Significant rate acceleration in carbonate synthesis from carbon dioxide and oxiranes using dimethyl carbonate as a recyclable medium

Article abstract of DOI:10.1016/j.tetlet.2011.10.082

The synthesis of cyclic carbonates by the reaction of oxiranes and carbon dioxide in the presence of catalytic amount of tetrabutylammonium bromide in dimethylcarbonate without any metal catalyst is reported. Significant rate acceleration in the reaction

Full text of DOI:10.1016/j.tetlet.2011.10.082

Tetrahedron Letters 52 (2011) 6957–6959
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Significant rate acceleration in carbonate synthesis from carbon dioxide and
oxiranes using dimethyl carbonate as a recyclable medium
⇑
⇑
Subodh Kumar, Suman L. Jain , Bir Sain
Chemical Sciences Division, Indian Institute of Petroleum (Council of Scientific and Industrial Research), Dehradun 248005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of cyclic carbonates by the reaction of oxiranes and carbon dioxide in the presence of cat-
alytic amount of tetrabutylammonium bromide in dimethylcarbonate without any metal catalyst is
reported. Significant rate acceleration in the reaction is observed in dimethylcarbonate as compared to
the other solvents. Under the reaction conditions of 100 °C and 2.1 MPa in dimethyl carbonate, maximum
conversion and selectivity is achieved. The dimethylcarbonate containing tetrabutylammonium bromide
catalyst can be easily recovered and reused for at least six recycles with the same selectivity.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 2 August 2011
Revised 13 October 2011
Accepted 15 October 2011
Available online 20 October 2011
Keywords:
Dimethylcarbonate
CO
2
Cyclic carbonates
Metal-free
Green synthesis
Utilization of carbon dioxide as a renewable and environmen-
tally friendly source of carbon for chemical transformation is of
paramount importance both from C1 chemistry as well as green
2
of CO with epoxides in presence of catalytic amount of tetrabutyl-
ammoium bromide (TBAB) in the absence of any metal based cat-
alyst (Scheme 1).
1
chemistry viewpoints. Chemical fixation of carbon dioxide in the
At first we carried out the reaction of styrene epoxide with CO
2
form of organic carbonates by the reaction with oxiranes is of great
interest due to the wide spread applications of these compounds as
synthetic intermediates, aprotic polar solvents, and as intermedi-
in presence of TBAB (1 mol %) under solvent-free conditions at
100 °C and 2.1 MPa pressure. The reaction was found to be very
slow and provided very poor yield of the corresponding carbonate
even after 10 h (Table 1, entry 1). Next, we performed the reaction
2
,3
ates in the production of pharmaceuticals and fine chemicals.
Nevertheless, the formation of cyclic carbonates form oxiranes
2 2
in a number of organic solvents including acetonitrile, CH Cl , DMC
and carbon dioxide is an old reaction4 and a number of inorganic
and toluene under similar reaction conditions. Surprisingly, the
reaction was found to be very fast in DMC and afforded almost
quantitative conversion of the desired carbonate selectively with-
out any evidence for the formation of any by-product. Whereas
the reaction was found to be very slow in dichloromethane and
there was no reaction occurred in toluene (Table 1, entry 1). This
observation established the promoting effect of the DMC on the
reaction rate. After completion of the reaction, the reaction mixture
was concentrated under vacuum to recover dimethyl carbonate.
The resulting residue was diluted with diethyl ether to recover
the TBAB by decantation. The combined ether layer was subjected
to usual work-up to obtain the product, while the recovered DMC
and TBAB catalyst were reused as such for subsequent experiments.
5
and organic compounds including amines, phosphanes, organotin
6
7
8
halides, transition metal complexes mainly salen complexes,
9
10
11
organocatalysts, ionic liquid and polymer grafted ionic liquid
are known to catalyze this transformation. The recent challenges
in chemical industry have focused on limiting the use of organic
solvents or replacing them with new, environmentally benign
media in a view to develop green chemical synthesis.12 In particu-
lar, the use of dimethyl carbonate (DMC) as a non-toxic, non-corro-
sive, and environmentally friendly solvent is gaining particular
interest in recent years.13 However, to the best of our knowledge,
there is no literature report on using DMC as reaction media for
2
the synthesis of carbonates from the reaction of CO and epoxides.
Continuing our search for new reaction media to perform
organic reactions, we herein report the first successful use of
dimethyl carbonate (DMC) as a recyclable reaction media as well
as promoter for the synthesis of cyclic carbonates from the reaction
O
+
-(1 mol%)
O
Bu N Br
4
+
CO2
O
O
R
DMC 100 ºC
.1 Mpa
2
R
⇑
Corresponding authors. Tel.: +91 135 2525901; fax: +91 135 2660202.
E-mail addresses: suman@iip.res.in (S.L. Jain), birsain@iip.res.in (B. Sain).
Scheme 1. Synthesis of carbonate under various conditions.
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.082

6
958
S. Kumar et al. / Tetrahedron Letters 52 (2011) 6957–6959
Table 1
the use of DMC in the present reaction not only enhances the
a
Metal free DMC promoted coupling of epoxides with CO
2
reaction rates but also provides the recycling of the catalyst.
Furthermore, the scope of the reaction was extended to various
epoxides using TBAB as catalyst and DMC as reaction media under
Entry Epoxide
Product
Reaction
Time (h)
Yieldb
(%)
1
4
described reaction conditions. The results of these experiments
are summarized in Table 1. After completion of the reaction, the
reaction product was isolated by extraction with diethyl ether,
however TBAB immobilized in DMC can be easily recovered and re-
used for subsequent experiment. In a controlled experiment in
DMC without using tetrabutylammonium bromide (1 mol %), the
reaction was found to be very slow and a very poor yield (20%)
of the desired product was obtained after 12 h. These results indi-
O
O
9
4
20 ,
—
2
6
c
5 ,
O
O
6.5
d
1
1
2
e
0f
O
O
O
O
2
4.5
5.0
8
98
97
75
2
cated that the presence of TBAB is essential for activation of CO in
Me
the present transformation. In contrast the presence of DMC signif-
icantly increased the reaction rate and afforded almost quantita-
tive conversion in shorter reaction time. Epoxides bearing an
electron-donating substituent (Table 1, entries 2 and 3) were con-
verted to the corresponding cyclic carbonates in high yields (95–
Me
O
O
O
O
3
9
9%) within a short reaction time (4–6 h). Whereas, epoxide with
MeO
an electron-withdrawing group (Table 1, entry 4) required a longer
reaction time (8 h) to give the product in a moderate yield (75%),
which is probably due to the reduced electron density of the epox-
ide oxygen atom. In all cases, cyclic carbonates were formed exclu-
sively; cyclic carbonate and unreacted epoxide could be obtained
after the reaction. Using this protocol, the reactions could be car-
MeO
O
O
O
O
4
O N
2
2
ried out efficiently under a 2.1 MPa CO pressure, making this
O N
2
method not perfect from synthetic viewpoints. Nevertheless,
avoidance of expensive metallic catalyst, toxic and volatile organic
solvent, use of environmentally benign recyclable reaction media,
easy work-up and high product yields reflects the merits of the
developed protocol over most of the existing ones.
O
O
O
5
6
4.0
3.5
92
90
O
O
O
O
The exact mechanism of the reaction is not known at this stage;
O
7c
however based on the known mechanism, the possible mecha-
O
O
O
nism of the reaction involves the epoxide ring opening by the
nucleophilic attack of bromide ion of quaternary salt. The promot-
ing effect of dimethyl carbonate is due to its non-bonding interac-
tion with the oxygen of epoxide, which may facilitate the ring
opening of the oxirane ring. The ring opening, leads to the forma-
tion of oxy anion species 2 which on subsequent reaction with
carbon dioxide and subsequent cyclization may give the corre-
sponding carbonate (Scheme 2).
7
2.5
3.0
98
94
O
1
5
O
O
Cl
8
O
O
Cl
O
O
In conclusion, we have demonstrated the first successful use of
DMC not only as a recyclable reaction media but also its promoting
effect on the reaction rates. The present reaction was carried out at
O
O
9
6.0
85
1
00 °C and 2.1 MPa by using TBAB (1 mol %) as catalyst under
O
O
1
00
n-Bu
1
0
1
O
O
2.0
2.0
98
95
n-Bu
O
80
60
O
1
O
O
a
Reaction conditions: substrate (10 mmol), TBAB (1 mol %, 0.1 mmol), at 100 °C
4
0
under pressure 2.1 MPa in DMC (10 ml).
b
Isolated yields.
In the absence of DMC.
By using dichloromethane is solvent.
Using toluene as solvent.
c
d
20
0
e
f
In the absence of TBAB catalyst.
0
1
2
3
4
5
6
The recycling of the developed catalytic system (TBAB in DCM) was
checked for six runs (Fig. 1). In all the cases the conversion of the
epoxide to cyclic carbonate was consistently reproducible. Thus,
Cycle
Figure 1. Results of recycling experiments.

S. Kumar et al. / Tetrahedron Letters 52 (2011) 6957–6959
6959
+
-
+
Et N Br
MeO
MeO
4
_
_NBu
4
+
Br
R
O
O
ONBu CO2
O
4
O
+
N Br-
+ Et
R
O
4
Br
O
R
O
O
R
1
2
Scheme 2. Plausible mechanism for DMC promoted synthesis of carbonates.
metal-free conditions. The DMC immobilized TBAB was successfully
recycled for several runs and almost quantitative conversion were
obtained with exclusive formation of cyclic carbonates without for-
mation of any by-product. This report describes the first example of
efficient use of DMC as an eco-friendly and recyclable solvent for the
cyclic carbonate synthesis using carbon dioxide and oxiranes. Fur-
ther applications of DMC for the chemical fixation of carbon dioxide
are currently under investigation in our laboratory.
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We acknowledge Director IIP for his kind permission to publish
these results. SK thanks to CSIR for his research fellowship. DST,
New Delhi is kindly acknowledged for financial assistance. We
are thankful to Dr. Babita Behra and Mr. G. M. Bahuguna for provid-
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14. General experimental procedure: A 50 mL stainless steel autoclave was charged
(
2
with epoxide (10.0 mmol), TBAB catalyst (0.1 mmol), DMC (10 ml) and then
CO
100 °C for the reaction time as mentioned in the Table 1. The reactor was
cooled in an ice bath for 30 min, and the product was dissolved in Et O, and
2
(initial pressure of 2.1 MPa). The mixture was heated with stirring at
(
2
2
.
separated by extraction. The remained DMC layer containing TBAB was dried
under vacuum and reused as such for subsequent experiment. The product was
analyzed by 1H NMR and GC analysis. In some cases the product was purified
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