Boosting the Catalytic Performance of Organic Salts for the Fast and Selective Synthesis of α-Alkylidene Cyclic Carbonates from Carbon Dioxide and Propargylic Alcohols

Article abstract of DOI:10.1002/cctc.201800063

The synthesis of α-alkylidene cyclic carbonates (αCCs) by the carboxylative coupling of CO₂ with propargylic alcohols is receiving increasing interest but requires the use of catalysts that are most often added in large quantities and/or lack selectivity. Herein, we describe the fine-tuning of the structure of organocatalysts of the ammonium-type that enables us to identify the important structural parameters that dictate their catalytic performance. Tetrabutylammonium oxalate was revealed to be one of the most attractive organocatalysts with a remarkably fast, complete and selective production of αCCs at a low catalyst loading (2.5 mol %) under solvent-free conditions. This study opens new avenues for the facile and selective synthesis of libraries of αCCs from CO₂ and propargylic alcohol by using simple organocatalysts.

Full text of DOI:10.1002/cctc.201800063

Accepted Article
Title: Boosting the catalytic performance of organic salts for the fast
and selective synthesis of α-alkylidene cyclic carbonates from
CO2 and propargylic alcohols.
Authors: Bruno Grignard, Charlene Ngassamtounzoua, Sandro
Gennen, Bernard Gilbert, Raphael Mereau, Christine Jerome,
Thierry Tassaing, and Christophe Detrembleur
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10.1002/cctc.201800063
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Boosting the catalytic performance of organic salts for the fast and
selective synthesis of α-alkylidene cyclic carbonates from CO2 and
propargylic alcohols.
Bruno Grignard[a], Charlène Gabriela Ngassamtounzoua[a], Sandro Gennen[a], Bernard Gilbert[c],
Raphaël Méreau[b], Christine Jerome[a], Thierry Tassaing[b], Christophe Detrembleur[a]*
Abstract: The synthesis of α-alkylidene cyclic carbonates (αCCs) by
carboxylative coupling of CO2 with propargylic alcohols is receiving
increasing interest but requires the use of catalysts that are most often
added in large quantity and/or are lacking of selectivity. Herein, we
describe that the fine-tuning of the structure of organocatalysts of the
ammonium-type enables to identify the important structural
high CO2 pressure and high temperature under moisture-free
conditions. Metal-free systems derived from (organic) bases such
as phosphines, guanidine, K2CO3, N-heterocyclic carbenes
(NHC) and olefins (NHO), or ionic liquids were also developed for
this reaction. Their catalytic activity generally arose from their
capability to activate the substrate or CO2 through acid-base
reaction or by forming stable zwitterionic guanidine, NHC or
NHO–CO2 adducts.18-22 However, most of these catalysts were
efficient at temperature between 60 to 100 °C and pressure up to
25 to 100 bar. Moreover, long reaction times at high catalyst
loading were required for reaching a high propargylic alcohol
conversion.In 2016, Wang et al. reported series of novel reusable
ionic liquids (ILs) derived from 1,8-Diazabicyclo[5.4.0]undec-7-
ene (DBU) that were able to promote the synthesis of αCCs at
60 °C and 25 bar.23 Even at ILs loading of 200 mol% compared to
propargylic alcohols, the coupling reaction remained slow as
attested by yields of 68 – 89 % after 24 h. This moderate activity
might be related to the low basicity of ILs. Indeed, Wang et al.
correlated the catalytic activity of phosphonium-based ILs with the
basicity of their counter-anion. Anions such as imidazolides with
a pKa value close to 7.4 represented the best compromise in
terms of reaction yield and selectivity.24 This trend was further
confirmed by our groups for (super)bases promoted CO2/alkynols
coupling reactions. Organic bases of high pKa values showed
outstanding catalytic activity but at the expense of the selectivity.25
Very recently, Liu et al. developed series of novel phosphonium-
based organocatalysts by introducing the concept of multiple-site
activation of CO2 via the use of anions derived from carboxylic
acid and/or hydroxyl multifunctional pyridines. With a loading of
10 mol%, these catalysts showed good activity at 1bar and 30 °C,
with yields between 33 and 91 % after 20h.26 The scarce
examples of the organocatalyzed coupling reactions promoted at
low CO2 pressure (1 bar) were performed with a large excess of
the catalyst compared to the reactant.27 New trends deal will the
development of binary catalysts combining metal species with
ionic liquids or organic bases 28-32. These catalysts work by the
synergistic activation of both the substrate and CO2, or by the
double activation of the substrate via the hydroxyl and alkyne
groups. As a typical example, Han et al. reported on the selective
synthesis of αCCs under solvent-free conditions by using a
combination of ZnI2 as catalyst and NEt3 as cocatalyst at room
temperature and low CO2 pressure (10 bar).28 However, a high
catalyst loading (20 mol%) was required and the cocatalyst was
used in equimolar amount with respect to the propargylic alcohol.
AgOAc/tetraalkylammonium bromide salt29, AgOAc/Schiff based
ligands30, AgI/AcO- catalysts31 or AgOAc/DBU32 were also
described as catalytic systems. Although some of them were
found very active for the selective formation of αCCs, one of the
component of the binary catalyst, generally the base, was used in
parameters
that
dictate
their
catalytic
performance.
Tetrabutylammonium oxalate revealed to be one of the most attractive
organocatalysts with a remarkable fast, complete and selective
production of αCCs at low catalyst loading (2.5 mol%) under solvent
free-conditions. This study clearly opens new avenues for the facile
and selective synthesis of libraries of αCCs from CO2 and propargylic
alcohol by using simple organocatalysts.
Introduction
The chemical conversion of CO2 into five-membered cyclic
carbonates (5CCs) represents one of the most efficient ways to
valorize this waste in useful organic synthons. Beside the
extensively studied epoxide/CO2 coupling reaction,1-10 the
synthesis of α-alkylidene cyclic 5-membered carbonates (αCCs)
by carboxylative coupling of CO2 with propargylic alcohols is
gaining recent strong interest. αCCs differ in their structure from
conventional 5-membered cyclic carbonates by the presence of
an exocyclic olefinic group that facilitates the ring-opening of the
cyclic carbonate by various nucleophiles such as amines and
alcohols while selectively forming one regioisomer.11 The
thermodynamic driving force for the αCCs ring-opening relies on
the formation of an enol species that rapidly rearranges into a
more stable β-keto tautomer.12,13 The exceptional reactivity of
αCCs compared to 5CCs was recently exploited to access to
novel classes of regioregular hydroxyl- or oxo-functional
polyurethanes and/or polycarbonates under mild experimental
conditions.11 αCCs are synthesized by coupling of CO2 with
propargylic alcohols. The first reported metal catalysts (Ru, Co,
Pd, Ag, …)14-17 only showed acceptable activity and selectivity at
[a]
Center for Education and Research on Macromolecules (CERM),
CESAM Research Unit, Chemistry Department, B6a, University of
Liège, 13, Allée du 6 Août, 4000 Liège, Belgium
* E-mail: christophe.detrembleur@uliege.be
[b]
[c]
Institut des Sciences Moléculaires (ISM), UMR5255 CNRS,
Université de Bordeaux, 351 Cours de la libération, F-33405
Talence Cedex, France
Chemistry Department, B6C, University of Liège, 13 Allée du 6 Août
13, 4000 Liège, Belgium
Supporting information for this article is given via a link at the end of
the document
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10.1002/cctc.201800063
ChemCatChem
FULL PAPER
equimolar amount or even in excess as compared to the
propargylic alcohol. This large co-catalyst content was required to Results and Discussion
perform the reaction at low temperature (< 45 °C) and pressure
(1-10 bar). Although huge progresses were made in the field of
organocatalysis, the rational correlation between their structure
and their influence on the reaction rate and selectivity is rarely
addressed, the presence of side products being not deeply
investigated.
1. Catalyst synthesis. Scheme 1 summarizes the library of
organocatalysts based on ammonium and phosphonium salts
with carboxylates, phenolates, boronate or imidazolate counter-
anions that were prepared. The ammonium and phosphonium
based catalysts were synthesized by
a two-step process
according to adapted procedures from the literature (Scheme 2).
In this work, various organic salts (Scheme 1) were prepared and
evaluated for the CO2/alkynols coupling reaction. The objective is
to identify the important structural parameters that dictate the
performances of the catalyst in terms of both the rate and the
selectivity of the reaction, while maintaining the catalyst loading
low (5 mol%). Tetrabutylammonium acetate was chosen as a
reference catalyst of moderate activity.33 By tuning the structure
of both the cation (ammonium, phosphonium, imidazolium or
guanidinium) and anion ((di-)carboxylates, phenolates, boronate,
imidazolide), the basicity of the catalyst was modulated, which
strongly affected the catalytic activity.
Scheme 2: General strategy for the synthesis of organic catalysts. R1 = R2 = R3
= R4 = -CH3, -C4H9, -C8H17. Nitrogen can be replaced by phosphorus.
The first step consisted in exchanging the halide anion of the
quaternary ammonium/phosphonium salts by OH- anion by
reaction with potassium hydroxide. Then, the quaternary
ammonium/phosphonium hydroxide salt/IL was neutralized by
addition of carboxylic acids of various structures, phenol
derivatives, phenyl boronic acid or imidazole.34-35
All synthetic procedures and characterizations are
detailed in supporting information.
2. Screening of the catalysts activity. The
basicity of the catalyst has been shown crucial for
the selective coupling of propargylic alcohols with
CO2.24,25 In this study, we focused first on
organocatalysts based on carboxylate anions; their
basicity can be tuned by varying the structure of
their alkyl substituent. The activity and selectivity of
carboxylate salts towards the formation of αCCs
was first screened using a model reaction between
2-methyl-3-butynol and CO2 under solvent-free
conditions at 50 bar, 80 °C for 6 h using 5 mol% of
catalyst (Scheme 1).
Tetrabutylammonium acetate (nBu4NOAc) was
first tested as the reference catalyst. Figure S3
illustrates the 1H NMR spectra of nBu4NOAc, 2-
methyl-3-butynol and the crude reaction mixture
obtained after 6 h. Formation of αCC, 4,4-dimethyl-
5-methylene-1,3-dioxolan-2-one
(Scheme
1,
product A) was attested by the appearance of
peaks at 4.33 ppm and 4.77 ppm typical of the
exocyclic olefinic group of the αCC, and one singlet
at 1.61 ppm assigned to the two methyl groups.
Meanwhile, complete consumption of 2-methyl-3-
butynol was highlighted by the absence of any
residual peaks of the reactant at 1.55, 2.16 and
1
2.44 ppm. However, the H NMR spectrum of the
crude sample also highlights the presence of two
acyclic carbonates by-products. Sample B, formed
by the regio-selective ring-opening of αCC with 2-
methyl-3-butynol, shows typical resonances of
methyl groups at 1.52, 1.72, 2.24 ppm, and one
Scheme 1: CO2/propargylic alcohol coupling reaction: design of
singlet at 2.54 ppm corresponding to the alkyne proton. By-
product C produced by the addition of α-hydroxyketone D (formed
organocatalysts
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28
29
nBu4N+
nBu4N+
OH*
Br-
/
/
-
-
-
-
-
-
-
-
by hydrolysis of αCC) onto αCC is also observed by the presence
of its characteristic methyl group peaks at 1.53 and 2.18 ppm.
Then, the conversion of 2-methyl-3-butynol and the selectivity
towards the formation of the (a)cyclic carbonates were
determined by comparing the relative intensities of the
integrations of characteristic peaks of each sample (see
experimental section ESI2 for details). nBu4NOAc afforded a
quantitative conversion of 2-methyl-3-butynol with the formation
of αCC (product A) with a selectivity of 93 %.
The catalytic performance of all organocatalysts was then tested
for the same model reaction. The conversion and the selectivity
towards the formation of product A were benchmarked with the
ones obtained for nBu4NOAc. The influence of the structural
parameters of the organocatalysts on their activity and selectivity
is discussed in details below.
Conditions: T = 80 °C, P = 50 bar, t = 6 h, 0.03 mol of propargylic alcohol, 5
mol% of catalyst, volume of the cell = 20 ml.
* catalyst partly soluble
a using 2.5 mol% of catalyst
b using 1.66 mol% of catalyst
c reaction in the presence of 18-crown-6, [KOAc]/[18-crown-6] =1, for 6h
d reaction in the presence of 18-crown-6, [KOAc]/[18-crown-6] =1, for 24h
* pKa values in water
3. Influence of the alkyl substituent of the ammonium. The
influence of the nature of the alkyl chain of the ammonium cation
and of its length on its catalytic performance was first evaluated
for various ammonium bearing AcO- anion (Table 1, entries 1-4).
As illustrated in Table 1, the evolution of the 2-methyl-3-butynol
conversion was found strongly dependent on the alkyl chain
length of the cation, e.g. 0 % with hydrogen, < 10 % with methyl
or octyl, and 100 % with n-butyl. Interestingly, the same evolution
pervades for the same ammoniums bearing oxalate counter-
anion as attested by a maximum conversion of 100% with a
nBu4N+ cation (Table 1, entries 18-20).
This trend was explained by the cation-anion interactions
impacting the ion pair separation and consequently the catalyst
activity. Electrostatic interactions between the acetate anion and
NH4+ or cations with short alkyl substituents in C1 are expected to
be strong, which disfavours the ion pair separation and lowers the
catalytic activity of the acetate. Indeed, the well accepted
mechanism 33 involves the deprotonation of the hydroxyl group of
the alkynoI by a basic catalyst and the simultaneous nucleophilic
attack of the so-produced alkoxide anion onto CO2. This first step
results in the formation of a cation-carbonate ionic pair and an
acid. Then, the ring closure and formation of the α-methylene
cyclic carbonate occur via an intramolecular nucleophilic addition
of the carbonate anion onto the C≡C bond with the simultaneous
pronation of the alkenyl anion by the acid (Scheme 3).
Table 1. Organocatalysts screening for the coupling of CO2 with 2-methyl-3-
butyn-1-ol.
Entry
[C+]
[A-]
pKa*
Selectivity (%)
Conv
(%)
SA
-
SB
-
SC
-
+
1
2
3
4
5
6
7
8
NH4
AcO-
AcO-
AcO-
AcO-
AcO-
AcO-
AcO-
AcO-
4.78
4.78
4.78
4.78
4.78
4.78
4.78
4.78
0
<6
100
<4
11
0
Me4N+
nBu4N+
Oct4N+
nBu4P+
Oct4P+
C4MIm+
DBUH+
93
93
n.d.
92
-
n.d.
2
n.d.
5
n.d.
8
n.d.
0
-
-
16
84
100
-
-
-
86
14
9
K+
K+
K+
AcO-
AcO-
AcO-
4.78
4.78
4.78
0
-
-
-
-
10c
11d
13
88
92
87
8
7
5
12
13
14
15
nBu4N+
nBu4N+
nBu4N+
nBu4N+
C2H5COO-
C3H7COO-
C5H11COO-
C9H19COO-
4.88
4.87
4.84
4.9
100
100
100
7
97
97
3
-
-
3
4
-
96
-
100
-
16
17
nBu4N+
nBu4N+
PhCOO-
4.19
0.05
95
<5
63
98
2
n.d.
-
-
n.d.
-
CF3COO-
n.d.
100
1.23,
4.19
18a
19a
Me4N+
nBu4N+
Oct4N+
nBu4N+
nBu4N+
Ox2-
Ox2-
Ox2-
Mal2-
Suc2-
100
< 4
100
44
>99
n.d.
>99
100
trace
n.d.
trace
-
-
1.23,
4.19
n.d.
1.23,
4.19
20a
-
-
2.83,
5.69
21a
4.16,
5.61
22a,*
23
nBu4N+
nBu4N+
PhO-
9.89
100
100
90
98
3
2
7
-
24a
Cath2-
9.3, 13
n.d.
Scheme 3. Proposed general mechanism for the organocatalytic carboxylative
coupling of CO2 with propargylic alcohols with ammonium salts (C+A-)
25b
26a
27
nBu4N+
nBu4N+
nBu4N+
Pyrog3-
PhBO2-
Im-
100
100
100
90
93
93
10
7
-
-
-
n.d
6.95
7
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From the optimized minimum energy structures of nBu4NOAc and
NH4OAc catalysts calculated by the M06-2X functional[36] using
the 6-311G(d,p) basis set (Figure 1), the presence of two
supplementary hydrogen bonds for NH4OAc shortens the
distance between the two ions of the ions pair from 4.22 Å for
nBu4NOAc to 2.82 Å for NH4OAc. This additional interaction
observed with NH4OAc strongly hampers the catalytic activity of
the acetate anion due to the energy needed to “break” this ion pair.
By substituting nBu4N+ for the phosphorous counterpart (nBu4P+),
the 2-methyl-3-butynol conversion strongly decreased from
100 % to only 11 % (Table 1, entry 5), highlighting the importance
of the nature of the cation on the catalytic performances. No
catalytic activity was also noted for Oct4POAc (Table 1, entry 6).
By using imidazolium acetate (C4MImAc; Table 1, entry 7), a low
conversion was also measured (16 %) with however the product
A
as the main compound. The catalytic performance of
nBu4NOAc was then benchmarked with that of 1,8-
diazabicyclo[5.4.0]-7-undecenium acetate (DBUHAc, Table 1,
entry 8)37 which was reported as a catalyst for the coupling of
propargylic alcohols with CO2 (although no data about the
potential formation of by-products was available).37 nBu4OAc was
found slightly more active than DBUHAc as evidenced by a 2-
methyl-3-butynol conversion of 100 % vs 84 %. However, the
major difference between the two organocatalysts arises from the
selectivity toward the formation of product A. While nBu4NOAc
yielded product A with a selectivity of 93 %, by-product B was
identified as the main compound with DBUHAc. This huge
difference in selectivity is assumed to be the result of the acid-
base equilibrium of DBUHA that releases DBU (a superbase) in
the reaction medium. Indeed, DBU (and other organic
superbases) were shown to promote the formation of the acyclic
carbonate at the expense of the cyclic one.25,38 Chen et al. also
demonstrated that the increase of pKa of amines for the catalyzed
propargylic alcohol/CO2 reaction was found to increase the
reaction rate but was detrimental to the selectivity.24 This study
highlights the impact of the cation structure of AcO- catalysts on
their performances and further confirms that nBu4N+ cation should
be privileged for tailoring efficient organocatalysts.
(A)
(B)
Figure 1: Optimized geometries (M06-2X/6-311G(d,p)) of the structures of
NH4OAc (A) and nBu4NOAc (B).
5. Influence of the R group of the carboxylate. We then
investigated the influence of the structure of the carboxylate on
the activity of the catalyst based on nBu4N+ cation. Although
increasing the length of the R alkyl chain of the carboxylate has
In contrast, while substituents in C8 should favour the ion pair
separation, their bulkiness brings steric hindrance that is
detrimental for the CO2/2-methyl-3-butynol coupling reaction. The
butyl group appears to offer the best compromise in terms of ion
only
a very limited influence on the pKa value of the
corresponding acid, the steric hindrance is expected to be
increased and might affect the catalytic activity. For R substituents
in C2, C3 or C5 (Table 1, entries 12-14), the activity of the
corresponding catalysts was similar to that of nBu4NOAc (Table
1, entry 3) with the quantitative conversion of 2-methyl-3-butynol
into product A with a selectivity above 96 %. However, with a C9
substituent (Table 1, entry 15), a very poor conversion was noted
(< 10%). This difference in catalytic activity is assumed to be the
result of the steric hindrance induced by the C9 chain. Similarly to
the case of the alkyl substituents of ammonium cations, there is
therefore an optimum carboxylate chain length beyond which the
conversion starts declining. Replacing acetate by a slightly less
basic benzoate (Table 1, entry 16) led to a slightly lower 2-methyl-
3-butynol conversion of 95% with a selectivity in product A of 98 %.
When the even less basic trifluoroacetate anion was used (Table
1, entry 17), only traces (< 5 %) of 2-methyl-3-butynol were
converted. This result highlights the crucial role of the basicity of
the carboxylate anion on the performances of the organocatalysts.
pair separation and steric hindrance with
a quantitative
conversion in 6 h. Note that the poor 2-methyl-3-butynol
+
conversion (< 6 %) obtained with NH4 , Me4N+ or Oct4N+ prevents
us from drawing any reliable conclusion regarding the influence of
the alkyl chain length of the ammonium cation on the selectivity
towards the formation of product A.
The crucial influence of the ion-pair separation on the catalyst
activity was further confirmed experimentally by carrying out the
coupling reaction in the presence of potassium acetate (KOAc) as
catalyst. This acetate salt displayed no catalytic activity (Table 1,
entry 9) under the experimental conditions that are optimal for
nBu4NOAc (Table 1, entry 3). Importantly, the addition of 18-
crown-6 ([KOAc]/[18-crown-6] = 1) activated this ammonium salt
for the coupling reaction. Indeed, 13 and 90 % of the propargylic
alcohol were converted after 6 and 24h, respectively, with a high
selectivity (~ 87%) in aCC (product A) (Table 1, entries 10-11).
The crown ether complexed the potassium cation, favoring the
ion-pair separation and therefore the reaction.
6. Influence of the nature of the anion. Tetrabutylammonium
type organocatalysts composed of various anions (dicarboxylates,
phenolate, catecholate, pyrogallolate, boronate, imidazolide)
4. Influence of the nature of the cation. The nature of the cation
on the activity of AcO--based organocatalysts was then evaluated.
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10.1002/cctc.201800063
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were then prepared and their activity compared under identical
experimental conditions (50 bar, 80 °C, 6 h). Oxalate (Ox2-) was
first tested as the anion (Table 1, entries 18-20). To take into
account the presence of 2 carboxylate groups (and thus 2 nBu4N+)
per oxalate, 2.5 mol% of catalyst was used instead of 5 mol% in
order to compare the activity of the catalyst with the reference
(nBu4NOAc). 2-methyl-3-butynol was fully converted into product
A with a selectivity superior to 99% by using (nBu4N)2Ox (93 %
for nBu4NOAc and full conversion), compared to 63 % conversion
with 100 % selectivity for (Me4N)2Ox. Substituting the butyl or
methyl groups of the ammonium by octyl ones was detrimental for
the catalytic activity, only less than 4 % of the propargyl alcohol
was converted (Table 1, entry 20).
The kinetics of the reaction was therefore monitored by following
the disappearance of the alkyne peak at 710 cm-1, and the
appearance of the peak at 649 cm-1 characteristics of the cyclic
carbonate as illustrated on a typical 3D superposition of the
Raman spectra recorded with time for the reaction catalysed by
nBu4NOAc (Figure 2a). The CO2 peak intensity varied slightly at
the early stages of the reaction and then reached a stable value
once the propargylic alcohol/acetonitrile liquid phase was
saturated by CO2. The kinetic profile of each monitored reaction
was obtained by plotting the evolution of the relative intensity of
the cyclic carbonate peak at 649 cm-1 with time.
Figure 2 shows the kinetic profiles recorded for reactions
catalysed by Me4NOAc, nBu4NOAc, (nBu4N)2Ox and nBu4NOPh.
Crucial information on the activity and selectivity of the catalysts
was deduced from the slope of the kinetic curves (that is linked to
We then investigated the influence of the spacer between the two
carboxylate groups on the catalyst performances by testing
tetrabutylammonium malonate and succinate (Table 1, entries 21
and 22). The former afforded full conversion of 2-methyl-3-butynol, the reaction rate) and from the intensity at which the plateau is
with a similar selectivity for A than (nBu4N)2Ox. Although
tetrabutylammonium succinate is also highly selective towards
the formation of A, the 2-methyl-3-butynol conversion only
reached 44 % under identical experimental conditions. This
observation is at first glance surprising based on the higher
basicity of succinate. However, the low solubility of this catalyst in
propargylic alcohol might account for this difference of activity.
The other catalysts were fully soluble in the reaction medium.
We then tested tetrabutylammonium phenolate (nBu4NOPh) as
potential catalyst (pKa of phenol = 9.8, Table 1, entry 23). A full
conversion of 2-methyl-3-butynol was measured, however with a
lower selectivity in A of 90 % as compared to 100% conversion
with (nBu4N)2Ox (pKa2 of oxalate = 4.19) and 100 % selectivity in
A. This result confirms that highly basic catalysts are very active
but at the expense of the selectivity as previously discussed.
Under identical experimental conditions, nBu4N+ with catecholate
(Cath2-), pyrogallolate (Pyrog3-), phenylboronate (PhBO2-) or
imidazolide (Im-) gave comparable results to nBu4NOPh in terms
of conversion and selectivity (Table 1, entries 24-27). No catalytic
activity was observed when the organic anion was substituted for
the hydroxyl or bromide one (Table 1, entries 28-29).
reached (that is related to the content in product A).
The catalysts screening enabled us to identify the most efficient
catalysts but full kinetics studies are required to class them
according to their catalytic activity and selectivity. Therefore,
kinetic studies of the organocatalyzed coupling of 2-methyl-3-
butynol with CO2 were carried out by online Raman spectroscopy
under similar experimental conditions (i.e. P = 50 bar, T = 80 °C,
catalyst loading of 5 or 2.5 mol% (the latter with oxalate based
organocatalysts). The only difference compared to previous
experiments arises from the addition of acetonitrile (acetonitrile/2-
methyl-3-butyn-2-ol composition of 50/50 v/v) that served as an
internal reference. Figure S5 shows the Raman spectra of the
starting product, 2-methyl-3-butyn-2-ol, and the pure final one,
product A, recorded by using a red ruby laser of λ = 785nm. The
Raman spectrum of 2-methyl-3-butyn-2-ol shows two peaks at
2117 cm-1 and 710 cm-1 which are characteristic of the
n stretching and the d C-H out of plane bending mode of the
alkyne group, respectively. The Raman spectrum of product A
shows a distinctive peak at 649 cm-1, characteristic of the C=O
out of plane bending vibration of the cyclic carbonate.
Figure 2: Addition of CO2 to 2-methyl-3-butynol in the presence of various
organocatalysts. (A) Evolution of the Raman spectra with time, (B)
Comparison of the kinetic profiles. Conditions: T = 80 °C, p = 50 bar, t = 6
h, 0.03 mol (2.6 ml) of propargylic alcohol, 5 mol% of nMe4OAc, nBu4OAc
and nBu4OPh and 2.5 mol% for (nBu4N)2Ox, volume of the cell = 20 ml, v
ACN = 2.6 ml.
These profiles could not be established for (nBu4N)2Cath and
(nBu4N)3Pyrog due to fluorescence of the reaction medium. From
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Figure 2, the organocatalysts were classified regarding their
activity in the order Me4NOAc < nBu4NOAc < (nBu4N)2Ox <
nBu4NOPh. The two latter organocatalysts (nBu4NOPh and
(nBu4N)2Ox) were identified as the most active ones as attested
by a higher slope and a plateau value that was reached after 50
min and 270 min, corresponding to a 12-fold and ~ 2.5-fold
increase of the activity as compared to nBu4NOAc, respectively.
Me4NOAc was significantly less active and no plateau was
reached after 800 mins of reaction.
decreases, especially at very low CO2 pressure. The higher
competition between the CO2 fixation on propargylic alcohol, and
the ring-opening of the αCC (product A) by unreacted propargylic
alcohol could be proposed as a plausible explanation for this trend.
Nonetheless, even at 5 bar, the (nBu4N)2Ox catalysts remained
quite selective as compared to nBu4NOPh.
Table 2. Influence of the experimental parameters on the coupling reaction
between CO2 and 2-methyl-3-butynol.
The selectivity of the reaction in product A was estimated from the
plateau value. With a relative intensity value of ~56000,
(nBu4N)2Ox was found to be the most selective catalyst with a
quantitative formation of the cyclic carbonate A as confirmed by
1H NMR spectroscopy of the reaction medium. All typical signals
of product A were observed with no side product and the absence
of any starting propargyl alcohol. With respective relative intensity
values of ~50000 and ~ 49000, nBu4NOPh and nBu4NOAc were
slightly less selective than (nBu4N)2Ox. By comparing the relative
intensity values reached with these catalysts with the one for
(nBu4N)2Ox, one estimates a selectivity towards product A of
approximately 90 % that is in agreement with that reported for the
catalyst screening experiments carried out under neat conditions
(Table 1, entries 23 and 3).
[C+]
[A-]
P
T
X(%)b
Selectivity (%)
(bar)
(°C)
SA
SB
-
SC
AcO-
5
1
nBu4N+
80
11
100
-
2a
3
nBu4N+
nBu4N+
nBu4N+
nBu4N+
nBu4N+
nBu4N+
nBu4N+
nBu4N+
Ox2-
PhO-
AcO-
Ox2-
5
80
80
80
80
80
60
60
60
100
100
20
89
35
5
65
5
-
6
-
5
4
15
15
15
50
50
50
95
-
5a
6
100
100
16
94
6
4
-
PhO-
AcO-
Ox2-
89
7
-
7
100
100
100
8a
9
24
-
-
PhO-
63
-
-
It is worth noting that the kinetic profile of the cyclic carbonate
formation for the nBu4NOAc catalyzed reaction displayed a
peculiar shape that did not fit a pseudo-first order kinetics, in
contrast to what is expected for a reaction carried out with an
excess of CO2. Indeed, the reaction rate increased with the
consumption of the propargylic alcohol. This was previously
explained by the solvation of OAc- by propargylic alcohol through
hydrogen bonding.33 This solvation shell stabilized OAc-, that
consequently lowered its catalytic activity as the result of a
decrease of its pKa value. This hydrogen bonding solvation shell
around OAc- was progressively weakened with the consumption
of propargylic alcohol, thus leading to the increased activity of the
counter-anion and of the reaction rate. Consequently, the
observed unusual kinetic behaviour for this reaction was due to
an evolution during the reaction of the hydrogen bond solvation
shell formed by the propargylic alcohol around OAc-. This
observation on the rate of reaction was also made for the CO2/2-
methyl-3-butynol coupling catalyzed by nBu4NOPh and
(nBu4N)2Ox.
Conditions: T = 80 °C, p = 50 bar, t = 6h, 0.03 mol of propargylic alcohol, 5 mol%
of catalyst, volume of the cell = 20 ml.
a using 2.5 mol% of catalyst
b Conversion and selectivity determined by 1H NMR
Influence of the temperature. The temperature can impact the
reaction in two ways, i.e. by affecting the rate of the reaction, and
by changing the solubility of CO2 in the propargylic alcohol phase.
We showed that for a constant pressure, the concentration of CO2
dissolved in the alcohol phase decreases with the decrease in the
temperature.39 Entries 7 to 9 in Table 2 summarize the results
obtained for experiments realized at 60°C and 50 bar for 6h using
nBu4NOAc, (nBu4N)2Ox and nBu4NOPh. As expected, lowering
the temperature slowed down the reaction as the conversions
close to 100% at 80°C with all catalysts decreased to 16, 24 and
63 by using respectively nBu4NOAc, (nBu4N)2Ox and nBu4NOPh.
8. Screening of the propargylic alcohols. In an effort to
broaden the scope of α-methylene cyclic carbonates that could be
produced by this synthetic approach, the (nBu4N)2Ox promoted
coupling of CO2 with various propargylic alcohols precursors was
investigated using the optimum experimental conditions, i.e. p =
50 bar, T = 80 °C for 6 h using 2.5 mol% of catalyst (relative to
the alcohol). The results obtained are summarized in Table 3.
No αCC was formed from prop-2-yn-1-ol or but-3-yn-2-ol
precursors, whereas 2-methyl-3-butynol was fully and selectively
converted into product A under similar experimental conditions.
These results can be explained by the Thorpe-Ingold effect (or
“gem effect”) which postulates that the mutual repulsion of gem
di-methyl groups favour the cyclization.40 When both methyl
7. Influence of reaction conditions.
Influence of the pressure. Our groups reported on the phase
behaviour of propargylic alcohols/CO2 biphasic systems and
highlighted that the concentration of CO2 dissolved in the
propargylic alcohol phase decreased with a decrease in pressure
at a constant temperature.39 This effect is reflected on the results
we obtained as shown in the Table 2 (entries 1-6).
At a constant temperature of 80°C, reducing the pressure slows
down the reaction rate as evidenced for experiments realized with
nBu4NOAc catalyst for which the conversion which is close to
100% at 50 bar (Table 1, entry 19) drops to 20 and 11% at 15 and
5 bar respectively (Table 2, entries 4 and 1 respectively). For the
other two catalysts, a decrease in the pressure is detrimental for
the selectivity towards the formation of the cyclic carbonate, which
groups of 2-methyl-3-butynol were replaced by
a bulkier
cyclohexyl substituent, the conversion was low (12 %) but the
reaction was also highly selective (Table 3, entry 4). These results
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10.1002/cctc.201800063
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show that the steric hindrance induced by bulkier substituents
may impede either the addition of CO2 onto the alcohol, or the
cyclization of the carbonate ion. Interestingly, a polymerizable bis-
αCC11 was synthesized from a dipropargylic alcohol with a
conversion of 35 % and a selectivity above 99 % (Table 3, entry
5). When one of the methyl groups of 2-methyl-3-butynol was
replaced by an ethyl group, the conversion decreased to 20% but
the selectivity remained excellent (100%) (Table 3, entry 6).
Complete conversion was however reached after 24 h of reaction
when using 5 mol% catalyst (Table 3, entry 7). Under these
conditions, other bulky propargylic alcohols were almost
quantitatively converted into the corresponding cyclic carbonates
with a high selectivity (60-96%) after only 6h of reaction (Table 3,
entries 8-11). These experiments clearly highlight that the catalyst
is active for the coupling reaction of CO2 with a broad range of
propargylic alcohols.
Conditions: T = 80 °C, p = 50 bar, t = 6 h, 0.03 mol of propargylic alcohols,
2.5 mol% of (nBu4N)2Ox, volume of the cell = 20 ml,
a experiment for 24 h
b5 mol% of (nBu4N)2Ox
Conclusions
In this work, we tailored various organic salts that were tested as
catalysts for the carboxylative coupling of CO2 with propargylic
alcohols. We have investigated the influence of the modulation of
the structure of the organocatalyst (mainly the type and structure
of the cation and anion) on the catalytic performances. Optimum
activity resulted from the best compromise between ion-pair
separation controlled by steric effects and the basicity of the anion.
Low ion-pair separation might limit the catalytic activity because
of stronger electrostatic interactions between the cation and the
anion and a lower availability of the anion for the deprotonation of
the alcohol. Organic salts with too basic anions increased the rate
of the reaction but at the expense of the selectivity. Among the
different salts that were tested, tetrabutylammonium oxalate
appeared to be the most active catalyst for the 2-methyl-3-
butynol/CO2 coupling reaction with quantitative conversion of the
starting product into the corresponding α-methylene cyclic
carbonate with a remarkable selectivity (> 99 %) in less than 6h
of reaction at 50 bar and 80 °C under solvent free conditions. Only
2.5 mol% of catalyst (compared to the propargylic alcohol) were
required for this performance. This study clearly opens new
Table 3: Synthesis of various alkylidene 5-membered cyclic carbonate
Reagent
Product
Conv.(%)
Selectivity
(%)
1
2
3
0
0
0
0
100
99
avenues for conceiving
efficient organocatalysts for
CO2/propargylic alcohol coupling reactions, with potentially less
toxicity than metal based ones because carboxylate anions favour
the biodegradability and decrease the toxicity of the
corresponding organic salts.41
4
12
35
100
99
Acknowledgements
5a
All co-authors thank Prof. B. Gilbert for designing and setting up the probe that
allowed to perform in-situ Raman spectroscopy under pressure, and for all his
advices and his help for the comprehension of the reaction kinetics. Prof. Gilbert,
who is co-author of this paper (and of many others with Liege’s group),
regrettably passed on the 17th on February 2018. The authors of Liège thank
the “Region Wallonne” and the FEDER in the frame of the CO2Green, Flycoat
and Prostem projects, the "Belgian Science Policy" in the frame of the
6
20
99
100
93
7a,b
"Interuniversity Attraction Poles Programme (IAP VII/5)
– Functional
8b
98
99
95
95
Supramolecular Systems" and the "Fonds National pour la Recherche
Scientifique” (F.R.S.-FNRS) for financial supports. This work was partly
supported by the Fonds de la Recherche Scientifique (FNRS) and the Fonds
Wetenschappelijk Onderzoek
– Vlaanderen (FWO) under EOS project
9b
n°O019618F (ID EOS: 30902231). C.D. is Research Director by F.R.S.-FNRS.
We also thank computational facilities provided by the MCIA (Mesocentre de
Calcul Intensif Aquitaine) of the University of Bordeaux financed by the ‘‘Conseil
Regional d’Aquitaine’’.
10b
11
98
85
60
Keywords: carbon dioxide • cyclic carbonate • organocatalyst
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Entry for the Table of Contents (Please choose one layout)
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Bruno Grignard, Charlene Gabriela
Ngassamtounzoua, Bernard Gilbert,
Raphael Mereau, Christine Jerome,
Thierry Tassaing, Christophe
Detrembleur*
Text for Table of Contents
How can we design highly active and
selective organocatalysts for the
carboxylative coupling of CO2 with
propargylic alcohols?
Page No. – Page No.
Boosting the catalytic performance of
organic salts for the fast and
selective synthesis of α-alkylidene
cyclic carbonates from CO2 and
propargylic alcohols
O
CO2
R
O
OH
O
R
Organocatalyst?
R
O
R
N
R
O
R
O
R
H
N
R
O
O
N
O
B
H3C
N
O
O
N
R
R
P
H
H3C
O
R
N
R
N
R
Layout 2:
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Author(s), Corresponding Author(s)*
((Insert TOC Graphic here; max. width: 11.5 cm; max. height: 2.5 cm))
Page No. – Page No.
Title
Text for Table of Contents
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