"This is the peer reviewed version of the following article: Curr. Opin. Green Sust. Chem. 2017, 3, 55-60, which
has been published in final form at DOI: 10.1016/j.cogsc.2016.11.005. This article may be used for non-commercial
purposes in accordance with the Terms and Conditions for Self-Archiving published by Elsevier at
https://www.sciencedirect.com/science/article/pii/S2452223616300669?via%3Dihub."

1

Recent Progress in Stereoselective Synthesis of Cyclic Organic
Carbonates and Beyond
José Enrique Gómez,a Arjan W. Kleij,a,b,*
a

b

Institute of Chemical Research of Catalonia (ICIQ), the Barcelona Institute of Science
and Technology, Av. Països Catalans 16, 43007 – Tarragona, Spain

Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23,
08010 – Barcelona, Spain
E-mail: akleij@iciq.es

Abstract: The recent developments in the stereoselective formation of cyclic organic
carbonates are discussed, together with their use as intermediates in stereoselective
synthesis of other valuable scaffolds.

Keywords: carbon dioxide; cyclic carbonates; homogeneous catalysis; post-synthetic
conversions; stereoselectivity

2

1. Introduction
Atmospheric carbon dioxide is an abundant and alternative source of carbon representing
about 750 billion tons of carbon in the atmosphere. Technologies towards both
sequestration and utilization of this non-toxic, renewable and cheap C1 waste product are
highly desirable in order to reduce the dependence of our society on the limited amount
of fossil fuels we have available. Therefore, valorization of CO2 is currently receiving
considerable attention from the scientific communities to convert it into organic
molecules with commercial value [1-6].
Despite the disadvantage of a high kinetic stability of carbon dioxide, with the rapid
development of organometallic chemistry and catalysis, efficient approaches to realize
CO2 utilization have been discovered in the past decades. Of particular interest is the
development of chemical methodologies towards the incorporation of this carbon-based
feedstock into useful synthetic organic molecules under sustainable mild conditions. The
synthesis of cyclic organic carbonates (COCs; see Figure 1) and related linear and
polycarbonates have been widely studied in this context of CO2 valorization. Cyclic
carbonates have been identified as key compounds in several applications ranging from
their use as (non-protic) solvents to synthetic intermediates in organic synthesis [7-12].

Figure 1. Recent progress in COC synthesis and their post-synthetic use.

3

Although the major portion of the reported catalytic approaches involving CO2 as key
reagent in the last years have focused on simple transformations for the synthesis of these
heterocyclic derivatives, less focus has been directed towards stereocontrolled
transformations (including diastereo- and enantioselective synthesis) for the preparation
of value-added scaffolds [13,14]. In this review article, we highlight the latest
achievements (20132016) in the area involving stereo-controlled COC synthesis, with a
special focus on those contributions that have helped to understand the importance of
COCs in synthetic chemistry by using these compounds as useful starting materials for
more complex organic molecules (Figure 1).
2. Enantioselective Synthesis
Chiral cyclic carbonate scaffolds are important substructures of different biological
compounds, and can also be regarded as useful synthetic intermediates in the
pharmaceutical industry (cf., masked 1,2-diols) and polymer industry [15]. The
preparation of COCs via the coupling reaction of CO2 and epoxides is one of the most
promising routes for CO2 conversion. However, despite the large diversity of catalyst
systems developed for these transformations, the majority of reactions using CO2 as
feedstock concern the preparation of “simple” achiral chemicals, and only a handful of
catalysts are able to mediate the synthesis of chiral COCs [14].
Since the seminal work reported by Yosihara [16] and Nguyen [17] in 1997 and 2001,
respectively, several catalytic systems for the preparation of enantiomerically enriched
COCs have been reported. The simplest way to prepare these enantiopure compounds is
to use enantio-specific approaches by coupling enantiopure epoxides with CO2, aiming
for a high level of retention of the original chiral configuration. A recent example was
provided by the Lu group who developed a bifunctional Al(III)salen complex with two
pendant quaternary ammonium salts. This system showed high catalytic activity for the
construction of COC with full retention of the initial configuration (>99% ee) in the
coupling of different chiral epoxides and CO2 through ring-opening at the non-substituted
carbon center of the oxirane unit [18]. More recently, Capacchione and co-workers
reported the use of a Fe(III) complex in combination with bromide as achiral binary
catalyst that allows for enantio-specific conversion of CO2 and chiral epoxides (i.e., (R)styrene oxide) giving moderates values of selectivity (i.e., 72% ee for COC formation)
[19].
One of the few reports not dealing with metal-catalyzed synthesis of enantiopure COCs
was presented by Jamison in 2013 [20]. In this contribution the authors described a
continuous flow process for the enantio-specific formation of COCs from oxiranes with
excellent retention of configuration. The enantiopure epoxides [(R)-styrene oxide and (S)phenyl glycidyl ether] were combined with CO2 in the presence of in situ produced
bromide radicals, and the corresponding COCs were obtained with full retention of the
stereochemical information.
In other recent contributions, optically active cyclic carbonates were prepared by catalytic
kinetic resolution of racemic epoxides in the presence of CO2 [21], with preferred
4

activation of one of the two enantiomers of the (rac) epoxide. After initial attempts
reported by Dibenedetto and Aresta [22] giving rather low ee (22%), important advances
have been published over the last years. Most of these focused on the use of metallosalen
complexes, and Co-based systems have shown exceptional potential for the kinetic
resolution of (rac) epoxides to afford chiral COCs [13, 21].
Jiang an co-workers have developed bifunctional Co(salen) complexes equipped with
imidazolium halide groups for catalytic asymmetric coupling of epoxides with CO2 [23].
These newly developed catalysts operate under mild reaction conditions (25ºC, 1.2 MPa)
providing the COCs in moderate ee´s of up to 57%, and showing high stability. Essential
in this design is the length of the linker between the imidazolium unit and the metal center,
with improved activities when using longer linkers but, unfortunately, at the expense of
the chiral induction efficiency.
Although a number of different metal-based salen complexes have been developed, only
a few catalytic systems gave comparable results to the Co-based ones. In 2015 North and
co-workers reported the use of Al(III) and Cr(III) salen complexes for the kinetic
resolution of terminal epoxides upon coupling with CO2 [24]. The results of this work
indicated that the combination of a Al(III) or Cr(III) salen catalyst and a n-Bu4NBr cocatalyst (chloride based nucleophile in the case of the Cr(III) complex) is one of the most
selective catalytic systems for the synthesis of chiral cyclic carbonates from (rac)
epoxides with enantiomeric ratios of up to 93:7 (86% ee; diphenyl-substituted
epoxyamine as substrate), although the kinetic resolution efficiency for less functional
epoxides was poorer.
Chiral mixed metal-organic frameworks (MOF) based on enantiopure nickel(II) salen
metalloligands combined with tetranuclear cadmium clusters have been reported as
recyclable catalysts for the enantioselective synthesis of COCs with modest levels of
chiral induction (45‒52% ee) when using (rac) propylene oxide [25]. The results
demonstrate that MOFs can act as self-supported, and comparatively very active
heterogeneous catalysts for the synthesis of chiral COCs with potential for further
optimization. In the context of heterogeneous systems for kinetic resolution of (rac)
epoxides, Qi and co-workers reported an elegant asymmetric auto-tandem
epoxidation/epoxide-CO2 coupling sequence catalyzed by chiral polyoxometalate-based
metal-organic frameworks (POMOFs). These systems incorporate a chiral
organocatalyst, POM units that consist of Lewis acid sites and POM linking units (aminobipyridines). The latter can interact with CO2 molecules leading to a synergistic activation
of a series of styrenes and ,-unsaturated aldehydes towards the one-pot formation of
chiral COCs in good yields and high ee´s of up to 80% [26].

5

Figure 2. Enantioselective organocatalytic preparation of six-membered COCs reported
by Johnston.
However, one of the most recent and important achievements in the enantioselective
conversion of CO2 was reported in 2015 by Johnston and co-workers (Figure 2) [27]. In
this contribution the authors described the use of pyrrolidine-substituted bis(amidine)s as
organocatalysts (Figure 2: 1, StilbPBAM) in the presence of a Brønsted acid (HNTf2)
than can mediate the three component reaction between homoallylic alcohols, CO2 and
an electrophilic source of iodine. This conceptual approach provides an effective route
towards the preparation of chiral six-membered COCs (16 examples, typical yields 7095%, ee´s typically 74-95%). The catalyst developed uses the combination of a dual
Brønsted acid/base activation to achieve high levels of enantioselectivity.

3. Diastereoselective Conversions
Diastereoselective control in COC synthesis has become a topic of increasing interest and
the relative orientation of the substituents in these heterocycles may influence properties
of the resultant materials upon ring-opening polymerization (ROP). Thus, it is important
to control the relative configuration of pending substituents and the degree of this
selectivity. In the most recent years various diastereoselective strategies for the synthesis
of such COCs have been developed, ranging from the use of unsaturated allylic or
homoallylic alcohols and propargylic (linear) carbonates to the use of internal epoxides
as substrates [13].
Amongst the most recent accomplishments in this area, it has been shown that for the
conversion of 2,3-disubstituted epoxides into their respective COCs (cis or trans) there
exist two stereo-divergent pathways [7]. Initial co-polymerization of the epoxide and CO2
followed by a depolymerization step, proceeds with the formal inversion of the initial
configuration. Direct cyclic carbonate formation, however, preserves the original
stereochemical configuration through two consecutive SN2 processes. The group of
Darensbourg has achieved extensive progress addressing the first conceptual approach
[28-33]. They developed effective catalytic systems based on combinations of Co(III) and
6

Cr(III) salen complexes with different co-catalysts (quaternary ammonium and
phosponium salts) which allow for the diastereoselective synthesis of COCs through a
metal-bound or metal-free backbiting mechanism depending on the substrate and the
reaction conditions. Both types of processes may occur through so-called “carbonate” or
“alkoxide” backbiting and the experimental conditions can be fine-tuned towards one
preferred pathway.
In the direct stereospecific synthesis of COCs from internal epoxides, various
contributions have been communicated. In 2013 North and co-workers presented a
bimetallic Al(salen) catalyst which is able to convert internal oxiranes into their
corresponding carbonates with overall retention of the configuration and in moderate to
high yields [34]. The same group has been also interested in exploring the use of other
metals than aluminum. In a subsequent contribution they showed that Cr(III) salophen
complexes are also effective for the stereospecific synthesis of internal cyclic carbonates
under relative mild conditions [35]. This work build on previous success with other Lewis
acid catalysts based on Fe(III) and Al(III) aminotriphenolate complexes [13, 36].
Other significant advances were made by the Kleij group [37]. They illustrated that a
single Fe(III) aminotriphenolate/TBAX (X = halide) binary catalyst system can achieve
high levels of stereospecificity in COC synthesis using pure cis- or trans-isomers of 2,3dimethylepoxybutane depending on the reaction conditions. Interestingly, both the trans
and cis epoxide could be converted into the cis- and trans-COC, respectively, with high
stereospecificity of up to >99%. Other relevant Fe(III) complexes were disclosed by
Capacchione and co-workers: a series of dinuclear iron (III) complexes bearing thioethertriphenolate ligands were prepared by this group and their use as catalysts for the coupling
of CO2 and diastereopure internal epoxides was reported with comparable results for the
known Fe(II) complexes [38].
In 2014 Ema et al. contributed to this sub-area by devising highly active bifunctional
metalloporphyrin catalysts (Figure 3) [39]. In their work they presented a Mg(II) and
Zn(II) porphyrin-based bifunctional system which was able to convert a trans-1,2disubstituted epoxide into its corresponding trans-COC with excellent retention of the
configuration (99:1 trans/cis). Alternatively, the Leitner group studied in detail the
combination of halide based nucleophiles and polyoxometalates (POMs) as binary
catalyst systems for the insertion of CO2 into cis-monoepoxides originating from plant
oils as biogenic feedstock towards the stereoselective synthesis of biobased COCs [40].
This latter study revealed that the combination of the classical ammonium halide catalysts
such as tetrabutylammonium bromide (TBAB) together with a tetraheptylammonium
silicotungstates (THA-Cr-Si-POM) as the polyoxometalate component particularly give
the best results in terms of selectivity (highest dr 96:4) and yield working under
supercritical CO2 conditions with equimolar amounts of both catalyst components (2.0
mol%) leading to high overall catalyst performance.

7

Figure 3. Bifunctional metalloporphyrin catalyst for the diastereoselective formation of
COCs.

In a similar approach, Werner and co-workers reported on binary catalysts to address the
stereoselective conversion of mono-, di- and tri-epoxides derived from fatty acid
precursors [41]. Albeit not with very high diasteroselectivities, oleo-chemical based COC
synthesis was realized through the use of MoO3 as highly active co-catalyst in
combination with tetra-n-butylphosphonium bromide ([Bu4P]Br) as nucleophile. The
chemo- as well as the stereoselectivity of the carbonated oleochemicals could be
controlled to a certain extent (typical dr´s 70:30) by a proper choice of the catalyst
components and reaction conditions. The same authors presented in a recent similar
contribution the use of other binary catalyst systems composed of [Oct4P]Br and FeCl3
for the COC formation from oleochemical epoxides [42]. Although this Fe-based binary
catalyst did not increase the diastereoisomeric control in these reactions ratios, enhanced
reaction rates could be attained. Judging from these recent results, there still remains a

8

need for the development of new active catalysts that can combine very high chemo- and
stereoselectivity.

4. Stereoselective Synthesis using Cyclic Carbonates as Synthons
The stereoselective synthesis of cyclic carbonates has attracted significant attention in the
last years as mentioned in the previous sections. However, little attention has been
focusing on the use of functional COCs as potential precursors or intermediates in organic
synthesis involving stereoselective transformations [43-45]. For example, COCs can be
regarded as masked syn-diols with CO2 acting as an easily removable protecting group:
this has allowed the chemo-selective preparation of a wide range of di- and tricyclic
organic carbonates in excellent diastereoselectivities providing, after base hydrolysis,
synthetically useful cis-1,2-diol scaffolds [46]. A cis-configured bi/tricyclic epoxide is
first converted into a cis-COC with retention of configuration under Al-catalysis followed
by treatment of the cyclic carbonate with a suitable base (NaOH or K2CO3) affording the
corresponding vicinal diols in high yields and stereoselectivities.
More recently, Kleij et al. reported another example of controlling stereoselectivity
utilizing organic carbonates as intermediates in synthetic organic chemistry [47]. In this
case, they reported a stereodivergent carbamate synthesis by in situ trapping of an organic
carbonate. Delicate optimization of the ratio and amount of catalyst components allows
for formation of either cyclic or polymeric carbonates which are conveniently intercepted
by amine reagents resulting in a controlled and selective aminolysis process forming
carbamate products with either cis (through a cyclic carbonate intermediate) or trans
(through a oligomeric/polymeric carbonate intermediate) stereo-chemistry. Thus, this
unprecedented approach allows for stereodivergence from a single oxirane substrate with
easy access to both cis and trans carbamate isomers in high stereoselectivity (dr´s >19:1).
This mode of stereocontrol represents a new tool in the synthesis of CO2-derived fine
chemicals.

Figure 4. Two different approaches using vinyl carbonate as substrates in stereoselective synthesis of various organic compounds.

9

Recently Zhang and co-workers reported a series of Pd-catalyzed decarboxylative
strategies using vinyl substituted COC as key reagents (Figure 4) [48]. Upon
decarboxylation, a postulated zwitterionic “allylPd” complex is formed in situ that was
combined with different electrophiles to accomplish the synthesis of various chiral
compounds. In general this strategy utilizes the vinyl functionality to induce highly
reactive pi-allylpalladium intermediates which display ambivalent reactivity. For
instance, in the presence of Michael acceptors (substituted methylenemalononitriles) as
coupling partners [49], simple access to chiral and highly substituted furans containing
two quaternary stereo-centres was achieved with high levels of diastereo- and
enantioselectivity (yields >90% and ee´s 8595% typically). These products can be
transformed straightforwardly into the corresponding chiral amino acids, which are useful
intermediates for the synthesis of natural products.
The same authors also used other electrophilic coupling partners such as formaldehyde
furnishing (R)-1,3-dioxolanes with high enantioselectivities (>90% ee) and yields (≥
84%) [50]. These products can be hydrolysed easily into the corresponding chiral vicinal
diols without loss of stereochemical information, and thus provide a practical entry for
the preparation of pharmaceuticals. Two other contributions reported by the same group
demonstrates the generality of the approach, and following similar experimental
conditions employing isocyanates [51] and imines [52], the preparation of chiral 4vinyloxazolidin-2-ones and 4-vinyloxazolidines, respectively, was attained in high yields
and high enantio- and diastereoselective control.
Although electrophiles were shown to be useful in these coupling reactions with vinyl
substituted COCs, the ambivalent character of the postulated Pd-allyl species formed in
situ can also serve to activate nucleophiles. This conceptually different strategy was
recently probed by Kleij and co-workers (Figure 4), who reported a general
stereoselective methodology for the formation of highly substituted (Z)-configured
olefinic compounds based on a Pd-catalyzed decarboxylative functionalization using
suitable nucleophiles. This protocol features vinyl carbonates and nucleophiles as starting
materials, where the formation of a (Z)-configured six-membered palladacyclic
intermediate was computationally determined to be key to the high level of stereocontrol.
Nucleophilic attack on the in situ formed (Z)-palladacycle by aromatic/aliphatic amines
gave access to an extensive series of challenging (Z) allylic amines with Z/E ratios >99:1
in most cases [53], whereas the use of H2O as nucleophile allowed the preparation of
elusive (Z)-1,4-but-2-ene-diols in good yields and high stereo-control as observed for the
allylic amines [54].
The examples of stereoselective synthesis based on the post-modification or use of
functional COCs showcase the increasing importance and versatility of these (mostly)
CO2 derived building blocks in synthetic chemistry. Since many different functional
groups can be envisaged, this area of research should have a bright future and more
interesting developments are expected.

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5. Summary and Conclusions
This concise review presents the latest progress in the CO2-mediated stereoselective
formation of COC and developments aiming for their post-synthetic use in organic
synthesis. From the examples discussed herein it is clear that synthetic chemists are
continuously improving on the use of CO2 to create more complex structures through
innovative catalysis procedures. Relevant progress has been reported in enantio- and
diasteroselective approaches towards COCs and derived products. A rather new and
exciting development is to use the potential of functionalized COCs as valuable
precursors in stereoselective transformations to produce useful intermediates for fine
chemical and pharmaceutical synthesis. However despite the impressive progress in the
the stereoselective conversion of CO2, there remain some challenges open. Among these
is the use of CO2 to target molecules and materials with a higher degree of relevance for
industrial applications and significant evolution of asymmetric methodology is still
necessary in COC synthesis and beyond. Important in this respect is a further optimization
of the catalysis of cyclic ether/CO2 coupling reactions to supply the synthetic
communities with COC scaffolds that incorporate additional and new functionality that
will amplify the accessibility of new synthons with added value in academic and
commercial laboratories.

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