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Cavitand based Polyphenols as Highly Reactive Organocatalysts
for the Coupling of Carbon Dioxide and Oxiranes
Luis Martínez-Rodríguez,[a] Javier Otalora Garmilla[a] and Arjan W. Kleij*[a][b]

Abstract: Various cavitand based polyphenols were prepared from
cheap and accessible aldehyde and resorcinol/pyrogallol reagents
giving their respective resorcin[4]- or pyrogallol[4]arenes. The preorganization of the phenolic units allows for intra- and intermolecular
hydrogen bond (HB) networks affecting both the reactivity and stability
of these HB donor catalysts. Unexpectedly, we found that
resorcin[4]arenes show cooperative catalysis behavior compared to
parent resorcinol in the catalytic coupling of epoxides and CO 2 with
significantly higher turnover. At elevated reaction temperatures, the
resorcin[4]arene based catalyst 3d displays the best catalytic
performance with both very high turnover numbers and frequencies
combining increased reactivity and stability compared to pyrogallol,
and ample substrate scope. This type of polyphenol structure thus
illustrates the importance of a new, highly competitive organocatalyst
design for devising sustainable CO2 conversion processes.

of efficient catalyst systems based on binary or bifunctional
systems comprising of azaphosphatranes,[8] polyphenols,[9]
phosphonium or ammonium alcohols,[10] silane diols[11] and
fluorinated alcohols.[12] Parallel to these activities other promising
organocatalytic systems have been recently communicated
presenting new catalyst designs based on the host-guest
complexation of nucleophilic reagents,[13] the use of phosphorus
ylides[14] and hydroxy-functionalized mono- and bis-imidazolium
bromides.[15]

Introduction
One of the biggest challenges associated with carbon dioxide
conversion is the design of appropriate catalytic systems that
show improved reactivity and selectivity behavior.[1] Up to date,
most of the attention in the area of CO2 catalysis has been
focused on (homogeneous) catalysts based on metal complexes
of various kinds showing in a few cases high reactivity and
unusual scope and/or selectivity.[2] Metal catalysts based on
abundant metals such as Zn,[3] Fe,[4] Al,[5] and Co[6] have by far
received most of the attention allowing for significant evolution in
both catalyst design as well as improving the product portfolio that
can be accessed from CO2 representing a renewable and cheap
carbon feed stock for chemical synthesis.
Organocatalysis has recently appeared on the radar of
synthetic chemists representing a sustainable alternative for
metal based approaches in CO2 conversion catalysis.[7] In
particular, the catalytic coupling of epoxides and CO2 has been
studied extensively and can be regarded as a benchmark process
for new catalyst development in non-reductive CO2 couplings.
Progress in this area has been considerable with the development
Figure 1. Schematic structures of resorcinol (1a), pyrogallol (1b), tannic acid (2)
and cavitand based polyphenols (3).
[a]

[b]

L. Martínez-Rodríguez, J. Otalora Garmilla, Prof. Dr. A. W. Kleij
Institute of Chemical Research of Catalonia (ICIQ)
The Barcelona Institute of Science and Technology
Av. Països Catalans 16, 43007 Tarragona (Spain)
E-mail: akleij@iciq.es
Prof. Dr. A. W. Kleij
Catalan Institute of Research and Advanced Studies (ICREA)
Pg. Lluís Companys 23, 08010 Barcelona (Spain)
Supporting information for this article is given via a link at the end of
the document.

We have become interested in the use of (natural)
polyphenols such as pyrogallol and tannic acid (see Figure 1; 1b
and 2) for the conversion of epoxides through hydrogen bond
(HB) activation. Their transformation into cyclic carbonates in the
presence of CO2 takes advantage of the extended HB network
that arises upon activation of the epoxide towards the formation
of key intermediates, and consequently lower kinetic barriers

FULL PAPER
allowed for either low temperature conversions (45ºC) [9b,c] or the
use of reduced polyphenol loading for effective catalytic
turnover.[9a] Nonetheless, there are still challenges to be met upon
using such polyphenols as at higher reaction temperatures (i.e.,
80ºC) some catalyst degradation through deprotonation and
formation of relatively inactive phenolate groups cannot be fully
avoided preventing the efficient recycling of these phenolic
additives and thus restricts the total turnover number (TON). [9a,c]

Figure 2. Intra- and intermolecular hydrogen-bonding networks in cavitand
based structures. Part of this figure has been reprinted with permission from
reference 19. Copyright (2013) American Chemical Society.

In our quest to develop thermally more robust polyphenol
based catalysts while maintaining high reactivity and privileged
substrate scope, we considered that cavitand structures (Figure
1, structures 3)[16] may deliver the appropriate combination of
activity and thermal/chemical stability. These cavitand structures,
including resorcin[4]arenes (3: X = H) and pyrogallol[4]arenes (3:
X = OH), give rise to pre-organized supramolecular structures,
often being hexameric in nature, in solution and solid state
through inter-molecular, water-assisted HB interactions (Figure 2,
right).[17] At the same time, the bowl shape of monomeric cavitand
molecules is also controlled through intra-molecular HBs between
adjacent resorcinol/pyrogallol units typically expressed in solvent
media such as alcohols and acetonitrile (Figure 2, left).[18] These
latter, intra-molecular HB patterns suggest a similar potential for
catalytic activation[19] of epoxides compared to pyrogallol/tannic
acid (Figure 1) with stabilization of key intermediate transition
states through multiple HB interactions being more efficient than
in the absence of such HB donors.
In this contribution we will show that cavitand based
polyphenols are excellent HB activators in the formation of cyclic
carbonates from epoxides and CO2 with unprecedented turnover
numbers and frequencies. Catalytic data in combination with
several control experiments support a cooperative catalytic effect
when using the resorcin[4]arene systems, which also show the
best combination of activity and stability at elevated temperatures.
In combination with the easy access to these modular and cheap
polyphenolic structures and catalytic scope, these cavitands
represent a new and powerful type of organocatalyst for the
conversion of CO2 into value added chemicals.

Results and Discussion
Resorcin[4]arenes 3ae, pyrogallol[4]arenes 3fi and the
octahydroxypyridine[4]arene 3j (Scheme 1) were prepared
according to previous reported procedures (Experimental
Section) and their molecular identity was established by 1H/13C
NMR and mass analysis: these analyses were in accordance with
the literature data (Supporting Information for details).
Compounds 3a3i were initially tested, in combination with NBu4X
(X = halide), as binary catalysts in the coupling of 1,2epoxyhexane 4a and CO2 at 50ºC using methylethyl ketone
(MEK) as solvent (Table 1).

Scheme 1. Resorcin[4]arenes 3ae, pyrogallol[4]arenes 3fi and the
octahydroxypyridine[4]arene 3j.

Table 1. Screening of conditions using a polyphenol/TBAX binary catalyst in the
coupling of 1,2-epoxyhexane and CO2 to afford cyclic carbonate 4b.[a]
Entry

Polyphenol

Amount
[mol %]

NBu4X
[mol %]

T
[ºC]

MEK
[mL]

Yield 4b
[%][b]

1

3b

1.5

I, 5.0

45

2.5

81

2

3b

1.5

I, 5.0

50

2.5

91

3

3b

1.5

I, 5.0

50

5.0

65

4

3b

1.5

Cl, 5.0

50

2.5

31

5

3b

1.5

Br, 5.0

50

2.5

65

6



0

I, 5.0

50

2.5

4

7

3b

1.5

0

50

2.5

0

8

1a

6.0

I, 5.0

50

2.5

47

9

1a

6.0

I, 5.0

50

5.0

24

10

1b

4.0

I, 5.0

50

2.5

99

[a] Reaction conditions: 1,2-epoxyhexane 1.0 mmol, p(CO2) = 10 bar, 18 h,
polyphenol amount normalized with respect to [OH] groups, p(CO2)º = 1.0 MPa,
18 h. [b] NMR yields based on mesitylene as internal standard, selectivity for 4b
was >99%.

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We first tested resorcin[4]arene 3b/NBu4I as a binary catalyst
for the synthesis of organic carbonates from epoxides and CO 2
under conditions closely related to those previously probed for a
binary pyrogallol based system (entry 1). [9c] At 45ºC, a NMR yield
of 81% was already achieved, and raising the temperature to
50ºC increased this yield to 91% (entry 2). Dilution of the reaction
mixture (entry 3) or changing the nature of the nucleophile (entries
4 and 5) gave poorer kinetics leading to lower yields of 4b. In the
absence of 3b (entry 6; 4%) or nucleophile (entry 7; 0%) very low
to no conversion of the epoxide substrate was noted showing the
imperative role of both catalyst components in this coupling
reaction. Comparison of the efficiency of resorcin[4]arene 3b
(entry 2) with that of the parent building unit resorcinol 1a (entry
7) showed a much higher yield of carbonate 4b for the cavitand
based system despite the use of a similar concentration of
diphenol units. This effect was maintained under more dilute
conditions (cf., entries 3 and 9). Pyrogallol 1b, a triphenol, gave
virtually quantitative yield (entry 10, 99%) under these conditions.
The remarkable yield of 4b in the presence of resorcin[4]arene 3b
suggests a cooperative effect between the 1,3-diphenol sites in
the catalytic activation of the epoxide and/or more efficient
stabilization of the intermediates of the carbonate formation
reaction.

yield of 4b was achieved with 3d (R = nonyl), a trend that was
also observed within the series of pyrogallol[4]arenes 3f3i
(entries 69).[20]

1b
3d

3i

1a

Figure 3. Comparative kinetics in the formation of carbonate 4b from 1,2epoxyhexane and CO2 using resorcinol 1a, pyrogallol 1b, resorcin[4]arene 3d
and pyrogallol[4]arene 3i. The polyphenol amount was normalized with respect
to the [OH] groups. Conditions used: 4 mol% 1b, 5 mol % 1a, 1.5 mol% 3d and
1.0 mol% 3i. For all reactions: 1,2-epoxyhexane 1.0 mmol, NBu4I 5 mol%, MEK
2.5 mL, p(CO2)º = 10 bar, 50ºC.
Table 2. Screening of cavitand/TBAI binary catalysts 3 in the coupling of 1,2epoxyhexane and CO2 to afford cyclic carbonate 4b.[a]

The best-performing nonyl-substituted polyphenols 3d and 3i
were then more closely examined and compared with the parent
Entry
Cavitand
Amount
NBu4I
T
R
Yield 4b
resorcinol 1a and pyrogallol 1b: the kinetic profiles of each binary
[mol %]
[mol %]
[ºC]
[%][b]
catalyst (upon combining with NBu4I) were determined (Figure 3).
93
1
3a
1.5
5.0
50
Me
Interestingly, the pyrogallol[4]arene 3i shows inferior catalytic
performance with respect to resorcin[4]arene 3d. The reason for
91
2
3b
1.5
5.0
50
Et
this behaviour is likely the competing self-assembly of the
94
3
3c
1.5
5.0
50
Bu
individual cavitand molecules of 3i into larger aggregates (i.e.
hexamers, cf. Figure 2). Cohen et al. compared the stability of
98
4
3d
1.5
5.0
50
Non
undecyl-substituted resorcin[4]arenes and pyrogallol[4]arenes.[21]
Titration studies involving these cavitand molecules
27
5
3e
1.5
5.0
50
Ph
demonstrated that upon increasing the polarity of the medium by
78
6
3f
1.0
5.0
50
Me
adding CD3OD to a solution of the cavitand in CDCl3, the
hexameric, aggregated state was fully disrupted for both types of
84
7
3g
1.0
5.0
50
Et
cavitand. However, essentially much lower amounts of CD 3OD
87
8
3h
1.0
5.0
50
Bu
were required in the case of the resorcin[4]arene in line with a
stronger self-assembly behaviour of the pyrogallol[4]arene.
93
9
3i
1.0
5.0
50
Non
Therefore, under the reaction conditions reported in Table 2 and
Figure 3, the poorer performance of 3f3i compared to the
[a] 1,2-epoxyhexane 1.0 mmol, polyphenol amount normalized with respect to
[OH] groups, MEK 2.5 mL, p(CO2)º = 1.0 MPa, 18 h. Abbreviations: Me = methyl, resorcin[4]arene series 3a3d is thus explained in terms of a
Et = ethyl, Bu = n-butyl, Non = n-nonyl, Ph = phenyl. [b] NMR yields based on stronger competing self-assembly. This behaviour competes with
mesitylene as internal standard, selectivity for 4b was >99%.
hydrogen bonding between the epoxide and the phenolic groups,
and thus slows down the reaction. In order to further support the
We then screened a series of nine cavitands (3a3i, Scheme
view that competitive HB interactions can slow down the catalytic
1) in the coupling of 1,2-epoxyhexane 4a and CO2 (Table 2) to
reaction, octahydroxypyridine[4]arene 3j (Scheme 1)[22] was also
investigate the role of the pendent R groups. Within the series of
tested as a HB donor system in the synthesis of carbonate 4b.
resorcin[4]arenes 3a3e (entries 15), the highest
The 2,6-dihydroxypyridine subunits in 3j are known to induce

FULL PAPER
intramolecular N···HO hydrogen bonds, and the significant lower
yield after 18 h (1.5 mol%, 60%) than observed for
resorcin[4]arene 3d (1.5 mol%, 98%) is a clear testament of
competitive H-bonding. Thus, the best catalytic performance
among the cavitand structures at 50ºC is noted for 3d.
In an effort to further increase the reactivity, the coupling of
1,2-epoxyhexane and CO2 was carried then out at 80ºC using
resorcin[4]arene 3d, pyrogallol[4]arene 3i and pyrogallol 1b
(Table 3, Figure 4). Under these conditions the nucleophilic
additive NBu4I alone produces poor catalysis (entry 1, 17% yield
of 4b). Various combinations of cavitand/nucleophile were probed
(entries 25) while maintaining a similar ratio between both
catalyst components (ratio NBu4I/[OH] groups  3.3). For the
resorcin[4]arene 3d based catalyst, the conditions reported in
entry 3 still produced quantitative yield of 4b, whereas further
lowering the amount of catalyst to 0.25 mol% 3d/ 0.8 mol% NBu4I
showed a modest decrease in yield to 80% (entry 4). For
comparison, upon using a similar amount of catalyst derived from
pyrogallol[4]arene 3i (cf., entries 3 and 5), a very high though not
quantitative yield of 4b was noted. Remarkably, under these latter
conditions, the pyrogallol 1b based catalyst produced a markedly
lower yield of 4b (77%; cf. entries 3 and 7) showing the superior
performance of the resorcin[4]- and pyrogallol[4]arene based
catalysts at 80ºC.

thereafter barely increases. This is in line with our previous results
using either pyrogallol 1b or tannic acid 2 as catalysts
components; both systems show inferior stability at this elevated
temperature causing side-reactions that involve the deprotonation
of the polyphenolic unit and replacement thereof by NBu4.[9a,b] The
formation of (deprotonated) phenolate groups causes a decrease
in the ability to form extended HB networks as to stabilize catalytic
intermediates, which results in higher kinetic barriers and thus
slower reactions. Consequently, both the nucleophile and
polyphenol concentration is negatively affected and the catalysis
is shut down in the case of pyrogallol. On the contrary, both
cavitand based catalysts based on 3d and 3i retain catalytic
activity after prolonged use and therefore are more effective
systems for cyclic carbonate preparation under elevated
temperature conditions with 3d performing slightly better than 3i
in the reported time span. Importantly, comparing the pKa values
of resorcinol 1a (9.20)[23] and pyrogallol 1b (9.01)[24] shows that
the pyrogallol unit is more acidic and likely to undergo deprotonation more facilely. This is likely causing a (much) shorter
lifetime of the catalyst whereas the resorcin[4]arene based
system 3d shows comparatively a longer lifetime. This results in
better potential for obtaining higher turnover numbers at elevated
reaction temperatures. Interestingly, the preorganization of less
active resorcinol units (cf., Figure 3, 1a versus 1b) in the cavitand
significantly increases their catalytic potential as compared with
the pyrogallol based one underlining the importance of the
catalyst structure for effective turnover.

3d
Table 3. Catalytic coupling of 1,2-epoxyhexane and CO2 at 80ºC to afford cyclic
carbonate 4b.[a]

3i
1b

Entry

Polyphenol

Amount
[mol %]

NBu4I
[mol %]

T
[ºC]

Yield 4b
[%][b]

1



0

1.6

80

17

2

3d

0.75

2.5

80

>99

3

3d

0.50

1.6

80

>99

4

3d

0.25

0.8

80

80

5

3i

0.33

1.6

80

93

6

1b

2.0

3.2

80

>99

7

1b

1.3

1.6

80

77

8

1b

0.66

0.8

80

55

[a] 1,2-epoxyhexane 1.0 mmol, polyphenol amount normalized with respect to
[OH] groups, MEK 2.5 mL, p(CO2)º = 1.0 MPa, 18 h. [b] NMR yields based on
mesitylene as internal standard, selectivity for 4b was >99%.

To investigate this in more detail the full kinetic profiles for
each of the catalyst systems reported in entries 3, 5 and 7 (Table
3) were determined (see Figure 4). The pyrogallol catalyst system
reaches a plateau conversion of around 70% after 6 h which

Figure 4. Comparative kinetics in the formation of carbonate 4b from 1,2epoxyhexane and CO2 at 80ºC using pyrogallol 1b (1.3 mol%), resorcin[4]arene
3d (0.50 mol%) and pyrogallol[4]arene 3i (0.33 mol%). Conditions used: 1,2epoxyhexane 1.0 mmol, NBu4I 1.6 mol%, 2.5 mL, p(CO2)º = 10 bar, 80ºC. Note
that in all reactions the same molar amount of phenol groups was used.

The influence of the time frame on the performance of the
polyphenol to act as an efficient HB donor in the activation of
epoxides was carried with resorcin[4]arene 3d and pyrogallol 1b
in the synthesis of carbonate 4b (Table 4; scale 10 mmol of 4a).
Solvent-less (neat) conditions were employed to favour kinetics
and the use of nucleophile alone again showed considerably

FULL PAPER
lower yield of 4b (entries 1 and 2; 7 and 32%, respectively)
compared with the use of both 3d and NBu4I combined (entries 2
and 4; 46 and 74%, respectively). Under these conditions, the
TON based on the total of phenol active sites amounted to 1225
(entry 5). Higher turnover numbers were thus simply achieved by
prolonging the reaction showing virtually full conversion after 30
h. The pyrogallol based catalyst (entries 6 and 7) showed lower
efficiencies with only a modest increase in the TON as defined
above after 18 h further confirming the favourable stability
features of the cavitand structure 3d at elevated temperature.
Thus the combination of the cooperative action of the resorcinol
units in 3d with a higher chemical stability compared to pyrogallol
1b makes this system among the most efficient organocatalyst
reported to date with very high TON. The turnover frequency of
the binary catalyst system based on 3d and NBu4I was estimated
after 1 h (entry 3, 46%) considering the much lower conversion
(entry 1, 7%) in the absence of 3d. When correcting for this
background conversion still a significant part of it may be
attributed to the binary catalyst (39%, TON = 488 based on phenol
group molar concentration, TOF/h/[OH] group = 488 h-1), reporting
thus the highest (initial) activity for a binary organocatalyst in this
area.

carbonates 4b22b in the presence of CO2. In order to produce
synthetically useful yields, 1.53.0 mol% of 3d was used together
with 5 mol% of NBu4X (X = I, Br) in MEK (2.5 mL). The use of
solvent was in some cases warranted to prevent solidification of
the reaction mixture and incomplete conversion of the substrate:
for the internal epoxides 18a22a neat conditions were used.

Table 4. Comparison between resorcin[4]arene 3d/NBu4I and pyrogallol 1b/
NBu4I as binary catalysts in the coupling of 1,2-epoxyhexane and CO2 at 80ºC
to afford cyclic carbonate 4b. N.a. stand for non-applicable.[a]
Entry

Cat.
[mol%]
[a]

OH
units
[mol%]

t
[h]

Yield
4b
[%][c]

TON

TONc

TOFc

[d]

[e]

[f]

1





1

7







2





18

32







3

3d, 0.010

0.080

1

46

575

488

488

4

3d, 0.010

0.080

18

74

925

525

29

5

3d, 0.010

0.080

30

98

1225

n.a.

n.a.

6

1b, 0.026

0.080

18

60

750

350

19

7

1b, 0.026

0.080

30

66

825

n.a

n.a

[a] 1,2-epoxyhexane 10.0 mmol, polyphenol amount normalized with respect
to [OH] groups (see third column), neat conditions, p(CO2)º = 1.0 MPa, NBu4I
1.6 mol%. [b] Total amount of OH (phenol) units. [c] NMR yields based on
mesitylene as internal standard, selectivity for 4b was >99%. [d] TON = total
turnover number based on molar amount of phenol groups. [e] Corrected TON
using the measured background conversions, see entries 1 and 2. [f]
Corrected average TOF/h using the measured background conversions, see
entries 1 and 2.

Motivated by these results, we then examined a wide scope
of epoxide substrates (4a22a) in the formation of their cyclic

Figure 5. Substrate scope in the conversion of various terminal and internal
epoxides 4a22a into their cyclic carbonates 4b22b using 3d/NBu4I as catalyst.
General conditions: epoxide 1 mmol, 1.5 mol% 3d, 5 mol% NBu4I, 18 h, 1 MPa,
50ºC, 2.5 mL of MEK. * Using 3 mol% of 3d, 5 mol% NBu4Br, 80ºC 18 h, neat.

At 50ºC and 1 MPa of pressure terminal epoxides 4a17a
were smoothly converted into their carbonates 4b17b in high
conversion (>99%) and isolated yields (9299%). The
temperature and pressure conditions are comparatively very mild
considering the use of an organocatalyst system, and prompted
us to examine more challenging internal epoxides (18a22a) as
reaction partners. To date limited progress has been achieved

FULL PAPER
using such epoxide substrates in organocatalytic approaches.
One promising example was recently reported by Tassaing et
al.[12b] who used a fluorinated alcohol as HB donor and achieved
at 100ºC and 2 MPa a conversion of 73% of cyclohexene oxide
(CHO; 18a in Figure 5) after 5 h. Werner et al. reported on the use
of bifunctional phosphonium salts that were effective for internal
epoxide conversion at temperatures in the range 90120ºC and 1
MPa.[10a] For CHO specifically, the best results in terms of yield
were obtained at 120ºC and 4 MPa (40 bar) producing the
carbonate 18b in 69% yield after 6 h.
We first screened potential conversion of CHO 18a at 50ºC
and 1 MPa but this afforded the carbonate 18b in only 6% yield
after 18 h. We were pleased to find that upon increasing the
temperature to 80ºC and using 3 mol% of cavitand 3d the
conversion of 18b could be significantly increased to 89%
(isolated yield 85%) using neat conditions. As a control
experiment, the reaction in the absence of 3d was also carried out
and gave only 8% yield (duplo experiment) showing the
importance of the cavitand 3d to achieve a high yield of 18b under
similar conditions.
Other internal epoxides (19a22a, Figure 5) were then also
subjected to these latter conditions including cyclic and acyclic
substrates. Whereas the 3,4-epoxyfuran 21a was converted in
high conversion (84%) and yield (79%), the corresponding
cyclopentene oxide 20a gave a much lower (reproducible) yield
(38%). The acyclic epoxides 19a and 22a were also converted
into their carbonates 19b and 22b showing the more general
potential of 3d/NBu4I as a binary organocatalyst in the conversion
of more challenging internal epoxides. All epoxides 18a22a were
converted with full retention of configuration (cis >99% for 18b,
20b and 21b or trans >99% for 22b) except for 19a (cis/trans =
8:2). Such loss of stereo-chemical information with this substrate
in the formation of its carbonate product in the presence of CO2
has been observed before,[25] and may be related to a partial SN1
character of the nucleophilic attack of the linear carbonate
intermediate onto the CBr bond initially formed in the ringopening of the epoxide by NBu4Br.

Conclusions
The use of easily accessible and modular cavitand structures
such as resorcin[4]arenes and pyrogallol[4]arenes constitute
interesting HB donor binary catalysts in combination with
ammonium halide salts. The pre-organization of the phenolic units
within the resorcin[4]arenes was proven to be beneficial for the
catalytic efficiency in organic carbonate formation from epoxides
and CO2, and resulted in cooperative effects (for 3d) leading to
significantly higher conversion rates compared to resorcinol 1a.
Upon raising the reaction temperature from 50 to 80ºC in order to
improve the overall reactivity, the cavitand structures showed
higher chemical stability than pyrogallol 1b making these former
systems more suitable for achieving both very high TONs. Also,
resorcin[4]arene 3d combined with NBu4I shows very high initial
turnover frequencies of almost 500 h -1 at 80ºC. This improved and
unparalleled reactivity was shown to be beneficial in the formation
of 19 different carbonates under comparatively mild reaction

conditions (50ºC, 1 MPa). Further to this, six di-substituted
epoxides (16a and 18a22a) were also screened as reaction
partners and efficiently converted under neat conditions at 80ºC
providing good to excellent isolated yields of 7994%. Compared
with the state-of-the-art in organocatalytic CO2/epoxide coupling
chemistry this is a remarkable result. Hence, cavitand based
binary organocatalysts have proven to be sustainable, versatile
and highly reactive alternatives for metal-based systems in the
catalytic coupling of epoxides and CO2. Further attention is now
on the combination of cooperative and bifunctional concepts to
develop organocatalytic processes with improved reactivity under
ambient conditions.

Experimental Section
General
Methyl ethyl ketone (MEK, Aldrich ACS reagent >99%) and
carbon dioxide (purchased from PRAXAIR) were used as
received without further purification or drying. All the
resorcin[4]arenes and pyrogallol[4]arenes were synthesized
following the classical condensation of aldehydes in the presence
of acid.[18] All other (polyphenolic) chemicals are commercially
available at Aldrich and were used as received. 1H and 13C1H
NMR spectra were recorded on a Bruker Avance 500 NMR
spectrometer at 297 K. Chemical shifts are reported in ppm
relative to the residual solvent peaks in CDCl 3 ( = 7.26 ppm) and
[DMSO]-D6 ( = 2.50 ppm). Mass analyses were carried out by
the High Resolution Mass Spectrometry Unit at the ICIQ in
Tarragona, Spain.
Cavitand synthesis
Typical experiment: a solution of one equivalent of
resorcinol/pyrogallol (6 mmol) in a solution of ethanol (95%, 75
mL) and concentrated hydrochloric acid (25 mL) was cooled to 2
ºC. Then the aldehyde reagent (6 mmol, 1 eq.) dissolved in
ethanol (95% 50 mL) was added drop-wise to the reaction mixture.
The resulting solution was stirred at 75ºC during a period of 18 h
up to 72 h depending on aldehyde substrate. Upon cooling to rt,
the precipitate that separated was washed repeatedly with cold
water and methanol and finally dried: the compounds were
recrystalized from acetonitrile. Analysis details of all cavitand
structures 3a3j can be found in the Supporting Information.
Catalytic experiments
Typical procedure: the organic cyclic carbonate synthesis from
epoxides and CO2 was carried out a 30 mL steel autoclave using
1,2-epoxyhexane (1 mmol, 1 eq), cavitand structure (1.0-1.5
mol %), NBu4I (5 mol %) and MEK (2.5 mL). The autoclave was
then subjected to three cycles of pressurization and
depressurization with CO2. Finally the autoclave was charged with
1 MPa (10 bar) of CO2, heated to 50ºC and the content stirred for
18 h. Hereafter, the autoclave was cooled to rt and carefully
depressurized. The volatiles were removed under reduced

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pressure and the product was purified by flash column
chromatography (1:1 hexane/ethyl acetate as eluent) to afford the
pure cyclic carbonate. Analytical details of all organic carbonates
4b22b can be found in the Supporting Information.

[5]

Acknowledgements
We thank ICIQ, ICREA, and the Spanish Ministerio de Economía
y Competitividad (MINECO) through project CTQ-2014-60419-R
and the Severo Ochoa Excellence Accreditation 2014–2018
through project SEV-2013-0319. Dr. Noemí Cabello, Sofía Arnal
and Vanessa Martínez are acknowledged for the mass analyses.
LMR thanks ICIQ for a predoctoral fellowship.

[6]

Keywords: carbon dioxide • cooperativity • organocatalysis •
pyrogallenes • resorcinarenes
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FULL PAPER
Entry for the Table of Contents:

FULL PAPER
Organize yourself ! Cavitand based
polyphenols in combination with NBu4X
(X = I, Br) are excellent organocatalysts
for the synthesis of organic carbonates
showing interesting cooperative
behaviour and high chemical stability.
The catalytic results demonstrate a
combination of high initial TOFs with an
exclusive substrate scope that includes
several internal epoxides. These new
metal-free, cheap and modular cavitand
based binary catalysts showcase the
importance of a pre-organization of
functional groups for maximum catalytic
efficiency.

Luis Martínez-Rodríguez, Javier Otalora
Garmilla and Arjan W. Kleij*

Page No. – Page No.

Cavitand based Polyphenols as
Highly Reactive Organocatalysts for
the Coupling of Carbon Dioxide and
Oxiranes