Cycloaddition of CO2 to epoxides over both homogeneous and silica-supported guanidine catalysts

Article abstract of DOI:10.1016/S0040-4039(03)00424-6

The synthesis of cyclic carbonates by CO₂ insertion into epoxides catalysed by both homogeneous MTBD and TDB supported on MCM-41 mesoporous silica is reported.

Full text of DOI:10.1016/S0040-4039(03)00424-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2931–2934
Cycloaddition of CO to epoxides over both homogeneous and
2
silica-supported guanidine catalysts
a
a
a
b
Alessandro Barbarini, Raimondo Maggi, Alessandro Mazzacani, Giovanni Mori,
a,
a
Giovanni Sartori * and Raffaella Sartorio
a
Clean Synthetic Methodologies Group, Dipartimento di Chimica Organica e Industriale dell’Universit a` , Parco Area delle Scienze
7/A, I-43100 Parma, Italy
1
b
Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Parco Area delle Scienze 17/A,
I-43100 Parma, Italy
Received 20 December 2002; revised 11 February 2003; accepted 12 February 2003
Abstract—The synthesis of cyclic carbonates by CO insertion into epoxides catalysed by both homogeneous MTBD and TDB
2
supported on MCM-41 mesoporous silica is reported. © 2003 Elsevier Science Ltd. All rights reserved.
The use of carbon dioxide in synthetic and industrial
applications represents an important goal since the
Less extensively studied was the use of ionic liquids as
8
9
solvent catalysts and heterogeneous catalysts. Due to
the great interest in this topic and in continuation of
our studies on the preparation and use of organic
catalysts supported on different polymeric materials, we
present here a comparative study of the catalytic
efficiency of guanidine MTBD (7-methyl-1,5,7-triazabi-
cyclo[4.4.0]dec-5-ene) 1 and its heterogeneous counter-
part 2 (Fig. 1), obtained by covalently bonding TBD to
output of CO from combustion into the atmosphere
2
continues to raise and minimisation of its emission is
desirable and can be achieved by direct fixation into the
1
target compounds. Thus, new strategies aimed at
recovering and utilizing CO have assumed increasing
2
importance.
CO is an inexpensive reagent, environmentally benign,
2
MCM-41 silica, in the cycloaddition of CO with epox-
2
non-flammable, ubiquitous and easily separable from
reaction mixtures. However, due to the inert nature of
10
ides to give cyclic carbonates.
CO , its activation and incorporation into organic sub-
strates still remains a difficult target.
2
The special goals of this study were: (i) to collect
information concerning the possibility of running the
homogeneous reaction under catalytic conditions with
maximum yield and selectivity; (ii) to compare the
catalytic efficiency of the homogeneous and hetero-
geneous catalysts; (iii) to apply the optimised processes
to the synthesis of a series of cyclic carbonates.
A particularly studied reaction involving CO is the
2
cycloaddition with epoxides to produce cyclic carbon-
ates. These compounds show interesting applications as
polar aprotic solvents, as precursors for polycarbonate
materials and, in general, as intermediates in organic
2
synthesis. The reaction was performed by using differ-
ent classes of catalysts including groups IVA, VA and
3
4
VIA organometallic halides and alkaline halides. Alu-
minium and chromium porphyrinates and phthalocyan-
5
inates have also been utilised as well as other transition
metal complexes of nickel, palladium, chromium and
6
copper. Particular conditions to perform direct synthe-
sis of polycarbonates by copolymerisation of CO with
2
7
epoxides have also been set up.
*
Corresponding author. Tel.: +39 0521 905551; fax: +39 0521
05472; e-mail: sartori@unipr.it
9
Figure 1. Catalysts utilised in the study.
0
040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00424-6

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A. Barbarini et al. / Tetrahedron Letters 44 (2003) 2931–2934
In a first series of experiments the cycloaddition of CO2
40 ml, 50 bar) to styrene oxide (26 mmol) in acetoni-
trile (5 ml) was studied as a function of the amount of
catalyst MTBD under homogeneous conditions in a
small autoclave at 140°C (Scheme 1 and Fig. 2).
However, the efficiency of MCM-41-TBD in the
present reaction was shown by the possibility of recy-
cling the catalyst for almost three further cycles (64, 61
and 63% yield, respectively) after filtration, washing
(
11
14
with DMF and immediately reusing.
The yield of product 4 increases by increasing MTBD/3
molar ratio and reaches a maximum value of 96% by
using 4% mol of MTBD with respect to the reagent 3.
The optimised conditions with both homogeneous
(MTBD) and heterogeneous catalysts (MCM-41-TBD)
were used for the preparation of some cyclic carbonates
15
from the corresponding epoxides (Table 1).
The selectivity value is very high (>98%) at MTBD/3
ratio values <0.04 and decreases together with yield by
using larger amounts of the catalyst, due to the compet-
itive isomerisation of styrene oxide 3 to acetophenone
and phenylacetaldehyde.
All terminal epoxides afforded the corresponding cyclic
carbonates in good yield and excellent selectivity with
both homogeneous and heterogeneous catalysts. As
expected, being the average pore diameter of meso-
porous silica 30 A, the diffusion of reagents and prod-
,
ucts through the material is completely unrestricted and
no remarkable steric limitations could be observed in
the reaction of different epoxides with the supported
catalyst. Lower yields have been observed with internal
epoxides even if the selectivity was still good. However,
it is important to stress that in both procedures the
catalyst amount (4% molar ratio with respect to the
epoxide) is crucial in order to avoid side reactions such
as the epoxide isomerisation.
The further goal was the heterogenisation of the
organic catalyst by tethering TBD to the surface of a
siliceous support through an easy methodology
1
2
reported in the literature. The mesoporous MCM-41
silica was utilised as inorganic support since the
anchored active TBD molecules are expected to be
more easily accessible in the mesoporous material than
1
3
in an amorphous one. The model reaction was succes-
sively performed by using the supported catalyst
(
MCM-41-TBD, loading 1.27 mmol/g) whose amount
Concerning the mechanism, one can imagine that the
presence on the solid catalyst of hydroxy groups such
as the alcohol on the arm and the silanols on the
support surface can activate the epoxide by H-bond
formation. Nevertheless, no direct evidence is available
to confirm this hypothesis. The sole information we can
give is based on some preliminary experiments concern-
was determined on the basis of the loading value in
order to have a supported-TBD/3 ratio equal to 0.04.
The yield of product 4 in the model reaction was
studied as a function of time (Fig. 3). As expected,
production of 4 during the time with the supported
catalyst MCM-41-TBD is lower than that obtained in
the reaction carried out with the homogeneous counter-
part, suggesting that the supported catalyst suffer from
some diffusion resistance.
ing the possible interaction of CO with MTBD. In
2
fact, by bubbling CO at rt into an acetonitrile solution
2
of MTBD, after a few minutes the formation of a white
crystalline precipitate was observed. Whereas it has
1
been impossible to collect comprehensible H NMR or
FT-IR spectra of this material as well as X-ray diffrac-
tion data due to its great instability, we succeeded in
trapping intermediate 5 by reaction with CH I; the
3
O-methylated cation derived from 5 was detected by
ESI-MS (positive ion) analysis. In addition, by stirring
overnight at rt this crystalline precipitate in the pres-
ence of styrene oxide in acetonitrile, compound 4 was
obtained. On the basis of this simple and preliminary
Scheme 1. Reaction between styrene oxide and carbon diox-
ide in the presence of MTBD.
experiment, it is possible to hypothesise that CO could
2
Figure 3. Comparison of the yield of product 4 as a function
Figure 2. Yield (ꢀ) and selectivity (ꢁ) of product 4 as a
of time under homogeneous (ꢀ) and heterogenous (ꢁ) cataly-
function of time under MTBD catalysis.
sis.

A. Barbarini et al. / Tetrahedron Letters 44 (2003) 2931–2934
2933
Table 1. Synthesis of cyclic carbonates 4
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6
. (a) Tascedda, P.; Dun a˜ ch, E. J. Chem. Soc., Chem.
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7
. See for example: (a) Darensbourg, D. J.; White Stafford,
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Scheme 2. Hypothesised MTBD-promoted CO2 activation.
2602; (c) Cheng, M.; Moore, D. R.; Reczek, J. J.; Cham-
berlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am.
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be activated through the formation of the zwitterionic
compound 5 which can add to the epoxide via nucle-
ophilic attack, as previously reported for solid basic
11008–11009; (e) Darensbourg, D. J.; Holtcamp, M. W.;
Struck, G. E.; Zimmer, M. S.; Niezgoda, S. A.; Rainey,
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9
catalysts (Scheme 2).
In conclusion, we have demonstrated that the synthesis
8
of cyclic carbonates by CO insertion into epoxides can
2
be carried out under catalytic conditions by using
guanidines; the reaction performed with heterogenous
MCM-41-TBD is slower than that with homogeneous
MTBD, but shows the great advantage that the catalyst
can easily be recovered and reused for at least three
further cycles.
9
. (a) Yano, T.; Matsui, H.; Koike, T.; Ishiguro, H.; Fuji-
hara, H.; Yshihara, M.; Maeshima, T. Chem. Commun.
1997, 1129–1130; (b) Yamaguchi, K.; Ebitani, K.;
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1
2
999, 121, 4526–4527; (c) Tu, M.; Davis, R. J. J. Catal.
001, 199, 85–91.
1
1
0. Polymer-supported TBD is now commercially available
Acknowledgements
(Aldrich).
1. The experimental parameters were selected on the basis
of previous experiments made in our laboratories on
similar reactions.
We acknowledge the support of the Ministero del-
l’Istruzione, dell’Universit a` e della Ricerca (MIUR),
Italy, and the University of Parma (National Project
Processi puliti per la Chimica Fine’). The help rendered
by Dr. C. Guzzon and Mr. P. A. Bonaldi (Lillo) is also
acknowledged.
12. Subba Rao, Y. V.; De Vos, D. E.; Jacobs, P. A. Angew.
‘
Chem., Int. Ed. Engl. 1997, 36, 2661–2663.
13. Carloni, S.; De Vos, D. E.; Jacobs, P. A.; Maggi, R.;
Sartori, G.; Sartorio, R. J. Catal. 2002, 205, 199–204.
1
4. A possible catalytic effect of the surface silanols of the
support was excluded by carrying out a comparative
experiment in the presence of unfunctionalised MCM-41
silica; product 4 was detected only in traces.
References
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. (a) Aresta, M.; Quaranta, E.; Tommasi, I.; Giannoccaro,
P.; Ciccarese, A. Gazz. Chim. Ital. 1995, 125, 509–538; (b)
Behles, J. A.; De Simone, J. M. Pure Appl. Chem. 2001,
15. General procedure for the preparation of cyclic carbon-
ates 4: a mixture of the selected epoxide (26 mmol) and
the selected catalyst (1 mmol, 0.15 g for MTBD, 0.79 g

2
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A. Barbarini et al. / Tetrahedron Letters 44 (2003) 2931–2934
for MCM-41-TBD) in acetonitrile (5 ml) was charged in
TBD, the catalyst was removed by filtration) and the
products purified over silica gel column by using hexane–
ethyl acetate mixtures. All products 4a–f were character-
ised (IR, H NMR and MS) and their mps compared
with literature reported mps.
a small autoclave; then CO was introduced (50 bar) and
2
the mixture was heated at 140°C for the selected time.
The reaction mixture was then cooled to rt, methyene
chloride was added (in the experiment with MCM-41-
1