An Aminopyridinium Ionic Liquid: A Simple and Effective Bifunctional Organocatalyst for Carbonate Synthesis from Carbon Dioxide and Epoxides

Article abstract of DOI:10.1002/cplu.202000367

An aminopyridinium ionic liquid is presented as a green, tunable, and active metal-free one-component catalytic system for the atom-efficient transformation of oxiranes and CO₂ to cyclic carbonates. Inclusion of a positively charged moiety into aminopyridines, through a simple single-step synthesis, provides a one-component ionic liquid catalytic system with superior activity; effective in ring opening of epoxide, CO₂ inclusion, and stabilization of oxoanionic intermediates. An efficiency assessment of a variety of positively charged aminopyridines was pursued, and the impact of temperature, catalyst loading, and the kind of nucleophile on the catalytic performance was also investigated. Under solvent-free conditions, this bifunctional organocatalytic system was used for the preparation of 18 examples of cyclic carbonates from a broad range of alkyl- and aryl-substituted oxiranes and CO₂, where up to 98 percent yield and high selectivity were achieved. DFT calculations validated a mechanism in which nucleophilic ring-opening and CO₂ inclusion occur simultaneously towards cyclic carbonate formation.

Full text of DOI:10.1002/cplu.202000367

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An Aminopyridinium Ionic Liquid: A Simple and Effective
Bifunctional Organocatalyst for Carbonate Synthesis from
Carbon Dioxide and Epoxides
Amirhossein Ebrahimi+,[a] Mostafa Rezazadeh+,[b] Hormoz Khosravi,[c] Ali Rostami,*[a] and
Ahmed Al-Harrasi*[a]
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An aminopyridinium ionic liquid is presented as a green,
tunable, and active metal-free one-component catalytic system
for the atom-efficient transformation of oxiranes and CO2 to
cyclic carbonates. Inclusion of a positively charged moiety into
aminopyridines, through a simple single-step synthesis, pro-
vides a one-component ionic liquid catalytic system with
superior activity; effective in ring opening of epoxide, CO2
inclusion, and stabilization of oxoanionic intermediates. An
efficiency assessment of a variety of positively charged amino-
pyridines was pursued, and the impact of temperature, catalyst
loading, and the kind of nucleophile on the catalytic perform-
ance was also investigated. Under solvent-free conditions, this
bifunctional organocatalytic system was used for the prepara-
tion of 18 examples of cyclic carbonates from a broad range of
alkyl- and aryl-substituted oxiranes and CO2, where up to 98%
yield and high selectivity were achieved. DFT calculations
validated a mechanism in which nucleophilic ring-opening and
CO2 inclusion occur simultaneously towards cyclic carbonate
formation.
Introduction
elevated-energy substrate preparation or reductive chemical
transformations.[5] In this regard, preparation of low-energy and
remarkably valuable cyclic carbonates have been actively
pursued in sustainable and green chemistry.[3] Cyclic carbonates
have broad applications as intermediates for chemical and
pharmaceutical industries,[6] harmless benign polar aprotic
solvents,[7] liquid electrolytes for lithium ion batteries,[8] and
monomers for a range of eco-friendly polymers such as
polycarbonates[9] and nonisocyanate poly(hydroxyurethanes)
(NIPU).[10]
One of the few examples of industrial application of CO2 lies
in the preparation of cyclic carbonates through atom-efficient
cycloaddition of CO2 and epoxides.[3] The development of
effective catalytic systems are crucial to achieve sustainable
processes based on CO2 chemistry.[11] In the past few years, an
array of catalytic methods for the above transformation have
evolved, including metal-mediated protocols,[12] ammonium,[13]
phosphonium,[14] pyridiniums,[15] and imidazolium salts,[16]
guanidines,[17] amidines,[18] and N-heterocyclic carbenes
(NHCs).[19a] Even though metal-based catalytic systems exhibit
superior activities compared to the organic equivalents, the
mandatory absence of refractory metal remnants, and effective
catalyst isolation from the final adduct, promote practicing
organocatalytic approaches.[19b] In this sense, devising mild,
cheap, sustainable, and metal-, solvent-, and co-catalyst-free
catalytic settings that will meet highly demanding environ-
mental regulations remain an attractive, yet unrealized goal.
Bifunctional strategies have recently become incredibly
popular in the coupling of CO2 and epoxides in which
incorporating a hydrogen-bond donor group into an ionic-
liquid core containing the nucleophilic component results in
enhanced catalytic activities.[20b] In these systems, a synergistic
effect among the two catalytic entities is involved whereas
The continuous emission of anthropogenic carbon dioxide, a
major greenhouse gas and an undesired environmental
pollutant, into the atmosphere has been identified as a major
factor in adverse global climate change.[1] Therefore, current
endeavors are concentrated on efficient processes for capture
and utilization of CO2 (CUC) to be applied alongside sustainable
energy and transport developments.[2] The conversion of the
captured CO2 that is easily accessible through carbonaceous
fuel combustion and human actions into value-added chemicals
has been a thriving area of research,[3] mainly due to merits
associated with CO2 as a non-toxic, renewable, inexpensive and
globally available C1 feedstock for chemical industry.[4] How-
ever, application of this well-oxidized and inert building block
in chemical production has been hampered owing to its
thermodynamic stability and chemical inertness, underlined by
[a] A. Ebrahimi,+ Dr. A. Rostami, Prof. A. Al-Harrasi
Natural and Medical Sciences Research Center (NMSRC)
University of Nizwa
616, Nizwa (Sultanate of Oman)
E-mail: arostami@unizwa.edu.om
aharrasi@unizwa.edu.om
[b] M. Rezazadeh+
Department of Polymer and Material Chemistry
Shahid Beheshti University
19839-4716 Tehran (Iran)
[c] H. Khosravi
Peptide Chemistry Research Center
K. N. Toosi University of Technology
P. O. Box 15875-4416 Tehran (Iran)
[+] AE and MR are contributed equally. Manuscript was written through con-
tributions of all authors. All authors have given approval to the final version
of the manuscript.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cplu.202000367
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activation and ring-opening of the epoxide and further
stabilization of the reaction intermediates result in lower energy
barriers associated with this process.[20,21]
explored, and thus the evaluation of their catalytic activities in
sustainable utilization of CO2 merits more investigation.
Pursuing our quest in inventing simple one-component
metal- and solvent-free platforms for mild chemical fixation of
CO2, we presumed that 2-aminopyridine – a cheap and
commercially available pyridine – with a suitable N-alkylated
substituent could promote the title reaction (Scheme 1b). This
could be achieved via the N-alkylated 2-aminopyridinium ionic
liquid[25] by (i) increasing the halide nueclophilicity for the ring-
opening process as a result of the weaker ion-paring effect; (ii)
stabilization of the epoxide-CO2 intermediate as a precursor to
its role as an oxygen nucleophile in the following carbon-
oxygen bond-making stage.
Towards the goal of reducing the cost, time, and energy
consumption in CO2 catalysis, we report here that the N-octyl-2-
aminopyridinium ionic liquid 3c, a simple and potent bifunc-
tional catalyst, could promote the reaction of oxiranes with CO2
under environmentally benign conditions and short reaction
times.
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Capable of high CO2 uptake, pyridinium ionic liquids have
emerged as a suitable medium for CO2 absorption.[22] This
feature has recently been applied to the use of pyridinium-
based poly(ionic liquid)s for CO2 capture and partitioning and
illustrates the importance of the pyridinium moiety in CO2
chemical fixation technologies.[23] Amongst various fascinating
organocatalytic systems explored to date (see above), there has
been less attention paid to systems containing the pyridinium
core (Scheme 1a).[15,24] Lately, Waser and Kass reported a binary
catalytic system containing N-methylpyridinium ions containing
thiourea and ammonium iodide as a binary catalytic system to
couple terminal epoxides and CO2 under fairly mild settings;
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however,
a multistep catalyst preparation involving toxic
thiophosgene was required.[24b] With the exceptions of the
recent account by Zhang et al on protonated-DMAP[15] and our
recent report on electrostatically enhanced phenol,[24c] bifunc-
tional platforms based on pyridinium salts that promote
chemical fixation of CO2 and epoxides, have not yet been
Results and Discussion
Catalyst screening and parameter optimization
Given the ongoing interest towards devising organocatalytic
platforms in CO2-based chemical reactions by employing the
merits of minimalism, the reactivity of N-alkylated amino
pyridinium salts under environmentally benign settings was
assessed. Several N-alkylated amino pyridinium salts 3a–e were
made by straightforward alkylation of cheap and commercially
accessible aminopyridines in
a standard and single-step
manner.[26] The entry into these varied and easily-modified salts
could be realized by straightforward change of both the
aminopyridine block or the alkyl halide segment, generating a
set of air-tolerant and architecturally distinct one-component
catalysts in high yields (Scheme 2).
Towards evaluating our assumption, we started to inves-
tigate atom-economic transformation of CO2 (balloon) and
styrene oxide (SO) 1a – used as a standard and commercially
available epoxide – to styrene carbonate (SC) 2a. At the outset,
we noticed that 3-amino-N-methyl pyridinium salt 3a was able
to promote this process under co-catalyst and solvent-free
conditions: CO2 pressure of 1 atm, and 5 mol% catalyst loadings
°
at 50 C, styrene oxide provided 36% yield of the corresponding
carbonate 2a in 24 h. The reaction yields were measured readily
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by evaluating the H NMR spectra with specific resonances that
match SO 1a (δ=3.90, 3.16, and 2.82 ppm) and SC 2a (δ=5.62,
4.74, and 4.26 ppm) in the presence of mesitylene as an internal
standard. Surprisingly, the other regioisomer, 2-amino-N-methyl
pyridinium salt 3b, provided selective formation of the
corresponding cyclic carbonate (>99%) in 89% yield under the
same reaction conditions. The observed higher conversion
provided by catalyst 3b could be due to the weaker ion-paring
effect between the iodide anion and the positively charged
pyridinium moiety via anion-binding interactions[27] and/or
steric and electronic effects. Rising the steric demand of the
Scheme 1. CO2 Fixation with Epoxides Catalyzed by Pyridinium-Based
Organocatalysts.
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hand, these findings show that even though an amine group is
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very competent in CO2 activation, the potential amine-CO2
carbamate intermediate[30] is inefficiently nucleophilic to suc-
cessfully ring expand the oxirane moiety under our optimized
reaction settings. An amine-free group, N-methyl pyridinium
salt, 3i, that is incapable of providing discrete hydrogen-bonds,
provided the resultant carbonate in only 27% yield, thus
validating the importance of the hydrogen-bond functionality
in anion-binding interaction, and successive stabilization of the
oxo-anionic intermediates. In general, hydrogen-bond donor
groups are believed to improve the catalyst potency by
modulating the halide nucleophilicity and subsequent stabiliza-
tion of the oxo-anionic intermediates, hence giving rise to
milder conditions.
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To underline the significance of the bifunctional character
of 3c, binary catalytic systems made of different aniline
derivatives (3f–h) and a quaternary ammonium iodide salt 3j
were investigated, and gave inferior yields as noted (Scheme 2).
These findings highlight the essential roles of both hydrogen-
bond donor functionality and nucleophilicity – the two essential
components of the bifunctional catalyst – on the catalytic
action in this coupling reaction. The effect of temperature on
reaction yield was also investigated: decreasing the temper-
°
ature to 40 C provided the styrene carbonate adduct in
quantitative yield while a further decrease in temperature to
°
30 C reduced the reaction yield to 73%. The influence of
different solvents with altered polarities (acetonitrile, dimeth-
ylformamide, and dimethyl sulfoxide) at mild conditions (30 C,
°
p(CO2)=1 atm) was also tested; however, none of the utilized
solvents provided yield enhancement, most likely due to their
hydrogen-bond accepting character resulting in catalyst
deactivation.[31] Unsurprisingly, the reaction yield was clearly
associated with the amount of catalyst employed: the impact of
Scheme 2. Effect of different aniline derivatives. a) Conditions: styrene oxide
°
1a (1 mmol), catalyst (0.05 mmol, 5 mol%), p(CO2) (1.0 atm, balloon), 50 C,
24 h, reaction was performed neat. b) Yield was verified by 1H NMR analysis
(CDCl3) using mesitylene (0.043 mmol, internal standard) as an internal
standard. The selectivity for 2a was >99%. c) The reaction was carried out
in the presence of n-tetrabutylammonium iodide (0.05 mmol).
°
the catalyst loading was evaluated using 3c (40 C, p(CO2)=
1 atm) in which by reducing the loadings by two-fold from 5 to
2.5 mol% the product yield dropped to 48%. The conversion
kinetics of SO 2a was explored by a sequential run of eight
uniform trials in which the reactions were halted following a
pyridinium core from methyl (3b) to octyl group (3c) provided
the SC 2a in quantitative yield and high selectivity (>99%). The
noted boost in the reaction yield can be justified by either
improved solubility of 3c, or greater nucleophilicity of the
iodide as the ionic interaction among the iodide anion and the
positively charged pyridinium unit is reduced by the increase in
the size of the cation.[14a–c,16,28] The choice of nucleophile
similarly induces a striking outcome on the yields of the
reaction: proceeding from iodide 3c to bromide 3d reduced
the yield to 89% presumably as a result of greater hydrogen-
bond accepting properties of the latter and the weakened
hydrogen-bond donor ability of the catalyst in activating the
epoxide.[14a,20h,29] This trend correlates well with the order of
catalytic activity since a strong interaction among the halide
anion and the hydrogen-bond donor motif reduces the catalytic
activity. In addition, 2-amino-N-methyl pyridinium salt 3e
equipped with tetraphenyl borate as the counterion (a non-
nulceophilic anion that cannot activate the epoxide), gave only
4% yield of expected product. Likely, the anion segment holds
an important share in starting the operation by ring-expansion
of the oxirane before the addition reaction to CO2. On the other
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given period and assessed by H NMR spectroscopy: 3c could
effectively mediate nearly quantitative conversion of styrene
oxide to styrene carbonate in 9 h (Figure 1a). Furthermore, the
confirmation of specific resonances of SO 1a and cyclic SC
adduct 2a was achieved by offline attenuated total reflectance
(ATR)-FTIR spectroscopy: the time-based evolution of the
process was examined by following the intensity of the
carbonyl vibration band at 1780 cmÀ 1 along with bending
vibrations at 1150 cmÀ 1 and 1040 cmÀ 1. Throughout the reac-
tion, the intensity of cyclic carbonate carbonyl band increased
significantly and reached its maximum; however, the intensity
of oxirane ring-deformation bands at 985 and 878 cmÀ 1,
decreased continuously, and eventually vanished after 9 h
(Figure 1b).[24c]
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mostly recognized as challenging substrates due to their low
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reactivity.[9c] This one-component catalyst was compatible with
challenging oxiranes equipped with heteroatoms – halogen,
oxygen and nitrogen electron-withdrawing groups – including
epichlorohydrin, phthalimide, and especially glycidyl derivatives
(2f–o) – cases with special interest due to their availability from
renewable resources.[32] Also, this catalytic system was applied
to epoxides with pendant olefin side chains (2h and 2i)
particularly the glycerol carbonate methacrylate (2i) that are
valuable building blocks for homo- and co-polymerization
reactions to generate polymers with a cyclic carbonate built-in
functionality in the backbone.[33] A recent life cycle study
showed the importance of preparation 2i through the cyclo-
addition of CO2 and 1i, a decline in global warming potential of
3% for the preparation of 2i from 1i could be reached (1i has
been synthesized from epichlorohydrin and methacrylic acid).[34]
Also, the alkyne substituted carbonate (2j) was synthesized in
distinguished yield of 90%. The preparation of biscarbonates
has recently gained considerable attention, as they have been
employed as a starting material for the synthesis of non-
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isocyanate polyurethanes (NIPUs).[35] It is notable that
a
challenging substrate that carries two epoxide functionality
such as bisphenol A diglycidyl ether experienced ring-opening
reaction providing the cyclic carbonate in 95% yield and high
selectivity (>99%).
In order to further understand the reaction pathway,
optically pure (R)- and (S)-(+)-glycidyl benzyl ether (1m and 1n)
was evaluated respectively under our optimized reaction
settings to assess the chirality transfer from the chiral oxiranes.
We obtained the cyclic carbonate products (2m and 2n) in
99% enantiomeric excess (ee) in each instance, with retention
of configuration originating from oxiranes with comparable ee
values, verifying the halide-induced-ring-opening of the oxirane
from the reduced-crowded site through SN2 pathway (favored
attack on the methylene carbon).[36] This one-component
catalytic setting offers an access to enantio-enriched cyclic
carbonates without stereochemical erosion. In order to broaden
the catalytic scope, we explored the cycloaddition of challeng-
ing 1,2-disubstituted epoxides (1q and 1r) due to their steric
hindrances (Scheme 3 and Scheme 4). Under slightly harsh
°
conditions of 10 mol% 3c at 80 C and under 1 atm CO2
Figure 1. i) Time-dependent conversion and ii) ATR-FTIR spectroscopy in situ
monitoring of styrene oxide 1a conversion in the presence of CO2 to cyclic
carbonate 2a. Conditions: styrene oxide 1a (1 mmol), 3c (0.05 mmol,
pressure for 24 h, the reactions took place slowly to provide the
desired cyclic carbonates (2q and 2r) in inferior yields, yet
excellent selectivity (>99%). In general, organocatalyzed con-
version of disubstituted epoxides to their corresponding cyclic
carbonates requires either high pressure (20–30 atm) CO2 or a
co-catalyst.[37a,39] The reactions of cis- and trans-stilbene oxide
1r, proceeded to give a thermodynamically stable trans-cyclic
carbonate 2r, as the sole product. The stereoselective outcomes
suggest the transformation of stilbene oxides into the resultant
cyclic carbonates could happen through different pathways,
involving rapid iodide-iodide anion exchange and/or SN1 type
manifolds. A closely related stereochemical outcome has been
previously described for the [3+2] cycloaddition of 1,2-
disubstituted epoxides and CO2.[37]
°
5 mol%), p(CO2) (1.0 atm, balloon), 40 C, 9 h, reaction carried out neat.
Substrate scope
Encouraged by these outcomes and with a desire to assess the
substrate scope of the 3c-catalyzed [3+2] cycloaddition
reaction as shown in Scheme 3, we evaluated numerous
terminal alkyl- and aryl-functionalized oxiranes (1a–q) under
°
our optimized catalytic conditions (5 mol% catalyst, 40 C and
9 h). Various styrene oxide derivatives (2a–c) and oxiranes
bearing a simple alkyl chain (2d and 2e) experienced ring-
opening event to provide the desired cyclic carbonates in high
yields and selectivity (>99%). Styrene oxide derivatives are
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Scheme 3. Cycloaddition of various epoxides 1 and CO2 catalyzed by 3c.[a] a) Conditions: epoxide 1 (2 mmol), 3c (0.1 mmol, 5 mol%), p(CO2) (1.0 atm, balloon),
°
40 C, 9 h, reaction was performed neat. b) Yields refer to the product isolated by column purification; yield and selectivity of all products were determined by
1
°
H NMR. c) Conditions: epoxide 1q (2 mmol), 3c (0.2 mmol, 10 mol%), p(CO2) (1.0 atm, balloon), 80 C, 24 h, reaction was performed neat. All products were
characterized by comparison of their 1H NMR and 13C NMR spectra with those of authentic samples (see the Supporting Information).
Mechanism
increasing the concentration of n-TBAI from 0:1 to 10:1 molar
ratio, distinct down-field shifts (from δ=8.33 to 8.51 ppm) of
the 3e NH2 proton resonance as well as pyridinium ring protons
resonances (Δδ=0.03 to 0.15 ppm) were observed.[38] These
data suggest that the styrene oxide 1a is unable to compete
with the iodide anion to form a hydrogen-bonded complex
3c:1a, indicating 3c catalytic activity is mostly related to its
control on the reactivity of the ion-pair through anion binding
as well as its hydrogen-bonding stabilization of oxoanionic
intermediates that form after iodide-mediated ring-opening of
the epoxide.[37a]
To gain further mechanistic insight into the nature of the
behavior of 3c, we examined the reaction of diastereo-isomeri-
cally pure cis-1r in the presence and the absence of CO2 (see
Scheme 4 and the Supporting Information): in the presence of
CO2 beside the trans-2r product, a mixture of unreacted
epoxides cis and trans-1r (67%, trans/cis=1.1:1) was obtained
indicating a substantial loss of epoxide stereo-information. Also,
Further studies for the hydrogen-bonding interaction with
epoxide by 2-amino-N-octyl pyridinium ionic liquid 3c was
verified by 1H NMR experiments (see the Supporting
Information).[38] These trials were attained using solutions of SO
1a to a 0.2 M solution of 3c in CDCl3 at ambient temperature.
Upon increasing the concentration of oxirane 1a from 0:1 to
10:1 molar ratio, Negligible down-field shifts (from δ=8.14 to
8.18 ppm) of the 3c NH2 proton resonance as well as minor up-
field shifts of the pyridinium ring protons resonances (Δδ=0.04
to 0.07 ppm) were observed. It seems that the interaction
between catalyst 3c and epoxide 1a is typically weak, a
phenomenon that was previously observed by Kleij and co-
workers.[37a]
On the other hand, anion-binding properties of the
tetraphenylborate salt of 2-amino-N-methyl pyridinium ionic 3e
was further probed by 1H NMR experiments whereas upon
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Scheme 4. Reactions of 1,2-disubstituted epoxides with CO2.[a] a) Conditions: epoxide 1r (2 mmol), 3e (0.2 mmol, 10 mol%), p(CO2) (1.0 atm, balloon), 80 C,
24 h, reaction performed neat. Yields refer to the product isolated by column purification. All products were characterized by comparison of their 1H NMR and
13C NMR spectra with those of authentic samples (see the Supporting Information).
°
in the absence of CO2, a similar loss of stereo-information was
noticed in which a mixture of cis and trans epoxides (1r: trans/
cis=1.7:1) favoring the thermodynamically more stable trans
diastereo-isomer, was obtained. The ring-opening of the
epoxide could proceed with the aid of iodide anion and be
reversible, and the alkyl halide intermediate could undergo a
rapid iodide-iodide anion exchange and/or SN1 processes
allowing epoxide β carbon scrambling..[37c,39,14d] In general, not
only alkyl halide is able to undergo a rapid iodide-iodide anion
exchange and/or a SN1 processes but also carbonate intermedi-
ate is able to undergo similar processes to furnish the
thermodynamically favored trans-carbonate product.
Based on the result for the conversion of the optically pure
glycidyl benzyl ethers (R)-1n and (S)-1o, in which the absolute
configuration was fully retained (see above), we propose the
SN2 manifold as the most probable reaction pathway for
epoxides that are not able to stabilize cationic intermediates
providing the overall retention of the absolute stereochemistry.
On the other hand, epoxides carrying substituents efficient in
stabilizing cationic intermediates could undergo addition reac-
tion via either a rapid iodide-iodide anion exchange process
and/or SN1 manifolds providing the thermodynamically favored
trans product as noticed in the case of cis-stilbene oxide (cis-1r)
(see Scheme 4 and the Supporting Information).
propylene oxide (PO) with the concomitant attack of the oxo-
anion to CO2 (Scheme 5, path A), whereas the second pathway
consist of two discrete steps including the nucleophilic addition
of IÀ to PO followed by CO2 incorporation to the formed
halohydrin intermediate (Scheme 5, path B). Although the
halide ring expansion of the oxirane is prone to happen via
both sides, yet the less substituted carbon is energetically
preferred owing to the less steric hindrance. Therefore, only
In order to achieve a profound insight into 2-amino-N-alkyl
pyridinium mediated ring expansion of the oxirane in the
presence of CO2, a detailed computational mechanistic study
was performed. Density functional theory (DFT) calculations
were carried out at the hybrid B3LYP functional with the
Gaussian 09 suite of programs (see Figure 2 and the Supporting
Information for computational details).[40] Former mechanistic
investigations have been conveyed for the reaction of CO2 and
oxiranes with two proposed routes.[41] The first pathway is based
on the nucleophilic attack of IÀ on the methylene carbon of the
Scheme 5. Proposed mechanistic pathways for the 3c-catalyzed cycloaddi-
tion of CO2 and epoxides.
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Figure 2. Potential energy surfaces of the cycloaddition of propylene oxide (PO) and CO2 by using 2-amino-N-methyl pyridinium iodide 3b (paths A and B).
For mechanistic details, see the Supporting Information.
mechanistic pathways including the attack on the secondary Conclusion
methylene carbon (β carbon) is studied.[42a] Optimized structures
and energetic information for transition state (TS) and inter-
mediates (Int.) for all possible mechanisms can be reached
through the Supporting Information. The first transition state
(TS1) of path (A) is destabilized by 17.1 kcalmolÀ 1 relative to the
reactants (Figure 2, path A) whereas, TS1 for path B is
destabilized by 19.1 kcalmolÀ 1 (Figure 2, path B). Thus, transi-
tion state (TS1 A) for path A (concerted process) is energetically
favored by 2.0 kcalmolÀ 1 relative to transition state (TS1 B) for
path B (stepwise process). Similar findings have been previously
reported by Dove and co-workers.[18a] In order to evaluate the
possibility of the stepwise pathway B in which a halohydrin
intermediate (Int. B) could be formed, we performed the same
reaction employing both aryl- and alkyl- substituted epoxide
(1a and 1e), catalyst 3c in the absence of CO2 however only
traces of the corresponding halohydrin intermediates were
formed, supporting the concerted pathway A (see the Support-
ing Information).[42b]
This implies that the reaction will generally carry on via the
concurrent iodide activation of the epoxide ring and CO2
inclusion, similar to the previously proposed mechanism for the
protonated amidinium iodide [HDBU]I mediated cycloaddition
reaction of CO2 and epoxides.[18a] As suggested earlier for other
catalytic platforms, these results indicate that the rate-determin-
ing step could be assigned to the ring-expansion of the
epoxide.[14] Based on the above experimental and computa-
tional studies, we have inferred a hydrogen-bonding/anion-
binding mechanism for the aminopyridinium ionic liquid-
catalyzed [3+2] addition of CO2 to epoxides wherein the
catalyst could simultaneously participate in anion-binding
processes, and CO2 inclusion in a synergistic fashion.
Although metallo-organo catalysts have been epitomized as the
most active catalytic platform for carbonate synthesis from CO2
and epoxides,[43] there is still exist a great motivation in devising
metal-free catalytic systems that could rival the metal-catalyst
efficiencies. In this area, a straightforward evaluation of reaction
parameters provided by various organocatalysts still remains a
challenge. For instance, in the presence of CO2 (balloon), when
TBAI was utilized with 2-pyridinemethanol as the hydrogen-
bond donor source in a binary fashion with 8 mol% loadings,
[44]
°
styrene carbonate was obtained in 85% yield at 45 C in 20 h.
On the other hand, a recently reported protonated amidinium
iodide [HDBU]I catalyst could mediate the same reaction at
°
higher temperature (70 C) and catalyst loading of 10 mol%,
albeit in shorter reaction time (4 h).[18a] In general, ammonium
halides have been widely exploited as the main co-catalysts, yet
evaluation of other halide sources such as pyridinium halide is
still in its infancy. The 2-amino-N-octyl pyridinium ionic liquid
(3c) explored here is a simple and effective one-component
catalytic system that induce epoxide ring opening and CO2
inclusion in a synergic manner. In this respect, an array of N-
alkylated aminopyridinium salts differing in pyridine regioisom-
ers, nitrogen substituents, and counter ions were prepared and
subsequently employed in [3+2] coupling between CO2 and
epoxides. This bifunctional ionic liquid mediator is able to
convert a broad variety of architecturally distinct oxiranes to
the corresponding cyclic carbonates in high-to-excellent yields
and high selectivity under mild reaction conditions (5 mol%
°
catalyst, 40 C, 9 h). DFT calculations to obtain reaction barriers
for various catalytic pathways were performed to give mecha-
nistic insight and supported the experimental findings in which
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Experimental Section
General procedure for the synthesis of cyclic carbonates: In an
oven-dried vial equipped with a stir bar was added epoxide 1
(2 mmol, 1 equiv.) and 3e (33 mg, 0.1 mmol, 5 mol%). The vial was
fitted with septa, degassed with carbon dioxide and let to stir
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°
under a positive pressure of carbon dioxide (balloon) at 40 C for
9 h. The carbonate products 2 were isolated via flash column
chromatography using silica gel and ethyl acetate/hexanes as
eluent. All isolated carbonate products matched the previously
reported characterization data.
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Supporting Information: (See footnote on the first page of this
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Supporting Information for this article is available at
Funding Sources
Support was provided by Sultanate of Oman Research Council
(TRC) (Grant # BFP/RGP/EI/18/021) and the University of Nizwa.
Acknowledgments
Financial support from Sultanate of Oman Research Council (TRC)
(Grant #BFP/RGP/EI/18/021) and the University of Nizwa is grate-
fully acknowledged. We would like to thank Dr. John Husband
(Sultan Qaboos University) for proofreading the manuscript and
providing access to the Gaussian 09 program.
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: carbon dioxide · hydrogen-bond donors · ionic
liquids · organocatalysis · ring-opening reactions
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a detailed computational studies for two other
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Manuscript received: May 11, 2020
Revised manuscript received: July 2, 2020
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