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

Article abstract of DOI:10.1002/cssc.201501463

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

Full text of DOI:10.1002/cssc.201501463

DOI: 10.1002/cssc.201501463
Full Papers
Cavitand-Based Polyphenols as Highly Reactive
Organocatalysts for the Coupling of Carbon Dioxide and
Oxiranes
[a]
[a]
[a, b]
Luis Martínez-Rodríguez, Javier Otalora Garmilla, and Arjan W. Kleij*
A variety of cavitand-based polyphenols was prepared from
cheap and accessible aldehyde and resorcinol/pyrogallol re-
agents to give the respective resorcin[4]- or pyrogallol[4]ar-
enes. The preorganization of the phenolic units allows intra-
and intermolecular hydrogen bond (HB) networks that affect
both the reactivity and stability of these HB-donor catalysts.
Unexpectedly, we found that the resorcin[4]arenes show coop-
erative catalysis behavior compared to the parent resorcinol in
higher turnover. At elevated reaction temperatures, the
resorcin[4]arene-based catalyst 3d displays the best catalytic
performance with very high turnover numbers and frequen-
cies, combining increased reactivity and stability compared to
pyrogallol, and an ample substrate scope. This type of poly-
phenol structure thus illustrates the importance of a new,
highly competitive organocatalyst design to devise sustainable
CO conversion processes.
2
the catalytic coupling of epoxides and CO with a significantly
2
Introduction
[
12]
One of the biggest challenges associated with CO conversion
orinated alcohols. Parallel to these activities, other promising
organocatalytic systems have been communicated recently
that present new catalyst designs based on the host–guest
2
is the design of appropriate catalyst systems that show good
[1]
reactivity and selectivity. To date, most attention in the area
[
13]
of CO catalysis has been focused on (homogeneous) catalysts
complexation of nucleophilic reagents, the use of phospho-
2
[
14]
based on metal complexes of various kinds that show high re-
rus ylides, and hydroxy-functionalized mono- and bisimida-
[
2]
[15]
activity and unusual scope and/or selectivity in a few cases.
zolium bromides.
[
3]
[4]
Metal catalysts based on abundant metals such as Zn, Fe,
We have become interested in the use of (natural) polyphe-
nols such as pyrogallol and tannic acid (1b and 2, respectively;
Figure 1) for the conversion of epoxides through hydrogen
bond (HB) activation. Their transformation into cyclic carbo-
[5]
[6]
Al, and Co have received much attention to allow for signif-
icant evolution in both catalyst design as well as the improve-
ment of the product portfolio that can be accessed from CO2,
which represents a renewable and cheap carbon feedstock for
chemical synthesis.
nates in the presence of CO takes advantage of the extended
2
HB network that arises upon the activation of the epoxide to-
wards the formation of key intermediates. Consequently, lower
Recently, organocatalysis has appeared on the radar of syn-
thetic chemists as it is a sustainable alternative to metal-based
kinetic barriers allow either low-temperature conversions
[7]
[9b,c]
approaches in the catalysis of CO conversion. In particular,
(458C)
or the use of a reduced polyphenol loading for ef-
2
[
9a]
the catalytic coupling of epoxides and CO has been studied
fective catalytic turnover. Nonetheless, there are still chal-
lenges to be met upon using such polyphenols as at higher re-
action temperatures (i.e., 808C) some catalyst degradation
through deprotonation and the formation of relatively inactive
phenolate groups cannot be fully avoided, which prevents the
efficient recycling of these phenolic additives and thus restricts
2
extensively and can be regarded as a benchmark process for
new catalyst development in nonreductive CO coupling. Prog-
2
ress in this area has been considerable with the development
of efficient catalyst systems based on binary or bifunctional
[
8]
[9]
systems that comprise azaphosphatranes,
polyphenols,
[10]
[11]
[9a,c]
phosphonium or ammonium alcohols, silane diols, and flu-
the total turnover number (TON).
In our quest to develop thermally more robust polyphenol-
based catalysts with a high reactivity and a privileged sub-
strate scope, we considered that cavitand structures (Figure 1;
[
a] 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. Pa ¨ı sos Catalans 16
[
16]
3
)
may deliver the appropriate combination of activity and
4
3007 Tarragona (Spain)
thermal/chemical stability. These cavitand structures, which in-
clude resorcin[4]arenes (3: X=H) and pyrogallol[4]arenes (3:
X=OH), give rise to preorganized supramolecular structures
that are often hexameric in nature in solution and in the solid
state through intermolecular, water-assisted HB interactions
E-mail: akleij@iciq.es
[
b] Prof. Dr. A. W. Kleij
Catalan Institute of Research and Advanced Studies (ICREA)
Pg. Lluís Companys 23
0
8010 Barcelona (Spain)
[
17]
(
Figure 2, right). At the same time, the bowl shape of mono-
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cssc.201501463.
meric cavitand molecules is controlled through intramolecular
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In this contribution we will show that cavitand-based poly-
phenols are excellent HB activators in the formation of cyclic
carbonates from epoxides and CO with unprecedented turn-
2
over numbers and frequencies. Catalytic data in combination
with several control experiments support a cooperative catalyt-
ic effect when using the resorcin[4]arene systems; the latter
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 3 f–I, and octahy-
droxypyridine[4]arene 3j (Scheme 1) were prepared according
to procedures reported previously (Experimental Section) and
1
13
their molecular identity was established by H and C NMR
spectroscopy and MS: these data were in accordance with the
Figure 1. Schematic structures of resorcinol (1a), pyrogallol (1b), tannic acid
2), and cavitand-based polyphenols (3).
(
Scheme 1. Resorcin[4]arenes 3a–e, pyrogallol[4]arenes 3 f–i, and the octahy-
droxypyridine[4]arene 3j.
literature data (Supporting Information). Compounds 3a–
i were initially tested in combination with NBu X (X=halide) as
4
binary catalysts in the coupling of 1,2-epoxyhexane (4a) and
CO at 508C using methyl ethyl ketone (MEK) as the solvent
2
(
Table 1).
We first tested 3b/NBu I as a binary catalyst for the synthesis
4
of organic carbonates from epoxides and CO under conditions
2
closely related to those tested previously for a binary pyrogal-
[
9c]
lol-based system (Table 1, entry 1). At 458C, a yield of 81%
measured by NMR spectroscopy) was already achieved, and
(
an increase of the temperature to 508C 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, which led 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 observed, which
shows the imperative role of both catalyst components in this
coupling reaction. A comparison of the efficiency of 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 concentra-
tion of diphenol units. This effect was maintained under more
Figure 2. Intra- and intermolecular HB networks in cavitand-based structures.
Part of this figure is reprinted with permission from Ref. [19]. Copyright
(
2013) American Chemical Society.
HBs between adjacent resorcinol/pyrogallol units typically ex-
pressed in solvent media such as alcohols and acetonitrile
[
18]
(
Figure 2, left).
These intramolecular HB patterns suggest
[19]
a similar potential for the catalytic activation of epoxides as
for pyrogallol/tannic acid (Figure 1) as the stabilization of key
intermediate transition states through multiple HB interactions
is more efficient than in the absence of such HB donors.
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The best-performing nonyl-substituted polyphenols 3d and
3i were examined more closely and compared with the pa-
rents 1a and 1b, and the kinetic profiles of each binary cata-
Table 1. Screening of conditions using a polyphenol/TBAX binary catalyst
[a]
in the coupling of 1,2-epoxyhexane and CO
2
to afford 4b.
lyst (in combination with NBu I) were determined (Figure 3). In-
4
[b]
Entry
Polyphenol Amount
mol%]
NBu
[mol%]
4
X
T
[8C]
MEK
[mL]
Yield of 4b
[%]
[
1
2
3
4
5
6
7
8
9
3b
3b
3b
3b
3b
–
3b
1a
1a
1b
1.5
1.5
1.5
1.5
1.5
0
1.5
6.0
6.0
4.0
I, 5.0
I, 5.0
I, 5.0
Cl, 5.0
Br, 5.0
I, 5.0
0
I, 5.0
I, 5.0
I, 5.0
45
50
50
50
50
50
50
50
50
50
2.5
2.5
5.0
2.5
2.5
2.5
2.5
2.5
5.0
2.5
81
91
65
31
65
4
0
47
24
99
1
0
[
a] Reaction conditions: 1,2-epoxyhexane 1.0 mmol, p(CO
polyphenol amount normalized with respect to [OH] groups, p(CO
.0 MPa, 18 h. [b] NMR yields based on mesitylene as internal standard,
selectivity for 4b was >99%.
2
)=10 bar, 18 h,
2
)=
1
Figure 3. Comparative kinetics in the formation of 4b from 1,2-epoxyhexane
and CO
ized with respect to the concentration of OH groups. Conditions: 4 mol%
b, 5 mol% 1a, 1.5 mol% 3d, and 1.0 mol% 3i. For all reactions: 1,2-epoxy-
hexane 1.0 mmol, NBu I 5 mol%, MEK 2.5 mL, p(CO )=10 bar, 508C.
2
using 1a, 1b, 3d, and 3i. The amount of polyphenol was normal-
1
dilute conditions (cf. entries 3 and 9). Pyrogallol (1b), a triphe-
nol, gave a virtually quantitative yield (entry 10, 99%) under
these conditions. The remarkable yield of 4b in the presence
of 3b suggests a cooperative effect between the 1,3-diphenol
sites in the catalytic activation of the epoxide and/or more effi-
cient stabilization of the intermediates of the carbonate forma-
tion reaction.
4
2
terestingly, 3i shows an inferior catalytic performance to 3d.
The reason for this behavior is likely the competing self-assem-
bly of the individual cavitand molecules of 3i into larger ag-
gregates (i.e., hexamers, cf. Figure 2). Avram and Cohen com-
pared the stability of undecyl-substituted resorcin[4]arenes and
We then screened
a series of nine cavitands (3a–i;
[
21]
Scheme 1) in the coupling of 4a and CO (Table 2) to investi-
pyrogallol[4]arenes. Titration studies on these cavitand mole-
cules demonstrated that if the polarity of the medium was in-
2
gate the role of the pendent R groups. Within the series of
3
3
a–e (entries 1–5), the highest yield of 4b was achieved with
d (R=nonyl), a trend that was also observed for 3 f–i (en-
creased by adding CD OD to a solution of the cavitand in
3
CDCl , the hexameric, aggregated state was fully disrupted for
3
[
20]
tries 6–9).
both types of cavitand. However, essentially much lower
amounts of CD OD were required in the case of the resorcin-
3
[
4]arene in line with a stronger self-assembly behavior of the
pyrogallol[4]arene. Therefore, under the reaction conditions re-
ported in Table 2 and Figure 3, the poorer performance of 3 f–
i compared to 3a–d is thus explained in terms of a stronger
competing self-assembly. This behavior competes with hydro-
gen bonding between the epoxide and the phenolic groups
and thus slows down the reaction. To further support the view
that competitive HB interactions can slow down the catalytic
Table 2. Screening of cavitand/TBAI binary catalysts 3 in the coupling of
[a]
1,2-epoxyhexane and CO
2
to afford cyclic carbonate 4b.
[b]
Entry
Cavitand
Amount
mol%]
NBu
[mol%]
4
I
T
[8C]
R
Yield 4b
[%]
[
[
22]
reaction, octahydroxypyridine[4]arene (3j; Scheme 1)
was
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3 f
3g
3h
3i
1.5
1.5
1.5
1.5
1.5
1.0
1.0
1.0
1.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
50
50
50
50
50
50
50
50
50
Me
Et
Bu
Non
Ph
Me
Et
93
91
94
98
27
78
84
87
93
tested as a HB-donor system in the synthesis of 4b. The 2,6-di-
hydroxypyridine subunits in 3j are known to induce intramo-
lecular N···HO hydrogen bonds, and the significantly lower
yield after 18 h (1.5 mol%, 60%) than that observed for 3d
(
1.5 mol%, 98%) is a clear testament for competitive H-bond-
ing. Thus, the best catalytic performance among the cavitand
structures at 508C is observed for 3d.
Bu
Non
In an effort to further increase the reactivity, the coupling of
[
a] 1,2-Epoxyhexane 1.0 mmol, polyphenol amount normalized with re-
1
,2-epoxyhexane and CO was performed at 808C using 3d,
2
spect to [OH] groups, MEK 2.5 mL, p(CO )=1.0 MPa, 18 h; abbreviations:
2
Me=methyl, Et=ethyl, Bu=n-butyl, Non=n-nonyl, Ph=phenyl. [b] NMR
yields based on mesitylene as internal standard, selectivity for 4b was
3i, and 1b (Table 3, Figure 4). Under these conditions, the nu-
cleophilic additive NBu I alone leads to poor catalysis (entry 1,
4
>
99%.
1
7% yield of 4b). Various combinations of cavitand and nucle-
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nents; both systems show inferior stability at this elevated
temperature, which causes side-reactions that involve the de-
protonation of the polyphenolic unit and its replacement by
2
Table 3. Catalytic coupling of 1,2-epoxyhexane and CO at 808C to afford
cyclic carbonate 4b.
[a]
[
9a,b]
NBu4.
The formation of (deprotonated) phenolate groups
causes a decrease in the ability to form extended HB networks
to stabilize catalytic intermediates, which results in higher ki-
netic barriers and thus slower reactions. Consequently, both
the nucleophile and polyphenol concentration is affected neg-
atively and the catalysis is shut down in the case of pyrogallol.
Conversely, both the catalysts based on 3d and 3i retain their
catalytic activity after prolonged use and, therefore, are more
effective systems for cyclic carbonate preparation at elevated
temperatures; compound 3d performs slightly better than 3i
in the reported time span. Importantly, a comparison of the
[
b]
Entry
Polyphenol
Amount
mol%]
NBu
[mol%]
4
I
T
[8C]
Yield 4b
[%]
[
1
2
3
4
5
6
7
8
–
0
1.6
2.5
1.6
0.8
1.6
3.2
1.6
0.8
80
80
80
80
80
80
80
80
17
>99
>99
80
93
>99
77
3d
3d
3d
3i
1b
1b
1b
0.75
0.50
0.25
0.33
2.0
1.3
0.66
55
[
23]
[24]
pK values of 1a (9.20) and 1b (9.01) shows that the pyro-
a
[
a] 1,2-Epoxyhexane 1.0 mmol, polyphenol amount normalized with re-
spect to [OH] groups, MEK 2.5 mL, p(CO )=1.0 MPa, 18 h. [b] NMR yields
based on mesitylene as internal standard, selectivity for 4b was >99%.
gallol unit is more acidic and likely to undergo deprotonation
more easily. This causes a (much) shorter lifetime of the cata-
lyst, whereas the system based on 3d shows a comparatively
longer lifetime. This results in a better potential to obtain
higher TONs at elevated reaction temperatures. Interestingly,
the preorganization of less active resorcinol units (cf., Figure 3,
2
1
a vs. 1b) in the cavitand increases their catalytic potential
significantly compared with the pyrogallol-based system,
which underlines the importance of the catalyst structure for
effective turnover.
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 investigated with 3d and 1b in the synthesis of
4
b (Table 4; scale 10 mmol of 4a). Solvent-free (neat) condi-
tions were employed to favor the kinetics, and the use of nu-
cleophile alone again showed a considerably lower yield of 4b
(
entries 1 and 2; 7 and 32%, respectively) compared with the
Figure 4. Comparative kinetics in the formation of 4b from 1,2-epoxyhexane
and CO at 808C using 1b (1.3 mol%), 3d (0.50 mol%), and 3i (0.33 mol%).
Conditions: 1,2-epoxyhexane 1.0 mmol, NBu I 1.6 mol%, 2.5 mL,
p(CO )=10 bar, 808C. Note that in all reactions the same molar amount of
phenol groups was used.
use of both 3d and NBu I in combination (entries 2 and 4; 46
4
2
4
2
Table 4. Comparison of 3d/NBu
coupling of 1,2-epoxyhexane and CO
b. n.a. stands
4
I and 1b/NBu
4
I as binary catalysts in the
2
at 808C to afford cyclic carbonate
[a]
4
for
not
applicable.
ophile were tested (entries 2–5) and a similar ratio was main-
tained between the catalyst components (ratio NBu I/[OH]
4
groupsꢀ3.3). For the catalyst based on 3d, the conditions re-
ported in entry 3 still produced a quantitative yield of 4b,
whereas a further decrease of the amount of catalyst to
[a]
[c]
[d]
[e]
[f]
Entry Catalyst
OH units
[mol%]
t
Yield of 4b
TON
TON
c
TOF
c
[mol%]
[h] [%]
0
.25 mol% 3d/0.8 mol% NBu I showed a modest decrease in
4
yield to 80% (entry 4). For comparison, we used a similar
amount of catalyst derived from 3i (cf., entries 3 and 5), and
a very high though not quantitative yield of 4b was observed.
Remarkably, under these conditions, the catalyst based on 1b
produced a markedly lower yield of 4b (77%; cf. entries 3 and
1
2
3
4
5
6
7
–
–
–
–
1
7
–
–
–
–
–
–
488
29
n.a.
19
n.a
18 32
46
3d, 0.010 0.080
3d, 0.010 0.080
3d, 0.010 0.080
1b, 0.026 0.080
1b, 0.026 0.080
1
575 488
925 525
1225
750 350
825
18 74
30 98
18 60
30 66
n.a.
n.a
7
), which shows the superior performance of the resorcin[4]-
and pyrogallol[4]arene-based catalysts at 808C.
To investigate this in more detail, the full kinetic profiles for
each of the catalyst systems reported in entries 3, 5, and 7
[a] 1,2-Epoxyhexane 10.0 mmol, polyphenol amount normalized with re-
spect to [OH] groups (see third column), neat conditions, p(CO
1.0 MPa, NBu 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 con-
versions, see entries 1 and 2. [f] Corrected average TOF per hour using
the measured background conversions, see entries 1 and 2.
2
)
=
4
I
[
(
Table 3) were determined (Figure 4). The pyrogallol catalyst
system reaches a plateau in the conversion of around 70%
after 6 h, which barely increases thereafter. This is in line with
our previous results using either 1b or 2 as catalyst compo-
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and 74%, respectively). Under these conditions, the TON based
on the total number of phenol active sites was 1225 (entry 5).
Higher TONs were thus achieved simply by prolonging the re-
action to give virtually full conversion after 30 h. The pyrogal-
lol-based catalyst (entries 6 and 7) showed lower efficiencies
with only a modest increase in the TON after 18 h; this further
confirms the favorable stability of 3d at elevated temperatures.
Thus, the combination of the cooperative action of the resorci-
nol units in 3d with a higher chemical stability than 1b makes
this system among the most efficient organocatalysts reported
to date with a very high TON. The turnover frequency of the
binary catalyst system based on 3d and NBu I was estimated
4
after 1 h (entry 3, 46%) by considering the much lower conver-
sion (entry 1, 7%) in the absence of 3d. If we corrected for this
background conversion, still a significant part of it may be at-
tributed to the binary catalyst (39%, TON=488 based on
phenol group molar concentration, TOF per hour/[OH] group=
À1
4
88 h ), which is the highest (initial) activity for a binary orga-
nocatalyst in this area.
Motivated by these results, we then examined a wide range
of epoxide substrates (4a–22a) in the formation of their cyclic
carbonates 4b–22b in the presence of CO . To produce syn-
2
thetically useful yields, 1.5–3.0 mol% of 3d was used together
with 5 mol% of NBu X (X=I, Br) in MEK (2.5 mL). The use of
4
solvent was in some cases warranted to prevent the solidifica-
tion of the reaction mixture and incomplete conversion of the
substrate. Neat conditions were used for the internal epoxides
18a–22a.
At 508C and under 1 MPa of pressure, terminal epoxides
a–17a were converted smoothly into the carbonates 4b–
7b with high conversions (>99%) and isolated yields (92–
4
1
9
9%). The temperature and pressure conditions are compara-
tively very mild for an organocatalyst system, which prompted
us to examine more challenging internal epoxides 18a–22a as
reaction partners. To date, limited progress has been achieved
with the use of such epoxide substrates in organocatalytic ap-
proaches. One promising example was reported recently by
Figure 5. Substrate scope in the conversion of various terminal and internal
epoxides 4a–22a into cyclic carbonates 4b–22b using 3d/NBu
alyst. General conditions: epoxide 1 mmol, 1.5 mol% 3d, 5 mol% NBu
8 h, 1 MPa, 508C, 2.5 mL of MEK. *With the use of 3 mol% of 3d, 5 mol%
NBu Br, 808C 18 h, neat.
4
I as the cat-
4
I,
1
[
12b]
4
Tassaing et al.
donor and achieved a conversion of 73% of cyclohexene oxide
18a in Figure 5) after 5 h at 1008C and 2 MPa. Werner et al.
who used a fluorinated alcohol as a HB
(
reported the use of bifunctional phosphonium salts that were
effective for internal epoxide conversion at temperatures in
a high conversion (84%) and yield (79%), cyclopentene oxide
(20a) gave a much lower (reproducible) yield (38%). The acy-
clic epoxides 19a and 22a were also converted into their car-
bonates 19b and 22b, which shows the general potential of
[10a]
the range 90–1208C and 1 MPa.
For 18a specifically, the
best results in terms of yield were obtained at 1208C and
4
MPa (40 bar) and produced 18b in 69% yield after 6 h.
3d/NBu I as a binary organocatalyst in the conversion of more
4
We first screened the potential conversion of 18a at 508C
challenging internal epoxides. All epoxides 18a–22a were con-
verted with the full retention of configuration (cis>99% for
18b, 20b, and 21b and trans>99% for 22b) except for 19a
(cis/trans=8:2). Such a loss of stereochemical information with
this substrate in the formation of its carbonate product in the
and 1 MPa, but this afforded 18b in only 6% yield after 18 h.
We were pleased to find that upon increasing the temperature
to 808C and using 3 mol% of cavitand 3d, the conversion of
8b could be increased significantly to 89% (isolated yield
5%) under neat conditions. As a control experiment, the reac-
tion in the absence of 3d was also performed and gave only
% yield (duplicate experiment), which shows the importance
1
8
[
25]
presence of CO has been observed before and may be re-
2
lated to a partial S 1 character of the nucleophilic attack of the
N
8
linear carbonate intermediate onto the CÀBr bond formed ini-
of 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 to test cyclic and acyclic
substrates. Although 3,4-epoxyfuran (21a) was converted with
tially in the ring-opening of the epoxide by NBu Br.
4
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Full Papers
were recrystallized from acetonitrile. Characterization details of
Conclusions
3
a–j are given in the Supporting Information.
Easily accessible and modular cavitand structures such as re-
sorcin[4]arenes and pyrogallol[4]arenes constitute interesting
hydrogen-bond-donor binary catalysts in combination with
ammonium halide salts. The preorganization of the phenolic
units within the resorcin[4]arenes was beneficial for the catalyt-
ic efficiency in organic carbonate formation from epoxides and
Catalysis experiments
Typically, the synthesis of organic cyclic carbonates from epoxides
and CO was performed in a 30 mL steel autoclave using 1,2-ep-
2
oxyhexane (1 mmol, 1 equiv.), cavitand (1.0–1.5 mol%), NBu4I
(
5 mol%), and MEK (2.5 mL). The autoclave was then subjected to
CO and resulted in cooperative effects (for 3d) that led to sig-
2
three cycles of pressurization and depressurization with CO . Finally
2
nificantly higher conversion rates compared to resorcinol (1a).
If the reaction temperature was increased from 50 to 808C to
improve the overall reactivity, the cavitand structures showed
a higher chemical stability than pyrogallol (1b), which makes
these former systems more suitable to achieve very high turn-
over numbers. Furthermore, resorcin[4]arene 3d combined
the autoclave was charged with 1 MPa (10 bar) of CO , heated to
2
5
08C, and the contents was stirred for 18 h. Thereafter, the auto-
clave was cooled to RT and carefully depressurized. The volatiles
were removed under reduced pressure, and the product was puri-
fied by flash column chromatography (1:1 hexane/ethyl acetate as
eluent) to afford the pure cyclic carbonate. Characterization details
of 4b–22b are given in the Supporting Information.
with NBu I shows very high initial turnover frequencies of
4
À1
almost 500 h at 808C. This improved and unparalleled reac-
tivity was shown to be beneficial in the formation of 19 differ-
ent carbonates under comparatively mild reaction conditions
Acknowledgements
(
50–808C, 1 MPa). Moreover, six disubstituted epoxides (16a
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 MS analyses.
L.M.R. thanks ICIQ for a predoctoral fellowship.
and 18a–22a) were also screened as reaction partners and
were converted efficiently under neat conditions at 808C to
provide good to excellent isolated yields (79–94%). Compared
with the state-of-the-art in organocatalytic CO /epoxide cou-
2
pling chemistry, this is a remarkable result. Hence, cavitand-
based binary organocatalysts are sustainable, versatile, and
highly reactive alternatives to metal-based systems in the cata-
Keywords: cavitands
· cooperitivity · hydrogen bonds ·
lytic coupling of epoxides and CO . Further attention is now
2
organocatalysis · polyphenols
on the combination of cooperative and bifunctional concepts
to develop organocatalytic processes with improved reactivity
under ambient conditions.
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Received: October 28, 2015
Revised: November 26, 2015
Published online on February 23, 2016
ChemSusChem 2016, 9, 749 – 755
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