New highly active and selective heterogeneous catalytic system for the synthesis of unsymmetrical organic carbonates: A green protocol

Article abstract of DOI:10.1002/ejoc.200400888

A green, efficient, and selective synthesis of unsymmetrical organic carbonates is realised in the liquid phase by direct condensation of an alcohol and diethyl carbonate, in an ecocompatible route, in the presence of a recyclable heterogeneous solid-base CsF/α-Al₂O₃ catalyst. The catalyst displayed unprecedented activity after activation at 393 K for 4 h and produced quantitative yields with a greater rate compared with solid bases reported so far. The present process is a potential alternative to replace soluble bases in commercial synthesis. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejoc.200400888

SHORT COMMUNICATION
New Highly Active and Selective Heterogeneous Catalytic System for the
Synthesis of Unsymmetrical Organic Carbonates: A Green Protocol
Bhaskar Veldurthy,[a] Jean-Marc Clacens,[a] and François Figueras*[a]
Keywords: Unsymmetrical organic carbonates / Cyclic carbonates / Alcohols / Diethyl carbonate / CsF on α-alumina /
Green protocol
A green, efficient, and selective synthesis of unsymmetrical
organic carbonates is realised in the liquid phase by direct
condensation of an alcohol and diethyl carbonate, in an eco-
compatible route, in the presence of a recyclable hetero-
geneous solid-base CsF/α-Al2O3 catalyst. The catalyst dis-
played unprecedented activity after activation at 393 K for
4 h and produced quantitative yields with a greater rate com-
pared with solid bases reported so far. The present process is
a potential alternative to replace soluble bases in commercial
synthesis.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
which render them un-suitable for “green technology”. Re-
cently, a liquid-phase synthesis of alkyl carbonates by coup-
ling of an alcohol, CO2 and alkyl halide in the presence
of Cs2CO3 at ambient temperatures[12] and a solid-phase
reaction involving an alcohol or amine, ligated to a resin
through a CO2 linker, in the presence of Cs2CO3 and tet-
rabutyl ammonium iodide (TBAI)[13] to produce corre-
sponding carbonates or carbamates were reported. The use
of more than stoichiometric amounts of soluble bases as
catalysts, long reaction times with moderate yields makes
these procedures unattractive in terms of eco-friendliness
and economy. Most recently, the synthesis of unsymmetri-
cal organic carbonates as illustrated by the reaction of vari-
ous alcohols with diethyl carbonate catalysed at ca. 338 K
by MCM-41-TBD[14] (1,5,7-triazabicyclo[4.4.0]dec-5-ene
anchored on mesoporous MCM-41 silica) required 15–24 h
for high yields. The difficulty in regenerating a catalyst con-
sisting of organics grafted on silicas and long reaction time
deems this procedure unviable. Therefore, the challenge is
to develop a heterogeneous catalyst for the selective synthe-
sis of organic carbonates by an eco-economic route with
high throughput and which adheres to environment protec-
tion laws.
In our continued efforts for the development of the solid
base catalysts for selective organic transformations,[15,16]
herein we report a fast and selective synthesis of unsymmet-
rical organic carbonates in the presence of a new, recyclable
CsF/α-Al2O3 solid base catalyst by direct condensation of
various alcohols and diethyl carbonate (DEC). The reaction
conditions are optimised, to obtain exclusively the unsym-
metrical organic carbonate in quantitative yields
(Scheme 1). In the present process, the catalyst can be acti-
vated by drying at 393 K for 4 h prior to the reaction, thus
avoiding the usual high activation temperatures, and DEC
Introduction
In recent years, increasingly demanding regulations and
economic pressure have led to intense research to develop
new “clean and green” methods for the minimization of
toxic waste and by-products arising from the chemical pro-
cesses.[1,2] Organic carbonates have been utilized ubiqui-
tously as intermediates in fine chemicals, biological, and
medicinal fields,[2,3] and have also played an important role
as plasticizers, synthetic lubricants,[4] monomers for organic
glass,[5] and solvents.[6] The most common procedures for
the synthesis of these compounds include the reaction of
phosgene with diols and the coupling of halo compounds
with alcohols and phenols.[2a,7] Both methods involve the
use of toxic and hazardous materials, thus creating a drive
to develop alternative procedures. The modern synthesis of
dimethyl carbonate (DMC)[8] attracted considerable atten-
tion, as it is an environmentally benign alternative to phos-
gene in several reactions, in particular for the preparation
of organic carbonates as reviewed by Shaik and Sivaram.[9]
The synthesis of unsymmetrical dialkyl carbonates by the
transesterification of carboxylic acid diesters with an allyl
alcohol in the presence of a metal base catalyst[10] (chosen
from inorganic carbonates, alcoholates and hydroxides) and
of a symmetrical dialkyl carbonate with an alcohol, using
cesium-based salts[11] as catalyst have been demonstrated.
Both the procedures utilize crown ethers or polyethylene
glycols as co-catalysts in combination with the inorganic
bases and produce large amounts of salts as toxic waste,
[a] Institut de Recherches sur la Catalyse du CNRS,
2 av. A. Einstein, 69626 Villeurbanne, France
Fax: +33-47-244-5399
E-mail: figueras@catalyse.cnrs.fr
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
1972
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200400888
Eur. J. Org. Chem. 2005, 1972–1976

Catalytic System for the Synthesis of Unsymmetrical Organic Carbonates
SHORT COMMUNICATION
in excess is used as solvent, which can be easily separated ascribed to a strong reaction between the fluoride moiety
by distillation and recycled, so that the process appears as with the surface leading to the formation of fluoroalumi-
solvent-free.
nate as confirmed by powder XRD and 19F MAS NMR
spectroscopic studies.[16] Similar differences are observed
between the two different forms of TiO2: Rutile and ana-
tase. The rutile form is known to be basic; the anatase form
is acidic. Indeed, KF/rutile is more active than KF/anatase
(Entry 6 and 7). In homogeneous reactions, equimolar
amounts of homogeneous analogues of the corresponding
solid bases were employed. Effectively, less than 5% pro-
duct was obtained with KF even after 24 h (Entry 11),
whereas, with CsF, a complete conversion towards the de-
sired product was obtained in 12 h (Entry 12). No detect-
able product formation was observed with α-Al2O3 (Entry
13) and a reaction performed without any catalyst (Entry
14). These results provide an evidence that the CsF/α-Al2O3
is the best catalyst and that the high activity is attributed
to the high dispersion of CsF on α-Al2O3 support.
In an effort to understand the scope of the reaction, a
variety of alcohols including, alkyl, cyclic, heterocyclic, or
aryl (Table 2) and diols (Table 3) with DEC in the presence
of the best evolved catalytic system were investigated. All
the substrates were selectively transformed to the corre-
sponding unsymmetrical carbonates in quantitative yields
with a greater rate, compared to earlier reports. There is no
significant effect of the substituent in case of 1-phenyl-
ethanol (Table 2, Entry 2 and 3). High selectivity was
achieved with allylic alcohols in present liquid phase reac-
tion (Entry 5 and 7). The inertness of the amine group has
been exploited for the selective preparation of correspond-
ing O-carbonate in case of 2-(4-aminophenyl)-ethanol (En-
try 4). Chiral menthol is converted into the corresponding
carbonate without any racemization or inversion (Entry 9).
The steric hindrance due to the proximity of the reactive
hydroxy group represents again a limiting factor (Entries 2,
3, 8, 9, and 11).
Scheme 1.
Results and Discussion
During the course of current study, a variety of soluble
and solid base catalysts have been explored on 1-phenyle-
thanol, chosen as a model reactant in the presence of DEC.
Selected solid bases were KF on different supports[15] (ex-
cept KF/CaF, a commercial catalyst), CsF on α- and γ-alu-
mina and HDT-F.[16] The catalysts CsF/α-Al2O3, KF/α-
Al2O3, HDT-F and the homogeneous analogues KF and
CsF were activated by drying at 393 K for 4 h, while the
rest by calcining under nitrogen at 673 K for 3 h prior to
the reaction.
Surprisingly, all the catalysts exhibited high selectivity in
producing the corresponding carbonate, despite the large
differences in their catalytic activity as shown in Table 1, in
which the time required for complete conversion or the de-
gree of conversion after 12 h are a measure of activity. The
results obtained correlate-well with the basicity of the cor-
responding solids as measured by volumetric adsorption of
CO2. Complete conversion towards the corresponding pro-
duct was obtained with CsF/α-Al2O3, KF/α-Al2O3, and
HDT-F catalysts (Entry 1, 2, and 5). For the same amount
of fluoride, 1 mmol/g, the turnover (TON, catalytic cycles
per metal centre) reached with CsF/α-Al2O3 is significantly
higher (greater than 60 mol/mol of Cs) than that of KF/α-
Al2O3. HDT-F contains about 3 wt% F corresponding to
1.5 mmol/g and also shows a lower activity.[16] Supported
It is noteworthy that, 1,2-diols led to the formation of
KF and CsF on γ-Al2O3, produced moderate yields (Entry cyclic carbonates (Table 3, Entry 12 and 13), whereas cyclic
3 and 4) in agreement with a lower basicity. This may be products were not formed when the number of methylene
Table 1. Catalytic properties of various solid bases and their homogeneous analogues in direct condensation of 1-phenyl ethanol with
DEC. Supported fluorides contain 1 mmol fluoride per gram of support.
Entry[a]
Catalyst
Number of basic sites (μmol/g)
Reaction time (h)
Yields (%)[b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
CsF/α-Al2O3
KF/α-Al2O3
CsF/γ-Al2O3
KF/γ-Al2O3
HDT-F
11.4
14
–
98
47
–
–
–
–
–
–
–
–
–
0.45
5.0
12
12
12
12
12
12
12
12
12
12
12
12
100
100
57[c]
36[c]
100
88[c]
8[c]
KF/TiO2 (Rutile)
KF/TiO2 (Anatase)
KF/ZrO2
44[c]
5[c]
KF/Al-grafted Si-MCM-41
KF/CaF (20 wt.-%)
KF (pure)
12[c]
Ͻ5[c]
100
NR
NR
CsF (pure)
α-Al2O3 (pure)
Blank[d]
[a] All the reactions performed on 2 mmol alcohol with 33 mmol of DEC and 0.1 g of catalyst in nitrogen atmosphere at 403 K. [b]
Yields are calculated with H1 NMR and GC. [c] Remaining is the starting material. [d] In absence of catalyst; NR represents no reaction.
Eur. J. Org. Chem. 2005, 1972–1976
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1973

B. Veldurthy, J.-M. Clacens, F. Figueras
SHORT COMMUNICATION
Table 2. Direct condensation of various alcohols with DEC with CsF/α-Al2O3 as catalyst in solvent-free system.
[a] All the reactions performed on 2 mmol alcohol with 33 mmol of DEC and 0.1 g of catalyst under nitrogen at 403 K. [b] Isolated
yields. [c] 2nd cycle. [d] 3rd cycle. [e] 4th cycle.
groups between the alcohol functions increased. Thus, 2,5- alkoxide such as 1 can react with DEC by forming an inter-
hexanediol and 1,4-cyclohexanediol yielded the correspond- mediate [2] to yield unsymmetrical alkyl carbonate 3 and
ing bis-carbonates as the sole reaction products (Entry 14 producing ethanol 4 as by-product, presumably because
and 15).
alkoxides conjugated with cesium are considered to consti-
A plausible mechanism could be represented by a typical tute “naked anions” exhibiting enhanced nucleophilicit-
transesterification process, where the more nucleophilic rea- ies.[17] Therefore, it is strongly believed that the Cs-alkoxides
gent displaces the less nucleophilic one or when both the constitute weakly coordinated species that enhance nucleo-
reagents have similar nucleophilicity, the less volatile com- philicity to a degree that affects the rate of nucleophilic at-
pound displaces the more volatile one. In the present pro- tack on DEC, resulting in higher yields.
cess, the nucleophilic displacement of the ethoxy group by
Recycling of the catalyst was investigated by taking out
a second molecule of the alcoholic reagent probably leads the reaction mixture after completion of the reaction, leav-
to the corresponding unsymmetrical carbonate. As shown ing the catalyst in the lowest possible amount of liquid and
in Scheme 2, the catalytic cycle maybe initiated by abstrac- then adding a new feed of reactants. As reported (Table 2,
tion of a proton by a negatively charged fluoride moiety Entries 2, 3 and 9) the catalyst can be recycled three times,
from an alcohol to generate an alkoxide anion stabilised but a longer reaction time is required for the third cycle.
at the cesium surface. Unlike other alkali equivalents, Cs- Interestingly, the chemical analysis of the used catalysts (af-
1974
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2005, 1972–1976

Catalytic System for the Synthesis of Unsymmetrical Organic Carbonates
SHORT COMMUNICATION
Table 3. Direct condensation of various diols with DEC with CsF/ cedure are (a) facile handling of the solid base, (b) greater
α-Al2O3 as catalyst in solvent-free system.
reaction rate and quantitative yields with all the substrates
tested, (c) elimination of further product purification, and
(d) solvent free and direct condensation optimising the
highest atom economy. The potential for commercial appli-
cation is strengthened by the high throughput of the prod-
ucts, lower process inventories and use of an easily recycla-
ble system.
Experimental Section
Supporting Information (see also the footnote on the first page of
this article): Literature procedures for known compounds used in
this work.
Catalyst: The catalyst was CsF/α-Al2O3 prepared by incipient wet-
ness impregnation using an aqueous solution of fluoride. 1 mmol
CsF or 1 mmol KF were loaded per gram of α-alumina (SPH, 512,
10.5 m2/g), γ-alumina (SCP, 350, 400 m2/g) from Rhône Poulenc.
The other supports were anatase (44 m2/g) and rutile (6 m2/g) from
Millenium Chemicals, and zirconia from MEL. The number of ba-
sic sites was measured by volumetric adsorption of CO2.
[a] All the reactions performed on 2 mmol alcohol with 33 mmol
Catalytic Reactions: The reactants (purum quality) were purchased
of DEC and 0.1 g of catalyst under nitrogen at 403 K. [b] Isolated
from Aldrich and used without any further purification. In a typi-
yields.
cal reaction, an excess of DEC (33 mmol) and 1-phenylethanol
(2 mmol), were placed in a three-neck round-bottomed flask
equipped with a condenser. Freshly activated catalyst (100 mg) was
added and the experiment started with stirring under nitrogen at
ca. 403 K. Stirring was continued until the completion of the reac-
tion, as monitored by thin-layer chromatography (TLC) (ethyl ace-
tate/hexane, 1:10). After completion of the reaction, reaction mix-
ture was filtered and the catalyst washed with DEC (2×2.5 mL) to
wash off the product adhered on the surface of the catalyst. The
combined filtrate fractions were concentrated by distillation under
reduced pressure to obtain almost clean product. The excess DEC
that is removed from the reaction mixture can be reused in subse-
quent reaction without further purification. The products were
analysed by 1H NMR and the structure and purity of the products
were confirmed by GC-MS analysis. For the recycling experiments,
the products and the starting materials were separated by the col-
umn chromatography in order to purify them. Silica gel (60–120
mesh) was used for the column and the mobile phase was 5 to 20%
ethylacetate in hexane system.
1H and 13C NMR spectra were recorded with a Bruker AM 250
spectrometer (250 and 62.5 MHz, respectively). The chemical shifts
are reported in ppm (δ scale) relative to internal TMS and coupling
constants are reported in Hertz (Hz). The elemental analysis and
mass spectrometry were carried out at Service Central d’Analyse
(CNRS), Echangeur de Solaize, BP 22 – Vernaison, France. Ele-
mental analysis (percentage of the C and H were determined with
infrared analysis of the gases produced upon combustion of the
product at 1000 °C and for the N: the NOx produced was reduced
to N2 gas and measured by GLC using catharometric detection).
Mass spectrometry was performed using LCT (Micromass –
Waters) electrospray positive mode (ESI) technique to determine
the molecular ion peak (single mass) for all the new compounds;
except compound 7, for which EI mass was performed at 70 eV.
Scheme 2. Plausible mechanism for the CsF/α-Al2O3 catalysed syn-
thesis of unsymmetrical organic carbonates.
ter 4th cycle) revealed that there was no loss of Cs, K, and
F. Therefore, the cause for the deactivation of the catalyst
may be related to partial hydrolysis of products due to the
traces of water present in the new feed of reactants which
would inhibit the reaction. Indeed, basic sites are defects of
the solid, which are very few (typically 30 μmol/g on CsF/
α-Al2O3) so that traces of acid formed from water in the
reactants are sufficient to kill the catalyst since they are
strongly adsorbed at the surface.
Characterizations data for the new compounds
In summary, we have shown that a new heterogeneous
solid base CsF/α-Al2O3 is a highly efficient and reusable
catalyst for the selective synthesis of unsymmetrical organic
Ethyl 1-(4-Chloro)phenethyl Carbonate (3): Colourless oil. 1H
NMR (250 MHz, CDCl3): δ [ppm] = 7.32 (s, 4 H, aromatic), 5.7
carbonates. The main advantages of the described pro- [q, J = 6.6 Hz, 1 H, CH(CH3)], 4.38–4.15 (m, 2 H, CH3CH2OCO2),
Eur. J. Org. Chem. 2005, 1972–1976
www.eurjoc.org © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1975

B. Veldurthy, J.-M. Clacens, F. Figueras
SHORT COMMUNICATION
1.58 (d, J = 6.6 Hz, 3 H, OCHCH3), 1.29 (t, J = 7.1 Hz, 3 H, OCO2CH2), 2.1–1.55 (m, 8 H, cyclic), 1.32 (t, J = 7.1 Hz, 6 H, 2
OCO2CH2CH3). 13C NMR (63 MHz, CDCl3): δ [ppm] = 154.43 OCO2CH2CH3). 13C NMR (63 MHz, CDCl3): δ [ppm] = 154.59
(OCO2), 139.67 (Cl–C), 133.9 [CCH(CH3)], 128.76–127.49 (CH, (OCO2), 74.33–73.58 (CHO), 63.82–63.77 (OCO2CH2CH3), 27.51–
aromatic), 75.43 (CH3CHOCO2), 64.03 (OCO2CH2CH3), 22.28 27.18 (CH2, cyclic), 14.28 (OCO2CH2CH3). MS (ES+): m/z =
[CH(CH3)], 14.23 (OCO2CH2CH3). MS (ES+): m/z = [MNa+] 251. [MH+] 261. C12H20O6: calcd. C 55.37, H 7.74; found C 55.33, H
C11H13O3: calcd. C 57.78, H 5.73; found C 58.10, H 5.75.
7.67.
Ethyl 2-(4-Amino)phenethyl Carbonate (4): Pale yellow oil. 1H
NMR (250 MHz, CDCl3): δ [ppm] = 7.02 (d, J = 8.5 Hz, 2 H,
aromatic), 6.65 (d, J = 8.3 Hz, 2 H, aromatic), 4.26 (t, J = 7.2 Hz,
2 H, OCO2CH2CH3), 4.18 (q, J = 7.1 Hz, 2 H, CH2CH2OCO2),
3.6 (br., 2 H, NH2), 2.85 (t, J = 7.3 Hz, 2 H, CH2CH2OCO2), 1.3
(t, J = 7.1 Hz, 3 H, OCO2CH2CH3). 13C NMR (63 MHz, CDCl3):
δ [ppm] = 155.15 (OCO2), 145.09 (CNH2), 129.75 (CH, aromatic),
126.94 (CCH2), 115.27 (CH, aromatic), 68.60 (CH2CH2OCO2),
63.86 (OCO2CH2CH3), 34.28 (CH2CH2OCO2), 14.27 (OC-
O2CH2CH3). MS (ES+): m/z = [MNa+] 232. C11H15O3N: calcd. C
63.14, H 7.23, N 6.69; found C 63.39, H 7.37, N 6.63.
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5.67 (m, 1 H, olefinic), 4.55 (d, J = 6.5 Hz, 2 H, CHCH2OCO2),
4.18 (q, J = 7.1 Hz, 2 H, OCO2CH2CH3), 2.04 (q, J = 7.1 Hz, 2
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(250 MHz, CDCl3): δ [ppm] = 4.25–4.05 (m, 5 H, CH3CH2OCO2,
CHOCH2OCO2, CHOCH2OCO2), 3.95–3.75 (m, 2 H, CH2OCH2),
2.1–1.85 (m, 3 H, furfuryl), 1.75–1.55 (m, 1 H, furfuryl), 1.3 (t, J
= 7.1 Hz, 3 H, CH3CH2OCO2). 13C NMR (63 MHz, CDCl3): δ
[ppm] = 155.12 (OCO2), 76.24 (OCH), 69.43–68.43–64.05 (OCH2),
27.87–25.64 (CH2), 14.26 (CH3CH2OCO2). MS (ES+): m/z =
[MNa+] 197. C8H14O4: calcd. C 55.17, H 8.1; found C 55.6, H 7.98.
Ethyl 1-Cyclohexylethyl Carbonate (11): Colourless oil. 1H NMR
(250 MHz, CDCl3): δ [ppm] = 4.64–4.52 (quintet, J = 6.3 Hz, 1 H,
CHO(CH3)), 4.24–4.12 (q, J = 7.1 Hz, 2 H, CH3CH2OCO2), 1.85–
1.6 (m, 4 H, cyclic), 1.59–1.42 (m, 1 H, cyclic CH), 1.31 (t, J =
7.1 Hz, 3 H, CH3CH2OCO2), 1.23 [d, J = 6.4 Hz, 3 HCHO(CH3)],
1.3–0.9 (m, 6 H, cyclic). 13C NMR (63 MHz, CDCl3): δ [ppm] =
155.08 (OCO2), 78.91 (OCHCH3), 63.60 (CH3CH2OCO2), 42.61
(CH, cyclic), 28.48–28.35–26.35–26.03–25.97 (CH2, cyclic), 17.00
[CH(CH3)], 14.31 (CH3CH2OCO2). MS (ES+): m/z = [MNa+] 223;
C11H20O3: calcd. C 65.99, H 10.07; found C 66.58, H 10.14.
2,5-Bis(ethoxycarbonyloxy)hexane (14): Colourless oil. 1H NMR
(250 MHz, CDCl3): δ [ppm] = 4.85–4.7 (m, 2 H, 2 CHOCH3), 4.2
(q, J = 7.1 Hz, 4 H, 2 OCO2CH2), 1.78–1.55 (m, 4 H, CH2CH2),
1.31 (t, J = 7.2 Hz, 6 H, 2OCO2CH2CH3), 1.29 (d, J = 6.2 Hz, 6
H, 2CHOCH3). 13C NMR (63 MHz, CDCl3): δ [ppm] = 154.78
(OCO2), 74.81–74.48 (CHOCO2), 63.74 (OCO2CH2CH3), 31.82–
31.39 (CHCH2), 19.98–19.86 (CHCH3), 14.27 (OCO2CH2CH3).
MS (ES+): m/z = [MNa+] 285; C12H22O6: calcd. C 55.95, H 8.45;
found C 55.94, H 8.60.
[17] a) C. Galli, Org. Prep. Proced. Int. 1992, 24, 285–307 .and refer-
ences cited therein; b) F. Chu, E. E. Dueno, K. W. Jung, Tetra-
hedron Lett. 1999, 40, 1847–1850.
1,4-Bis(ethoxycarbonyloxy)cyclohexane (15): White crystalline so-
lid, m.p. 46 °C (uncorrected). 1H NMR (250 MHz, CDCl3): δ [ppm]
= 4.78–4.65 (br., 2 H, 2 CHOCO2), 4.2 (q, J = 7.1 Hz, 4 H, 2
Received: December 16, 2004
1976
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