (&)
Chemical Sciences Division, CSIR-Indian Institute of Petroleum,
Dehradun 248005, India
e-mail: suman@iip.res.in
123

S. Kumar, S. L. Jain
Scheme 1
Table 1 Optimization experiments using 1-butanol as substrate
moderate product yields make these procedures unattrac-
tive. Furthermore, the synthesis of unsymmetrical organic
carbonates is achieved by the reaction of various alcohols
with diethyl carbonate in the presence of a catalytic amount
of MCM-41-TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene
anchored on mesoporous MCM-41 silica) at ca. 338 K
[16]. Since organically modified silicas are difficult to
regenerate, the development a simple and facile alternative
methodology is desirable. Consequently Kantam et al. [17]
reported the use of nanocrystalline MgO and Zhou et al.
[18] described the use of metal organic frameworks
(MOFs) as heterogeneous catalysts for the synthesis of
organic carbonates by the reaction of diethyl carbonate
with alcohols. MOFs are difficult to synthesize and require
expensive chemicals. Most recently, Veldurthy et al. [19]
reported an efficient and green approach for the synthesis
of unsymmetrical carbonates by using alcohols and diethyl
carbonate in the presence of the heterogeneous base cata-
lyst CsF/Al2O3. In this procedure the reactions were carried
out at 130 °C and under a nitrogen atmosphere. Song et al.
[20] reported the synthesis of unsymmetrical organic car-
Entry
Base
Base amounta
Time/min
Conversion/
selectivity/%b
1
2
3
4
5
6
7
8
9
–
–
60
30
15
35
40
10
15
8
–
NaOH
KOH
0.2
0.2
0.2
0.2
0.2
0.1
0.3
0.5
68/98
90/100
48/95
52/98
92/100
90/100
94/100
97/100
K2CO3
Et3N
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
5
1-Butanol/DMC (1:5) at room temperature
a
Equiv. with respect to alcohol
b
Determined by GC
present work is the high yield of products (90–95 %) in
most cases with very high purity ([98 %) after a simple
workup.
bonates catalyzed by
a sulfonic acid-functionalized
zirconium phosphonate. Recently Tundo et al. [21] repor-
ted the methylation of primary alcohols with DMC for the
synthesis of methyl ethers in the presence of basic alumina
or hydrotalcite as catalyst at 200 °C. The methyl carbon-
ates formed in the first step decarboxylate in the second
step to yield the corresponding methyl ethers. In another
report, Tundo et al. [22] studied the reaction of dialkyl
carbonates with alcohols to investigate the behavior of
preferential leaving and entering groups for the newly
synthesized carbonates. Very recently Mutlu et al. [23]
reported the use of TBD as an organocatalyst for the syn-
thesis of unsymmetrical carbonates. However the reactions
were carried out at 80 °C and afforded symmetrical car-
bonates as the by-products.
Results and discussion
Initially we studied the effect of various bases such as
KOH, K2CO3, and KOt-Bu for the reaction of 1-butanol
(1a) with DMC (2) in a molar ratio of 1:5 in the presence
of base (0.2 eq) at room temperature. The results of these
experiments are summarized in Table 1 (entries 2–6). In a
controlled blank experiment, no reaction occurred in the
absence of base (Table 1, entry 1). Among the various
bases KOt-Bu and KOH were found to be the catalysts of
choice. Other selected bases such as NaOH, K2CO3, and
Et3N were also found to be quite effective but required
longer reaction times. If we compare the time required for
complete reaction and the strength of the bases, it can be
concluded that the reaction is faster with stronger bases.
We used excess DMC (fivefold more than alcohol) and
this can easily be recovered by distillation after the
reaction. We also performed the reaction of 1-butanol
with DMC (molar ratio 1:5) by varying the amount of
catalyst KOt-Bu from 0.1 to 0.5 eq under otherwise
similar experimental conditions. The results are summa-
rized in Table 1 (entries 7–9). Higher loadings of the
In view the aforementioned limitations of the existing
methods, there is still a need to develop a simple and
cost-effective methodology for the synthesis of unsym-
metrical carbonates. In the present paper we report an
easy base-assisted (such as potassium t-butoxide, KOt-Bu)
methodology for the synthesis of unsymmetrical carbon-
ates from the reaction of alcohols with DMC at room
temperature (Scheme 1). A remarkable outcome of the
123

An easy base-assisted synthesis
Table 2 KOt-Bu-catalyzed
synthesis of unsymmetrical
carbonates
Entry
1
Substrate
Product
Time/min
10
Conv/select /% Yield /% b
O
OH
OMe
92/100
90
O
1a
1b
3a
O
OH
2
3
4
O
10
12
15
92/100
96/100
94/100
90
93
92
OMe
3b
3c
3d
O
OH
O
OMe
1c
O
OH
O
OMe
O
1d
5
OH
OCH3
O
15
89/100
86
octan-1-ol
methyl octyl carbonate
1e
3e
O
CH3
CH3
6
7
OH
O
OMe
OMe
10
15
86/100
84/100
85
82
1f
3f
O
OH
O
1g
3g
O
OH
O
OCH3
O
8
9
20
20
87/98
85c
88
1h
1i
3h
3i
CH2OH
CH2OH
CH2O
OMe
OMe
90/100
O
CH2O
CH3
10
11
15
15
94/100
96/100
93
95
CH3
1j
3j
O
CH2OH
CH2O
OMe
OMe
OMe
1k
3k
O
CH2OH
CH2O
OMe
H3C
H3C
12
13
20
30
90/100
86/100
88
82
CH3
CH3
O
1l
3l
CH2OH
CH2O
OMe
Cl
1m
1n
3m
3n
Cl
O
OH
O
OCH3
90d
87d
14
15
25
35
92/97
90/89
O
OH
O
OCH3
Cl
1o
1p
1q
3o
3p
3q
Cl
O
OH
16
17
O
OCH3
O
30
30
91/100
92/100
89
90
O
OH
H
3CO
aReaction conditions: alcohol/ DMC (1:5), KOt-Bu (0.2 eq) at room temperature; bIsolated yields;
cSymmetrical carbonate remained; dCorresponding styrenes wereobtained as by-products.
123

S. Kumar, S. L. Jain
Table 3 Comparison of different catalysts with the present methodology
Entry Catalyst
Reaction conditions
Conversion/selec- Advantage
tivity/%
Disadvantage
1
MCM-41-TBD
125 °C, 8 h, 0.1 g
catalyst
96/99
Facile recovery and recycling Multi-step synthesis, difficult
to regenerate and high cost
of the precursors
of the catalyst
2
3
4
5
6
Mg/La metal oxide
CsF/Al2O3
TBD
125 °C, 3.5 h, 0.1 g
catalyst
98
Higher product yield, easily
recyclable catalyst system
Multi-step synthesis and
expensive metal oxides
130 °C, 0.5 h, 0.1 g
catalyst
98/100
Faster reaction, high product
yield, recyclable catalyst
High cost of the catalyst
80 °C, 1 h, 0.1 mol% 99/97
catalyst
Faster rates, high product yield High cost and limited
accessibility of the catalyst
High cost and multi-step
Bu2SnMoO4
t-BuOK
130 °C, 8 h
90/95
–
synthesis of the catalyst
Comparatively low cost and
high reactivity and mild
experimental conditions
High catalyst loading
7
KOH
Inexpensive, easily available,
highly reactive
Scheme 2
catalyst reduced the reaction time, whereas lower amounts
of base increased the reaction time but not to a great
extent.
carbonates 3f and 3g in 85 and 82 % yields, respectively
(entries 6, 7). Among all the alcohols studied, the activity of
the primary alkyl alcohols was found to be highest and they
were all transformed into the corresponding unsymmetrical
carbonates selectively (Table 2, entries 1–7). The activity
order of alkyl alcohols was primary alcohol [ secondary
alcohol (Table 2, entry 8), which could result mainly from
steric hindrance. Tertiary alcohols such as tert-amyl alcohol
did not undergo any reaction under the described experi-
mental conditions. Benzylic and aromatic substituted
alcohols (Table 2, entries 9–15) had lower activity than the
alkyl alcohols because of the delocalization of electrons in
the benzene ring, which reduces the nucleophilicity of the
generated alkoxide ion. In the case of cyclic aliphatic
alcohols (Table 2, entries 16, 17), the lower activity was
due to the steric hindrance of the ring. In all cases except
2-hydroxyoctanol (Table 2, entry 8), benzhydrol (entry 14),
and 4-chlorobenzhydrol (entry 15), the reaction was selec-
tive and afforded unsymmetrical carbonates in high to
excellent yields.
We explored the general scope of the reaction by using
different aliphatic, aromatic, and cyclic alcohols with DMC
under the optimized reaction conditions, e.g., alcohol 1
(1 eq.), KOt-Bu (0.2 eq), and DMC (5 eq) at room tem-
perature. All the substrates were selectively transformed to
the corresponding unsymmetrical carbonates 3 in high
yields within 10–30 min as summarized in Table 2. The
reaction of various aliphatic alcohols such as 1-butanol
(1a), 1-pentanol (1b), 1-hexanol (1c), 1-heptanol (1d), and
1-octanol (1e) with DMC (2) afforded the corresponding
alkyl methyl carbonates selectively, i.e., 1-butyl methyl-
carbonate (3a), 1-pentyl methylcarbonate (3b), 1-hexyl
methylcarbonate (3c), 1-heptyl methylcarbonate (3d), and
1-octyl methylcarbonate (3e) in 90, 90, 93, 92, and 86 %
yields, respectively (entries 1–5). Branched alcohols such as
6-methyl-1-heptanol (1f) and 5-ethyl-1-heptanol (1g) affor-
ded the corresponding unsymmetrical alkyl methyl
123

An easy base-assisted synthesis
The present methodology is advantageous as it uses an
inexpensive base catalyst, mild reaction conditions, and
allows the selective formation of unsymmetrical carbonates
in high yields. To establish the superiority of the developed
methodology, a comparison of different catalysts with the
present methodology is shown in Table 3.
acetate and washed several times with water. The organic
layer was dried over sodium sulfate and concentrated in
vacuo. Purification by silica gel column chromatography
with hexane/ethyl acetate (90:10, v/v) gave the corre-
sponding methyl carbonates in the yields mentioned in
Table 2. The conversion and purity of the product were
determined by GC–MS and identity was confirmed by
comparing the spectral data with those reported [9, 24].
Copies of the spectra and GC–MS chromatograms of the
products are given in the Supplementary Material.
Althoughtheexact mechanism ofthe reactionisnotclearat
this stage, a possible mechanistic pathway is shown in
Scheme 2. We assume that the first step is the generation of an
alkoxide ion (RO-) through proton abstraction from the
alcohol bythebase. Thenthe alkoxideionattacksthe carbonyl
carbon of DMC and forms a tetrahedral intermediate ion,
which then generates a methyl ester molecule (ROCO2Me)
and methanol as shown in Scheme 2. Strong bases may
facilitate the abstraction of the proton from the alcohol to give
an alkoxide ion and therefore enhance the reaction rates.
Acknowledgments We are thankful to the Director, IIP for his kind
permission to publish these results. SK acknowledges the CSIR, New
Delhi for the award of his Research Fellowship.
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Experimental
All the reagents and substrates are commercially available
and used as received. Anhydrous K2CO3 and KOH flakes
were used as received from Merck Chemicals. Potassium t-
butoxide was obtained from Acros Organics and used as
received. 1H and 13C NMR spectra were recorded for CDCl3
solutions at 500 and 125 MHz, respectively. GC–MS (HP
5890, series II) analyses were carried out by using a mass-
selective detecter (MSD) (30 m 9 0.30 mm; 50–250 °C,
8 °C/min) and GC (GC, Agilent 6820) analysis was carried
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a silicon OV-17 column (50 m 9 0.26;
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To a mixture of alcohol 1 (1 mmol) and DMC (5 mmol)
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resulting mixture was stirred at room temperature. Progress
of the reaction was monitored by TLC (SiO2). After
completion, the unreacted DMC and methanol were iso-
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`
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123