Free-radical-mediated copper-catalyzed triazole formation under UV-irradiation

Article abstract of DOI:10.1002/aoc.4733

Methoxy-polyethylene glycol (MPEG)-doped Cu (I)-polyaniline acts as an efficient catalyst for the 1,2,3-triazole formation reaction. The catalyst shows excellent performance when the reaction was carried out under ultraviolet irradiation conditions due to

Full text of DOI:10.1002/aoc.4733

Received: 13 August 2018
DOI: 10.1002/aoc.4733
Revised: 1 November 2018
Accepted: 4 November 2018
F U L L P A P E R
Free‐radical‐mediated copper‐catalyzed triazole formation
under UV‐irradiation
Debkumar Nandi1 | Abu Taher1 | Rafique Ul Isalm1,2 | Kaushik Mallick1
1 Department of Chemistry, University of
Methoxy‐polyethylene glycol (MPEG)‐doped Cu (I)‐polyaniline acts as an effi-
Johannesburg, P.O. Box 524, Auckland
Park 2006, South Africa
2 Department of Chemistry, School of
cient catalyst for the 1,2,3‐triazole formation reaction. The catalyst shows
excellent performance when the reaction was carried out under ultraviolet
Physical and Material Sciences, Mahatma
irradiation conditions due to the photo‐induced free‐radical formation from
Gandhi Central University, Motihari,
Bihar, India
the MPEG. A detailed mechanistic discussion on the free‐radical generation,
free‐radical‐mediated hydrogen abstraction and triazole formation has been
outlined in this manuscript. The catalyst system is equally active for one‐pot
multi‐component strategies for the title reaction.
Correspondence
Kaushik Mallick, Department of
Chemistry, University of Johannesburg, P.
O. Box 524, Auckland Park 2006, South
Africa.
KEYWORDS
Email: kaushikm@uj.ac.za
Cu (I) catalyst, free‐radical, triazole formation, ultraviolet irradiation
1 | INTRODUCTION
support materials play significant roles in the trapping of
light energy, which was converted to chemical energy for
The conversion of light energy to chemical energy through
a catalytic process is a relatively modern approach for
organic transformation reactions.[1] The reaction between
aryl iodide and arylboronic acid with the formation of
the corresponding coupling product has been reported
under light irradiation conditions using a palladium‐
carbon nitride‐based Mott–Schottky photocatalyst.[2] The
Mott–Schottky effect results in a higher Schottky barrier
that enhances the charge separation at the contacting
interface of metal and carbon nitride. The above‐
mentioned catalyst system was also efficient in facilitating
the photochemical hydrogenation of nitrobenzene using
formic acid as hydrogen source at room temperature.[3] A
gold–palladium bimetallic system was reported as a cata-
lyst for Suzuki coupling reactions under visible[4] and
visible‐to‐near‐infrared light[5] irradiation where the plas-
monic component gold was used for light absorption and
the palladium acts as the catalyst. The in situ formation
of the Cu (I) catalyst from the Cu (II) species in the pres-
ence of ultraviolet (UV) or sunlight irradiation for the Cu
(I)‐catalyzed azide‐alkyne cycloaddition reaction has been
reported using graphitic carbon nitride as a heterogeneous
photocatalyst.[6] Electronic properties of the catalyst or the
the construction of the desired organic molecules. The
presence of light shifts the reaction towards a kinetically
favorable direction by avoiding the introduction of hazard-
ous chemicals and thermal energy, which make the photo-
chemical reaction both environmentally and economically
viable. Several initiatives have been undertaken for the
utilization of the conduction band electron of the semicon-
ductor photocatalysts for the light‐induced organic
reactions. Semiconductor metal oxides with large band
gaps demonstrate as efficient photocatalysts under UV‐
irradiation, and the photocatalytic activity is usually
associated with free‐radical reaction processes for the
photo‐degradation of organic pollutants.[7] The band gap
of the metal oxides can be manipulated by taking advan-
tage of the various kinds of synthesis approaches. It is
reported that rutile TiO2 nanorods have been used for
the aerobic oxidation of benzyl alcohols to benzaldehydes,
yielding a high selectivity of 99% under visible light
irradiation.[8] Metal oxide‐supported gold nanoparticle,
plasmonic photocatalyst, is an excellent example of hetero-
geneous catalysis for the aerobic oxidation of alcohols in
toluene under the irradiation of natural sunlight.[9]
Cerium oxide‐supported gold particles have been reported
Appl Organometal Chem. 2019;e4733.
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© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4733

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NANDI ET AL.
for the selective oxidation of 4‐aminobenzyl alcohol to 4‐
aminobenzaldehyde through the photo‐induced Au–H
intermediate under UV and visible light irradiation.[10]
The free‐standing gold particles could also act as a
photocatalyst for the oxidation of alcohols to the corre-
sponding carbonyl compounds in the presence of peroxide
under LED irradiation (530 nm).[11]
cycloaddition reaction,[21–23] we have found the crucial
role of light energy on the yield of the reaction.[24,25]
Due to the light excitation, the conduction band electron
of the Cu (I) sulfide quantum dot acts as a scavenger of
the terminal proton of the alkyne in the presence of
organic azide with the formation of 1,4‐disubstituted
1,2,3‐triazoles, where the Cu (I) species of Cu2S acts as
a catalyst for the reaction.[24] We also have demonstrated
that the composite of copper nanoparticles and carbon
nitride performs as an effective system for the cycloaddi-
tion reaction between azide and alkyne molecules under
UV‐radiation conditions where copper nanoparticles
serve as active catalyst and the photoactive support mate-
rial (carbon nitride) plays the role of a promoter.[25]
In this current work, we like to report the effect of
UV‐irradiation on the triazole formation in the
presence of methoxy‐polyethylene glycol (MPEG; CH3O‐
Organic coupling reactions with high activation bar-
riers of the substrates are mostly dependent on relatively
high temperatures for activation.[12] Reports have also been
published regarding the formation of the C‐C bond at room
temperature over the heterogeneous palladium catalysts by
light‐mediated catalyst activation.[2] Bimetallic gold–
palladium systems showed remarkable performance for
the Suzuki coupling reaction of bromo‐/iodo‐benzenes
and aromatic boronic acids to biphenyls under light irradi-
ation conditions.[4,5] It was also reported that the size of the
catalyst has an effect on the reaction performance. Due to
the presence of light, the conduction electron of the gold
produces energetic electrons on the palladium surface that
enhance the intrinsic catalytic activity of the palladium and
promote the coupling reaction.[13] Metal‐catalyzed
cycloaddition reactions are another important protocol in
synthetic organic chemistry, as these reactions are vital to
the modern synthesis of natural products and biologically
active substances. The copper‐catalyzed azide‐alkyne
cycloaddition or the click reaction for the triazole forma-
tion continues to be of much interest[14] as it offers an
opportunity for chemists to connect two potentially com-
plex building blocks under mild conditions.[15] The Cu (I)
catalysts could facilitate the cycloaddition between an
azide and a terminal or internal alkyne in a regiospecific
manner to give only 1,4‐disubstituted triazole,[14] and have
attracted significant attention due to their potential
application in the fields of pharmaceutical, agrochemical,
dye, corrosion inhibitor, biochemical, polymer and other
functional materials.[16]
[OCH2C2H]n‐OH),
a
dopant, within the Cu (I)‐
polyaniline, Cu‐PA, catalyst system. In our earlier work,
we reported a single‐step synthesis route of Cu (I)‐
polyaniline composite and showed its potential applica-
tion as a catalyst for the click reaction under ambient[21]
and also under microwave irradiation[22] conditions. The
introduction of MPEG as a dopant to the Cu (I)‐
polyaniline system, Cu‐PA‐MPEG, shows a substantial
impact on the amount of product formation for the title
reaction in the presence and absence of UV‐irradiation.
2 | RESULTS AND DISCUSSION
The transmission electron microscopy (TEM) studies
were carried out at an accelerated voltage of 200 kV using
a Philips CM200 TEM equipped with a LaB6 source. The
UV–Vis spectra were recorded using a Shimadzu UV‐
1800 spectrophotometer with a quartz cuvette. Infrared
spectra, in the region 4000–500 cm−1, were obtained from
a Perkin‐Elmer 2000 FT‐IR spectrometer operating at a
A photochemical protocol has been developed[17] to
catalyze the reaction between azides and alkynes by in
situ generation of Cu (I) from Cu (II) complex in the pres-
ence of UV light, whereas a photolithographic technique
has been employed for the comprehensive spatial and
temporal control of the Cu (I)‐catalyzed azide‐alkyne
cycloaddition reaction by photochemical reduction of
Cu (II).[18] Fullerene‐containing macromolecule network
also applied for the visible‐light‐induced Cu (I)‐catalyzed
azide‐alkyne cycloaddition via an electron transfer mech-
anism.[19] Apart from those, an important review article
highlighted some fundamental aspects of light‐induced
click reactions and discussed the potential for this
methodology for the study of biomolecular systems.[20]
In association with our ongoing research on the
development of efficient catalysts for the azide‐alkyne
resolution of 4 cm−1
.
The TEM images (Figure 1) of (a) the Cu (I)‐
polyaniline and (b) the MPEG doped Cu (I)‐polyaniline
composite show no evidence of the formation of copper
nanoparticles. Figure 2A shows the UV–Vis spectra of
Cu (I)‐polyaniline and MPEG‐doped Cu (I)‐polyaniline.
Both the samples have identical features, with the high‐
intensity absorption peak at 315 nm, and this corresponds
to the π‐π* transition centered on the benzenoid unit.[26]
There is also a broad absorption band within the range
350–600 nm with the absorption maxima at 395 nm that
has also been observed for both samples, and could be
assigned for polaron‐π* transition for polyaniline.[27] X‐
ray photoelectron spectroscopy (Figure 2B) for Cu2p
shows that the binding energies of the photoelectron

NANDI ET AL.
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FIGURE 1 The transmission electron
micrsoscopy (TEM) image of the Cu (I)‐
polyaniline (A) and 10 wt% of methoxy‐
polyethylene glycol (MPEG)‐doped Cu (I)‐
polyaniline (B) composites
peaks of 2p3/2 and 2p1/2 are 932.7 eV and 952.2 eV,
respectively, for the Cu‐PA‐MPEG sample, indicating
copper exists in the form of Cu (I).[28] The molecular
structures of the resulting (a) Cu (I)‐polyaniline and (b)
MPEG‐doped Cu (I)‐polyaniline were further character-
ized by the Fourier transform‐infrared (FT‐IR) spectros-
copy technique (Figure 2C). The characteristic bands of
(a) at wavenumber of 1627 and 1501 cm−1 are assigned
for the C=C stretching of the quinoid and benzenoid
rings, respectively. The shifting of the band responsible
for C=C stretching of the quinoid rings at 1602 cm−1
was noticed for the MPEG‐doped Cu (I)‐polyaniline (a).
The C–N stretching of the secondary aromatic
amine was observed at the wavenumbers of 1273 and
1260 cm−1 for (a) and (b), respectively. The vibrational
peak at about 1115 cm−1 is associated with ‐N=Q=N‐
(Q refers to the quinoid ring), and observed only in the
MPEG‐doped sample.[29] In the spectrum (b), a character-
istic peak at 1027 cm−1 represents the vibration of ‐C‐O‐
C‐ in MPEG‐doped Cu (I)‐polyaniline. The detailed
characterization of the Cu (I)‐polyaniline system is
available elsewhere.[21–24]
The UV–Vis spectra show identical peak positions for
both spectra, but the intensity of the Cu (I)‐polyaniline
spectrum is higher compared with the Cu‐PA‐MPEG
spectrum. The FT‐IR spectra shows the high‐intensity
spectrum of the Cu‐PA‐MPEG as compared with Cu (I)‐
polyaniline spectrum. The shifting and formation of some
of the vibrational bands have also been observed in the
FT‐IR spectra of the Cu‐PA‐MPEG sample, which indi-
cates the successful doping of MPEG in the Cu (I)‐
polyaniline sample. The inductively coupled plasma mass
spectrometry technique was used to determine the copper
concentration within the MPEG‐doped Cu (I)‐polyaniline
hybrid system.
FIGURE 2 (a) Ultraviolet (UV)‐visible spectra of (a) Cu (I)‐
polyaniline and (b) methoxy‐polyethylene glycol (MPEG)‐doped
Cu (I)‐polyaniline composites, and (b) X‐ray photoelectron
spectroscopy for Cu‐2p core‐level binding energy. (c) Fourier
transform‐infrared (FT‐IR) spectra of (a) the Cu (I)‐polyaniline and
(b) the MPEG‐doped Cu (I)‐polyaniline composites

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NANDI ET AL.
terms of yield percentage, during the reaction between
1,3‐dipolar compound, azidomethyl benzene (1a) and
dipolarophile, 1‐ethynyl‐4‐methoxybenzene (2a) under
different conditions in the presence of MPEG‐doped Cu
(I)‐polyaniline, Cu‐PA‐MPEG (5 mol% of copper). When
the above reaction was performed in the presence of
triethylamine (Et3N) and methanol (solvent) under both
daylight and dark conditions, about 65% of the isolated
yield of the desired product, 3aa, was obtained after 3
hr, for both experiments. A yield of 98% of 3aa was
obtained when the reaction was performed under UV‐
irradiation (UV‐C) in the absence of Et3N for 3 hr. With
the addition of base, in the presence of UV‐irradiation,
the above reaction did not show any further positive
impact, in terms of the speed of the reaction as well as
the amount of yield obtained. It is important to mention
that the Cu (I)‐polyaniline (5 mol% of copper) system
(without MPEG) produced a yield of 65% of 1‐benzyl‐
4‐(4‐methoxyphenyl)‐1H‐1,2,3‐triazole in triethylamine
and methanol as solvent under UV‐irradiation, and also
under daylight and dark conditions. For both Cu‐PA‐
MPEG and Cu (I)‐polyaniline systems, the above
reaction showed less than 10% of yield in the absence
of base (Et3N) under daylight and also under dark
conditions.
2.1 | Mechanism of formation of Cu (I)‐
polyaniline composite
Copper sulfate‐mediated synthesis of polyaniline is an
example of the ‘in situ polymerization and composite for-
mation’ (IPCF) technique, where the organic component
(aniline) and the metal salt (CuSO4, 5H2O) were chosen
to facilitate this type of reaction, and copper sulfate acts
as an oxidizing agent for this type of oxidative polymeri-
zation process. The mechanism of the IPCF type of poly-
merization involves the release of electrons during the
reaction between monomer and metal salt (oxidizing
agent). In general, the released electrons reduce the metal
ions, such as, gold, silver and palladium, with the forma-
tion of their corresponding nanoparticles.[30–32] However,
in the present experiment we found the reaction between
aniline and copper sulfate evidences the partial reduction
of metal salt with the formation of Cu (I) and
polyaniline.[21–23] Aniline has several amine as well as
imine moieties that can act as a macro ligand,[33] that
coordinate with the Cu (I) species. The example of the
partial reduction of palladium[34] and copper[35] is also
available in the literature.
From the above optimization reaction, it is evident
that MPEG has a distinct positive role on the title
reaction under UV‐irradiation conditions. We also have
checked the current reaction with various solvents, such
as dichloromethane, chloroform, tetrahydrofuran and
water, and concluded that methanol was the best
2.2 | Optimization of the reaction
conditions
The optimization study was performed based on
the amount of product formation, 1‐benzyl‐4‐(4‐
methoxyphenyl)‐1H‐1,2,3‐triazole, 3aa, (Table 1), in
TABLE 1 Substrate scope for Cu‐PA‐MPEG catalyzed cycloaddition reaction under different reaction conditions
Reaction conditions: (i) azide (1.0 mmol), alkyne (1.0 mmol), MeOH (4.0 mL) and Cu‐PA‐MPEG (5 mol% of Cu); (ii) isolated yields (%): (a) = UV (absence of
Et3N), (b) = daylight + Et3N (1.0 mmol), (c) = dark + Et3N (1.0 mmol), (d) = daylight (absence of Et3N) and (e) = dark (absence of Et3N).

NANDI ET AL.
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performer as a solvent. A series of organic as well as inor-
ganic bases, such as, diethylamine, triethylamine (Et3N),
di‐isopropylamine, hydrazine monohydrate, potassium
carbonate and potassium hydroxide have been tested,
and Et3N was determined as the most successful base
for the reaction. We found that 5 mol% of copper was
the optimum amount of catalyst for the title reaction in
this current experiment. Below that amount a lesser
yield% was obtained and above that amount no substan-
tial improvement, either in terms of yield% or the velocity
of the reaction, was noticed. A doping concentration of
10 wt% of MPEG produced the optimum result for the
reaction.
participated in the title reaction with simple
phenylacetylene (2d) or an electron‐withdrawing group
attached alkyne such as 1‐ethynyl‐4‐nitrobenzene (2b)
and 1‐ethynyl‐4‐(trifluoromethoxy) benzene (2e), or an
electron‐donating group attached alkyne such as 1‐ethy-
nyl‐4‐methylbenzene (2c) to yield the expected product
1‐benzyl‐4‐(4‐nitrophenyl)‐1H‐1,2,3‐triazole (3ab, 95%
and 73%), 1‐(4‐methylbenzyl)‐4‐(4‐nitrophenyl)‐1H‐1,2,3‐
triazole (3bb, 90% and 62%), 1‐(4‐methylbenzyl)‐4‐p‐
tolyl‐1H‐1,2,3‐triazole
(3bc,
97%
and
78%),
1‐(naphthalen‐1‐yl)‐4‐phenyl‐1H‐1,2,3‐triazole (3cd, 91%
and 78%), 1‐(2‐bromobenzyl)‐4‐(4‐(trifluoromethoxy)phe-
nyl)‐1H‐1,2,3‐triazole (3de, 83% and 50%) and 1‐(2‐
bromobenzyl)‐4‐phenyl‐1H‐1,2,3‐triazole (3dd, 88% and
69%), under two different reaction conditions. Again,
bromobenzylazide (1d) underwent reaction with aliphatic
alkyne 1‐pentyne (2f), six‐member lactone attached
dipolarophile, i.e. 6‐methyl‐4‐(prop‐2‐ynyloxy)‐2H‐pyran‐
2‐one (2g), and cyclic alcohol attached dipolarophile, i.e.
1‐ethynylcyclohexanol (2h) for the respective 1,2,3‐
triazole molecules 1‐(2‐bromobenzyl)‐4‐propyl‐1H‐1,2,3‐
triazole (3df, 87% and 51%), 4‐((1‐benzyl‐1H‐1,2,3‐
2.3 | Substrate scope and comparative
study under different optical conditions
In this section, we compared the performance of the Cu‐
PA‐MPEG catalyst due to the effect of UV‐irradiation and
daylight conditions on the amount of product formation,
in the absence and presence of base, respectively.
The optimized reaction condition has now been explored
for various substituted 1,3‐dipolar molecules and
dipolarophile to test the feasibility of the reaction and
the substrate scope. The summarized data have been pre-
sented in Table 1. A graphical representation (Figure 3)
shows the comparative yields for the cycloaddition prod-
ucts under the event of base‐free UV‐irradiation and base
(Et3N)‐mediated daylight reaction conditions.
triazol‐4‐yl)methoxy)‐6‐methyl‐2H‐pyran‐2‐one
(3dg,
85% and 58%) and 1‐(1‐benzyl‐1H‐1,2,3‐triazol‐4‐yl)
cyclohexanol (3dh, 80% and 49%). In all cases, an average
of 30–40% higher yield was obtained when the reactions
were performed under UV‐irradiation conditions in the
absence of base, as compared with the daylight‐driven
reaction in the presence of triethylamine. The percentage
within the brackets indicates the isolated yield for the
reactions performed under the condition of UV‐
irradiation in the absence of triethylamine (a) and day-
light in the presence of triethylamine (b).
Benzylazide (1a), 1‐(azidomethyl)‐4‐methylbenzene
(1b),
1‐azidonaphthalene
(1c)
and
ortho‐
bromobenzylazide (1d) as 1,3‐dipolar counterpart
The unique properties of the carbohydrate moiety have
attracted increasing attention in various fields of science,
with particular interest to the biological sciences.[36] The
carbohydrate units critically control the specific biological
functions of cells and also play an important role in cell‐
to‐cell recognition mechanisms.[37] In the current study
we have chosen anomeric β‐anzide substituted glucopyra-
nose tetra acetate (1e) and anomeric β‐anzide substituted
galactopyranose tetra acetate (1f) as 1,3‐dipolar molecules
to react with both electron‐withdrawing and electron‐
donating groups substituted aromatic alkyne, such as, 1‐
ethynyl‐4‐nitrobenzene (2b), 1‐ethynyl‐4‐methylbenzene
(2c)
and
di‐substituted
1‐ethynyl‐4‐methoxy‐2‐
methylbenzene (2i) as dipolarophile. Results of the isolated
yields of the products in two different conditions have been
summarized in Table 2 and also in Figure 3. Both 1e and 1f
successfully react with 1‐ethynyl‐4‐nitrobenzene (2b) to
form the products 4‐(4‐nitro‐phenyl)‐1‐(2,3,4,6‐tetra‐O‐
acetyl‐β‐D‐glucopyranosyl)‐1H‐1,2,3‐triazole (3eb) and
4‐(4‐nitro‐phenyl)‐1‐(2,3,4,6‐tetra‐O‐acetyl‐β‐D‐
FIGURE 3 Graphical representation showing the comparative
yields for the cycloaddition products under base‐free ultraviolet
(UV)‐irradiation and base‐mediated daylight conditions

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NANDI ET AL.
TABLE 2 Substrate scope for Cu‐PA‐MPEG catalyzed cycloaddition of sugar azide with alkyne under base‐free UV‐irradiation and base‐
mediated daylight conditions
Reaction conditions: (i) sugar azide (373 mg, 1.0 mmol), alkyne (1.0 mmol), MeOH (4 mL) and Cu‐PA‐MPEG (5 mol% of Cu); (ii) isolated yields (%): (a) = UV
and (b) = daylight + Et3N (1 mmol).
galactopyranosyl)‐1H‐1,2,3‐triazole (3fb) with 89% and
85% of isolated products under UV‐irradiation in the
absence of triethylamine, and 57% or 59% of isolated prod-
ucts under daylight in the presence of triethylamine,
a single‐step one‐pot reaction and similar types of proto-
col are also evident in the literature.[38,39]
In this work, we were also interested to implement
Cu‐PA‐MPEG catalyst for the single‐step multi‐
component reaction using aryl halide, sodium azide and
alkyne under different photonic conditions. The results
are shown graphically in Figure 3 and also in Table 3.
Two aryl halides, such as benzylbromide (4a) and ortho‐
respectively. In
aromatic alkynes 2c and 2i to yield their corresponding
click products 4‐p‐tolyl‐1‐(2,3,4,6‐tetra‐O‐acetyl‐β‐D‐
glucopyranosyl)‐1H‐1,2,3‐triazole (3ec, 96% and 66%)
and 4‐(4‐methoxy‐2‐methyl‐phenyl)‐1‐(2,3,4,6‐tetra‐O‐
a similar way, 1e reacted with
acetyl‐β‐D‐glucopyranosyl)‐1H‐1,2,3‐triazole (3ei, 98%
and 64%), respectively. Again, when 1f was reacted with
aromatic alkynes 2c and 2i to yield their corresponding
TABLE 3 Substrate scope for Cu‐PA‐MPEG catalyzed one‐pot
multi‐component cycloaddition reaction under base‐free UV‐irra-
diation and base‐mediated daylight conditions
click
galactopyranosyl)‐1H‐1,2,3‐triazole (3fc, 94% and 63%)
and 4‐(4‐methoxy‐2‐methyl‐phenyl)‐1‐(2,3,4,6‐tetra‐O‐
products
4‐p‐tolyl‐1‐(2,3,4,6‐tetra‐O‐acetyl‐β‐D‐
acetyl‐β‐D‐galactopyranosyl)‐1H‐1,2,3‐triazole (3fi, 94%
and 61%), respectively, were formed.
A single‐step reaction is more attractive than a multi‐
step one in terms of overall yield of the target molecule,
and is also beneficial for the enlargement of substrate
scopes depending on the choice of reaction condition.
Also, attention has been given for the development of
single‐step multi‐component reactions, because of the
high degree of atom economy, applications in combinato-
rial chemistry and diversity‐oriented synthesis.[28] Multi‐
component strategies for click reaction where inorganic
azide salts, such as sodium azide and alkyl halide, may
participate in an in situ nucleophilic substitution reaction
to produce organic azide molecules that further could
undergo cycloaddition with the alkyne dipolarophile in
Reaction conditions: benzyl bromide (1 mmol), NaN3 (78 mg, 1.2 mmol),
alkyne (1 mmol), MeOH (4 mL) and Cu‐PA‐MPEG (5 mol% of Cu), (ii) iso-
lated yields (%): (a) = UV, (b) = daylight + Et3N (1 mmol).

NANDI ET AL.
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bromobenzylbromide (4b), were used in this experiment
for the multi‐component reaction. In each separate
experiment,
phenylacetelene
(2d),
1‐ethynyl‐4‐-
(trifluoromethoxy) benzene (2e), 3‐ethynylthiophene (2j)
and propargyl alcohol (2k) reacted with 4a in the pres-
ence of sodium azide, and the corresponding products 1‐
benzyl‐4‐phenyl‐1H‐1,2,3‐triazole (5ad, 90% and 65%), 1‐
benzyl‐4‐(4‐(trifluoromethoxy) phenyl)‐1H‐1,2,3‐triazole
(5ae, 88% and 55%), 1‐benzyl‐4‐(thiophen‐3‐yl)‐1H‐1,2,3‐
triazole (5aj, 83% and 58%) and (1‐benzyl‐1H‐1,2,3‐
triazol‐4‐yl) methanol (5ak, 78% and 42%), respectively,
were obtained successfully when MPEG‐doped Cu‐
polyaniline was used as a catalyst. In a similar way,
ortho‐bromobenzylbromide (4b) reacted with 1‐ethynyl‐
4‐methoxybenzene (2a) to yield 1‐((2‐bromobenzyl)‐4‐(4‐
methoxy) phenyl)‐1H‐1,2,3‐triazole (5ba, 88% and 55%).
The respective yield% values are mentioned within the
bracket, where the higher yields of products were
achieved under UV‐irradiation conditions in the absence
of base, while lower yields of the isolated products were
achieved in the presence of triethyl ammine under
daylight conditions.
SCHEME 1 Mechanism for the triazole formation
through the photolysis of MPEG, doped in the Cu (I)‐
polyaniline system.
The photo‐ and thermal‐induced free‐radical forma-
tion of polyethylene glycol is well documented in the
literature.[42,43] A similar kind of photolysis mechanism
can also be considered for methoxy polyethylene
glycol (R − [OCH2CH2]n − OH) with the formation of
À
_
oxygen‐centered −OCH2CH2 − Þ and carbon‐centered
2.4 | Mechanistic discussion of the
reaction
À
_
−OCH2CH − Þ free‐radicals due to the exposure of UV
light. The free‐radical‐mediated hydrogen abstraction
route is observed in biological systems, with the specific
examples of dehydrogenation of C‐H bond of a lipid
molecule[44] and the radical‐mediated dehydrogenation
of bile salts.[45] Free‐radical‐mediated organic transforma-
tion reactions are also common in literature.[43,46]
In the current study, the free‐radical‐induced hydro-
gen abstraction from the copper‐alkyne π‐complex (A)
can be proposed with the formation of the copper‐
acetylide complex (B), Scheme 1, step (II) (b) and (III).
In the presence of the azide molecule, the rest of the reac-
tion followed the above‐mentioned pathway and formed
the triazole molecule (C). Free‐radicals have more reac-
tive unpaired electrons than lone pairs of electrons and,
because of that, the proton abstraction process from the
alkyne molecule was faster and consequently produced
a higher amount of triazole product for a particular time
period.
We found that the triazole product (3aa) was also
formed, although a very small amount of yield (< 10%),
both under daylight as well as dark conditions in the
absence of base (Et3N), indicating that the amine group
of polyaniline is mainly responsible for the deprotonation
from the alkyne molecule.[26] It is also reported that the
amine moiety in graphitic carbon nitride was also in con-
trol for the proton abstraction process.[28] The Cu‐PA‐
MPEG composite system can act as a heterogeneous
The alkyne‐azide cycloaddition reaction needs alkaline
conditions to initiate the process in the presence of a Cu
(I) catalyst,[14,40] which allows the reaction to proceed at
room temperature with an increased rate of reac-
tion.[22,41] In this report, MPEG‐doped Cu (I)‐polyaniline
(Cu‐PA‐MPEG) was used as a catalyst for the title
reaction.
The mechanism for the copper‐catalyzed cycloaddi-
tion reaction for triazole synthesis is illustrated in
Scheme 1. A π‐complex (A), step (I), has been formed
between the Cu (I) and alkyne molecule, which results
in the lowering of the pKa value of the terminal proton,
a typical Lewis acid character. The π‐complex facilitated
the deprotonation of the alkyne molecule under basic
conditions (triethylamine), through step (II) (a) and
(III), with the formation of copper‐acetylide complex
(B). In the presence of the azide molecule, step (IV), the
copper‐acetylide forms a couple of intermediate com-
plexes that subsequently form the 1,2,3‐triazole (C)
through protonation along with the elimination of the
catalyst. When the reaction was carried out under the
exposure of UV‐irradiation in the absence of base, a
higher amount of yield (triazole product) was obtained.
The UV light source, UV‐C, within the range of 6.53–
4.43 eV and 190–280 nm wavelength, possesses a suffi-
cient amount of energy to produce the free‐radicals

8 of 9
NANDI ET AL.
catalyst for the current triazole formation reaction. Under
UV‐irradiation conditions, deactivation of the catalyst is
faster compared with daylight and dark environment,
which could be the effect of dissolution of MPEG in the
presence of solvent with time. The Cu‐PA‐MPEG catalyst
can be recycled five times without considerable deactiva-
tion under daylight and dark conditions.
4.2 | General procedure for the click
reaction
In a round‐bottomed glass flask (for daylight and dark
experiments) or round‐bottomed quartz flask (for the
UV‐irradiation experiment), azide, 1a–1d, (1 equivalent),
alkyne, 2a–2h, (1 equivalent), were charged in 4 mL of
methanol. To this reaction mixture, the catalyst Cu‐PA‐
MPEG was added and stirred under the following
conditions.
3 | CONCLUSION
This report demonstrates the catalytic role of the MPEG‐
doped Cu (I)‐polyaniline system for the cycloaddition
reaction between azide and alkyne molecules under UV‐
radiation conditions, where the polyaniline acts as a
support material for both Cu (I) and MPEG. The UV‐
irradiation plays a crucial role in generating free‐radicals
through the photolytic fission of MPEG for the radical‐
induced hydrogen abstraction from the alkyne molecule.
Copper serves as a catalyst for the title reaction, whereas
MPEG scavenges the free‐radicals with the formation of
stable carbon‐centered free‐radicals that abstract the H‐
atom from the alkyne molecule by means of a hydrogen
atom transfer mechanism, and finally proceed for the tri-
azole product in the presence of an azide molecule. In
association with our ongoing research on the fabrication
of effective catalysts for the cyclo‐addition of azide and
alkyne molecules,[21–25] further efforts will undertake in
future for the development of new materials as catalyst
to perform the similar kind of reactions by involving the
heterocycle substituted azide molecules.
a. UV light (in the absence of Et3N): a 50‐mL round‐
bottomed quartz cell was used for the reaction. The
UV‐light source was a Philips UV‐C (TUV T8;
germicidal) lamp. The optical intensity value of 40
mW cm−2 adjacent to the quartz reaction chamber
was measured by a hand‐held optical power meter
(Newport) during the course of the reaction.
b. Daylight + Et3N: a 50‐mL round‐bottomed glass flask
was used for the reaction. A Philips LED bulb
(8718291753032), 6.5 W, was considered as equiva-
lent to the daylight source. The optical intensity adja-
cent to the glass reaction chamber was 3.5 mW cm−2
.
c. Dark + Et3N: the reaction was performed in a dark
room. All other reaction conditions remained identi-
cal as mentioned in condition (b).
d. Daylight in the absence of Et3N: all reaction condi-
tions remained identical as mentioned in (b) in the
absence of Et3N.
e. Dark (in the absence of Et3N): all reaction conditions
remained identical as mentioned in (c) in the absence
of Et3N.
4 | EXPERIMENTAL
The reaction was monitored using the thin‐layer chroma-
tography technique and stirred for 3 hr. The reaction mix-
ture was then filtered, and the filtrate was dried. The solid
material was diluted with distilled water and extracted
with ethyl acetate. The combined organic layer was col-
lected and further purified by recrystallization or by the
column chromatography technique.
4.1 | Preparation of Cu‐PA‐MPEG
composite materials
In a typical experiment, 0.093 g of aniline was diluted in
methanol. On the other hand, in a separate 25‐mL bea-
ker, 0.004 g of MPEG was dissolved in hot water. MPEG
solution was added drop‐wise to the methanolic solution
aniline monomer solution under stirring conditions and
allowed to stir for 30 min. A solution of CuSO4·5H2O
(10 mL, 10−2 mol dm−3) was added slowly under continu-
ous stirring conditions to the mixture solution of aniline
and MPEG solution. During the addition, the solution
took on a green color while, at the end, a greenish precip-
itation was formed, indicating the formation of Cu (I)‐
polyaniline composite, at the bottom of the conical flask.
The ntire reaction was performed under ambient condi-
tions, and the product was dried at 60ºC under vacuum
conditions. The material was used as a catalyst for the
azide‐alkyne cycloaddition reaction (5 mol% of Cu).
1
The H and 13C NMR spectra of all compounds are
included in the supporting information.
ACKNOWLEDGEMENTS
This study was financially supported by the Faculty of
Science, University of Johannesburg, and by the Global
Excellence and Stature Programme, University of
Johannesburg.
CONFLICT OF INTEREST
There are no conflicts to declare.

NANDI ET AL.
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
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How to cite this article: Nandi D, Taher A,
Isalm RU, Mallick K. Free‐radical‐mediated
copper‐catalyzed triazole formation under UV‐
irradiation. Appl Organometal Chem. 2019;e4733.
https://doi.org/10.1002/aoc.4733
[24] D. Nandi, A. Taher, R. U. Islam, M. Choudhary, S. Siwal, K.
Mallick, Sci. Rep. 2016, 6, 33 025.
[25] D. Nandi, A. Taher, R. U. Islam, S. Siwal, M. Choudhary, K.
Mallick, R. Soc. Open Sci. 2016, 3, 160 580.