Pseudopeptidic macrocycles as cooperative minimalistic synzyme systems for the remarkable activation and conversion of CO2in the presence of the chloride anion

Article abstract of DOI:10.1039/d0gc01449d

A series of pseudopeptidic compounds have been assayed as organocatalyts for the conversion of CO2 into organic carbonates through a cooperative multifunctional mechanism. Conformationally constrained pseudopeptidic macrocycles 3a and 3b have been revealed to be excellent synzymes for this purpose, being able to provide a suitable preorganization of the different functional elements and reaction components to activate the CO2 molecule and stabilize the different anionic intermediates involved, through a series of cooperative supramolecular interactions. As a result, remarkable catalytic efficiencies are found at low CO2 pressures and moderate temperatures, with TON and TOF values surpassing those reported for other organocatalytic supramolecular systems under similar conditions. The process works well for monosubstituted epoxides. The involvement of the different structural elements has been analyzed in detail and preliminary studies show the potential for recovery and reuse of these catalytic systems.

Full text of DOI:10.1039/d0gc01449d

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Pseudopeptidic macrocycles as cooperative
minimalistic synzyme systems for the remarkable
activation and conversion of CO2 in the presence
of the chloride anion†
Cite this: DOI: 10.1039/d0gc01449d
b
Ferran Esteve, a Belén Altava,a M. Isabel Burguete,a Michael Bolte,
a
Eduardo García-Verdugo *a and Santiago V. Luis
*
A series of pseudopeptidic compounds have been assayed as organocatalyts for the conversion of CO2
into organic carbonates through a cooperative multifunctional mechanism. Conformationally constrained
pseudopeptidic macrocycles 3a and 3b have been revealed to be excellent synzymes for this purpose,
being able to provide a suitable preorganization of the different functional elements and reaction com-
ponents to activate the CO2 molecule and stabilize the different anionic intermediates involved, through a
series of cooperative supramolecular interactions. As a result, remarkable catalytic efficiencies are found
at low CO2 pressures and moderate temperatures, with TON and TOF values surpassing those reported
for other organocatalytic supramolecular systems under similar conditions. The process works well for
monosubstituted epoxides. The involvement of the different structural elements has been analyzed in
detail and preliminary studies show the potential for recovery and reuse of these catalytic systems.
Received 27th April 2020,
Accepted 27th June 2020
DOI: 10.1039/d0gc01449d
rsc.li/greenchem
Developing abiotic systems able to mimic the efficiency of
these natural structures is an important goal in Green
1. Introduction
Nature has developed smart and complex systems working Chemistry,3 and a broad range of synthetic supramolecular
under elaborate self-regulation protocols and mild conditions systems displaying enzymatic activity, often referred to as syn-
and producing reduced waste. Many of them are based on the zymes, has been reported.4 As in natural enzymes, the catalytic
self-organization and self-assembly of a few simple building activity of these systems can provide selective substrate reco-
blocks.1 Clear examples are provided by enzymes, which are gnition and is dominated by supramolecular interactions.5 In
highly sophisticated catalysts that Nature has optimized over this field, macrocyclic cavities have been often exploited as
billions of years. Their unique self-organization affords minimalistic enzymatic mimics, taking advantage of the higher
specific 3D-arrangements of individual functional groups degree of preorganization of the functionalities present, which
allowing the required supramolecular and cooperative inter- represents a key element of the active sites of enzymes.6
play of the various active-site functionalities within the
enzyme. This often enables a precise location of the reacting macrocyclizations often represent
Achieving such a preorganization is not always easy and
synthetic challenge.7
a
substrates/reagents within the 3D-environment of the active However, efficient macrocyclization strategies based on confor-
site of the enzyme leading to highly efficient transformations mational, configurational or template-induced self-organization
in terms of both activity and selectivity.2
have been developed for the preparation of minimalistic macro-
cyclic pseudopeptides.8,9 Despite their simplicity, as well as the
high molecular diversity attainable and the presence of a high
level of well-defined functionality,10 these macrocycles have not
been yet exploited as supramolecular organocatalytic systems.
On the other hand, the conversion of CO2 into valuable
chemicals represents a challenge of great current interest in
Green Chemistry, from different perspectives.11 A variety of
catalytic approaches have been reported for the reaction
aDepartamento de Química Inorgánica y Orgánica, Universitat Jaume I, Av. Sos
Baynat s/n, 12071 Castellón, Spain. E-mail: cepeda@uji.es, luiss@uji.es
bInstitut fur Anorganische Chemie, J. W. Goethe-Universitat Frankfurt, 60438
Frankfurt/Main, Germany
†Electronic supplementary information (ESI) available: Synthesis of pseudopep-
tides; comparison of catalytic systems; solvent and cation effect; studies for the
interaction with anions; kinetic profiles; conformational study of the macrocyclic
compounds; study of the catalytic mixtures; substrate scope for the CO2
addition; 1H, 13C NMR, and HRMS spectra for 5; XRD and computational data
(PDF). CCDC 1995455. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/d0gc01449d
between CO2 and epoxides to form cyclic carbonates,12–15
a
family of compounds of interest for a variety of different appli-
cations.16 Here we report the initial results for the evaluation
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Scheme 1 Cycloaddition of carbon dioxide to styrene oxide catalysed
by the supramolecular systems 3 : R4NX.
of some macrocyclic pseudopeptidic systems (3, Scheme 1), in
the presence of tetra-alkyl-ammonium chlorides, as supramo-
lecular synzymes for the activation and conversion of CO2 into
carbonates in the presence of epoxides. When appropriately
adjusted, the different structural elements of these minimalis-
tic pseudopeptides like the nature of the central spacers, the
amino acid side chains, and the groups attached to the amino
functions, can facilitate a precise and well-defined self-assem-
bly of the halide, the epoxide and CO2 in a minimalistic coop-
erative supramolecular interplay with the macrocyclic host,
providing a highly efficient catalytic transformation of CO2
into the corresponding cyclic carbonates.
Scheme 2 Synthesis of macrocyclic systems and related analogues. (i)
CH3CN, 2–3 h, 90 °C. (ii) CH3CN, 5 h, 90 °C. (iii) CH3OH, 1 h, 25 °C.
in acetonitrile (65 mL) in the presence of Cs2CO3 (764 mg,
2.346 mmol), and the reaction mixture was refluxed on a
heating mantle with magnetic stirring for 5 hours. The solvent
was then evaporated under vacuum and the resulting residue
was treated with basic water (pH ≈ 11), to afford pure 5 as a
solid after centrifugation of the resulting suspension at 3000
rpm for 8 min. Yield (140 mg, 77.3%, 0.302 mmol); m.p. =
168–169 °C; [α]2D5 = −41.7° (c = 0.4, CH3CN); IR (ATR): 3297,
2959, 1641, 1544 cm−1; 1H NMR (400 MHz, CD3CN) δ = 0.91 (d,
J = 6.7 Hz, 6H), 0.98 (d, J = 6.8 Hz, 6H), 2.86 (d, J = 6.7 Hz, 2H),
3.29–3.33 (m, 4H), 3.95–4.03 (m, 8H), 7.00 (s, 2H), 7.15–7.22
(m, 8H), 13C{1H}-NMR (100 MHz, CD3CN) δ = 18.4, 20.4, 29.8,
39.9, 56.5, 74.1, 123.1, 127.5, 140.8, 172.4; HRMS (ESI/Q-TOF)
m/z: [M + H]+ calcd for C28H38N4O2 463.3703; found 463.3706.
2. Experimental
General
NMR experiments were carried out at 500, 400 or 300 MHz for
1H and 125, 100 or 75 MHz for 13C. Chemical shifts are
reported in ppm from tetramethyl silane, using the residual
solvent resonance as the internal standard. Fourier transform
infrared spectra (FT-IR) were recorded using an attenuated
total reflection (ATR) adapter. High resolution mass spec-
trometry (HRMS) was recorded with a Q-TOF instrument.
Rotatory power was determined with a digital polarimeter (Na:
589 nm). Melting points were measured using a standard
apparatus and are uncorrected.
Open chain pseudopeptidic compounds 2 were prepared
following literature procedures,8c as well as the macrocyclic
structures 3 (Scheme 2).17
Two different set-ups were used for the cycloaddition reac-
tion with CO2. The first one, employed for the reaction at
atmospheric pressure, used a standard round bottom flask
with a CO2 balloon (100% CO2) as gas supply. The second one,
for the reactions under pressure, used a Berghof R-300 high
pressure reactor connected to a pressurized CO2 source and a
back-pressure regulator from Jasco (see Fig. S11†)
General procedure for the reaction of 6 with CO2 in the pres-
ence of pseudopeptides and Bu4NCl
8.7 mmol of styrene oxide, 0.087 mmol of Bu4NCl and
0.0087 mmol of the corresponding pseudopeptide were added
to a 25 mL twin-necked round bottom flask. The system was
purged with dry N2 and CO2, leaving two CO2 balloons (100%
CO2) as gas supply. The mixture was refluxed on a heating
mantle with magnetic stirring for 5 hours at 100 °C. Finally, a
sample of the crude was collected and analyzed by NMR. The
conversion of 6 into 7 could be calculated, considering the
integration of the signal at 5.67 ppm for the disappearance of
6 and the one for the signal at 3.88 ppm for the formation
of 7.
IR kinetic experiments
The same procedure above was carried out, but using just one
CO2 balloon (100% CO2) and closing the second neck with a
septum. Samples at defined times were directly collected
Synthesis of (2S,2′S)-N,N‘-(ethane-1,2-diyl)bis(2-(isoindolin-2-
yl)-3-methylbutanamide
Synthesis of 5. Compound 2a (101 mg, 0.391 mmol) and through the septum with a syringe and directly analysed by
α,α′-dibromo-o-xylene (4) (215 mg, 0.782 mmol) were dissolved FT-IR. The conversion of 6 into 7 could be calculated, using
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the area of the bands at 1440–1510 cm−1 (νC–C stretching, aro-
matic ring) as the reference and the area of the band at
1020–1100 cm−1 (νC–O asymmetric vibration) for the for-
mation of 7.
Crystal structures
Single crystals suitable for X-ray crystallography were obtained
by slow evaporation of a methanol solution of [3a·2HCl]. A
crystal was selected and mounted on a SuperNova, Dual, Cu at
zero, Atlas diffractometer. The structure was solved with the
SHELXT 2014/518 structure solution program and refined with
the SHELXL-2018/319 refinement package. Artwork represen-
tations were processed using MERCURY20 software. The
refined structure of [3a·2HCl] has been registered in CCDC
with the deposition number 1995455.†
Fig. 1 Pseudopeptidic macrocycles as minimalistic synzymes for the
conversion of CO2 through a cooperative multifunctional activation
mechanism. A1: preorganization of the reactive components (amide and
amine sites). A2: activation of the inert CO2 species by a Lewis base
(amine sites).
Molecular modelling
Lowest energy conformations for the different species con-
sidered were calculated at the MMFF level of theory using
Spartan08.21 Stationary points were confirmed by subsequent
frequency calculation. All vibrational frequencies were positive.
and potentially with the oxygen atom of epoxides, and in close
proximity of conformationally restricted amino groups that
3. Results and discussion
In previous supramolecular catalytic systems evaluated for this could participate in the activation of CO2 molecules.17
process, a common strategy has been the use of supramolecu-
Following this design, the pseudopeptidic macrocycle 3a
lar hosts able to strongly bind cations, affording in this way was initially tested in combination with Bu4NX salts for the
activated anions (e.g. “naked” X−),22–24 In the present case, synthesis of the organic carbonate 7 (Scheme 1). The selection
however, the design elements for the organocatalytic pseudo- of the L-valine derivative was based in the excellent results
peptides considered took into account their multifunctional observed in the synthesis of 3a from 2a and in the suitability
and highly preorganized character and their capacity to of its solubility properties, as most of these pseudopeptidic
develop cooperative supramolecular interactions with the three compounds have a rather limited solubility in styrene oxide
species involved in the process: the nucleophilic species (SO). The results obtained from the initial screening at 100 °C
(halide anions), the epoxide and the CO2 molecules. Such and ambient pressure (CO2 balloon) using 3a and SO as an
interactions
could
include
NHamide⋯X−⋯HNamide
,
epoxide of moderate-low reactivity,13a are summarized in
Oepoxide⋯HNamide and Namine⋯CO2 and cooperate to locate the Table 1. After 5 h and in the absence of the macrocycle, conver-
three components in close proximity (A1, Fig. 1), within a sions into carbonate were 49% for Bu4NI, 59% for Bu4NBr and
supramolecular complex, and with an appropriate orientation 65% for Bu4NCl (entries 1–3, Table 1). It must be noted that in
as to facilitate low energy pathways for the desired reaction, the absence of the tetrabutylammonium salt no reaction took
fully mimicking the behaviour of many enzymatic sites.
place when 3a was added (entry 4, Table 1). The use, under the
A common mechanism reported for the activation of CO2 same conditions, of an equimolecular mixture of both 3a and
has been its interaction with a Lewis base, in particular tertiary Bu4NX (1 mol% each relative to 6) led to a significant increase
amines.25,26 Thus, the Namine⋯CO2 interaction can also con- in the activity in the case of Bu4NCl, reaching a 93% conver-
tribute to this activation (A2, Fig. 1). Besides, Brønsted or sion (entry 7, Table 1) while the observed improvements where
Lewis acid sites have been used to activate the epoxide,27 minor for Bu4NBr and Bu4NBr (entries 5 and 6, Table 1). A
sometimes involving bifunctional or multifunctional complete selectivity towards the formation of 7 was observed
systems,15b,28,29 and accordingly the Oepoxide⋯HNamide can and the formation of other species was not detected by NMR.
contribute to activate the epoxide. The selected synzymatic The decomposition of tetraalkylammonium salts has been
structure 3 has a high modularity (size of the macrocyclic reported in some catalytic processes involving the same reac-
cavity, conformational preferences, polarity, etc.) as to enable a tion, but the formation of tributyl amine was not detected up
fast optimization of the catalytic efficiency.
to the detection limit of the techniques used.30 It should be
Constrained macrocycles 3, prepared as previously reported also noted that the crude of reactions with conversion close or
from open chain C2-symmetric pseudopeptides 2 and tetrakis(- lower than 50% were also analysed by chiral HPLC. In all the
bromomethyl)arenes 1 (Scheme S1†), have two amide groups cases, neither the epoxide nor the carbonate showed any
appropriately located as to cooperatively interact with anions enantioselectivity excluding the enantiopreference of the
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Table 1 Screening of 3a/Bu4NX mixtures for the reaction between
styrene oxide (6) and CO2 to afford 7 a
The catalytic performance of several tetraalkylammonium
chlorides (R4NCl) in the presence or absence of 3a was also
assayed (Table S3†). For both Bu4NCl and Et4NCl the presence
of the macrocycle 3a (0.1 mol%) led again to an increase in
conversion of ca. 1.5 times and to good TON values of 930
and 800, respectively (entries 4 and 5, Table S3†). However, no
reaction was observed for Me4NCl, most likely for the lack of
solubility of this salt in the reaction medium (entry 6,
Table S3†).
Although the initial solvent free conditions should be pre-
ferred according to the principles of green chemistry, the reac-
tion was also studied in 2-MeTHF and acetonitrile (Table S4†).
Both are non protic polar solvents in which nucleophiles can
be relatively “free” making them more reactive. The lack of an
adequate solubility of the ammonium salt and the macrocycle
precluded an efficient reaction in 2-MeTHF. Results improved,
however, in acetonitrile. At 1 bar of CO2 and 80 °C, with a 2.4
M concentration of epoxide, the conversion was 18% after 3 h
in the presence of 1 mol% of Bu4NCl and was increased more
than three times when 0.1 mol% of 3a was added (60% conver-
Bu4NX
(mol%)
3a
(mol%)
Conversionb
(%)
TON
(Bu4NX)
TON
(3a)
Entry
1
2
3
4
X = I, (1)
—
—
—
1
49
59
65
4
49
59
65
—
—
—
—
4
X = Br, (1)
X = Cl, (1)
—
5
6
7
8
X = I, (1)
1
1
1
0.1
0.1
—
—
0.1
0.01
53
67
93
93
22
16
45
73
76
53
67
93
93
220
160
450
730
760
53
67
93
930
220
—
—
730
7600
X = Br, (1)
X = Cl, (1)
X = Cl, (1)
X = Cl, (0.1)
X = Cl, (0.1)
X = Cl, (0.1)c
X = Cl, (0.1)c
X = Cl, (0.1)c
9
10
11
12
13
a 1 mL epoxide 6 (8.7 mmol), p(CO2) = CO2 balloon, 100 °C, 5 h.
b Conversions determined by 1H NMR, selectivity for 7 was >99.9% in
all cases. c p(CO2) = 10 bar.
macrocycle for a given enantiomeric epoxide, which seems sion, entries 5 and 6, Table S4†). When using only 0.01 mol%
reasonable considering the temperatures used.
of macrocycle 3a, the conversion after 3 h reached 39%, which
The same high conversion was maintained when the again corresponds with excellent TON and TOF values of 3900
amount of macrocycle was reduced from 1 to 0.1 mol%, while and 1300 h−1, respectively (entry 7, Table S4†).
keeping a 1 mol% loading of Bu4NCl (entry 8, Table 1).
As mentioned above, entries 1–3 in Table 1 reveal that for
However, in this case, the TON with respect to the 3a was sig- the 3a : Bu4NX system the reactivity order was Cl− > Br− ≫ I−.
nificantly higher, reaching a value of 930. Reducing also the This follows the basicity order and not the nucleophilicity
loading of Bu4NCl to 0.1 mol%, keeping a 1 : 1 3a : Bu4NCl order as found for other supramolecular receptors.23c Being
molar ratio, led to an important reduction in conversion (22%, the more basic anion, chloride is expected to interact stronger
entry 9, Table 1). When the same experiment was performed at with receptors 3 through hydrogen bonding to the amide NH
10 bar of CO2, a 73% conversion was observed. This result fragments, as shown in related pseudopeptides,8,32 and this
highlighted again the synergic effects between Bu4NCl and the should make chloride less available to react. Thus, the mecha-
macrocycle 3a, as in the absence of the macrocycle only a 45% nism involved cannot rely on the activation of the nucleophile,
conversion was observed (entries 11 and 12, Table 1).
but on the capacity of 3a to correctly preorganize all the react-
A ca. 1.5 times increase in conversion was always observed, ing species at short distances and with the correct orientation
for all the conditions assayed, when 1 equivalent of 3a was as to significantly enhance the reaction rate. The stronger
added to Bu4NCl. At 10 bar of CO2 the amount of macrocycle binding of Cl− (in comparison with Br− and I−) can favour the
could be further reduced to 0.01 mol%, while keeping a 1 : 10 adoption of the required conformation in the macrocyclic
3a : Bu4NCl molar ratio, with the conversion to carbonate pseudopeptide not achievable with the other two anions.
being 76% after 5 h. This represents achieving excellent TON
Thus, 1H NMR titrations in benzene-d6 (ESI, Fig. S1†)
and TOF values of 7600 and 1520 h−1. Although some compar- showed that upon addition of Bu4NCl, the NH amide proton
able or higher TON and TOF values have been described for signal underwent a large downfield shift (Δδ = 2.74 ppm). This
transition metal-based catalysts (Table S2†), often at higher variation is consistent with the development of strong
pressures and temperatures,14,31 the results presented here sig- NHamide⋯Cl−⋯HNamide H-bonding interactions (log β = 3.15
nificantly surpass those found for organocatalytic and supra- 0.01). The interaction of 3a with other halide anions was much
molecular catalytic systems, in particular when epoxides of smaller. After addition of 10 equivalents of Bu4NX the
moderate-low reactivity like SO are involved (for a detailed observed downfield shifts for the amide protons (Δδ (NH)) were
comparison see Table S1†).22,23,27–29 Some remarkable 1.43 and 0.19 ppm for Br− and I−, respectively (Table S5†). The
examples of organocatalytic systems being able to achieve high corresponding binding constants were too low to be accurately
conversions/yields of cyclic carbonates (>90%) at ambient determined. Significant changes were also observed for the
pressure and temperature have been reported. It must be signals corresponding to the methine of the stereogenic
noted, however, that most often they involve the use of more carbon (Δδ = 0.87 ppm), to the aromatic protons (Δδ =
reactive epoxides like glycidyl or propylene oxides and, 0.39 ppm) and to one of the isoindolinic protons -appearing as
besides, commonly use 2–10% catalyst loadings and extended two AB systems in the 3.3–4.6 ppm region-that shifted down-
reaction times (20–24 h) which leads to significantly reduced field from 3.42 ppm to 4.32 ppm, strongly reducing the aniso-
TON and TOF values.25,27–29
chrony of the AB system (ESI, Fig. S1 and S2†). Overall, this
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location of the epoxide on the same side of the halide and
close to the activated CO2 as envisaged in the catalyst design.
The level of structural preorganization of 3a, being a confor-
mationally constrained macrocycle can be relevant for the
results obtained. To evaluate a possible “macrocyclic effect”,34
the open chain pseudopeptidic compound 5, displaying the
same functional groups and the same connectivity than 3a,
but lacking the constraints associated to the macrocyclic struc-
ture, was synthetized (Scheme S1†) and tested.
The course of the model reaction catalysed by Bu4NCl or by
this salt in the presence of 3a or 5 (0.1 equivalents relative to
Bu4NCl) was monitored by FT-ATR-IR spectroscopy (ESI,
Fig. S5†). The conversion vs. time profiles confirmed the
importance of the macrocyclic structure, showing that the
higher conversion obtained after 5 h for the supramolecular
system 3a : Bu4NCl (65% for Bu4NCl alone, 73% in the pres-
ence of 5 and 93% in the presence of 3a) was associated to a
faster reaction rate (Fig. 4). Thus, for instance, after one hour
of reaction TOF values reached 380 h−1 for Bu4NCl, 480 h−1 for
5 : Bu4NCl and 630 h−1 for 3a : Bu4NCl.
Fig. 2 CD spectra in benzene (0.5 mM) for 3a (red) and 3a + 10 equiva-
lents of Bu4NCl (black).
suggests important conformational changes upon complexa-
tion with Cl−.
The results shown in Fig. 4 also confirmed the importance
of the tertiary amine groups in the activation of CO2
molecules.25,26 When the related pseudopeptide 2a presenting
primary amino groups was assayed, an irrelevant catalytic
effect was observed (Fig. 4). Furthermore, the double salt
[3a·2HCl], was obtained by treatment of 3a with the stoichio-
metric amount of HCl in methanol (Scheme S1†). This salt
could maintain the main structural features of [3a + Cl−] but
In the same way, the strong negative signal at 285 nm
observed in the CD of 3a in benzene (0.5 mM) essentially dis-
appeared after addition of 10 equivalents of Bu4NCl (Fig. 2)
while the effect was less intense in the presence of Bu4NBr and
minor for Bu4NI (ESI, Fig. S3†). This CD signal is assignable to
π–π* transitions of the aromatic group located in a chiral
environment and, as well known from studies related to pro-
teins and other biomolecules, is very sensible to changes in
the environment and mobility and to the presence of
additional groups at short distances.33
1
lacking the activating role of the tertiary amines. The H NMR
spectrum of this salt in DMSO-d6 (ESI, Fig. S6†) showed the
appearance of a new signal for the ammonium proton (R3N+H)
at 10.45 ppm. Additionally, the amide signal was shifted down-
Molecular modelling of the [3a + Cl−] complex shows that
in the most stable conformation both amide groups of the
macrocycle adopt a syn disposition and strongly interact with
the halide anion (Fig. 3).21 The conformation observed in the
macrocycle is similar to the one found in its X-Ray structure,17
although the two carbonyl groups display longer distances to
the corresponding methylene groups of the isoindolinic rings,
in particular in one of the cases (ESI, Fig. S4†) which agrees
well with the changes observed in 1H NMR.
field (ΔδNH
=
1.3 ppm), indicating the presence of
NHamide⋯Cl− interactions. The X-Ray crystal structure of
[3a·2HCl] contained two molecules in the asymmetric unit,
with syn and anti-arrangements of the amide groups (ESI,
In contrast, for the most stable conformation calculated for
the [3a + I−] complex, the amide groups adopt an anti-disposi-
tion, where only one amide group is interacting with the
iodide anion (Fig. 3). It must be noted that only in the syn-dis-
position of the amide groups it is possible to facilitate the
Fig. 4 Conversion vs time profiles obtained for the reaction between 6
Fig. 3 Lowest energy conformers calculated for (a) [3a + Cl−] complex and CO2 as monitored by FT-ATR-IR. Catalysts: 1 mol% Bu4NCl and
highlighting the amide groups are in syn-disposition. (b) [3a + I−]
complex highlighting the amide groups in anti-disposition.
0.1 mol% of 2a, 3a or 5. Reaction conditions: solventless, 100 °C, 5 h,
CO2 balloon.
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Fig. S7, see also Fig. S12†). Both structures exhibit be associated to the coexistence of several conformations as
Cl−⋯HNamide hydrogen bonds, particularly the one with the suggested by X-Ray data and molecular modelling studies,17
syn amide groups that seems to display an appropriate preorga- and no significant changes were observed in the presence of
nization of the cavity. However, no formation of the carbonate chloride anion (ESI, Fig. S3†). Thus, the formation of a strong
7 was obtained when 6 was heated in the presence of 1 mol% [3a + Cl−] complex displaying a highly restricted conformation-
of salt [3a·2HCl], highlighting the importance of these tertiary al mobility and an appropriate preorganization seems to be a
amino groups.
requisite for the higher activity of 3a in this catalytic process,
The length of the central aliphatic spacer linking the two despite the reduced reactivity of the anion in such a complex.
amino acid fragments is another structural element defining The larger spacers would allow, for instance, and anti-disposi-
the conformational flexibility of compounds 3 and, accord- tion of the amide groups and the location of chloride and
ingly, their level of preorganization. Hence, important differ- epoxide coordinated on opposite sides of the macrocyclic
ences were found as a function of the length of the spacer in cavity, precluding a direct interaction of these two components
macrocycles 3a–e demonstrating that a reduction in the preor- of the reaction.
ganization of the macrocyclic cavity is critical, leading to less
According to literature data, hydrogen bonding between the
active systems. Compounds with shorter spacers (n = 0 or 1, 3a oxygen atom of SO and the amide hydrogens can activate the
and 3b) afforded efficient catalytic systems (conversion > 90%, epoxide.15,28,29 1H NMR titrations of 3a with 6 in C6D6 did not
entries 1 and 2, Table 2), while for spacers of intermediate show significant changes that could be attributed to the for-
length (n = 2 or 3, 3c and 3d) some decrease in conversion was mation of the corresponding complex. However, 1H NMR
observed (ca. 80% conversion, entries 3 and 4, Table 2). spectra obtained for the [3a + Cl− + 6] system in benzene-d6 in
Finally, the macrocycle with the largest spacer (n = 8) afforded ratios similar to those in the catalytic experiments (2 mM of
a strong decay in activity (52% conversion, entry 5, Table 2).
3a, 20 mM of Bu4NCl and 240 mM of 6) suggested the for-
The increase in mobility associated to the length of the mation of a ternary complex (ESI, Fig. S8†). Thus, the complex
spacer in compounds 3 was highlighted by the changes signal for the methylene protons closer to the ammonium
observed for the signals from the four protons of the isoindoli- (R-CH2-N+R3) experienced an upfield shift (Δδ = 0.05 ppm),
1
nic rings in H NMR spectra. As mentioned, they are observed associated to the weakening of the ion pair, that is appreciably
in all solvents as two well-differentiated AB systems, the larger than the one observed in the absence of 6. Despite the
signals for the protons directed towards the macrocyclic cavity, large excess of 6 present, a minor downfield shift was also
and accordingly affected by the shielding cone of the observed for the protons of the epoxide ring suggesting the
CvOamide fragments, appearing at higher field. In CD3OD presence of a weak interaction Cl−⋯HCHORepoxide
(ESI, Fig. S2†), the larger anisochrony was observed for com-
.
In agreement with this, molecular modelling for [3a + Cl−
+
pounds with the shorter spacers (3b, Δδ = 0.59 and 0.37) and 6] showed that in the lowest energy species the rigidly pre-
decreased significantly for the macrocycles with the larger organized cavity of 3a facilitates a cooperative but asymmetric
spacers (3d and 3e). For 3e the spectrum changes to an appar- H-bonding of both syn-amide groups with the chloride anion,
ent AB single system, suggesting a high degree of confor- while 6 is located displaying short NHamide⋯O distances,
mational flexibility allowing the rotation of the aromatic unit, adopting a suitable conformation for the nucleophilic attack
accompanied by the inversion at the nitrogen atoms, with on the less hindered carbon atom and contributing to the
respect to the macrocyclic main plane.35
shielding of the anion from its interaction with Bu4N+
Besides, the CD spectrum for the less active macrocycle 3e (Fig. 5a).
(larger spacer) was like that of 3a but, in this case, the effect of
In contrast, in the lowest energy species for [3e + Cl− + 6]
the addition of Bu4NCl was minor. For the intermediate only one of the two anti-amides interacts with the chloride
macrocycle 3d the CD signal was essentially absent, which can anion, while the epoxide is located closer to the macrocycle
but adopting an orientation much less favourable for the reac-
tion to occur, as the CO2 molecule that could be interacting
with one of the amino groups should not be appropriately
located (Fig. 5b).
Table 2 Screening of macrocyclic catalysts for the reaction between
styrene oxide (6) and CO2 to afford 7 a
In the light of the former results, the observed catalytic
activity seems to rely on a cooperative multifunctional mecha-
nism based on an efficient preorganization of the different
components and participating groups: amide, tertiary amine,
anion, epoxide and CO2 (ESI, Fig. S9†). According to the
general mechanism established for the synthesis of cyclic car-
bonates from epoxides and carbon dioxide, the mechanism
depicted in Fig. 6 can be considered.30 After formation of the
[3a + Cl−] complex (A), interaction with the epoxide can lead to
the formation of the [3a + Cl− + 6] supramolecular species (B),
while CO2 can be activated by tertiary amino groups in close
Spacer
–(CH2)n+2
3 : Bu4NCl
molar ratio
Conversionc
(%)
Entry
Macrocycleb
–
1
2
3
4
5
3a
3b
3c
3d
3e
0
1
2
3
8
1 : 10
1 : 10
1 : 10
1 : 10
1 : 10
93
96
83
79
52
a 1 mL of 6 (8.7 mmol), 1 mol% of BuN4Cl, 1 : 10 macrocycle : BuN4Cl
°
molar ratio, CO2 balloon, 100 C, 5 h. Conversion achieved for BuN4Cl
alone 65%. b Obtained as previously reported in ref. 17. c Conversions
determined by 1H NMR; selectivity for 7 was >99.9 % in all the cases.
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Fig. 7 Structures of the epoxides assayed.
Fig. 5 Lowest energy conformers calculated (MMFF level of theory) for
the supramolecular species (a) [3a + Cl− + 6] and (b) [3e + Cl− + 6].
Non-essential hydrogen atoms are omitted for clarity.
Table 3 Cycloaddition of CO2 to different epoxides catalysed by 3a :
BuN4Cla
Entry
Epoxide
Conversionb (%)
TON (3a)
TON (Bu4NCl)
1
2
3
4
5
6
8
9
10
11
93
94
99
930
940
990
93
94
99
98
980
98
18 (99)c
180 (990)c
18 (99)c
a Reaction conditions: solventless, 100 °C, 5 h, CO2 balloon; 1 mol% of
BuN4Cl, 0.1 mol% of 3a. b Conversions determined by 1H NMR.
c Conversions determined by 1H NMR after 15 h of reaction without
solvent, 100 °C using a CO2 balloon.
was observed (18%), as a result of its lower reactivity associated
to the steric hindrance present (entry 5, Table 3).36 In this
case, 15 hours were required for a complete conversion of the
epoxide in the desired carbonate.
Finally, the reusability of the catalyst was also assayed using
styrene oxide as the substrate at the optimized reaction con-
ditions (Fig. S10†). The pseudopeptidic macrocycle 3a could be
recovered almost quantitatively by precipitation with cyclopen-
tyl methyl ether (CPME) and could be used after centrifugation
and decantation of the solution with only a minor decrease in
the conversion of 6 into 7 being observed after three reuses.
After the recycling protocol, the integrity of the organocatalyst
was corroborated by NMR and MS analyses, with no significant
changes observed as compared to the spectra of freshly pre-
pared 3a. Considering the small scale of the experiments,
these results support the feasibility of this methodology to
isolate and reuse the catalytic system.
Fig. 6 Proposed mechanism for the cycloaddition of styrene oxide and
CO2 catalysed by the supramolecular system 3a : R4NX.
proximity leading to the intermediate C. The attack of the
chloride to the less hindered carbon atom of the epoxide pro-
duces the requisite intermediate D, which subjected to the
insertion of CO2 generates the intermediate E. Both, the alkox-
ide and the alkylcarbonate anions, in intermediates D and E,
can be stabilised by interaction with the amide groups, redu-
cing the energy required for the ring-opening of the epoxide,
which is considered the rate determining step. Finally, the
corresponding cyclic carbonate is obtained through an intra-
4. Conclusions
molecular cyclization step to give the [3a + Cl− + 7] species (F), Overall, the present results show that pseudopeptidic macro-
from which the catalytic [3a + Cl−] complex (A) is regenerated. cycles represent a remarkable and unique scaffold for the
Nevertheless, more detailed studies, including in depth kinetic development of a synzymatic approach allowing the efficient
analyses and high level computational studies, are needed to conversion of CO2 into organic carbonates achieving high TON
fully analyze these mechanistic details, although this is out of and TOF values surpassing the best values reported for supra-
the scope of this initial work.
molecular systems under related conditions. Despite the
The coupling reactions of CO2 with several epoxides (Fig. 7) reduced reactivity that could be associated to the interaction of
were also investigated under the optimized conditions. In the chloride anion with the amide NH fragments, the confor-
general, excellent conversions (>90%) were found for reactive mational restrictions defined by the macrocyclic structure,
and moderately reactive monosubstituted epoxides (entries along with the very high functional density, facilitate an appro-
1–4, Table 3). Only for cyclohexene oxide a lower conversion priate supramolecular preorganization, with suitable distances
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and orientations, of the three components involved: the
nucleophilic anion, the epoxide and one molecule of CO2.
Simultaneously, the tertiary amino groups can activate the CO2
molecules, while the amide functionalities could activate the
epoxide and stabilize the different anionic intermediates
formed, leading to the cooperative involvement of the different
structural elements, fully mimicking the behaviour found in
enzymatic sites. Similar catalytic activities are observed for
monosubstituted epoxides, while the activity decreases with
cyclohexene oxide. A preliminary approach has been developed
allowing the recovery and reuse of the catalytic system.
Additional work is needed to fully disclose the mechanistic
aspects of the process involved, but the present results provide
a new promising entry to the development of efficient metal-
free processes for the conversion of CO2 into cyclic carbonates
from epoxides.
5 (a) Y. Zhu, J. Rebek and Y. Yu, Chem. Commun., 2019, 55,
3573–3577; (b) N. W. Wu, I. D. Petsalakis,
G. Theodorakopoulos, Y. Yu and J. Rebek, Angew. Chem.,
Int. Ed., 2018, 57, 15091–15095; (c) N. W. Wu and J. Rebek,
J. Am. Chem. Soc., 2016, 138, 7512–7515.
6 (a) B. D. Nath, K. Takaishi and T. Ema, Catal. Sci. Technol.,
2020, 10, 12–34; (b) M. Raynal, P. Ballester, A. Vidal-Ferran
and P. W. N. M. van Leeuwen, Chem. Soc. Rev., 2014, 43,
1734–1787.
7 V. Martí-Centelles, M. D. Pandey, M. I. Burguete and
S. V. Luis, Chem. Rev., 2015, 115, 8736–8834.
8 (a) V. Marti-Centelles, M. I. Burguete and S. V. Luis, J. Org.
Chem., 2016, 81, 2143–2147; (b) V. Marti-Centelles,
M. I. Burguete and S. V. Luis, Chem. – Eur. J., 2012, 18,
2409–2422; (c) J. Becerril, M. Bolte, M. I. Burguete,
F. Galindo, E. Garcia-España, S. V. Luis and J. F. Miravet,
J. Am. Chem. Soc., 2003, 125, 6677–6686.
9 (a) M. Bru, I. Alfonso, M. Bolte, M. I. Burguete and
S. V. Luis, Chem. Commun., 2011, 47, 283–285;
(b) I. Alfonso, M. Bolte, M. Bru, M. I. Burguete, S. V. Luis
and J. Rubio, J. Am. Chem. Soc., 2008, 130, 6137–6144.
10 S. V. Luis and I. Alfonso, Acc. Chem. Res., 2014, 47, 112–124.
11 (a) L. Delafontaine, T. Asset and P. Atanassov,
ChemSusChem, 2020, 13, 1688–1698; (b) M. Ronda-Lioret,
G. Rothenberg and N. R. Shiju, ChemSusChem, 2019, 12,
3896–3914; (c) P. Tomkins and T. E. Mueller, Green Chem.,
2019, 21, 3994–4013; (d) Y. Chen and T. Mu, Green Chem.,
2019, 21, 2544–2574; (e) J. Leclaire and D. J. Heldebrant,
Green Chem., 2018, 20, 5058–5081; (f) Q.-W. Song,
Z.-H. Zhou and L.-N. He, Green Chem., 2017, 19, 3707–
3728; (g) G. Fiorani, W. Guo and A. W. Kleij, Green Chem.,
2015, 17, 1375–1389.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work has been partially supported by projects UJI-B2019-
40 (Pla de Promoció de la Investigació de la Universitat Jaume
I) and RTI2018-098233-B-C22 (Ministerio de Ciencia,
Innovación y Universidades). F.E. acknowledges MECD for the
FPU fellowship. The authors are grateful to the SCIC of the
Universitat Jaume I for technical support.
12 (a) R. R. Shaikh, S. Pornpraprom and V. D’Elia, ACS Catal.,
2018, 8, 419–450; (b) M. Alves, B. Grignard, R. Mereau,
C. Jerome, T. Tassaing and C. Detrembleur, Catal.: Sci.
Technol., 2017, 7, 2651–2684; (c) H. Buttner, L. Longwitz,
J. Steinbauer, C. Wulf and T. Werner, Top. Curr. Chem.,
2017, 375, 50.
Notes and references
1 (a) Z. Huang, S. K. Kang, M. Banno, T. Yamaguchi, D. Lee,
C. Seok, E. Yashima and M. Lee, Science, 2012, 337, 1521–
1526; (b) V. Schaller and A. R. Bausch, Nature, 2012, 481,
268–269; (c) K. H. Smith, E. Tejeda-Montes, M. Poch and 13 (a) S. Subramanian, J. Oppenheim, D. Kim, T. S. Nguyen,
A. Mata, Chem. Soc. Rev., 2011, 40, 4563–4577;
(d) J. S. Moore and M. L. Kraft, Science, 2008, 320, 620–621;
(e) R. A. Segalman, Science, 2008, 321, 919–920.
W. M. H. Silo, B. Kim, W. A. Goddard III and C. T. Yavuz,
Chem, 2019, 5, 3232–3242; (b) Y. Xie, Z. Zhang, T. Jiang,
J. He, B. Han, T. Wu and K. Ding, Angew. Chem., Int. Ed.,
2007, 46, 7255–7258.
2 A. Fersht, Structure and Mechanism in Protein Science: A
Guide to Enzyme Catalysis and Protein Folding, 14 (a) K. Takaishi, B. D. Nath, Y. Yamada, H. Kosugi and
W. H. Freeman, New York, 1999.
T. Ema, Angew. Chem., Int. Ed., 2019, 58, 9984–9988;
(b) J. A. Castro-Osma, K. J. Lamb and M. North, ACS Catal.,
2016, 6, 5012–5025; (c) C. Maeda, T. Taniguchi, K. Ogawa
and T. Ema, Angew. Chem., Int. Ed., 2015, 54, 134–138;
(d) M. M. Dharman, J.-I. Yu, J.-Y. Ahn and D.-W. Park,
Green Chem., 2009, 11, 1754–1757.
3 (a) L. Wang, R. Zhang, Q. Han, C. Xu, W. Chen, H. Yang,
G. Gao, W. Qin and W. Liu, Green Chem., 2018, 20, 5311–
5317; (b) X. Cao, S. P. Teong, D. Wu, G. Yi, H. Su and
Y. Zhang, Green Chem., 2015, 17, 2348–2352;
(c) G. Rothenberg and J. H. Clark, Green Chem., 2000, 2,
248–251.
4 (a) E. Kuah, S. Toh, J. Yee, Q. Ma and Z. Gao, Chem. – Eur.
J., 2016, 22, 8404–8430; (b) Z. Dong, Q. Luo and J. Liu,
Chem. Soc. Rev., 2012, 41, 7890–7908; (c) M. J. Wiester,
15 (a) H. Zhou, H. Zhang, S. Mu, W.-Z. Zhang, W.-M. Ren and
X.-B. Lu, Green Chem., 2019, 21, 6335–6341; (b) Y.-D. Li,
D.-X. Cui, J.-C. Zhu, P. Huang, Z. Tian, Y.-Y. Jia and
P.-A. Wang, Green Chem., 2019, 21, 5231–5237.
P. A. Ulmann and C. A. Mirkin, Angew. Chem., Int. Ed., 16 (a) A. J. Kamphuis, F. Picchioni and P. P. Pescarmona,
2011, 50, 114–137.
Green Chem., 2019, 21, 406–448, DOI: 10.1039/c8gc03086c;
Green Chem.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Green Chemistry
Paper
(b) X. Liu, J. G. de Vries and T. Werner, Green Chem., 2019,
21, 5248–5255; (c) N. Yadav, F. Seidi, D. Crespy and
V. D’Elia, ChemSusChem, 2019, 12, 724–754; (d) Y. Ding,
J. Tian, W. Chen, Y. Guan, H. Xu, X. Li, H. Wu and P. Wu,
Green Chem., 2019, 21, 5414–5426.
K. Suvarnapunya, S. V. C. Vummaleti, S. Wannakao,
L. Cavallo, A. Poater and V. D′ Elia, Adv. Synth. Catal., 2019,
361, 366–373; (c) O. Sodpiban, S. Del Gobbo, S. Barman,
V. Aomchad, P. Kidkhunthod, S. Ould-Chikh, A. Poater,
V. D’Elia and J.-M. Basset, Catal. Sci. Technol., 2019, 9,
6152–6165; (d) K. Takaishi, T. Okuyama, S. Kadosaki,
M. Uchiyama and T. Ema, Org. Lett., 2019, 21, 1397–1401;
(e) X. Wu, C. Chen, Z. Guo, M. North and A. C. Whitwood,
ACS Catal., 2019, 9, 1895–1906; (f) S. Arayachukiat,
C. Kongtes, A. Barthel, S. V. C. Vummaleti, A. Poater,
S. Wannakao, L. Cavallo and V. D’Elia, ACS Sustainable
Chem. Eng., 2017, 5, 6392–6397.
17 F. Esteve, B. Altava, M. Bolte, M. I. Burguete, E. García-
Verdugo and S. V. Luis, J. Org. Chem., 2020, 85, 1138–1145.
18 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found.
Crystallogr., 2008, 64, 112–122.
19 G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem.,
2015, 71, 3–8.
20 C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington,
P. McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, 28 S. Liu, N. Suematsu, K. Maruoka and S. Shirakawa, Green
J. van de Streek and P. A. Wood, J. Appl. Crystallogr., 2008,
41, 466–470.
21 (a) B. J. Deppmeier, A. J. Driessen, T. S. Hehre, W. J. Hehre,
Chem., 2016, 18, 4611–4615.
29 N. Liu, Y.-F. Xie, C. Wang, S.-J. Li, D. Wei, M. Li and B. Dai,
ACS Catal., 2018, 8, 9945–9957.
J. A. Johnson, P. E. Klunzinger, J. M. Leonard, I. N. Pham, 30 (a) W. Clegg, R. W. Harrington, M. North and R. Pasquale,
W. J. Pietro and Y. Jianguo, Calculated using Spartan08 soft-
ware at the MMFF level of theory. Spartan ‘08, build 132
Chem. – Eur. J., 2010, 16, 6828–6843; (b) M. North and
R. Pasquale, Angew. Chem., Int. Ed., 2009, 48, 2946–2948.
(Mar 27 2009), Wavefunction Inc., Irvine CA, 2009; 31 (a) L. Wang, C. Xu, Q. Han, X. Tang, P. Zhou, R. Zhang,
(b) T. A. Halgren, J. Comput. Chem., 1996, 17, 490–519.
22 (a) A. Mirabaud, A. Martinez, F. Bayard, J.-P. Dutasta and
V. Dufaud, New J. Chem., 2018, 42, 16863–16874; (b) J. Wang,
Y. Liang, D. Zhou, J. Ma and H. Jing, Org. Chem. Front., 2018, 5,
G. Gao, B. Xu, W. Qin and W. Liu, Chem. Commun., 2018,
54, 2212–2215; (b) C. J. Whiteoak, N. Kielland, V. Laserna,
E. C. Escudero-Adán, E. Martin and A. W. Kleij, J. Am.
Chem. Soc., 2013, 135, 1228–1231.
741–748; (c) A. Mirabaud, J.-C. Mulatier, A. Martinez, 32 (a) I. Martí, M. Bolte, M. I. Burguete, C. Vicent, I. Alfonso
J.-P. Dutasta and V. Dufaud, Catal. Today, 2017, 281, 387–391;
(d) A. Mirabaud, J.-C. Mulatier, A. Martinez, J. P. Dutasta and
V. Dufaud, ACS Catal., 2015, 5, 6748–6752.
and S. V. Luis, Chem. – Eur. J., 2014, 20, 7458–7464;
(b) I. Martí, J. Rubio, M. Bolte, M. I. Burguete, C. Vicent,
R. Quesada, I. Alfonso and S. V. Luis, Chem. – Eur. J., 2012,
18, 16728–16741.
23 (a) C. Maeda, S. Sasaki, K. Takaishia and T. Ema, Catal. Sci.
Technol., 2018, 8, 4193–4198; (b) L. Martínez-Rodríguez, 33 (a) G. Pescitelli, L. Di Bari and N. Berova, Chem. Soc. Rev.,
J. O. Garmilla and A. W. Kleij, ChemSusChem, 2016, 9, 749–755;
(c) J. Shi, J. Song, J. Ma, Z. Zhang, H. Fan and B. Han, Pure
Appl. Chem., 2013, 85, 1633–1641; (d) J. Song, Z. Zhang, B. Han,
S. Hu, W. Li and Y. Xie, Green Chem., 2008, 10, 1337–1341.
24 W. Desens, C. Kohrt, M. Frank and T. Werner,
ChemSusChem, 2015, 8, 3815–3822.
2011, 40, 4603–4625; (b) B. Ranjbar and P. Gill, Chem. Biol.
Drug Des., 2009, 74, 101–120; (c) B. M. Bulheller, A. Rodger
and J. D. Hirst, Phys. Chem. Chem. Phys., 2007, 9, 2020–
2035; (d) S. M. Kelly, T. J. Jess and N. C. Price, Biochim.
Biophys. Acta, 2005, 1751, 119–139; (e) S. M. Kelly and
N. C. Price, Curr. Protein Pept. Sci., 2000, 1, 349–384.
25 L. Wang, G. Zhang, K. Kodama and T. Hirose, Green Chem., 34 (a) H. Guo, L.-W. Zhang, H. Zhou, W. Meng, Y.-F. Ao,
2016, 18, 1229–1233.
D.-X. Wang and Q.-Q. Wang, Angew. Chem., Int. Ed., 2020,
59, 2623–2627; (b) N. H. Evans and P. D. Beer, Angew.
Chem., Int. Ed., 2014, 53, 11716–11754; (c) D. K. Cabbiness
and D. W. Margerum, J. Am. Chem. Soc., 1969, 91, 6540–
6541.
26 (a) C. Chen, J. Zhang, G. Li, P. Shen, H. Jin and N. Zhang,
Dalton Trans., 2014, 43, 13965–13971; (b) Z. Z. Yang,
L. N. He, Y. N. Zhao, B. Li and B. Yu, Energy Environ. Sci.,
2011, 4, 3971–3975; (c) J. C. Meredith, K. P. Johnston,
J. M. Seminario, S. G. Kazarian and C. A. Eckert, J. Phys. 35 I. Alfonso, M. I. Burguete, F. Galindo, S. V. Luis and
Chem., 1996, 100, 10837–10848. L. Vigara, J. Org. Chem., 2007, 72, 7947–7956.
27 (a) K. A. Andrea and F. M. Kerton, ACS Catal., 2019, 9, 36 N. A. Tappe, R. M. Reich, V. D’Elia and F. E. Kühn, Dalton
1799–1809; (b) P. Yingcharoen, C. Kongtes, S. Arayachukiat,
Trans., 2018, 47, 13281–13313.
This journal is © The Royal Society of Chemistry 2020
Green Chem.