Amidinate Aluminium Complexes as Catalysts for Carbon Dioxide Fixation into Cyclic Carbonates

Article abstract of DOI:10.1002/cctc.201702014

A series of inexpensive and sustainable amidinate aluminium complexes has been developed as catalysts for the chemical fixation of carbon dioxide into cyclic carbonates. The reactions using terminal epoxides as substrates were carried out at room temperat

Full text of DOI:10.1002/cctc.201702014

Accepted Article
Title: Amidinate aluminium complexes as catalysts for carbon dioxide
fixation into cyclic carbonates
Authors: Danay Meléndez, agustin Lara-Sánchez, Javier Martínez,
Xiao Wu, Antonio Otero, Jose Castro-Osma, Michael North,
and Rene Rojas
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Amidinate aluminium complexes as catalysts for carbon dioxide
fixation into cyclic carbonates
Danay Osorio Meléndez,[a] Agustín Lara-Sánchez,[c] Javier Martínez,[c] Xiao Wu,[d] Antonio Otero,[c]
José A. Castro-Osma,*[b] Michael North,*[d] and René S. Rojas*[a]
Abstract:
A
series of inexpensive and sustainable amidinate
polycarbonates (Scheme 1).[6,7,8] The synthesis of ethylene and
propylene carbonates has been commercialised for over 50
years,[9] and nowadays cyclic carbonates are widely used as
polar aprotic solvents,[10] electrolytes for lithium ion batteries[11]
and organic synthetic intermediates.[12] Aliphatic polycarbonates
have promising applications including as precursors for the
synthesis of non-isocyanate derived polyurethanes[13] and are
currently being commercialized as replacements for aromatic
polycarbonates.[14,15]
aluminium complexes has been developed as catalysts for the
chemical fixation of carbon dioxide into cyclic carbonates. The
reactions using terminal epoxides as substrates were carried out at
room temperature and one bar of carbon dioxide pressure in the
presence of tetrabutylammonium iodide as co-catalyst in the
absence of solvent. Under these reaction conditions, excellent
conversions and selectivities were achieved for a broad range of
terminal epoxides. Moreover, the optimal catalyst could be used for
the synthesis of disubstituted cyclic carbonates from internal
epoxides and carbon dioxide, highlighting the potential of these
amidinate aluminium complexes as catalysts.
Introduction
Carbon dioxide is considered to be an inexpensive, nontoxic and
sustainable C1 feedstock for the synthesis of value-added
products, and its chemical utilization is a rapidly growing
research area in the scientific community.[1,2] Although being
chemically attractive, the inherent thermodynamic and kinetic
stability of carbon dioxide is the major impediment for its
incorporation into organic frameworks. To date, a number of
valuable chemicals have been produced by incorporating carbon
dioxide, these include amides,[3] methanol[4] and formic acid.[5]
Meanwhile, the reaction between epoxides and carbon dioxide
has also been developed into a successful carbon dioxide
utilisation process, producing cyclic carbonates and
Scheme 1. Reaction between epoxides and carbon dioxide.
Over recent years, many research groups have made
significant contributions and a variety of catalytic systems have
been developed for the synthesis of cyclic carbonates from
epoxides and carbon dioxide, some of which operate under
ambient reactions (25 °C, carbon dioxide pressure ≤ 1 bar).
These systems are mainly based on metal complexes,[16] metal
organic frameworks[17] and some organocatalysts.[18] In particular,
metal-salen complexes have been well investigated and among
them, aluminium,[19] chromium[20] and cobalt[21] complexes have
been most widely studied for the reaction between epoxides and
carbon dioxide. These systems usually require a halide salt as a
cocatalyst, though single-component, bifunctional catalysts in
which one or more quaternary ammonium or phosphonium
halides are covalently linked to a catalyst have also been
developed.[22] For example, the combination of 2.5 mol% of a
bimetallic aluminium(salen) complex and tetrabutylammonium
bromide catalyses the synthesis of cyclic carbonates from
terminal epoxides at 25 oC and one bar carbon dioxide
pressure.[19] It is worth noting that the bimetallic aluminium(salen)
complex is capable of carrying out the kinetic resolution of the
epoxides[ 23 ] and single-component versions of this catalyst
system have been developed.[ 24 ] Furthermore, acen[ 25 ] and
heteroscorpionate[ 26 ] and aminophenolate[ 27 ] complexes have
also been studied, and showed high catalytic activity towards
cyclic carbonate formation under mild reaction conditions.
[a]
Dr. D. A. Meléndez, Prof. R. S. Rojas*
Nucleus Millennium Chemical Processes and Catalysis (CPC),
Laboratorio de Química Inorgánica, Facultad de Química
Universidad Católica de Chile
Casilla 306, Santiago-22 6094411, Chile.
E-mail: rrojasg@uc.cl
[b]
Dr. J. A. Castro-Osma*
Departamento de Química Inorgánica, Orgánica y Bioquímica, and
Centro de Innovación en Química Avanzada (ORFEO-CINQA),
Facultad de Farmacia
Universidad de Castilla-La Mancha
02071-Albacete, Spain.
E-mail: JoseAntonio.Castro@uclm.es
[c]
Dr. A. Lara-Sánchez, Dr. J. Martínez, Prof. A. Otero
Departamento de Química Inorgánica, Orgánica y Bioquímica, and
Centro de Innovación en Química Avanzada (ORFEO-CINQA),
Facultad de Ciencias y Tecnologías Químicas
Universidad de Castilla-La Mancha
Campus Universitario, 13071-Ciudad Real, Spain.
Dr. X. Wu, Prof. M. North*
Green Chemistry Centre of Excellence, Department of Chemistry
The University of York
York, YO10 5DD (UK).
E-mail: Michael.north@york.ac.uk
A generally accepted mechanism for the metal-mediated
coupling reaction between epoxides and carbon dioxide to afford
cyclic carbonates is shown in Scheme 2. First, the Lewis-acidic
metal center coordinates to the epoxide followed by ring-opening
by the cocatalyst to give an alkoxide ion. Then, carbon dioxide is
inserted followed by ring-closing to afford the five-membered
cyclic carbonate. This also leads to the regeneration of both the
catalyst and the cocatalyst.
[d]
Supporting information including: experimental, copies of the 1H and
13C NMR spectra for cyclic carbonates and details of the kinetic
analyses for this article is given via a link at the end of the document
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10.1002/cctc.201702014
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Scheme 2. General mechanism for cyclic carbonate synthesis.
Amidinate metal complexes have been shown to catalyse
28
]
important
reactions
including:
polymerisations,[
hydrogenations,[ 29 ] hydrosilylations,[ 30 ] Kharasch reactions,[ 31 ]
amidations[32] and hydrophosphination reactions.[33] In our effort
to investigate and take advantage of the catalytic properties of
amidinate systems as described above, recently we have
reported the application of a series of aluminium bisamidinates
towards ring-opening polymerization (ROP) of cyclic esters.[34] It
was observed that all complexes were active for the ROP of ε-
caprolactone. Moreover, the most active catalyst displayed
significant catalytic activity towards the ROP of L-lactide and
copolymerization of ε-caprolactone and L-lactide, producing
biodegradable materials with narrow polydispersities. Taking into
account the high catalytic activity shown by these complexes for
the ROP of cyclic esters, their catalytic activity for cyclic
carbonates formation from epoxides and carbon dioxide was
investigated and the results are reported herein.
Figure 1. Aluminium based bis(amidinate) catalysts.
Scheme 3. Synthesis of cyclic carbonates 8a–j.
Table 1. Conversion of styrene oxide 7a into styrene carbonate 8a using
catalyst 1-6 and Bu4NBr.[a]
Conversion
(%)[b] 3h
Conversion
(%)[b] 6h
Conversion
(%)[b] 24h
Results and Discussion
Entry
1
Complex
1
2
12
16
10
20
5
27
30
23
31
11
18
4
56
67
53
57
22
34
8
Aluminium complexes 16 (Figure 1) were synthesized by
reaction of the bis(amidine) precursors and a trialkylaluminium in
toluene in excellent yield and purity, as previously reported.[34]
2
A
catalyst screening for the conversion of styrene oxide 7a and
CO2 into styrene carbonate 8a was initially carried out using a
combination of 5 mol% of complexes 14 or 10 mol% of
complexes 56 and 5 mol% of tetrabutylammonium bromide
(TBAB) as the catalyst system at 25°C and 1 bar of CO2 for 24
hours under solvent free conditions (Scheme 3). Each reaction
was analysed by 1H-NMR spectroscopy to determine the
conversion of epoxide 7a into cyclic carbonate 8a (Table 1). In
all cases, the selectivity towards the cyclic carbonate was higher
than 99%.
3
3
4
4
5
5[c]
6[c]
-
6
8
7[d]
8[e]
2
2
0
0
0
[a] Reactions carried out at 25°C and 1 bar of CO2 for 24 hours using 5 mol%
of both complex 14 and Bu4NBr. [b] Conversion determined by 1H-NMR
spectroscopy of the reaction mixture. [c] 10 mol% of monometallic complex
was used. [d] 5 mol% of Bu4NBr added. [e] No Bu4NBr was added.
As can be seen from entries 1–6, bimetallic
complexes 14 were more active than either of the monometallic
complexes 56 using the same aluminium loading. Moreover,
ethyl containing complexes 2, 4 and 6 are more active than
methyl complexes 1, 3 and 5 (entries 1–6). Amongst the
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Table 2. Influence of cocatalyst on the synthesis of styrene carbonate 8a
catalysed by complex 2.[a]
Table 3. Conversion of styrene oxide 7a into styrene carbonate 8a using
catalyst 1-6 and TBAI.[a]
Conversion
(%)[b] 3h
Conversion
(%)[b] 6h
Conversion
(%)[b] 24h
Entry
Cocatalyst
Bu4NF
Conversion
(%)[b] 3h
Conversion
(%)[b] 6h
Conversion
(%)[b] 24h
Entry
Catalyst
0
9
0
0
1
2
3
4
5
6
1
2
3
4
5
6
15
18
12
11
6
30
42
25
32
13
21
58
87
53
56
27
38
1
2
3
4
5
6
Bu4NCl
Bu4NBr
Bu4NI
13
30
42
10
0
38
67
87
25
0
16
18
6
PPNCl[c]
DMAP[d]
0
10
[a] Reactions were carried out at 25°C and 1 bar CO2 for 24 hours using 5
mol% of both complex 2 and cocatalyst. [b] Conversion determined by 1H-
NMR spectroscopy of the reaction mixture. [c] Bis(triphenylphosphine)iminium
chloride . [d] 4-Dimethylaminopyridine.
[a] Reactions carried out at 25°C and 1 bar of CO2 for 24 hours using 5 mol%
of catalyst and TBAI. [b] Conversion determined by 1H-NMR spectroscopy of
the reaction mixture.
The most active complex 2 has four ethyl-aluminium bonds
which could hydrolyse during the reaction. This has been
previously observed for heteroscorpionate aluminium complexes
where water has been shown to have a significant influence on
the catalytic activity.[26] Therefore, to further optimize the reaction
conditions, the addition of known, small amounts of water to the
reaction mixture was investigated. As shown in Figure 2, under
the standard reaction conditions, (25°C, 1 bar CO2, 24 hours, 5
mol% of 2 and tetrabutylammonium iodide, using styrene oxide
as substrate) the addition of just 1 mol% of water, the amount
needed to hydrolyse just one in every twenty aluminium-ethyl
bonds had a dramatic effect on the catalytic activity, decreasing
the conversion from 87% to 54%. However, the addition of up to
8 mol% of water, enough to hydrolyse one in every 2.5
aluminium-ethyl bonds, had no further impact on the catalytic
activity.
catalysts studied, complex 2 was the most effective catalyst for
this transformation. Control experiments (entries 7–8) showed
that the combination of an aluminum complex and a nucleophile
source is needed in order to achieve good conversions, which is
consistent with other aluminum systems, where reaction at room
temperature only occurs in the presence of a cocatalyst.[23-27]
A combination of complex 2 with a range of nucleophile
sources was chosen as a catalyst system to optimize the
cocatalyst for styrene carbonate synthesis as shown in Table 2.
It was found that the combination of complex
2 and
tetrabutylammonium iodide (TBAI) formed an efficient catalyst
system for styrene carbonate synthesis at 25 oC and 1 bar
carbon dioxide pressure (Table 2, entry 4). Other cocatalysts
such as tetrabutylammonium fluoride, tetrabutylammonium
chloride,
4-dimethylaminopyridine
and
bis(triphenylphosphine)iminium chloride were also tested, but
lower activities were obtained (Table 2, entries 1, 2, 5 and 6).
Thus, the best results were obtained when the cocatalyst has
the potential to generate a good leaving group. Hence, the
results suggest that for catalysis using complex 2, the final step
of the catalytic cycle shown in Scheme 2 (ring-closure with
elimination of the nucleophile) is the rate determining step of the
catalytic cycle.
In order to establish, if the change of cocatalyst from
tetrabutylammonium bromide to tetrabutylammonium iodide has
a positive effect for each of complexes 1-6, the conversion of
styrene oxide 7a into styrene carbonate 8a was investigated
under same reaction conditions and the results are summarized
in Table 3. For complexes 1 and 3-6 there was no significant
difference between the two cocatalysts, whilst for complex 2,
tetrabutylammonium iodide was significantly more active than
tetrabutylammonium bromide (Table 1 and 3 respectively).
These results may be due to a change in the relative rates of
epoxide ring-opening, in which the halide of the cocatalyst acts
as a nucleophile (rate limiting for catalysts 1 and 3-6) and cyclic
carbonate ring-closure, in which the halide acts as a leaving
group, (rate limiting for catalyst 2).
90
85
80
75
70
65
60
55
50
0
2
4
6
8
Water (mol%)
Figure 2. Effect of added water on the conversion of styrene oxide 7a into
cyclic carbonate 8a catalysed by 5 mol% of complex 2 and TBAI.
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Having determined the optimal reaction conditions, the
been described for this application.[6,23-27] Due to the lower
reactivity of these epoxides, reactions were performed using 7
mol% of complex 2 and tetrabutylammonium iodide as catalyst
system for 24 hours at 50°C and 10 bar CO2, except when
epoxide 9e was used as a substrate for which a temperature of
90°C was used. Cyclic carbonates 10bf were synthesized in
good yields (Table 5).
formation of ten monosubstituted cyclic carbonates (8aj), from
their corresponding terminal epoxides and carbon dioxide was
carried out using 5 mol% of catalysts 2 and 4 with TBAI as a
cocatalyst. Reactions were carried out at 25°C and 1 bar CO2,
except when propylene oxide 7b was used as a substrate when
the reaction was carried out at 0°C due to the volatility of
epoxide 7b. As can be seen in Table 4, both catalysts converted
the terminal epoxides into their corresponding cyclic carbonates
in good yields under mild reaction conditions.
Table 4. Epoxide conversion into cyclic carbonates using catalyst 2 and 4.a]
Complex 2
Conversion Yield
(oC) (%),[b] [TOF][c] (%)[c] (%),[b] [TOF] [c] (%)[d]
Complex 4
T
Conversion
Yield
Entry
Epoxide
R=Ph (7a)
R=Me (7b)
R=Et (7c)
25
0
87 [0.7]
--[e] [0.8]
53 [0.4]
97 [0.8]
78 [0.7]
92 [0.8]
52 [0.4]
69 [0.6]
40 [0.3]
51 [0.4]
80
97
46
84
71
89
43
66
35
47
57 [0.5]
--[e] [0.7]
38 [0.3]
81 [0.7]
29 [0.2]
91 [0.8]
51 [0.4]
36 [0.3]
53 [0.4]
60 [0.5]
53
81
30
73
20
87
42
32
42
53
1
2
Scheme 4. Synthesis of disubstituted cyclic carbonates from internal epoxides.
25
25
25
25
3
R=Bu (7d)
R= Oct (7e)
R=CH2Cl (7f)
4
Table 5. Synthesis of disubstituted cyclic carbonates using complex 2 as
catalyst.[a]
5
Entry
Substrate
Conversion (%)[b] [TOF][c]
100[e]
Yield (%)[d]
6
9a
9b
9c
83[e]
80
1
2
3
4
5
6
R=CH2OH (7g) 25
R=CH2OPh (7h) 25
R= 4-ClC6H4 (7i) 25
R= 4-BrC6H4 (7j) 25
7
85 [0.5]
8
--[f] [0.5]
85
9
9d
9e[g]
9f
--[f] [0.5]
87
10
62 [0.4]
57
[a] Reactions carried out at 1 bar CO2 for 24 hours, using 5 mol% of catalyst 2
or 4 and TBAI. [b] Conversion determined by 1H-NMR of the reaction mixture.
[c] TOF (h-1) = moles of product/(moles of catalyst x reaction time). [d]
Determined on purified product. [e] Conversion could not be determined
because of the volatility of the substrate.
81 [0.5]
76
[a] Reactions carried out at 10 bar CO2 for 24 hours, using 7 mol% of both
complex 2 and TBAI. [b] Conversion determined by 1H-NMR of the crude
product. [c] TOF (h-1) = moles of product/(moles of catalyst x reaction time). [d]
Determined on the purified product. [e] Polycyclohexene oxide was obtained
as product. [f] Conversion could not be determined because of the substrate
volatility. [g] Reaction carried out at 90°C.
In general, good to excellent conversions and yields were
obtained when non-functionalized and functionalized aliphatic
epoxides were used (Table 4, entries 2-8). However, lower
conversions were obtained when using functionalized aromatic
epoxides 7i and 7j (Table 4, entries 9 and 10) due to the
solidification of the reaction mixture which could affect the
catalytic conversion. Moreover, this study, allowed the activity of
catalysts 2 and 4 to be compared using a wider range of
epoxides since the catalytic activity will depend on the solubility
of the complex in the epoxide. In general, both catalysts showed
good catalytic activity. However, complex 2 was a better catalyst
for aliphatic epoxides 7b-7h while complex 4 showed better
catalytic activity towards functionalized aromatic epoxides 7i-j.
In order to expand the range of substrates, the use of
internal epoxides was investigated (Scheme 4). As most of the
disubstituted epoxides are aliphatic, complex 2 was chosen for
this study. Internal epoxides are less active than terminal
epoxides, but recently some efficient catalytic systems have
When epoxide 9a was used as the substrate,
polycyclohexene oxide was obtained, indicating that the ring
opening polymerisation of the epoxide took place without
insertion of carbon dioxide under these reaction conditions. On
the other hand, when epoxides 9b-f were used as substrate, the
cyclic carbonate products were obtained with 100% selectivity.
Under these reactions conditions, epoxide 9b was an excellent
substrate, allowing the formation of cyclic carbonate 10b in 80%
isolated yield (Table 5, entry 2). Cyclic carbonates 10c and 10d
were obtained in high yield, considering the volatility of epoxides
9c,d (Table 5, entry 3 and 4). When the cis-isomer 9c was used,
a mixture of cis- and trans-cyclic carbonates 10c,d was obtained
in a 36:64 cis:trans ratio. However, when epoxide 9d was used
as a substrate, cyclic carbonate 10d was obtained with a
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0.011
0.01
stereoselectivity higher than 95%. In the case of stilbene oxide
9e (Table 5, entry 5) it was necessary to increase the
temperature since the substrate is solid at lower temperatures,
and under these reaction conditions 57% of 10e was obtained.
Finally, sterically hindered cyclic carbonate 10f was obtained in
76% yield with 100% retention of the stereochemistry.
0.009
0.008
0.007
0.006
0.005
0.004
0.003
0.002
y = 0.0119x + 0.0002
R² = 0.99
To achieve a better understanding of the catalyst system,
a kinetic study for the synthesis of styrene carbonate 8a using
catalyst 2 and tetrabutylammonium iodide was carried out at
50°C and 1 bar CO2 in the absence of a solvent (See Supporting
information). Samples were collected from the reaction mixture
y = 0.0093x + 0.0006
R² = 0.99
k0obs
k1obs
1
every 30 minutes and were analysed by H-NMR spectroscopy
0.2
0.4
0.6
[TBAI]
0.8
1
to determine conversion of styrene oxide 7a into styrene
carbonate 8a. It has been shown in previous reports[26] that,
under these reaction conditions, the reaction follows zero order
kinetics at low conversions of the epoxide since it acts as both
substrate and reaction solvent. However, when the cyclic
carbonate is the main product in the reaction mixture, the
reaction follows first order kinetics.
Figure 4. Plots of k0obs and k1obs vs [TBAI]. Reactions were carried out at 50 °C
under solvent-free conditions with [2] = 440 mM and [TBAI] = 262–873 mM.
Data shown are the average of two experiments, with the individual
experiments being used to indicate the error bars.
The order with respect to the concentrations of TBAI and
log [2]
catalyst 2 were determined and the plots of log[TBAI] vs logk0obs
,
-1.2
-1
-0.8
-0.6
-0.4
-0.2
log[TBAI] vs logk1obs, log[2] vs logk0obs and log[2] vs logk1obs have
gradients of 0.86, 1.01, 0.90 and 1.23 respectively (Figures 3
and 5). This indicates that the reaction is first order with respect
to the concentration of both TBAI and complex 2. This was
confirmed by plotting k0obs or k1obs vs [TBAI] and k0obs or k1obs vs
[2] (Figures 4 and 6), which showed a good fit to a straight line
with intercepts very close to the origin.
-2
-2.2
-2.4
-2.6
-2.8
-3
y = 1.23x - 1.82
R² = 0.99
y = 0.90x - 2.00
R² = 0.99
log [TBAI]
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
-1.9
-2
K0obs
k1obs
-3.2
Figure 5. Logarithmic plots of k0obs and k1obs vs [2]. Reactions were carried out
at 50 °C under solvent-free conditions with [2] = 87–612 mM and [TBAI] = 440
mM. Data shown are the average of two experiments, with the individual
experiments being used to indicate the error bars.
-2.1
-2.2
-2.3
-2.4
-2.5
-2.6
y = 1.01x - 1.91
R² = 0.99
y = 0.86x - 2.01
R² = 0.99
0.009
0.008
0.007
k0obs
k1obs
0.006
0.005
0.004
0.003
0.002
0.001
0
y = 0.0142x - 0.0006
R² = 0.99
Figure 3. Logarithmic plots of k0obs and k1obs vs [TBAI]. Reactions were carried
out at 50 °C under solvent-free conditions with [2] = 440 mM and [TBAI] = 262
– 873 mM. Data shown are the average of two experiments, with the individual
experiments being used to indicate the error bars.
y = 0.0100x + 0.0003
R² = 0.99
k0obs
k1obs
0.05
0.15
0.25
0.35
0.45
0.55
0.65
[2]
Figure 6. Plots of k0obs and k1obs vs [2]. Reactions were carried out at 50 °C
under solvent-free conditions with [2] = 87–612 mM and [TBAI] = 440 mM.
Data shown are the average of two experiments, with the individual
experiments being used to indicate the error bars.
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Based on the experimental results showing that cyclic
Acknowledgements
carbonate formation occurs with retention of the epoxide
stereochemistry and that the kinetic studies indicate that
reactions are first order with respect to the concentration of the
catalyst and tetrabutylammonium iodide, we propose that the
reaction follows the mechanism shown in Scheme 2 with
complex 2 acting as the Lewis acid. The epoxide ring-opening
and cyclic carbonate ring-closing steps both involve an inversion
of configuration and result in overall retention of stereochemistry.
The first order kinetics with respect to complex 2 suggest that
only one of the aluminium atoms within the complex is acting as
a Lewis acid at any time. This is intriguing given that complex 2
is twice as active as the corresponding monometallic complex 6
(Table 1, entries 2 and 6). This difference in reactivity may be
related to solubility or stability differences between the
monometallic and bimetallic complexes under the reaction
conditions.
This work was supported by the Ministerio de Economía y
Competitividad (MINECO), Spain (grant nos. CTQ2014-51912-
REDC and CTQ2014-52899-R), Nucleus Millenum CPC (ICM
No. 120082), FONDECYT project No. 1161091, and CONICYT
(grant no 21130376), Chile.
Keywords: Carbon dioxide • sustainable catalysis • cyclic
carbonates • aluminium • epoxide
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Amidinate aluminium complexes 16, in the presence of
tetrabutylammonium iodide as
a
cocatalyst, have been
developed as active and versatile catalyst systems for cyclic
carbonates formation from terminal epoxides and carbon dioxide.
The catalyst systems formed by the combination of complex 2 or
complex 4 and tetrabutylammonium iodide display good catalytic
activity at 25 oC and one bar carbon dioxide pressure in the
absence of a solvent. These catalyst systems displayed a broad
substrate scope catalysing the formation of a cyclic carbonates
from aryl, alkyl and functionalized terminal epoxides in good to
excellent yields. Moreover, the combination of complex 2 and
tetrabutylammonium iodide displayed good catalytic activity
towards the synthesis of disubstituted cyclic carbonates from
their corresponding internal epoxides and carbon dioxide under
mild reaction conditions. The use of internal epoxides confirmed
that cyclic carbonate synthesis occurs with retention of the
epoxide stereochemistry.
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Amidinate Et-Al catalyse cyclic
carbonate synthesis. In the
Author(s), Corresponding Author(s)*
Page No. – Page No.
presence of a tetrabutylammonium
iodide cocatalyst, bimetallic amidinate
aluminium complexes are effective
catalysts for cyclic carbonate
synthesis from carbon dioxide and
both terminal and internal epoxides.
Amidinate aluminium complexes as
catalysts for carbon dioxide fixation into
cyclic carbonates
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