Heterogeneous Aza-Michael Addition Reaction by the Copper-Based Metal–Organic Framework (CuBTC)

Article abstract of DOI:10.1007/s10562-020-03459-7

Abstract: The copper benzene-1, 3, 5-tricarboxylate metal–organic framework (CuBTC) was found to be an effective heterogeneous catalyst for the aza-Michael addition reaction of the four types of amines to electron deficient alkenes at room temperature. The catalytic protocol showed high product yields and outstanding chemo selectivity. The cyclic amines (piperidine and pyrrolidine) and aliphatic amines (n-dibutylamine) provided aza-Michael addition with a high yield of product (?98%) within shorter reaction period (2?h) at room temperature under mild reaction conditions using CuBTC. However, it was observed that the aza-Michael reaction proceeded more slowly, giving 62% yield of product after 24?h in the case of aromatic amine (aniline) with n-butyl acrylate in the presence of CuBTC under identical reaction conditions. The catalyst could be reused four recycles without losing its initial catalytic activity and selectivity. XRD and SEM analysis further confirmed that the crystallinity of catalyst was retained during the reaction. A reaction mechanism is proposed for the aza-Michael addition reaction over heterogeneous CuBTC catalyst. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03459-7

Catalysis Letters
https://doi.org/10.1007/s10562-020-03459-7
Heterogeneous Aza‑Michael Addition Reaction by the Copper‑Based
Metal–Organic Framework (CuBTC)
Samiran Bhattacharjee · Aftab Ali Shaikh2,4 · Wha‑Seung Ahn3
1
Received: 7 September 2020 / Accepted: 8 November 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
The copper benzene-1, 3, 5-tricarboxylate metal–organic framework (CuBTC) was found to be an eꢀective heterogeneous
catalyst for the aza-Michael addition reaction of the four types of amines to electron deꢁcient alkenes at room temperature.
The catalytic protocol showed high product yields and outstanding chemo selectivity. The cyclic amines (piperidine and pyr-
rolidine) and aliphatic amines (n-dibutylamine) provided aza-Michael addition with a high yield of product (⁓98%) within
shorter reaction period (2 h) at room temperature under mild reaction conditions using CuBTC. However, it was observed
that the aza-Michael reaction proceeded more slowly, giving 62% yield of product after 24 h in the case of aromatic amine
(
aniline) with n-butyl acrylate in the presence of CuBTC under identical reaction conditions. The catalyst could be reused
four recycles without losing its initial catalytic activity and selectivity. XRD and SEM analysis further conꢁrmed that the
crystallinity of catalyst was retained during the reaction. A reaction mechanism is proposed for the aza-Michael addition
reaction over heterogeneous CuBTC catalyst.
Graphic Abstract
Keywords Metal–organic frameworks · CuBTC · Aza-michael reaction · Heterogeneous catalyst
*
Samiran Bhattacharjee
s.bhattacharjee@du.ac.bd
1
Introduction
1
2
3
4
Centre for Advanced Research in Sciences (CARS),
University of Dhaka, Dhaka 1000, Bangladesh
The Aza-Michael addition reaction is an eꢀective and sim-
plest protocol for the preparation of β–amino carbonyl com-
pounds from amines with electron-deꢁcient alkenes, which
was discovered in 1874 [1, 2]. These compounds are one of
the most valuable intermediates for the preparation of a wide
range of bioactive molecules, antibiotics, peptide analogues,
chiral auxiliaries, and other nitrogen-containing compounds
Department of Chemistry, University of Dhaka, Dhaka 1000,
Bangladesh
Department of Chemistry and Chemical Engineering, Inha
University, Incheon 402-751, Republic of Korea
Present Address: BCSIR, Dr.Qudrat-E-Khuda Road,
Dhaka 1205, Bangladesh
Vol.:(0
1
123456789)
3

S. Bhattacharjee et al.
[
3]. The reaction can also generate a large variety of inter-
reaction [40]. In continuation of our work on the MOF-based
catalysts for ꢁne chemicals preparation, we report here the
possibility and eꢂciency of CuBTC as a reusable catalyst
for the Aza-Michael reaction using a variety of amines, such
as cyclic, aliphatic and aromatic with diꢀerent electron-deꢁ-
cient alkenes at room temperature under mild reaction condi-
tions. In this work, we have expanded the range of substrates
to examine the chemo-selectivity of CuBTC to give high
product yields and good chemo-selectivity.
mediates viz., sol–gel precursors, coupling agents, reactive
diluents, and silicon-based (macro) materials [4].
Traditionally, 1,4-addition of amines with electron-deꢁ-
cient alkenes have been carried out using a stoichiometric
amount of Lewis acids, viz., NiCl .7H O, [5] LiClO , [6]
3
2
4
Cu(OTf) , [7] Na SnO , CeCl .7H O-NaI [8] and BiNO
2
2
3
3
2
3
[
9]. However, the above processes have many drawbacks,
such as, diꢂculty in recovery and reuse of the catalyst, long
reaction time, and high temperature, which make the proce-
dure ecologically unacceptable [10, 11]. To overcome the
above drawback, ionic liquids or supported ionic liquids
have been applied as an eꢂcient catalyst for the formation
of α, β–unsaturated compounds; they can be reused with-
out loss of activity, which are time-consuming and costly
2 Experimental
2.1 Materials
[
12–15]. Recently, several heterogeneous catalysts for the
aza-Michael reaction have been studied, such as Cu-Al
hydrotalcite, [16] vinyl sulfones, Amberlyst-15, [17] phos-
phate impregnated titania, [18] silica-supported aluminum
chloride, [19] acidic alumina, [20] Al-KIT5 nano-cage
mesoporous, [21] MCM-41 Supported Phenanthrolinium
Dibromide, [22] polymer‐shell-encapsulated magnetite
nanoparticles, [23] and copper–amine complex supported on
mesoporous silica SBA-15 [24]. Further eꢀorts to develop
simple and environmentally friendly green protocols are
highly desirable.
Copper(II) nitrate hydrate (98%), 1,3,5-benzenetricarbox-
ylic acid (95%), piperidine (99%), pyrrolidine (99%), dibu-
tylamine (99.5%), aniline (ACS reagent, 99.5%), 4-methy-
laniline (99.6%), 4-chloroaniline (99%), n-butyl acrylate
(99%), methyl acrylate (99%) and n-dodecane (99%) were
purchased from Sigma-Aldrich and were used as received.
2.2 Synthesis of Cu(BTC)
During the past decade, metal–organic frameworks
The MOF structure was prepared according to our previ-
ously published method using the ethanol reꢃux method
[41]. Brieꢃy, copper(II) nitrate hydrate (0.875 g) and tri-
mesic acid (0.42 g) in 25 ml ethanol was heated under
reꢃux with constant stirring for 24 h and cooled to room
temperature. The blue precipitated was separated by ꢁltra-
tion, washed with water and then with ethanol and dried at
100 ℃ under vacuum for 5 h.
(
MOFs) have gained much attention as potential heterogene-
ous catalysts for various organic transformation owing to the
large surface area, ordered crystalline structure, structural
diversity and tunable pore sizes [25–34]. MOFs have vari-
ous types of heterogeneous active sites, among them, open-
metal sites/coordinatively unsaturated metal sites are one
of the most regularly studied ones for catalysis applications
[
35–37]. Recently, Rostamnia et al. reported the catalytic
activity of open metal site iron-based MOF (MIL-100(Fe))
for the aza-Michael reaction as an eꢂcient green catalyst
2.3 Characterization
[
38]. BASF has recently commercialized some important
MOFs as adsorbents or catalysts for industrial applications.
Among them, CuBTC (copper benzene-1, 3, 5-tricarboxy-
late) is one of the well-known MOFs for potential adsorbents
or catalysts owing to its simple preparation procedure, high
yield, excellent textural properties, and stability [35]. This
MOF is known as Basolite(TM) C300, which was composed
of a cluster of two copper ions with a paddlewheel unit,
having unsaturated open metal sites. The material showed
good catalytic activity and selectivity for ꢁne chemical
preparation by the Lewis acidic or redox properties of the
material [35]. Savonnet et al. [39] have reported on the cata-
lytic application of the MOFs as an eꢂcient catalyst for the
aza-Michael addition reaction. Nguyen et al. reported that
XRD pattern of the sample was obtained using a Rigaku
UltimaIV diꢀractometer with CuKα radiation (λ = 1.54)
at 0.2 0 min-1. SEM micrographs were taken on a JEOL
(JSM-6490LA) instrument. The N adsorption–desorption
2
measurements were measured on a BELsorp Mini (BEL
Corporation, Japan) instrument at 77 K. The surface areas
were calculated by the BET (Brunauer–Emmett–Teller)
method, and the pore volumes and pore diameters were esti-
mated from desorption branch of the isotherm based on BJH
(Barrett-Joyner-Halenda) model. Prior to the measurement,
the sample was degassed at 150 ℃ under vacuum for 3 h.
FTIR spectra were recorded as KBr discs on a IRPrestige 21
(Shimadzu, Japan) spectrometer. Cu content in the reaction
product solution was measured by atomic absorption spec-
troscopy using Perkin-Elmer AAS AAnalyst 200.
5
mol% CuBTC was found to be highly active and reusable
in selected organic reactions for the aza-Michael addition
1
3

Heterogeneous Aza-Michael Addition Reaction by the Copper-Based Metal–Organic Framework…
2.4 Catalytic Reaction
3 Results and Discussion
The aza-Michael addition reaction of amines with alkenes
over CuBTC as catalyst was carried out in a round-bottom
ꢃask equipped with a condenser. In a typical run, 2 mmol
of piperidine, 5 mol% of CuBTC, methanol (4 mL), and
The copper benzene-1, 3, 5-tricarboxylate (CuBTC)
metal–organic framework structure has been readily pre-
pared and well characterized [35]. Ahn et al. described the
preparation method on a bench scale having good textural
properties by the solvothermal method using a 1 L-capacity
Pyrex reactor in ethanol medium [41]. The material was
0
.88 mmol of n-dodecane as an internal standard were
stirred at room temperature for 20 min. n-Butyl acrylate
(
2 mmol) was added to the mixture with constant stirring.
characterized by XRD, SEM, N adsorption–desorption
2
After completion of the reaction, ethyl acetate (5 mL) was
added and then the catalyst was ꢁltered oꢀ and the material
washed with methanol and dried under vacuum for 5 h at
isotherm and FT-IR. As shown in Figs. 1 and 2, XRD and
SEM of present material conꢁrmed that the MOF showed
characteristic XRD and morphology as reported previously
1
00 ℃, and reused for further reaction. The ꢁltrate was dried
[35, 41]. N adsorption–desorption isotherms and pore size
2
over Na SO4, and the conversion and product selectivity
distribution curves are shown in Fig. 3. The material showed
2
were measured using a GC (Clarus 500, Perkin-Elmer) ꢁtted
with a high performance HP-1 capillary column and FID.
A hot ꢁltering experiment of the aza-Michael addition
reaction of piperidine and n-butyl acrylate was carried out
by separating the CuBTC from the reaction mixture after
a type I pattern [35]. The BET surface area and pore vol-
2
3
ume were 1304 cm /g and 0.650 cm /g, respectively. As
shown in the Fig. 3, the pore size distribution curve was
centered at 1.5 nm. The CuBTC showed strong bands at
−
1
1652, 1450 and 1374 cm in the IR spectrum (Fig. 4a) due
6
0 min of reaction. The ꢁltrate mixture was then stirred for
a further 65 min at room temperature. The conversion and
selectivity were measured by GC.
Fig. 1 Powder X-ray diꢀraction patterns of (a) fresh CuBTC and (b)
Fig. 3 N adsorption–desorption isotherms of CuBTC; inserted are
2
reused CuBTC after the ꢁfth run
their pore size distribution curves
Fig. 2 Scanning electron
microscopy images of a fresh
CuBTC and b catalyst after the
ꢁ
fth run
1
3

S. Bhattacharjee et al.
and ꢁve-member cyclic amine (pyrrolidine), pyrrolidine
was suitable to activate methyl acrylate (Table 1). Moreo-
ver, aliphatic amine, such as dibutylamine with methyl
acrylate gave the corresponding addition product in high
yield with a longer period (150 min) than the other two
cyclic amines as shown in Table 1.
Subsequently, the chemoselectivity of CuBTC was
examined using a mixture of 2 mmol of piperidine, 2 mmol
of aniline, and 2 mmol of n-butyl acrylate as shown in
Fig. 7. After 125 min, the aza-Michael addition product of
piperidine was formed in high yield (98%) and no product
was obtained with aniline under the same reaction condi-
tions. On the other hand, the catalytic activity of CuBTC
between aromatic anime, such as aniline with n-butyl
acrylate as a Michael acceptor showed that aniline aꢀorded
only 62% yield of addition product after 24 h (Fig. 8). We
then carried out the aza-Michael reaction using substi-
tuted aniline such as 4-chloroaniline and 4-methylaniline
with n-butyl acrylate under the same reaction conditions
in order to establish the good activity of CuBTC as cata-
lyst. The catalytic activity was found to be inꢃuenced by
the type of substituted aniline, and the product yield was
increased in the following order: 4-chloroaniline < ani-
line<4-methylaniline (Table 1). Similar observations were
reported earlier for other catalysts [38, 42].
Fig. 4 FT-IR spectra of (a) fresh CuBTC and (b) the reused catalyst
after ꢁfth run
to stretching modes of the C-O group, as well as the absence
−
1
of strong bands in the region 1760–1690 cm assigned to
C = O stretching vibration, indicated the deprotonation of
–
COOH in the trimesic acid upon the reaction with copper
cations [40].
The prepared CuBTC material was tested as a catalyst
for the aza-Michael reaction of various amines to electron-
deꢁcient alkenes at room temperature. The reaction was car-
ried out using 5 mol% CuBTC under the identical conditions
previously reported by Nguyen et al. [40] and the results are
listed in Table 1. Using 5 mol% catalyst, the catalyst formed
a mono-addition product only in all cases. Initially, blank test
experiments were carried out using piperidine with n-butyl
acrylate under identical reaction conditions as described in
Table 1 in the absence of CuBTC. Piperidine was converted
to a product with 9% yield after 125 min. After 125 min,
piperidine was converted to mono-addition products with
The heterogeneity of the catalyst was carried out
through the catalyst recycling test (Table 1). The mate-
rial was separated after each reaction by simple ꢁltra-
tion, washed with methanol, and dried under vacuum for
5 h at 100 ℃. The recovered material was reused several
times without losing any signiꢁcant activity (Table 1).
No Cu leaching in the ꢁltrate mixture was found by the
atomic absorption spectrometer. XRD and SEM results
of reused catalyst suggesting that the stability of CuBTC
was retained during the reaction (Figs. 1 and 2). The IR
spectrum of the reused catalyst was also indistinguishable
to the fresh CuBTC (Fig. 4b).
9
8% yield at room temperature in the presence of 5 mol%
To further examine the leaching of active metal in homo-
geneous phase, the aza-Michael addition reaction of piperi-
dine with n-butyl acrylate was performed at room tempera-
ture using CuBTC as a catalyst and the catalyst was ꢁltered
oꢀ after 60 min of reaction time. The ꢁltrate mixture (with-
out catalyst) was continued for reaction with stirring at room
for further 65 min as shown in Fig. 9. No signiꢁcant increase
activity was found after the separation of catalyst, indicating
the reaction proceed through Cu sites in the MOF matrix.A
proposed reaction mechanism for the aza-Michael addition
reaction of amine to electron-deꢁcient alkene over hetero-
geneous CuBTC catalyst is shown in Fig. 10. The mecha-
nism involves the initial coordination of carbonyl oxygen of
Michael acceptor to unsaturated Cu(II) open metal sites to
CuBTC under the same reaction conditions (Fig. 5). The
inꢃuence of reaction time on the catalytic performance is
shown in Fig. 6. The product yield gradually increased with
increasing reaction time.
To examine the inꢃuence of diꢀerent alkenes, the reac-
tion was performed between piperidine and methyl acrylate
as the Michael acceptor, forming a corresponding mono-
addition product with slightly shorter period (110 min) to
achieve a yield around 98% (Fig. 6) than n-butyl acrylate
(
reaction time 125 min) under identical reaction condi-
tions, which indicated that methyl acrylate is suitable to
activate the piperidine for aza-Michael addition reaction.
In a comparison of six-member cyclic amine (piperidine)
1
3

Heterogeneous Aza-Michael Addition Reaction by the Copper-Based Metal–Organic Framework…
Table 1 Aza-Michael addition reaction of various amines with alkenes using CuBTC
Entry
1
Runa
Amine
Alkene
Time (min)
125
Yield (%)b
98c
1
2
3
5
1
97
97
96
98
2
3
110
90
1
99
2
5
1
98
97
97
4
5
150
5
1
95
1440
62, 60d
6
7
a
1
1
1320
1680
67
50
Reaction conditions: amine (2 mmol), alkene (2 mmol), CuBTC (5 mol%), methanol (5 mL) and room temperature
b
c
d
GC yield
Blank test in the absence of CuBTC (Yield: 9%)
Fifth run
rise the electrophilicity of the β-carbon (I) [38]. This inter-
4 Conclusion
mediate species reacts with the amine to generate a new
carbon–nitrogen bond (II), and simultaneously form (III)
by intermolecular proton transfer. In the ꢁnal step, the cor-
responding product was formed and releasing the CuBTC
In summary, we have described an eꢂcient heterogeneous
catalyst system for the Aza-Michael reaction for cyclic,
aliphatic, and aromatic amines with electron-deꢁcient alk-
enes using CuBTC under mild reaction conditions. Cyclic
[
38, 43].
1
3

S. Bhattacharjee et al.
O
NH2
NH
+
H C
2
O
O
CH3
+
H C
2
O
CH3
CuBTC
Methanol, Room temp., 125 min
CuBTC
Methanol, Room temp., 24 h
N
O
CH3
O
O
Yield: 98%
Fig. 5 Aza-Michael addition reaction of piperidine with n-butyl
N
H
O
CH3
acrylate using CuBTC
Yield: 62%
1
00
Fig. 8 Aza-Michael addition reaction of aniline with n-butyl acrylate
using CuBTC
8
6
4
2
0
0
0
0
0
1
00
a
b
Table 1: Entry
8
6
4
2
0
0
0
0
0
1
2
3
4
0
30
60
90
120
150
Time (min)
Fig. 6 Eꢀect of diꢀerent amines and alkenes on reaction conver-
sion: (1) piperidine with n-butyl acrylate, (2) piperidine with methyl
acrylate, (3) pyrrolidine with methyl acrylate, and (4) dibutylamine
with methyl acrylate
0
30
60
Time (min)
90
120
Fig. 9 Aza-Michael addition reaction between piperidine and n-butyl
acrylate: (a) with CuBTC and (b) ꢁltrate (CuBTC ꢁltered oꢀ after
NH
2
6
0 min of reaction time
O
NH
+
+
2
H C
O
CH
3
and aliphatic amines were better to activate alkenes over
CuBTC with high product yield (⁓99%) at room tempera-
ture within 2 h reaction time. The catalyst showed high
product yields and excellent chemo-selectivity between
the aromatic amine and cyclic amine. The catalyst could
be successfully recycled for at least four recycles without
any loss of catalytic activity. CuBTC can be easily pre-
pared and showed high stability in C–C bond formation,
which makes this protocol attractive for modern chemical
industries.
CuBTC
Methanol, Room temp., 125 min
N
O
O
CH
3
+
N
H
4 9
OC H
O
Yield: 98%
Yield: 0%
Fig. 7 Chemoselective aza-Michael addition reaction of cyclic amine
in the presence of aromatic amine using CuBTC
1
3

Heterogeneous Aza-Michael Addition Reaction by the Copper-Based Metal–Organic Framework…
Fig. 10 The proposed reaction
mechanism for the aza-Michael
addition of cyclic amine with
methyl acrylate over CuBTC
Acknowledgements This work was supported by University of Dhaka
8. Bartoli G, Bosco M, Marcantoni E, Pertrini M, Sambri L, Tor-
(
CARS) internal fund and in part by the Basic Science Research Pro-
regiani E (2001) Conjugate Addition of Amines to α, β-Enones
Promoted by CeCl3·7H2O−NaI System Supported in Silica Gel.
J Org Chem 66:9052–9055
gram through the National Research Foundation of Korea (NRF).
9
.
Srivastava N, Banik BK (2003) Bismuth Nitrate-Catalyzed Ver-
satile Michael Reaction. J Org Chem 68:2109–2114
Compliance with Ethical Standards
1
0. Giovanna B, Roderick A (2016) Aza-Michael Mono-addition
Using Acidic Alumina under Solventless Conditions. Molecules
Conflict of interest The authors have no conꢃicts to declare.
2
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