Synthesis of symmetrical organic carbonates via significantly enhanced alkylation of metal carbonates with alkyl halides/sulfonates in ionic liquid

Article abstract of DOI:10.1021/jo051722h

We report a new phosgene-free method for the synthesis of symmetrical organic carbonates via alkylation of metal carbonate with various alkyl halides and sulfonates in 1-n-butyl-3-methyl-imidazolium hexafluorophosphate, [bmim] [PF₆], as an ecofriendly reaction media. Alkylation of metal carbonate in various ionic liquids with 1-bromo-3-phenylpropane (1a) as a model reactant has thoroughly been investigated. Potassium and cesium carbonates appeared to be the most suitable metal carbonate due to their high solubility in ionic liquids. Besides good to excellent yields, this simple and convenient methodology is devoid of highly toxic and harmful chemicals such as phosgene and carbon monoxide, which is an additional advantage.

Full text of DOI:10.1021/jo051722h

Synthesis of Symmetrical Organic Carbonates via Significantly
Enhanced Alkylation of Metal Carbonates with Alkyl Halides/
Sulfonates in Ionic Liquid
Yogesh R. Jorapur and Dae Yoon Chi*
Department of Chemistry, Inha University, 253 Yonghyundong Namgu, Inchon 402-751, Korea
dychi@inha.ac.kr
Received August 14, 2005
We report a new phosgene-free method for the synthesis of symmetrical organic carbonates via
alkylation of metal carbonate with various alkyl halides and sulfonates in 1-n-butyl-3-methyl-
imidazolium hexafluorophosphate, [bmim][PF6], as an ecofriendly reaction media. Alkylation of
metal carbonate in various ionic liquids with 1-bromo-3-phenylpropane (1a) as a model reactant
has thoroughly been investigated. Potassium and cesium carbonates appeared to be the most suitable
metal carbonate due to their high solubility in ionic liquids. Besides good to excellent yields, this
simple and convenient methodology is devoid of highly toxic and harmful chemicals such as phosgene
and carbon monoxide, which is an additional advantage.
Introduction
are synthesized by utilizing alkyl halides and alcohols
using the carbonate and bicarbonate salts of alkali metals
and silver.8 Organic carbonates have been synthesized
via oxycarbonylation of alcohols9 and also by carbon
dioxide insertion into epoxides followed by transesteri-
fication of cyclic carbonates with appropriate alcohols.10
The alternative approach is by the carbonate exchange
reaction.11 Recently, Rossi et al.12 reported synthesis of
Alkyl organic carbonates have played a very important
role in the area of synthetic organic chemistry and have
become excellent templates for the formation of carbon-
carbon and carbon-hetero bonds. They have been used
as novel protecting groups,1 as lubricants and hydraulic
fluid,2 as herbicides, acaricides, fungicides, and seed
disinfectants,3 as octane enhancers for gasoline,4 and in
cosmetics and pharmaceuticals.5 Carbonates have also
been used in industry as a solvent for the separation of
phenols and cyclohexanones.6 Common synthetic meth-
ods leading to the carbonate moiety include the use of
classical toxic and harmful chemicals such as phosgene,
pyridine, and carbon monoxide.7 Additionally, carbonates
(7) (a) Gen. Patent 109,933 (Chemische Fabrik von Heydon) 1900
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1900 Friedl, 1904, 6, 1160. (b) Schnell, H. Chemistry and Physics of
Polycarbonates; Interscience Publishers: New York, 1964; Vol. 9, p
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Kenichi, F.; Shigeo, Y.; Hideo, T.; Hiseo, K. Kogyo Kagaku Zasshi 1960,
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* To whom correspondence should be addressed. Tel: +82-32-860-
7686. Fax: +82-32-867-5604.
(1) (a) Hegarty, A. F. In Comprehensive Organic Chemistry; Suth-
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A.-A. G.; Sivaram, S. Chem. Rev. 1996, 96, 951-976.
(2) (a) Ishida, N.; Hasegawa, H.; Sasaki, U.; Ishikawa, T. U.S. Patent
5,391,311, 1995. (b) Ishida, N.; Sakamoto, T.; Hasegawa, H. U.S. Patent
5,370,809, 1994.
(3) (a) Gier, D. W. U.S. Patent 3,348,939, 1964; Chem. Abstr. 1964,
68, 77729f. (b) Pianka, M.; Sweet, P. J. J. Sci. Food Agric. 1968, 19,
667-681. (c) Pianka, M. J. Sci. Food Agric. 1966, 17, 47-56. (d) Becher,
M.; Sehring, R. DE-OS 20,54,255, 1970; Chem. Abstr. 1972, 76,
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Chem. Abstr. 1970, 80, 36857a.
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(11) (a) Buysch, H.-J. In Ullman’s Encyclopedia of Industrial
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10.1021/jo051722h CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/22/2005
10774
J. Org. Chem. 2005, 70, 10774-10777

Alkyl Carbonate from Metal Carbonate
TABLE 1. Symmetrical Organic Carbonate Synthesis
from 1-Bromo-3-phenylpropane (1a) in the Presence of
K2CO3 under Various Reaction Conditionsa
FIGURE 1. Ionic liquids.
organic carbonates from alkyl halides and tetrabutyl-
ammonium alkyl carbonates at room temperature. How-
ever, it requires initial synthesis of noncommercially
available highly hygroscopic reagents. Further, carbon-
ates have been synthesized from primary or secondary
alcohols and carbon dioxide via unstable methanesulfonyl
carbonates.13 Moreover, some of the above methods suffer
in practicality since carbonate formation occurs under
harsh reaction conditions such as high temperature of
150 °C and above. Furthermore, most of the above
processes require external source of carbon dioxide with
a need for special experimental assembly. The lack of
facile synthetic methodologies for the preparation of
symmetrical and unsymmetrical alkyl carbonates moti-
vated us to develop some efficient, convenient, and
moreover an environmentally friendly procedures for the
synthesis of organic carbonates.
yieldb (%)
[bmim][X]
(mL)
CH3CN
(mL)
entry
time (h)
1a
2a
3a
1
2c
3
4
5
6
7
8
[PF6] (1.5)
[PF6] (1.5)
30
48
96
48
32
32
36
48
89
trace
92
90
69
1.5
1.25
2
12
83
85
80
22
2
[PF6] (0.25)
[SbF6] (1.5)
[BF4] (1.5)
[OTf] (1.5)
[NTf2] (1.5)
trace
trace
2
4
2
58
a All reactions were carried out on a 2.0 mmol reaction scale of
1-bromo-3-phenylpropane (1a) and 1.0 mmol of K2CO3 at 110 °C.
b Isolated yield. c Reaction was carried out in the absence of K2CO3.
Recently, our research group built a strong and en-
couraging foundation for state-of-the-art ionic liquid
technologies.14 Ionic liquids composed of an organic cation
and an inorganic anion have been found to provide
precise tuning of reactions by suitable combinations of
cation and anions (Figure 1).15 Herein, we report the
newer synthetic method toward the synthesis of sym-
metrical alkyl carbonates via alkylation of various metal
carbonates with various alkali halides and sulfonates in
various ionic liquids. Our initial studies proved how
efficiently ionic liquids enhanced the nucleophilicity of
K2CO3 toward 1-bromo-3-phenylpropane (1a) as a model
compound.
TABLE 2. Symmetrical Organic Carbonate Synthesis
from 1-Bromo-3-phenylpropane (1a) in the Presence of
Various Metal Carbonates in [bmim][PF6]a
yieldb (%)
entry
MxCO3
time (h)
1a
2a
3a
1
2c
3
4
5
6
BaCO3
48
48
31
48
48
48
92
44
Ag2CO3
Cs2CO3
CaCO3
20
15
3
86
94
94
92
Na2CO3
NaHCO3
trace
4
a All reactions were carried out on a 2.0 mmol reaction scale of
alkyl bromide (1a) with 1.0 mmol of MxCO3 in 1.5 mL of
[bmim][PF6] at 110 °C. b Isolated yield. c Since silver carbonate is
light sensitive, the reaction was carried out in the dark.
Results and Discussion
Table 1 illustrates the synthesis of symmetrical alkyl
carbonate from 1a in the presence of K2CO3 under
various reaction conditions. In [bmim][PF6], the carbon-
ate synthesis completed within 30 h at 110 °C with an
excellent yield of 89% of 2a (entry 1), whereas the same
in a conventional organic solvent such as CH3CN at 110
°C occurred hardly even after 4 days (yield 2%, entry 3).
Entry 2 clearly indicates that the carbonate synthesis
proceeds via alkylation of metal carbonate.
To verify the catalytic activity of [bmim][PF6], the
reaction was performed with 0.25 mL of [bmim][PF6] (1.2
mmol), which showed sluggish behavior with only 12%
of desired product (entry 4). Further, to shed light on
various other ionic liquids in comparison to [bmim][PF6],
we have investigated the synthesis of symmetrical alkyl
carbonates using four other ionic liquids. In entries 5-7,
[bmim][SbF6], [bmim][BF4], and [bmim][OTf] were used
as ionic liquids, and we obtained consistent results with
desired product in 83, 85, and 80% yields, respectively,
whereas [bmim][NTf2] showed a slow behavior with only
22% of desired product (entry 8). To investigate the
recyclability of [bmim][PF6], we carried out symmetrical
carbonate synthesis repeatedly under the conditions of
entry 1 in Table 1 by recycling [bmim][PF6]. The reaction
proceeded very well without the degradation of ionic
liquid even after three runs.16
To thoroughly understand the alkylation of metal
carbonate in [bmim][PF6], we carried out the alkylation
with various metal carbonates. As shown in Table 2, it
is clear that among the group IA alkali metal carbonates
Cs2CO3 proved to be best, presumably due to less tight-
ness of ion pairs17,18 (yield 86%, entry 3). Entries 1 and 4
indicate the poor ability of alkali earth metal with no
desired product, whereas transition-metal carbonate
(13) Bratt, M. O.; Taylor, P. C. J. Org. Chem. 2003, 68, 5439-5444.
(14) (a) Kim, D. W.; Song, C. E.; Chi, D. Y. J. Am. Chem. Soc. 2002,
124, 10278-10279. (b) Kim, D. W.; Song, C. E.; Chi, D. Y. J. Org. Chem.
2003, 68, 4281-4285. (c) Kim, D. W.; Choe, Y. S.; Chi, D. Y. Nucl.
Med. Biol. 2003, 30, 345-350. (d) Kim, D. W.; Hong, D. J.; Seo, J. W.;
Kim, H. S.; Kim, H. G.; Song, C. E.; Chi, D. Y. J. Org. Chem. 2004, 69,
3186-3189. (e) Boovanahalli, S. K.; Kim, D. W.; Chi, D. Y. J. Org.
Chem. 2004, 69, 3340-3344. (f) Jorapur, Y. R.; Lee, C.-H.; Chi, D. Y.
Org. Lett. 2005, 7, 1231-1234.
(15) For reviews, see: (a) Sheldon, R. Chem. Commun. 2001, 2399-
2407. (b) Wasserscheid, P. J.; Keim, W. Angew. Chem., Int. Ed. 2000,
39, 3772-3789. (c) Welton, T. Chem. Rev. 1999, 99, 2071-2083.
(16) 1H, 13C, and 19F NMR spectra incorporated in the Supporting
Information.
(17) Galli, C. Org. Prep. Proceed. Int. 1992, 24, 285-307.
J. Org. Chem, Vol. 70, No. 26, 2005 10775

Jorapur and Chi
TABLE 3. Symmetrical Organic Carbonates from
Ag2CO3 generated desired product 2a in 20% yields (entry
2). The reactions with either Na2CO3 or NaHCO3 did not
proceed to symmetrical organic carbonates (entries 5 and
6).19
Various Alkyl Halides/Mesylates and Tosylates in
[bmim][PF6]a
To ascertain the scope of this novel method, we
examined several other electrophilic moieties as shown
in Table 3. Our methodology proved to be best as it
worked for various alkyl halides and sulfonates (entries
1-4). In the case of chloride 1b, a slower reaction rate
was observed compared to those of iodide, tosylate, and
mesylate to yield 45% of the desired carbonate. On the
other hand, iodide, mesylate, and tosylate reacted within
24, 34, and 42 h to produce carbonate 2a in 84, 83, and
82% yield, respectively (entries 2-4). Our attempt to
synthesize symmetrical carbonates from acetate failed
(entry 5). Alkyl halides 1g and 1h reacted within 55 and
31 h, respectively, to furnish 4 and 6 with good yields,
78 and 82%, respectively (entries 6 and 7). Alkyl halide
1i gave a low yield (51%) of desired product 8 together
with the unwanted styrene as elimination compound
(entry 8). Active alkyl chloride 1j reacted within 36 h to
generate 10 in 80% yield (entry 9), whereas in the case
of alkyl halide 1k the reaction was found to be slow with
a yield of 12 of 59%, presumably due to poor solubility
in ionic liquid (entry 10).
The ionic liquids acted not only as a solvent but also
as an activater for K2CO3, which further generates a
complex of increased lipophilicity, and “naked” carbonate
anion underwent nucleophilic substitution with an alkyl
halide or sulfonate. Although there has been a contro-
versy regarding the enhancement of nucleophilicity in
ionic liquid,20 symmetrical carbonate could be prepared
in high yield by alkylation using potassium carbonate,
which is generally considered as a poor nucleophile. Our
protocol is supported by the recent studies on remarkable
solubility of CO2 in imidazolium based ionic liquids.21
Our attempt to synthesize unsymmetrical organic
carbonates via alkylation of K2CO3 by two different
reactants (1:1) (alkyl halide or sulfonate) in [bmim][PF6]
as expected did not produce consistent results. We
observed a low yield of unsymmetrical organic carbonates
along with the mixture of two symmetrical organic
carbonates generated from two different reactants.
In conclusion, we displayed a new methodology toward
the synthesis of symmetrical organic carbonates in ionic
liquid under mild conditions. The strategy is based on
the alkylation of a cheap, nontoxic, stable, and easy to
handle potassium carbonate. This method is devoid from
the use of extremely toxic phosgene and carbon monoxide.
The experimental and workup procedure is very simple,
and convenient with no special equipments. Apart from
the alkyl bromides, this method works well for the other
alkyl halides or sulfonates, which are found to be an
additional practical incentive. Thus, our industrially
feasible protocol opens avenues for today’s organic chem-
a All reactions were carried out on a 2.0 mmol reaction scale of
alkyl halides/mesylates and tosylates 1 with 1.0 mmol of K2CO3
in 1.5 mL of [bmim][PF6] at 110 °C. b Isolated yield. c Styrene was
obtained in 32% yield.
ist who is concerned with safety and toxic waste mini-
mization. It is expected that reactions using polymer-
supported ionic liquid (PSIL)22 instead of ionic liquid
might provide excellent yields with low reaction time for
the synthesis of organic carbonates. In addition, we are
(18) For the alkyl carbonates synthesis using cesium carbonate,
see: (a) Kim, S.-I.; Chu, F.; Dueno, E. E.; Jung, K. W. J. Org. Chem.
1999, 64, 4578-4579. (b) Cella, J. A.; Bacon, S. W. J. Org. Chem. 1984,
49, 1122-1125.
(19) For dialkyl carbonate synthesis using potassium bicarbonate
in the presence of phase-transfer catalyst, see ref 18b.
(20) Lancaster, N. L. J. Chem. Res. 2005, 413-417.
(21) Cadena, C.; Anthony, J. L.; Shah, J. K.; Morrow, T. I.;
Brennecke, J. F.; Maginn, E. J. J. Am. Chem. Soc. 2004, 126, 5300-
5308.
(22) Kim, D. W.; Chi, D. Y. Angew. Chem., Int. Ed. 2004, 43, 483-
485.
10776 J. Org. Chem., Vol. 70, No. 26, 2005

Alkyl Carbonate from Metal Carbonate
NMR (400 MHz, CDCl3) δ 1.97-2.04 (m, 4H), 2.72 (t, J ) 7.6
Hz, 4 H), 4.15 (t, J ) 6.4 Hz, 4H), 7.18-7.30 (m, 10 H); 13C
NMR (100 MHz, CDCl3) δ 30.2, 31.9, 67.2, 126.0, 128.3, 128.4,
140.9, 155.3; MS FAB 299.0 (M + H+), 199.7 (100). Anal.
Calcd: C, 76.48; H, 7.43. Found: C, 76.29; H, 7.49. CAS
Registry No.: 102162-49-2.
trying to improve our prodigious ionic liquid methodology
toward the synthesis of unsymmetrical organic carbon-
ates.
Experimental Section
Typical Procedure for Di(3-phenylpropyl)carbonate
(2a). 3-Bromopropylbenzene (1a, 398.0 mg, 2.0 mmol) was
added to the mixture of K2CO3 (138.2 mg, 1.0 mmol) and 1-n-
butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6]
(1.5 mL). The mixture was stirred over 30 h at 110 °C. The
reaction mixture was extracted from the ionic liquid phase
with ethyl ether (5 mL × 5). The organic layer was dried over
anhydrous sodium sulfate and evaporated under reduced
pressure. The residue was purified by flash column chroma-
tography (silica gel) (5% EtOAc/Hx) to obtain 265.2 mg (89%)
of di(3-phenylpropyl)carbonate (2a) as a colorless liquid: 1H
Acknowledgment. This work was supported by the
Korea Science and Engineering Foundation (M2-0504-
07-0003-05-A07-07-003-11).
Supporting Information Available: Experimental pro-
cedures including characterization of all compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO051722H
J. Org. Chem, Vol. 70, No. 26, 2005 10777