Chitosan-supported copper as an efficient and recyclable heterogeneous catalyst for A3/decarboxylative A3-coupling reaction

Article abstract of DOI:10.1016/j.tetlet.2018.03.053

Chitosan-supported copper (chit@copper) based heterogeneous catalysts have been explored for A³-coupling and decarboxylative A³-coupling. The developed protocol employs low catalyst loading, solventless condition and easy work-up for

Full text of DOI:10.1016/j.tetlet.2018.03.053

Tetrahedron Letters xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Chitosan-supported copper as an efficient and recyclable heterogeneous
3
3
catalyst for A /decarboxylative A -coupling reaction
Pavneet Kaur , Bhupinder Kumar , Vinod Kumar , Rakesh Kumar a,
a
b
b,
⇑
⇑
a
Laboratory of Organic Chemistry, Department of Chemical Sciences, Central University of Punjab, Bathinda 151001, India
Laboratory of Organic and Medicinal Chemistry, Department of Pharmaceutical Sciences and Natural Products, Central University of Punjab, Bathinda 151001, India
b
a r t i c l e i n f o
a b s t r a c t
3
Article history:
Chitosan-supported copper (chit@copper) based heterogeneous catalysts have been explored for A -cou-
Received 1 February 2018
Revised 14 March 2018
Accepted 19 March 2018
Available online xxxx
3
pling and decarboxylative A -coupling. The developed protocol employs low catalyst loading, solventless
condition and easy work-up for the synthesis of diversely substituted propargylamines. More impor-
tantly, the catalyst could be recovered and reused without any significant loss in the activity. This offer
3
huge advantages as recyclability issues are rarely addressed in decarboxylative A -coupling. Leaching
studies were carried out using AAS and ICPMS analysis. It is envisaged that chit@copper catalysts can
have potential applications in terms of efficiency and recyclability in the emerging area of decarboxyla-
tive CAH bond activation/functionalization strategies.
Keywords:
Chitosan
Decarboxylative A -coupling
Propargylamines
Heterogeneous catalyst
Recyclability
3
Ó 2018 Elsevier Ltd. All rights reserved.
Environmental degradation is one of the major concerns1 of
environmentalists and political discourse at the international level.
The toxic waste generated by the chemical industries is one of the
major threats to human, animals and plant lives on the earth.
2
co-workers have reported chit@Cu(OAc) as a highly active and
recoverable heterogeneous catalyst for the CAS cross-coupling
reaction which has led to the green synthesis of antiulcer drug
1
8
4
zolimidine. Similarly, chit@CuSO as a heterogeneous catalyst
2
Novel reagents, solvent free synthesis and recyclable catalytic sys-
has been employed for the azide-alkyne cycloaddition and for
3
15
tems are being developed to minimize the waste in chemical syn-
the synthesis of 1,2,3-triazoles using aryl boronic acids.
4,5
thesis. From the last few years, heterogeneous catalysis has been
proved to be an effective strategy to improve the efficiency and
recyclability of the catalysts. In recent years, abundantly available
and renewable biopolymers such as starch, cellulose, chitin, chi-
tosan etc., have received a phenomenal attention for their use as
a support for metals and their application in heterogeneous
The transition metal-catalyzed three-component coupling of an
3
aldehyde, amine and alkyne (A -coupling) represents an important
strategy for the synthesis of diversely substituted propargylamines
1
9–22
3
in an atom economic way.
Similarly, decarboxylative A -cou-
pling (coupling of aldehydes, amines and alkynyl carboxylic acids)
is a useful and challenging transformation wherein alkynyl car-
6
–10
catalysis.
boxylic acids act as economical and stable surrogate for polymer-
8
23–26
Chitosan, an amino-polysaccharide, is the second most abun-
dant biopolymer after cellulose. This biodegradable polymer is
insoluble in most of the organic solvents as well as in water and
ization prone alkynes via in situ decarboxylation.
There are
3
only few protocols for decarboxylative A -coupling which mainly
focus on substrate scope without addressing the recyclability
aspect of the catalyst. Thus, development of efficient and recy-
clable heterogeneous catalytic system in such coupling is an
important objective as generation of huge amount of metal waste
is incongruous to the principles of Green Chemistry. This in partic-
ular is highly desired during synthesis of drug like molecules
where even trapping of traces of metal impurity in the final pro-
duct could be potentially harmful.
1
1,12
can act as a suitable material for the heterogeneous catalysis.
The presence of free amino and hydroxyl groups on chitosan
enables it to coordinate with different metal atoms/ions and make
it an attractive polymeric support for the immobilization of metal
1
3
14
15
16
17
catalysts such as Pd, Rh, Cu, Ti, Ru, etc. Furthermore,
chitosan based catalysts exhibit high thermal stability and can be
used in various organic transformations. Recently, Zhang and
Following our research interest on the development of environ-
2
7–29
mentally friendly methodologies,
we herein report chitosan-
⇑
Corresponding authors.
E-mail addresses: vinod.kumar@cup.edu.in (V. Kumar), rakesh.kumar@cup.edu.
supported copper(I) as an efficient recyclable heterogeneous
catalyst for A -coupling reaction. The versatility of developed
3
in (R. Kumar).
https://doi.org/10.1016/j.tetlet.2018.03.052
0
040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
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2
P. Kaur et al. / Tetrahedron Letters xxx (2018) xxx–xxx
chit@Cu(I) catalytic system was successfully extended towards
decarboxylative A -coupling. To the best of our knowledge this is
ionic liquid [bmim]OH could provide 4a only in 29% yield (entry
6). Interestingly, reaction under solventless conditions dramati-
cally enhanced the product yield (4a, 72%, entry 7). Further, the
catalyst amount in the reaction was optimized (entries 8–11) and
it was observed that the reaction proceeded efficiently when the
catalyst amount was reduced from 100 mg to 10 mg (4a, 92%, entry
3
the first report wherein recyclable chitosan-supported copper
3
catalyst has been reported for the decarboxylative A -coupling.
Results and discussion
1
1
0). Further, when the reaction temperature was increased from
00 °C to 140 °C the reaction completes within 45 min using 10
3
In A -coupling, mechanistically, aldehyde and amine react to
mg of the catalyst (entry 13, 4a, 93%). As expected, no product
generate imine/iminium ion, followed by nucleophilic addition of
3
19
could be detected when A reaction was performed with chitosan
alkyne (in situ generated metal-alkyne complex). Recently, the
research group of Dekamin12 has demonstrated that OH and NH
alone (entry 15). The reaction was also conducted under micro-
wave irradiations and a significant reduction in the reaction time
was observed (15 min) while the product 4a was obtained in com-
parative yield (90%, entry 14). So, the final optimized conditions for
the reaction were 1a (0.77 mmol), 2a (0.85 mmol) and 3a (1.00
mmol), chit@CuI (10 mg, 0.04 mmol%) at 140 °C under nitrogen
atmosphere. Use of 1:1:1 of 1a:2a:3a under similar reaction condi-
tion, provided comparatively lower yield of the product. Here it is
2
group on biopolymer chitosan facilitate the imine formation from
corresponding aldehyde and amine via hydrogen bonding network.
3
These findings and mechanistic rationale of A -coupling incited us
to explore the chitosan-supported metal catalysts for this reaction.
Various chitosan-supported copper catalysts (chit@CuI, chit@CuBr,
2 4
chit@Cu(OAc) and chit@CuSO
) were prepared.18 These catalysts
were analysed by FT-IR, FE-SEM-EDS, TGA, ICP-MS and AAS spec-
troscopy. For instance, the amount of Cu content of freshly pre-
pared chit@CuI catalyst, as measured by ICPMS and AAS was
found to be 2 wt%. FT-IR spectra of chitosan showed characteristic
overlapping absorption bands of OAH and NAH stretching vibra-
important to mention that while conducting the reaction under N
2
atmosphere (entries 1–15), we haven’t observed the formation of
any homocoupling product of alkyne (i.e., diphenylbutadiyne).
3
However, when A -coupling was performed under air, a small
À1
amount of homocoupling product was detected (<4% yield, GC–
MS) along with desired 4a in 87% yield (entry 16).
tions around 3429 cm which became a bit sharper and stronger
in case of chit@CuI catalyst with a little shift in the position of
other characteristics bands (See; SI). FE-SEM analysis of chit@CuI
catalyst clearly showed the morphological changes on the surface
of chit@CuI catalyst when compared with pure chitosan polymer
The scopes and limitations of the developed reaction conditions
(
Table 1, entry 13) were ascertained using a variety of optionally
substituted aldehydes, amines and alkynes as coupling partners
Table 2). The chit@CuI catalyst was found compatible with alipha-
(
(
Fig. 1a and b). The FE-SEM-EDS analysis clearly illustrate the
presence of copper and iodine in the chitosan supported catalyst
Fig. 1c).
FE-SEM-EDS elemental maps were recorded at a selected region
tic as well as aromatic benzaldehydes (irrespective of o,m,p-posi-
tion of substituents) bearing electron-donating (EDG’s) or
electron-withdrawing groups (EWG’s). However, reaction of ben-
(
zaldehyde containing strong EWG, NO
sponding product could be isolated in comparatively lower yield
4e, 58%) along with unreacted starting material and unidentified
2
was found slow and corre-
of chit@CuI catalyst which clearly represent the dispersity of cop-
per on the surface of chitosan (Fig. 2).
30
(
TGA was used to determine the thermal stability of chitosan
and chit@CuI catalyst (See; SI). Two well defined signals
were obtained at heating rate of 10 °C/min. The first signal at
T < 100 °C is connected to loss of water which is loosely bound
on chitosan surface whereas second signal (254–580 °C) represents
side products. On the other hand, hydroxy substituent on ben-
zaldehyde 4f and sterically hindered aromatic aldehydes 4g were
well tolerated. In comparison to aromatic aldehydes, the reaction
of an aliphatic aldehyde along with morpholine and pheny-
3
lacetylene (4h,76%) provided A product in comparatively lower
6
0.7% weight loss due to the release of volatile components by the
yield. The catalytic system was found compatible with different
types of cyclic secondary amines such as morpholine, piperidine,
N-methyl/phenyl piperazine and acyclic secondary amine such as
thermal degradation of chitosan.
To evaluate the efficacy of the synthesized catalysts (Table 1), a
model reaction employing 3-chlorobenzaldehyde (1a), morpholine
N-methylbenzyl amine. However, reaction with primary amines
(2a) and phenylacetylene (3a) as coupling partners was conducted.
3
(
4q, 4r and 4 s) as one of the A component could not proceed
Initially, it was planned to check the ability of chitosan@copper
catalysts for this reaction using benign solvents such as water or
ionic liquids. The screening of various chitosan@copper catalysts
under present reaction condition. Notably, aryl alkyne provided
comparatively better yield than aliphatic alkynes (4u). No product
could be isolated using trimethyl silylacetylene as a coupling part-
ner (4v). The reaction was also conducted using acetophenone
(
100 mg per 0.77 mmol of 1a) was carried out in water (2 mL) at
1
00 °C (Table 1, entries 1–4), wherein chit@CuI provided 4a in
(
replacing aldehyde with ketone), amine and phenylacetylene but
1
7% yield (entry 3). Use of chit@CuI catalyst along with ionic liquid
no product (4w) formation was observed.
[
bmim]Br increased the yield of 4a up to 42% (entry 5). However,
Fig. 1. (a) FE-SEM image of chitosan, (b) FE-SEM image of chit@CuI catalyst, (c) FE-SEM-EDS spectrum of chit@CuI catalyst.
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P. Kaur et al. / Tetrahedron Letters xxx (2018) xxx–xxx
3
Fig. 2. (a) FE-SEM image of chit@CuI catalyst, selected area elemental maps showing the presence of O, Cu, I, N and C, (b) FE-SEM image of the selected area depicting the
overall elemental maps.
procedure after each cycle, the catalyst was reused efficiently for
six consecutive cycles to afford 4a in 93%, 92%, 88%, 85%, 83%
and 78% yields (Fig. 3).
Table 1
Screening of chitosan@copper catalysts for A -coupling.
3
a
3
Chit@CuI catalyst in decarboxylative A -coupling and its
recyclability
To check the efficacy and recyclability of our chit@CuI catalyst in
3
decarboxylative A -coupling, a mixture of 1a, 2a and phenylpropi-
olic acid (5a) was heated at 140 °C (using 10 mg catalyst) and corre-
sponding propargylamine 4a was obtained in 86% yield (Scheme 1).
3
1
Reuse of this catalyst for the next cycle produced the desired pro-
duct nearly in same yield (85%). The catalyst could be recycled and
reused successfully up to four cycles (yield ranging from 86% to
th
7
8%). In the 5 cycle 4a was obtained in 65% yield (Scheme 1). Here
it is worth to mention that a majority of the protocols reported for
3
decarboxylative A -coupling mainly focus on substrate scope
without addressing the recyclability aspect of the catalyst.
2
Also, there is no need for N atmosphere and the reaction pro-
ceeded well in air without formation of any homocoupling product,
diphenylbutadiyne. Mechanistically, it is presumed that incipient
chit@Cu-alkyne complex B (formed in situ via chit@CuI assisted
decarboxylation of phenylpropiolic acid) undergoes addition to
the iminium ion A (formation assisted by OH and NH
2
group of
1
2
chitosan
Scheme 2).
The reaction was successfully extended to other substituted
) resulting in the formation of propargylamines
2
3–26
(
aGeneral condition: 1a (0.77 mmol), 2a (0.85 mmol), 3a (1.00 mmol),
benzaldehydes containing EWG’s and EDG’s including
heteroaldehyde and an aliphatic aldehyde (Table 3).
a
chitosan@copper catalyst (5–100 mg) at 100–140 °C under N
2
atmosphere.
b
Yield of isolated product after column chromatography.
Encouraged by above results, we tried to explore the potential
of this catalyst for a novel domino oxidative-decarboxylative A3
coupling using benzylalcohol in place of aldehyde (Scheme 3).
However, the desired product could be isolated only in 23% yield.
Finally, we carried out the leaching studies of Cu metal from the
c
Ionic liquid, 1-butyl-3-methylimidazolium bromide.
d
Ionic liquid, 1-butyl-3-methylimidazolium hydroxide.
e
Without any solvent.
Microwave irradiation at 250 W, 140 °C.
f
g
Using chitosan support only i.e., without copper loading, nd: not detected.
h
2
In air (without N atm.).
3
3
catalyst in case of A -coupling as well as in decarboxylative A -cou-
3
st
nd
rd
pling. In A -coupling, after the completion of 1 , 2 and 3 cycle,
the quantitative analysis of the recovered catalyst by ICPMS
revealed no change in wt% of Cu content. We further analysed the
extracted product of the 3 cycle and no leaching of Cu metal
was detected. After 6 cycle, the amount of Cu content as measured
3
Recyclability of chit@CuI in A -coupling
rd
Reusability and recyclability of the transition metal catalyst is
highly desired, from both economical and environmental point of
view. To assess the reusability of the catalyst, we performed the
th
by ICPMS in the recovered catalyst was 1.93 wt% as compared to
2 wt% in the freshly prepared chit@CuI catalyst. This confirms that
chitosan provides enough binding sites on its surface to minimize
deterioration and metal leaching and facilitates its efficient
recycling.
3
A -coupling reaction under the optimized conditions (Table 1,
entry 13). After the completion of reaction, the product was
extracted with diethyl ether and the catalyst was recovered, dried
on rotary evaporator and reused for next cycle.31 Using the above
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P. Kaur et al. / Tetrahedron Letters xxx (2018) xxx–xxx
Table 2
Scope of chit@CuI catalyzed coupling of aldehyde, amine and alkyne under solventless conditions.a
aGeneral condition: aldehyde (1, 0.77 mmol), amine (2, 0.85 mmol), alkyne (3, 1.00 mmol), chit@CuI (10 mg) at 140
°
2
C for 45 min under N atmosphere, yield in parenthesis represents the yield of isolated product after column
chromatography.
b
Heating at 140 °C for 2 h.
c
Inseparable side products obtained, nd: not detected.
3
st
Similarly, in case of decarboxylative A -coupling, after 1 and
2nd cycle no leaching of the Cu content was observed in the recov-
ered catalyst as well as in the extracted product as determined by
th
ICPMS analysis. However, after the 5 cycle, the amount of Cu con-
tent was reduced to 1.8% as compared to 2 wt% in the freshly pre-
pared chit@CuI catalyst. The above results were further confirmed
using AAS analysis. Despite negligible metal leaching, the loss in
the activity of catalyst (during recycling experiments) may be
attributed to the poisoning of the surface site of the catalyst by
3
contact with H
A -coupling).
2
O and CO
2
(released during A /decarboxylative
3
3
3
Although various A /decarboxylative A -methodologies using
copper based catalytic systems besides expensive metal catalysts
3
Fig. 3. Recyclability of chit@CuI catalyst in A -coupling.
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P. Kaur et al. / Tetrahedron Letters xxx (2018) xxx–xxx
5
3
Scheme 1. Recyclability of chit@CuI catalyst in decarboxylative A -coupling.
3
Scheme 2. Plausible mechanism of decarboxylative A -coupling.
Table 3
Chit@CuI catalyzed decarboxylative A -coupling.
3
a
aGeneral condition: aldehyde (1, 0.77 mmol), amine (2, 0.85 mmol), phenylpropiolic acid (5, 1.00 mmol), chit@CuI
10 mg), 140 °C for 45 min.
(
b
Yield in parenthesis represents the yield of isolated product.
of gold and silver have been explored, however, recyclability of
these catalysts, particularly in case of decarboxylative A -coupling
has rarely been addressed. Furthermore, easy synthesis of the
catalyst from abundantly available biopolymer besides waste-free
synthesis of propargylamines under solventless conditions are
some other advantages of our developed protocol.
3
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6
P. Kaur et al. / Tetrahedron Letters xxx (2018) xxx–xxx
3
Scheme 3. Tandem oxidative-decarboxylative A -coupling.
In conclusion, we have successfully explored the role of chi-
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tosan@copper based heterogeneous catalysts for A -coupling and
9.
3
decarboxylative A -coupling reaction. The developed methodology
2
provided a green route for the synthesis of diversely substituted
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1
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in decarboxylative A -coupling. The present catalytic system could
2
be a versatile strategy for the development of various other green
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2
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31. Catalyst recycling: after completion of the reaction between 1a, 2a and 3a
Table 1, entry 13), the crude mixture was extracted with diethyl ether and the
2
3
3
2
substituted benzaldehyde
A. Supplementary data
(
Supplementary data (experimental details, 1H and 13C NMR
data, NMR spectra of some representative compounds) associated
with this article can be found, in the online version, at
https://doi.org/10.1016/j.tetlet.2018.03.052.
(
(
chit@CuI catalyst was separated by simple filtration. The catalyst was dried on
rotary evaporator at 60 °C for 4 h and then used for the next cycle. Using the
same procedure after each cycle, the catalyst could be successfully reused.
Similarly, the above method was applied for the recyclability of the catalyst in
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Please cite this article in press as: Kaur P., et al. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.03.052