An efficient (NHC) Copper (I)-catalyst for azide–alkyne cycloaddition reactions for the synthesis of 1,2,3-trisubstituted triazoles: Click chemistry

Article abstract of DOI:10.1016/j.ica.2017.06.065

In this study a series of new benzimidazolium salts (2a–c) were synthesized from the reaction of 5,6-dimethyl-1-(alkylbenzyl)-1H-benzo[d]imidazole with various alkyl halides. These salts were used to synthesize cupper N-heterocyclic carbene (Cu-NHC) compl

Full text of DOI:10.1016/j.ica.2017.06.065

Accepted Manuscript
Research paper
An efficient (NHC) Copper (I)-catalyst for azide–alkyne cycloaddition reactions
for the synthesis of 1,2,3-trisubstituted triazoles: click Chemistry
Nedra Touj, Ismail Özdemir, Sedat Yaşar, Naceur Hamdi
PII:
S0020-1693(17)30411-5
http://dx.doi.org/10.1016/j.ica.2017.06.065
ICA 17714
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
15 March 2017
10 June 2017
28 June 2017
Please cite this article as: N. Touj, I. Özdemir, S. Yaşar, N. Hamdi, An efficient (NHC) Copper (I)-catalyst for
azide–alkyne cycloaddition reactions for the synthesis of 1,2,3-trisubstituted triazoles: click Chemistry, Inorganica
Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.06.065
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An efficient (NHC) Copper (I)-catalyst for azide–alkyne cycloaddition
reactions for the synthesis of 1,2,3-trisubstituted triazoles : click Chemistry
Nedra Touj 1, Ismail Özdemir 2, Sedat Yaşar*3 and Naceur Hamdi 1,3
*
1Research Laboratory of Environmental Sciences and Technologies (LR16ES09), Higher
Institute of Environmental Sciences and Technology, University of Carthage, Hammam-
Lif 2050, Tunisia;
2
İnönü University, Faculty of Science and Art, Department of Chemistry, Malatya 44280,
Turkey;
3
Chemistry Department, College of Science and Arts, Qassim University, Al-Rass, 51921,
Kingdom of Saudi Arabia
* Correspondence: naceur.hamdi@isste.rnu.tn; syasar44@gmail.com
Abstract
In this study a series of new benzimidazolium salts (2a–c) were synthesized from the reaction
of 5,6-dimethyl-1-(alkylbenzyl)-1H-benzo[d]imidazole with various alkyl halides. These salts
were used to synthesize cupper N-heterocyclic carbene (Cu-NHC) complexes 3-4. The
obtained (NHC) Copper (I) complexes 3-4 were characterized by FT-IR, NMR (1H and 13C)
spectroscopic methods, mass spectrometry (EI-MS and HRMS) and elemental analysis. These
novel cupper complexes 3-4 were used as a catalyst for alkyne – azide cycloaddition
(CuAAC) reaction. Several triazoles 7 have been synthesized. This catalytic system fulfils the
requirements of “click chemistry” with its mild and convenient conditions, notably in water at
room temperature with low catalyst loading and simple isolation with no purification step.
Keywords: Alkynes, Azides, Click chemistry, Homogeneous catalysis, Copper-N-heterocyclic
carbene complex
Introduction
1,2,3-Triazoles are attractive molecules and widely used in organic synthesis, materials
sciences, drug development, and bioconjugation chemistry1. Thus, the privileged 1,2,3
triazole motif has not only been extensively utilized by synthetic organic chemists, but also by
chemists working in fields as diverse as bio-conjugation2 and anion/cation recognition,3just to
name a few relevant research fields. A close look at the 1,2,3-triazole ring also shows the
1

attractive features that this ring, in combination with relevant substituents has to offer to
coordination and organometallic chemists (Scheme 1).
- Coordination to a metal center
- H-bonding acceptor
- Coordination to a metal center
- Alkylation
- H-bonding acceptor
N3
2N
R1
N
- Steric and electronic tuning
- Additional donor atom
- Steric and electronic tuning
- Additional donor atom
R2
H
- H-bonding donor
- Deprotonation: Generation of a carbanion or a carbene
Scheme 1: Various functionalities of the 1,2,3- triazoles ring
Thus, the 1,2,3-triazole can act as a monodentate ligand by coordination via the N3 nitrogen,
which is the most basic donor atom in the ring.4The ring in its deprotonated form
(triazolides) can also act as a monodentate carbanionic donor via C–.5However, more
interesting are the possibilities of introducing additional donor substituents on the triazole ring
for generating chelating ligands, incorporating multi triazole rings within a ligand backbone
for generating multi-dentate triazole ligands, or methylating the N3 nitrogen and
deprotonating the triazole C–H ring proton to generate species that have been variously called
abnormal N-heterocyclic carbenes (aNHC) or mesoionic carbenes (MIC). In addition, the
modular synthesis of such substituted triazole rings offers unending possibilities for steric and
electronic tuning of the ligands and in turn their metal complexes.
The traditional preparation methods of 1,2,3-triazoles by 1,3-dipolar cycloaddition of
azides and alkynes under thermal conditions6. Great developments have been realized as click
chemistry, copper-catalyzed azide–alkyne cycloaddition (CuAAC), which provides a simple
method to join together organic molecules efficiently under mild conditions, has been
explored in depth7
The copper and ruthenium catalyzed azide alkyne cycloaddition reactions are powerful
strategies for the regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles and 1,5-
2

8
disubstituted 1,2,3-triazoles. However, internal alkynes are rarely developed and 1,4,5-
trisubstituted 1,2,3-triazoles are difficult to synthesize through this strategy. Furthemore
functionalized 1,2,3-triazole derivatives have emerged as interesting ligands for transition
metals and organometallic species due to their potential to act as N donors.9 A wide range of
mono-, bis-, tris-, and polydentate ligands containing the 1,2,3-triazole skeleton has been
synthesized and studied for their coordination chemistry.10 The preparation of this type of
ligands has become more accessible due to the discovery of the 1,3-dipolar Cu(I) catalyzed
alkyne–azide cycloaddition (CuAAC) reaction which yields 1,4-disubstituted 1,2,3-triazoles.11
Late in the past decade, a new subclass of NHCs, namely 1,2,3-triazol-5-ylidenes (F), was
discovered and even isolated in their free form.12 This new generation of N-heterocyclic
carbenes (NHCs) has been termed as “mesoionic carbenes” (MICs) because no uncharged
resonance forms can be generated from these systems.13 Due to the combination of their
strong donor character (higher compared to classical NHCs) and the vast synthetic flexibility
of their ligand precursors via click chemistry, their use in the area of catalysis has raised a
great deal of interest in both academia and industry.
The Cu(I)‐catalyzed Huisgen 1,3‐dipolar cycloaddition reaction between alkynes and azides
(CuAAC) for constructing triazole cycles, which was independently reported by Sharpless et
al.14 and Meldal et al.15 in 2002, represents the most efficient essence of “click chemistry”
proposed by Sharpless et al.16 in 2001. This reaction has been widely applied in medicinal,
bioorganic, and materials chemistry, as well as many other areas of research over the last
decade 17,18. Although the CuSO4/sodium ascorbate system is still the most frequently used
method, numerous efficient catalysts have been developed for improving the present reaction
conditions 19. For example, the use of ligands decreases the amount of Cu catalyst needed and
enhances its catalytic activity owing to stabilization of the Cu(I) center20-28. Moreover, various
20-21
Cu complexes, especially Cu(I) complexes bearing different ligands such as amines
,
29-30
N‐heterocycles 22-24, phosphines, and phosphonites
and N‐heterocycle carbenes (NHCs)
31 have been developed and have shown satisfactory results.
Recently, N-heterocyclic carbenes (NHC) is firmly occupied an important place in the
ancillary ligand for homogeneous catalysis partially for substitution of the phosphine
ligands.32-35 The NHC has superior advantageous due to their high thermal stability, nontoxic
chemistry, resistance to oxidation,36,37 strong σ-donor ligands ability38 with weak π-acceptor
capability, tuneable of electronic and steric properties and easily handling process. The
3

commonly used NHC ligands for transition metals are generating from imidazolium,
imidazolidinium and benzimidazolium based salts in presence of strong bases. In 1993, the
first NHC–Cu complex was reported by the Arduengo group39, soon after their ground
secceed isolation of a free NHC 40. Advances in NHC–copper chemistry were then steadily
reported, including better synthetic routes to well-defined complexes,41 and theoretical studies
of the NHC–Cu bond that allowed for a better understanding of NHC–metal interactions.42
Despite these developments, the first known application in catalysis of a NHC–Cu complexes
reported in 2001 by Woodward’s for conjugate addition.43 NHC-Cu complexes have been
used successfully as effective catalyst in various catalytic reaction such as reduction of
carbonyl compounds44, carbonyl hydrosilylation45, hydroboration and diboration46-47, cross-
coupling reactions48-52, Miscellaneous reactions53-57 and [3 + 2] Cycloaddition reaction 58-60
.
We herein report the synthesis of benzimidazolium salts 2a-c bearing symmetrical and
unsymmetrical ligands and their (NHC) Copper (I) complexes 3-4. Further we investigate
the catalytic activity of these complexes for the Huisgen cycloaddition reaction of insitu
generated azides and terminal alkynes to synthesize triazoles.
2. Results and discussion
a- Synthesis and characterization of benzimidazolium salts 2a-c precursors and their
(NHC) Copper (I) 3-4
The benzimidazolium salts (2a–c) as NHC-precursors, which are stable against light, air and
humidity both in the hard form and in the solution state, were obtained by quaternization of
5,6-dimethyl-1-(alkylbenzyl)-1H-benzo[d]imidazole 1 with various alkyl halides in DMF at
70°C for 48h (Scheme 2). These salts were soluble in polar solvents such as dichloromethane,
ethyl alcohol and methanol but were not soluble in common slightly polar or non polar
solvents like diethyl ether and hexane.
4

R1
R1
R1
R3
H
N
N
N
N
N
X
X
(X= Cl, Br)
X
EtOH, KOH,
Reflux
DMF, 70°C
N
R2
R2
R2
1
2
R3
2.a: R1= R3= 2,3,4,5,6-pentamethyl, R2= 5,6-dimethyl
2.b: R1=R3= 2,3,4,5,6-pentamethyl, R2= H
2.c: R1=R3= 4-tertbutyl, R2= H
N
Cl
N
N
N
Br
Cl
N
N
2.a
2.b
2.c
Scheme 2. The Synthesis of N,N- substituted new benzimidazolium salts 2a-c
1
The structures of benzimidazolium salts 2a-c were verified by H NMR, 13C NMR, FT_IR
1
and elemental analyses. The H NMR spectra of the benzimidazolium salts further supported
the assigned structures; the resonances for C(2)–H were observed as sharp singlets at 9.85,
13
10.34, and 11.83ꢀppm, respectively, for 2a–c. C NMR chemical shifts were consistent with
1
the proposed structures; the imino carbon are typical singlets in the H-decoupled mode at
141.3, 143.0 and 143.1 ꢀppm, respectively, for benzimidazolium chlorides 2a–c. The IR data
for benzimidazolium salts 2a–c clearly indicate the presence of –C=N– with a ν(C=N) at
1440.9 1454.8 and 1427.8ꢀcm−1, respectively, for 2a–c. The NMR and IR values are in
accordance with the literature 61-62
.
b. Synthesis and characterization of (NHC) Copper (I) 3-4
Metal‐NHC compounds are in general prepared by treatment of either in situ generated NHC
or isolated NHC, from the corresponding imidazolium salts using strong bases such as
KOBut, NaH/KH or Na[N(SiMe3)2], with the corresponding metal precursors63-65. The
complexes (3a–c) were synthesized by treatment of benzimidazolium salts 2a-c with K2CO3
5

in acetone at reflux in the presence of CuI to form the Cu-NHC complexes 3, in good yields
the reaction mixture was mixed for 24ꢀh at 60°C under argon. Treatment of
the benzimidazolium salts 2a-c with Cu2O in water at reflux afforded quantitatively the
expected complexes 4a–c after 24ꢀh. Cu–NHCs 4a–c were obtained as white solids in 63–
74% yield. The Cu–carbene complexes 3-4 were generally soluble in polar solvents especially
in dichloromethane and methanol. Scheme 3
R1
N
Cu
I
N
O
C
R2
K
,
I
u
C
R1
3
R3
N
N
(X= Cl, Br)
X
R2
2
R3
R1
C
u
O
N
N
Cu
X
(X= Cl, Br)
R2
2.a: R1= R2= 2,3,4,5,6-pentamethyl
2.b: R1=R2= 2,3,4,5,6-pentamethyl
2.c: R1=R2= 4-tertbutyl
R3
4
Scheme 3. Synthesis of (NHC) Copper (I) 3-4.
The structures of complexes 3-4 were determined by their characteristic spectroscopic data
and elemental analyses. In the 1H NMR spectra, in CDCl3 the lack of a downfield NCHN
signal shows the successful formation of Cu-NHC complexes 3-4. The signal of benzylic
proton (N–CH2–Ar) belonging to Cu-NHC 3a-c complexes was observed as a sharp singlet
signal at around δ 5.39ꢀppm, 5.47ppm and 5.71 respectively. Further, the carbene carbon
peaks of Cu-NHC 3a-c resonates at 184.9, 186.2 and 194.2ꢀꢀppm respectively. The FT-IR data
clearly indicated the presence of ν(CN) at 1448.5, 1419.0 and 1476.5ꢀcm−1 for the Cu-NHC
complexes 3a-c respectively. The absence of a peak in the range 8–10 ppm indicates a
complete removal of C2-H proton from the benzimidazolium salt 2a-c by Cu2O and the
formation of NHC–Cu (I) catalysts 4a-c. However, the resonances of aromatic, benzylic
methylene and aliphatic protons of NHC–Cu (I) catalysts 4 were observed with no or
6

13
negligible changes from their respective benzimidazolium 2a-c. The C NMR resonances of
the carbene carbons in NHC–Cu (I) catalysts 4a-c occur in the δ range of 182.6–1861.0 ppm.
CHa,b,c,a’,b’,c’)
CH
CH
H1,1’
Harom
Figure 1: 1HNMR spectra (CDCl3, 400MHz) of (NHC) Copper (I) 4a
132.8
132.4
132.0
f1 (ppm)
131.6
131.2
16.3
16.1
f1 (ppm)
15.9
210
200
190
180
170
160
150
140
130
120
110
100
f1 (ppm)
90
80
70
60
50
40
30
20
10
0
-10
-20
Figure 2: 13C NMR spectra (CDCl3, 100MHz) of (NHC) Copper (I) 4a
7

The remarkable stability of the NHC–Cu (I) catalysts 3-4 toward heat, oxygen and moisture
prompted us to test them in the Huisgen cycloaddition of benzyl azide and phenyl-acetylene
at room temperature in water. Although organic azides are generally safe and stable toward
water and oxygen, 66 those of low molecular weight can be difficult to handle.67 Therefore, a
methodology that avoids their isolation is desirable.68
The catalytic potential of the complex 3a was explored in a cycloaddition reaction of an 4-
methylbenzyl bromide (1 mmol) and phenylacetylene (1.05 mmol) with NaN3 (1.05 mmol) at
room temperature to form 1,2,3-triazole (Scheme 4). Initially, 4-methylbenzyl bromide and
phenylacetylene were chosen as a model reactant to optimize the reaction conditions.When
the reaction was carried out with 5 mol% catalyst loading 3a (Table 1, entries 1,2) under room
temperature, product is isolated in 100% yield after 24h and 2h respectively. It was observed
that the reaction was completed in 2 h using 3 mol % of complex 3a as a catalyst in water
under atmospheric conditions at room temperature. In order to compare the efficacity of
complex 3a with others simple complexes of Copper such us CuCl, CuBr and CuI we have
running the following reactions. Using 3 mol% of the copper catalysts CuCl or CuBr or CuI,
virtually no conversion of the starting materials was observed in water after 2h. Table 1
( entries 5,6 ) when we used CuCl as catalyst, under the same conditions complex CuI afford
the triazole with a 27% yiled ( Table 1, entry 7) . Satisfyingly, conversion of benzyl chloride
into triazole was obtained on water (Table 1, entry 1). However, reactions of 4-methoxy
benzylchloride in water were not reproducible to afford the corresponding triazole. Similar
results were observed for 4-chlorobenzyl chloride, 4-methoxybenzyl chloride, 4-
isopropylbenzyl chloride, 9-chloromethyl anthracene, chloromethyl ethyl ether, bromomethyl
cyclohexane and pentaerythritol tetrabromide.
2
N
1
N
N3
NaN3
r.t
5
+
4
Br
Scheme 4: Cycloaddition reaction of 4-methylbenzyl bromide and phenylacetylene with
NaN3 at room temperature.
8

Table 1: Synthesis of triazole by using 4-methylbenzyl bromide and phenylacetylene with
NaN3 at room temperature
Entry
Benzylbromide
Catalyst
Alkyne
Time
24 h
2h
Yield
100%
1
2
3
4
5
6
7
3.a (5 mol %)
3.a (5 mol%)
3.a (3 mol%)
3.a (3 mol%)
CuCl (3 mol%)
CuCl (3 mol%)
CuI (3 mol %)
100%
24h
2h
100%
100%
2h
No Reaction
No Reaction
27%
2h
2h
Table 2: Synthesis of triazoles by using differents substrates under the optimum conditions
Entry
1
Benzylbromide
Catalyst
3.a
Alkyne
Time
2h
Yield
32%
2
3
OMe
Cl
3.a
3.a
2h
2h
No Reaction
No reaction
Cl
4
9

Cl
3.a
2h
No reaction
5
6
3.a
3.a
2h
2h
No raction
No reaction
Br
7
3.a
2h
No reaction
Reaction conditions:[Cu] (3 mol %.), benzylbromide (1 mmol.), sodium azide (1,05
mmol.), phenylacetylen (1,05 mmol) water (1 ml), Room T°, conversion of the triazole was
determined by GC
Therefore different substituted benzylbromide bearing electron-donating and one alkyne were
used were to synthesize different triazoles. It was found that electron donating groups at para
position in benzylbromide could be efficiently converted to the desirable 1.4 triazoles 7 with
various NHC copper (I) 3-4 as catalysts at room temperature (Table 3, entries 1-6, entries 13-
18). On the other hand, between phenylacetylene and respectively 3.5 dimethyl
benzylbromide and 2.4.6- trimethyl benzylbromide the later gave better yield (Table 3, entries
7-12, entries 19-24) (Scheme 5).
N
N
N
[Cu(NHC)]
NaN3, Room T°, 2 h
R
Br
R
6
5
7
10

2
2
3'
1'
3'
1'
(a)
N
N
1
N
1
N
4'
2'
2'
4'
N 3
N 3
(a)
(b)
5'
5'
5
5
4
8'
4
7'
7'
6'
6'
1"
1"
2"
2"
(c)
(b)
6"
6"
7a
3"
3"
7b
5"
5"
4"
4"
(a)
2
3'
1'
2
1'
N
3'
1
N
4'
2'
N
1
N
2'
4'
N 3
N 3
5'
5
7'
5'
4
7'
5
4
(a)
6'
(b)
(c)
1"
6'
2"
1"
2"
6"
6"
3"
7c
3"
7d
5"
4"
5"
4"
2
1'
2
3'
1'
3'
N
1
N
4'
2'
N
1
N
4'
2'
N 3
N 3
7'
5'
5
7'
5'
4
5
4
H3CO
6'
1"
6'
1"
2"
2"
6"
7f
6"
3"
7e
3"
5"
5"
4"
4"
Scheme 5: NHC copper (I) 3-4 catalyzed formation of triazoles 7
Table 3. NHC copper (I) catalyzed click reactiona
Entry
Substrat
Triazole
Yieldb
Catalyst
(Cu-NHC)
1
2
3
4
5
6
7
3a
3b
3c
4a
4b
4c
3a
90%
54%
94%
81%
85%
64%
96%
2
3'
1'
(a)
N
Br
1
N
2'
4'
N 3
5a
(a)
(b)
5'
5
4
8'
7'
6'
1"
2"
(c)
6"
7a
3"
5"
4"
11

2
3'
1'
8
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
N
89%
59%
94%
94%
76%
100%
93%
88%
92%
100%
95%
84%
100%
99%
87%
45%
100%
85%
98%
97%
88%
60%
100%
1
N
4'
2'
N 3
5'
5
9
4
7'
6'
1"
2"
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
(b)
6"
Br
3"
7b
5"
4"
5b
2
3'
1'
N
1
N
2'
4'
N 3
5'
5
4
7'
Br
(a)
6'
1"
2"
5c
6"
3"
7c
5"
4"
(a)
2
1'
3'
N
1
N
4'
2'
N 3
7'
5'
5
4
(b)
(c)
Br
6'
1"
2"
6"
5d
3"
7d
5"
4"
2
1'
3'
N
1
N
4'
2'
OMe
N 3
7'
5'
5
4
Br
H3CO
6'
1"
2"
5e
6"
3"
7e
5"
4"
12

Br
31
32
33
34
35
36
3a
3b
3c
4a
4b
4c
88%
98%
95%
85%
75%
100%
2
1'
3'
N
1
N
2'
4'
N 3
7'
5'
5
4
6'
1"
2"
7f
6"
3"
5"
4"
5f
a [Cu] (3 mol %.), benzylbromide (1 mmol.), sodium azide (1,05mmol.), phenylacetylen (1,05
mmol) water (1 ml), rt. b Conversion of the triazole was determined by GC.
1
The structures of the prepared compounds 7 were confirmed by H and 13C-NMR
1
spectroscopic methods, and IR spectra. A singlet observed in the HNMR spectrum at δ =
8.10–8.13 ppm confirmed the presence of the triazolyl hydrogen, supported by the signals in
the 13CNMR spectrum at δ = 129.34ppm. The signals for the quaternary carbon C4 of the
triazole ring appeared at δ =137.82 ppm in the 13C-NMR spectrum. These chemical shift
values are consistent with those reported for 1,4-disubstituted 1,2,3-triazoles 69-72
.
CH3 (a,b)
H3’ ,7’
H5
H2’’,6’’
H3’’,5’’
H5
H4’’
Figure 3: 1HNMR spectra (CDCl3, 400MHz) of (NHC) Copper (I) 7b
H1’
13

Figure 4: 13C NMR spectra (CDCl3, 100MHz) of (NHC) Copper (I) 7b
A plausible mechanism for Cu(I)-catalyzed synthesis of 1,4- triazoles cycloaddition is
proposed in Scheme 6. In a first stage, the starting alkyne would react with the copper(I)
species to form an acetylide–copper complex that would interact with the azide activating it
toward nucleophilic attack of the acetylide carbon to the ‘external’ nitrogen atom of the azide
and generating a metallacycle. Subsequent ring contraction would generate a copper-
triazolide, direct precursor of the reaction product upon protonation. The last step of the
catalytic cycle is the exclusion of the Cu(I) through reductive elimination, to give the desired
1,4-disubstituted triazoles.
14

Scheme 6: Proposed mechanism cycle of the CuAAC
Conclusions
In conclusion air and water stable benzimidazolium salts (2a–c) were synthesized and
1
characterized by H NMR, 13C NMR, HRMS, FT-IR and elemental analyses. We have
developed a versatile and highly efficient catalytic system for the Huisgen cycloaddition. In
situ generated azides transformed 1,2,3-triazoles in water at room temperature conditions in
extremely short reaction times by Cu-NHC complexes with lower loading than literature.
NHC–Cu (I) catalysts 3 showed high catalytic activities tha NHC–Cu (I) catalysts 4
The unprecedented use of an internal alkyne in this copper-catalyzed transformation has also
been described. Further synthetic applications and mechanistic studies addressing the role of
the NHC ligand in this transformation are currently ongoing in our laboratories.
15

Experimental Section
Materials and methods
All procedures were carried out under an inert atmosphere using standard schlenk line
techniques. Chemicals and solvents were purchased from Sigma-Aldrich. All solvents were
1
purified and dried by MBraun SPS 800 solvent purification system. H NMR and 13C NMR
was recorded respectively at 300 and 400 MHz for (1H) and 75 and 101 MHz for (13C) in
CDCl3and DMSO-d6 with TMS as an internal reference. Signals are quoted in parts per
million as δ downfield from TMS (δ 0.00) as an internal standard. FT-IR spectra were
recorded on a Mattson 1000 spectrophotometer, wavenumbers in cm-1.
Synthesis of ligands (1a-c)
A standard schlenk was charged with 5,6-dimethylbenzimidazole (4.38 g, 30 mmol,),
potassium hydroxide (1.68g, 30 mmol) and EtOH (30ml) and stirred at room temperature for
1h. after this time the corresponding aryl bromide or chloride (30 mmol) was added slowly
and the resulting mixture was refluxed at 80°C for 2-3 days. After reaction finished, the
reaction mixture was allowed to cool down to room temperature then the solvent was
removed under reduced pressure. The white solid obtained was washed with DCM (20-30ml)
and filtred through filter paper. The CH2Cl2 was removed under reduced pressure and the
crude product was dried under vaccum.
5,6-dimethyl-1-(2,3,4,5,6-pentamethylbenzyl)-1H-benzo[d]imidazole 1.a
This NHC precursor was synthesized according to published procedure and found NMR data
shown below.62
1
m.p. 173°C. Yield (97%). ν(CN)= 1448.65 cm-1. H NMR (CDCl3, 400MHz) δ (ppm) 2.11(s,
6H, CH3(c,g)), 2.19(s, 6H, CH3(d,f)), 2.23(s, 3H, CH3(e)), 2.32(s, 3H, CH3(a)), 2.37(s, 3H,
CH3(b)), 5.16(s, 2H, CH2(1’)), 7.20(s, 1H, Harom(7)), 7.24(s, 1H, Harom(4)), 7.30(s, 1H, H2).13C
NMR(CDCl3, 101MHz)δ (ppm) 16.51(C(c,g)), 16.87(C(d,f)), 17.16(C(e)), 20.33(C(a)),
20.66(C(b)), 44.27(C1’), 109.77(C7), 120.29(C4), 127.37(C5,8), 131.21(C4’,5’,6’), 131.97(C6),
133.38(C3’,7’), 133.56(C9), 136.13(C2’), 141.10(C2).
1-(2,3,4,5,6-pentamethylbenzyl)-1H-benzo[d]imidazole 1.b
This NHC precursor was synthesized according to published procedure and found NMR data
shown below.62
1
m.p. 121°C. Yield (95%). ν(CN)= 1454.34 cm-1. H NMR (CDCl3, 400MHz) δ (ppm) 2.21(s,
6H, CH3(a,e)), 2.27(s, 6H, CH3(b,d)), 2.31(s, 3H, CH3(c)), 5.31(s, 2H, CH2(1’)), 7.35(t, 2H,
Harom(5,6)), 7.43(s, 1H, Harom(7)), 7.56(s, 1H, Harom(4)), 7.85(s, 1H, H2).13C NMR(CDCl3,
16

100MHz)δ (ppm) 16.55(C(a,e)), 16.89(C(b,d)), 17.19(C(c)), 44.42(C1’), 109.77(C7), 120.29(C4),
122.37(C5), 122.87(C6), 127.12(C8,9), 133.47(C4’,5’,6’), 133.55(C3’,7’), 136.28(C2’), 141.96(C2).
1-(4-(tert-butyl)benzyl)-1H-benzo[d]imidazole 1.c
This NHC precursor was synthesized according to published procedure and found NMR data
shown below.63
m.p. 123.6°C. Yield (92%). ν(CN)= 1457.59 cm-1. 1H NMR (CDCl3, 400MHz) δ (ppm) 1.22(s,
9H, CH3(a,b,c)), 5.25(s, 2H, CH2(1’)), 7.06(d, 2H, Harom(3’,7’)), 7.20(d, 1H, Harom(5,6)), 7.29(d, 1H,
Harom(4’,6’,7)), 7.77(s, 1H, Harom(4)), 7.88(s, 1H, H2).13C NMR(CDCl3, 100MHz)δ (ppm)
31.28(C(a,b,c)), 34.60(C8’), 48.53(C1’), 110.08(C7), 120.39(C4), 122.25(C5), 123.06(C6),
125.97(C4’,6’), 126.92(C3’,7’), 132.46(C2’,8), 143.20(C2,9), 151.40(C5’).
Synthesis of ligands (2a-c)
A mixture of benzimidazolium salt (1a-1c) (1 g) and the corresponding benzyl bromide or
chloride (1eq) in DMF (2 ml) was stirred at 70°C for 2-3 days. After that time, the white solid
formed was washed with diethyl ether (20 ml) and stirred for couple hours. Then the reaction
mixture was filtred through filter paper and the white solid was dried under vaccum then
crystallized with DCM-ether (1:3) for further purification.
5,6-dimethyl-1,3-bis(2,3,4,5,6-pentamethylbenzyl)-1H-benzo[d]imidazol-3-ium chloride
2a
This NHC precursor was synthesized according to published procedure and found NMR data
shown below.62
m.p. 307.7°C. Yield (96%). ν(CN)= 1440.99 cm-1. 1H NMR (CDCl3, 400MHz) δ (ppm) 2.22(s,
30H, CH3), 2.28(s, 6H, CH3(a,b)), 5.8(s, 4H, CH2(1’,1”)), 7.11(s, 2H, Harom(4,7)),9.85(s, 1H, H2).
13CNMR(CDCl3, 101MHz) δ (ppm) 17.01(CH3), 16.92(C(c,g,c’,g’)), 17.01 (C(d,f,d’,f’)),
17.28(C(e,e’)), 20.77(C(a,b)), 48.10(C1’;1”), 113.41(C4;7), 125.58(C8;9), 130.42(C4’;5’;6’;4”;5”;6” ,
)
133.50(C3’;7’;3”;7”), 133.74(C5;6), 136.88(C2’;2”),141.35(C2).
1,3-bis(2,3,4,5,6-pentamethylbenzyl)-1H-benzo[d]imidazol-3-ium chloride 2b
This NHC precursor was synthesized according to published procedure and found NMR data
shown below.62
m.p. 210.6°C. Yield (92%). ν(CN)= 1454.84 cm-1. 1H NMR (CDCl3, 400MHz) δ (ppm) 2.15(s,
12H, CH3(a,e,a’,e’)), 2.18(s, 12H, CH3(b,d,b’,d’)), 2.20(s, 6H, CH3(c,c’)), 5.84(s, 4H, CH2), 7.2(d,
2H, Harom(5,6)), 7.34(d, 2H, Harom(4,7)), 10.34(s, 1H, H2). 13C NMR(CDCl3,100MHz) δ
17

(ppm) 16.95(Ca,e,a’,e’), 17.06(Cb,d,b’,d’), 17.30(Cc,c’), 48.58(C1’;1”), 113.72(C4;7), 125.36(C5;6),
126.88(C8;9),131.85(C5’;5”),133.49(C4’;6’;4”;6”), 133.82(C3’;7’;3”;7”), 137.07(C2’;2”), 143(C2).
1,3-bis(4-(tert-butyl)benzyl)-1H-benzo[d]imidazol-3-ium bromide 2c
This NHC precursor was synthesized according to published procedure and found NMR data
shown below.63
1
m.p. 296.5°C. Yield (89%). ν(CN)= 1427.88 cm-1. H NMR (CDCl3, 400MHz) δ (ppm) 1.2 (s,
18H, CH3), 5.75(s, 4H, CH2), 7.33(d, 4H, Harom(4’,6’,4”,6”)),7.39(d, 4H, Harom(3’;7’3”,7”)), 7.46(d,
2H, Harom(5;6)), 7.55(d, 2H, Harom(4;7)), 11.83(s, 1H, H2).13CNMR(CDCl3,101 MHz) δ (ppm)
31.18(CH3), 34.67(C8’;8”),
51.27(C1’;1”), 113.78(C4;7), 126.34(C4’;6’;4”;6”), 127.09(C5,6),
128.17(C3’;7’;3”;7”), 129.56(C8;9),131.41(C2’;2”), 143(C2),152.46(C5’;5”).
Synthesis of [(NHC)CuI] (3a-c)
A standard schlenk was charged with ligand (1mmol), CuI (1 mmol) and K2CO3 (2 mmol).
The resulting mixture was refluxed in acetone under Argon at 60°C for 24h. After this time,
the mixture was filtered through silica. The pad of silica was concentrated with rotary
evaporator and dried under vacuum.
(5,6-dimethyl-1,3-bis(2,3,4,5,6-pentamethylbenzyl)-2,3-dihydro-1H-benzo[d]imidazol-2-
yl)copper(II) iodide 3a
1
m.p. 265.6°C.Yield (68%). ν(CN)= 1448.51 cm-1. H NMR (CDCl3, 400MHz) δ (ppm) 2.12(s,
12H, CH3(c,g,c’,g’)), 2.16(s, 12H, CH3(d,f,d’’,f’)), 2.19(s, 6H, CH3(e,e’)), 2.21(s, 6H, CH3(a,b)), 5.39
(s, 4H, CH2(1’,1”)), 6.86(s, 2H, Harom(4,7)). 13C NMR (CDCl3, 100MHz) δ (ppm) 16.10(C(c,g,c’,g’)),
16.25(C(d,f,d’,f’)), 16.36(C(e,e’)),19.46(C(a,b)), 47.8(C1’;1”),
110.85(C4;7), 126.35(C8;9),
131.79(C4’;6’;4”;6”), 131.9 (C5’;5”), 132.09(C5;6),132.91(C3’;7’;3”;7”), 135.67(C2’;2”), 184.99(C2).
Anal. Calc. for C33H42N2ICu (%): C, 60.31; H, 6.44; N, 4.26. Found (%): C, 60.39; H, 6.52;
N, 4.34. HR-MS(ESI), m/z=529,2614 [M-I]+ (calcd for C33H42N2Cu:529,2644).
(1,3-bis(2,3,4,5,6-pentamethylbenzyl)-2,3-dihydro-1H-benzo[d]imidazol-2-yl)copper(II)
iodide 3b
m.p. 245.3°C. Yield (72%). ν(CN)= 1419.02 cm-1.1H NMR (CDCl3, 400MHz) δ (ppm) 2.13(s,
12H, CH3(a,e,a’,e’)), 2.17(s, 12H, CH3(b,d,b’,d’)), 2.22(s, 6H, CH3(c,c’)), 5.47 (s, 4H, CH2), 7.08(d,
2H, Harom(5;6)), 7.15(d, 2H, Harom(4;7)).
18
13CNMR (CDCl3, 100MHz) δ (ppm)

156.11(C(a,e,a’,e’)), 16.26 (C(b,d,b’,d’)), 16.38 (C(c,c’)), 48.13 (C1’;1”), 110.61(C4;7), 122.76(C5;6),
126.13(C5’;5”), 132.08(C4’;6’;4”;6”), 132.98(C3’;7’;3”;7”), 133.22(C8;9), 135.79(C2’;2”), 186.25(C2).
Anal. Calc. for C31H38N2ICu (%): C, 59.19; H, 6.09; N, 4.45. Found (%): C, 59.25; H, 6.17;
N, 4.56.HR-MS(ESI), m/z=501,2398 [M-I]+ (calcd for C31H38N2Cu:501,2331).
(1,3-bis(4-(tert-butyl)benzyl)-2,3-dihydro-1H-benzo[d]imidazol-2-yl)copper(II) iodide 3c
1
m.p. 208.1°C. Yield (66%). ν(CN)= 1476.56 cm-1. H NMR (DMSO_d6, 400MHz) δ (ppm)
1.20(s, 18H, CH3), 5.71(s, 4H, CH2), 7.28(s, 6H, Harom(4’,6’,4”,6”,5,6)), 7.39(d, 4H, Harom(3’,7’,3”,7”)),
7.28(d, 2H, Harom(4,7)),13C NMR (DMSO_d6, 101MHz) δ(ppm) 36.23(CH3), 39.43(C8’;8”),
55.85(C1’;1”), 117.18(C4;7), 128.74(C5;6), 130.57(C4’;6’;4”;6”), 132.52(C3’;7’;3”;7”), 138.63(C8;9),
138.71(C2’;2”), 155.47(C5’;5”), 194.20(C2). Anal. Calc. for C29H34N2ICu (%): C, 57.95; H, 5.70;
N, 4.66. Found (%): C, 58.03; H, 5.76; N, 4.74. HR-MS(ESI), m/z=473,2044 [M-I]+ (calcd
for C29H34N2Cu:473,2018).
Synthesis of [(NHC)CuX] (4a-c)
A 50 ml bottom flask was charged with ligand (2 mmol), Cu2O (1.3 mmol) and deionized
water (6 ml). The mixture was refluxed for 24h with vigorous stirring. After this time period,
the reaction mixture was allowed to cool down to room temperature. The reaction mixture
was diluted with CH2Cl2 and the organic fraction was extracted. The organic extract was dried
with MgSO4 and concentrated with rotary evaporator. The crude product was dissolved in a
minimal amount of CH2Cl2 and was eluted with CH2Cl2 over a silica plug to afford the
desired product as an off-white solid.
(5,6-dimethyl-1,3-bis(2,3,4,5,6-pentamethylbenzyl)-2,3-dihydro-1H-benzo[d]imidazol-2-
yl)copper(II) chloride 4a
m.p. 265.6°C. Yield (70%). ν(CN)= 1447.07 cm-1. 1H NMR (CDCl3, 300MHz) δ (ppm) 2.12(s,
12H, CH3(c,g,c’,g’)), 2.16(s, 18H, CH3(d,e,f,d’,e’,f’)), 2.21(s, 30H, CH3(a,b)), 5.39 (s, 4H, CH2), 6.82
(s, 1H, Harom(4,7)).13CNMR (CDCl3,75MHz) δ (ppm) 15.99(C(c,g,c’,g’)), 16.21(C(d,e,f,d’,e’,f’)),
19.45(C(a,b)), 48(C1’;1”), 110.77(C4;7), 126.49(C8;9), 131.78(C4’;6’;4”;6”), 131.83(C5’;5” ,
)
132.13(C5;6),132.71(C3’;7’;3”;7”), 135.38(C2’;2”), 182.69(C2). Anal. Calc. for C33H42N2ClCu (%):
19

C, 70.06; H, 7.48; N, 4.95. Found (%): C, 70.15; H, 7.59; N, 5.05. HR-MS(ESI),
m/z=529,2619 [M-Cl]+ (calcd for C33H42N2Cu:529,2644).
(1,3-bis(2,3,4,5,6-pentamethylbenzyl)-2,3-dihydro-1H-benzo[d]imidazol-2-yl)copper(II)
chloride 4b
M.P= 238.8°C. Yield (74%). ν(CN)= 1435.31 cm-1.1H NMR (CDCl3, 300MHz) δ (ppm) 2.22(s,
12H, CH3(a,e,a’,e’)), 2.25(s, 12H, CH3(b,d,b’,d’)), 2.30(s, 6H, CH3(c,c’)), 5.48 (s, 4H, CH2), 7.04(d,
4H, Harom(5,6)), 7.10(d, 4H, Harom(4,7)). 13CNMR (CDCl3,75MHz) δ(ppm) 17.05(C(a,e,a’,e’)), 17.26
(C(b,c,d,b’,c’,d’)), 49.37(C1’;1”), 111.57(C4;7), 123.72(C5;6), 127.26(C5’;5”), 133.13(C4’;6’;4”;6”),
133.84(C8;9), 134.28(C3’;7’;3”;7”), 136.58(C2’;2”), 186.03(C2). Anal. Calc. for C31H38N2ClCu
(%): C, 69.25; H, 7.12; N, 5.21. Found (%): C, 69.37; H, 7.19; N, 5.34. HR-MS(ESI),
m/z=501,2391 [M-Cl]+ (calcd for C31H38N2Cu:501,2331).
(1,3-bis(4-(tert-butyl)benzyl)-2,3-dihydro-1H-benzo[d]imidazol-2-yl)copper(II) bromide
4c
1
M.P= 208.1°C. Yield (63%). ν(CN)= 1479.23 cm-1. H NMR (CDCl3, 300MHz) δ (ppm)
13
1.21(s, 18H, CH3), 5.55(s, 4H, CH2), 7.19-7.34(m, 12H, Harom). CNMR (CDCl3,75MHz) δ
(ppm) 30.23(CH3), 33.56(C8’;8”), 51.58(C1’;1”), 110.96(C4;7), 123.00(C5;6), 124.96(C4’;6’;4”;6”),
126.27(C3’;7’;3”;7”), 130.99(C8;9), 132.79(C2’;2”) 150.49(C2),184.60(C5’;5”). Anal. Calc. for
C29H34N2BrCu (%): C, 62.87; H, 6.19; N, 5.06. Found (%): C, 62.96; H, 6.26; N, 5.16. HR-
MS(ESI), m/z=473,2043 [M-Cl]+ (calcd for C29H34N2Cu:473,2018).
Synthesis of Triazoles 7a-f
A vial was charged with benzimidazolium salt (1 mmol), sodium azide (1.05 mmol), alkyne
(1.05 mmol), [Cu(NHC)] (3 mol%) and water (1 ml). The reaction was stirred vigorously at
room temperature for 2h. After this time, EtOAc was added and the organic fraction was
extracted. The organic extract was dried over MgSO4, filtered and concentrated under reduced
pressure to give the expected triazoles.
1-(4-(tert-butyl)benzyl)-4-phenyl-1H-1,2,3-triazole 7a
m.p. 162.8°C. Yield (68%). ν(C=CH)= 2957.93 cm-1, ν(C(aryl)-N)= 1104.93 cm-1. 1H NMR (CDCl3,
400MHz) δ (ppm) 1.22 (s, 9H, CH3), 5.43 (s, 2H, CH2), 7.16(d, 2H, Harom(3’;7’)), 7.22(d, 2H,
Harom(4’;6’)), 7.31(t, 4H, Harom(3”;4”;5”)), 7.58(s, 1H, H5), 7.72(d, 2H, Harom(2”;6”)). 13C NMR
20

(CDCl3, 101MHz) δ (ppm) 30.22(CH3), 33.6(C8’), 52.88(C1’), 124.65(C4’;6’), 125.02(C3’;7’),
126.85(C2”;6”), 127.07(C4”), 127.74(C3”;5”), 129.55(C1”,5), 130.62(C2’), 150.85 (C4,5’).
1-(3,5-dimethylbenzyl)-4-phenyl-1H-1,2,3-triazole 7b
m.p. 121.4°C. Yield (76%). ν(C=CH)= 2915.89 cm-1,ν(C(aryl)-N)= 1024.62 cm-1. 1H NMR (CDCl3,
400MHz) δ (ppm) 2.22(s, 6H, CH3), 5.39(s, 2H, CH2), 6.83(s, 2H, Harom(3’;7’)), 6.9(s, 2H,
Harom(5’)), 7.22(t, 1H, Harom(4”)),7.31(t, 3H, Harom(3”;5”)), 7.58(s, 1H, H5), 7.73(d, 2H,
Harom(2”;6”)). 13C NMR (CDCl3, 100MHz) δ (ppm) 20.17 (CH3), 53.18 (C1’), 124.64(C2”;6”),
124.84(C3’;5’;7’), 127.07(C4”), 127.75(C3”;5”), 129.34(C1”,5), 133.49(C2’;4’;6’), 137.82(C4).
1-(4-methylbenzyl)-4-phenyl-1H-1,2,3-triazole 7c
m.p. 108.9°C. Yield (80%). ν(C=CH)= 2970.38 cm-1, ν(C(aryl)-N)= 1075.44 cm-1.1H NMR (CDCl3,
400MHz) δ (ppm) 2.29 (s, 3H, CH3), 5.46(s, 2H, CH2), 7.13(s, 4H, Harom(3’,4’,6’,7’)), 7.19(s, 1H,
H5), 7.24(t, 1H, Harom(4”)),7..39(t, 3H, Harom(3”;5”)), 7.74(s, 2H, Harom(2”,6”)). 13C NMR (CDCl3,
100MHz) δ(ppm) 20.14(CH3), 53.12(C1’), 124.62(C3’;7’;2”;6”), 127.10(C4”), 127.13(C3”;5”),
127.77(C4’;6’), 128.79(C1”,5), 130.60(C2’), 137.74(C4,5’).
4-phenyl-1-(2,4,6-trimethylbenzyl)-1H-1,2,3-triazole 7d
m.p. 145.7°C. Yield (72%). ν(C=CH)= 2971.28 cm-1, ν(C(aryl)-N)= 1044.88 cm-1.1H NMR (CDCl3,
400MHz) δ (ppm) 2.24(s, 6H, CH3), 5.52(s, 2H, CH2), 6.87(s, 2H, Harom(4’;6’)), 7.20(t, 2H,
Harom(5’)), 7.25(s, 1H, H5), 7.25(t, 3H, Harom(3”;5”)), 7.67(d, 2H, Harom(2”;6”)). 13C NMR (CDCl3,
100MHz) δ(ppm) 18.65(CH3(a,c)), 20.01(CH3(b)), 47.27(C1’), 117.52(C2”;6”), 124.59(C4’;6’),
126.99(C4”), 127.70(C3”;5”), 128.68(C1”,5), 129.61(C5’), 136.86 (C3’;7’), 138.04(C2’),
146.52(C4).
1-(4-(methoxy)benzyl)-4-phenyl-1H-1,2,3-triazole 7e
m.p. 170°C. Yield (85%). ν(C=CH)= 2956 cm-1, ν(C(aryl)-N)= 1106 cm-1. H NMR (CDCl3,
1
400MHz) δ (ppm) 3.75 (s, 3H, OCH3), 5.45 (s, 2H, CH2), 7.20(d, 2H, Harom(3’;7’)), 7.24(d, 2H,
Harom(4’;6’)), 7.35(t, 4H, Harom(3”;4”;5”)), 7.61(s, 1H, H5), 7.74(d, 2H, Harom(2”;6”)). 13C NMR
(CDCl3, 101MHz) δ (ppm) 57.22(OCH3), 33.5(C8’), 52.82(C1’), 124.75(C4’;6’), 125.12(C3’;7’),
128.75(C2”;6”), 128.17(C4”), 127.75(C3”;5”), 129.50(C1”,5), 130.65(C2’), 150.75 (C4,5’).
1-(4-(isopropyl)benzyl)-4-phenyl-1H-1,2,3-triazole 7f
m.p. 175°C. Yield (92%). ν(C=CH)= 2956 cm-1, ν(C(aryl)-N)= 1106 cm-1. H NMR (CDCl3,
1
400MHz) δ (ppm) 1.22 (s, 6H, CH3), 5.45 (s, 2H, CH2), 7.20(d, 2H, Harom(3’;7’)), 7.25(d, 2H,
Harom(4’;6’)), 7.35(t, 4H, Harom(3”;4”;5”)), 7.60(s, 1H, H5), 7.75(d, 2H, Harom(2”;6”)). 13C NMR
(CDCl3, 101MHz) δ (ppm) 30.20(CH3), 33.7(C8’), 52.85(C1’), 124.75(C4’;6’), 125.22(C3’;7’),
126.75(C2”;6”), 127.17(C4”), 127.75(C3”;5”), 129.65(C1”,5), 130.65(C2’), 150.75 (C4,5’).
21

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27

Graphical abstract
R1
N
N
Cu
I
x
u
R2
l
f
e
R
,
C
°
0
6
,
e
R1
n
3
o
t
e
R3
c
A
N
(X= Cl, Br)
X
N
R2
2
R3
R1
w
a
t
e
r
,
1
0
0
°
C
,
R
N
N
e
f
l
u
x
Cu
X
(X= Cl, Br)
R2
R3
4
28

In this study
-
A series of new benzimidazolium salts (2a–c) were synthesized from the reaction of
5,6-dimethyl-1-(alkylbenzyl)-1H-benzo[d]imidazole with various alkyl halides.
These salts were used to synthesize cupper N-heterocyclic carbene (Cu-NHC)
complexes 3-4.
-
-
The obtained (NHC) Copper (I) complexes 3-4 were characterized by FT-IR, NMR
(1H and 13C) spectroscopic methods, mass spectrometry (EI-MS and HRMS) and
elemental analysis.
These novel cupper complexes 3-4 were used as a catalyst for alkyne – azide
cycloaddition (CuAAC) reaction.
29