Cellulose-immobilized NHC-Cu(i) complex: An efficient and reusable catalyst for multicomponent synthesis of 1,2,3-triazoles

Article abstract of DOI:10.1039/c5ra19075d

A novel cellulose supported copper NHC complex has been prepared by the reaction of cellulose supported imidazolium salt with copper(i) iodide. The catalyst is active in the synthesis of 1,2,3-triazoles via a one-pot reaction of alkyl/benzyl halides or to

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Cellulose‐Immobilized NHC–Cu(I) Complex: An Efficient and
Reusable catalyst for Multicomponent Synthesis of 1,2,3‐Triazoles
Ali Pourjavadi* and Zahra Habibi
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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A novel cellulose supported copper NHC complex has been anticancer drugs, as well as to the development of a latent
prepared by the reaction of cellulose supported imidazolium salt catalyst for this transformation.10
with copper(I) iodide. The catalyst is active in the synthesis of 1,2,3‐
Recently, biopolymers such as alginate, cellulose, chitosan,
triazoles via a one‐pot reaction of an alkyl/benzyl halides or gelatine, starch and wool have been used as supports for
tosylates and terminal alkynes, with sodium azide in water.
catalytic applications.11 Several interesting features of the
biopolymers for example, bio‐degradable, environmentally
safe, high sorption capacity, physical and chemical versatility
make them attractive to use as supports.
Huisgen 1,3‐dipolar cycloadditions are exergonic fusion
processes that unite two unsaturated reactants and provide fast
access to an enormous variety of five‐membered heterocycles.1
The cycloaddition of azides and alkynes to give triazoles is
Abdelmouleh et al. reported the surface modification of
12
cellulose (CL) fibers with silanes coupling agents. Recently,
2
arguably the most useful member of this family. The classical
Koga et al. reported in situ modification of cellulose paper with
method for their synthesis involved thermal 1,3‐dipolar
3
‐aminopropyltrimethoxysilane for catalysis Knoevenagel
3
cycloaddition. However, this procedure was associated with
condensation between aldehydes or ketones and active
elevated temperature, low yields and lack of selectivity.
Independent discoveries by Sharpless et al. and Meldal et al.
demonstrated that in the presence of a Cu(I) catalyst, this
cycloaddition could be performed regioselectively, affording
exclusively the 1,4 disubstituted 1,2,3‐triazoles.2 This
chemistry has found wide application in various disciplines
including materials science, chemical biology, and medicinal
13
methylene compounds.
Based on the understanding described above, herein, we
report the synthesis of novel heterogeneous catalyst containing
NHC anchored in CL, as well as its application for the
multicomponent 1,3‐dipolar cycloaddition of terminal alkynes
and organic azides generated in situ from sodium azide and
different organic halides or tosylates.
The process for the preparation of CL supported copper NHC
complex is schematically described in Scheme 1. First, the
grafting of the imidazolium salt (NHC ligand precursor) onto the
CL support followed by reaction with sub‐stoichiometric
amount of copper(I) iodide and sodium tert‐butoxide, to afford
NHC‐Cu/CL
,4
5
chemistry. Several members of the 1,2,3‐triazole family have
indeed shown interesting biological properties, such as anti‐
allergic, anti‐bacterial, and anti‐HIV activity.6
During the last 20 years, N‐Heterocyclic Carbenes (NHCs)
have been extensively studied in organometallic synthesis and
7
catalysis. As strong σ‐donors and weak π‐acceptors, the NHC
ligands have a stronger interaction with the metal center
compared to phosphine ligands, thereby enhancing the stability
8
of NHC complexes toward heat and moisture. Recently, several
papers were reported about the remarkable activity of
supported and unsupported [(NHC)CuX] complexes (X=Cl, Br)
9
in the 1,3‐dipolar Huisgen cycloaddition. Such catalytic systems
have already been applied to the preparation of triazole‐ Scheme 1. Schematic description for the preparation of NHC‐Cu/CL
containing carbanucleosides, porphyrins, and platinum‐based
FT‐IR spectroscopy of CL‐IL showed a new medium
‐
1
a.
absorption band at 1572 cm for C=N stretching (Fig. 1).
Polymer Research Laboratory, Department of Chemistry, Sharif University of
Technology, Azadi Avenue, P. O. Box 11365‐9516, Tehran, Iran E‐mail:
purjavad@sharif.edu
†
Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
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Thermogravimetric analysis (TGA) was further used to study
the thermal behavior and stability of the NHC‐Cu/CL. TGA
indicates the catalyst stable up to 231 °C. (Fig. 2).
The morphology of Fig. 3a‐c shows representative SEM
images of the as‐received CL and NHC‐Cu/CL. The density and
distribution of the NHC‐Cu group on the NHC‐Cu/CL
nanocomposite were evaluated by quantitative energy
dispersive X‐ray spectroscopy (EDS) mapping. As can be seen in
Fig. 3d–g, rather than only being located at the edges of the
graphene sheets, the elements Cu, Cl, I and Si were found to be
uniformly dispersed on the whole surface of the NHC‐Cu/CL
nanocomposite indicating the homogeneous distribution of the
NHC‐Cu.
Fig. 3 SEM images of (a) CL and (b‐c) NHC‐Cu/CL, and corresponding quantitative EDS
elemental mapping of (d) Cu, (e) Cl (f) I and (g) Si.
The catalytic activity of the NHC‐Cu/CL composite as catalyst
was then tested in multicomponent 1,3-dipolar cycloaddition of
terminal alkynes and organic azides generated in situ from sodium
azide and different organic halides or tosylates. To optimize the
reaction conditions, benzyl bromide, sodium azide and
phenylacetylene were tested as model substrates in the
presence of various solvents (Table 1). The results indicate that
both reaction temperature and solvent significantly influence
the product yield in the coupling reaction. After several
screening experiments with different combinations, the best
ones were proved to be NHC‐Cu/CL (1 mol% of Cu), benzyl
bromide (1 mmol), sodium azide (1.2 mmol), phenylacetylene
(
1.3 mmol) and H
2
O (3 mL) at 50 °C for 1 h.
Fig. 1 FT‐IR spectra of cellulose (blue), grafted imidazolium salt (red), and NHC‐Cu/CL
a
Table 1. Screening of the reaction conditions
(green).
b
Entry
Solvent
Temp. (ºC)
Yield (%)
1
2
3
4
5
6
7
8
H
2
O
50
50
50
50
50
50
60
40
98
83
61
20
65
73
98
81
H
2
O/EtOH (1:1)
EtOH
PhCH
CHCl
3
3
CH
H
3
2
2
CN
O
H
O
a
benzyl bromide, (1.0 mmol), sodium azide (1.2 mmol), phenylacetylene (1.0
b
mmol), solvent (3 mL), NHC-Cu/CL (1 mol% of Cu), 1 h Isolated yield
Fig. 2 TGA plots of CL and NHC‐Cu/CL
Using the optimized reaction conditions, it was then
attempted to expand the scope of organic halides or tosylates
and terminal alkynes in water at 50 °C, using 1 mol% NHC‐Cu/CL
(
(
Table 2). Electron donating substituents like methyl, methoxy
entries 2 and 3), and electron withdrawing substituents such as
acetyl, nitro groups at para position of benzyl bromide (entry 4
were equally effective toward the nucleophilic substitution of
)
azide, followed by 1,3‐dipolar cycloaddition. However, in case
of non‐activated alkyl halides like n‐hexyl bromide as well as n‐
octyl bromide, the reaction required longer reaction time and
furnished corresponding triazoles in lower yields (Table 2,
2
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Table 3. Reusability of the NHC-Cu/CL composite in multicomponent 1,3-
dipolar cycloaddition
entries 5 and 6). Unfortunately, the catalytic system was less
effective for the reaction of benzyl chlorides (Table 2, entries 7‐
a
9
). Benzyl and allyl tosylate reacted well and giving good yields
1st
2nd
3rd
4th
5th
6th
7th
Reaction cycle
(
Table 2, entries 10 and 11). Furthermore, by using various
b
substituted phenylacetylenes with substituents that have
different electronic properties, different yields of the final
products were achieved. The aryl acetylene bearing electron‐ mmol), H
Yield (%)
98
98
98
97
97
96
96
a
benzyl bromide, (1.0 mmol), sodium azide (1.2 mmol), phenylacetylene (1.0
b
2
O (3 mL), NHC-Cu/CL (1 mol% of Cu), 1 h, 50 ºC. Isolated yield
.
3
donating group (p‐Me) and electron withdrawing group (p‐CF )
reacted well and giving good yields (Table 2, entries 12 and 13).
Table 2. Multicomponent 1,3-dipolar cycloaddition catalyzed by NHC-
Cu/CL
Conclusions
a
We have successfully developed a novel, practical and
environmentally friendly method for the multicomponent 1,3‐
dipolar cycloaddition by using NHC‐Cu/CL as catalyst. In
addition, this methodology offers the competitiveness of
recyclability of the catalyst without significant loss of catalytic
activity, and the catalyst could be readily recovered and reused
for seven cycles, thus making this procedure environmentally
more acceptable. Further studies are currently underway in our
laboratories and results will be disseminated in due course.
1
2
b
Entry
1
R
X
Br
R
Yield
98%
Ph
4-CH
4-CH OC
4-NO
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2
3
4
5
6
3
C
6
H
4
Br
Br
94%
91%
98%
82%
84%
90%
79%
85%
98%
97%
98%
90%
3
6
H
4
2
C
6
H
4
Br
C
C
5
H
H
11
Br
7
15
Br
Notes and references
c
7
Ph
Cl
1
2
3
R. Huisgen, in 1,3‐Dipolar Cycloaddition Chemistry ed. A.
Padwa, (Wiley, New York, 1984.)
c
8
4-CH
3
OC
6
H
4
Cl
c
9
4-NO
Ph
CH =CH
2
C
6
H
4
Cl
V.V. Rostovtsev, L.G. Green, V.V. Fokin and K.B. Sharpless,
Angew. Chem. Int. Ed., 2002, 41, 2596.
1
1
1
1
0
1
2
3
OTs
OTs
Br
(a) R. Huisgen, G. Szeimies and L. Moebius, Chem. Ber., 1965,
2
98, 4014. (b) R. Huisgen, Pure Appl. Chem., 1989, 61, 613. (c)
Ph
Ph
4-CH
3
C
H
6 4
R. Huisgen, Angew. Chem., Int. Ed. Engl., 1963,
C.W. Tornøe, C. Christensen and M. Meldal, J. Org. Chem.,
002, 67, 3057.
2, 565.
4
Br
4-CF
3 6 4
C H
2
5
6
M. Meldal and C. W. Tornøe Chem. Rev., 2008, 108, 2952.
G. C. Tron, T. Pirali, R. A. Billington, P. L. Canonico, G. Sorba
and A. A. Genazzani, Med. Res. Rev., 2008, 28, 278.
aorganic halide or tosylate, (1.0 mmol), sodium azide (1.2 mmol), terminal
b
2
alkyne (1.0 mmol), H O (3 mL), NHC-Cu/CL (1 mol% of Cu), 1 h Isolated
c
yield. reaction time = 3 h.
7
(a) D. T. Cohen and K. A. Scheidt, Chem. Sci., 2012,
3, 53. (b) A.
The heterogeneous nature of the catalysis was proved using a hot
filtration test and AAS analysis. To determine whether the catalyst is
actually functioning in a heterogeneous manner or whether it is merely
a reservoir for more active soluble copper species, we performed a hot
filtration test in the multicomponent 1,3-dipolar cycloaddition of
benzyl bromide, sodium azide and phenylacetylene after ~50% of the
coupling reaction is completed. The hot filtrates were then transferred
Grossmann and D. Enders, Angew. Chem. Int. Ed., 2012, 51
,
314. (c) G. C.; Fortman and S. P. Nolan, Chem. Soc. Rev., 2011,
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8
9
to another flask containing H
2
O at 50 °C. Upon the further heating of
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Chem. Commun., 2013, 49, 11358. (b) M.‐L. Teyssot, A.
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catalyst-free solution for 6 h, no considerable progress (~3% by GC
analysis) was observed. Moreover, using AAS of the same reaction
solution at the midpoint of completion indicated that no significant
quantities of cooper (~1%) are lost to the reaction liquors during the
process.
The recyclability of the NHC-Cu/CL composite was also examined
by the multicomponent 1,3-dipolar cycloaddition. It was found that
the recovery can be successfully achieved in seven successive reaction
runs (Table. 3).
Mullangi, and S. K. Mandal, Adv. Synth. Catal., 2013, 355
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Cellulose-Immobilized NHC–Cu(I) Complex: An Efficient and Reusable
catalyst for Multicomponent Synthesis of 1,2,3-Triazoles
Ali Pourjavadi and Zahra Habibi