Schiff-base molecules and COFs as metal-free catalysts or silver supports for carboxylation of alkynes with CO2

Article abstract of DOI:10.1039/d1gc02118d

Carboxylation of terminal alkynes with CO2 to produce propiolic acids is an atom economical and high-value route for CO2 fixation and utilization, but the conversion under mild conditions needs transition metal catalysts. In this article, we demonstrated for the first time the transition-metal-free organocatalysts for the reaction. The efficient catalysts are Schiff bases derived from 1,3,5-triformylphloroglucinol (Tp), either homogeneous (discrete molecules) or heterogeneous (covalent organic frameworks, COFs). The key catalytic sites are phenoxo and imine groups, which activate CO2 through phenoxo-CO2 complexation and also activate the C(sp)-H bond through bifurcate C-H?Nimine and C-H?Ophenoxo hydrogen bonds. The 2,2′-bipyridyl sites in the COF also contribute to the catalytic performance. The COF catalyst is less active than the molecular one but has the advantages of heterogeneous catalysis. Higher performance was also demonstrated by combining silver nanoparticles (AgNPs) with the intrinsically catalytic COF. This work opens up the potential of developing transition-metal-free catalysts for the CO2 conversion reaction and demonstrates the new prospects of COFs as tailorable platforms for heterogeneous catalysis.

Full text of DOI:10.1039/d1gc02118d

View Article Online
View Journal
Green
Chemistry
Cutting-edge research for a greener sustainable future
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: L. Zhang, R. Bu, X.
Liu, P. Mu and E. Gao, Green Chem., 2021, DOI: 10.1039/D1GC02118D.
Volume 20
This is an Accepted Manuscript, which has been through the
Number
May 2018
Pages 1919-2160
9
7
Royal Society of Chemistry peer review process and has been
accepted for publication.
Green
Chemistry
Cutting-edge research for a greener sustainable future
rsc.li/greenchem
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1463-9262
PAPER
Paul T. Anastas et al.
The Green ChemisTREE: 20 years after taking root with the 12
principles
rsc.li/greenchem

Please do not adjust margins
Page 1 of 9
Green Chemistry
View Article Online
DOI: 10.1039/D1GC02118D
ARTICLE
Schiff-base molecules and COFs as metal-free catalysts or silver
supports for carboxylation of alkynes with CO2
Lin Zhang,a Ran Bu,a Xiao-Yan Liu,a Peng-Fei Mua and En-Qing Gao*a
Received 00th January 20xx,
Accepted 00th January 20xx
Carboxylating terminal alkynes with CO2 to produce propiolic acids is an atom-economic and high-value route for CO2 fixation
and utilization, but the conversion under mild conditions needs transition metal catalysts. In this article, we demonstrated
for the first time the transition-metal-free organocatalysts for the reaction. The efficient catalysts are the Schiff bases
derived from 1,3,5-triformylphloroglucinol (Tp), either homogeneous (discrete molecules) or heterogeneous (covalent
organic frameworks, COFs). The key catalytic sites are the phenoxo and imine groups, which activate CO2 through phenoxo-
CO2 complexation and also activate the C(sp)-H bond through bifurcate C-HNimine and C-HOphenoxo hydrogen bonds. The
2,2’-bipyridyl sites in the COF also contribute to the catalytic performance. The COF catalyst is less active than the molecular
one but has the advantages of heterogeneous catalysis. Higher performance was also demonstrated by combining silver
nanoparticles (AgNPs) with the intrinsically catalytic COF. This work opens up the potentials of developing transition-metal-
free catalysts for the CO2 conversion reaction and demonstrates the new prospects of COFs as tailorable platforms for
heterogeneous catalysis.
DOI: 10.1039/x0xx00000x
these troublesome problems, current research has been
devoted to the development of efficient catalytic systems. The
catalysts thus far reported for direct C-H carboxylation of
terminal alkynes are transition-metal catalysts, mostly silver
and copper (Scheme 1a, route D).19-21 The catalysis involves the
base-assisted formation of metal acetylide intermediate and
subsequent CO2 insertion to the M-C bond.22, 23 The catalytic
transformation can occur under more friendly and less energy-
consuming conditions with improved group tolerance. For
Introduction
CO2 is the chief culprit of the greenhouse effect and also a
valuable single-carbon (C1) source for its ubiquity, renewability,
nontoxicity and cheapness.1, 2 Therefore, the use of CO2 for
green and economical organic synthesis has attracted
continuous interest in the community of sustainable
chemistry.3-5 In this context, various C-C, C-N and C-O coupling
reactions of CO2 fixation have been developed in recent years.6-
8 Here we are focused on the carboxylation of terminal alkynes
with CO2. The reaction provides a straightforward atom-
economic route to propiolic acids, which are valuable
feedstocks for chemical and pharmaceutical industries.9-11
A general difficulty in CO2 conversion is its intrinsic inactivity
arising from the high thermodynamic stability and kinetic
inertness.12, 13 Classical carboxylation reactions with CO2 make
use of highly reactive and stoichiometric nucleophiles (bases)
such as organolithium or Grignard reagents to deprotonate
alkynes (Scheme 1a, route A).14, 15 The methods have a poor
compatibility with functional groups and require harsh
conditions (anhydrous, anaerobic and so on), which offset the
benefits of CO2. Carboxylation of terminal alkynes can proceed
smoothly in the presence of excess weak bases like Cs2CO3,16
DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene)17 and TBD (1,5,7-
triazabicyclo[4.4.0]dec-1-ene),18 but high temperature and high
CO2 pressure are needed (Scheme 1a, route B). To overcome
R'Li or R'MgX
CO2 (1 atm)
(a)
A
B
Cs2CO3, TBD or DBU
CO2 ( 2.5 ~ 120 atm), 80 ~ 120 o
Noncatalytic
H
HO
C
O
Trimethylsilylacetylene & 18-crown-6
or triethoxymethylsilane (5 equiv)
CsF or t-BuOK, CO2 ( 1 atm), 40 o
C
H+
R
C
Transition metal catalysts
(Ag, Cu, Au, Co, Nd, Mo)
Cs2CO3, CO2 (1 atm), 25 ~ 80 o
R
D
E
C
Catalytic
SalAn, TpAn, TpBpy, or Ag@TpBpy
this work
Cs2CO3, CO2 (1 atm), 60 o
C
(b)
O
O
H
N
O HN
O
N
N
NH
N
H
N
N
O
O
NH O
O
O
HN
H
N
O
O
N
N
HN
HN
O
TpBpy
N
N
TpAn
NH
O
O
NH
O
O HN
O
N
H
N
N
N
N
OH
N
O
NH O
O
SalAn
N
H
b. Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, Shanghai
200062, China. E-mail: eqgao@chem.ecun.edu.cn
†Electronic supplementary information (ESI) available: Experimental and
characterization data. See DOI: 10.1039/x0xx00000x
O
Scheme 1 (a) Routes for the carboxylation of terminal alkynes with CO2. (b)
The Schiff-base molecules and COF catalysts used in this article.
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 2 of 9
ARTICLE
Journal Name
simpler operation and better economy, recyclable imine groups, which cooperate to activate the C(sp)-H bond and
View Article Online
heterogeneous catalysts have been obtained by loading Ag, Cu CO2. The results pay the way for the deveDloOpI:m10e.1n0t3o9/fDt1rGaCn0si2t1i1o8nD-
and Au species (mostly silver nanoparticles, AgNPs) on various metal-free catalysis for the CO2-alkyne reaction, either
matrices such as inorganic oxides, organic polymers, carbon homogeneous or heterogeneous.
nitride and metal-organic frameworks (MOFs).24-27 However,
the use of transition metals also causes practical problems such
as complicated preparation, high cost, toxic metal leaching and
Results and discussion
contamination. Therefore, transition-metal-free reactions
under mild conditions are desired. In this respects, two
alkynylsilane-mediated carboxylation approaches have been
reported (Scheme 1a, route C), which involve two sequential
base-promoted (CsF or t-BuOK) reactions: silylation of terminal
alkynes and CO2 insertion into the Si-C bonds.28, 29 However,
approaches significantly reduce the atom economy and
enhance the cost of the excess use and unrecoverable
consumption of silylating reagents in these of production and
waste disposal. Obviously, the more environmentally benign
and cost-effective protocols lie in the use of transition-metal-
free catalysts or organocatalysts, preferably heterogeneous. To
our knowledge, however, such catalysts for direct carboxylation
of alkynes remain undeveloped.
Synthesis and characterization
TpBpy was synthesized from 2,2'-bipyridine-5,5'-diamine
(Bpyda) and 1,3,5-triformylphloroglucinol (Tp), using
previously reported mechanochemical method47 with some
modification. Ag@TpBpy was obtained by impregnation of
TpBpy with AgNO3 solution and subsequent reduction with
hydrazine.
a
The as-synthesized TpBpy exhibits the X-ray diffraction (XRD)
profile characteristic of the crystalline 2D hexagonal COFs (Fig.
1a). Pawley refinements of the XRD pattern produced a good
agreement with the eclipsed AA stacking of 2D hexagonal layers
in the P6/m space group with a = 29.3 Å and c = 3.5 Å. The
theoretical channel size is about 2.5 nm (HH distance between
opposite edges of the hexagonal window). The π–π stacking
distance between the COF layers is 3.5 Å using the d spacing
between the (001) planes (2 = 26.7 ± 2°). As can be seen from
Fig. 1b, the crystalline framework retains its integrity after
impregnation with AgNO3 and subsequent reduction. After
reduction, the characteristic (111) and (200) diffraction peaks of
metallic Ag appear at 2 = 38° and 44.2°, respectively, indicating
the successful immobilization of AgNPs.48
In the FT-IR spectrum of TpBpy (Fig. 1c), there are no bands
characteristic of the NH2 group (3205 and 3313 cm-1) for Bpyda
and the formyl C=O vibration (1644 cm-1) of Tp, indicating the
occurrence of imine condensation. The peaks at 1570 and 1442
cm-1 are attributable to the skeleton C=C and C=N stretching
vibrations of the pyridyl and benzene rings of the COF. The
strong absorption observed at 1604 cm-1 is generally ascribed to
ν(C=O) of the Tp-based COFs in the keto-enamine tautomeric
form. These results confirmed the successful preparation of
TpBpy as proposed. The IR spectrum after AgNPs loading is
overall similar (Fig. 1d). However, a shoulder absorption can be
discerned at the low-wavenumber side of the 1570 cm-1 band,
and the relative intensity of the 1570 cm-1 band to the 1604 cm-1
band is reversed. Always observed for different batches of Ag-
loaded COFs, these changes in IR spectra could be indicative of
the interactions between AgNPs and the organic support.
The elemental composition and states were investigated
using X-ray photoelectron spectroscopy (XPS) (Fig. S1, in the
ESI). The N 1s XPS spectrum of TpBpy (Fig. 1e) shows two peaks
at 399.9 eV and 398.7 eV, corresponding to secondary amine
and pyridyl nitrogens, respectively. After immobilization of
AgNPs, the N 1s profile changes, and the deconvolution reveals
a new peak at 399.3 eV, which could be due to the interaction
between Ag and N. The success of Ag loading is confirmed by
the presence of the XPS signals of Ag, and the gap (6.0 eV)
between the 3d3/2 (374.0 eV) and 3d5/2 (368.0 eV) is
characteristic of metallic Ag(0) (Fig. 1f).48 The small
bathochromic shift of the binding energy compared with the
Covalent organic frameworks (COFs) are crystalline organic
porous materials constructed from modular building blocks
with specific functionalities that afford dynamic and self-
regulating covalent assembly.30, 31 Since the first report by Yaghi
et al. in 2005,32 the study of COFs has rapidly grown into a
multidisciplinary field and provided versatile materials
platforms for many potential applications.33-35 In particular, the
inherent advantages make COFs intriguing for applications in
heterogeneous catalysis. They can exhibit excellent chemical
stability.36 The ordered structure allows uniform dispersion of
functional sites for the benefit of catalytic efficiency.37 The
permanent porosity and high surface area permit rapid mass
transport of substrates/products and high accessibility of
functional sites.38 The modular and tailorable structure can be
modified at the molecular level through reasonable strategies
to achieve new or better catalytic performance.39 COF-based
heterogeneous catalysis can be achieved by (i) utilizing the
active sites at the frameworks, either intrinsic or elaborately
incorporated, and (ii) using the porous organic matrices to
support metal nanoparticles or other catalytic species.40
A
number of COFs have been used for catalytic CO2 conversion,
mainly including photo-/electrocatalytic reduction and
cycloaddition with epoxides.41-43 There have been only a few
studies of alkyne-CO2 carboxylation using COF-supported metal
catalysts.44, 45
Our previous study suggested that Cu(I)-modified COFs can
promote the carboxylation reaction with concomitant Cu(I)-to-
Cu(II) oxidation and that the catalysts can be regenerated by
reduction with KI.46 Here, we report the extension of the study
to COF-supported AgNPs (Ag@TpBpy, Scheme 1a, route E),
which are more efficient and more facile to regenerate. More
interestingly, we demonstrate that the pristine imine-linked
organic framework and the corresponding discrete Schiff-base
molecules can intrinsically catalyse the carboxylation reactions.
By comprehensive comparative studies, we reveal that the key
active sites in the pure organic catalysts are the phenolato and
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 9
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
(a)
(b)
(c)
View Article Online
DOI: 10.1039/D1GC02118D
Tp
-NH2
Bpyda
TpBpy
Ag@TpBpy
Pawley refinement
AgNO3@TpBpy
Experiment
Simulated
Difference
Bragg Positions
TpBpy
Ag@TpBpy
5
10 15 20 25 30 35 40 45 50
5
10 15 20 25 30 35 40 45 50 3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
2θ (deg.)
2θ (deg.)
399.9 eV
398.7 eV
(e)
N 1s
(d)
(f)
Ag 3d
TpBpy
TpBpy
Ag@TpBpy
Ag@TpBpy
399.3 eV
1442
1604
6 eV
372
1570
402
401
400
399
398
397
1800 1650 1500 1350 1200
Wavenumber (cm-1)
380
376
368
364
Binding Energy (eV)
Binding Energy (eV)
Fig.1 (a) Experimental and simulated XRD patterns of TpBpy. The Pawley refinement is also provided (Rp=4.96%, Rwp=7.07%). (b) XRD patterns of TpBpy,
AgNO3@TpBpy and Ag@TpBpy. (c) FT-IR spectra of the precursors, TpBpy and Ag@TpBpy. (d) Magnified FT-IR spectra of TpBpy and Ag@TpBpy (e) N 1s XPS spectra
of TpBpy and Ag@TpBpy. (f) Ag 3d XPS spectra for Ag@TpBpy.
reported value for Ag(0) (3d5/2 = 368.3 eV)49 could indicate weak and 1d) show that AgNPs were well dispersed in the substrate
interactions between AgNPs and organic ingredients. with particle sizes 3 - 6 nm. The lattice fringes of metallic Ag are
The scanning electron microscope (SEM) image of TpBpy clear, with the interplanar spacing of 0.23 and 0.20 nm
indicates an irregular morphology (Fig. 2a). There is no change corresponding to the (111) and (200) planes, respectively. It is
in the morphology from the SEM images after metal impossible that the relatively large AgNPs are enclosed within
modification (Fig. 2b). The high-resolution transmission the channels or intercalated between layers in a COF crystallite.
electron microscope (HRTEM) images of AgNPs@TpBpy (Fig. 1c It is most likely that the post-synthetically introduced AgNPs are
(b)
(c)
(a)
(d)
0.20 nm
(200)
0.23 nm
(111)
20 nm
1μm
2 nm
1μm
(e)
70
(g)
(f)
100
80
60
40
20
0
800
600
400
200
0
TpBpy
Ag@TpBpy
60
50
40
30
20
10
0
TpBpy
TpBpy
Ag@TpBpy
Ag@TpBpy
2.1 wt%
0
100 200 300 400 500 600 700 800
Temperature (°C)
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
P/P0
Fig.2 (a-d) Microscopic images of TpBpy (a, SEM) and Ag@TpBpy (b, SEM; c and d, TEM). (e) TGA of TpBpy and Ag@TpBpy. (f) N2 absorption-desorption isotherms
collected at 77 K. (g) CO2 sorption isotherms at 273 K.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 4 of 9
ARTICLE
Journal Name
located at the surfaces of crystallites and/or between the COF Ag(I) state is stabilized by its coordination wiVthiewtAhrteicleNO,nNlin'e-
crystallites.50
chelating bipyridyl linker.
DOI: 10.1039/D1GC02118D
Thermogravimetric analysis (TGA) indicates high thermal
stability (up to 400 °C) for both TpBpy and Ag@TpBpy. (Fig. 2e).
The pure organic framework decomposes completely without
any residue above 640 °C. Differently, perhaps owing to an Ag-
catalysed process, the decomposition of the framework in
Ag@TpBpy is more rapid and gives a constant-weight residue
above 560°C (2.1 wt%). The residue should be elemental silver,
the amount being in good agreement with the Ag content (1.9
wt%) measured by inductively coupled plasma atomic emission
spectrometer (ICP-AES).
N2 adsorption measurements at 77 K were performed with
both TpBpy and Ag@TpBpy (Fig.2f). The COF exhibits a type-I
adsorption isotherm, and the Brunauer–Emmett–Teller (BET)
surface area is 1480 m2 g-1. Ag@TpBpy also shows a type-I
adsorption isotherm, verifying the retention of the
microporosity. The BET surface area is reduced to 761 m2 g-1
after Ag loading, which is due to pore blocking by AgNPs. CO2
adsorption was tested at 273K and 298K (Fig. 2g and Fig. S2a,
ESI). TpBpy displays an adsorption capacity of 68.3 cm3 g-1 for
CO2 at 273 K and ambient pressure. The capacity decreases to
59.4 cm3 g−1 for the AgNPs-loaded COF. The adsorption heat
calculated using the Clausius-Clapeyron equation at low CO2
pressure is 27.7 kJ mol-1 for TpBpy and 33.0 kJ mol-1 for
Ag@TpBpy (Fig. S2b in the ESI). The increased adsorption heat
suggests the higher affinity of AgNPs for CO2 molecules.
For comparative catalytic studies, a series of similar COFs and
the corresponding Ag@COFs were synthesized by similar
methods, where the 2,2'-bipyridyl or hydroxyl groups in TpBpy
were replaced by biphenyl or methoxyl, respectively (Scheme
2). The characterization data are provided in the ESI (Fig. S3). It
is notable that impregnation of TpBD with AgNO3 directly led to
Ag@TpBD without any reduction procedure (Fig. S3a). The
spontaneous reduction from Ag(I) to Ag(0) has been observed
Catalysis with Ag@COF
The carboxylation of phenylacetylene with CO2 under the
atmospheric pressure was selected as model reaction for
catalytic tests. The results of the control tests with various
catalysts or under different conditions are summarized in Table
1. Previous studies have established that N,N-
dimethylformamide (DMF) and dimethylsulfoxide (DMSO) are
the favourable solvents for the catalytic reaction (ESI, Table S2).
According to entries 1 and 2 in Table 1, Ag@TpBpy performs
much better in DMSO (yield 93% after 6 h in the presence of
Cs2CO3 at 60 °C) than in DMF (62%). Therefore, DMSO was
chosen for further tests. The control reaction in the presence of
Cs2CO3 but in the absence of the catalyst proceeded slowly with
a yield of 30% under the same conditions (Entry 3). The reaction
without Cs2CO3 gave no conversion, no matter whether
Ag@TpBpy was used or not (Entries 4 and 5). The results
confirm the crucial role of Cs2CO3 and the catalytic activity of
Ag@TpBpy. The reaction over Ag@TpBpy is significantly more
efficient than that over pure TpBpy (Entries 1 and 6), clearly
indicating that AgNPs play an important catalytic role.
Comparing the reactions with and without TpBpy (Entries 3 and
6) suggests the COF itself is also active, which is to be discussed
later. Here we focus on the effect of the matrices on the
performance of Ag@COFs. A series of isoreticular and iso-
dimensional COF matrices were used to support AgNPs (Scheme
2). First, the hydroxyl groups in TpBpy were partially or
completely changed to methoxyl. The resultant Ag catalysts,
Ag@TpTpOMeBpy (Ag content 3.2 wt%) and Ag@TpOMeBpy
Table 1 Catalytic data for carboxylation of phenylacetylene with CO2.a
catalyst
HCl
COOH
+ CO2
balloon
DMSO
for
the
imine-based
COF
derived
from
1,3,5-
benzenetricarbaldehyde and p-phenylenediamine,51 where the
residual amine groups at the edges of the COF was assumed to
be the reductant. The reductive Ag(I) incorporation observed
Entry
Catalyst
Base
Time (h)
Yieldd (%)
1
2b
3
Ag@TpBpy
Ag@TpBpy
-
Cs2CO3
Cs2CO3
Cs2CO3
-
6
6
93
62
30
-
for TpBD could arise from
a similar mechanism. The
spontaneous reduction was also observed for impregnation of
TpOMeBD with AgNO3, but no Ag(I) reduction was observed for
all of the three COFs derived from Bpyda (Fig. S3 and Fig. 1b).
Therefore, the reductivity of imine-based COFs is dependent
upon the diamine linkers used. A possible reason is that the
6
4
-
6
5
Ag@TpBpy
TpBpy
-
6
-
6
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
6
67
86
64
66
80
7
Ag@TpTpOMeBpy
Ag@TpOMeBpy
Ag@TpBD
Ag@TpBD
12
12
6
8
9
10c
6
a Reaction condition: phenylacetylene (0.5 mmol), Cs2CO3 (1.5 mmol), CO2
(balloon), catalysts (20 mg, unless otherwise specified), 60 °C, DMSO (3 mL).
b
The solvent is DMF (3 ml) instead of DMSO. c Ag@TpBD (76 mg), the Ag
Scheme 2 Various COFs and Ag@COFs synthesized in this work.
amount is equal to that for Ag@TpBpy (20 mg). d Yield of isolated product.
This journal is © The Royal Society of Chemistry 20xx
4 | J. Name., 2012, 00, 1-3
Please do not adjust margins

Page 5 of 9
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
View Article Online
Table 2 Metal-free catalysis for carboxylation of phenylacetylene with CO2.a
DOI: 10.1039/D1GC02118D
12 h
1 h
(a)
120
100
80
60
40
20
0
catalyst
HCl
COOH
+ CO2
100
100
Cs2CO3, DMSO
96
95
balloon
93
Entry
Catalyst
Yield (%)
1
2
3
4
5
6
7
8
9
None
TpBpy
TpBD
TpOMeBpy
TpOMeBD
TpAn
SalAn
PhAn
phloroglucinol
2,2’-bipyridine
30b/35c
67b/70c
52b/57c
48b
42
41
41
39
38
33b
90c
68c
22c
1
2
3
4
5
37c
Cycle
10
27c
(b)
a Reaction conditions: phenylacetylene (0.5 mmol), Cs2CO3 (1.5 mmol), CO2
(balloon), DMSO (3 mL), 60 °C, 6 h. The catalyst dosage was 20 mg for COFs
and TpAn. Other catalysts were controlled to be equivalent to TpAn in
amount of active sites. b Isolated yield. c Yield based on GC.
I
Transition-metal-free Catalysis
As mentioned above, the pure COF can catalyse the
carboxylation reaction. Although the activity is lower than that
of Ag@TpBpy, the finding is exciting because the transition-
metal-free organocatalysts for the reaction are unprecedented.
The finding is consistent with the fact that the COF matrices
strongly affect the catalytic activities of Ag@COFs. These
inspired us to further explore the transition-metal-free catalysis
(Table 2). As can be seen, TpBD and TpOMeBpy also show
intrinsic activity, which however is lower than that of TpBpy
(Entries 3 and 4). The high intrinsic activity of TpBpy compared
to other COFs could be responsible for its superiority as support
for AgNPs (vide supra). Furthermore, the reaction over
TpOMeBD gave a conversion comparable to that without any
catalyst (Entry 5). The above results suggest that both
phloroglucinol and bipyridyl moieties contribute to the intrinsic
catalytic activity of TpBpy.
II
III
10
20
2 (deg.)
30
40
50
Fig.
3 (a) Recycling tests of the Ag@TpBpy catalyst (reaction conditions:
phenylacetylene (0.5 mmol), Cs2CO3 (1.5 mmol), CO2 (balloon), Ag@TpBpy (20
mg), DMSO (3 mL), 60 °C, 12 h or 1 h). (b) XRD patterns of the fresh catalyst (I) and
those after 5 catalytic runs, the reaction time being 1 h (II) or 12 h (III).
(4.2 wt%), have higher Ag content than Ag@TpBpy (1.9 wt%).
However, the carboxylation reactions using the methoxyl-
containing catalysts gave significantly lower yields even though
the reaction time was doubled (Entries 7 and 8). Significant yield
decrease was also observed for Ag@TpBD (Ag content 0.5 wt%)
(Entries 9 and 10), in which the COF has biphenyl linkers in the
place of bipyridyl. Therefore, the catalytic activity is strongly
affected by the COF matrix that supports AgNPs. Ag@TpBpy is
the best catalyst, which could be simply because its activity
arises not only from the metal site but also from the organic
matrix (vide infra).
The recyclability of Ag@TpBpy was studied (Fig. 3a). The
catalyst isolated from a catalytic run was directly used for the
next run after simple washing and drying. When each run was
performed for 1 h, the yields for 5 consecutive runs were kept
around 40±2 %, suggesting no degradation in intrinsic activity.
When the reaction time was increased to 12 h for each run,
complete or nearly complete conversions were observed for 5
runs. XRD measurements with the used catalysts confirm that
the crystalline framework is retained (Fig. 3b). ICP-AES analysis
indicates that the Ag content decreases from 1.9 to 1.5 wt%
after 5 catalytic runs (12 h for each run), which accounts for the
slight yield decrease observed for the long-time recycling
experiments.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 6 of 9
ARTICLE
Journal Name
View Article Online
100
(a)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
DOI: 10.1039/D1GC02118D
Table 3 Carboxylation of terminal alkynes catalysed by TpBpy.a
(b)
80
60
40
20
60 °C
50 °C
TpBpy
HCl
R
COOH
R
+ CO2
balloon
Cs2CO3, DMSO
Entry
1
Alkyne
Product
Yield b (%)
67
COOH
0
0
2
4
6
8
10 12 14
0
2
4
6
8
10 12 14
Time (h)
Time (h)
80
(c)
67
(d)
Cl
Cl
COOH
2
3
4
5
6
72
70
47
42
77
69
67
66
65
60
40
20
0
O2N
O2N
COOH
COOH
TpBpy
TpBpy ( after 5 runs)
1
2
3
4
5
10
20
30
2 (deg.)
40
50
H3CO
H3CO
COOH
Cycle
COOH
Fig. 4 (a) Kinetic plots for the carboxylation of phenylacetylene with CO2.
Conditions: phenylacetylene (0.5 mmol), CO2 (1 atm, balloon), TpBpy (20 mg),
Cs2CO3 (1.5 mmol), DMSO (3 mL). (b) The fit of the data in (a) to the first-order
kinetics. (c) Recycling tests under the aforementioned conditions (60 °C, 6 h for
each run). (d) XRD patterns of the fresh and used catalysts.
a
Reaction conditions: terminal alkynes (0.5 mmol), TpBpy (20 mg), Cs2CO3
(1.5 mmol), CO2 (balloon), DMSO (3 mL), 60 °C, 6 h. b Yield of isolated product.
To further determine the effective catalytic sites in TpBpy, we
performed control experiments with several model molecules
that contain specific functional sites similar to those in the COF.
The vertex structure of TpBpy is modelled by the Schiff base
(TpAn) derived from Tp and aniline (An). The control catalytic
test suggests that TpAn has more prominent activity than TpBpy
for the CO2 insertion reaction. The model monomer affords a
high yield of 90% (Entry 6), which is comparable to that achieved
with Ag@TpBpy under identical conditions. The simpler model
phenol-imine molecule (SalAn) derived from salicylaldehyde
and aniline also significantly catalyses the reaction. With the
catalyst dosage being controlled to be equivalent in hydroxyl
and imine amount, the yield of the reaction using SalAn is
obviously lower than that with TpAn (Entry 7). This suggests that
the tri(enol-imine) core of TpAn is more efficient than the
discrete enol-imine structure of SalAn. Further tests were
performed with the imine molecule (PhAn) derived from phenyl
aldehyde and aniline, lacking the phenolic hydroxyl group. PhAn
shows no catalytic effect and even has some inhibitory effect as
compared to the reaction with Cs2CO3 alone (Entry 8). On the
other hand, phloroglucinol shows a very weak promoting effect
(Entry 9). The comparison suggests that the phenolic and imine
sites cannot efficiently promote the CO2 insertion reaction
when standing alone, but they cooperate in the enol-imine
structure to afford a prominent catalytic activity. The yield of
the catalytic reaction with 2,2’-bipyridine is similar to that
without it, so 2,2’-bipyridine alone is inactive (Entry 10).
However, the comparisons between TpBpy and TpBD and
between TpOMeBpy and TpOMeBD suggest that the 2,2’-
bipyridyl moieties in the COFs does contribute to the catalytic
reaction. This reflects that the heterogeneous catalytic process
in the confined space of the porous solid is different from the
homogeneous one. Therefore, the catalytic activity of TpBpy
mainly arises from the enol-imine group at the corner of the
hexagonal channel. The discrete Schiff-base counterpart TpAn
shows a higher activity, but TpBpy is superior to TpAn with
respect to operational facileness and environmental
friendliness because the COF is insoluble and permits
heterogeneous catalysis.
Our further studies were focused on TpBpy. The solvent and
base effects on the metal-free catalyst were investigated (Table
S1). The catalytic reaction in presence of Cs2CO3 can also
proceed in DMF and propylene carbonate (PC), but the
conversion is significantly slower than that in DMSO. Only minor
conversion was observed for the reaction with acetonitrile or
dioxane as solvent. Therefore, DMSO is the optimal solvent for
TpBpy. No product was detected for the catalytic reaction in the
presence of other bases, including Na2CO3, K2CO3, KOH or
triethylamine (Entries 6-9, Table S1). The observation indicates
the uniqueness of Cs2CO3 for the reaction. Not only does it serve
to provide the basic condition, but the Cs+ cation also plays a
crucial role in the catalytic process, irreplaceable by Na+ and K+.
Kinetic studies for the catalytic carboxylation reaction of
phenylacetylene were carried out at 50 and 60 °C in DMSO-d6.
1
The yield was analysed at given time intervals using H NMR.
The yield at 60 °C reached 95 % in 14 h (Fig. 4a). The metal-free
heterogeneous catalytic efficiency is even higher than many Ag
and Cu catalysts,44, 52 which often need a longer time to reach a
conversion above 90%. Decreasing the reaction temperature by
10 C leads to a significant reduction in yield (76 %). As
demonstrated in Fig. 4b, the conversions at both temperatures
follow the first-order kinetics with reaction rate constants of
0.00164 and 0.00304 s-1 for 50 and 60 °C, respectively (R2 > 0.99
for the linear regression). The activation energy was calculated
to be 51.3 kJ mol-1 according to the Arrhenius equation.
The substrate scope of the metal-free heterogeneous
catalysis was examined. Under the mild conditions (60 °C, 1 atm
CO2 balloon), different terminal alkynes, including aliphatic and
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 9
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
aromatic ones, can smoothly undergo the CO2 insertion
reaction. The reactivity of different alkynes over the catalyst
was checked by comparing the intermediate conversions at
fixed reaction time. As can be seen from Table 3, the
phenylacetylene derivatives with electron-withdrawing groups
are more reactive than those with electron-donating groups.
The substituent effects suggest that the reactivity is associated
with the polarity or acidity of the terminal C-H bond: the
electron-withdrawing substituents increase the polarity, and a
higher polarity is beneficial to activation of the bond through
hydrogen bonding.
As shown in Fig. 4c, the activity of the metal-free COF catalyst
is fully retained in the 5 cycles tested. XRD measurements
confirm that the crystalline framework remains unchanged
after use (Fig. 4d). The results show the good recyclability of
TpBpy, whose active sites are intrinsic to the framework and
unleachable.
View Article Online
DOI: 10.1039/D1GC02118D
(a)
A
B
C
D
1020 cm-1
C-O
E
2000
1600
1200
800
400
Wavenumber (cm-1)
(b)
H
The catalytic performance of Ag@TpBpy and TpBpy are
compared in Table S2 with that of previously reported Cu and
Ag catalysts with various solid supports. Generally, Ag is
superior to Cu in term of turnover frequency (TOF). Ag@TpBpy
outperforms most previous Ag catalysts, including two previous
COF-supported (Entries 8 and 9).44, 45 Free of metal, TpBpy is
superior to some supported Ag catalysts in term of mass
efficiency, which we define as the amount of product yielded
per unit of catalyst mass in unit time. Although TpBpy is still less
efficient than most metal catalysts, our finding opens the
possibility of metal-free catalysts for direct alkyne
carboxylation.
N
H
N
N
H
O
H
9.0 8.5 8.0 7.5 7.0
3.2
f1 (ppm)
3.1
3.0
Fig.5 (a) FT-IR spectra of various samples of TpBpy. (A) as-prepared; (B) treated with
CO2; (C-E) samples after base treatment (Cs2CO3/DMSO), washing (DMSO and
acetone) and drying (C), CO2 treatment (D), and heating in vacuum (E). (b) 1H NMR
of phenylacetylene in CDCl3 before and after addition of 2,2’-bipyridine and SalAn.
Mechanism consideration
The catalytic mechanism of solid-supported Ag NPs for terminal
alkyne carboxylation has been well established, including four
main steps: (i) activation of the terminal C(sp)–H bond through
-complexation, (ii) simultaneous deprotonation and M–C(sp)
bond formation, (iii) insertion of CO2 to M–C(sp) to generate M-
propiolate, and (iv) decomposition of M-propiolate to release
propiolate and the catalytic metal center.22, 23
phenolates. This is reminiscent of the Kolbe-Schmitt reaction, a
long-known carboxylation reaction for synthesis of
hydroxybenzoic acids from alkali metal phenolates (PhOA, with
A = alkali metal ion) and CO2.56 Experimental and computational
studies support that CO2 and PhOA form an intermediate PhOA-
CO2 complex at the first stage of the reaction (Scheme 3a).57, 58
Similar phenoxo-CO2 complexes have been proposed to be
responsible for the reversible equimolar capture of CO2 with
phosphonium phenolate ionic liquids (Scheme 3b).59
Accordingly, in the catalytic carboxylation of terminal alkynes,
we can assume that CO2 is activated by forming similar
intermediate complexes with the phenoxo sites of the catalysts.
To probe the interactions of CO2 with TpBpy, the COF was pre-
treated with the saturated solution of Cs2CO3 in DMSO,
thoroughly washed with DMSO and then stirred in CO2
atmosphere for 30 min without any solvent. As shown in Fig. 5a,
no appreciable change in IR spectra was observed after
treatment with Cs2CO3 and washing, but the subsequent
treatment with CO2 led to a new IR peak appears at 1020 cm-1,
which disappeared after heating. The new IR absorption can be
attributed to weak C-O bonds between CO2 and phenoxo. In
contrast, the COF without Cs2CO3 pretreatment does not show
any IR changes after stirred in CO2 atmosphere. Therefore, we
can assume that the COF is deprotonated to generate phenoxo
For the metal-free catalytic activity observed for TpBpy, TpAn
and SalAn, we have demonstrated (vide supra) that the key is
the combination of the phenol and imine groups in these
structures. The phenol-imine combinational site may serve to
activate both CO2 and terminal alkynes. Considering the pKa
-
values, 8.8 for phloroglucinol53 (9.9 for phenol54), 10.3 for HCO3
,
and ~28.7 for phenylacetylene,55 it is not easy for
phenylacetylene to be deprotonated by Cs2CO3, but
phloroglucinol or phenol can be deprotonated to form cesium
O
O
O
A
O
O
O
(a)
(b)
Scheme 3 Intermediates proposed for the Kolbe-Schmitt reaction (a, A =
alkali metal) and for CO2 capture by phenolate-based ionic liquids (b).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 8 of 9
ARTICLE
Journal Name
Cs2CO3
CsHCO3
Conclusions
View Article Online
Some new catalysts have been descriDbOedI: 10fo.1r03t9h/De1GcCa0ta21ly18tiDc
carboxylation of terminal alkynes with CO2. We found that the
phenol-imine Schiff bases, either as discrete molecules or COFs,
can efficiently promote the C-C coupling reaction. This is the
first demonstration of transition-metal-free organocatalysts,
either homogeneous or heterogeneous, for the reaction. The
effective catalytic sites are the phenoxo and imine groups,
which activate CO2 through phenoxo-CO2 complexation and
also activate the C(sp)-H bond through hydrogen bonds. The
molecular catalyst TpAn is more active than the COF catalyst
TpBpy, but the latter has the advantages of heterogeneous
catalysis and therefore is superior in sustainability. Combining
both metal sites and organic sites, Ag@TpBpy affords an activity
higher than the pure COF and comparable to the homogeneous
organocatalyst, at the cost of using precious metals. This work
provides optional catalysts for the CO2-alkyne reaction and
especially sheds new light on the developing of transition-
metal-free catalysts for the reaction.
N
N
N
H
O
O-Cs+
OH
O
C
O
R
R
R
N
H
O
Cs+O-
O
O
H
O-
Cs+
Scheme 4 The carboxylation reaction mechanism for the metal-free catalyst
used in this study.
groups, which form PhO-CO2 (or PhOCs-CO2) complexes to
activate CO2. The presence of basic phenoxo groups is
supported by the fact that soaking the Cs2CO3-treated and
washing TpBpy in water leads to a basic solution (no pH change
was observed in the control test with the pristine COF).
Furthermore, CO2 adsorption experiments reveal that the COF
after Cs2CO3 treatment shows much higher adsorption heat at
low CO2 loading than the pristine COF (ESI, Fig. S4), which
confirms greatly enhanced affinity to CO2 owing to
deprotonation.
On the other hand, the C(sp)-H group of terminal alkynes can
form bifurcate hydrogen bonds with the chelating phenoxo-
imine sites. The interaction was confirmed by 1H NMR (Fig. 5b).
The terminal H signal of phenylacetylene shifts from 3.062 to
3.090 ppm after the addition of SalAn, indicating the hydrogen
bonding of C(sp)-H with the phenol-imine sites. We found that
the imine linkage of SalAn is broken after treated with Cs2CO3,
which prevents us from detecting the NMR change using
deprotonated SalAn. However, the phenol group in TpBpy can
be deprotonated without breaking the linkage. The
deprotonated phenoxo-imine sites in the COF should be more
prone to form hydrogen bonds.
On the basis of the above analysis, we proposed Scheme 4 for
the metal-free catalytic mechanism through the phenol-imine
sites. The key is the formation of the intermediate complex in
which the C(sp)-H group is activated by chelating hydrogen
bonding and CO2 is activated by phenoxo. The intermediate
complex can undergo proton transfer from C(sp)-H to phenoxo
oxygen and simultaneous nucleophilic addition of alkynyl to
activated CO2, yielding the propiolate product and regenerating
the phenol-imine sites. Additionally, the 2,2’-bipyridyl sites in
TpBpy can provide an alternative approach for C(sp)-H
activation. As shown in Fig. 5b, the 1H NMR signal of C(sp)-H is
shifted from 3.062 to 3.087 ppm upon adding 2,2’-bipyridine to
the phenylacetylene/CDCl3 solution, suggesting intermolecular
interactions. Probably, the C(sp)-H group can form bifurcate
hydrogen bonds with 2,2’-bipyridine.
Experimental
Synthesis of TpBpy
342.4 mg (1.8 mmol) of p-toluene sulfonic acid monohydrate
was thoroughly mixed was with Bpyda (83.8 mg, 0.45 mmol) in
a mortar. Tp (63 mg, 0.3 mmol) was added and mixed
adequately, and then 10 ~ 50 μL of water was added. The
mixture was continuously ground until the formation of a
dough. The dough was sealed in a glass vial and heated for 3 h
at 120 °C. After cooling to room temperature, a deep reddish
hard solid was obtained. The product was ground to a powder,
washed with hot water, DMF and acetone, and dried in a
vacuum oven.
Synthesis of Ag@TpBpy
To an aqueous suspension of TpBpy (10 mL, 6 mg mL-1), the
AgNO3 aqueous solution (10 mL, 8.8 mmol L-1) was added. The
mixture was stirred at room temperature in the dark for 30 min.
The solid (AgNO3@TpBpy) was filtered out, washed with water
and acetone, and dried in a vacuum oven. AgNO3@TpBpy was
redispersed in water (10 mL), and hydrazine hydrate (85%, 500
μL) was added. After stirring for 30 min at room temperature,
Ag@TpBpy was filtered, washed with water and acetone, and
dried in a vacuum oven.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (Grant No. 21773070 and 21971069).
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 9
Green Chemistry
Please do not adjust margins
Journal Name
ARTICLE
33. Q. Sun, B. Aguila, J. Perman, L. D. Earl, C. W. AbneVyi,ewY.ACrthicelenOgn,liHne.
Wei, N. Nguyen, L. Wojtas and S. Ma, JD. OAIm: 1.0C.1h0e3m9/.DS1oGcC.,022101187D,
139, 2786-2793.
34. X. Wang, L. Chen, S. Y. Chong, M. A. Little, Y. Wu, W.-H. Zhu, R.
Clowes, Y. Yan, M. A. Zwijnenburg, R. S. Sprick and A. I. Cooper,
Nat. Chem., 2018, 10, 1180-1189.
35. Q. Xu, H. Zhang, Y. Guo, J. Qian, S. Yang, D. Luo, P. Gao, D. Wu,
X. Li, Z. Jiang and Y. Sun, Small, 2019, 15, 1905363.
36. Z. Fu, X. Wang, A. M. Gardner, X. Wang, S. Y. Chong, G. Neri, A.
J. Cowan, L. Liu, X. Li, A. Vogel, R. Clowes, M. Bilton, L. Chen, R.
S. Sprick and A. I. Cooper, Chem. Sci., 2020, 11, 543-550.
37. W. K. Haug, E. R. Wolfson, B. T. Morman, C. M. Thomas and P.
L. McGrier, J. Am. Chem. Soc., 2020, 142, 5521-5525.
38. X. Zhao, P. Pachfule, S. Li, T. Langenhahn, M. Ye, C. Schlesiger,
S. Praetz, J. Schmidt and A. Thomas, J. Am. Chem. Soc., 2019,
141, 6623-6630.
39. Y. Zhi, Z. Wang, H.-L. Zhang and Q. Zhang, Small, 2020, 16,
2001070.
40. J. Guo and D. Jiang, ACS Cent. Sci., 2020, 6, 869-879.
41. W. Zhong, R. Sa, L. Li, Y. He, L. Li, J. Bi, Z. Zhuang, Y. Yu and Z.
Zou, J. Am. Chem. Soc., 2019, 141, 7615-7621.
42. H.-J. Zhu, M. Lu, Y.-R. Wang, S.-J. Yao, M. Zhang, Y.-H. Kan, J. Liu,
Y. Chen, S.-L. Li and Y.-Q. Lan, Nat. Commun., 2020, 11, 497.
43. P. Puthiaraj, H. S. Kim, K. Yu and W.-S. Ahn, Microporous
Mesoporous Mater., 2020, 297, 110011.
44. Q.-Q. Dang, C.-Y. Liu, X.-M. Wang and X.-M. Zhang, ACS Appl.
Mater. Interfaces, 2018, 10, 27972-27978.
45. X. Lan, C. Du, L. Cao, T. She, Y. Li and G. Bai, ACS Appl. Mater.
Interfaces, 2018, 10, 38953-38962.
46. R. Bu, L. Zhang, L.-L. Gao, W.-J. Sun, S.-L. Yang and E.-Q. Gao,
Mol. Catal., 2021, 499, 111319.
47. S. Karak, S. Kandambeth, B. P. Biswal, H. S. Sasmal, S. Kumar, P.
Pachfule and R. Banerjee, J. Am. Chem. Soc., 2017, 139, 1856-
1862.
48. D. Chakraborty, P. Shekhar, H. D. Singh, R. Kushwaha, C. P.
Vinod and R. Vaidhyanathan, Chem. - Asian J., 2019, 14, 4767-
4773.
49. N. Salam, S. K. Kundu, R. A. Molla, P. Mondal, A. Bhaumik and S.
M. Islam, RSC Adv., 2014, 4, 47593-47604.
50. J. J. Jarju, A. M. Lavender, B. Espina, V. Romero and L. M.
Salonen, Molecules, 2020, 25.
51. L. Wang, H. Xu, Y. Qiu, X. Liu, W. Huang, N. Yan and Z. Qu, J.
Hazard. Mater., 2020, 389, 121824.
52. M. Trivedi, G. Singh, A. Kumar and N. P. Rath, Dalton Trans.,
2015, 44, 20874-20882.
References
1. L.-J. Zhou, W. Sun, N.-N. Yang, P. Li, T. Gong, W.-J. Sun, Q. Sui
and E.-Q. Gao, ChemSusChem, 2019, 12, 2202-2210.
2. E. A. Quadrelli and G. Centi, ChemSusChem, 2011, 4, 1179-1181.
3. Q.-W. Song, Z.-H. Zhou and L.-N. He, Green Chem., 2017, 19,
3707-3728.
4. M. He, Y. Sun and B. Han, Angew. Chem., Int. Ed., 2013, 52,
9620-9633.
5. Q. Liu, L. Wu, R. Jackstell and M. Beller, Nat. Commun., 2015, 6,
5933.
6. A. J. Kamphuis, F. Picchioni and P. P. Pescarmona, Green Chem.,
2019, 21, 406-448.
7. W.-S. Liu, L.-J. Zhou, G. Li, S.-L. Yang and E.-Q. Gao, ACS
Sustainable Chem. Eng., 2021, 9, 1880-1890.
8. S.-L. Hou, J. Dong and B. Zhao, Adv. Mater., 2020, 32, 1806163.
9. Y. Yuan, C. Chen, C. Zeng, B. Mousavi, S. Chaemchuen and F.
Verpoort, ChemCatChem, 2017, 9, 882-887.
10. H. Cheng, B. Zhao, Y. Yao and C. Lu, Green Chem., 2015, 17,
1675-1682.
11. G. Bresciani, F. Marchetti and G. Pampaloni, New J. Chem., 2019,
43, 10821-10825.
12. M. Poliakoff, W. Leitner and E. S. Streng, Faraday Discuss., 2015,
183, 9-17.
13. C.-X. Guo, B. Yu, J.-N. Xie and L.-N. He, Green Chem., 2015, 17,
474-479.
14. B. Pieber, T. Glasnov and C. O. Kappe, RSC Adv., 2014, 4, 13430-
13433.
15. A. Polyzos, M. O'Brien, T. P. Petersen, I. R. Baxendale and S. V.
Ley, Angew. Chem., Int. Ed., 2011, 50, 1190-1193.
16. D. Yu and Y. Zhang, Green Chem., 2011, 13, 1275-1279.
17. F.-W. Li, Q.-L. Suo, H.-L. Hong, N. Zhu, Y.-Q. Wang and L.-M. Han,
Tetrahedron Lett., 2014, 55, 3878-3880.
18. X. Wang, Y. N. Lim, C. Lee, H.-Y. Jang and B. Y. Lee, Eur. J. Org.
Chem., 2013, 2013, 1867-1871.
19. S. Ghosh, A. Ghosh, S. Riyajuddin, S. Sarkar, A. H. Chowdhury, K.
Ghosh and S. M. Islam, ChemCatChem, 2020, 12, 1055-1067.
20. Z. Wu, L. Sun, Q. Liu, X. Yang, X. Ye, Y. Hu and Y. Huang, Green
Chem., 2017, 19, 2080-2085.
21. D. J. Shah, A. S. Sharma, A. P. Shah, V. S. Sharma, M. Athar and
J. Y. Soni, New J. Chem., 2019, 43, 8669-8676.
22. F. Manjolinho, M. Arndt, K. Goossen and L. J. Goossen, ACS
Catal., 2012, 2, 2014-2021.
23. D. Yu and Y. Zhang, Proc. Natl. Acad. Sci. U. S. A., 2010, 107,
20184-20189.
24. P. Yang, S. Zuo, F. Zhang, B. Yu, S. Guo, X. Yu, Y. Zhao, J. Zhang
and Z. Liu, Ind. Eng. Chem. Res., 2020, 59, 7327-7335.
25. W. Zhang, Y. Mei, X. Huang, P. Wu, H. Wu and M. He, ACS Appl.
Mater. Interfaces, 2019, 11, 44241-44248.
26. W.-J. Sun and E.-Q. Gao, Appl. Catal., A, 2019, 569, 110-116.
27. A. H. Chowdhury, U. Kayal, I. H. Chowdhury, S. Ghosh and S. M.
Islam, ChemistrySelect, 2019, 4, 1069-1077.
28. B. Yu, P. Yang, X. Gao, Z. Yang, Y. Zhao, H. Zhang and Z. Liu, Sci.
China: Chem., 2018, 61, 449-456.
29. B. Yu, P. Yang, X. Gao, Z. Z. Yang, Y. F. Zhao, H. Y. Zhang and Z.
M. Liu, New J. Chem., 2017, 41, 9250-9255.
30. K. Geng, T. He, R. Liu, K. T. Tan, Z. Li, S. Tao, Y. Gong, Q. Jiang
and D. Jiang, Chem. Rev., 2020, 120, 8814-8933.
31. S. Kandambeth, K. Dey and R. Banerjee, J. Am. Chem. Soc., 2019,
141, 1807-1822.
53. M. Lohrie and W. Knoche, J. Am. Chem. Soc., 1993, 115, 919-924.
54. J. M. Sanchez-Ruiz, J. Llor and M. Cortijo, J. Chem. Soc., Perkin
Trans. 2, 1984, 2047-2051.
55. F. G. Bordwell, G. E. Drucker, N. H. Andersen and A. D.
Denniston, J. Am. Chem. Soc., 1986, 108, 7310-7313.
56. R. Schmitt, Journal für Praktische Chemie, 1885, 31, 397-411.
57. Y. Kosugi, M. A. Rahim, K. Takahashi, Y. Imaoka and M. Kitayama,
Appl. Organomet. Chem., 2000, 14, 841-843.
58. Z. Markovic, S. Markovic, N. Manojlovic and J. Predojevic-
Simovic, J. Chem. Inf. Model., 2007, 47, 1520-1525.
59. C. Wang, H. Luo, H. Li, X. Zhu, B. Yu and S. Dai, Chem. - Eur. J.,
2012, 18, 2153-2160.
32. A. P. Cote, A. I. Benin, N. W. Ockwig, M. O'Keeffe, A. J. Matzger
and O. M. Yaghi, Science, 2005, 310, 1166-1170.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins