Biguanide-functionalized hierarchical porous covalent organic frameworks for efficient catalysis of condensation reactions

Article abstract of DOI:10.1016/j.mcat.2021.111663

Covalent organic frameworks (COFs) can be rationally designed with desired physicochemical properties for a far-ranging application in catalytic systems. Herein, a biguanide-functionalized covalent organic framework was designed and prepared via N-alkylat

Full text of DOI:10.1016/j.mcat.2021.111663

Molecular Catalysis 509 (2021) 111663
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Biguanide-functionalized hierarchical porous covalent organic frameworks
for efficient catalysis of condensation reactions
,
Kai Gong1 *, Daquan Zhang1, Yunyun Wang, Cunhao Li, Huimin Zhang, Haoran Li,
Huiru Feng
School of Pharmaceutical Sciences, Jiangnan University, Wuxi, 214122, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Covalent organic frameworks (COFs) can be rationally designed with desired physicochemical properties for a
far-ranging application in catalytic systems. Herein, a biguanide-functionalized covalent organic framework was
designed and prepared via N-alkylation reaction, exhibiting a granular aggregate structure with high specific
surface area. The biguanide-decorated DG–COF can be used as a highly efficient basic catalyst for the Knoeve-
nagel condensation and Knoevenagel-Michael-cyclocondensation reaction. The present methodology is of
excellent yields, short reaction times and simple operational procedure. DG–COF catalytic performance does not
change after eight-times regeneration.
Covalent organic frameworks
Knoevenagel condensation reaction
Knoevenagel-Michael-cyclocondensation
reaction
Biguanide
Heterogeneous basic catalyst
1. Introduction
overcome these issues, various heterogeneous catalysts have been
developed via the immobilization catalytic sites into porous materials,
Condensation reactions have attracted increasing attentions owing
to their merits of good atomic economy, high selectivity and simple
procedures [1,2]. Researchers have made great efforts on the conden-
sation reactions to develop a wide range of applications in organic,
pharmaceutical, and material chemistry [3-6]. Knoevenagel condensa-
tion is a powerful approach to form carbon–carbon double bonds, which
involves carbonyl compounds reacting with active methylene com-
pounds [7,8]. The Knoevenagel products, as Michael acceptors, are high
active olefins to further react with various nucleophiles for producing a
variety of organic compounds. Tandem reaction is that three or more
reactants are performed in one-pot, which can eliminate the tedious
process for purification and separation of intermediate byproducts to
such as silica, carbon materials (carbon nanotubes), oxides nano-
particles, polymers, metal organic frameworks (MOFs), and covalent
organic frameworks (COFs) [15-21]. Even so, developing a versatile
heterogeneous catalyst is still in great demand for the condensation
reactions.
Among of aforementioned porous materials, COFs have gained more
interest in developing of heterogeneous catalysts for synthetic method-
ologies because they can be predesigned and controllably synthesized
with precise tailored microstructure and devisable catalytic functions
[22,23]. A variety of catalytic sites, such as amines, porphyrins, sulfonic
acid, bipyridines, melamine, amino acids and pyrrolidines, have been
introduced to functionalized COFs as heterogeneous catalysts for various
organic reactions [24-28]. From the perspective of organic structural
chemistry, there are many more types of functionalized COFs than have
been reported so far. Therefore, developing more novel functionalized
COFs as heterogeneous catalysts is worth looking forward to. The
rational design of COFs is of significant importance and desired in both
academic research and industrial applications.
reduce
economic
costs
and
minimize
pollution
[9].
Knoevenagel-Michael-cyclocondensation reaction is a typical tandem
reaction between aldehydes, malononitrile and dimedone for the syn-
thesis of 4H-benzopyran derivatives [10,11]. The Knoevenagel
condensation and Knoevenagel tandem reactions are traditionally per-
formed in acidic or basic conditions, such as pyridine, piperidine and
amino acids [12-14]. Regretfully, these homogeneous catalysts suffer
from one or more fatal problems including long reaction time, higher
reaction temperature, undesired side reactions, tedious work-up, espe-
cially in non-reusable catalyst and environmental pollution. To
Guanidine, as nitrogen-enriched base group, have been widely used
as homogeneous basic catalysts in organic synthesis and as ligand in
organometallic chemistry [29–33]. To improve the catalytic cycle per-
formance, guanidine has been introduced to the appropriate porous
* Corresponding author.
E-mail address: kingong222@163.com (K. Gong).
1
These authors contributed equally.
https://doi.org/10.1016/j.mcat.2021.111663
Received 14 March 2021; Received in revised form 11 May 2021; Accepted 18 May 2021
Available online 4 June 2021
2468-8231/© 2021 Published by Elsevier B.V.

K. Gong et al.
Molecular Catalysis 509 (2021) 111663
materials as carriers, such as silica, fullerene (C60), resin, Fe3O4 nano-
particles and MOFs (ZIF-90) [34–38]. But it is seldom reported that the
experiment research on COFs functionalized by guanidine groups. COFs
exhibit superiority due to uniform adjustable hole structure, structural
and chemical stability. As known to all, the catalytic performance of
guanidine based hybrid is highly dependent on the type of employed
supports with various porous structure and physicochemical property.
Therefore, seeking of novel specially constructed COFs is very important
to enhance the overall catalytic performance of guanidine based
catalysts.
We are much concerned that the fabrication of novel functionalized
COFs with exclusive catalytic sites and their application in condensation
reactions. Combining the advantages of COFs and guanidine, we hope
that COFs functionalized by guanidine will succeed in the heterogeneous
Scheme 1. Synthetic route for DG-COF
Fig. 1. FT-IR spectra of (a) N, N’-Bis(4-bromophenyl) biguanide and (b) DG-COF
2

K. Gong et al.
Molecular Catalysis 509 (2021) 111663
and used without further purification.
Fourier transform infrared (FT-IR) spectra were recorded on a Bruker
1
Tensor II spectrometer. H NMR and 13C NMR spectra were obtained
from solution DMSO-d6 or CDCl3 with TMS as internal standard using a
BRUKER AVANCE III (400 MHz) spectrometer. Solid-state nuclear
magnetic resonance (SS NMR) spectra were recorded on an Agilent
600M spectrometer. Thermogravimetric Analysis (TGA) was obtained
under nitrogen using a NETZSCH STA 449F3 thermal analyzer. The
surface areas were evaluated by the Brunauer-Emmett-teller (BET,
Micromeritics ASAP 2460). The Barrett-Joyner-Halenda (BJH) method
was used to calculate the pore volume and pore sizes. The Hammett
indicator approach was employed to assess the basic strength of the COF
catalyst using phenolphthalein (H_, 9.8), 2,4-dinitroaniline (H_, 15.0),
and 4-nitroaniline (H_,18.4) as Hammett indicators. The NH3
temperature-programmed-desorption (TPD) was obtained with
a
chemisorption analyzer (Bel Cata II) to evaluate the basicity of the
prepared COF catalyst. The morphologies of DG-COF were visualized by
using scanning electron microscopy (SEM) taken with a Hitachi SU8010
microscope. Transmission electron microscopy (TEM) images were ob-
tained using a JEOL JEM 2100 PLUS microscope.
Fig. 2. Solid-state 13C NMR of DG-COF.
2.2. Synthesis of N, N’-bis(4-bromophenyl) biguanide
catalytic system for organic reactions. Herein, we would be happy to
report on the synthesis of biguanide-functionalized COF (DG-COF) and
its application in the Knoevenagel condensation and the Knoevenagel-
Michael-cyclocondensation reaction. As far as we know, this work
firstly reported the experimental investigation of biguanide-
functionalized COF, exhibiting high efficient catalytic performance,
which could benefit from its specific hierarchical structure.
A mixture of sodium dicyanamide (0.49 g, 5.5 mmol) and 4-bromoa-
niline (2.06 g, 12 mmol) was added to 20 mL water in 50 mL round-
bottomed flask. The mixture was heated at 100◦C. And then, hydro-
chloric acid solution (1 M, 25 mL) was slowly dropwise added and
refluxed for 3 h. After the reaction, the solvent was removed by rotary
evaporation, and then hot ethanol (25 mL) was added. The white solid
was filtered while it was hot, washed twice with ethanol, and dried to
obtain a white powder, namely N, N’-bis(4-bromo phenyl) biguanide,
which can be used directly in the next reaction.
2. Experimental section
2.1. Materials and methods
All chemicals and reagents were obtained from commercial sources
Fig. 3. (a) and (b) SEM images of DG-COF. (c), (d) and (e) TEM images of DG-COF.
3

K. Gong et al.
Molecular Catalysis 509 (2021) 111663
Fig. 4. N2 adsorption-desorption isotherms and pore size distribution of DG-COF.
2.3. Synthesis of DG-COF
H2O (5 mL) were added to a reaction tube. The mixture was stirred at
room temperature for an appropriate time (monitored by TLC). After
completion of the reaction, ethyl acetate was added to the reaction tube.
DG-COF was separated by filtration, washed with ethyl acetate, dried
under in vacuum to be recycled and directly reused in the next reaction.
The combined organic solvent was concentrated under reduced pres-
sure. The resulting crude product was purified using flash column
chromatography to afford the desired product. The recycle experiments
were performed for 8 times, and the yield of each reaction is calculated.
Knoevenagel-Michael-cyclocondensation reaction: Benzaldehyde
(106 mg, 1 mmol), malononitrile (73 mg, 1.1 mmol), dimedone (154
mg, 1.1 mmol), DG-COF (2 mol%, 7 mg) and H2O (5 mL) was added to a
reaction tube. The resulting mixture was stirred at room temperature for
an appropriate time (monitored by TLC). After the reaction, the purifi-
cation method of the product and the recovery method of DG-COF are
the same as 2.3, and the catalytic cycle is 8 times.
Melamine (42 mg, 0.33 mmol), N, N’-bis(4-bromophenyl) biguanide
(206 mg, 0.5 mmol), copper iodide (19 mg, 0.1 mmol), potassium car-
bonate (138 mg, 1 mmol) and DMSO (20 mL) were added to a Shrek
reaction tube, and reacted at 110◦C for 48 h under nitrogen protection.
After the reaction, additional ethanol (25 mL) was added. The resulting
mixture was stirred overnight at room temperature and was centrifuged
to obtain crude solid product. The solid was washed with water and
ethanol for three times and then dried under high temperature vacuum
to obtain dark brown powder, namely the covalent organic framework
DG-COF.
2.4. Catalytic performance
Knoevenagel condensation reaction: Benzaldehyde (106 mg, 1
mmol), malononitrile (73 mg, 1.1 mmol), DG-COF (2 mol%, 7 mg) and
3. Results and discussion
DG-COF was synthesized using the reaction of N, N’-bis(4-bromo-
phenyl) biguanide with melamine (Scheme 1). To verify the structural
features, DG-COF was characterized by FT-IR, solid-state NMR, SEM,
TEM, BET, TPD, Hammett indicator titration and TGA.
The FT-IR spectra of N, N’-bis(4-bromophenyl)biguanide (a) and DG-
COF (b) are shown in Fig. 1. The distinct peaks at 3174 and 1598 cmꢀ 1
–
– – –
can be attributed to the NH and
C NH stretching vibrations of N,
–
N’-bis(4-bromophenyl)biguanide. In the FT-IR spectra of DG-COF, the
stretching vibration near 3337 cmꢀ 1 can be assigned to NH2 and
–
ꢀ 1
–
NH, and the tensile vibration stretching vibration at 1598 cm is
–
–
attributed to the
C NH group. Both N, N’-bis(4-bromophenyl)
–
biguanide and DG-COF exhibited the benzene ring skeleton vibration
and the benzene ring C H bending vibration at about 1485 cmꢀ 1 and
–
830 cmꢀ 1. It proves that there is a biguanide structure in the DG-COF.
In order to further verify the structure of DG-COF, solid state NMR
was used to characterize it. As shown in Fig. 2, the solid-state 13C NMR
spectrum of DG-COF presented a typical peak at 198 ppm, correspond-
ing to the biguanide carbon. The peaks at 186, 178, 170, 149, and 124
Fig. 5. TGA curve of DG-COF.
4

K. Gong et al.
Molecular Catalysis 509 (2021) 111663
Table 1
Optimization of reaction conditions for Knoevenagel condensation reactiona.
Entry
DG-COF (mol%)
T (◦C)
Solvent
Time (min)
Yieldb (%)
1
2
-
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
H2O
10
10
10
10
10
10
10
10
10
10
-
1
2
5
10
2
2
2
2
2
H2O
87
99
99
99
99
92
67
88
99
3
H2O
4
H2O
5
H2O
6
EtOH
DCM
7
8
MeCN
MeOH
EtOH/H2O
9
10
a
Reaction conditions: benzaldehyde (1 mmol, 1 equiv.), malononitrile (1.2 mmol, 1.2 equiv.), solvent (5 mL), DG-COF.
GC yield.
b
Table 2
Knoevenagel condensation reaction catalyzed by DG-COFa.
a
Reaction conditions: aromatic aldehydes (1 mmol, 1 equiv.), malononitrile (1.1 mmol, 1.2 equiv.), H2O (5 mL), DG-COF (2 mol%). b GC yield. c Malononitrile (2.2
mmol, 2.2 equiv.). d Ethyl cyanoacetate (1.1 mmol, 1.1 equiv.).
5

K. Gong et al.
Molecular Catalysis 509 (2021) 111663
ppm were assigned to the aromatic carbons. The above all indicated that
the biguanide structural units were successfully integrated into DG-COF
skeleton.
DG-COF has a 2% loss rate at about 100 ℃, which may be due to the
analysis of water in DG-COF. At 400◦C, DG-COF still maintains more
than 90% of its weight. It shows that DG-COF can be applied to most of
the catalytic reactions, even at high temperatures. Subsequently, due to
the decomposition of biguanide, DG-COF showed a relatively uniform
decline process. At 800◦C, DG-COF still maintained 62% of its weight,
indicating that DG-COF has excellent thermal stability.
The morphology and structure of DG-COF were characterized by
SEM and TEM (Fig. 3). SEM showed that the state of DG-COF was the
accumulation of fluffy nano-scale granular particles. The TEM showed
that DG-COF had a lamellar structure with a diameter of about 100 nm,
which was then stacked into a relatively regular hierarchical aggregate
structure. The unique stacked structure of DG-COF provides a larger
surface area. The small diameter and high surface area of DG-COF are
beneficial to fully utilize the catalytic activity of the biguanide group in
the structure of DG-COF, and its regular pores are also conducive to
catalytic activity in base-catalyzed reactions, so as to provide more
active center.
After various characterization of DG-COF, the catalytic performance
of DG-COF was evaluated in Knoevenagel condensation reaction
(Table 1). Using benzaldehyde and malononitrile as a template reaction,
the effects of catalyst amount, temperature, solvent and reaction time on
Knoevenagel condensation reaction were investigated. Without the
presence of DG-COF, Knoevenagel condensation reaction cannot pro-
ceed. The best addition of DG-COF is 2 mol%, and the yield can reach
99% (Table 1, entry 3). In the temperature comparison, the Knoevenagel
condensation reaction catalyzed by DG-COF only needs 10 min at room
temperature to achieve high yield and complete conversion. Under these
conditions, a very high yield has been reached, and prolonging the re-
action time and/or increasing the reaction temperature cannot distinctly
change the yield of the reaction. In the selection of solvents, both H2O
and EtOH showed extremely high yields. Obviously, from the perspec-
tive of environmental friendliness, H2O is the best solvent.
Fig/ 4 shows the nitrogen adsorption isotherm measurement of DG-
COF at 77 K. The nitrogen adsorption isotherm is closely related to type
IV isotherm in IUPACS classification, with a convex upward at low P/P0
and a hysteresis in the high P/P0 region, indicative of a porous material.
In the test, the Langmuir Surface Area of DG-COF is 875 m2/g, which is
mainly due to its single-particle smaller nanostructure and fluffy stacked
morphology. The extremely high specific surface area and relatively
uniform pore diameter of DG-COF provide better conditions for
improving its catalytic effect. The meso/macroporous coexisting hier-
archical porous architectures of DG-COF are proved by the pore size
distribution results, which is consistent with the results shown in SEM
and TEM. From NH3 TPD, the basicity of DG-COF was determined to be
0.44±0.02 mmol/g. The basic strength (H_) of DG-COF measured by the
Hammett indicator method was shown to be 9.8-15.0. The results
indicated that DG-COF can be a solid base catalyst for organic reactions.
Catalyst stability is a very important point in actual catalytic appli-
cations and the thermostability is a premise of catalytic stability. As
shown in Fig. 5, we used TGA to test the thermal stability of DG-COF.
After determining the optimal reaction conditions, different aro-
matic aldehyde derivatives and active methylene compounds were
selected to evaluate the catalytic performance of DG-COF (Table 2).
Almost all aromatic aldehydes, whether they have electron-donating or
electron-withdrawing substituents, were smoothly produced to the
desired products with excellent yields within 10 min, indicating that DG-
COF has excellent catalytic performance in Knoevenagel condensation
reaction.
The above remarkable results encouraged us to explore further ap-
plications for DG-COF. The Knoevenagel-Michael-cyclocondensation
Table 3
Knoevenagel-Michael-cyclocondensation catalyzed by DG-COFa.
a
Reaction conditions: aryl aldehydes (1 mmol, 1 equiv.), malononitrile (1.1 mmol, 1.1 equiv.), dimedone (1.1 mmol, 1.1 equiv.), H2O (5 mL). b GC yield.
6

K. Gong et al.
Molecular Catalysis 509 (2021) 111663
HN
HN
HN
NH
NH
R
R
1
N
N
H
O
H
C
C
H
N
CH
C
OH
C
C
H
C
N
N
C
H
2
I
II
N
O
H
HN
NH
NH
NH
H2N
NH
NH
NH
-H2O
H
O
HN
HN
H3C
CH3
R
2
H
N
C
O
O
O
H
C
N
3
H3C
O
H3C
O
O
CH3
CH3
CH3
CH3
IV
III
Michael addition
R
R
R
Cyclization
Tautomerization
O
O
H
O
N
N
H
H
N
C
C
C
C
N
H3C
O
NH2
H3C
O
NH
H3C
O
CH3
CH3
CH3
5
VI
V
Scheme 2. Plausible mechanism for Knoevenagel condensation and Knoevenagel-Michael-cyclocondensation reaction catalyzed by DG-COF.
Fig. 6. DG-COF catalytic cycle performance.
7

K. Gong et al.
Molecular Catalysis 509 (2021) 111663
condensation and Knoevenagel-Michael-cyclocondensation reaction, we
recovered DG-COF through simple filtration and washing after the re-
action and used it for the next same catalytic reaction. There was almost
no loss of DG-COF recycled each time, and it reached 8 cycles in the
catalytic reaction without significant activity changes (Fig. 6). It showed
that the biguanide in the structure of DG-COF is very stable, and the
catalyzed reaction system has almost no negative effect on DG-COF. In
addition, the DG-COF recovered after 8 cycles was characterized by FT-
IR spectra and TEM. The FT-IR spectra of the recovered DG-COF in Fig. 7
shows almost no change, and the TEM recovered in Fig. 8 shows that the
morphology of DG-COF has not significantly changed, it also further
confirmed the excellent cycle stability of DG-COF.
To illustrate the advantage of this methodology, we carefully
compared the catalytic performance of our catalyst with other reported
catalysts in Knoevenagel condensation and Knoevenagel-Michael-
cyclocondensation reaction (Table 4). It was evidently found that DG-
COF displayed excellent catalytic activity as compared to those docu-
mented previously.
Fig. 7. FT-IR spectra of the fresh DG-COF (a) and the reused DG-COF after the
8th run (b).
4. Conclusion
In our research, the covalent organic framework (DG-COF) con-
structed from biguanide was successfully synthesized and used in the
catalytic reaction of Knoevenagel condensation and Knoevenagel-
Michael-cyclocondensation reaction. DG-COF has undergone a series
of characterization, showing its good structure and potential perfor-
mance. In the condition screening of the two reactions, the substrate
expansion and the cycle performance test, DG-COF showed very excel-
lent catalytic effects. After the cycle performance test, the morphology
and catalytic activity of DG-COF hardly changed after 8 time of catalytic
reactions.
reaction for the synthesis of 4H-benzopyran derivatives can be per-
formed using base catalysts. Therefore, we next explored the catalytic
application of DG-COF in Knoevenagel-Michael-cyclocondensation re-
action. The reaction of benzaldehyde, malononitrile and dimedone was
carried out under the aforesaid Knoevenagel optimal conditions. Sur-
prising, the reaction smoothly occurred to obtain the desired product
with 99% yield within 20 min at room temperature. As shown in Table 3,
various aromatic aldehydes bearing either electron-donation or
electron-withdrawing groups gave the desired products with excellent
yields in short reaction time. These results further indicated that DG-
COF as base catalyst have outstanding catalytic performance.
CRediT authorship contribution statement
The proposed mechanism for the condensation reactions is shown in
Scheme 2. Firstly, malononitrile (2) was converted to an intermediate
anion I in the presence of DG-COF. This intermediate reacted with an
aromatic aldehyde, and the resulting aldol underwent subsequent base-
induced elimination to obtain the Knoevenagel condensation product
(3). Dimedone was converted to an enolate anion (IV) by eliminating
one proton (H+) in the presence of DG-COF. The enolate anion (IV) at-
tacks to the intermediate 3 as Michael acceptor to give intermediate V.
Finally, further aromatization and cyclocondensation of V produced VI
which was converted to corresponded product (5).
Kai Gong: Methodology, Investigation, Formal analysis, Writing –
original draft. Daquan Zhang: Methodology, Investigation, Formal
analysis, Writing – original draft. Yunyun Wang: Visualization, Inves-
tigation. Cunhao Li: Visualization, Investigation. Huimin Zhang:
Software, Validation. Haoran Li: Software, Validation. Huiru Feng:
Software, Validation.
Declaration of Competing Interest
The recycling performance is also a very important index to evaluate
the catalyst in practical applications. In the selected Knoevenagel
The authors declare no competing financial interest.
Fig. 8. TEM image of reused DG-COF after the 8th run.
8

K. Gong et al.
Molecular Catalysis 509 (2021) 111663
Table 4
Comparison of DG-COF with diverse reported catalysts for Knoevenagel condensation and Knoevenagel-Michael-cyclocondensation reaction.
Entry
Catalyst
Solvent
T (◦C)
Time
Yield a (%)
Refs.
Knoevenagel condensation reaction c
1
2
3
4
5
PANTTF (5 mol%)
ZIF-8 (5 mol%)
H2O
20
r.t
r.t
60
r.t
1.5 h
1 h
99
[39]
[40]
[41]
[42]
[37]
Toluene
93
ZnMOF (5 mol%)
1-NO3 (0.3 mol%)
Fe3O4-MDI-Gn (50 mgc)
MeOH
2 h
100 b
98 b
95
Solvent-free
1 h
EtOH/H2O
10 min
(1:1)
6
Guanidine 3 (10 mol%)
MgAl-LDH (10 mol%)
NC-700 (20 mg)
DCM
r.t.
40
40
25
r.t.
2 h
91.9
94
[43]
7
THF
40 min
1 h
[44]
8
Ethanol/water (v/v, 1:3),
99 b
99 b
99 b
[16]
9
Nb6 (0.5 mol%)
Ehanol
H2O
45 min
10 min
[45]
10
DG-COF (2 mol%)
This work
Knoevenagel-Michael-cyclocondensation reaction d
1
[cmmim]Br (10 mol%)
Fe3O4-MDI-Gn (50 mgc)
Fe3O4@ SiO2@TiO2 (10 mgc)
Na2SeO4 (100 mgc)
Solvent-free
EtOH/ H2O(1:1)
Solvent-free
EtOH/ H2O(1:1)
PEG/ H2O(1:1)
Solvent-free
Solvent-free
H2O
110
r.t
5 min
10 min
20 min
1 h
90
93
93
97
95
91
84
94
90
99
[10]
2
[37]
3
95
[11]
4
reflux
r.t
[46]
5
MNPs-Guanidine (5 mgc)
[Pyridine-SO3H]Cl (10 mol%)
NH4H2PO4/Al2O3 (30 mg)
Fe3O4@SiO2–imid–PMAn (20 mg)
KF–alumina (20 mol%)
DG-COF (2 mol%)
15 min
9 min
20 min
20 min
2 h
[47]
6
95
[48]
7
80
[49]
8
reflux
reflux
r.t
[50]
9
EtOH
[51]
10
H2O
20 min
This work
a
Isolated yield.
b
c
GC yield.
The substrate is benzaldehyde and malonitrile.
The amount of substrate is 1 mmol.
d
eThe substrate is benzaldehyde, malonitrile and dimedone.
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