Molybdenum carbide as an efficient and durable catalyst for aqueous Knoevenagel condensation

Article abstract of DOI:10.1039/c9nj04647j

By following green chemistry principles, various molybdenum compounds as cost-effective, commercially available, heterogeneous, and benign catalysts were used for the Knoevenagel condensation reaction. Among the compounds (Mo₂C, MoS₂, MoB, MoSi₂), molybdenum carbide showed efficient performance for the Knoevenagel condensation in aqueous media at room temperature, affording the corresponding products in high yields within a short reaction time. Notably, using this commercially available heterogeneous catalyst, the deacetalization-Knoevenagel condensation of benzaldehyde dimethyl acetal and malononitrile at room temperature could successfully proceed in excellent yield. Molybdenum carbide could be recovered five times without loss of activity with an operationally simple procedure.

Full text of DOI:10.1039/c9nj04647j

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Molybdenum carbide as an efficient and durable
catalyst for aqueous Knoevenagel condensation†
Cite this: New J. Chem., 2019,
43, 16437
Mina Tavakoliana and Mohammad Mahdi Najafpour
*
abc
Received 9th September 2019,
Accepted 19th September 2019
DOI: 10.1039/c9nj04647j
rsc.li/njc
By following green chemistry principles, various molybdenum (TMCs) have higher activity and durability compared to their parent
compounds as cost-effective, commercially available, heterogeneous, metals and could show a behavior similar to noble metals in surface
and benign catalysts were used for the Knoevenagel condensation catalysis. These unique properties arise from the addition of carbon
reaction. Among the compounds (Mo2C, MoS2, MoB, MoSi2), molyb- that causes an alteration in the electronic properties of these
denum carbide showed efficient performance for the Knoevenagel materials.9
condensation in aqueous media at room temperature, affording the
The Knoevenagel reaction is a powerful synthetic method for
corresponding products in high yields within a short reaction time. the formation of the CQC bond.10,11 The valuable a,b-unsaturated
Notably, using this commercially available heterogeneous catalyst, the products obtained by this transformation are important intermedi-
deacetalization-Knoevenagel condensation of benzaldehyde dimethyl ates in the synthesis of natural products,12 therapeutic agents,13
acetal and malononitrile at room temperature could successfully polymers with different functional groups,14 cosmetics, and
proceed in excellent yield. Molybdenum carbide could be recovered perfumes.15 Although this reaction seems to be convenient and
five times without loss of activity with an operationally simple simple, the disadvantages of applied catalytic systems including
procedure.
using expensive metal-based catalysts, tedious steps for the
preparation of heterogeneous catalysts, high temperature, and
performing in non-green solvent cause it to still be a matter of
interest to chemists.16–19
Introduction
Herein, we studied the efficiency of various molybdenum
compounds as inexpensive and commercially available catalysts to
perform the Knoevenagel condensation and one-pot deacetalization-
Knoevenagel condensation of benzaldehyde dimethyl acetal in water
as a green solvent at room temperature.
Firstly, to obtain the best reaction conditions, the reaction of
benzaldehyde with malononitrile was chosen as the model
reaction, the results are summarized in Table 1. Initially studies
revealed that among the various applied molybdenum compounds
(Mo2C, MoS2, MoB, MoSi2) used as the catalysts, molybdenum
carbide was the best choice, affording the corresponding
Knoevenagel adduct in 97% yield in water at room temperature
(Table 1, entry 1).
These results for the model reaction were in agreement with
the high ability of molybdenum carbide catalysts related to
their unique d-band electronic structures, similar to group 8
metals (Pt, Pd, Rh, etc.).9
From Table 1, we found that the nature of the solvents had a
great effect on the reaction yield. In this way, polar solvents
such as H2O and EtOH could afford the desired product in high
yields, but in toluene and n-hexane as the representatives of
non-polar solvents no promising results were obtained within
Catalysts, as the heart of chemical reactions, play a critical and
key role in chemical processes in both academic and industrial
research labs. For this reason, many attempts have been devoted
to the use of simple, efficient, economic, and eco-friendly
catalysts which are easily prepared and can be extracted from
the reaction mixture without tedious steps.
In recent years, molybdenum compounds have attracted
significant attention as earth-abundant, inexpensive, and benign
catalysts.1–3 Particularly, molybdenum carbides have been success-
fully used in various reactions such as desulfurization,4,5 water–gas
shift,6 and hydrogenation reactions.7,8 It was found, by using
experimental and theoretical studies, that transition metal carbides
a Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
Gavazang, Zanjan 45137-6731, Iran. E-mail: mmnajafpour@iasbs.ac.ir;
Tel: +98-24-3315-3201
b Research Center for Basic Sciences & Modern Technologies (RBST), Institute for
Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
c Center of Climate Change and Global Warming, Institute for Advanced Studies in
Basic Sciences (IASBS), Zanjan, 45137-66731, Iran
† Electronic supplementary information (ESI) available: Including spectroscopic
data for products. See DOI: 10.1039/c9nj04647j
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Table 1 Optimization of the reaction conditions for the Knoevenagel
condensation of benzaldehyde and malononitrilea
Table 2 Scope of the Knoevenagel condensation reaction by Mo2C at
room temperaturea
Entry
Catalyst
Solvent
Yielda (%)
1
2
3
4
5
6
7
8
Mo2C
MoS2
MoSi2
MoB
Mo2C
Mo2C
Mo2C
Mo2C
Mo2C
Mo2C
Mo2C
H2O
H2O
H2O
H2O
97
93
90
71
80
96
3
2
6
97
53
THF
EtOH
n-Hexane
PhCH3
—
H2O
H2O
9
10b
11c
a
Reaction conditions: benzaldehyde (1 mmol), malononitrile (1.1 mmol),
b
and 2.5 mL H2O, GC yields. 10 mol% (20 mg) of the catalyst was used.
c
2.5 mol% (5 mg) of the catalyst was used.
12 h (Table 1, entries 5–8). In addition, the reaction could not
successfully proceed in the absence of solvent (Table 1, entry 9).
Further studies revealed that increasing the loading of catalyst
from 5 mol% to 10 mol% had no positive effect on the yield and
time of the reaction (Table 1, entry 10). Also, decreasing the
amount of the catalyst to 2.5 mol% caused a great drop in the
yield, 53% in 2 h (Table 1, entry 11).
a
Reaction conditions: benzaldehyde (1 mmol), malononitrile (1.1 mmol),
Mo2C (5 mol%, 10 mg) and 2.5 mL H2O, GC yields.
With the optimal reaction conditions in hand (Table 1, entry 1),
we then explored the substrate scope of the reaction for other
aldehydes (Table 2). Aromatic aldehydes bearing either electron-
donating or electron-withdrawing substituents in the para, meta
and ortho positions furnished the corresponding Knoevenagel
products in high yields (Table 2, 3a–h), without being affected by
the electronic effect of the substituent. Interestingly, a 97% yield was
obtained even for the challenging 2,4-Me-benzaldehyde that suffers
from steric hindrance (Table 2, 3b). Notably, our procedure was
efficient in the case of 1-naphthaldehyde and E-cinnamaldehyde as
well as a heteroaryl aldehyde like 2-thiophenecarboxaldehydeand
3-pyridinecarboxaldehyde (Table 1, 3i–l). Also, the aliphatic alde-
hydes that are classified as challenging precursors showed good
results as can be seen in Table 2, 3m, albeit with longer reaction
Scheme 1 Deacetalization-Knoevenagel reaction of benzaldehyde dimethyl
acetal using molybdenum carbide.
Notably, most of the previous systems employed organic
times than the aromatic aldehydes (3h). Furthermore, the reaction solvents and the reaction was performed at high temperature to
was also performed with ketones, namely, cyclohexanone and 4- achieve satisfactory results (Table 3). However, we used a green
nitro-benzophenone (Table 1, 3n–o), which resulted in a very good route with much lower amounts of catalyst (10 mg) at room
yield of the products with a longer reaction time than the temperature in water.
aldehydes, indicating the high efficiency of molybdenum carbide
as a catalyst for Knoevenagel condensation reactions.
In the next part of this study, we examined the reusability of
this commercially available catalyst in the model reaction between
Encouraged by these results, we turned our efforts toward benzaldehyde and malononitrile under optimized reaction condi-
using this catalyst to the one-pot deacetalization-Knoevenagel tions (Table 1, entry 1). When the first run of the reaction was
reaction of benzaldehyde dimethyl acetal. Surprisingly, 5 mol% completed, ethyl acetate was added to the reaction mixture and
of Mo2C could successfully furnish the corresponding Knoevenagel owing to the good dispersion of Mo2C in water, the product and
product in high yield at room temperature in 6 h (Scheme 1). As unreacted substrates were successfully extracted. Then, for the next
shown in Table 3, this catalytic activity compared to other run, only substrates were added to the aqueous phase containing the
reports which used synthesized heterogeneous catalysts is very catalyst. Using this simple recycling method, the catalyst could be
impressive.
recovered five times while maintaining high activity (Fig. 1).
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Table 3 Comparing the catalytic activity of the present system with previous catalysts in the one-pot deacetalization-Knoevenagel reaction of
benzaldehyde dimethyl acetal (1 mmol) and malononitrile
Entry
Catalyst (mol%)
Conditions
Yield (%)
Ref.
1
2
3
4
5
6
7
MIL-101 MOFs (0.04 mmol)
Ln cluster-based MOFs (100 mg)
OMCN-1 (60 mg)
MOF (JUC-199) (100 mg)
PCN-222-Co@TpPa-1 (50 mg)
BNP (40 mg)
DMF, 2 h, 90 1C
DMSO-d6, 24 h, 50 1C
Toluene, 24 h, 80 1C
1,4-Dioxane, 4 h, 90 1C
DMSO-d6, 10 h, 50 1C
Toluene, 24 h, 80 1C
99
97
99
99
99
99
94
20
21
22
23
24
25
26
SO3H-AA@MNP (10 mg)
Toluene, H2O (11 mmol), 2 h, 90 1C
Fig. 1 Recovery of Mo2C catalyst in the Knoevenagel condensation
reaction between benzaldehyde and malononitrile.
Scheme 2 Suggested mechanism for the Knoevenagel reaction cata-
lyzed by Mo2C.
Next, the XRD pattern of the Mo2C catalyst after six runs in
the reaction was recorded and compared with that of the fresh
catalyst, Fig. 2. Both XRD patterns revealed that Mo had not
changed during the reaction, so losing some of the catalyst
during the recovery process could probably be considered
responsible for the deactivation observed.27
However, the proposed mechanism for the role of the
catalyst is shown in Scheme 2. Generally, the reactant mole-
cules were attached to the Mo2C surface, and a C–C bond was
formed due to the bifunctional properties of the catalyst.28
Simultaneously, polar solvents such as water bonded to nearby
Mo atoms and accelerated the formation of the product.
In summary, we used molybdenum carbide as a recyclable
and commercially available catalyst for the Knoevenagel con-
densation reaction. The reaction performed well over a range of
aldehydes, especially aliphatic and heteroaromatic aldehydes,
giving the corresponding products with up to 95% yield. Also,
cyclohexanone and 4-nitro-benzophenone as representatives of
ketones, gave the desired products in high yields. Particularly, using
this catalytic system, the one-pot deacetalization-Knoevenagel
reaction of benzaldehyde dimethyl acetal proceeded very well
with similar conditions and yields to the benzaldehyde sub-
strate albeit with longer reaction time. The catalyst could be
recycled five times without significant loss of catalytic activity.
Thus, this commercially available and recoverable catalyst can
be considered as a promising catalytic system, especially in
industrial applications.
Experimental
General information
Reagents obtained from commercial sources were used without
further purification. Molybdenum compounds and malono-
nitrile were purchased from Aldrich Company. Gas chromato-
graphy analyses were obtained on a Varian CP-3800 using a
flame ionization detector (FID). 1H-NMR spectra were recorded
on commercial instruments (400 MHz). Chemical shifts were
reported in ppm from tetramethylsilane with the solvent resonance
as the internal standard (CDCl3: d = 7.29). 13C-NMR spectra were
collected on commercial instruments (100 MHz) with complete
proton decoupling.
General procedure for the Knoevenagel condensation reaction
using Mo2C catalyst
A 5 mL round-bottomed flask was charged with a carbonyl
compound (1 mmol), malononitrile (1.1 mmol), Mo2C (10 mg)
and H2O mixture (2.5 mL). Then it was stirred at room
temperature under argon atmosphere. After completion of the
reaction (monitored by GC), the mixture was extracted by ethyl
acetate (2 Â 5 mL). The organic phase was dried with anhydrous
Fig. 2 XRD patterns of fresh and reused Mo2C.
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