Metal-free synthesis of cyclic and acyclic carbonates from CO2 and alcohols

Article abstract of DOI:10.1002/ejoc.201400031

Diverse cyclic and acyclic carbonates such as ethylene carbonate, propylene carbonate, glycerol carbonate, and dimethyl carbonate were synthesized in moderate to good yields by the direct coupling of the corresponding alcohols with carbon dioxide in the absence of metal catalysts and inorganic bases. The direct carbonation mechanism of alcohols in the presence of 1,8-diazabicyclo[5. 4.0]undec-7-ene, 1-butyl-3-methylimidazolium hexafluorophosphate, and dibromomethane was probed by ¹⁸O-labeling experiments and chiral alcohol experiments.

Full text of DOI:10.1002/ejoc.201400031

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201400031
Metal-Free Synthesis of Cyclic and Acyclic Carbonates from CO2 and
Alcohols
Yu Na Lim,[a] Chan Lee,[a] and Hye-Young Jang*[a,b]
Keywords: Carbon dioxide / Carbonates / Ionic liquids
Diverse cyclic and acyclic carbonates such as ethylene carb-
onate, propylene carbonate, glycerol carbonate, and di-
methyl carbonate were synthesized in moderate to good
yields by the direct coupling of the corresponding alcohols
with carbon dioxide in the absence of metal catalysts and
inorganic bases. The direct carbonation mechanism of
alcohols in the presence of 1,8-diazabicyclo[5.4.0]undec-7-
ene, 1-butyl-3-methylimidazolium hexafluorophosphate, and
dibromomethane was probed by 18O-labeling experiments
and chiral alcohol experiments.
and reactions performed with Cs2CO3 generate inorganic
salt wastes. Cyclic carbonates have been synthesized by
Lewis acid and transition-metal-catalyzed addition of CO2
to oxiranes.[2] Although the direct couplings of diols to CO2
has been reported, the efficiency of the process is not as
high as that in which oxiranes and CO2 are used.[5a,6,7]
However, the direct conversion of diols into cyclic carbon-
ates would provide an efficient carbonation method that
utilizes various alcohols, for example, ethylene glycol, pro-
pylene glycol, and glycerol (byproducts of the biodiesel pro-
cess). Thus, an assortment of cyclic carbonates can be pro-
duced from various diols by their direct conversion into cy-
clic carbonates.
Our research group has been interested in organic-com-
pound-mediated CO2 conversion leading to the investiga-
tion of metal-free carbonation reactions.[8] In this study, we
present the metal-free conversion of alcohols into cyclic and
acyclic carbonates by using CO2 and commercially valuable
ethylene carbonate (EC), propylene carbonate (PC), glyc-
erol carbonate (GC), and dimethyl carbonate (DMC). The
reactions of CO2 with the alcohols were performed in the
presence of an organic base and dibromomethane (CH2Br2)
as the solvent and alcohol activator in the absence of metal
species.
Introduction
Employing chemical conversion reactions of carbon di-
oxide (CO2) to synthesize value-added products in chemical
industries is attractive, because of the important economic
and environmental effects of such a conversion.[1] Among
the chemical conversion processes involving CO2, the pro-
duction of dimethyl carbonate, ethylene carbonate, and
polycarbonates from CO2 has been actively investigated,
some of which have been already commercialized.[2,3] An
environmentally friendly synthesis of carbonates from CO2
could replace processes that use toxic chemicals such as
phosgene (COCl2) and carbon monoxide (CO). In addition
to the environmental benefits, the emerging demand for
CO2-incorporated carbonates for the fabrication of lithium-
ion battery electrolytes and for the production of polycarb-
onates has attracted research into the chemical conversion
of CO2 into carbonates.[2a,2e]
Most acyclic carbonates are synthesized from alcohols
and CO2 by heterogeneous catalysis by using metal oxides,
zeolites, and metal complexes; however, efficient dehy-
dration in these syntheses presents a hurdle in obtaining
carbonates in high yields.[2h,4] To avoid this dehydration
problem and to promote an efficient and direct coupling of
alcohols and CO2, the Mitsunobu reagent (triphenylphos-
phine and diethyl azodicarboxylate) and a combination of
dichloromethane/Cs2CO3 were introduced to afford acyclic
carbonates in good yields.[5] However, reactions performed
with the use of the Mitsunobu reagent generate a large
amount of phosphine oxide/hydrazine dicarboxylate waste,
Results and Discussion
Optimization of the synthesis of EC from ethylene glycol
and CO2 is summarized in Table 1. The reaction of ethylene
glycol with CO2 was initiated by an organic base, 1,8-diaza-
bicyclo[5.4.0]undec-7-ene (DBU), in CH2Br2 at 70 °C. DBU
is assumed to deprotonate the hydroxy protons of ethylene
glycol, which would render it more nucleophilic. In ad-
dition, DBU is known to form an adduct with CO2, which
thereby increases the susceptibility of CO2 to the addition
of ethylene glycol. CO2 adducts of amidine-type organic
[a] Division of Energy Systems Research, Ajou University,
Suwon 443-749, Korea
E-mail: hyjang2@ajou.ac.kr
http://ajou.ac.kr/~hyjang2
[b] Korea Carbon Capture & Sequestration R&D Center,
Deajeon 305-343, Korea
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201400031.
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Y. N. Lim, C. Lee, H.-Y. Jang
SHORT COMMUNICATION
bases, for example, DBU, have been reported as key inter- sure did not improve the conversion yield (Table 2, en-
mediates in organic-base-mediated CO2 conversions.[9] EC tries 3–5). A higher temperature and lower loading of DBU
(1b) was obtained in 24% yield in the presence of DBU resulted in lower yields (Table 2, entries 6 and 7). Instead
under 5 bar CO2 pressure (Table 1, entry 1; 1 bar = of CH2Br2, CH2Cl2 was utilized to afford 2b in 25% yield
100 kPa).
(Table 2, entry 8).
Using the optimized conditions, the direct coupling of
various alcohols with CO2 was examined to afford the cor-
responding cyclic and acyclic carbonates (Table 3). The
alcohols were subjected to the optimized reaction condi-
Table 1. Synthesis of ethylene carbonate (1b) from ethylene glycol.
Table 3. Synthesis of various carbonates from CO2.
[a] Yield of isolated product. [b] No bmimPF6. [c] 1a (1 mmol).
Ionic liquids are known to increase the solubility of CO2
in the reaction media in the synthesis of cyclic carbonates
from oxiranes and CO2.[10] Therefore, an ionic liquid was
added to the reaction mixture to increase the yield. 1-Butyl-
3-methylimidazolium hexafluorophosphate (bmimPF6) dra-
matically increased the yield of 1b to 54% (Table 1, entry 2).
The yield of 1b increased to 74% if the pressure of CO2 was
increased to 10 bar (Table 1, entry 3). Organic bases that
are known to form an adduct with CO2, such as 1,5-diaza-
bicyclo[4.3.0]non-5-ene (DBN) and 1,5,7-triazabicy-
clo[4.4.0]dec-1-ene (TBD), were subjected to the carbonate
synthesis conditions, and 1b was afforded in 38 and 11%
yield, respectively (Table 1, entries 4 and 5). Instead of
CH2Br2, CH2Cl2 was applied to afford 1b in 46% yield
(Table 1, entry 6).
The synthesis of acyclic carbonates was conducted by
using benzyl alcohol and DBU in CH2Br2 at 70 °C. At
10 bar CO2 pressure, dibenzyl carbonate (2b) was obtained
in 25% yield (Table 2, entry 1). Similar to cyclic carbonate
1b, the yield of 2b increased to 69% upon adding bmimPF6
(Table 2, entry 2). An increase or decrease in the CO2 pres-
Table 2. Synthesis of dibenzyl carbonate (2b) from benzyl alcohol
and CO2.
[a] Yield of isolated product. [b] No bmimPF6. [c] 100 °C.
[a] Yield of isolated product. [b] GC conversion yield.
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Metal-Free Synthesis of Cyclic and Acyclic Carbonates
tions: DBU, bmimPF6, and CH2Br2 under 10 bar CO2 pres- the above-mentioned experiments, a mechanism was pro-
sure. Glycerol underwent cyclization to afford GC (3b) in posed, as shown in Scheme 2. The primary alcohol attacks
86% yield (Table 3, entry 1). Phenyl- and methyl-substi- the DBU–CO2 adduct to form carbonate I, which then re-
tuted ethylene diols 4a and 5a underwent cyclization to af- acts with CH2Br2 to form reactive carbonate II. DBU–CO2
ford 4b and 5b in 79 and 67% yield, respectively (Table 3, adducts are known to react with nucleophiles.[8a,9] The ad-
entries 2 and 3). Cyclic diol 6a also underwent cyclization dition of a primary alcohol to CO2 is known to be favored
to afford 6b in 73% yield (Table 3, entry 4). To form a six- over the addition of a secondary alcohol to CO2.[11] The
membered cyclic carbonate, 2,2-dimethyl-1,3-propanediol intramolecular addition of the secondary alcohol followed
(7a) was used to afford 7b, although it was obtained in a by the elimination of HOCH2Br affords 4b. The intramolec-
moderate yield (Table 3, entry 5). Cinnamyl alcohol (8a) af- ular addition of the carbonate nucleophile in intermediate
forded acyclic carbonate 8b in 56% yield (Table 3, entry 6). III may not be possible, as shown by the experimental result
Benzyl alcohol derivatives possessing various substituents obtained with the use of (S)-4a.
at the phenyl ring were subjected to the reaction conditions.
On the basis of the results shown in Table 3, entries 7–10,
the position of the substituents and the electron-donating/
withdrawing groups on the phenyl ring did not greatly af-
fect the yield. Compared to the aromatic carbonates, ali-
phatic carbonates 13b and 14b were formed in moderate
yield (Table 3, entries 11 and 12).
A plausible reaction mechanism for the formation of the
carbonates by using CO2 was investigated by 18O-labeling
experiments with the use of 18O-labeled 4a and optically
active (S)-4a (Scheme 1). 18O-Labeled 4a was converted
into 4b in 70% yield, and the oxygen 18O isotope was not
Scheme 2. Plausible mechanism for carbonate formation.
exchanged with 16O during the reaction. The cyclic carbon-
ate obtained from optically active (S)-4a was analyzed by
HPLC analysis; the chirality of the secondary alcohol was
Conclusions
retained without racemization or inversion of stereochemis-
try. In addition, experiments with no CH2Br2, 1 equiv. of
CH2Br2, 1 equiv. of CH3I, and CH2Cl2 (solvent) were per-
formed, which showed the effect of the halogen in the alkyl
halides in the reaction (Scheme 1). With 1 equiv. of CH2Br2,
4b was formed in 56% yield, and without CH2Br2, no prod-
uct was formed. Instead of CH2Br2, another reactive alkyl
halide, that is, CH3I, was tested to form 4b, but no product
was formed. Using a less-reactive alkyl dihalide, that is,
CH2Cl2, a lower yield (37%) was observed. On the basis of
We report the transition-metal-free synthesis of cyclic
and acyclic carbonates by using nontoxic carbon sources,
such as CO2 and alcohols. The direct coupling of alcohols
with CO2 was achieved by using CH2Br2 without the use of
Mitsunobu-type reagents or separate steps for alkyl halide
preparation. Diverse alcohols, such as glycerol, dimethyl
propanediol, benzyl alcohols, allyl alcohols, aliphatic
alcohols, and methanol were converted into cyclic and acy-
clic carbonates in moderate to good yields. In particular,
glycerol carbonate and dimethyl carbonate, which are at-
tractive and versatile compounds for use in diverse chemical
industries, were prepared in moderate to good yields. A
plausible mechanism for this reaction was proposed on the
basis of an 18O-labeling experiment and a chiral alcohol
experiment.
Experimental Section
General Methods: Anhydrous solvents were transferred by using
oven-dried syringes. 1H NMR spectra were recorded by using a
Varian Mercury Plus (400 MHz) spectrometer. Chemical shifts are
reported in part per million (ppm) downfield from tetramethylsil-
ane. 13C NMR spectra were recorded by using a Varian Mercury
Plus (100 MHz) spectrometer. Chemical shifts are reported in ppm
relative to the center of the triplet at δ = 77.00 ppm for deuterio-
chloroform. Commercially available chemicals were used as re-
ceived without further purification.
General Procedure for Carbonation of Alcohols by CO2: An auto-
clave reactor was charged with a solution of the alcohol (0.5 mmol)
in CH2Br2 (0.5 m), bmimPF6 (0.1 mL), and DBU (2 equiv.) under
Scheme 1. Control experiments.
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Acknowledgments
This study was supported by the Korea Research Foundation
(grant numbers 2009-0094046 and 2013008819) and by the Korean
Government, Ministry of Education, Science and Technology
[Korea CCS R&D Center (KCRC), grant number 2012-0008935].
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Received: January 8, 2014
Published Online: February 13, 2014
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