Aromatic guanidines as highly active binary catalytic systems for the fixation of CO2 into cyclic carbonates under mild conditions

Article abstract of DOI:10.1039/c9cy00667b

We have synthesised a set of aromatic mono- and bis(guanidines) which are highly effective binary catalytic systems (guanidine/cocatalyst) for the formation of cyclic carbonates. The presence of multiple N-H bonds causes a modification in the traditional

Full text of DOI:10.1039/c9cy00667b

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J. Martínez, R. S. Rojas, F. CARRILLO-HERMOSILLA, A. Ramos, R. Fernandez-Galan and A. Antiñolo, Catal.
Sci. Technol., 2019, DOI: 10.1039/C9CY00667B.
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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or lgo i ng ys
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DOI: 10.1039/C9CY00667B
ARTICLE
Aromatic guanidines as highly active binary catalytic system for
the fixation of CO into cyclic carbonates under mild conditions
2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
Ángela Mesías-Salazar, Javier Martínez,* René S. Rojas,* Fernando Carrillo-Hermosilla, Alberto
b
b
b
Ramos, Rafael Fernández-Galán and Antonio Antiñolo*
DOI: 10.1039/x0xx00000x
We have synthesised a set of aromatic mono- and bis(guanidines) which are highly effective binary catalytic system
guanidine/cocatalyst) for the formation of cyclic carbonates. The presence of multiple NH bonds causes a modification in
(
the traditional mechanism proposed for the synthesis of cyclic carbonates catalysed by guanidines, through the formation
of hydrogen bonds between the oxygen atom of the epoxide and the NH groups of the guanidines. This change allows
reducing considerably the reaction temperature and the CO
2
pressure employed in this process.
hydrogen bonds with the epoxide that would facilitate its opening,
6
Introduction
CO2 is an inexpensive, abundant, renewable and easily available
which is the rate-limiting step for this process.
1
2
3
4
5
1-5
Guanidines, of general formula R N=C(NR R )(NR R ) (R = H, alkyl,
aryl), are an attractive and versatile class of organic molecules
which have recently received increasing attention due to their
tuneable steric and electronic properties. The preparation of
guanidines by reaction between an amine and a carbodiimide is
an atom-economical process which has been a successful area of
source for the formation of value-added C building blocks which
1
has attracted a great interest from the scientific and industrial
7
1
community. Nevertheless, CO is an inert molecule and its capture,
2
activation or insertion into organic molecules in an industrial scale
is still a complicated challenge. Taking that into account, it is
essential to develop new catalytic processes that allow the
transformation of CO into different products, such as methanol,
formic acid, and amines. One of the most promising reactions
involving the use of CO2 is the preparation of cyclic carbonates,
which present a wide variety of applications, by cycloaddition of
2
7
research in recent years. In particular, the preparation of
guanidines from aromatic amines (anilines) requires the use of a
2

7b d
3
catalyst due to their lower nucleophilicity.
Commonly, to obtain
an outstanding variety of mono- and bis(guanidines) from aromatic
amines the use of metal catalysts is necessary. This catalytic process
has been carried out by a large variety of metal complexes such us
4
CO to epoxides (Scheme 1).
2
8
9
10
11
aluminium, zinc , alkaline-earth and rare-earth metals.
Scheme 1. Synthesis of cyclic carbonates.
5
Normally, this reaction has been catalysed by metals. However, in
recent years the use of organocatalysts has been extensively
Fig 1. Mono- and bis(guanidines) 1ai used for the synthesis of cyclic
carbonates
studied for this transformation, which supposes an effective and
6
interesting alternative to metal compounds. Additionally, to Guanidines have been widely studied as organocatalysts for the
12
improve the catalytic activity of these systems for the synthesis of preparation of cyclic carbonates from epoxides and CO2. However
cyclic carbonates, the research in this field is focusing on the design these reactions often require high temperatures (100-140 :C) and
of new compounds bearing OH or NH groups able to form CO2 pressures (10-50 bar) and proceed mainly through the
activation of CO molecule in the first step of the catalytic cycle
2
12
(
Scheme 1S on ESI). Moreover, the formation of hydrogen bonds
has only been proposed as a secondary mechanism in some of the
12b,fi
latter instances
Thus, bearing this in mind, we have focused
our efforts on the preparation of a series of less-nucleophilic
aromatic mono- and bis(guanidines), unable to react directly with
CO and containing several NH bonds in an attempt to change the
2
main mechanism in the synthesis of cyclic carbonates by the
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formation of multiple hydrogen bonds between the oxygen atom of the formation of a hydrogen bond between the NH group of the
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6
l,14
DOI: 10.1039/C9CY00667B
the epoxide and the NH groups of the guanidine moiety with the guanidine and the O-donor group of styrene oxide,
whereas light
principal objective to significantly reduce the reaction temperature electron donating groups (EDG) such as methyl groups resulted in a
and the CO pressure used in this process.
little decrease of the activity (Table 1, entry 3), although the steric
hindrance could also play an important role in this matter.
2
On the other hand, bis(guanidines) 1fi were found to be more
active than monoguanidines for the transformation of styrene oxide
into styrene carbonate (Table 1, entries 69). In general,
Results and Discussion
Synthesis and catalytic activity of mono- and bis(guanidines)
i
bis(guanidines) substituted with Pr groups (1f and 1h) displayed a
Accordingly, for the preparation of aromatic mono- and
bis(guanidines) 1ai (Fig. 1), which present some NH bonds, a
single or double catalytic addition was performed by reaction of
aromatic mono- or diamines with one or two equivalents of the
corresponding carbodiimide in toluene catalysed by ZnEt , Zn(OTf)
higher catalytic activity than bis(guanidines) with Cy moieties (1g
and 1i) probably due to the greater steric hindrance presented by
these groups which made its union with the epoxide more difficult.
Among them, bis(guanidine) 1f was selected as the most active
substrate achieving a 92 % of styrene carbonate formation with the
2
2
9a,13
or B(C F ) as reported in the literature (Table S1).
-1
6
5 3
highest value of TOF, 3.83 h (Table 1, entry 6).
Having prepared the guanidines 1ai, we decided to explore their
potential utility as dual catalyst systems for the preparation of cyclic
carbonates from epoxides and CO . Firstly, the transformation of
Compound 1f was used to study the effect of the cocatalyst for the
preparation of styrene carbonate (Table 2). The best result was
achieved with TBAI at 70 :C and 1 bar of constant CO2 pressure
2
styrene oxide (2a) into styrene carbonate (3a) catalysed by
guanidines 1ai was chosen to optimise the reaction conditions.
The reactions were carried out at 70 :C and 1 bar of constant CO2
pressure under solvent-free conditions for 24 h, using 2 mol% of
tetrabutylammonium iodide (TBAI) as cocatalyst and 2 mol% of
monoguanidines 1ae or 1 mol% of bis(guanidines) 1fi as catalyst
load keeping the number of NH bonds constant since all these
bonds are considered active centers for the synthesis of cyclic
carbonates. In this particular case, it is worth noting that the
combination of guanidines and TBAI were completely soluble into
styrene oxide at 70 :C at the beginning of the reaction. Conversions
(
Table 2, entry 4). With respect to other ammonium salts, it was
found that the order of catalytic activity was I > Br > Cl > F (Table 2,
entries 14). These results were also observed in our previous
work.
(
15
Bis(triphenylphosphoranylidene)ammonium
chloride
PPNCl) displayed moderate catalytic activity (Table 2, entry 5)
whereas 4-dimethylaminopyrirdine (DMAP) was not good
a
cocatalyst for this transformation (Table 2, entry 6) owing to the
formation of an adduct through hydrogen bonding between the
NH group of the guanidine 1f and the nitrogen atom from the
pyridine moiety (See Fig. S1 and Fig. S2 on ESI for further details).
The formation of this adduct was confirmed by the reaction
between guanidine 1f and DMAP in a 1:1 molar ratio in which a
slight variation of the resonances was observed in the
1
were determined by H-NMR spectroscopy and the results are
shown in Table 1.
a
1
Table 1. Styrene carbonate 3a synthesis catalysed by guanidines 1ai
corresponding H NMR spectrum (Fig. 1Sb and Fig. 2Sb in ESI) with
respect to the H NMR signals of the DMAP product (Fig. 1Sa and Fig
1
2
Sa on ESI). Moreover, we noted the broadening of the NH signal
1
in the H NMR spectrum of this adduct (See page S38 on ESI) which
supported the existence of an interaction between 1f and DMAP. In
addiction, in an attempt to clarify if this adduct was formed under
the presence of the epoxide, we carried out an experiment amongst
b
c
-1
d
Entry
Guanidine
Conversion (%)
TON
TOF (h )
1
f, DMAP and styrene oxide 2a in a 1:1:1 molar ratio and we
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
70
73
54
78
77
92
80
81
62
35.0
36.5
23.0
39.0
38.5
92.0
80.0
81.0
62.0
1.45
1.52
1.13
1.63
1.60
3.83
3.33
3.38
2.58
1
observed as well the appearance of this adduct in its H NMR
spectrum (Fig. 1Sc and Fig. 2Sc). As a conclusion, this fact surely
limited the formation of a hydrogen bond amongst guanidine 1f and
styrene oxide 2a what affected its catalytic activity. Thus, TBAI was
found to be the best cocatalyst for the formation of styrene
carbonate (3a). Control experiments were carried out to
demonstrate that both bis(guanidine) 1f and TBAI are necessary to
a
Reactions were carried out at 70 :C and 1 bar CO pressure for 24 h using 2 prepare the cyclic carbonate product (Table 2, entries 7 and 8).
2
mol% of monoguanidines 1ae or 1 mol% of bis(guanidines) 1fi and 2 mol% of
tetrabutylammonium iodide (TBAI) under solvent free conditions unless specified
Having defined the optimal reaction conditions for the synthesis of
styrene carbonate, we subsequently focused our effort on the
preparation of a wide range of cyclic carbonates 3ak from their
corresponding epoxides 2ak employing 1 mol% of bis(guanidine)
b
1
otherwise. Determined by H NMR spectroscopy of the crude reaction mixture.
c
d
TON = moles of product/moles of catalyst. TOF = TON/time (h).
Monoguandines 1ae displayed high to moderate conversions
7854 %) for the synthesis of styrene carbonate 3a (Table 1,
1f and 2 mol% of TBAI as catalyst system under solvent-free
(
conditions (Fig. 2). Reactions were performed at 70 :C and under
entries 15). It is remarkable that the presence of electron releasing
groups (ERG) in the guanidine, such as CN and CF , caused a slight
increase in the catalytic activity (Table 1, entries 4 and 5) as
expected owing to an increase of the aniline acidity which facilitates
constant CO pressure of 1 bar in a single glass reaction tube of a
2
3
multipoint reactor (See ESI for further details). No polycarbonate
formation was obtained and the selectivities towards cyclic
carbonate preparation were up to 99%. Finally, It must be noted
2
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that guanidine 1f and TBAI were perfectly soluble into the different Given the elevated catalytic activity of this catalyst system (1f/TBAI)
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DOI: 10.1039/C9CY00667B
epoxides at 70 :C.
for the transformation of terminal epoxides into their
corresponding cyclic carbonates, we then tried to extend further
the substrate scope using internal or disubstituted epoxides as
substrates. Firstly, we optimised the reaction conditions by
employing cyclohexene oxide 4a catalysed by guanidine 1f and
TBAI. The results are presented in Table 3. Low conversions were
achieved for cyclohexene carbonate formation 5a at 70 :C at
different catalyst and cocatalyst loadings while keeping constant
the 1f/TBAI molar ratio at 1:2 (Table 3, entries 13). In order to
increment the catalytic activity, the effect of increasing the reaction
temperature to 85 :C was subsequently studied. Higher conversions
were obtained under these reaction conditions in which we were
Table 2. Cocatalyst election for the synthesis of 3a catalysed by
a
bis(guanidine) 1f
b
c
-1
d
Entry
Cocatalyst
Conversion (%)
TON
3
40
69
92
44
7
TOF (h )
e
1
2
3
4
5
6
7
8
TBAF
3
40
69
92
44
7
0.13
1.67
2.87
3.83
1.83
0.29
0
TBACf
g
TBAB
TBAI
h
-1
pleased to find that the highest value of TOF (1.83 h ) was afforded
i
PPNCl
using 2 mol% of guanidine 1f and 4 mol% of TBAI (Table 3, entry 6).
Control experiments were performed to confirm that both
components (guanidine and nucleophile) were essential to carry out
this catalytic transformation (Table 3, entries 7 and 8). It is
important to note that 1f/TBAI synthesised selectively the cyclic
carbonate product and the formation of polycarbonate was no
DMAPj
-
-
0
TBAI
5
5
0.21
a
Reactions were carried out at 70 ºC and 1 bar CO
2
pressure for 24 h using 1
mol% of bis(guanidine) 1f and 2 mol% of cocatalyst under solvent free conditions
b
1
unless specified otherwise. Determined by H NMR spectroscopy of the crude
c
d
reaction mixture. TON = moles of product/moles of catalyst. TOF = TON/time
e
f
(
h). TBAF: tetrabutylammonium fluoride. TBAC: tetrabutylammonium chloride, observed although this substrate also has the ability to form
g
h
TBAB: tetrabutylammonium bromide.
PPNCl: Bis(triphenylphosphoranylidene)ammonium chloride. DMAP: 4-
dimethylaminopyridine.
TBAI: tetrabutylammonium iodide.
j
polycarbonates.
I
a
Table 3. Cyclohexene carbonate 5a synthesis catalysed by guanidine 1f
It is worth highlighting that cyclic carbonates 3ak were achieved
with excellent conversions and yields using the combination of
bis(guanidine) 1f and TBAI as catalyst system (Fig. 2). It is
remarkable that this catalyst system (1f/TBAI) is tolerant to several
functional groups such us alkyls, aryls, alcohols, halides and ethers,
which demonstrates its catalytic potential for the synthesis of a
large variety of cyclic carbonates.
-1
c
Entry
1f
mol%)
1.0
1.5
2.0
1.0
1.5
2.0
2.0
0
TBAI
(mol%)
2.0
3.0
4.0
2.0
3.0
4.0
0
T (:C)
Conversion
TOF (h )
b
(
(%)
1
2
3
4
5
6
7
70
70
70
85
85
85
85
85
15
25
37
41
66
91
0
0.63
0.69
0.77
1.71
1.83
1.09
0
8
4.0
2
0.02
a
Reactions were carried out at 7085 :C and 10 bar CO
2
pressure for 24 h using
1
2 mol% of guanidine 1f and 24 mol% of tetrabutylammonium iodide (TBAI)
b
1
under solvent free conditions unless specified otherwise. Determined by
NMR spectroscopy of the crude reaction mixture.
product/(moles of catalyst x time).
H
c
TOF = moles of
Having selected the optimal reaction conditions for the synthesis of
cyclohexane carbonate 5a, we focused on the preparation of others
disubstituted cyclic carbonates from internal epoxides and CO . The
2
results are shown in Fig. 3. The reactions were carried out
employing 2 mol% of guanidine 1f and 4 mol% of TBAI at 85 :C and
1
0 bar of CO pressure in a stainless steel reactor with a magnetic
2
stir bar for 24 h. In general, good isolated yields (8259 %) were
found for cyclic carbonates 5ad. The synthesis of cis- and trans-
cyclic carbonates 5c and 5d were isolated as a 94:6 mixture
cis/trans-cyclic carbonate for 5c product and a 4:96 mixture
cis/trans-cyclic carbonate for 5d, respectively (See ESI for further
details). These results suggest that this transformation happened
with retention of the epoxide stereochemistry.
a
1
Fig. 2. Conversion was determined by H NMR spectroscopy of the
b
crude reaction mixture. Isolated yield obtained from purified cyclic
carbonate.
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cyclic carbonates formation (Table 4, entries 6, 8 and 9) showed
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higher catalytic activity than the previous guanidines (Table 4,
entries 1, 3 and 4), although these substrates also required high
reaction temperatures and CO2 pressure to afford these catalytic
results. Finally, when we compare our catalytic results with some of
the best organocatalysts reported to date in the literature (Table 4,
entries 1114), we can conclude that our binary catalyst system is
less active than these highly active organocatalysts for the synthesis
of cyclic carbonates, although these systems required higher
reaction temperatures and CO pressures. It is noteworthy that in
2
general the organocatalysts which present a hydroxyl group on its
structure (Table 4, entries 1114) are more active than those which
present an amino moiety probably due to the greater
electronegativity of the oxygen atom causes the formation of
stronger hydrogen bonds.
a
1
Fig. 3. Conversion was determined by H NMR spectroscopy of the
b
crude reaction mixture. Isolated yield obtained from purified cyclic
carbonate.
According to the literature, the catalyst systems reported to date
based on guanidines required elevated temperatures and CO2
pressures to carry out the preparation of cyclic carbonates as
aforementioned, thus we wanted to compare their catalytic activity
with our catalytic results and with other highly active
organocatalysts for this catalytic transformation. It is important to
note that for the guanidine systems, we are going to distinguish
between two different types of catalysts: guanidines that
performed the formation of cyclic carbonates with or without the
assistance of hydrogen bonds. These catalytic comparisons are
shown in Table 4 (the structures of these organocatalyst are shown
in Fig. S7 on ESI). Firstly, in the comparison of our catalytic results
with those guanidines that carried out the preparation of cyclic
carbonates in the absence of hydrogen bonds (Table 4, entries 1, 3
To investigate the reaction mechanism of this catalytic
transformation, we have carried out a set of reactions between
bis(guanidine) derivative 1f and CO or 2a and TBAI using a Young
2
valve NMR tube and the results were determined spectroscopically.
We first studied the possible formation of hydrogen bonding
between the NH groups of the bis(guanidine) 1f and the oxygen
atom of the styrene oxide 2a at different molar ratios (Fig. S3S6 on
ESI). As can be seen on Fig. S3, the signal around 3.52 ppm
corresponding to the NH group of the bis(guanidine) 1f broadened
and it was observed a slight change in the chemical shift of this
1
signal in their corresponding H NMR spectra at different molar
ratios of 1f:2a (Fig. S4). On the other hand, it is worth noting that a
slight variation of the carbon resonances of the oxirane were also
13
1
noticed in the C{ H} NMR spectra (Fig. S5 and Fig. S6 on ESI).
Based on these NMR experiments, it is possible to confirm the
formation of a hydrogen bond between the guanidine and the
epoxide.
12j
and 4), it should be pointed out that only one guanidine displayed
an extraordinary catalytic activity (Table 4, entry 3) whereas the
rest of this type of guanidines exhibited similar activities than our
binary catalyst system, although guanidine 1f was able to achieve
these catalytic results under relatively mild reaction conditions
On the other hand, we carried out other NMR experiments to try to
clarify the reaction mechanism for this process (Fig. 4). Firstly, we
(
Table 4, entries 2 and 5).
performed the reaction among 1f and CO and no modification was
2
1
noted in the H-NMR spectrum (Fig. 4a) which confirmed that there
Table 4. Comparison of catalytic activities of different organocatalysts
Entry
Guani.
Epoxide
T (:C)
[Pres. (bar)]
150 (50)
70 (1)
100 (7)
140 (50)
70 (1)
Yield
TOF
(h )
1.3
3.9
37.5
1.2
3.6
8.5
3.9
5.8
was no activation of the CO molecule on this catalytic cycle. After
2
f
-1
h
(
mol%)
3.2
1.0
0.1
4.0
1.0
7.2
1.0
3.0
0.5
1.0
2.0
3.0
5.0
2.0
1.0
(%)
100
that, when the reaction amongst 1f, styrene oxide 2a and TBAI took
place at 70 :C for one hour, the nucleophilic attack of the iodide
anion to the epoxide did not occur under these reaction conditions
12e
a
g
1
2b
2
2b
2a
2a
2a
94
1
2j
g
3
4
90
12k
(Fig. 4b) highlighting the importance of the presence of CO in this
95
88
92
93
87
99
83
97
2
5
catalytic transformation. The epoxide ring-opening was observed
12f
b
6
2g
100 (10)
70 (1)
when CO was added and the intermediate specie 1f’’ was obtained
2
7
2g
2f
2f
2f
2a
1
(
Fig. 4c). The appearance of two new resonances in the H-NMR
1
2g
8
130 (30)
130 (20)
70 (1)
90 (10)
60 (20)
45 (10)
80 (10)
70 (1)
1
2i
spectrum around 3.83 and 2.13 ppm, respectively (Fig. 4c),
corresponding to the CH and CH protons of the open epoxide,
9
49.5
3.5
1
0
2
6
o
1
1
16.2
20.8
0.92
50.0
3.8
supported the formation of intermediate 1f’’. Moreover, DEPT-135
experiments exhibited a negative peak at 66.8 ppm and a positive
14a
c
1
1
2
3
2b
2b
2d
100
83
100
6m
d
6
n
e
one at 34.0 ppm belonging to the CH
and CH of the opened
2
1
4
epoxide, respectively, indicating that the iodide anion attacks at the
1
5
2d
90
a
d
b
c
Used 15 mL of DCM as solvent. Used ZnI
2
as cocatalyst. Used 3 mol% of TBAB. most hindered carbon (See ESI, pages S44 and S45) as previously
e
6e,16
Used 5 mol% of TBAI as cocatalyst and 5 mL of MEK as solvent. Used TBAI as has been reported.
These results suggest that the formation of a
f
g
1
cocatalyst. Isolated yield from purified cyclic carbonate. Determined by H NMR
spectroscopy of the crude reaction mixture. TOF = moles of product/(moles of
catalyst x time).
hydrogen bond and the presence of CO may be the key for the
2
h
activation of the epoxide on the synthesis of cyclic carbonates
catalysed by bis(guanidine) 1f.
On the other hand, in general guanidines in which the presence of
hydrogen bonds assists in the activation of the CO molecule for the
2
4
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(
d)
(
(
b)
c)
(
a)
1
1
Fig. 4. (a) H NMR spectrum of compound 1f and CO
2
at 70 :C for 24 hours in CDCl
3
. (b) H NMR spectrum of 1f, styrene oxide 2a and TBAI in a molar ratio
1
1
:4:2 (1f:2aꢀTBAI) at 70 :C for one hour in CDCl
two hour in CDCl
3
. (c) H NMR spectrum of 1f, styrene oxide 2a, TBAI and CO
2
in a molar ratio 1:4:2 (1f:2aꢀTBAI) at 70 :C for
1
3
. (d) H NMR spectrum of styrene carbonate 3a in CDCl
3
.
Based on the experimental data and other results previously
6
,16
reported,
a proposed mechanism for the preparation of styrene
carbonate 3a catalysed by guanidines 1ai is showed in Scheme 2.
Firstly, the formation of a hydrogen bonding amongst the oxygen
atom of the epoxide and the NH group of the guanidine 1 is
observed. The presence of CO2 is crucial for the formation of
intermediate specie 1’’ through the nucleophilic attack of the iodide
at the most hindered carbon atom of the epoxide to create an iodo-
alkoxide. Finally, CO inserts into OH bond to form a carbonate
2
intermediate 1’’’ which quickly cyclizes to afford the styrene
carbonate product and the binary catalytic system (1/TBAI).
Conclusions
In summary, we have prepared a set of aromatic mono- and
bis(guanidines) that can act as efficient dual catalytic system
for the formation of a large variety of cyclic carbonates 3a
and 5a
the epoxide occurred via hydrogen bonding between the N

k
d. Mechanistic studies confirmed that the activation of

H
groups of the guanidines and the oxygen atom of the epoxide,
instead of through the activation of the CO molecule on the
2
first step of the catalytic cycle, which implies a change in the
reaction mechanism of the synthesis of cyclic carbonates
catalysed by guanidines reported to date. It is worth
highlighting that this mechanistic modification allowed us to
Scheme 2. Plausible and general mechanism for the synthesis of
styrene carbonate 3a catalysed by guanidines 1a
i.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or lgo i ng ys
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ARTICLE
Journal Name
ChemSusChem, 2017, 10, 1152–1159; (b) Y. KumVieawtAarbticaleraO,nMline.
decrease significantly the reaction temperature (from 140-100 :C to
Okada and S. Shirakawa, ACS Sustainable Chem. Eng., 2017,
DOI: 10.1039/C9CY00667B
70-85 :C) and the CO2 pressure (from 10-50 bar to 1-10 bar)
5
, 7295−7301; (c) H. Büttner, J. Steinbauer, C. Wulf, M.
employed to date in this process. Furthermore, the development of
new guanidines containing multiple NH bonds to enhance the
catalytic activity for the preparation of cyclic carbonates is ongoing
in our research group.
Dindaroglu, H.-G. Schmalz and T. Werner, ChemSusChem,
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J. Martínez, F. de la Cruz-Martínez, M. P. Caballero, J.
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Conflicts of interest
There are no conflicts to declare
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981−1987; (f) N. Liu, Y.-F. Xie, C. Wang, S.-J. Li, D. Wei, M. Li
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Acknowledgements
This work was supported by FONDECYT, Project No. 1161091. A. M.-
S. acknowledges funding from Ph. D. CONICYT fellowship No.
Detrembleur, Catal. Sci. Technol., 2017,
Büttner, L. Longwitz, J. Steinbauer, T. Werner, Top. Curr.
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21150322.
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acknowledges
funding
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6
CONICYT/FONDECYT/Postdoctorado Project (No. 3180073). We
gratefully acknowledge financial support from the Ministerio de
Jerome and B. Grignard, ChemSusChem, 2015,
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, 1845–1849;
(
m) C. J. Whiteoak, A. Nova, F. Maseras and A. W. Kleij,
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ChemSusChem, 2012,
5, 2032–2038; (n) S. Sopeña, E.
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Catalysis Science & Technology
Page 8 of 8
View Article Online
DOI: 10.1039/C9CY00667B
Aromatic guanidines as highly active binary catalyst system for the
fixation of CO into cyclic carbonates under mild conditions
2
a
a
a
Angela Mesías-Salazar, Javier Martínez,* René S. Rojas,* Fernando Carrillo-
b
b
b
b
Hermosilla, Alberto Ramos, Rafael Fernández-Galán and Antonio Antiñolo*
The formation of hydrogen bonding causes a considerable decrease in the reaction temperature
and CO pressure used in this process
2
This work: The presence of NH groups causes a change
in the mechanism through hydrogen bonding formation
R'
O
R
H
N
N
H
H
N
O
R
R
T = 70-85 ºC
R
CO2 (1-10 bar)
R
N
+
R N
1
5 examples;
N
H
R
R'
R
up to 94 % yield
R'
O
O
R
O
O
R
R'
N
CO (10-50 bar)
O
2
N
O
+
T = 100-140 ºC
R
R'
N
R
N
N
R
N
O
Previous work
FOR TABLE OF CONTENTS USE ONLY