An efficient synthesis of organic carbonates: atom economic protocol
with a new catalytic system
Bhaskar Veldurthy and François Figueras*
Institut de Recherches sur la Catalyse du CNRS, 2 avenue A. Einstein, 69626 Villeurbanne, France
Received (in Cambridge, UK) 9th December 2003, Accepted 27th January 2004
First published as an Advance Article on the web 18th February 2004
Selective and solvent free synthesis of unsymmetrical organic
carbonates catalysed by a reusable MgLa mixed oxide is
achieved for the first time via direct condensation of an alcohol
and diethyl carbonate in economic route with excellent yields.
natural montmorillonite. The solid bases were Na-Laponite, KF/a-
Al2O3,22 HDT-F23 and MgLa mixed oxides.18,24 The reaction was
performed at ~ 125 °C in a three-necked round bottomed flask
equipped with a condenser. An excess of DEC (4 ml) and substrate
(2 mmol) were placed in the reactor. Freshly activated catalyst (0.1
g) was added and the experiment started with stirring in a nitrogen
atmosphere. Stirring was continued until the completion of the
reaction, as monitored by thin-layer chromatography (TLC). After
completion of the reaction, the catalyst was filtered off and the
products were analysed by 1H NMR. The structure and the purity of
the products were confirmed by GC-MS analysis.
The reaction gives selectively the unsymmetrical carbonate
when basic catalysis is used and a mixture of carbonate and ether/
olefin when acid catalysis is involved. Solid bases gave the
unsymmetrical carbonate not only with high selectivity but with a
faster rate. If we compare the time required for complete reaction
with the basic strength of the solids, we can conclude that the
reaction is faster with stronger bases. The reaction works efficiently
with a variety of alcohols including aromatic, cyclic, heterocyclic
or aliphatic (Table 2). The reactivity is higher with glycols
whatever their structure. In the series of substituted 1-phenyl-
ethanols, small changes in reactivity are observed with the nature of
substituents and an electron withdrawing group tends to give an
increase in rate (entry 3 and 4). High selectivity was achieved with
allylic alcohols in the present liquid phase reaction (entries 6 and 9).
Amine groups do not react under similar reaction conditions. This
inertness has been exploited in the corresponding selective O-
carbonate preparation in the case of 2-(4-aminophenyl)ethanol
(entry 5). The steric hindrance in proximity to the reactive hydroxyl
group represents again a limiting factor. It is noteworthy that,
whereas 1,2-diols gave cyclic carbonates (Table 3 entries 1 and 3),
cyclic products are not formed when the number of methylene
Organic carbonates have been utilized ubiquitously as inter-
mediates for the synthesis of fine chemicals,1,2 pharmaceuticals,3
plasticizers, synthetic lubricants,4 monomers for organic glasses,
and solvents.5,6 The reaction of phosgene with diols and the
coupling of halo formates with isolated alcohols and phenols are the
most common procedures,1,2,7–9 but these methods involving toxic
materials are not environmentally satisfactory. The modern synthe-
ses of dimethyl carbonate (DMC) have recently been re-
viewed.10–12 DMC is an environmentally friendly reactant which
can be substituted for phosgene in several reactions, in particular
the preparation of organic carbonates reviewed by Shaik and
Sivaram.13 The preparation of alkyl carbonates from alkyl halides
and alcohols has been performed using alkali metal carbonates or
carbon dioxide as an environmentally benign alternative. However,
these reactions produce stoichiometric amounts of salts and would
not meet environmental regulations. Recent reports describe the
liquid-phase synthesis of alkyl carbonates via coupling of an
alcohol, CO2 and alkyl halide in the presence of Cs2CO3 at ambient
temperatures.14 A solid-phase reaction of an alcohol or amine
ligated to a resin through a CO2 linker in the presence of caesium
carbonate and tetrabutylammonium iodide (TBAI) has also been
described.15 The main drawbacks of these procedures are the usage
of more than stoichiometric amounts of base and reagents, high
reaction time and moderate yields which make them unattractive
for green technology. Most recently, the reactions of several
alcohols on diethyl carbonate catalysed at ~ 125 °C by MCM-
41-TBD (1,5,7- triazabicyclo[4.4.0]dec-5-ene anchored on me-
soporous MCM-41 silica) requires 15–24 h.16 Since organics
grafted on silicas are not easily regenerated, a drive to develop
alternative methods is required.
We herein report an efficient method for the selective synthesis
of unsymmetrical organic carbonates in quantitative yields via
direct condensation of various alcohols with diethyl carbonate
(DEC) in the presence of a new reusable solid base MgLa mixed
metal oxide. This catalytic process (Scheme 1) reported for the first
time allows an eco-friendly and economic technology. MgLa
mixed oxide (Mg/La atomic ratio 3) was prepared as reported
earlier and calcined at 550 °C.17,18 It could then be regenerated at
high temperature by the oxidation of organic contaminants.
We have explored a variety of solid acid and basic catalysts for
the preparation of the corresponding carbonate using 1-(4-chloro-
phenyl)ethan-1-ol as a model reactant in presence of DEC (Table
1). The chosen solid acids were selected from catalysts which have
dual Lewis and Brønsted acidic sites as described earlier: sulfated
alumina,19 MCM-41 (Si/Al = 40), Cu2+ -K10, Zn2+-K1020,21 and
Table 1 Catalytic properties of various solids in direct condensation of
1-(4-chlorophenyl)ethan-1-ol with DEC: a model reaction for catalyst
screening
Yield a(%)
Reaction
Entry
Catalyst
time/h
I
II
1
2
3
4
5
6
7
8
9
MgLa mixed oxides
KF/a-Al2O3
3.5
5
15
2
0.5
0.5
0.3
0.4
15
100
100
—
—
—
HDT-F
46b
Laponite
95
31
—
—
—
5c
Al2O3·SO4(h = 1)
MCM-41 (Si/Al = 40)
Cu+2-K10 mont
Zn+2-K10 mont
Natural mont
69d
100e
100e
100e
No reaction
a Yield from H1NMR. b Remainder is the starting material. c p-Chlorostyr-
ene. d Olefin and corresponding di-ether. e Olefin, ether and unidentified
polymeric product.
Scheme 1
734
C h e m . C o m m u n . , 2 0 0 4 , 7 3 4 – 7 3 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4