Merging sustainability with organocatalysis in the formation of organic carbonates by using CO2 as a feedstock

Article abstract of DOI:10.1002/cssc.201200255

The use of phenolic compounds as organocatalysts is discussed in the context of the atom-efficient cycloaddition of carbon dioxide to epoxides, forming useful cyclic organic carbonate products. The presence and cooperative nature of adjacent phenolic groups in the catalyst structure results in significantly enhanced catalytic efficiencies, allowing these CO₂ fixation reactions to operate efficiently under virtually ambient conditions. The cooperative effect has also been studied by computational methods. Furthermore, when the cycloaddition reactions are carried out on a larger scale and under solvent-free conditions, further enhancements in activity are observed, combined with the advantageous requirement of reduced loadings of the binary organocatalyst system. The reported system is among one of the mildest and most effective metal-free catalysts for this conversion and contributes to a much more sustainable development of organic carbonate production; this feature has not been the main focus of previous contributions in this area. In a fix: A new organocatalytic method for organic carbonate synthesis is reported and allows attractive conditions (25 °C, 10 bar, no solvent) to be used (see picture). Cyclic carbonates produced from CO₂ and epoxides are isolated in high yields. The mild nature of this process increases the overall process sustainability for this type of widely studied carbon dioxide fixation process.

Full text of DOI:10.1002/cssc.201200255

DOI: 10.1002/cssc.201200255
Merging Sustainability with Organocatalysis in the
Formation of Organic Carbonates by Using CO2 as
a Feedstock
Christopher J. Whiteoak,[a] Ainara Nova,[a] Feliu Maseras,*[a, b] and Arjan W. Kleij*[a, c]
The use of phenolic compounds as organocatalysts is dis-
cussed in the context of the atom-efficient cycloaddition of
carbon dioxide to epoxides, forming useful cyclic organic car-
bonate products. The presence and cooperative nature of adja-
cent phenolic groups in the catalyst structure results in signifi-
cantly enhanced catalytic efficiencies, allowing these CO2 fixa-
tion reactions to operate efficiently under virtually ambient
conditions. The cooperative effect has also been studied by
computational methods. Furthermore, when the cycloaddition
reactions are carried out on a larger scale and under solvent-
free conditions, further enhancements in activity are observed,
combined with the advantageous requirement of reduced
loadings of the binary organocatalyst system. The reported
system is among one of the mildest and most effective metal-
free catalysts for this conversion and contributes to a much
more sustainable development of organic carbonate produc-
tion; this feature has not been the main focus of previous con-
tributions in this area.
Introduction
Carbon dioxide fixation, which is universally defined as conver-
sion of this greenhouse gas into value-added (in)organic mate-
rials, is presently receiving enormous and increasing attention
from the chemical community.[1–6] Current emissions of carbon
dioxide into the earth’s atmosphere have been raising several
environmental and societal concerns connected with its effect
on global warming. On the other hand, carbon dioxide can
also be regarded as a potentially unlimited carbon feedstock
that may provide an alternative for the chemical industry be-
cause our current carbon-based feedstocks from fossil fuels are
predicted to be depleted in decades to come.[7] Carbon dioxide
with sufficient purity for use as a reagent is currently available
as a byproduct from many industrial processes, for example,
the synthesis of ammonia and fermentation processes. Exam-
ples of successful (industrial) products that can be obtained
from carbon dioxide include organic (poly)carbonates,[8,9]
methanol,[10] and urea.[11] Since carbon dioxide represents a ki-
netically and thermodynamically stable end product of all com-
bustion processes based on organic matter, its conversion into
more complicated (although useful) products involves a huge
energy barrier. To reduce the kinetic hurdle, catalysis has been
demonstrated to be a key technology that allows conversion
of CO2 under much milder conditions.[12] Another aspect that
may add to an overall improved process sustainability is the
use of green media and reagents.[13] In this context, organoca-
talysis has proven to be an innovative area in catalysis science
that avoids the utilization of metal-containing species.[14–18]
Organocatalysis is attracting increased attention as a result of
increasingly demanding impurity specifications, which require
lower amounts of metal residues to be present in end prod-
ucts, and thus, organocatalysis can be regarded as a highly at-
tractive method for catalytic CO2 conversion.
The formation of five- and six-membered organic carbonates
is a rapidly developing area of research and the most elegant
way of producing these carbonates is through an atom-effi-
cient cycloaddition of CO2 to terminal or multiply substituted
epoxides in the presence of a suitable catalyst.[19,20] While (tran-
sition) metal catalysis is able to mediate this conversion with
different degrees of success,[21–26] organocatalyzed procedures
are limited to very few examples, probably as a result of signifi-
cantly reduced efficiency for substrate activation potential
under mild reaction conditions compared with strong Lewis
acidic metal catalysts.[27–33] Work by Shi and co-workers has
shown that phenolic compounds in combination with organic
bases provide a two-component catalyst system that can be
used to catalyze cyclic organic carbonate formation from epox-
ides and CO2, albeit under elevated reaction conditions (T=
100–1208C, pCO =35 bar, 48 h) as a result of the low efficiency
2
of this catalytic system.[34] These conditions make this organo-
catalytic protocol far from ideal upon consideration of the
overall sustainability. Another drawback of this system is that
optimization of the catalytic efficiency is limited because more
acidic phenols are involved in proton transfer to the cocatalyst
[a] Dr. C. J. Whiteoak, Dr. A. Nova, Prof. Dr. F. Maseras, Prof. Dr. A. W. Kleij
Institute of Chemical Research of Catalonia (ICIQ)
Av. Paꢀsos Catalans 16, 43007 Tarragona (Spain)
Fax: (+34)977-920-828
E-mail: akleij@iciq.es
[b] Prof. Dr. F. Maseras
Departament de Quꢁmica, Universitat Autꢂnoma de Barcelona
08193 Bellaterra (Spain)
[c] Prof. Dr. A. W. Kleij
Catalan Institute of Research and Advanced Studies (ICREA)
Pg. Lluꢁs Companys 23, 08010 Barcelona (Spain)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200255.
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F. Maseras, A. W. Kleij et al.
[which in this example contains an organic base, that
is, 4-dimethylaminopyridine (DMAP)], thereby inhibit-
ing turnover because the mode of activation of
the epoxide substrate is comprised of hydrogen
bonding.
Other organocatalysts have also been reported for
the formation of cyclic organic carbonates and relat-
ed products, including ionic-liquid-based sys-
tems[35–37] and simple ammonium halides;[38] however,
in the majority of these cases, the overall sustainabili-
ty is significantly compromised by high reaction tem-
peratures, high pressures, high catalyst loadings, or
a combination thereof. This not only negatively influ-
ences the energy balance of the process, but also
leads to a relatively large net output of carbon diox-
ide, and hence, an undesirable influence on the
carbon cycle. With respect to recycling of the catalyst
system, lower reaction temperatures are preferred
particularly in those cases where ammonium halides
are employed. Reaction temperatures that are too
high may lead to fast(er) cocatalyst deactivation by
Hofmann elimination; a feature that has been recent-
ly reported in detail by North and Pasquale.[39] There-
fore, with the aforementioned concerns in mind, we
set out to explore new (binary) organocatalysts that
are able to convert CO2 and epoxides under highly
mild reaction conditions.
Herein, we report on the development of a new,
binary catalyst system comprising of commercially
available (substituted) phenolic compounds com-
bined with nBu4NI as a cocatalyst required for the ep-
oxide ring-opening step. These multiphenolic sys-
tems have an interesting synergistic effect, resulting
in increased hydrogen-bonding abilities towards O-donor sys-
tems (epoxides).[40] The strong intramolecular hydrogen-bond
patterns lead to large stabilization effects and different pKa
values that are potentially of use in catalytic processes.[41]
Furthermore, the presence of a quaternary ammonium iodide
as the cocatalyst allows more acidic phenols to be used,
higher reactivity to be attained, and hence, significantly milder
reaction conditions (=lower energy input, higher catalyst sta-
bility) to be used, marking this procedure as a highly attractive,
sustainable, and cost-efficient catalytic system.
Figure 1. Organocatalyst structures 1–23.
screening conditions, the nBu4NI cocatalyst displays no activity
(Table 1, entry 1), and therefore, all activities observed are
a consequence of the binary-catalyst system rather than the
use of only nBu4NI.
Initially, a series of phenols with variation of the para-sub-
situent (1–4) were tested as catalysts; a significant increase in
product yield was found for the more electron-deficient
system 4 compared with the parent phenol 1.[42] Following
this, a series of catechols (1,2-dihydroxybenzenes, 5–9) were
then examined.
Interestingly, in comparison with 1, an unexpected and re-
markable fivefold increase in product yield was observed
(75%, in the case of 5), which is significantly higher than the
sum of the activity of two independent phenol units (i.e.,
30%). In contrast with the series 1–4, a decrease in product
yield is evident for catechols containing electron-withdrawing
substituents, 7 and 8; this is ascribed to a rearrangement
effect that results in significant population of the quinone
form of these catechols, and thus, reduces the presence of
phenolic groups. Note that the presence of an additional but
distant OH group (cf., 9, 74%) does not appreciably increase
the yield in this reaction when compared with 5 (74%).
A similar synergistic effect to that observed with the cate-
chols was also noted within the series of naphthols 13–16. The
Results and Discussion
The screening of various phenolic organocatalyst systems
(Figure 1, 1–23) using nBu4NI as the cocatalyst, was carried out
with 1,2-epoxyhexane as a benchmark substrate, methylethyl
ketone (MEK) as a relatively green and good CO2-solubilizing
solvent.[26] In a typical experiment, a stainless-steel autoclave
was charged with 1,2-epoxyhexane, nBu4NI cocatalyst, MEK,
and mesitylene (internal standard) followed by three cycles of
pCO at 5 bar with intermediate release of the pressure and final
2
stabilization to pCO of 10 bar. The conversions were obtained
2
1
by H NMR spectroscopy using mesitylene as the internal stan-
dard for the screening are shown in Table 1. Under these
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Formation of Organic Carbonates
substrate conversion. Screening results obtained when using
the easily prepared “bis-catechol” system 20 proved to be less
effective; this can be rationalized by competitive intramolecu-
lar OH···N=CH interactions, which involve the 2-positioned phe-
nolic groups. This effectively reduces the hydrogen-bonding
ability towards substrates, leading to lower catalytic activities.
In relation to this, “bis-pyrogallol” 23 showed activity lower
than that observed for 22 for the same reason.
To ascertain whether the mode of activation was more gen-
eral, a thiourea (19) was also screened as a potential catalyst
because thioureas have been applied with prevalent success in
organocatalysis due to their ability to hydrogen bond to, and
thus activate, various substrate molecules.[43] We therefore
tested 19 as a catalyst in the reaction shown in Scheme 1.
However, the low product yield (12%) again underlines the
unique ability of phenolic groups to activate the epoxide in
this reaction.
Table 1. Screening of catalysts 1–23 when using 1,2-epoxyhexane as
a substrate.[a]
Catalyst
T
[8C]
pCO
[bar]
Yield
[%][b]
2
–
1
1
1
2
3
4
5
5
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
22
22
22
22
23
45
45
25
45
45
45
45
25
45
45
45
45
45
45
45
45
45
45
45
45
45
45
45
45
45
45
25
45
45
45
45
45
10
10
10
10
10
10
10
10
5
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
2
0[c]
0[d]
3
15
12
27
75
39
18
75
91
73
60
74
14
65
23
70
24
36
23
9
0
12
8
30
63
54
72
0[d]
100 (90)[e]
74 (70)[e]
To further investigate the wider applicability of the newly
developed binary-catalyst system based on 22, and the possi-
bility of applying even milder reaction conditions (258C), the
substrate scope for compound 22 was examined (Table 2). As
clearly observed, compound 22 is an effective catalyst for
Table 2. Synthesis of 4-substituted 1,3-dioxolan-2-ones when using cata-
lyst 22 under optimized conditions.[a]
Substrate
Yield[b] [%]
258C
Yield[c]
[%]
5
458C
100
100
100
54
10
10
10
63
63
42
27
83
93
91
52
[a] Conditions: 1,2-epoxyhexane (0.002 mmol), 458C, pCO =10 bar,
2
5 mol% catalyst/nBu4NI cocatalyst, MEK (5 mL), 18 h. [b] Reported yields
1
are based on H NMR spectroscopic analysis using mesitylene as an inter-
nal standard. [c] With only nBu4NI (5 mol%). [d] With only the catalyst
(5.0 mol%). [e] In brackets, isolated yields after extraction of the product
from the catalyst system; selectivity ꢀ99% for the carbonate product.
64
81
64
45
48
56
100
100
83
94
88
62
80
94
85
parent compound 13, with two adjacent phenol units, showed
the highest activity (70% yield, close to that of 5, 75%), where-
as 14–16 gave poorer results. Control experiments with diphe-
nol compounds 10–12 further confirmed that the presence of
two neighboring OH fragments was beneficial for catalytic ac-
tivity, and 11 showed, in line with this structural prerequisite,
comparable activity as that observed for 5 and 13. When the
OH groups were replaced by NH2 (17) or SH (18), only very low
amounts or no product was observed, indicating the specific
requirement for phenolic groups. Remarkably, a further in-
crease in product yield could be realized by applying pyrogal-
lol (22), which, under our screening conditions, resulted in
quantitative conversion of the epoxide into its respective cyclic
organic carbonate product. Again, the presence of adjacent
phenolic fragments seems to be crucial for high activity be-
cause isomeric 21 (phloroglucinol) showed a markedly lower
93
96
88
45
92
90
51
22
50
93
63
99
81
51
89
[a] Conditions: epoxide (0.002 mmol), pCO =10 bar, 5 mol% catalyst/
2
nBu4NI cocatalyst, MEK (5 mL), 18 h. [b] Reported yields for cyclic organic
carbonate products based on 1H NMR spectroscopic analysis using mesi-
tylene as an internal standard. [c] Isolated yields for reactions at 458C; se-
lectivity ꢀ99% for the carbonate product.
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F. Maseras, A. W. Kleij et al.
the opportunity for full conver-
sion of these epoxides under
neat conditions unless high tem-
peratures required for “catalysis
in the melt” are applied, and
therefore, the use of a solvent
(MEK) can be beneficial in these
cases.
Pure isolated organic cyclic
carbonate products could be
easily obtained by simple sol-
vent extraction from the crude
reaction residue. After removal
of the solvent under vacuum,
the cyclic organic carbonate was
extracted by using toluene/hep-
tane (3:1 v/v)[44] and 1H and
13C NMR spectra for all isolated
compounds were obtained (see
the Supporting Information), and
additionally IR spectroscopy fur-
ther confirmed the formation of
these carbonate structures.
The residue remaining after
extraction of the cyclic organic
carbonate product contained
the
original
binary-catalyst
1
system, as supported by H NMR
spectroscopy. To gain insight
into the stability of the catalyst
system, the residue was then
reused as a catalyst in a second
run. It was found that indeed
this residue was catalytically
active, but resulted in a some-
what lower isolated yield (78%)
than that observed in the first
catalytic run (90%). Therefore,
the 22/nBu4NI binary catalyst
system may show potential as
a recyclable catalyst if appropri-
ately supported.
Scheme 1. Gibbs energy profiles for carbonate formation catalyzed by a) iodide (in black) and b) iodide plus 22
(X=Y=OH, in blue), catechol (X=OH, Y=H, in green), or phenol (X=Y=H, in red).
After obtaining successful re-
sults when using the 22/nBu4NI
a number of substrates (Table 2), giving high to quantitative
yields of cyclic organic carbonate products at 458C and result-
ing in comparable activities (i.e., yields in the range 60–100%,
and selectivities of more than 99%) to those reported for
metal-based catalytic systems, for example, the ZnII–salphen
catalyst system under comparable conditions (i.e., MEK as the
binary-catalyst system under our standard screening condi-
tions, we further examined whether the catalyst system could
be employed on a larger scale. Reactions were performed at
25 and 458C in neat propylene oxide (1 g, 17 mmol scale, com-
pared with 2 mmol in the screening experiments) by using a re-
duced loading of only 2.0 mol% of the binary-catalyst system.
After 18 h, the isolated yield of propylene carbonate was very
high (93–96%, Table 3), suggesting that quantitative conver-
sion of the epoxide had taken place. When the reaction in
neat propylene oxide was performed with only 22 or nNBu4NI
(2 mol%) for 18 h, conversions of 0 and 26% were obtained,
respectively, which are significantly lower than the isolated
yields obtained for the binary-catalyst system. Shorter reaction
solvent, 458C, pCO =10 bar, 18 h reaction time).[22,23] Upon low-
2
ering the temperature to 258C, an expected decrease in yield
was observed, although reasonable levels of product formation
were still achieved. It should be noted that during the scoping
experiments we were able to obtain high yields for products
containing CH2OCH2Ph and CH2O-4-tBuPh groups; the solid
nature of these cyclic organic carbonate products usually limits
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Formation of Organic Carbonates
take place simultaneously in TS1, with a free energy of
38.9 kcalmolÀ1 (1 cal=4.2 J). Afterwards, the transition state for
the carbonate ring-closure step (TS2) has an energy of
30.8 kcalmolÀ1 above the separate reactants. The rate-limiting
step is thus marked by TS1 and the barrier of 38.9 kcalmolÀ1 is
consistent with a sluggish reaction under these experimental
conditions.
Table 3. Synthesis of propylene carbonate when using the binary-catalyst
22/nNBu4NI system in neat propylene oxide.[a]
T
t
22
nBu4NI
Yield
[%][b]
[8C]
[h]
[mol%]
[mol%]
This mechanism changes when a phenol derivative is intro-
duced as the catalyst in addition to iodide (Scheme 1b). A new
intermediate I1X,Y appears, resulting from ring opening of the
epoxide upon interaction with the two catalysts. The largest
effect is observed for 22 (blue profile in Scheme 1b), which is
discussed herein in some detail. In this case, one proton from
the central hydroxyl group of 22 is transferred to the alkoxylic
center, resulting from the ring opening of the epoxide. The OH
distances for this O···HÀO fragment are 1.50 and 1.03 ꢁ in
I1OH,OH; this shows a strong hydrogen-bonding interaction. The
newly formed OH group is stabilized by another OH group of
22, while the remaining OH group stabilizes the oxygen atom
of the phenoxide. Intermediate I1OH,OH is formed from the reac-
tants through TS1OH,OH, with a relative energy of 27.3 kcal
45
45
45
25
45
45
45
45
25
18
18
18
18
1
2
3
4
6
2.0
–
–
0
26
93
96
43
65
78
84
69
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
[a] Conditions: propylene oxide (17 mmol), pCO =10 bar (at t=0 h),
2
2.0 mol% catalyst/nBu4NI cocatalyst, no co-solvent. [b] Isolated yields; se-
lectivity ꢀ99% for the carbonate product.
times were then investigated and the pressure drop in the re-
actor was monitored over time (Figure 2). At 458C, after only
4 h the isolated yield of product was marginally lower (84%)
than that obtained after 18 h (93%). At 258C, as expected,
after 6 h the isolated yield of the cyclic carbonate was lower
(69%). Isolated yields obtained after 1, 2, and 3 h at 458C were
molÀ1. Afterwards, CO2 addition takes place through TS2OH,OH
,
with a relative energy of 29.7 kcalmolÀ1. Catalyst 22 then dis-
sociates from the reacting system, which reverts to I1 and
evolves to products through TS2. All attempts to keep 22 as-
sociated to the system after TS2OH,OH resulted in species such
as I2OH,OH (Scheme 2) with higher free energies than the corre-
sponding ones in Scheme 1. The key result from the full
energy profile is that the highest energy point is TS2OH,OH, with
an energy of 29.7 kcalmolÀ1 above the separate reactants. This
is much lower than 37.7 kcalmolÀ1 required for the mechanism
without 22 and explains the catalytic role of this molecule. It is
also worth noting that the main role of 22 is the stabilization
of intermediate I1OH,OH, and the two transition states connect-
ed to it, but it does not assist the ring-closure step.
As shown in Scheme 2, the three hydroxyl groups of 22 par-
ticipate in the stabilization of I1OH,OH. Thus, replacement by hy-
drogen of one or two of them (that is, replacing 22 with cate-
chol or phenol) should have consequences on the rate of the
reaction. This is indeed the trend observed in the profiles in
Figure 2. Pressure development over time at 25 and 458C for the conversion
of propylene oxide into its carbonate. See Table 3 for details and isolated
yields.
also obtained (Table 3) and followed a reverse trend of CO2
uptake observed during the reaction (Figure 2). These results
indicate that under neat conditions the 22/nBu4NI binary-cata-
lyst system is more active than when used with a co-solvent
and marks this system as a rare example of an organocatalyst
system that is active under mild conditions for this cycloaddi-
tion reaction.
The mechanism for carbonate formation in MEK was studied
computationally by DFT (B3LYP) calculations. Preliminary calcu-
lations (see the Supporting Information) indicated no involve-
ment of the R4N+ cation, which was consequently neglected
from the most elaborate calculations reported herein. With
iodide as the only catalyst (Scheme 1a, energy profile in black),
nucleophilic addition, epoxide ring opening, and CO2 addition
Scheme 2. Proposed catalytic cycle when using binary catalyst 22/nBu4NI.
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F. Maseras, A. W. Kleij et al.
lyzed by means of 1H NMR spectroscopy using [D6]DMSO as the
solvent and the yield was determined by using mesitylene as the
Scheme 1, the energies associated to the highest energy
points TS2OH,H and TS2H,H are 32.9 and 37.7 kcalmolÀ1, respec-
tively. This is fully consistent with the yields obtained with cat-
alysts 1, 5, and 22 at 458C and 10 bar of CO2 of 15, 75, and
100% respectively, as shown in Table 1. The high computed
barriers for all processes are also consistent with the require-
ment of high CO2 pressure and more than one equivalent of
catalyst and cocatalyst. These computational results show that
the key role of the OH groups in 22 is in stabilizing intermedi-
ates and transition states by means of inter- and intramolecu-
lar hydrogen bonds,[40,41] and provide a mechanistic picture
that explains all of the experimental observations.
1
internal standard. Isolated yields and H/13C{1H} NMR spectra of all
products synthesized at 458C are reported in Table 2 and were ob-
tained by removing the solvent, mesitylene, and unreacted sub-
strate under vacuum. The residue was then dissolved in toluene
(10 mL) and heptane was added until a precipitate formed. The
mixture was then filtered and the solvent was removed from the
filtrate to yield the pure, substituted, organic cyclic carbonate.
4-(Hydroxymethyl)-1,3-dioxolan-2-one required a slightly different
extraction from the catalytic system due to its polarity: after the in-
itial vacuum step, the residue was dissolved in chloroform (20 mL)
and heptane (4 mL) was added. The mixture was then filtered and
the solvent was removed under vacuum. Acetone (0.5 mL) and di-
ethyl ether (25 mL) were added to this residue. After 3 h at À308C,
the mixture was filtered and the solvent was removed from the fil-
trate to yield pure 4-(hydroxymethyl)-1,3-dioxolan-2-one (24g). The
identity of each of the organic cyclic carbonate products was con-
firmed by comparison to the literature values previously reported
(see also the Supporting Information).
Conclusions
Catechol and/or pyrogallol (22) scaffolds are excellent organo-
catalysts for the preparation of various cyclic organic carbo-
nates from epoxides and carbon dioxide as starting materials.
The occurrence of synergistic effects translates into a catalytic
system with a favorable energy balance and allows carbon di-
oxide fixation into useful organic matter under very mild reac-
tion conditions. The binary organocatalyst system 22/nBu4NI is
a powerful catalyst with ample substrate scope under very
Typical NMR spectroscopy data for organic cyclic carbonate prod-
uct: 4-Methyl-1,3-dioxolan-2-one: Yield: 83%; 1H NMR (CDCl3,
3
500 MHz): d=4.90–4.82 (m, 1H; OCH2CHO), 4.56 (dd, J(H,H)=8.4,
7.8 Hz, 1H; OCH2CHO), 4.03 (dd, 3J(H,H)=8.4, 7.2 Hz, 1H;
3
OCH2CHO), 1.50 ppm (d, J(H,H)=6.3 Hz, 3H; CHCH3); 13C{1H} NMR
mild reaction conditions (25–458C, pCO =10 bar, 2–5 mol% cat-
2
(CDCl3, 125 MHz): d=155.1 (C=O), 73.6, 70.7, 19.4 ppm. For com-
plete data for all other products from Table 2, see the Supporting
Information.
alyst, and no need for a solvent) compared with the current
state of the art. It therefore represents a new attractive orga-
nocatalytic protocol that can be operated under conditions
that are desirable from an energy and CO2-emission point of
view, adding up to a higher degree of sustainability. Further-
more, this work also demonstrates that commercially available
22 may hold promise as a catalyst that can also activate other
O- and N-containing substrates through hydrogen bonding.
Currently we are designing other types of polyphenolic struc-
tures useful as hydrogen-bond activators in the context of CO2
conversion.
Synthesis and characterization of compound 23: A solution of
(R,R)-1,2-diphenyl-ethanediamine (117.4 mg, 0.553 mmol) in iPrOH
(20 mL) was combined with 2,3,4-trishydroxybenzaldehyde
(232.0 mg, 1.51 mmol) and briefly heated to reflux. After cooling to
ambient temperature, a yellow solution was obtained that upon
further cooling precipitated a yellow solid. Filtration afforded 23,
which was dried in air and obtained as a yellow powder (248.7 mg,
1
74%, corrected for iPrOH inclusion). H NMR ([D6]DMSO, 500 MHz):
d=13.73 (brs, 2H; OH), 9.45 (brs, 2H; OH), 8.35 (brs, 2H; OH),
8.26 (s, 2H; CH=N), 7.31–7.23 (m, 7H; ArH), 7.20–7.16 (m, 2H;
ArH), 6.59 (d, 3J(H,H)=8.5 Hz, 2H; ArH), 6.25 (d, 3J(H,H)=8.4 Hz,
2H; ArH), 4.98 ppm (s, 4H; NCH2CH2N); 13C{1H} NMR ([D6]DMSO,
125 MHz): d=166.5, 152.6, 149.9, 140.8, 132.9, 128.6, 128.2, 127.7,
123.4, 111.8, 107.6, 77.2 ppm; HRMS (MALDI+, dctb): m/z calcd for
485.1713 [M]+; found: 485.1750; elemental analysis calcd (%) for
C28H24N2O6·2iPrOH·0.5H2O: C 66.54, H 6.73, N 4.56; found: C 66.90,
H 6.87, N 4.85. The presence of iPrOH in the product was con-
firmed by both 1H and 13C{1H} NMR spectroscopy analysis.
Experimental Section
General: MEK and carbon dioxide (purchased from PRAXAIR) were
used as received without further purification or drying prior to use.
All phenolic compounds (except for 20 and 23) were commercially
purchased from Sigma Aldrich and used without any further purifi-
cation. Compound 20 was previously reported and prepared as
such.[45] The synthesis of 23 is reported herein. The organic prod-
ucts 24a–n have been reported previously.[46–52] 1H NMR spectra
were recorded on a Bruker AV-400 or AV-500 spectrometer and ref-
erenced to the residual deuterated solvent signals. Elemental anal-
ysis was performed by the Unidꢂd de Anꢂlisis Elemental at the Uni-
versidad de Santiago de Compostela. MS analysis was performed
by the Research Support Group at the ICIQ.
Computational details: All calculations were performed with the
Gaussian 09 package[53] of programs with the hybrid B3LYP func-
tional.[54] The basis set was the ECP-adapted SDDALL[55] with a set
of polarization functions for I,[56] the all-electron 6-31G(d,p)[57] for H
and C, and the 6-31+G(d,p)[58] for O. Full optimization of geometry
was performed in MEK by using the continuum SMD model[59]
without any symmetry constraints, followed by analytical computa-
tion of the Hessian matrix to identify the nature of the located ex-
trema as minima or transition states. Each transition state was re-
laxed toward reactant and product by using the vibrational data to
confirm its nature. The zero-point, thermal, and entropy corrections
were evaluated to compute enthalpies and Gibbs free energies
(T=298 K, p=1 bar).
Typical catalytic experiment:
A solution of nBu4NI (5 mol%,
37.6 mg), 22 (5 mol%, 12.6 mg), 1,2-epoxyhexane (0.002 mmol,
200 mg), and mesitylene (0.002 mmol, 244 mg) in MEK (5 mL) were
added to a stainless-steel reactor. Three cycles of pressurization
and depressurization of the reactor (with pCO =5 bar) were carried
2
out before finally stabilizing the pressure at the required pressure.
The reactor was then heated to the required temperature and left
stirring for a further 18 h. An aliquot of the solution was then ana-
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Formation of Organic Carbonates
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Acknowledgements
We thank ICIQ, ICREA, and the Spanish Ministerio de Economꢁa y
Competitividad (MINECO) through projects CTQ-2011-27385,
CTQ2011-27033, Consolider Ingenio 2010 CSD2006-0003, and
a Juan de la Cierva contract for A.N. for support.
Keywords: carbon dioxide fixation · cooperative effects ·
cycloaddition · organocatalysis · phenols
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Published online on && &&, 0000
ChemSusChem 0000, 00, 1 – 8
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
&
7
&
ÞÞ
These are not the final page numbers!

FULL PAPERS
C. J. Whiteoak, A. Nova, F. Maseras,*
A. W. Kleij*
In a fix: A new organocatalytic method
for organic carbonate synthesis is re-
ported and allows attractive conditions
(258C, 10 bar, no solvent) to be used
(see picture). Cyclic carbonates pro-
duced from CO2 and epoxides are isolat-
ed in high yields. The mild nature of
this process increases the overall pro-
cess sustainability for this type of widely
studied carbon dioxide fixation process.
&& – &&
Merging Sustainability with
Organocatalysis in the Formation of
Organic Carbonates by Using CO2 as
a Feedstock
&8
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ChemSusChem 0000, 00, 1 – 8
ÝÝ
These are not the final page numbers!