FULL PAPER
“This is the peer reviewed version of the following article: ChemSusChem 2015, 8, 3248-3254, which has been published in final form
at DOI: 10.1002/cssc.201500710. This article may be used for non-commercial purposes in accordance with Wiley Terms and
Conditions for Self-Archiving published at http://olabout.wiley.com/WileyCDA/Section/id-820227.html."

FULL PAPER
Highly Efficient Organocatalyzed Conversion of Oxiranes and
CO2 into Organic Carbonates
Sergio Sopeña,[a] Giulia Fiorani,[a] Carmen Martín[a] and Arjan W. Kleij*[a][b]

Abstract: The use of a binary catalyst system based on tannic
acid/NBu4X (X = Br, I) is presented being a highly efficient
organocatalyst at very low catalyst loading for the coupling between
carbon dioxide and functional oxiranes affording their organic
carbonates in good yields. The presence of multiple polyphenol
fragments within the tannic acid structure is considered to be
beneficial for synergistic effects leading to higher stabilization of the
catalyst structure during catalysis. The observed TOFs exceed 200 h1
which is among the highest reported to date for organocatalysts in
this area of CO2 conversion. The current organocatalyst system
presents a useful, readily available, cheap but above all reactive
alternative for most of the metal-based catalyst systems reported to
date.

Brøndstedt acids,[26-31] (supported) phosphonium, ammonium or
imidazolium salts and derivatives[32-38] and others.[39-44] However,
organocatalysts are usually much less effective in the activation
of organic substrates and require longer reaction times, much
higher reaction temperatures and/or (much) higher loadings for
effective turnover.[4] Thus, the generally observed lower reactivity
of organocatalysts is still posing a major challenge to be able to
compete with metal-based catalysts eventually providing more
sustainable catalysis solutions.

Introduction
Current (catalytic) research centered on the use of carbon dioxide
(CO2) as a cheap and renewable source of carbon focuses on the
incorporation into other organic scaffolds as to be able to partially
replace fossil fuel based chemistries.[1-9] Considerable progress in
this area has been achieved leading to a wide variety of organic
structures one may obtain using CO2 as a molecular synthon.[1014]
Among these organic products, organic carbonates [7,8,15-17]
have conquered a prominent position being useful in a number of
applications including their use as fuel additives, as non-protic
solvents and as monomer intermediates for polymeric
structures.[18] In the last decade, highly efficient catalytic methods
for their preparation have emerged with those incorporating Lewis
acidic metal ions probably among the most active reported to
date.[15,19-22] Notwithstanding, the use of metal-based catalysts in
industrial settings is not always desired and the presence of trace
amounts of (toxic) metals in final products is subject of an
increasingly lower limit accepted by (end-)users in commercial
settings.
Therefore, from this viewpoint one would ideally like to use
organocatalysis for the formation of organic carbonates and
progress in this respect is characterized by the use of various
types of catalysts based on ionic liquids, [23-25] (poly)alcohols and

[a]

[b]

S. Sopeña, Dr. G. Fiorani, Dr. C. Martín, Prof. Dr. A. W. Kleij
Institute of Chemical Research of Catalonia (ICIQ)
Av. Països Catalans 16, 43007 – Tarragona (Spain)
E-mail: akleij@iciq.es
Prof. Dr. A. W. Kleij
Catalan Institute for Research and Advanced Studies (ICREA)
Pg. Lluis Companys 23, 08010 – Barcelona (Spain)
Supporting information for this article is given via a link at the end of
the document.

Scheme 1. (a) Chemical structure of tannic acid (TA) and pyrogallol (PG). (b)
Stabilization of intermediates in organic carbonate synthesis through
Hbonding using PG.

In the course of our studies devoted to the development of
new and more efficient organocatalytic methodology for organic
carbonate formation, we and others have shown that activation of
oxiranes through hydrogen-bonding using polyphenolic
compounds, fluorinated alcohols and silanediols is very
attractive.[45-48] In particular, pyrogallol PG (i.e., 1,2,3-

FULL PAPER
trihydroxybenzene) is an example of a highly efficient
organocatalyst system able to mediate the coupling between
terminal epoxides and CO2 under extremely mild reaction
conditions (2545ºC, 210 bar CO2 pressure, 2 mol% catalyst).
Of particular importance is the cooperative nature of the adjacent
phenol groups that allows for an extended hydrogen-bond
network upon activation of the oxirane thereby lowering the
activation barrier for its ring-opening by an external nucleophile
(Scheme 1b, below) and beyond.[31,42] We thought that an even
higher local concentration of phenolic sites would be beneficial for
catalytic turnover and therefore considered tannic acid (i.e. TA,
Scheme 1a) as catalyst additive being a naturally occurring plant
phenol that is commercially available and cheap. The presence of
multiple phenol fragments in tannic acid should facilitate highly
efficient activation of oxiranes through similar synergistic effects
as noted for PG.
Herein we report on the use of binary catalyst systems derived
from tannic acid and suitable nucleophiles for the coupling of CO 2
and various epoxides; these new catalyst systems are among the
most active organocatalysts reported to date[44] with appreciable
turnover frequencies able to compete with those reported for
many known metal-based catalysts. The observation of
synergistic effects for improved catalyst stability and lifetime
creates new potential for new organocatalyst design in this area
of CO2 catalysis.

effect on the conversion rate (see entries 14 and 15 versus 3) and
80ºC seems to be rather optimal for this catalyst system.
Table 1. Screening of conditions and (co)catalyst loadings using various
nucleophiles, tannic acid 1 and 1,2-epoxyhexane (2a) as substrate.[a]

NBu4X
[mol%]

Solvent

T
[ºC]

Yield
[%][b]

1

0.50[c]

I (5.0)

MEK

45

16

2

2.0[c]

I (5.0)

MEK

80

93

3

0.05

I (5.0)

MEK

80

>99[d]

4

0

I (5.0)

MEK

80

47

5

0

I (2.0)

MEK

80

41

6

0.05

0

MEK

80

0

7

0.05

I (4.0)

MEK

80

>99

0.05

I (3.0)

MEK

80

>99

9

0.05

I (2.0)

MEK

80

>99

10[e]

First we combined tannic acid TA with NBu4I to obtain a binary
catalyst system capable of mediating the coupling between 1,2epoxyhexane 1a (a benchmark substrate) and CO2. Our previous
result using pyrogallol/NBu4I as binary catalyst system (5 mol%
each with respect to the same substrate, MEK as solvent, MEK =
2-butanone)[49] gave a yield of 100% (93% isolated) at 45ºC after
18 h. Thus, we first attempted to use similar conditions (Table 1,
entry 1) with a slightly lower amount of hydrogen-bond donor, i.e.
TA (0.50 mol%). However, we observed that under these
conditions, the tannic acid was not fully soluble therefore
complicating epoxide turnover. We were pleased to find that an
increase of the reaction temperature to 80ºC gave a
homogeneous catalyst solution when the catalyst loading was
kept low enough, i.e. at 0.05 mol% of TA (entry 3) leading to
quantitative formation of the cyclic carbonate 1b in high selectivity
(>99%). The use of the nucleophilic reagent alone (see entries 4
and 5) led to a much lower yield in cyclic carbonate 1b,
emphasizing the imperative role of the tannic acid to mediate this
conversion. In the presence of tannic acid TA and absence of cocatalyst (NBu4I) no conversion was noted (entry 6).
We then varied the co-catalyst loading (entries 7-11) while
keeping [TA] at 0.05 mol%, and observed that quantitative
conversion of 1a into 1b could still be achieved at an NBu4I
loading of 2 mol%. Interestingly, the initial TOF under these
conditions was quite high (236 h-1, entry 10; t = 2 h); in the
absence of TA only an 8% yield of 1b was noted after 2 h. The
catalyst loading, [TA], could be further reduced to around 0.03
mol% (entry 12) to give substantially higher conversion of 1a
compared to the turnover facilitated by the co-catalyst NBu4I
alone (cf., entry 5). The reaction temperature had a significant

TA
[mol%]

8

Results and Discussion

Entry

0.05

I (2.0)

MEK

80

24

11

0.05

I (1.0)

MEK

80

82

12

0.03

I (2.0)

MEK

80

79

13

0.01

I (2.0)

MEK

80

47

14

0.05

I (5.0)

MEK

70

76

15

0.05

I (5.0)

MEK

60

53

16[f]

0.05

I (5.0)

MEK

80

>99

17

0.05

Br (2.0)

MEK

80

>99

18

0.05

Cl (2.0)

MEK

80

70

19

0.05

I (2.0)

ACE

80

>99

20[g]

0.025

I (2.0)

MEK

80

>99

21[h]

0.15

I (2.0)

MEK

80

62

22[i,j]

0.03

I (2.0)

MEK

80

35

23[h,i]

0.15

I (2.0)

MEK

80

33

24[h,i]

0.30

I (2.0)

MEK

80

55

[a] General conditions: 1,2-epoxyhexane (8.3 mmol), p(CO2)0 = 10 bar, cocatalyst NBu4X (amount indicated), 18 h, 30 mL autoclave as reactor. MEK
= 2-butanone and ACE = acetone (5 mL). [b] Yield determined by 1H NMR
(CDCl3) using mesitylene as an internal standard. Selectivity for the cyclic
carbonate was >99%. [c] Not fully homogeneous. [d] Isolated yield 99%. [e]
Using 2.5 mL of MEK, t = 2 h; TON = 472, TOF = 236 h-1; the reaction in
absence of TA afforded only 8% of 1b. [f] p(CO2)0 = 6 bar. [g] Using only 2.5

FULL PAPER
mL of solvent. [h] Using pyrogallol as catalyst. [i] Reaction time 6 h. [j] Average
TOF/h/TA = 195 h-1.

When the initial pressure was lowered to 6 bar, the yield of 1b
remained quantitative (entry 16). Upon changing the nature of the
nucleophile (entries 17 and 18), a lower yield of 1b was apparent
when chloride was used, whereas the bromide based binary
catalyst gave a similar result, i.e. quantitative conversion of
epoxide 1a could be attained (cf., entry 9).[50] The use of an
alternative solvent (acetone; entry 19) also gave productive
catalysis in line with our previous results on Zn-catalyzed
carbonate formation.[51] Further lowering the tannic acid TA
loading to 0.025 mol% and performing the catalysis in only 2.5 mL
of MEK still gave quantitative conversion (entry 20).[52]
Finally, a comparison was made between the tannic acid
based binary catalyst TA/NBu4I and our previous reported binary
couple pyrogallol/NBu4I (Table 1, entries 21-24). Since the tannic
acid structure is based on five (substituted) pyrogallol units,[53] the
comparison was thus made with the synthesis of carbonate 1b
mediated by 5 equiv of pyrogallol. After 18 h, a difference between
the yield of 1b promoted by tannic acid (entry 12: 79%) and
pyrogallol (5 equiv, entry 21: 62%) was observed, and when
reducing the reaction time to 6 h a much smaller difference in the
yield of 1b was noted (cf, entries 22 and 23; 35% versus 32%
yield). The tannic acid derived binary catalyst system (i.e.
TA/NBu4I) displayed under these conditions still an appreciably
high average TOF of 195 h-1. Then we decided to make a further
comparison between TA and various polyphenol based structures
including pyrogallol (PG), catechol (CC) and propyl-gallate (PGA),
see Table 2.
For the comparative studies we used a high-throughput
experimentation platform (AMTEC reactor, see Supporting
Information) and estimated the reactivity of the polyphenols under
similar reaction conditions (entries 1-11; reaction time 4 h).
Moreover, for completion, the conversion obtained in the absence
of the polyphenol additive was also examined (entry 12). In the
latter case very low conversion was noted (8%) and the
production of carbonate 1b noted under these conditions is thus
the effect of using the binary catalysts comprising of the
polyphenol structures. Comparisons were made between the
conversion/activity of the TA based catalyst (entry 1) and those
consisting of 1, 5 or 10 equiv of polyphenols PG, CC or PGA
(entries 311). From the data noted in Table 2 it is clear that the
TA based systems show favorable comparative reactivity
behavior with high molecular TONs and TOFs. It should be
mentioned that is difficult to use a correct reference system for TA
as PG and CC are electronically different from the pseudo PG
units within the TA structure, and PGA probably represents a
better electronic match. Furthermore, the TA structure contains a
significant amount of water (12% weight loss upon drying) [53] and
reported TON/TOF values in Table 2 are uncorrected. The
reactive polyphenol units within TA are non-randomly distributed
compared to the other investigated catalyst systems during
catalysis which likely reduces their accessibility. We hypothesize
that intramolecular H-bonding is in fact controlling the accessibility
of the polyphenol units, a phenomenon which cannot be (fully)
counterbalanced by the use of a moderately polar solvent such as
MEK. Since the reactions in MEK needed an increased reaction
temperature (80ºC) for full dissolution of both catalyst

components, it seems plausible to assume that this solvent
indeed is not able to break up intra/intermolecular H-bonding
between the separate polyphenol units at lower temperatures.
Despite these features, at very low loading of TA (0.03 mol%) the
relative reactivity seems to indicate that the high local
concentration of phenol groups provides some degree of synergy
leading to efficient catalysis behavior. Thus, the overall catalytic
effect should be taken into account rather than trying to
quantitatively correlate the findings in Table 2.
Table 2. Screening of various polyphenols in the synthesis of carbonate
1b.[a]

Entry

Phenol

Amount
[mol%]

Conv.
[%]

Yield
[%][b]

TON[c]

TOF[d]

1

TA

0.03

21

20

634

159

2

TA

0.15

47

44

295

74

3

PG

0.03

10

9

303

76

4

PG

0.15

32

30

200

50

5

PG

0.30

43

41

138

34

6

CC

0.03

15

14

461

115

7

CC

0.15

42

41

272

68

8

CC

0.30

51

48

158

40

9

PGA

0.03

11

10

344

86

10

PGA

0.15

46

44

290

72

11

PGA

0.30

65

64

218

54

12[e]



0

8

7





[a] General conditions: 1,2-epoxyhexane (4.15 mmol), p(CO2)0 = 10 bar,
NBu4I (2.0 mol%), 4 h, 80ºC, MEK (2.5 mL), AMTEC reactor. [b] Yield
determined by 1H NMR (CDCl3) using mesitylene as an internal standard.
Selectivity for the cyclic carbonate was >99%. [c] Total turnover number per
molecule of catalyst based on reported yields. [d] Average turnover frequency
per molecule of catalyst based on reported yields. [e] Only 2.0 mol% NBu4I
used.

Remarkably, upon comparing the reactivity of PG, CC and
PGA as catalyst additives (cf., entries 3-11), one can note the
lower efficiency of PG among the polyphenols studied. This result
contrasts our previous findings[45] where the catalytic efficiency of
PG proved to be markedly better than that observed for CC at
45ºC. Intrigued by this discrepancy, we decided to investigate the
long-term temperature effect on the catalytic performance of the
polyphenol additives in more detail by measuring the conversion

FULL PAPER
of 1,2-epoxyhexane 1a into carbonate 1b at 80ºC at various time
intervals (full data in the Supporting Information, Tables S1-S3).
First we compared the kinetic profiles of TA, PG, CC and PGA
during the first 6 h using equimolar amounts of polyphenol (0.03
mol%; see Figure 1). Interestingly, both triphenolic derivatives PG
and PGA show inferior catalytic behavior as the conversion
already seems to reach a plateau level after 4 h at this catalyst
loading, whereas the tannic acid TA and catechol CC still retain
good activity. These results seem to indicate some catalyst
degradation for both PG and PGA based binary catalysts under
the operative conditions.

further supports the view that the TA is a more stable catalyst
under these conditions and has a longer life-time compared to PG.
The more effective catalytic behavior of TA is probably the result
of a higher local concentration of phenol groups present in the
catalyst structure which likely does not induce strong
intermolecular effects. As previously reported by us,[45] in the
pyrogallol case (Scheme 1b) potential catalyst degradation may
occur via irreversible proton transfer of the phenol to the substrate
with the nucleophilic additive also being involved. This eventually
translates into lower catalytic efficiencies and higher reaction
temperatures in combination with very low catalyst loadings
(0.030.15 mol%) may quickly lead to unproductive catalysis
behavior and incomplete substrate conversion. It therefore seems
that tannic acid TA holds promise as a catalytic additive under
dilute conditions, whereas for the other polyphenols much higher
concentrations are required to maintain similar effective turnover.
To probe the hypothesis that indeed the disappearance of phenol
sites may be responsible for losing catalytic activity, we decided
to investigate the recyclability of the binary catalyst TA/NBu4I in
the conversion of 1,2-epoxy-dodecane 2a (Scheme 2) into
carbonate 2b at two different reaction temperatures (45 and 80ºC).

Figure 1. Comparison of the catalytic performance of polyphenol based binary
catalysts in the conversion of 1a into 1b. For reactions conditions: see Table 2.

Scheme 2. Recycling studies using 1,2-epoxy-dodecane 2a (6 mmol) as
substrate and TA/NBu4I (0.25-0.50 and 2.55.0 mol%, respectively) in MEK (15
mL) at p(CO2) = 10 bar.

Figure 2. Comparison of the catalytic performance of TA (0.03 mol%) and CC
(0.15 mol%) based binary catalysts in the conversion of 1a into 1b. For reactions
conditions: see Table 2.

In order to make the comparison more realistic we also
followed the performance of 0.03 mol% of TA and 0.15 mol% of
PG during 18 h (Figure 2). As can be noted, after about 5 h the
reactivity of the PG-based binary catalyst is drastically reduced
while the TA-based system still shows appreciable activity. This

At both reaction temperatures (see Supporting Information for
details), the catalyst was easily separated from the productcontaining hexane phase. The hexane solution was then
concentrated and showed virtually pure carbonate product 2b
indicating that no significant catalyst components were extracted.
Two solid catalyst fractions (FR1 and FR2, Scheme 2) could be
separated and were reused in a second catalyst cycle. The
catalyst recycled at 80ºC showed a significant drop in conversion
(8927%) whereas at 45ºC a similar though slightly reduced
conversion drop (5424%) was noted. The catalyst structure
after the second cycle was separated from the product/substrate
phase and subjected to 1H NMR, IR and TGA (thermogravimetric) analysis. The combined analytic data clearly showed
a loss of reactive phenol units likely caused by competing
reactions between the polyphenol and the substrate/nucleophile
(see for analytical data the Supporting Information), giving
halohydrin and NBu4-based TA salt byproducts which were
identified by 1H NMR as also previously reported for the
degradation of PG under forcing conditions.[46] The regeneration
of the TA structure was probed by treatment of the isolated solid
fraction from the recycling experiments under acidic conditions.
Treatment of the recycled material with concentrated HCl (37%)

FULL PAPER
regenerated the TA species as evidenced by 1H NMR and IR
analysis showing very high similarity to the spectroscopic footprint
of the commercial product. Thus, the acid treatment indicates the
possibility of catalyst regeneration (see the Supporting
Information). When the recycled binary catalyst was reused in the
synthesis of cyclic carbonate 2b (Scheme 2), an improved yield
of 51% (versus 24% for the untreated recovered TA) was
determined at 80ºC.
The mass balance for both TA (FR1, Scheme 2) and NBu4I
(FR2, Scheme 2) was then carefully checked. Whereas for FR1 a
virtually complete isolation of the original TA amount (24.6 versus
25.7 mg; 96%) was noted, a clear loss of the co-catalytic NBu4I
(FR2, 35.2 versus 56.8 mg; 62%) was apparent. Finally, we then
checked separately the activity of a regenerated TA sample with
a fresh amount of NBu4I in the synthesis of cyclic carbonate 1b
and compared the conversion with the original data using 0.03
mol% of TA at 80ºC for 18 h (79% conversion; see entry 12 Table
1). Fortunately, we found a comparable conversion (75%) for the
regenerated TA catalyst supporting the view that it can be easily
recycled upon acid treatment.
Table 3. Tannic acid mediated synthesis of carbonate 1b under various
conditions.[a]

Entry

TA
[mol%]

NBu4I
[mol%]

t
[h]

Conv.
[%]

Yield
[%][b]

TON[c]

TOF[d]

1

0.05

0.5

24

96

86

1721

72

2



0.5

24

36

32





3

0.05

0.5

66

100

100

1985

30

4



0.5

66

76

76





5

0.01

0.1

24

26

24

2220

92

6



0.1

24

18

16





7

0.01

0.1

66

53

45

4458

68

8



0.1

66

55

48





[a] General conditions: 1,2-epoxyhexane (10 mmol), p(CO2)0 = 10 bar, 80ºC,
MEK (5 mL), AMTEC reactor. [b] Yield determined by 1H NMR (CDCl3) using
mesitylene as an internal standard. Selectivity for the cyclic carbonate was
>99%. [c] Total turnover number per TA equivalent based on reported yields.
[d]
Average turnover frequency per TA equivalent based on reported yields.

We then further investigated the application of the binary
TA/NBu4I catalyst couple in the synthesis of carbonate 1b using
relatively low amounts of TA (Table 3, 0.010.05 mol%) combined
with 10 molar equiv of NBu4I (with respect to TA) as nucleophile.
At 0.05 mol% of TA (entry 1), carbonate 1b was produced in 86%
yield after 24 h with a high TON of 1721 and an average TOF/h
of 72. Notably, in the absence of TA (entry 2) carbonate 1b is
produced only in 32% yield. Increasing the total reaction time to
66 h (entries 3 and 4) reduces this difference as expected which
may also be an effect of partial catalyst degradation. When the

TA loading was further reduced to 0.01 mol% (entries 5 and 7)
the difference between the binary couple and the catalyst system
comprising of the iodide nucleophile alone becomes less
significant, despite the higher and valuable TONs observed (2220
after 24 h, 4458 after 66 h). Obviously the long-term stability plays
a key role to attain high average activity.
Next the substrate scope was investigated (see Figure 3)
using conditions that would allow for high conversion of the
epoxide substrates 1a16a at the reported temperature (80ºC)
and pressure (10 bar). All carbonate products 1b16b were
produced in a high-throughput reactor system at a 2 mmol
substrate scale. Most of the substrates were converted in high
conversion/selectivity with good to excellent isolated yields of up
to 94% (except for 6b; 57%). Note that under these reaction
conditions the synthesis of 1b was also probed in the absence of
tannic acid TA providing only a low yield of 22%. The binary
catalyst TA/NBu4I tolerates a number of functional groups
including alcohol, alkyl halide, heterocyclic ring systems,
carbamate, ether, alkene and alkyne groups. Of further note is
that the sterically more hindered substrate 9a could also be
converted (69%) with 40% isolated yield of carbonate 9b,
whereas the internal epoxide 10a gave, as expected, much poorer
results in line with the more challenging nature of this conversion
for organocatalytic catalyst systems.[43]

FULL PAPER
Figure 3. Substrate scope for the TA-mediated synthesis of organic carbonates
1b-16b. General conditions: 2 mmol of epoxide, 0.5 mol% TA, 5 mol% NBu4I,
80ºC, p(CO2)º = 10 bar, 18 h, MEK (5 mL), AMTEC reactor. Note that under
these conditions, the yield obtained for 1b was only 22% in the absence of TA.

The conversion of internal epoxides was recently computed to
be more energetically demanding (cf., higher kinetic barriers) as
compared to terminal epoxides.[54] Nonetheless, the formation of
all carbonates was mediated by only 0.5 mol% TA which is an
attractive feature within the context of providing a sustainable and
reactive alternative for metal-based carbonate formation
reactions.

Conclusions
In summary, we here present a novel binary catalyst system
based on a naturally occurring and fairly cheap polyphenol, i.e.
tannic acid, which shows excellent catalytic reactivity at
exceptionally low loadings being thus an attractive and
sustainable organocatalytic alternative. Comparative catalysis
studies have indicated that some degree of synergy between the
various poly(phenol) units within the TA structure may help to
increase catalyst lifetime, providing conceptually an interesting
approach to further improve the potential of polyphenol-based
organocatalysis in the area of CO2 conversion. Our future work
will be focusing on merging these concepts with the design of
organocatalyst systems with improved reactivity and stability for
similar type of CO2 conversions, with a preferable use of
hydrogen-bonding as a substrate activation strategy.

Experimental Section
General
Methylethyl ketone (MEK) and carbon dioxide (purchased from PRAXAIR)
were used as received without further purification or drying prior to use. All
phenolic compounds were commercially purchased from Sigma Aldrich
and used without any further purification; the tannic acid TA was reagent
grade, see also footnote 46. NMR spectra were recorded on a Bruker AV400 or AV-500 spectrometer and referenced to the residual deuterated
solvent signals. FT-IR measurements were carried out on a Bruker Optics
FTIR Alpha spectrometer equipped with a DTGS detector, KBr beam
splitter at 4 cm-1 resolution.
Standard autoclave screening experiments
All reactions were performed in a 30 ml stainless steel reactor. In a typical
experiment, a solution of tannic acid TA (0.050 mol%, 7.05 mg), NBu4I
(2.00 mol%, 61.3 mg), 1,2-epoxyhexane (8.30 mmol, 831 mg) and
mesitylene (1.00 mL, 7.18 mmol) in MEK (5 mL) was added to a stainless
steel reactor. Three cycles of pressurization and depressurization of the
reactor (with p(CO2) = 5 bar) were carried out before finally stabilizing the
pressure at 10 bar. The reactor was then heated to the required
temperature and left stirring for a further 18 hours. Then the reactor was
cooled down, depressurized and an aliquot of the solution was analyzed
by means of 1H NMR spectroscopy using CDCl3 as the solvent. The yield
was determined using mesitylene as the internal standard. In all cases,
selectivity for the cyclic carbonate products was determined to be >99%.
Substrate scope experiments

All reactions were performed in an SPR16 Slurry Phase Reactor (Amtec
GmbH). First, tannic acid TA (0.500 mol%, 17.0 mg) and NBu4I (5.00 mol%,
36.9 mg) were put into reactors. Then, the AMTEC reaction vessels were
tested for leaks charging with 1.5 MPa of N2 to finally reduce the pressure
to 0.2 MPa. After injecting into the reactors the chosen epoxide (example:
2.00 mmol, 200 mg in the case of 1,2-epoxyhexane) in MEK (5 mL) and
using mesitylene (10.0 mol%, 24.0 mg) as internal standard (IS), the
vessels were heated to the desired reaction temperature (T = 80 ºC). Once
reaching the operating temperature, the CO2 pressure was raised to 1 MPa
and the reaction mixture was stirred at the appropriate temperature for 18
h. At the end of the reaction analysis of the crude product was done as
reported above in the screening phase. Isolated yields and 1H/13C NMR
spectra and IR spectra of all products prepared this way (cf., carbonates
1b-16b) were obtained by removing the solvent and unreacted substrate
under vacuum (at 0.5 mbar). The residue was then dissolved in DCM
(except for the conversion of substrate 4a whose solvent was ethyl
acetate) and filtered through a path of silica; after removal of the solvent,
the pure cyclic carbonate products were then obtained. The identity of
each of the carbonate products was confirmed by comparison to previously
reported literature data, and full tabular data and copies of all spectra are
reported in the Supporting Information.
Recycling experiments
All reactions were performed in a 45 ml stainless steel reactor. In a typical
experiment, a solution of tannic acid TA (0.250 mol%, 25.5 mg), nBu4NI
(2.50 mol%, 55.4 mg) and 1,2-epoxy-dodecane (6.00 mmol, 1.25 g) in
MEK (15 mL) was added to a stainless steel reactor. Three cycles of
pressurization and depressurization of the reactor (with p(CO2) = 0.5 MPa)
were carried out before finally stabilizing the pressure at 1 MPa. The
reactor was then heated to the required temperature and left stirring for an
additional 18 h. Then the reactor was cooled down, depressurized and the
reaction mixture was separated from the precipitate (Solid 1; FR1) and
moved to a flask. The solvent was removed under vacuum and hexane (80
mL) was added. Then the hexane solution was cooled to 30ºC. After 3 h,
the flask was then warmed up to room temperature and the solution was
then filtered and the collected precipitate (Solid 2; FR2) was washed with
hexane (80 mL). The combined organic phases were then concentrated
under vacuum to get the pure cyclic carbonate. For a new reaction cycle,
solid 1 (FR1) was put into a stainless steel reactor and solid 2 (FR2) was
dissolved in MEK (15 mL) and moved to the same reactor. Then a new
portion of 1,2-epoxydodecane (6.00 mmol, 1.25 g) was added. Finally, the
procedure continues as indicated above. To regenerate the catalyst, FR1
was treated with concentrated HCl for 18 hours. Then the mixture was
filtered and washed with diethyl ether and hexane. The obtained
precipitate was then dried under vacuum and combined with FR2 in a new
catalytic cycle in MEK (15 mL) and transferred to a pressure reactor. Then
1,2-epoxydodecane (6.0 mmol, 1.25 g) was added. Finally the procedure
continues as described above.

Acknowledgements
C.M. gratefully thanks the Marie Curie COFUND Action from the
European Commission for co-financing a postdoctoral fellowship.
G.F. kindly acknowledges financial support from the European
Community through a Marie Curie Intra-European Fellowship
(FP7-PEOPLE-2013-IEF,
project
RENOVACARB,
Grant
Agreement no. 622587). Further acknowledgements go to ICIQ,
ICREA, and the Spanish Ministerio de Economía y
Competitividad (MINECO) through project CTQ2014-60419-R,
and support through Severo Ochoa Excellence Accreditation
2014-2018 (SEV-2013-0319). Dr. Marta Giménez and Cristina

FULL PAPER
Rivero are thanked for their help regarding the high-pressure
experiments.

[37]
[38]

Keywords: carbon dioxide • hydrogen bonding • organic
carbonates • synergy • tannic acid • organocatalysis
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]

Carbon Dioxide as Chemical Feedstock (Ed.: M. Aresta), Wiley-VCH,
Weinheim, 2010.
M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann, F. E. Kühn,
Angew. Chem. Int. Ed. 2011, 50, 8510.
M. Peters, B. Köhler, W. Kuckshinrichs, W. Leitner, P. Markewitz, T. E.
Müller, ChemSusChem 2011, 4, 1216.
G. Fiorani, W. Guo, A. W. Kleij, Green Chem. 2015, 17, 1375.
M. Mikkelsen, M. Jørgensen, F. C. Krebs, Energy Environ. Sci. 2010, 3,
43.
D. R. Darensbourg, Chem. Rev. 2007, 107, 2388.
M. North, R. Pasquale, C. Young, Green Chem. 2010, 12, 1514.
P. P. Pescarmona, M. Taherimehr, Catal. Sci. Technol. 2012, 2, 2169.
N. Kielland, C. J. Whiteoak, A. W. Kleij, Adv. Synth. Catal. 2013, 355,
2115.
O. Jacquet, X. Frogneux, C. Das Neves Gomes, T. Cantat, Chem. Sci.
2013, 4, 2127.
H. Mizuno, J. Takaya, N. Iwasawa, J. Am. Chem. Soc. 2011, 133, 1251.
M. D. Greenhalgh, S. P. Thomas, J. Am. Chem. Soc. 2012, 134, 11900.
T. León, A. Correa, R. Martin, J. Am. Chem. Soc. 2013, 135, 1221.
Y. Li, I. Sorribes, T. Yan, K. Junge, M. Beller, Angew. Chem. Int. Ed.
2013, 52, 12156.
C. Martín, G. Fiorani, A. W. Kleij, ACS Catal. 2015, 5, 1353.
B. Schäffner, F. Schäffner, S. P. Verevkin, A. Börner, Chem. Rev. 2010,
110, 4554.
J. H. Clements, Ind. Eng. Chem. Res. 2003, 42, 663.
See for example: D. J. Darensbourg, A. I. Moncada, W. Choi, J. H.
Reibenspies, J. Am. Chem. Soc. 2008, 130, 6523.
C. J. Whiteoak, N. Kielland, V. Laserna, E. C. Escudero-Adán, E. Martin,
A. W. Kleij, J. Am. Chem. Soc. 2013, 135, 1228.
T. Ema, Y. Miyazaki, S. Koyama, Y. Yano, T. Sakai, Chem. Commun.
2012, 48, 4489.
J. Meléndez, M. North, P. Villuendas, Chem. Commun. 2009, 2577.
J. Song, Z. Zhang, S. Hu, T. Wu, T. Jiang, B. Han, Green Chem. 2009,
11, 1031.
J. Sun, S. Zhang, W. Cheng, J. Ren, Tetrahedron Lett. 2008, 49, 3588.
Z.-Z. Yang, Y.-N. Zhao, L.-N. He, J. Gao, Z.-S. Yin, Green Chem. 2012,
14, 519.
Z. Z. Yang, L.-N. He, C.-X. Miao, S. Chanfreau, Adv. Synth. Catal. 2010,
352, 2233.
J. L. Song, Z. F. Zhang, B. X. Han, S. Q. Hu, W. J. Li, Y. Xie, Green
Chem. 2008, 10, 1337.
S. G. Liang, H. Z. Liu, T. Jiang, J. L. Song, G. Y. Yang, B. X. Han, Chem.
Commun. 2011, 47, 2131.
C. Qi, J. Ye, W. Zeng, H. Jiang, Adv. Synth. Catal. 2010, 352, 1925.
J. Sun, W. G. Cheng, Z. F. Yang, J. Q. Wang, T. T. Xu, J. Y. Xin, S. J.
Zhang, Green Chem. 2014, 16, 3071.
J. Sun, L. J. Han, W. G Cheng, J. Q. Wang, X. P. Zhang, S. J. Zhang,
ChemSusChem 2011, 4, 502.
J. Q. Wang, J. Sun, W. G. Cheng, K. Dong, X. P. Zhang, S. J. Zhang,
Phys. Chem. Chem. Phys. 2012, 14, 11021.
J. Q. Wang, D. L. Kong, J. Y. Chen, F. Cai, L.-N. He, J. Mol. Catal. A:
Chem. 2006, 249, 143.
Z.-Z. Yang, Y.-N. Zhao, L.-N. He, J. Gao, Z.-S. Yin, Green Chem. 2012,
14, 519.
Z.-Z. Yang, L.-N. He, C.-X. Miao, S. Chanfreau, Adv. Synth. Catal. 2010,
352, 2233.
V. Caló, A. Nacci, A. Monopoli, A. Fanizzi, Org. Lett. 2002, 4, 2561.
X. Chen, J. Sun, J. Q. Wang, W. G. Cheng, Tetrahedron Lett. 2012, 53,
2684.

[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]

[48]
[49]

[50]

[51]
[52]

[53]

[54]

T. Werner, H. Buettner ChemSusChem 2014, 7, 3268. See also: C. Kohrt,
T. Werner, ChemSusChem 2015, 8, 2031.
T. Werner N. Tenhumberg, H. Buettner ChemCatChem 2014, 6, 34933500.
Y. Tsutsumi, K. Yamakawa, M. Yoshida, T. Ema, T. Sakai, Org. Lett.
2010, 12, 5728.
R. A. Watile, K. M. Deshmukh, K. P. Dhake, B. M. Bhanage, Catal. Sci.
Technol. 2012, 2, 1051.
Y. M. Shen, W. L. Duan, M. Shi, Adv. Syn. Catal. 2003, 345, 337.
B. Chatelet, L. Joucla, J.-P. Dutasta, A. Martinez, K. C. Szeto, V. Dufaud ,
J. Am. Chem. Soc. 2013, 135, 5348.
B. Chatelet, L. Joucla, J.-P. Dutasta, A. Martinez, V. Dufaud, Chem. Eur.
J. 2014, 20, 8571.
M. E. Wilhelm, M. H. Anthofer, M. Cokoja, I. I. E. Markovits, W. A.
Herrmann, F. E. Kühn, ChemSusChem, 2014, 7, 1357.
C. J. Whiteoak, A. Nova, F. Maseras, A. W. Kleij, ChemSusChem 2012,
5, 2032.
C. J. Whiteoak, A. H. Henseler, C. Ayats, A. W. Kleij, M. A. Pericàs,
Green Chem. 2014, 16, 1552.
Quite recently, Mattson et al. reported on interesting and highly active
silanediol catalysts for organic carbonate formation, see: A. M. HardmanBaldwin, A. E. Mattson, ChemSusChem 2014, 7, 3275.
S. Gennen, M. Alves, R. Méreau, T. Tassaing, B. Gilbert, C. Detrembleur,
C. Jerome, B. Grignard, ChemSusChem 2015, 8, 1845.
MEK was previously used by us and others as it is a good medium for
CO2 dissolution: A. Decortes, M. Martínez Belmonte, J. Benet-Buchholz,
A. W. Kleij, Chem. Commun. 2010, 46, 4580.
Compared to entry 5 of Table 1, the background reaction in the presence
of 2 mol% of NBu4Cl (44%) or 2 mol% NBu4Br (60%) show similar or
improved conversion rates. However, in the presence of TA the chloride
based binary catalyst (cf., entry 18) is somewhat less effective whereas
the bromide/iodide based ones (entries 9 and 17) give both quantitative
conversions. Since our previous studies were done with NBu4I as
nucleophilic additive, for comparative reasons we continued with this cocatalytic mediator.
A. Decortes, A. W. Kleij, ChemCatChem 2011, 3, 831.
Possible product inhibition was also probed using a 1:1 mixture of the
cyclic carbonate 1b and its precursor 1a taking the reaction in the
absence of the product as a reference. In both cases a similar conversion
was noted at the end of the reaction indicating no (significant) product
inhibition (Supporting Information for details).
Commercially available tannic acid is usually a combination of polygalloyl
glucoses or polygalloyl quinic acid esters with the number of galloyl
moieties per molecule varying depending on the plant source. Here we
have used the commercial product from Aldrich (ACS reagent, 500 g =
77.50 Euro) which contains about 12% of weight loss upon drying.
Further to this, considering the general structure of tannic acid that
contains five pseudo pyrogallol units, we have thus compared the
catalytic performance of tannic acid (0.03 mol%; 1 equiv) with that of
pyrogallol (0.15 mol%; 5 equiv).
F. Castro-Gómez, G. Salassa, A. W. Kleij, C. Bo, Chem. Eur. J. 2013, 19,
6289.

FULL PAPER
FULL PAPER
A highly efficient binary catalyst
system is reported consisting of
naturally occurring polyphenol, tannic
acid, and a suitable nucleophile
additive. This two component catalyst
is highly active towards the formation
of organic carbonates at very low
loadings of the polyphenol with
maximum TOFs exceeding 200 h-1.
The high reactivity of the tannic acid is
ascribed to the high local concentration of polyphenol units, showing
some degree of synergy and improved
catalyst stability compared to pyrogallol.

Sergio Sopeña, Giulia Fiorani, Carmen
Martín and Arjan W. Kleij*

Page No. – Page No.

Highly Efficient Organocatalyzed
Conversion of Oxiranes and CO2 into
Organic Carbonates