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Green Chem., 2014, 16, 1-3 | 1

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Green Chemistry

Cite this: DOI: 10.1039/x0xx00000x

Sustainable Conversion of Carbon Dioxide: The
Advent of Organocatalysis

Received 00th January 2014,
Accepted 00th January 2014

Giulia Fiorani,a Wusheng Guoa and Arjan W. Kleij a,b*

DOI: 10.1039/x0xx00000x
www.rsc.org/

The conversion of carbon dioxide (CO 2), an abundant renewable carbon reagent, into chemicals
of academic and industrial interest is of imminent importance to create a higher degree of
sustainability in chemical processing and production. Recent progress in this field is
characterised by a plethora of organic molecules able to mediate the conversion of suitable
substrates in the presence of CO 2 into a variety of value-added commodities with advantageous
features combining cost-effectiveness, metal-free transformations and general substrate
activation profiles. In this review, the latest developments are discussed in the field of CO 2
catalysis with a focus on organo-mediated conversions and their increasing importance as to
serve as practicable alternatives for metal-based processes. Also a critical assessment of the
state-of-the-art is presented with attention on those features that need further attention to lift the
usefulness of organocatalysis in the production of organic molecules of potential commercial
interest.

1. Introduction
Our current chemical needs depend highly on the availability of
fossil fuels as feed stock. Predictions have been made that
somewhere in the near future we will be running out of these
reserves and/or their immediate availability can no longer fulfil
the increasing global demand for these raw materials. As such,
contemporary chemists are facing a huge challenge to develop
more sustainable alternatives for chemical production. Carbon
dioxide is a waste product from all combustion processes and
represents a potential and alternative carbon feed stock1,2 for the
preparation of a variety of useful chemicals including MeOH,3,4
urea,5 lactones,6 various heterocycles,7-11 biodegradable
polymers,12-16 and carboxylated structures17-21 among others.22-24
In particular, direct CO 2 utilisation for the preparation of
polycarbonates, poly(ether)carbonates and polyurethanes is a
viable technology to access novel tailor-made CO2-based
materials.25 The development of cost- and energy-efficient
synthetic methodologies to exploit CO 2 as a C1 synthon could
eventually loosen the grip on our fossil fuel dependence,
contributing to a reduction of the overall CO2 emissions and
offering tangible alternatives to physical sequestration
methods.26,27
In nature, CO2 is captured, converted and/or reduced to
carbohydrates by a series of cascade reactions carried out by
specific metalloenzymes present in photosynthetic organisms. 28
Similarly, the field of CO 2 conversion catalysis is dominated by
the use of transition metal-based catalysts being either
heterogeneous or homogeneous in nature.29-32 While the use of a
catalytic system generally implies a sustainability improvement,
some marked differences can be foreseen between choosing
metal-based or organo-catalysts. It has been shown that
activation and/or stabilization of substrates/intermediates
through metal complexes or immobilized metal species via
coordination motifs is far more efficient compared with organobased catalytic mediators. Organocatalysts, on the other hand,

2 | Green Chem., 2014, 00, 1-3

are generally low cost, non-toxic molecules, characterized by a
good stability and inertness towards moisture and air. As such,
from a reaction design point of view, they can be regarded as
safer reaction components. Moreover, in some cases, they can be
readily derived from renewable feed stocks, devising technically
and economically practicable ad hoc procedures starting from
biomass and CO2, thus eventually developing “carbon-neutral”
processes to produce sustainable chemicals, fuels, and
materials.33
It is known that organocatalysis can sometimes lead to
additional energy expenditure during reaction and work-up time,
requiring higher catalyst loadings, higher operating temperatures
and prolonged reaction times. Nevertheless, in some cases, (e.g.
when using ionic liquids as reaction solvents and/or catalysts) the
reagents/products can be recovered by simple distillation
devising more safe and atom-economical processes while
avoiding commonly employed waste generating work-up
procedures. Catalyst heterogenation has been extensively
exploited to increase the lifetime of organo-catalyst simplifying
reaction monitoring and work-up procedures, while providing a
possible route towards the development of continuous flow
reactions. Organocatalysis is especially advantageous to develop
metal-free products or processes, since the incorporation of
chemicals into consumer products require that the amount of
metal residues to be present as low as possible to avoid any
toxicity issues. Since these requirements are becoming more
stringent by the year, alternative processes that can circumvent
the use of metal catalysts would provide a means to produce
targeted chemicals in a safer and potentially greener way. In this
respect, organo-based catalysts (i.e., organocatalysts) are of
growing interest and importance in the area of CO2 catalysis and
conversion. Recently, the pace with which organocatalytic
procedures are being utilized in organic preparations that involve
CO2 as a key reagent is unquestionably increasing. There are a
number of structurally different organocatalyst systems that are
considered as catalytic mediator including those based on ionic

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GREEN CHEMISTRY
liquids (ILs),33-35 N-heterocyclic carbenes (NHCs),36 phenolic
compounds37 and sophisticated azaphosphatranes.38 This is a true
testimony of the increasing prominence of organocatalysis in the
area of CO2 fixation to create value-added organic matter from a
waste material.
This review intends to showcase the most recent
developments in CO 2 catalysis where organocatalytic systems
have helped to increase the overall sustainability of the process
in terms of more efficient energy usage, raw material conversion,
avoiding solvent and/or metals and improvement of
selectivity/reactivity features. A complete overview of all the
literature is not feasible and therefore avoided; instead, the key
contributions in specific areas of CO2 research are highlighted.
Also, a critical evaluation of the reported organocatalytic
methodologies is a requisite for comparison with metal-based
systems. This may help to identify, where possible,
improvements that lead to a more widespread use of
organocatalysts in CO 2 conversion. Other excellent reviews
focusing on the use of hetero- and/or homogenous catalyst
systems have recently appeared,4,7-10,18,19,39-42 and may serve as a
reference point.
In the first section, attention is given to the various activation
modes that are the basis for the organocatalytic transformations.
Hereafter, the main types of organocatalysed CO2 coupling
reactions are discussed followed by a section on the latest
developments concerning new types of reactivity.

2. Activation modes of organocatalysts
Organocatalysis has become an essential tool for synthetic
organic chemists as testified by the variety of organocatalysed
organic transformations that can be performed with high chemo, regio- and enantio-selectivities.43-45 Although the currently
available CO2 conversion reactions are relatively simple from a
chemical reactivity point of view, the continuous development of
new and more efficient metal- and/or organo-catalysis is
essential due to the kinetic inertness of CO2. Although the array
of available organo-catalysed CO2 valorisation reactions is
expanding constantly, currently the field is dominated by
reductive and non-reductive coupling processes (see Figure 1).

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Fig. 1 CO2 transformations mediated by organocatalysts.

Up to now, the reported organocatalysts active in CO2
conversion catalysis can be roughly divided into three distinct
categories: (1) nitrogen-based heterocycles (including organic
bases and N-heterocyclic carbenes, NHCs); (2) organic salts,
molten salts and ionic liquids (ILs); and (3) (poly)phenolic and
poly-alcohol compounds. Most of these catalysts are active in a
small series of CO2 based synthetic processes which are
illustrated in Figure 1. Beside the established heterocyclic
synthesis based on CO2 coupling to epoxides and aziridines
(Figure 1, top), organocatalysis has also proved to be particularly
attractive for CO2 reduction (Figure 1, bottom). However,
depending on the type of reaction (non-reductive vs. reductive
coupling), the mechanistic hypothesis and the role of the catalyst
can be quite different.
Similarly to what has been reported for metal-based catalysts,
insights into the operative mechanism of organocatalysed
CO2/epoxide coupling reactions to afford cyclic carbonates have
shown that the role of the catalytic species can be either (a) direct
CO2 activation; (b) activation of high energy co-reactants (e.g.,
epoxides, aziridines, oxetanes and azetidines); or (c) dual
activation of both CO 2 and the associated high-energy reactant
as summarised in Scheme 1. Evidence of a dual activation
mechanism has been seldom reported for CO2 conversion
catalysts; notable examples include polyoxometalate based
systems developed by Mizuno46 and Leitner,47 respectively.
While a general reactivity pattern may be readily derived for
organocatalysed non-reductive CO2 coupling reactions, a general
mechanism for reductive CO2 couplings has not yet been defined
given the different reducing agents employed.

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Scheme 1 General organocatalytic activation modes for the synthesis of cyclic carbonates from epoxides and CO2.

2.1. Organic N-heterocyclic bases
Organic bases are probably the simplest organocatalysts studied
in the context of CO 2 conversion as a result of their availability
and structural diversity including alkyl/aryl mono- and
polyamines, N-heterocyclic derivatives and amidine and
guanidinelike structures. So far, these compounds have proven
to be useful in the epoxide/CO 2 couplings and reduction of CO2
to methanol. Concerning the scope of this review, the most
important class of organic bases reported to date are based on Nheterocyclic scaffolds, particularly the structures depicted in
Figure 2.
The most commonly employed and effective organic
catalysts for CO 2 valorisation are 4-dimethylamino-pyridine
(DMAP) and amidine- and guanidine-derived superbases. Given
the differences in Lewis basicity strength, the catalytic role of
these two classes of compounds is quite different. DMAP has
proven to be an efficient homogeneous 48 and heterogeneous49
catalyst for the synthesis of cyclic carbonates from epoxides,
even though high operating temperatures (120 ºC) and relatively
high CO2 pressures (1740 bar) were required; however, in
presence of Lewis acid co-catalysts such as phenols, this dual
catalytic system showed a good catalytic activity for the
preparation of cyclic carbonates from epoxides. 37,50 A definitive
mechanism for the activation by DMAP has not been elucidated

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yet. One mechanistic hypothesis relies on the sole activation of
the epoxide as depicted in Scheme 1. An alternative mechanism
based on a dual activation mode that involves both the epoxide
and the CO2 molecule has also been postulated. 50

Fig. 2 N-heterocyclic bases commonly employed as organocatalysts. Note that the
reactivity generally is higher when the pKa value is lower.

On the other hand, organic superbases such as DBU, TBD
and N-methyl TBD (MTBD) (Figure 2) have been extensively
employed as catalysts for CO 2 coupling reactions. As a typical
example, Sartori et al. reported the use of MTBD as the sole
catalyst for the synthesis of cyclic carbonates from CO 2 and
epoxides.51 When using both homogeneous and heterogeneous
silica-supported MTBD catalysts, different terminal and internal

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GREEN CHEMISTRY
epoxides were converted to the corresponding cyclic carbonate
with good to excellent yield (60-98%) and high chemoselectivities (90-98%) although the reaction conditions were
quite harsh (T = 140 ºC, p(CO2) = 50 bar). Catalyst
heterogenisation has been exploited as a straight-forward,
feasible way to address recyclability issues and to work under
milder conditions (T = 50 ºC),52,53 while the addition of a cocatalyst has been generally shown to improve reaction rates,
yields and selectivities. Recently, renewable based molecules
such as cellulose, were successfully employed as co-catalytic
mediators for the synthesis of cyclic carbonates. 54
The catalytic role of guanidine-derived organic superbases is
not limited to non-reductive CO2 coupling reactions: these
compounds are currently conquering a prominent role as CO 2
catalysts for the synthesis of methanol by reductive coupling of
CO2 with organosilanes55 and organoboranes.56 Regardless of the
hydride donor employed, formation of a superbase-CO2 salt is
crucial to start the catalytic cycle forming a catalytically active
CO2 complex (Scheme 2).

Scheme 2 CO2 reduction catalysed by TBD.

Whether involved in catalytic CO2 coupling or CO2 reduction
processes, the catalytic cycle involving superbases generally
starts with an initial “coordination” of the CO 2 molecule. Stable
organic superbase-CO2 adducts have been isolated and fully
characterized,57,58 thus providing experimental proof that this
class of compounds is capable of direct interaction with CO2.

2.2. N-heterocyclic carbenes (NHCs)
Since their first isolation by Arduengo in 1991, NHCs have
proved to be versatile ligands for homogeneous metal catalysis
and organocatalysis.59 Being highly nucleophilic in nature,
NHCs need to bind suitable electrophiles to form air- and
moisture-stable adducts: a classic example is the formation of a
zwitterionic NHCCO2 adducts (Scheme 3). 60 These adducts are
labile in solution, making them suitable for catalytic turnover.

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Scheme 3 NHC-CO2 adduct formation.

NHC-promoted CO2 activation allows for CO2 coupling
reactions with high-energy reaction partners, including the
preparation of cyclic carbonates and oxazolidinones from
epoxides and aziridines, 61,62 respectively, and the synthesis of methylene-carbonates and -methylene-oxazolidinones from
propargylic alcohols.62,63
To further improve catalyst lifetime and recyclability, NHCbased catalysts have been heterogenised. When NHC scaffolds
(Scheme 3; R1 = i-Pr, R2 = H) were grafted onto a silica MCM41 support, the resultant catalyst was active towards cyclic
carbonate formation at 0.5 mol % catalyst loading under
relatively mild conditions (T = 120 ºC, p(CO2) = 20 bar, t =
2448 h), whereas this supported catalyst was also used for the
synthesis of oxazolidinones under even milder conditions (T =
80-100 ºC) affording N-substituted oxazolidinones in excellent
yields.64 A different NHC (R1 = 2,6(i-Pr)2C6H5; R2 = H) was
covalently embedded within a tubular, micro-porous organic
network. When using this heterogeneous catalytic system,
propylene oxide (PO) was quantitatively converted to propylene
carbonate in 10 h with a catalyst load of only 0.065 mol % (Yield
= 92%; TOF = 142 s-1, T = 160 ºC, p(CO2) = 30 bar).65
Since their catalytic activity is based on direct CO2
activation, NHCs were also the first class of organocatalysts to
show catalytic activity towards CO 2 reduction with different
reduction partners, including mild reducing agents such as
silanes. Cascade reactions have been reported in the presence of
both a silane reductant (including polymethylhydrosiloxane
(PHMS, an industrial by-product) and appropriate nucleophiles
(amines, anilines, imines, hydrazines, hydrazones, Nheterocycles), thus employing CO2 as a versatile C1 synthon for
the preparation of, inter alia, formamidine derivatives (Scheme
4).23,66

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Scheme 4 CO2 reduction catalysed by a NHC catalyst.

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In most reported cases, the heterocycle/CO2 coupling product
can be recovered quantitatively from the reaction mixture by
simple distillation. To address catalyst stability/recyclability
issues, efforts have been made to develop heterogeneous ILbased
catalysts
including
silica-supported,72,80,81
co73,82-84 PEG-ylated85 and dendrimeric ILs. 86,87
polymerized,
Moreover,
task-specific
hydroxy-88,89
and
carboxy90,91 have been designed, synthesized and
functionalized ILs
tested as catalysts. These ILs have shown improved activity,
mainly due to more efficient epoxide activation through Hbonding interactions (Scheme 5), and subsequently have been
developed in their heterogeneous form. 81,92-94

2.3. Catalysis by organic salts and ionic liquids
As highlighted in detail in a recent review by Kerton et al.,32 the
use of organic salts/ionic liquids as organocatalysts for CO2
conversion has come a long way since the seminal work of Caló
et al. published in 2002.67 These catalysts have almost
exclusively been employed for the coupling of CO2 with strained
heterocycles such as epoxides and aziridines. Their nucleophilic
character mediates the ring-opening of the heterocycles while
stabilizing the ring-opened alkoxide intermediate by ion-pairing
interactions with the cationic part of the salt/IL.68 Beside
commercially available quaternary ammonium68-70 and
phosphonium halide salts, 69,71-73 a number of phosphonium, 74
imidazolium,75-77
pyridinium,76
pyrrolidinium78
and
10,79 derived ILs (including those based on DABCO,
superbases
DBU and TBD; Figure 4) have also been prepared and employed
as CO2 coupling catalysts obtaining with, in some cases,
excellent results in terms of conversion and selectivity. It is
generally assumed that catalysis occurs through an
epoxide/aziridine activation pathway (Scheme 1, right),
however, depending on the chosen anion/cation combination
some peculiar behaviour has been observed.

Scheme 5 Proposed mechanism for the activation of epoxides by hydroxyfunctionalised ILs.

Direct CO2 activation has also been achieved with Frustrated
Lewis Pairs (FLPs), i.e. molecular systems incorporating Lewis
acid and Lewis base functions which cannot interact directly due
to steric constraints and/or backbone rigidity.95 Their mechanism
of action, as illustrated in Scheme 6 for aryl-bridged phosphineborane FLPs, involves CO2 binding and formation of a
zwitterionic carboxylated FLP adduct.96 Depending on the
reaction partner, FLPs can be catalytically active towards CO 2
coupling to epoxides97 or, in presence of an appropriate reducing
agent, i.e. trialkylsilanes, towards its reduction.98 Interestingly,
up to date organo-catalysed reduction of CO 2 to methane has
only been achieved in presence of an FLP catalyst99
demonstrating their exceptional catalytic potential in CO 2
conversion catalysis.

Scheme 6 CO2 activation by FLPs: reversible activation of CO2 by an aryl-bridged
phosphine-borane ambiphilic molecule.

2.4. Phenolic and polyhydroxy compounds
Fig. 4 Quaternary ammonium/iminium salts and ILs used as catalysts for CO 2
coupling reactions.

6 | Green Chem., 2014, 00, 1-3

This broad class of organocatalysts is quite attractive since most
of them are naturally occurring or readily available from
renewable resources. The term “polyhydroxy” compounds refers
to several structurally different compounds (Figure 5) including
pyrogallol,100
linear
non-conjugated
polyols101
and

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polysaccharides.102-106 Due to their potential to form an
(extended) hydrogen-bond network upon activation of the
substrate, these catalysts have been employed almost exclusively
for epoxide/CO2 coupling reactions. This activation mechanism
(vide infra) can be applied to all hydroxyl and polyhydroxy
compounds, including the polysaccharide-based catalytic
systems (cellulose, CMC and chitosan). In general, their catalytic
behaviour is based on epoxide activation through the formation
an H-bonding intermediate or network as for hydroxylfunctionalised ILs (Scheme 5).

REVIEW
way of producing these heterocyclic scaffolds involves coupling
reactions of CO2 with readily available epoxides or aziridines in
the presence of a suitable catalyst. One should keep in mind that
non-exclusive selectivity in the ring-opening step of a monosubstituted aziridine may result in two regio-isomeric structures.
However, virtually one isomeric form (generally the 5substituted oxazolidinone) may be obtained when suitable
reaction parameters and catalysts are used.

Fig. 5 Phenolic, polyhydroxy- and polysaccharide-based organocatalysts for CO 2
coupling reactions.

The extent of the H-bonding network depends on the type of
polyhydroxy catalyst chosen. Generally, halide salts or organic
bases are required as co-catalytic additives to ensure efficient
catalysis. In the case of polysaccharides, however, the cocatalyst moiety can be embedded within the polysaccharide
backbone by direct quaternization of the biopolymer, thus
resulting in a bifunctional catalyst system. Polysaccharides
(cellulose, carboxymethylcellulose (CMC) and chitosan) are
particularly interesting for practical applications: they are easily
obtained from renewable resources and they can be further
functionalized by physical and chemical methods to improve the
overall catalytic activity. These functionalization methods are
based on the impregnation of ILs, electrostatic interactions with
ILs,102 organic superbases54 or covalent incorporation of
nucleophilic
moieties
within
the
macromolecular
catalyst.103,104,107 The possibility of multiple interactions between
the catalyst and the substrates is exemplified in Scheme 7 for a
heterogeneous chitosan catalyst functionalized with imidazolium
moieties.103

3. Heterocyclic structures based on CO2
Heterocyclic structures that incorporate a “carbonate” or
“carbamate” fragment as highlighted in Scheme 8, can serve as
valuable monomers of polyurethanes and polycarbonates, drugs,
electrolytes or reagents in a wide range of chemical
transformations,108-112 and therefore have attracted a great deal of
interest in industrial and academic research. The most elegant

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Scheme 7 Proposed mechanism for CS-EMIm-Br catalysed CO2 coupling reactions.
CS stands for cellulose and EMIm-Br for an imidazolium bromide unit.

Scheme 8 Preparation of heterocyclic structures from the reactions of CO 2 and
epoxides or aziridines.

Undoubtedly, ILs are the most widely studied
organocatalysts for the formation of heterocyclic structures from
CO2 which has been reviewed in detail by Kerton and co-workers
recently.32 In this review, most of the ionic organocatalysts
reported in the last eight years were listed and discussed, ranging
from simple “onium” species to supported IL catalysts. In order
to avoid unnecessary duplication, only some selected examples
of IL catalysts are discussed here in the context of heterocyclic
synthesis using CO2.

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Scheme 9 PEG-bridged TBD as catalyst for CO2-epoxides coupling reaction. PEG =
polyethyleneglycol.

For instance, He’s group reported85 on the use of polyethylene-glycol (PEG) bridged basic ionic liquids: the most
important structural feature is an ethylene glycol that ties two
quaternary 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) units
(Figure 2, Scheme 9). These TBD units are known to interact
with CO2 to form an activated carbamate intermediate that may
activate an epoxide substrate through H-bonding. Nucleophilic
ring-opening of the activated epoxide may then easily occur
using a bromide nucleophile providing a complementary
activation pathway. The attack of the carbamate unit on the ringopened epoxide (i.e., an alkoxide) produces a hemi-carbonate
that after ring-closure leads to a cyclic carbonate. This work is
exemplary for the majority of the organocatalytic promoters
reported to date: compared to metal based catalysts, the substrate
activation potential is lower, generally requiring higher reaction
temperatures for efficient catalytic turnover. However, despite
this kinetic drawback, most organocatalysts are readily available
and accessible from cheap and relatively non-toxic scaffolds.
A large improvement on the kinetic limitation in the
organocatalyzed conversion of epoxides and CO 2 was provided
by a pyrogallol (1,2,3-trihydroxybenzene) based binary catalyst
comprising Bu4NI.100 Pyrogallol is a commercially available
polyphenol and its excellent catalytic performance is a result of
the adjacent nature of the phenolic groups. As with many other
systems, the mode of activation of the epoxide substrate is
through hydrogen-bonding. The three phenol units are all being
involved in the stabilization of key intermediates (Scheme 10).
Interestingly, the synthesis of the cyclic carbonate products
could be done under highly mild conditions (T = 2545 oC,
p(CO2) = 10 bar), providing a low net CO2 emission reaction
setup. When compared to a previously reported metal based
catalytic system, comprising of a Lewis acidic Zn(salphen)
catalyst and using Bu4NI as co-catalyst113 employed under
similar conditions (i.e., MEK as solvent, T = 45 ºC, p(CO2) = 10
bar, t = 18 h), both the metal based and organocatalytic systems
showed similar activities: when using 2 mmol of 1,2epoxyhexane as substrate in 5 mL of MEK, quantitative
conversion to the corresponding cyclic carbonate (conversion >
99%) was observed with Zn(salphen)/Bu4NI (both 2.5 mol%) as
well as with pyrogallol/Bu4NI (both 5.0 mol%). This simple
comparison is further evidence of the exceptional potential of

8 | Green Chem., 2014, 00, 1-3

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pyrogallol as a CO 2 coupling catalyst, exhibiting activities
comparable to metal-based systems.
At a later stage,114 a bifunctional, polystyrene-supported
version of this pyrogallol catalyst was also successfully applied
under comparable mild conditions with the advantage that the
catalyst could be efficient recycled producing a more stable
system and producing thus higher turnovers. Additionally, this
organocatalysis protocol allowed for a multi-substrate campaign
with minimal cross-contamination upon recycling the supported
catalyst. These pyrogallol based catalysts are probably among
the most active reported to date considering the low operating
temperatures that are required. Other examples of
phenol/catechol-mediated synthesis of cyclic carbonates have
also appeared in recent literature, 37,115 though being generally
less efficient compared to the aforementioned pyrogallol
systems.

Scheme 10 Stabilization of a key intermediate through hydrogen-bonding involved
in the synthesis of cyclic carbonates.

Dufaud et al.116 recently developed a green method using
azaphosphatranes as catalysts. This method allows for the
efficient coupling of epoxides and CO2 at 80oC and atmospheric
pressure of CO2 using a 0.1 mol % of catalyst loading.
Azaphosphatranes can be considered structurally tunable
catalysts, and as such their catalytic activity can be optimised by
changing the substituents attached to the peripheral N-atoms (R,
Scheme 11). Whereas the presence of methyl substituents (R =
Me) results in catalyst degradation during the reaction, bulkier R
substituents such as p-methoxybenzyl and neo-pentyl result in
azaphosphatranes with increased stability, thus developing
longer-lived systems. Detailed optimization of the reaction
conditions showed that the azaphosphatrane bearing pmethoxybenzyl groups exhibits the highest catalytic activity.
Insights into the reaction mechanism were provided on the
basis of a series of detailed kinetic studies with the reaction
between styrene oxide (SO) and CO 2 serving as a benchmark
model. From the kinetic data it was deduced that SO first forms
an adduct through a hydrogen-bond to the cationic P center. This
first step is then followed by insertion of the CO 2 into the PN
bond, resulting in an unusual tricyclic phosphoryl- carbamate
intermediate which is highly reactive and sensitive toward
hydrolysis (Scheme 11). This intermediate can be stabilized
more efficiently by bulkier R groups and the reaction then

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GREEN CHEMISTRY
proceeds to the product via a nucleophilic attack of the chloride
anion, and subsequent attack of the resultant alkoxide species on
the activated CO 2.
Given the low kinetic stability of the tricyclic
phosphorylcarbamate intermediate in the absence of any steric
protection, the same authors have come up with an alternative,
successful strategy to stabilize this intermediate and to avoid
degradation of the catalyst through encapsulation by means of
self-assembly
giving
a
nano-cage.38
Thus,
several
hemicryptophane-caged azaphosphatranes were designed and
tested in the coupling reaction of CO2 and epoxide under
reasonably mild conditions (T = 100 oC; p(CO2) = 1 bar; 0.1
mol % of catalyst loading) taking the corresponding non-caged
catalysts
as
references.
Indeed,
the
encapsulated
azaphosphatrane systems showed much better catalytic activity
and higher stability when compared to the corresponding noncaged catalysts. This study on supramolecular catalysis also
provided new evidence concerning the previously proposed
mechanism involving the simultaneous dual activation of the
epoxide and CO2.116 The hemicryptophane was carefully chosen
as a building block to construct the cages. This aspect deserves
to be highlighted as it allowed the use of only 0.1 mol % of
catalyst, although most of the known supramolecular catalysts
suffer from product inhibition and have to be used in
stoichiometric amounts. This strong structure-activity
correlation sheds light on the design of new advanced,
supramolecular catalysts for the synthesis of heterocyclic
carbonates from CO2.

Scheme 11 Azaphosphatranes acting as catalysts in the coupling reaction between
CO2 and epoxides.

Alternative organocatalytic methodologies make use of
NHCs and their derivatives as catalysts in the reaction of CO 2
and epoxides. Ikariya et al.62 reported on imidazolium-2carboxylates (NHC-CO2 adducts) that can be employed as NHC
surrogates to catalyze the cycloaddition of CO 2 and epoxides.
The proposed mechanism involves the nucleophilic addition of
the NHCCO2 adduct to the epoxide, followed by an
intramolecular cyclization of the alkoxide intermediate (Scheme
12). The nucleophilic attack of the CO 2 moiety bound to the
NHC on the substrates is thought to be the rate-limiting step: this
hypothesis seems to be supported by a significant positive effect
of electron-donating alkyl substituents on the NHC heteroatoms.
Previously, Lu and coworkers60 reported on the use of 1,3-

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diarylimidazolium-2-carboxylates as catalysts in similar
transformations. At a later stage, the same group has shown that
a NHC-CO2 adduct could also be immobilized on a MCM-41
support and the catalytic activity of this hybrid material in the
coupling between CO2 and epoxides was investigated. 64
Although applicable, these protocols based on NHC catalysts
and their corresponding derivatives require high CO 2 pressures
(> 20 bar) and temperatures (> 100 oC). The hybrid silica
material is indeed recyclable; however, its multi-step synthesis
and isolation may result in extra energy and cost consumption,
compromising to some extend the overall process sustainability.

Scheme 12 Nucleophilic addition of NHC-CO2 adducts to epoxides.

It is worth mentioning that cyclic -alkylidene carbonates,
can also be prepared from the reaction of propargylic alcohols
with CO2 (p(CO2) = 45 bar) under NHC or NHCCO2 catalysis
at 60 oC.56 Promisingly, Lu’s group117 found that N-Heterocyclic
Olefin-CO2 adducts (NHOCO2) work more efficiently as
catalysts in this transformation (Scheme 13) under basically
ambient reaction conditions. X-ray analysis of the NHO-CO2
adduct revealed that the length of the C(carboxylate)-C(NHO) bond is
significantly longer than that of the corresponding C(carboxylate)C(NHC) bond. Therefore, the higher activity of the NHOCO2
adduct was tentatively ascribed to its lower stability compared to
NHC-CO2 adducts, thus facilitating the release of the CO2 and/or
the coupling product which may possible be rate-limiting in the
catalytic cycle.

Scheme 13 Preparation of cyclic -alkylidene carbonates from propargylic
alcohols and CO 2 under NHO-CO2 catalysis.

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REVIEW
A creative approach towards cyclic carbonate synthesis using
organic molecules as mediators was described by Jamison and
his group.118 Combination of N-bromosuccinimide (NBS) with a
radical initiator (benzoyl peroxide, BPO) in the presence of
dimethylformamide (DMF) as solvent in a continuous flow set
up allows for the efficient coupling reaction between epoxides
and CO2. Under optimized conditions, high conversion levels
and yields can be attained for various epoxide substrates. A series
of (kinetic) experiments suggested epoxide activation by
electrophilic attack from the bromine formed in situ. A slight
drawback from this approach may be presented by the use of
DMF as solvent/reactant requiring higher reaction temperatures
(120 ºC) as opposed to reactions carried out under milder, neat
conditions. However, this method could pave the way towards
continuous flow methodology for larger scale syntheses.
In contrast to epoxide/CO2 couplings, aziridine/CO2
couplings have been much less investigated, probably as a result
of the easier access to epoxide reagents. As further detailed in
the review by Kerton and coworkers,32 a number of ILs and their
polymer-supported derivatives has been used as catalysts in these
aziridine/CO2 coupling reactions. However, most of the reported
catalytic systems either require high reaction temperatures (>
120 oC) and CO2 pressures (> 50 bar) or deliver the heterocyclic
products in low yields/conversions and with poor regioselectivities.10,119-121
Gao’s group recently reported 122,123 an alternative strategy
for the selective preparation of 3-substituted cyclic carbamates
from the reaction between a cyclic carbonate (made using CO2)
and aromatic amines using 1-butyl-3-methylimidazolium acetate
(BmimOAc) as catalyst (Scheme 14). DFT calculations revealed
that both the cationic and the anionic moieties of the catalyst
activate in a cooperative fashion the substrates (i.e., cyclic
carbonates and aromatic amines) by means of hydrogenbonding. Despite the fact that this procedure is CO2-free and
requires relatively high reaction temperatures (140 oC), the use
of metals and high-pressure reaction conditions can be avoided,
thus developing a novel green method for the preparation of
cyclic carbamates. Moreover, cyclic carbonates, which can be
easily produced via direct CO2 coupling reactions with epoxides
and/or oxetanes, are conveniently used as substrates for this type
of reaction. Organocatalysts other than ILs including phenolic
compounds50 and NHCs64 also have found applicability in this
transformation but similar selectivity issues, as aforementioned,
have been encountered.
The synthesis of five-membered cyclic carbonates and
carbamates has been a highly active research area over the last
five years. However, the formation of six-membered cyclic
heterocycles based on direct coupling between suitable
precursors (oxetanes or azetidines) and CO2 is generally more
difficult. It is further complicated by the lower accessibility and
reactivity
of
oxetanes/azetidines
compared
with
oxiranes/aziridines.
Thus,
this
subfield
of
organic
carbonate/carbamate formation is still in its infancy, though very
recently several research teams have been able to design
transition metal based catalytic protocols that allow for
(substituted) oxetane-into-carbonate conversions.29,124-125

10 | Green Chem., 2014, 00, 1-3

Green Chemistry

Scheme 14 Preparation of cyclic carbamates from the reaction of cyclic carbonates
and aromatic amines with BmimOAc as catalyst.

4. Organocatalysed CO2 reduction
Organocatalysis has also proven to be a powerful tool for CO2
reduction. Various CO2 reduction products have already been
obtained including carbon monoxide (CO), formic acid,
formamidine derivatives, methoxysilanes and methoxyboranes,
of which the latter two can be easily converted to methanol and
methane. Both MeOH and methane are energy vectors and the
formation of these higher free energy molecules starting from
CO2 represents a possible alternative to fossil fuel based energy
supply.
N-heterocyclic carbenes (NHCs) have been extensively
applied as catalysts for the nucleophilic activation and
subsequent reduction of CO2. For example, NHC catalysts have
been employed for the CO2 reduction to CO in presence of
inexpensive aldehyde reaction partners as (sacrificial)
reductants. Although this reaction does not fall into the
“reductive CO2 coupling” category, it does show the potential of
organocatalytic systems to access other type of transformations
with formal reduction of the oxidation state of the CO2 reactant.
Initially, Zhang and Gu achieved the NHC-mediated catalytic
reduction of CO2 with aromatic aldehydes as oxygen
acceptors.126 The authors proposed that the NHC catalyst reacts
with CO2, resulting in an imidazolium carboxylate intermediate
(Scheme 15, below left; cf. Scheme 12); this species attacks the
2-position of cinnamaldehyde realising through a [1,5] Hshift,
the corresponding oxidation product as a potassium salt. The
catalyst is regenerated by a quick decay of the NHCCO adduct
intermediate, with concomitant release of CO.

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GREEN CHEMISTRY

REVIEW
independently by different research groups led to distinct
conclusions: Wang and co-workers postulated that the catalytic
cycle starts with an initial attack of the NHC on the silane
generating a pentavalent silicon species able to deliver a highly
nucleophilic hydride to CO2,134 while a computational study by
Zhang et al. supports an initial formation of an IMesCO2 adduct
followed by a rate-determining [1,5] H-shift to give the
corresponding formylsilane by hydrosilylation,135 in agreement
with the mechanism proposed initially.40

Scheme 15 Postulated intermediates for the oxidation of inactivated aldehydes in
the presence of CO 2.

In 2010 Nair et al. reported a similar NHCmediated
oxidation of aromatic aldehydes with CO 2 leading to carboxylic
acids.127 The expected aryl glyoxylic acid product, resulting from
trapping of the Breslow intermediate (Scheme 15, centre) with
CO2, was not observed. Different from what was previously
reported, the postulated reaction mechanism relies on an initial
addition of the catalyst to the aldehyde to form a nucleophilic
Breslow intermediate which then reacts with CO 2. However, in
2011, both suggested mechanisms and experimental results were
questioned by Bode and co-workers.128 After detecting nonneglectable traces of air (and therefore oxygen) in both reported
procedures, they suggested that exogenous molecular oxygen
was likely the actual stoichiometric oxidant in the abovediscussed reactions. In presence of oxygen, the NHC catalyst
triggers the formation of NHC-acyl intermediates (Scheme 15,
right), which are capable of transferring their acyl group to a
nucleophile (traces of water) to produce the corresponding
carboxylic acids.129 In the mechanistic hypothesis of Bode et al.
CO2 only accounts for the reduction of aldehyde dimerization or
oligomerisation side products. This particular chemistry
therefore represents an interesting subject of debate likely to
induce further detailed investigations into the role of the CO 2
reagent as oxidant.
Although organocatalyzed CO2 reduction to MeOH is still
not as powerful as transition metal based methodologies,130-133
organocatalysis offers tangible and diverse approaches to
reduced forms of carbon. For example, NHC catalysts have been
employed for the reduction of CO 2 to methanol. Ying et al.
reported on the reduction of imidazolium carboxylates by silanes
yielding methanol.36 The reaction is particularly interesting
because the mesitylimidazolylidene carboxylate NHC catalyst
(IMesCO2) employed is air stable, can be used in low
concentration (0.05 mol %) while maintaining high TONs and
TOFs (1840/25.5 h-1, respectively). All these features help to
overcome the typical drawbacks (catalyst deactivation,
requirement of an air- and oxygen-free atmosphere) of metalcatalysed CO2 reduction protocols. The proposed reaction
mechanism, based on experimental and spectroscopic evidence,
is based on the initial formation of a NHC-CO2 adduct.
Computational
and
experimental
studies
performed

This journal is © The Royal Society of Chemistry 2014

Scheme 16 Proposed NHC-catalysed CO2 hydrosilylation intermediates.

Another class of organocatalysts active for the reduction of
CO2 to methoxysilanes and methoxyboranes (and thus, to
methanol) are guanidine-based organic superbases (TBD and
MTBD; see Figure 2). In an effort to broaden the range of
compounds available from CO2 reduction, Cantat et al.
combined both reduction of CO2 and formation of CC, CN,
and CO bonds reporting on the organocatalysed synthesis of
formamidine derivatives starting from CO 2, trialkylsilane as
reducing agents and N-based nucleophiles: with this approach
they were able to synthesize different formamidine derivatives
of amines, anilines, imines, hydrazines, hydrazones and Nheterocycles (Scheme 2 for an example).55,66 This process is
particularly appealing from a practical point of view because of
its sustainable nature; in particular, it has been optimized to reuse
two common industrial wastes: CO2 being a by-product from all
combustion processes involving organic matter and
polymethylhydrosilane (PMHS).23
It has been recently demonstrated that rather mild (ambient)
conditions can be used for organocatalytic CO 2 conversions.
TBD or TMBD catalysts (0.025 mol%) have shown to be active
towards
CO2
reduction
using
9BBN
(9borabicyclo[3.3.1]nonane) as reducing agent giving the
corresponding dialkylmethoxyborane in >90% yield with TONs
and TOFs up to 548 and 33 h-1, respectively.56 Although for both
guanidine based catalysts the reduction of CO 2 with R2BH and
thus the first CH bond formation is rate determining, with
experimental and computational investigations revealing that
TBD and MTBD follow different mechanisms. MTBD was
proposed to promote the reduction of CO 2 through direct
activation of the hydroborane reagent and formation of ion pairs,
while TBD is converted into an FLP that first activates CO2 and
facilitates the hydride transfer from the boron to the carbon
center (see Scheme 17).

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REVIEW

Green Chemistry
hydrogen source) reaching similar rates to the ones reported for
transition-metal based catalysts.140,141

Scheme 17 Rate-determining step for the catalytic hydroboration of CO 2 with 9BBN using (M)TBD as a catalyst precursor: CO 2 activation based mechanism (TBD,
left) and hydroborane activation mechanism (MTBD, right).
Scheme 19 CO2 reduction with cathecolborane catalysed by an FLP
organocatalyst.

In both cases formate type (HCOOBR2) and acetal-like
H2C(OBR2)2 derivatives were identified as reaction
intermediates.
In addition, stoichiometric and catalytic
reductions of CO 2 have been reported which utilise frustrated
Lewis pairs (FLPs) for CO 2 activation and dihydrogen, silanes,
or boranes as the hydrogen source. 98 The first evidence of FLPs
catalysing CO2 reduction was reported by O’Hare et al. in 2009:
when heated at 110 ºC at 12 bar of H2 pressure, B(C6F5)3 and
TMP (TMP = 2,2,6,6-tetramethylpiperidine) were shown to bind
CO2 stoichiometrically and generate a formato-borate product.
Extended heating at 160 ºC for several days yielded, after
isolation by distillation, methanol as the sole C 1 product (Scheme
18, left).61 Piers et al. reinvestigated the system using
hydrosilanes as reductants in the presence of additional
equivalents of the Lewis acid, and under those conditions the
system effectively catalysed the reduction of CO 2 to methane
(Scheme 18, right).99 The reaction could be monitored
spectroscopically (NMR), allowing the authors to propose a
stepwise hydrosilylation process that was later supported
computationally.136 Unfortunately, the calculated turnover
numbers are still far from those reached with transition metalbased systems based on, for instance, iridium.132,137-139

Scheme 18 FLP catalysed stoichiometric reduction of CO 2: to methanol with H2 as
a reductant (left), and to methane with triethylsilane as a reductant (right).

Milder Lewis acid/base FLP partners have also been
proposed as catalyst systems. The preparation of 1cathecolboryl-2-diphenylphosphinobenzene was reported and
these kinds of FLP compounds catalyse the hydroboration of
CO2 in the presence of catecholborane to afford cat-BOB-cat and
MeOB-cat (as depicted in Scheme 19), and the latter could be
hydrolysed to methanol.96 The reaction was performed under
mild conditions (p(CO2) = 1 bar, Tmax = 70 ºC) observing a
maximum TOF of 973 h-1 (TONs up to 2950 using BH3·SMe2 as

12 | Green Chem., 2014, 00, 1-3

This transformation was proposed to occur through
simultaneous nucleophilic activation of the borane and
electrophilic “fixation” of CO2. It was postulated that the
reduction process occurs through a stepwise process: initial CO 2
coordination forming a formatoborate intermediate which, upon
rearrangement, loses the formaldehyde fragment necessary to
generate the final methoxide derivative. More recently, the same
authors reported a novel in situ generated FPL catalyst active
towards reductive hydroboration of CO2 based on a relatively
non-nucleophilic
proton
sponge
[=
1,8bis(dimethylamino)naphthalene].142 In the presence of
stoichiometric quantities of a BH3·SMe2 reductant, at 80 ºC and
1 bar of CO2 this system reached a TOF value of 64 h-1. Here the
postulated mechanism relies on the activation of BH 3·SMe2 and
converting it into a boronium–borohydride ion pair which, in the
presence of CO 2, can evolve into a formate derivative. Final
BH2+ abstraction from another FLP borane-proton sponge
complex leads to elimination of the methoxyborane product.
The organocatalyzed N-methylation of amines using CO2 and
suitable reducing agents (hydroboranes) has recently emerged as
another attractive catalysis route towards CO 2 valorisation.
Contributions from Cantat et al.143 and Dyson and co-workers144
have set the stage for new conversions mediated by
proazaphosphatrane superbases and NHC based organocatalysts.
The results highlight the versatile catalytic use of NHCs towards
the conversion of CO 2 into a variety of organic molecules such
as alkylated amines, MeOH, formamidines and organic
carbonates. As such they represent privileged catalyst systems
with high potential in other types of CO 2 transformations.

5. Other reactions
The use of organocatalysis for other types of CO 2 conversion
reactions is briefly discussed. Beside the formation of organic
carbonates and products derived from reductive couplings, the
formation of quinazoline2,4(1H,3H)diones (see Figure 1,
Scheme 20) has also drawn considerable attention. Similar to
organic carbonate formation, the formal oxidation state of the
C(CO2) carbon centre does not change, however, mechanistic
proposals145 seem to indicate that water may play a role in the

This journal is © The Royal Society of Chemistry 2014

GREEN CHEMISTRY
formation mechanism involving H2CO3 or bicarbonate. Though
suggested by DFT analysis of the mechanism, it remains an open
question whether the CO 2 is fully incorporated into the substrate.
Whereas metal based systems are known to catalyse the
efficient formation of these heterocyclic structures from CO 2
generally using 2-amino-benzonitriles as reaction partners,11,146
organocatalytic methods have been investigated successfully
providing green alternatives. Quinazoline-2,4(1H,3H)-diones
are considered as biologically and pharmacological relevant and
thus represent interesting synthetic targets. To date various
organocatalysts for the reaction between 2-amino-benzonitriles
and CO2 have been reported including IL based systems,147-150
amine-based heterocycles151,152 and carbonate bases/base
impregnated clay minerals. 153,154 The use of water as a medium
has been proposed for the non-catalytic formation of
quinazoline2,4(1H,3H)diones.155 In this latter case, the crucial
role of the formation of H 2CO3 acting as an intermediate was
proposed, though higher reaction temperatures/pressures were
required and relative longer reaction times were needed for high
substrate conversion.

Scheme 20 Some of the reported organocatalysts used in the formation of
quinazoline2,4(1H,3H)diones starting from 2-amino-benzonitriles and CO 2.

6. Summary and outlook
In the last five to ten years, the field of CO 2 chemistry and
conversion catalysis has expanded enormously. Whereas
homogeneous and heterogeneous metal catalysis still fulfil a
prominent position within this vivid area of research,
organocatalysis is emerging as a potential and useful alternative
tool in specific CO 2 conversion reactions.
The current review shows that some organocatalyst
structures such as NHCs and ILs can be regarded as “privileged”
systems, as these are widely applicable systems for different
organic transformations that are not limited to CO2 couplings.
However, the chemistry of other organocatalyst structures
(including azaphosphatrane derivatives) seems highly
promising, and the quest for a more diverse set of organocatalysts
able to compete with metal catalysis is an ongoing challenge. In
order to meet the challenges of sustainable though highly
effective catalysis, new and conceptually different approaches

This journal is © The Royal Society of Chemistry 2014

REVIEW
may be warranted making clever use of cooperative,
confinement and/or siteisolation effects. Without a doubt, there
is a bright future for organocatalysis as a green alternative in CO 2
conversion catalysis, and this review will hopefully spur the
catalysis community to develop new methodology of common
interest to academic and industrial experts.

Acknowledgements
GF kindly acknowledges financial support from the European
Community
through
FP7-PEOPLE-2013-IEF
project
RENOVACARB (grant agreement N°622587). WG thanks the
CELLEX foundation for financial support. ICIQ, ICREA and the
Spanish Ministerio de Economía y Competitividad (MINECO;
through the Severo Ochoa Excellence Accreditation 2014-2018
(SEV-2013-0319) and project CTQ2011-27385 are also
acknowledged for support.

Giulia Fiorani studied chemistry at the
Università degli Studi di Roma “Tor
Vergata”, where she obtained her MSc
(Hons.) in 2006. In 2010 she completed her
PhD in Chemical Sciences in the same
University working on the synthesis and application of ionic liquids. Several different postdoctoral research assignments followed, working on nano-structured
hydrid materials (Università degli Studi di Padova), functional
hybrid membranes (ITM-CNR, Padova) and synthesis and
synthetic applications of organic carbonates (Ca’ Foscari
University of Venice). Since March 2014, Giulia is working at
ICIQ in the group of Prof. Arjan Kleij as a Marie Curie IEF
Postdoctoral Fellow. Her research interests focus on the
preparation of new molecules and materials based on CO 2 and
renewable based feed stocks.

Wusheng Guo earned his MSc from South
China University of Technology in 2010,
and a PhD degree in organic chemistry in
June 2014 from the Autonomous University
of Barcelona under supervision of Prof.
Roser Pleixats and Dr. Alexandr Shafir. His
research focused on the use of hypervalent organoiodanes in
oxidative processes and the use of metal nanoparticles based on
Pd, Au and Rh in catalysis. Then, he joined the group of Prof.
Arjan Kleij at the Institute of Chemical Research of Catalonia
(ICIQ) in Tarragona as a CELLEX-supported postdoctoral
associate working on the conversion of CO2 for the formation of
various value-added chemicals with carefully designed catalysts.

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REVIEW

Arjan W. Kleij received his MSc (1996) and
PhD degree (2000) in Chemistry from
Utrecht University (The Netherlands)
under the supervision of Prof. Gerard van
Koten. Then he held several appointments
as a postdoctoral fellow in Madrid (Prof.
Javier
de Mendoza) and Amsterdam (Prof. Joost Reek and Prof. Piet
W.N.M. van Leeuwen), and as a scientist/project leader in
industry at Avantium Technologies and Hexion Specialty
Chemicals. In 2006 he was appointed Group Leader at ICIQ and
ICREA fellow, and was promoted to ICREA professor in 2011.
His main research interests are in the field of CO2 conversion
catalysis with a focus on heterocyclic synthesis, (bio)polymers
and the development of new catalysts and new reactivity
patterns.

Notes and references
a

Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans
16, 43007 – Tarragona, Spain. Fax; (+34)-977920823; Tel.: (+34)977920247; E-mail: akleij@iciq.es
b
Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís
Companys 23, 08010 – Barcelona, Spain.
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