Catalytic activity of MCM-41-TBD in the selective preparation of carbamates and unsymmetrical alkyl carbonates from diethyl carbonate

Article abstract of DOI:10.1006/jcat.2001.3439

The synthesis of carbamates 3 and unsymmetrical alkyl carbonates 5 by reaction of diethyl carbonate with aliphatic amines or alcohols has been realized by using as heterogeneous catalyst a hybrid organic-inorganic material prepared by anchoring TBD to MCM-41 silica. Products are obtained in high yield and very good selectivity and the solid catalyst can be recovered simply by filtration and reused for different cycles without apparent lowering of activity. A supported N -carbethoxyguanidinium active intermediate is proposed, and some spectroscopic data are shown to support the mechanistic hypothesis.

Full text of DOI:10.1006/jcat.2001.3439

Journal of Catalysis 205, 199–204 (2002)
doi:10.1006/jcat.2001.3439, available online at http://www.idealibrary.com on
Catalytic Activity of MCM-41–TBD in the Selective Preparation
of Carbamates and Unsymmetrical Alkyl Carbonates
from Diethyl Carbonate
,1
Silvia Carloni, Dirk E. De Vos,† Pierre A. Jacobs,† Raimondo Maggi, Giovanni Sartori,
and Raffaella Sartorio
Dipartimento di Chimica Organica e Industriale dell’Universita`, Parco Area delle Scienze 17A, I-43100 Parma, Italy; and †Center for Surface Science
and Catalysis, K. U. Leuven, Kardinaal Mercierlaan 92, B-3001 Leuven, Belgium
Received July 12, 2001; revised October 8, 2001; accepted October 10, 2001
tion requires a very large amount of catalyst and no proof
of the effective heterogeneity of the process was given.
The transesterification of dialkyl carbonates with alco-
hols or phenols represents a fundamental and safe route to
different symmetrical alkyl and aryl carbonates (12). The
green character of this approach is particularly evident, tak-
ing into account the fact that dimethyl carbonate is now pre-
pared on a large scale from methanol and carbon monoxide
in the presence of transition metal catalysts, which avoids
the use of dangerous reagents such as phosgene (13).
In past decades metal and nonmetal catalysts bound to
the surface of solid supports have attracted great inter-
est since they combine the advantages of homogeneous
catalysis with the easy recovery and reuse of heteroge-
neous materials (see e.g., 14). According to this approach,
a few years ago some of us reported the preparation of
a new solid-base catalyst obtained by immobilizing 1,5,7-
triazabicyclo[4.4.0]dec-5-ene (TBD) on the mesoporous
MCM-41 silica [MCM-41–TBD] (15). The catalytic effi-
ciency of this material in promoting classical aldol-like and
epoxidation reactions was also shown. More recently dif-
ferent guanidines immobilized on siliceous supports were
reported as efficient epoxidation catalysts for electron de-
ficient alkenes (16).
These results prompted us to investigate the use of silica-
supported TBD as catalyst for the phosgene-free synthesis
of carbamates 3 and unsymmetrical alkyl carbonates 5 by
reaction of diethylcarbonate 1 with an amine or an alcohol
(Scheme 1).
These reactions represent typical aminolysis or transes-
terification processes, where the more nucleophilic reagent
displaces the less nucleophilic one, or if both reagents have
similar nucleophilicity, where the less volatile compound
displaces the more volatile one (see, e.g., 17). Moreover,
since both reactions occur in steps, the formation of 3 or 5
can be followed by a further nucleophilic displacement of
the ethoxy group by a second molecule of the amine or of
the alcoholic reagent giving the corresponding urea or the
The synthesis of carbamates 3 and unsymmetrical alkyl carbon-
ates 5 by reaction of diethyl carbonate with aliphatic amines or
alcohols has been realized by using as heterogeneous catalyst a
hybrid organic–inorganic material prepared by anchoring TBD to
MCM-41 silica. Products are obtained in high yield and very good
selectivity and the solid catalyst can be recovered simply by filtra-
tion and reused for different cycles without apparent lowering of
activity. A supported N-carbethoxyguanidinium active intermedi-
ate is proposed, and some spectroscopic data are shown to support
the mechanistic hypothesis.
c
2002 Elsevier Science
Key Words: heterogeneous basic catalyst; MCM-41; tethering;
carbamates; transesterification; TBD.
1. INTRODUCTION
Carbamates (1) and alkyl carbonates (2) are compounds
of great interest in organic chemistry because they find
many important applications in pharmacology (3) as phar-
maceuticals, in agriculture (4) as herbicides, fungicides, and
pesticides, and in chemical industry (5) as intermediates.
Their syntheses generally proceed through the condensa-
tion of amines or alcohols with toxic carbonyl compounds
such as phosgene (6) or more stable and less harmful
reagents, the vast majority of which are still produced from
phosgene (7). Following a milder and safer approach, car-
bamates are prepared by reaction of amines with tetraethy-
lammonium hydrogen carbonate (8) or carbon dioxide (9)
and alkyl halides. While these reactions can be regarded as
highly efficient processes utilizing clean and safe reagents,
the production of a stoichiometric amount of salts brings
about an increase in the E factor (10) and limits their large-
scale applicability. More recently the synthesis of carba-
mates from amines and dimethyl carbonate was performed
with -alumina as a catalyst (11). Unfortunately the reac-
1 To whom correspondence should be addressed. Fax: +39 0521 905472.
E-mail: sartori@unipr.it.
199
0021-9517/02 $35.00
2002 Elsevier Science
All rights reserved.
c

200
CARLONI ET AL.
2.4. Reaction Procedure for Unsymmetrical Alkyl
Carbonate Synthesis
RNH2
O
2
+ EtOH
+ EtOH
RNH
RO
OEt
OEt
A mixture of the selected alcohol (10 mmol), DEC
(20 ml), and MCM-41-TBD (0.100 g) was stirred at reflux
( 125 C) for 15 h. After cooling to rt, the reaction mix-
ture was filtered and the catalyst was washed with diethyl
ether; the solvent and the residual DEC were removed
under reduced pressure and the crude product was chro-
matographed on silica gel using a mixture of hexane : ethyl
acetate, 80 : 20, as eluent.
3
O
Solid base
EtO
OEt
ROH
4
O
1
5
SCHEME 1
2.5. Spectroscopic Analysis
symmetric carbonate. Selective production of compounds
3 and 5 can be achieved by carefully controlling the nature
of reagents and the experimental conditions.
1
Melting and boiling points are uncorrected. H NMR
spectra were recorded at 300 MHz. Mass spectra were ob-
tained in EI mode at 70 eV. Microanalyses were carried out
at the Dipartimento di Chimica Generale ed Inorganica,
ChimicaAnalitica, ChimicaFisicadell’Universita` diParma,
Italy. Fourier transform IR (FT-IR) spectra were recorded
on a Nicolet PC 5 spectrophotometer.
To the best of our knowledge, a detailed characterization
of the below-reported products has never been published
before and therefore it is included here.
2. EXPERIMENTAL
2.1. Materials
All reagents were of commercial quality and were taken
from freshly opened containers.
2.2. Catalyst Preparation
2-(N-Ethylamino)-ethyl-carbamic acid ethyl ester (3i):
white solid, m.p. 51.5-52.5 C; H NMR (CDCl3) 5.1 (br
1
The synthesis of KG-60–TBD and MCM-41–TBD was
performed as previously reported in the literature (15).
The synthesis of 7-(2-hydroxy-3-isopropoxy-propyl)-
1,5,7-triazabicyclo[4.4.0]dec-5-ene (HI–TBD) was per-
formed as follows: A mixture of TBD (10 mmol, 1.39 g)
and glycidyl isopropyl ether (12 mmol, 1.39 g, 1.52 ml) in
tetrahydrofuran (10 ml) was stirred at room temperature
(rt) for 15 h; the solvent was removed under reduced pres-
sure and the product purified by distillation. Colorless oil,
b.p. 175–178 C/1 mm Hg; 1H NMR (CDCl3): 3.8-3.7 (m,
1H, CHOH), 3.55 (m, 1H, J = 6.1 Hz, (CH3)2CH), 3.5-3.2
(m, 8H, 2 CH2N, NCH2CH(OH), OCH2CHOH), 3.09 (t,
2H, J = 6.2 Hz, CH2N), 3.07 (t, 2H, J = 6.2 Hz, CH2N),
1.92 (m, 2H, J = 11.2 and 1.5 Hz, NCH2CH2CH2N), 1.77
(m, 2H, J = 5.8 Hz, NCH2CH2CH2N), 1.12 (d, 3H, J =
6.1 Hz, CH3), 1.11 (d, 3H, J = 6.1 Hz, CH3); IR (NaCl)
3132, 1591cm 1;MSm/z (M+ + 1)256(100), (M+)255(15),
254 (25), 212 (18), 182 (19). Anal. Calcd. for C13H25N3O2:
C, 61.15; H, 9.87; N, 16.46. Found: C, 61.29; H, 9.78; N, 16.55.
s, 1H, NHCO), 4.09 (q, 2H, J = 7.0 Hz, CH3CH2O),
3.25 (br q, 2H, J = 5.6 Hz, CH2CH2NHCO), 2.72 (t, 2H,
J = 5.9 Hz, CH2CH2NHCO), 2.63 (q, 2H, J = 7.1 Hz,
CH3CH2NH), 1.22 (t, 3H, J = 7.0 Hz, CH2CH2O), 1.08 (t,
1
3H, J = 7.1 Hz, CH3CH2NH); IR (NaCl) 3398, 1700 cm
;
MS m/z (M+ + 1) 161 (15), 115 (20), 86 (65), 84 (100). Anal.
Calcd. for C7H16N2O2: C, 52.48; H, 10.07; N, 17.49. Found:
C, 53.01; H, 9.88; N, 17.05.
Ethyl octyl carbonate (5n): colorless oil (37), b.p. 91–
1
92 C/4 mm Hg; H NMR (CDCl3) 4.18 (q, 2H, J = 7.1
Hz, CH3CH2O), 4.11 (t, 2H, J = 7.0 Hz, CH2CH2O),
1.66 (m, 2H, J = 7.0 Hz, CH2CH2O), 1.4-1.2 (m, 13H,
CH3CH2CH2CH2CH2CH2CH2), 0.87 (t, 3H, J = 7.0 Hz,
CH3CH2O); IR (NaCl) 1748 cm 1; MS m/z (M+ + 1) 203
(12), (M+) 202 (100), 173 (16). Anal. Calcd. for C11H22O3:
C, 65.31; H, 10.96. Found: C, 65.25; H, 10.83.
Bicyclo[2.2.1]hept-2-yl ethyl carbonate (5o): colorless
1
oil, b.p. 65–68 C/0.02 mm Hg; H NMR (CDCl3) 4.52
(br d, 1H, J = 7.0 Hz, CHOCO), 4.16 (q, 2H, J = 7.1
Hz, CH3CH2O), 2.38 (d, 1H, J = 4.4 Hz, CH), 2.28 (m,
2.3. Reaction Procedure for Carbamate Synthesis
1
1H, CH), 1.74 (ddd, 1H, J = 13.5, 7.0 and 2.5 Hz,
/
/
2
1
1
A mixture of the selected amine (10 mmol), diethyl car- CHCH2CHOCO), 1.6-1.4 (m, 4H,
/ CHCH2CH,
2
2
bonate (DEC; 20 ml), and MCM-41–TBD (0.100 g) was CHCH2CH2CH, 1/2 CHCH2CHOCO), 1.30 (t, 3H, J = 7.1
1
1
stirred at reflux ( 125 C) for 15 h. After cooling to rt, the Hz, CH3CH2O), 1.2-1.0 (m, 3H,
/
CHCH2CH,
/
2
2
reaction mixture was filtered and the catalyst was washed CHCH2CH2CH); IR (NaCl) 1739 cm 1; MS m/z (M+ + 1)
with diethyl ether; the solvent and the residual DEC were 185 (15), 111 (14), 95 (100). Anal. Calcd. for C10H16O3: C,
removed under reduced pressure by distillation and the 65.19; H, 8.75. Found: C, 65.28; H, 8.88.
crude product was chromatographed on silica gel using a
mixture of hexane : ethyl acetate, 80 : 20, as eluent.
Ethyl (1R)-menthyl carbonate (5p): 1H NMR (CDCl3)
4.50 (td, 1H, J = 11.4 and 4.4 Hz, CHOCO), 4.17 (q, 2H,

CATALYTIC ACTIVITY OF MCM-41–TBD IN SELECTIVE PREPARATIONS
201
J = 7.1 Hz, OCH2CH3), 2.07 (dtd, 1H, J = 11.4, 3.9 and
BzNH2
2a
O
O
O
2.6 Hz, CHCH(CH3)2), 1.96 (m, 1H, J = 6.7 and 2.6 Hz,
CH(CH3)2), 1.30 (t, 3H, J = 7.1 Hz, OCH2CH3), 1.8-0.8
(m, 7H, CH2CH2CH(CH3)CH ), 0.91 (d, 3H, J = 6.7 Hz,
+
EtO
OEt
BzHN
OEt
BzHN
NHBz
Catalyst
1
3a
CH CHCH3), 0.89 (d, 3H, J =26.7 Hz, CH3CHCH3), 0.78
3
1
(d, 3H, J = 7.0 Hz, CH3CHCH2); IR (NaCl) 1742 cm
;
MS m/z (M+ + 1) 229 (3), 227 (5), 139 (100). Anal. Calcd.
for C13H24O3: C, 68.38; H, 10.60. Found: C, 68.29; H, 10.78.
1,3-Bis-ethoxycarbonyloxy-propane (5s): colorless oil,
Catalysts:
N
1
O
b.p. 80–84 C/0.02 mm Hg; H NMR (CDCl3) 4.23 (t,
MCM-41
KG-60
MCM-41-TBD
O
Si
O
N
N
4H, J = 6.3 Hz, 2 CH2CH2O), 4.18 (q, 4H, J = 7.1 Hz,
2 CH3CH2O), 2.04 (m, 2H, J = 6.3 Hz, OCH2CH2CH2O),
1.30 (t, 6H, J = 7.1 Hz, 2 CH3); IR (NaCl) 1746 cm 1; MS
m/z (M+ +1) 221 (2), 131 (100). Anal. Calcd. for C9H16O6:
C, 49.09; H, 7.32. Found: C, 48.93; H, 7.19.
O
OH
N
O
KG-60-TBD
PS-TBD
1,6-Bis-ethoxycarbonyloxy-hexane (5t): colorless oil, b.p.
115–120 C/0.02 mm Hg [lit. (18) b.p. 130–140 C/0.8 mm
O
Si
O
N
N
O
OH
Hg]; 1H NMR (CDCl3)
4.18 (q, 4H, J = 7.1 Hz, 2
CH3CH2O), 4.12 (t, 4H, J = 6.6 Hz, 2 CH2CH2CH2O),
1.8-1.6 (m, 4H, 2 CH2CH2CH2O), 1.5-1.3 (m, 4H, 2
CH2CH2CH2O), 1.30 (t, 6H, J = 7.1 Hz, 2 CH3); IR (NaCl)
1745 cm 1; MS m/z (M+ + 1) 263 (100), 173 (22). Anal.
Calcd. for C12H22O6: C, 54.95; H, 8.45. Found: C, 55.11; H,
8.53.
N
Pol
N
N
3. RESULTS AND DISCUSSION
Pol
N
PS-DMAP
HI-TBD
3.1. Catalyst Effect on Carbamate Synthesis
N
To check the feasibility of our approach, the model
reaction between benzylamine 2a and diethylcarbonate
1 (DEC) employed as solvent–reagent was performed
in the presence of two solid catalysts prepared by im-
mobilization of TBD on mesoporous MCM-41 silica
(MCM-41–TBD) (19) or amorphous silica KG-60 (KG-
60–TBD) (20) following the methodology reported in the
literature (15). These prepared catalysts were compared
with two commercially available basic solid catalysts
usually employed in trans-esterification reactions, namely
1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine poly-
styrene-bound (PS–TBD; Aldrich) and dimethylaminopy-
ridine polystyrene-bound (PS–DMAP; Aldrich). All the
reactions were carried out at 125 C for 4 h with 0.100 g
N
O
N
N
OH
N
DMAP
N
SCHEME 2
of MCM-41–TBD, 0.220 g of KG-60–TBD, 0.04 g of 40, 6, and 53% yield, respectively. At this point we decided
PS–TBD, or 0.07 g of PS–DMAP, corresponding to the to focus our attention on the activity of the three best cata-
same amount of supported base (0.10 mmol). For com- lysts. Yields of product 3a versus time for the three reactions
parison the reaction was performed under homogeneous are shown in Fig. 1.
conditions using 7-(2-hydroxy-3-isopropoxy-propyl)-1,5,7-
The activity of MCM-41–TBD strongly parallels that of
triazabicyclo[4.4.0]dec-5-ene (HI–TBD) (0.10 mmol, the corresponding homogeneous counterpart, whereas the
0.026 g) containing the same molecular framework as the KG-60–TBD shows lower efficiency. These results seem
supported TBD and 4-dimethylaminopyridine (DMAP) to confirm that the level of structural order of the inor-
(0.10 mmol, 0.012 g) (see Scheme 2).
ganic support can influence the catalyst efficiency since
The preliminary results showed the total inertia of the supported active sites are more easily accessible in
DMAP and PS–DMAP, whereas MCM-41–TBD, KG-60– the mesoporous structure than in the amorphous material
TBD, PS–TBD, and HI–TBD afforded carbamate 3a in 56, (21). In fact, diffusion of reagents and products through the

202
CARLONI ET AL.
O
100
80
60
40
20
0
Et O
OEt
O
1
O
O
O
O
O
Si
Si
MCM-41-TBD
HI-TBD
O
O
KG-60-TBD
HO
_
HO
0
4
8
12
16
O
N
Time (h)
OEt
N
N
N
N
+
N
FIG. 1. Reactivity of benzylamine 2a with diethylcarbonate 1 over
different TBD-based catalysts as a function of time.
OEt
6
long-range ordered mesoporous material MCM-41–TBD
1
with a surface area of 385 m2 g is expected to be almost
H
N
H
completely unrestricted, while KG-60–TBD with a com-
pletely disordered pore distribution and a surface area of
155 m2 g 1 can suffer from some diffusion resistance, which
can render more difficult the access to the active sites. Fi-
nally, all reactions are characterized by high selectivity since
the carbamate 3a is the sole reaction product detected.
The solid catalyst could be efficiently reused for almost
three further cycles after filtration, washing with diethyl
ether and drying at rt under vacuum, giving product 3a
in 97, 97, and 93% yield, respectively (total TON [in mol
of product (mol of catalyst) 1] evaluated for four cycles is
385).
OEt
EtOH + Ph
Ph
N
H
O
3a
2a
SCHEME 3
nitrogens, promotes the nucleophilic attack to the carbonyl
by enhancing its electrophilic character in a way similar to
that observed with acylpyridinium salts (25).
Of course an EtO counterion may be the actual basic
catalyst, and some reactions were performed to test this
hypothesis. However, 3a is only produced in traces ( 5%)
in the presence of EtONa (0.10 mmol, 0.007 g) in DEC
(20 ml), proving that ethoxide is not the catalyst.
3.2. Leaching Test
To support this mechanistic hypothesis we compared the
FT-IR spectra of MCM-41–TBD material before and after
treatment with DEC and benzylamine at 125 C for 15 h
(Fig. 2). The spectrum of the material treated with DEC
To exclude the possible leaching of any active catalytic
species in solution, the model reaction was examined by
following the standard procedure suggested by Lempers
and Sheldon (22). Thus the reaction mixture was filtered at
125 C after 4 h (when 3a was produced in 58% yield) and
the filtrate was further heated at 125 C for 12 h. Product
3a was detected in 60% total yield (58% + 2%). In con-
trast, addition of both DEC (20 ml) and benzylamine 2a
(10 mmol, 1.07 g, 1.09 ml) to the recovered solid catalyst
and heating at 125 C for 16 h afforded the carbamate 3a
in 95% yield. These results confirm that the reaction really
occurs on the supported TBD active sites (23).
1
(Fig. 2B) shows the appearance of a band at 1676 cm of
==
==
the C O stretching, while the band of the C N stretching
1
shifts from 1636 to 1599 cm as a consequence of the
double bond weakening in the guanidinium ion 6. The
A
B
C
3.3. The Mechanism
Although work on mechanistic details is still in progress,
it is hypothesized that the present transformation may have
resulted from a catalytic cycle, depicted in Scheme 3.
As a strong base (24), the supported TBD is sufficiently
nucleophilic to attack the carbonyl group of diethyl car-
bonate, giving the guanidinium salt 6. Subsequent attack
of the amine on the activated carbonyl group of 6 gives the
product 3a and restores the catalyst. This step is particularly
2800
2300
cm-1
1800
1300
FIG. 2. FT-IR spectra of MCM-41–TBD (A), MCM-41–TBD treated
with DEC (B), and MCM-41–TBD treated with DEC and benzylamine
favored since the positive charge, delocalized over the three (C).

CATALYTIC ACTIVITY OF MCM-41–TBD IN SELECTIVE PREPARATIONS
203
TABLE 2
catalyst treated with DEC and washed with diethyl ether to
remove physically absorbed DEC was treated with benzy-
lamine under general reaction conditions (Fig. 2C), giving
the product 3a detected in the reaction mixture by GLC
analysis. Obviously, the FT-IR spectra of the catalyst before
andaftertreatmentwithbenzylaminearequitesimilarsince
the reaction is carried out in DEC as solvent–reagent and
consequently the catalyst is always present in the reaction
mixture as N-carbethoxyguanidinium 6.
Synthesis of Unsymmetrical Alkyl Carbonates 5
3.4. Synthesis of Carbamates and Unsymmetrical
Carbonates
With the reaction conditions for the model compound
optimized, we sought application in the synthesis of carba-
mates and unsymmetrical carbonates. Accordingly, DEC
was reacted with aliphatic amines in the presence of the
MCM-41–TBD catalyst, affording carbamates (Table 1)
(26). The reaction seems to be of general applicability with
respect to the amine and proceeds efficiently and selectively
at 125 C in 15 h. Only with an increased steric hindrance
in the proximity of the amino group are longer reaction
times and larger quantities of catalyst required (Table 1,
TABLE 1
Synthesis of Carbamates 3
a
Carried out for 24 h.
entries b, j, and k). Similarly, the primary amine becomes
the sole reaction site in a substrate containing both primary
and secondary amino groups (Table 1, entry i). Finally, as
expected, chiral 1,2-aminoalcohols gave the corresponding
oxazolidin-2-ones with complete retention of the optical
activity (Table 1, entries j and k).
A quite similar behavior was observed in the reaction of
DEC with alcohols to give unsymmetrical alkyl carbonates.
By using benzyl alcohol as the model substrate, the reac-
tion carried out with both HI–TBD and MCM-41–TBD was
faster (96 and 93% yield, respectively, after 8 h) than the
similar reaction for the preparation of carbamates (92 and
99% yield, respectively, after 15 h). However, at shorter
reaction times (3 h), the homogeneous catalyst HI–TBD
afforded the unsymmetrical carbonate 5l in higher yield
(72%) than did the heterogeneous MCM-41–TBD (46%).
Different unsymmetrical alkyl carbonates were synthe-
sized in good yield and excellent selectivity (Table 2). The
steric hindrance in proximity to the reactive hydroxy group
represents again a limiting factor (Table 2, entries o, p, and
q). It is noteworthy that whereas 1,2-diols gave the cyclic
carbonates (Table 2, entries q and r), cyclic products are not
formed when the number of methylene groups between the
alcohol functions increases. Thus, with 1,3- and 1,6-diols,
a
Carried out for 24 h with 200 mg of MCM-41–TBD.

204
CARLONI ET AL.
the linear biscarbonates become the sole reaction products
(Table 2, entries s and t).
6. (a) Crowther, H. L., and McCombie, H., J. Chem. Soc. 103, 27 (1913);
(b) Ben–Ishai, D., J. Am. Chem. Soc. 78, 4692 (1956).
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8. Inesi, A., Mucciante, V., and Rossi, L., J. Org. Chem. 63, 1337 (1998).
9. (a) McGhee, W., Riley, D., Christ, K., Pan, Y., and Parnas, B., J. Org.
Chem. 60, 2820 (1995); (b) Yoshida, M., Hara, N., and Okuyama, S.,
Chem. Commun. 151 (2000).
4. CONCLUSIONS
In conclusion, we have shown that the hybrid organic–
inorganic material prepared by anchoring TBD to MCM-41 10. Sheldon, R. A., Chem. Ind. (London) 12 (1997).
11. Vauthey, I., Valot, F., Gozzi, C., Fache, F., and Lamaire, M.,
Tetrahedron Lett. 41, 6347 (2000).
12. Ono, Y., Appl. Catal. A 155, 133 (1997).
silica can be utilized as efficient heterogeneous basic cata-
lystforthe synthesisof carbamatesandunsymmetricalalkyl
carbonates by reaction of DEC with aliphatic amines or
alcohols. Products are obtained in high yield and selectivity
and the solid catalyst can be recovered simply by filtration
and reused for different cycles without apparent lowering
of activity. A supported N-carbethoxyguanidinium active
intermediate is proposed, and some spectroscopic data are
shown to support the mechanistic hypothesis.
13. Saikh, A.-A. G., and Sivaram, S., Chem. Rev. 96, 951 (1996).
14. (a) Lindner, E., Schneller, T., Auer, F., and Mayer, H. A., Angew.
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Jacobs, Eds.). VCH, Weinheim, 2000.
15. Subba Rao, Y. V., De Vos, D. E., and Jacobs, P. A., Angew. Chem. Int.
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K. R., and Tinker, J. F., J. Org. Chem. 24, 64 (1958).
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(1930).
19. The loading of MCM-41–TBD catalyst is 1.04 mmol/g, the surface area
is 385 m2 g 1, and the material is thermally stable up to 200 C.
20. The loading of KG-60–TBD catalyst is 0.47 mmol/g, the surface area
is 155 m2 g 1, and the material is thermally stable up to 200 C.
21. (a) Climent, M. J., Corma, A., Guil–Lo´pez, R., Iborra, S., and Primo,
J., J. Catal. 175, 70 (1998); (b) Macquarrie, D. J., and Jackson, D. B.,
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ACKNOWLEDGMENTS
The authors acknowledge the support of the Ministero dell’Universita`
e della Ricerca Scientifica e Tecnologica (MURST), Italy; the Consiglio
Nazionale delle Ricerche (CNR), Italy; and the University of Parma
(National Project “Processi Puliti per la Chimica Fine”). The authors are
grateful to the Centro Interdipartimentale Misure (CIM) for the use of
NMR and mass instruments and to Mr. Pier Antonio Bonaldi (Lillo) for
technical collaboration.
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