Regioselective synthesis of 1,4-disubstituted triazoles using bis[(L)prolinato-N,O]Zn complex as an efficient catalyst in water as a sole solvent

Article abstract of DOI:10.1002/aoc.1816

A concise, convenient and mild route for one-pot regioselective synthesis of N-aryl- and N-alkyltriazoles in water as a sole solvent is reported. The methodology involves a three-component reaction comprising aryl/alkyl-alkyne, sodium azide and aryl/alkyl

Full text of DOI:10.1002/aoc.1816

Full Paper
Received: 30 March 2011
Revised: 21 May 2011
Accepted: 21 May 2011
Published online in Wiley Online Library: 8 July 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1816
Regioselective synthesis of 1,4-disubstituted
triazoles using bis[(L)prolinato-N,O]Zn complex
as an efficient catalyst in water as a sole solvent
Mazaahir Kidwai∗ and Arti Jain
A concise, convenient and mild route for one-pot regioselective synthesis of N-aryl- and N-alkyltriazoles in water as a sole
solvent is reported. The methodology involves a three-component reaction comprising aryl/alkyl-alkyne, sodium azide and
aryl/alkyl/allyl halide catalyzed by zinc(II) L-prolinate. Prominent features of our protocol are incorporation of transition metal
catalyst other than copper, water as the reaction medium, recyclability of catalyst and avoidance of hazardous aryl azide as a
c
reactant. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: bis[(L)prolinato-N,O]Zn; regioselective; 1,4-disubstituted triazole; water as a sole solvent; recyclability
Introduction
The importance of Zn for all kinds of life processes[30–32]
is generating new activities in the once-neglected field of Zn
coordination chemistry. One aspect of this should be the study of
Zn-amino acid and peptide complexes.
The vast majority of nature’s molecules, including proteins, nucleic
acid and biologically active compounds, contain nitrogen. There
is a growing interest in using specific biologically validated
substructures as starting point for library design.[1] The resulting
libraries can then be used to address a variety of biological targets.
Triazoles have attracted interest over the past few years
as a building block for the synthesis of pharmacologically
active compounds and are associated with a wide range of
activities including anti-HIVactivity,[2] selective β3-adrenergic
receptorinhibition,[3] antibacterialactivity,[4] potentantihistamine
activity[5–7] and anticancer activity.[8]
These moieties have been widely used in synthetic intermedi-
ates and have industrial applications such as for the preparation of
dyes, anticorrosive agents, photostabilizers, photographic materi-
als and agrochemicals.[8] These heterocycles are rigid linking units
that can mimic the atom placement and electronic properties of a
peptide bond without any susceptibility to hydrolytic cleavage.[9]
In 2002, Sharpless et al. discovered that Cu(I) catalyzes the
1,3- dipolar cycloaddition of azides and alkynes to form 1,4-
disubstituted triazoles, which greatly contributed to the pop-
ularization of click chemistry as a highly effective method for
functionalization.[10] The conditions used to conduct such reac-
tions can be addition of copper (I) salts in organic or aqueous
systems, often in conjunction with a base,[11–13] a copper (II)
salts/ascorbic acid system (to generate the copper (I) species
in situ),[14–16] copper salts adsorbed on zeolites,[17] charcoal[18] or
clay,[19] Al2O3,[20] copper wire,[21–23] nanoparticles/clusters[24,25]
and Cu-based ionic liquid.[26] Use of water as a solvent has also
been explored.[27,28] In contrast to the copper-catalyzed version,
ruthenium complexes ([Cp∗RuCl (PPh3)2] and [Cp∗RuCl]) were
found to be efficient catalysts for producing 1,5-disubstituted tri-
azoles regioselectively.[29] Until now no other method has been
explored in which a catalyst other than copper has been used. We
report for the first time zinc(II) L-prolinate-catalyzed synthesis of
1,4-disubstituted triazole.
Recently, proline and chiral zinc complexes have been shown
to act as enantioselective catalysts for aldol reactions.[33,34]
Bis[(L)prolinato-N,O]Zn complex, i.e zinc(II) L-prolinate, has also
been used for the synthesis of 1,5-benzodiazepines.[35] This
complex is soluble in water but is not soluble in organic solvents,
which allows simple and quantitative recovery of the catalyst. In
recent years, organic reactions in aqueous media have attracted a
great deal of attention. Water is no doubt cheap, safe and environ-
mentally friendly when compared with organic solvents, and has
uniquereactivityandselectivitythatcannotbeobtainedinorganic
solvents. The main advantage of our catalyst is the accessible
precursor for the formation of zinc(II) L-prolinate complex.
As part of our ongoing research program to devise greener
chemical transformations,[36–38] we reveal herein for the first time
a simple, convenient and efficient method for the preparation of
triazoles using zinc(II) L-prolinate as a catalyst in water as a solvent.
Experimental Section
All chemicals were purchased from Sigma–Aldrich and Lancaster
and were used as received. All reactions and purity of terminal
alkynesandhalidesweremonitoredbythin-layerchromatography
(TLC) using aluminum plates coated with silica gel (Merck) in
30% ethylacetate and 70% hexane. The isolated products were
further purified by column chromatography using silica gel
∗
Correspondenceto:Mazaahir Kidwai, Green Research Laboratory, Department
of Chemistry, University of Delhi, Delhi 110 007, India.
E-mail: kidwai.chemistry@gmail.com
GreenResearchLaboratory,DepartmentofChemistry,UniversityofDelhi,Delhi
110 007, India
c
Appl. Organometal. Chem. 2011, 25, 620–625
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

Regioselective synthesis of 1,4-disubstituted triazoles
H
H
Ph
Zinc(II) L-prolinate
water, 2 hrs.
Br
N
NaN3
+
+
N
N
Ph
3
2
1
Scheme 1. Model reaction between phenylacetylene, benzylbromide and sodium azide in the presence of zinc(II) L-prolinate in water as a sole solvent.
(Sigma–Aldrich 24, 217–9, 70, 35–70, mesh 40 Å surface area
Table 1. Model reaction in water catalyzed by various catalystsa
675 m2/g). IR spectra were recorded on a Perkin–Elmer FTIR-
1710 spectrophotometer using Nujol film and a KBr pellet. 1H
NMR and 13C NMR spectra were recorded on a Bruker Avance
Spectrospin 300 (300 and 75 MHz) using Tetramethylsilane (TMS)
as the internal standard, and chemical shifts are in δ. HR–MS mass
spectra were recorded on a Waters LCT Micromass. The melting
points of the compounds were measured through a Thomas-
Hoover melting point apparatus and are uncorrected. Powder
X-ray diffraction (XRD) was done on Bruker D8 Discover at 1600 W.
The Transmission electron microscopy (TEM) images were taken
on a Technai G2 30 U-TWIN at 300 kV.
Sample no.
Catalyst
Timeb
Yieldc (%)
1
Zn(CH3COO)2
Zn(OTf)2
Zn (ClO4)2
Zn(NO3)2
ZnSO4
5
6
40
43
40
35
–
2
3
6
4
5.5
12
12
5
6
7
ZnCl2
–
8
L-Proline
Zn(Lys)2
Zn(Arg)2
Zn(His)2
Zn(L-Pro)2
38
30
50
40
91
9
10
6
10
11
12
10
2
Synthesis and Characterization of Zinc(II) L-prolinate
The zinc amino complex was prepared by adding Et3N (0.6 ml)
to the amino acid (4.34 mmol) in MeOH (10 ml), followed, after
10 min, by zinc acetate (2.17 mmol). After stirring for 45 min,
a white precipitate was collected by filtration (25–95% yield).
Complexes were characterized by 1H- NMR, IR and ESI-MS.[31]
a Reactions were performed with phenylacetylene (0.01 mol), ben-
zylbromide (0.01 mol), sodium azide (0.01 mol) and various catalysts
(2 mol%) in water by refluxing the mixture for x h. b Reaction progress
monitored by TLC. c Isolated yield.
General Procedure for the Three-component Reactions
catalyst is water-soluble). Water has clear advantages as a solvent
because it is cheap, readily available and nontoxic. Moreover, it
has been argued that reactants that dislike water tend to come
closer (the hydrophobic effect) and complete the reaction faster
than in the absence of water. Interestingly, the three-component
click chemistry when carried out in water using zinc(II) l-prolinate
furnished 3 in a good isolated yield of 90% (Scheme 1).
The catalytic activities for the 1,3-dipolar cycloaddition of 1a to
2a in water were also compared with various catalysts. Therefore,
the same reaction was initially tested with zinc acetate and proline
individually. However, encouraging results were not found. Then,
thecatalyticabilitiesofotherZn-(L)-aminoacidcomplexes(2 mol%
of catalyst, H2O) were investigated. The complexes were prepared
and isolated as described for Zn-proline. It was found that zinc
acetatae is not able to catalyze the reaction efficiently, as the
four-membered ring formed by the acetate group on chelation
with zinc is not as stable as the five-membered ring formed by
the proline group. Moreover only in zinc(II) L-prolinate did the
amino acid contain secondary amine, which ultimately enhances
the catalytic efficiency of zinc(II) L-prolinate.Other catalysts do not
contain any secondary amine. To explain the dominance of our
catalyst we tested our model rection with other Zn salts. It was
found that the other salts used here were inferior to the zinc(II)
L-prolinate. The results are reported in Table 1.
An appropriate halide (1.0 mmol), terminal alkyne (1.1 mmol) and
sodium azide (1.1 mmol) were added to the round-bottom flask.
A 3 ml aliquot of water as solvent and 2 mol% zinc(II) L-prolinate
wereaddedandrefluxed.ThereactionwasmonitoredbyTLC.After
cooling to room temperature, ethyl ether (2 × 4 ml) was added to
the mixture to extract the product. The aqueous part was further
used for consecutive runs as the catalyst is water soluble. The
combined organic layers were washed with water and brine, dried
with anhydrous MgSO4 and evaporated under reduced pressure.
The residue was finally purified by column chromatography on
silica gel to give the desired product.
Results and Discussion
In an initial endeavor, a blank reaction was carried out using 1
equiv. each of phenylacetylene, benzylbromide and sodium azide.
Thesewererefluxedinethanol.After20 honly65%oftheexpected
product was obtained. The same reaction was then carried out
using zinc(II) l-prolinate as catalyst in ethanol. Surprisingly, a
significant improvement was observed and the yield of product
was dramatically increased to 85% after refluxing the mixture
in water for only 2 h. After successfully optimizing the reaction
conditions for the synthesis of N-aryltriazole 3, we then decided to
carry out the reaction using water as the sole solvent (because our
The main point of focus in this reaction is the solubility of the
catalyst in the reaction mixture. Since our catalyst is water-soluble,
c
Appl. Organometal. Chem. 2011, 25, 620–625
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
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M. Kidawi and A. Jain
Table 2. Solvent system for model reactiona
Table 3. One-pot synthesis of 1,2,3-triazoles from alkyl/aryl/allyl
halides, sodium azide and terminal alkynesa
Sample no.
Solvent system
Timeb
Yieldc (%)
Sample
no.
Time
(h)
R1
Halide
Product Yieldb
1
2
3
4
5
6
7
8
9
Ethanol
Ethanol–water
Water
4.5
2
85
92
91
85
78
60
60
50
80
1
Ph
Ph
PhCH2Br
PhCH2Cl
3a
3a
3b
3c
3d
3e
3f
91
91
89
85
85
89
82
90
88
78
87
89
89
88
88
78
85
88
80
2
2
2
2
Acetonitrile
THF
2
3
Ph
n-C4H9Br
2
3
4
Ph
p-(NO2)C6H4CH2Cl
CH2 CHCH2Br
CH3I
2.5
2.5
2
Toluene
DMF
5
5
Ph
5
6
Ph
DMSO
5
7
Ph
n-C6H13Br
PhCH2Br
2.5
1.5
1.5
2
PEG-400
3.5
8
COOC2H5
COOC2H5
3g
3h
3i
a Reactions were performed with the phenylacetylene (0.01 mol),
benzylbromide (0.01 mol), sodium azide (0.01 mol), zinc(II) L-prolinate
(2 mol%) as catalyst and solvent system (3 ml) by refluxing the mixture
for x h. b Reaction progress monitored by TLC. c Isolated yield.
9
n-C4H9Br
10
11
12
13
14
15
16
17
18
19
COOC2H5 p-(NO2)C6H4CH2Cl
COOC2H5
COOC2H5
COOC2H5
CH2OH
CH2 CHCH2Br
CH3I
3j
2
3k
3l
2
C6H13Br
2
PhCH2Br
3m
3n
3o
3p
3q
3r
2.5
3
CH2OH
n-C4H9Br
ifthereactionwerecarriedoutinwaterasthesolesolvent,thenthe
catalystwouldbesolubleandthecatalystwouldbehomogeneous
with the solvent but not with the reactant. On the other hand,
if the reaction were carried out in ethanol, then the catalyst
would be heterogeneous to the catalyst. Therefore, to investigate
the better result, the model reaction was first carried out in
water, ethanol and water–ethanol systems individually. When the
reactionwascarriedoutinethanol, thereactionoccurredbutmore
slowly. This particular observation explained the lesser selectivity
and activity of heterogeneous catalyst as compared with the
homogeneous reaction. Other organic solvents were also tested.
Although polar solvents such as ethanol and acetonitrile were
much better than nonpolar solvents like THF in terms of better
yields and shorter reaction times, owing to the enhanced solubility
of the reactants. Low yields were observed in the case of nonpolar
solvents owing to the failure of the substrate to access the catalyst.
Therefore we focused our attention on the aqueous medium.
Through these observations, we concluded that reaction in water
as the sole solvent would be the preferable option compared with
water–ethanol owing to environmental considerations (Table 2).
To further evaluate the scope of this zinc(II) L-prolinate complex-
catalyzed process, reactions of benzyl halide with several aliphatic
and aromatic terminal alkynes substituted by electron-donating
as well as electron-withdrawing groups were carried out. Likewise,
the reactivity of phenylacetylene with various alkyl and aryl
halides was also studied. In all cases, very good yields of a
single regioisomer, namely 1,4-disubstituted 1,2,3-disubstituted
triazoles, were obtained (Scheme 2).
CH2OH
p-(NO2)C6H4CH2Cl
CH2 CHCH2Br
CH3I
3
CH2OH
2.5
2.5
3
CH2OH
CH2OH
C6H13Br
a Reactions were performed with the terminal alkyne (0.01 mol), halide
(0.01 mol), sodium azide (0.01 mol) and zinc(II) L-prolinate (2 mol%) in
water by refluxing the mixture. b Isolated yield.
A halide with an electron-withdrawing nitro group [p-
(NO2)C6H4CH2Cl] gave a lower yield (Table 3). In our subsequent
experiments, we conducted a broader investigation of this reac-
tion in water by utilizing a variety of alkynes and alkyl halides
with the aim of synthesizing both N-aryl- and N-alkyltriazoles. The
crude N-derivatized triazoles were purified on silica gel column
furnishing pure compounds in 80–91% isolated yields.
A proposed mechanism for this transformation is depicted in
Scheme 3. Since the reaction is taking place in aqueous medium,
bis[(L)prolinato-N,O]Zn complex will exist with the metal ion in the
form of hydrated cation and the corresponding amino acid in the
form of anion as shown in Scheme 3. The bis[(L)prolinato-N,O]Zn
complex would be transformed into an acetylide by reaction with
the alkyne.[39,40] Upon the interaction of acetylide with azide,
the reaction would follow the pathway commonly accepted for
this transformation to form a triazolide intermediate, which then
H
R1
R2
Zinc(II) L-prolinate
X
NaN3
R
2
+
+
H
R1
water, 2 hrs.
N
N
N
1a-c
2a-g
3a-r
R1, a= Ph, b= COOC2H5, c= CH2OH
R2= Ph, C3H7, 4-NO2-C6H4, CH2CH, H, C5H11
X= Br, Cl, I
Scheme 2. Reaction between terminal alkynes, halides and sodium azide in the presence of zinc(II) L-prolinate in water as a sole solvent.
c
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 620–625

Regioselective synthesis of 1,4-disubstituted triazoles
O
NH
O
O
NH
O
H O
2
H
R1
H
R
1
N
Zn2+
(C4H8NCOO-)2
N
R2
N
Zinc diprolinate
(C4H8NCOO-)2
N1
(C4H8NCOO-)2
R1
5
4
R1
R2
3 N N2
N3
R
2
5
5
4
(C4H8NCOO-)2
R2
R1
R1
4
(C4H8NCOO-)2
N R2
1
3N
1
N
N
N
N
2
2
3
Scheme 3. Proposed mechanism.
d values, which were used for its characterization, and peaks
obtained from powder XRD were matched with the data card[41]
47-1919JCDPS. These interpreted peaks revealed that the crystal
is orthorhombic in shape.
To check the crystalline nature of the catalyst, TEM images of
the catalyst were taken on a carbon-coated grid. By these images it
was found that the compound is crystalline in nature. TEM images
are given in Fig. 2.
Thermal analysis
To estimate the thermal stability of zinc(II) L-prolinate complex,
Thermogravimetricanalysis/Differentialthermalanalysis(TG/DTA)
and differential scanning calorimetry (DSC) experiments were
carried out and the thermogram is illustrated in Fig. 3. The sample
was heated at the rate of 10 K/min in an inert atmosphere. An
endothermic peak that was not sharp at 100.62 ◦C in the DTA
curve may be due to the release of adhered solvent molecules.
The sharpness of the endothermic peak observed in the DTA
curve showed a good degree of crystallinity of the material.
The DTA curve gave one sharp endothermic peak at 342.81 ◦C,
which corresponds to the melting point of the complex. The TG
curve gives the detailed decomposition of the complex. The DSC
study was performed in the temperature range 20–500 ◦C at a
heating rate of 10 K/min in the nitrogen atmosphere (Fig. 3b).
The complex underwent an irreversible endothermic transition at
342.81 ◦C, which corresponds to the melting point of the crystal.
The melting point of the substance was confirmed using a melting
Figure 1. Powder XRD of zinc(II) L-prolinate.
ultimately forms the expected triazole and the regeneration of the
catalyst.
Characterization of the Catalyst
Powder X-ray diffraction
X-ray patterns of the zinc(II) L-prolinate complex recorded in the
2θ = 0–100 range are shown in Fig. 1. The complex has specific
c
Appl. Organometal. Chem. 2011, 25, 620–625
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
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M. Kidawi and A. Jain
Figure 2. TEM images of zinc(II) L-prolinate.
Table 4. Catalyst recycling studiesa
Catalyst recycle
Yieldb (%)
1
2
3
4
5
90
87
85
85
70
a Reactions were performed with the terminal alkyne (0.01 mol), halide
(0.01 mol), sodium azide (0.01 mol) and zinc(II) L-prolinate (2 mol%) in
water by refluxing the mixture.
b Isolated yield.
FTIR spectral analysis of zinc(II) L-prolinate
The characterization vibration of the complex was compared with
that of L-proline and the shift observed confirms the formation
of the title compound. The peak at 3422 cm−1 was assigned to
the OH stretching vibration of COOH grouping. The very strong
peak at 3216 cm−1 occurred owing to NH stretching vibration.
The lack of any strong IR band at 1700 cm−1 clearly indicates
the existence of the COO− ion in zwitter ionic form. The peak at
1600 cm−1 wasassignedtotheCOOstretchingoftheCOOHgroup.
The peaks observed between 1330–1450 and 2800–3216 cm−1
were characteristic of L-proline. The COO− vibrations showed a
characteristic peak at 1412 cm−1. The other peaks at 1330 and
1300 cm−1 were assigned to the wagging of CH2 group of the
complex. The twisting NH2 vibration was well established owing
to the peak at 1170 cm−1. The absorption bands owing to the C–N
stretch produced peaks at 1076 and 1062 cm−1. CH2 rocking (937
and 847 cm−1), in-plane deformation of COO− (784 cm−1), COO−
scissoring (707 cm−1) and rocking COO− (529 cm−1) vibrations
were observed.
Figure 3. (a) TG/DTA graph of zinc(II) L-prolinate. (b) Differential scanning
calorimetry graph of zinc(II) L-prolinate.
Catalyst Recycling Ability
point apparatus, and was noted at about 342 ◦C, almost coinciding
with the DTA trace. This is in good agreement with the reported
DSC value. Therefore, it is clear that the material is stable up to its
melting point, making it suitable for possible applications where
the complex is required to withstand high temperatures.
Catalyst recycling ability was checked under the same reaction
conditions. The reaction of phenylacetylene, benzylbromide and
sodiumazide was considered as a model reaction for recycling
studies. The results are reported in Table 4. These results showed
that the catalyst exhibits good catalytic ability for up to five cycles.
c
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Appl. Organometal. Chem. 2011, 25, 620–625

Regioselective synthesis of 1,4-disubstituted triazoles
Conclusions
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groups, inexpensive reagents, high yields, short reaction times
and easy work-up procedure, make this protocol very attractive
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