Convenient and efficient Suzuki–Miyaura and Heck–Mizoroki cross-coupling reactions catalyzed by 1,3,4-trisubstituted-1,2,3-triazolium iodide and palladium salt systems

Article abstract of DOI:10.1080/00958972.2019.1571583

A series of 1,3,4-trisubstituted-1,2,3-triazolium iodide salts (4a–c) were synthesized via a three-step reaction sequence. Corresponding anilines (1a–c) were converted to azides (2a–c) which were then treated with phenylacetylene with “Click” chemistry to access 1,4-disubstituted-1,2,3-triazoles (3a–c). Subsequent methylation of 1,4-disubstituted-1,2,3-triazoles (3a–c) yielded 1,3,4-trisubstituted-1,2,3-triazoliumiodide salts (4a–c) in appreciable yields. All the synthesized compounds were characterized by ¹H and ¹³C NMR, ATR–IR spectroscopic techniques and elemental analyses. Additionally, the structure of 1-(4-chlorophenyl)-4-phenyl-1,2,3-triazole (3b) was confirmed by single crystal X-ray diffraction analysis. The catalytic activity of 4a–c in a catalytic system consisting of 1,3,4-trisubstituted-1,2,3-triazoliumiodide salt/palladium(II) acetate/base were investigated toward Suzuki–Miyaura and Heck–Mizoroki cross-coupling reactions. The Suzuki–Miyaura cross-coupling reactions were carried out under mild reaction conditions with good to excellent yields, whereas Heck–Mizoroki cross-coupling reactions were performed at elevated temperature with moderate yields. Further, in situ method skips the synthetic procedure of preparing the palladium(II) complexes and hence is more economical and less tedious. (Figure presented.).

Full text of DOI:10.1080/00958972.2019.1571583

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Convenient and efficient Suzuki–Miyaura and
Heck–Mizoroki cross–coupling reactions catalyzed
by 1,3,4–trisubstituted–1,2,3–triazolium iodide and
palladium salt systems
Shahini C. R., Gautam Achar, Srinivasa Budagumpi, Ramesh B. Dateer, Helge
Müller–Bunz, Matthias Tacke & Siddappa A. Patil
To cite this article: Shahini C. R., Gautam Achar, Srinivasa Budagumpi, Ramesh B.
Dateer, Helge Müller–Bunz, Matthias Tacke & Siddappa A. Patil (2019): Convenient
and efficient Suzuki–Miyaura and Heck–Mizoroki cross–coupling reactions catalyzed by
1,3,4–trisubstituted–1,2,3–triazolium iodide and palladium salt systems, Journal of Coordination
Chemistry, DOI: 10.1080/00958972.2019.1571583
To link to this article: https://doi.org/10.1080/00958972.2019.1571583
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Convenient and efficient Suzuki–Miyaura and Heck–Mizoroki
cross–coupling reactions catalyzed by 1,3,4–trisubstituted–1,2,3–
triazolium iodide and palladium salt systems
Shahini C. R.a, Gautam Achara, Srinivasa Budagumpia, Ramesh B. Dateera, Helge Müller–
Bunzb, Matthias Tackeb, Siddappa A. Patila*
aCentre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura,
Ramanagaram, Bangalore 562112, India
bSchool of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
*Corresponding author: Dr. Siddappa A. Patil
Tel: +91 80 27577254
Fax: +91 80 27577211
E–mail: p.siddappa@jainuniversity.ac.in / patilsiddappa@gmail.com
1

ABSTRACT
A series of 1,3,4–trisubstituted–1,2,3–triazolium iodide salts (4a–c) were synthesized via a
three–step reaction sequence. Corresponding anilines (1a–c) were converted to azides (2a–c)
which were then treated with phenylacetylene with “Click” chemistry to access 1,4–
disubstituted–1,2,3–triazoles (3a–c). Subsequent methylation of 1,4–disubstituted–1,2,3–
triazoles (3a–c) yielded 1,3,4–trisubstituted–1,2,3–triazoliumiodide salts (4a–c) in
1
13
appreciable yields. All the synthesized compounds were characterized by H and C NMR,
ATR–IR spectroscopic techniques and elemental analyses. Additionally, the structure of 1–
(4–chlorophenyl)–4–phenyl–1,2,3–triazole (3b) was confirmed by single crystal X–ray
diffraction analysis. The catalytic activity of 4a–c in a catalytic system consisting of 1,3,4–
trisubstituted–1,2,3–triazoliumiodide salt/palladium(II) acetate/base were investigated
towards Suzuki–Miyaura and Heck–Mizoroki cross–coupling reactions. The Suzuki–Miyaura
cross–coupling reactions were carried out under mild reaction conditions with good to
excellent yields, whereas Heck–Mizoroki cross–coupling reactions were performed at
elevated temperature with moderate yields. Further, in situ method skips the synthetic
procedure of preparing the palladium(II) complexes and hence is more economical and less
tedious.
Keywords: 1,2,3–triazole; 1,2,3–triazolium iodide salt; palladium(II) acetate; crystal
structure; Suzuki–Miyaura cross–coupling; Heck–Mizoroki cross–coupling
2

1. Introduction
Organometallic chemistry involving N–heterocyclic carbenes (NHCs) and their metal
complexes have been dominated by imidazole and its derivatives including xanthines,
oxazoles, thiazoles, 1,2,4–triazoles, pyrimidines and related saturated systems that have been
employed in wide ranges of applications such as pharmaceutical, catalysis, luminescence,
functional materials, etc.[1–4]. However, the mainstream NHC chemistry diverged towards
an interesting contrast type, the abnormal NHCs that were first described by Crabtree et al. in
2001 and received enormous attention due to their stronger electron–donating abilities
compared to normal NHCs, a result of decreased π–stabilizing and inductive effects due to
lower number of adjacent heteroatoms that renders them a competitive class of ligands [5–8].
Although normal NHCs have been explored with various transition metal centres towards
broad applications, the abnormal analogues are relatively unexplored. Nevertheless, since the
introduction of “Click” chemistry involving CuAAC (Copper–Catalyzed Azide–Alkyne
Cycloaddition) to access 1,4–disubstituted 1,2,3–triazoles by Sharpless et al. in 2001, the
1,3,4–trisubstituted–1,2,3–triazol–5–ylidene species have been extensively studied, instigated
by Albrecht et al. in 2008, employing various substitutions which act as excellent ligand
species and possess applications such as optics, redox sensing, biomedicine and catalysis
upon complexation with a metal centre [9–13]. Further, a library of abnormal NHCs can be
derived from imidazolium, thiazolium, oxazolium, pyrazolium, isothiazolium, pyridinium
and quinolinium salts among others [14–16]. However, 1,3,4–trisubstituted–1,2,3–triazolium
species are of interest owing to new synthetic and catalytic opportunities [17–24]. Among the
transition metals for complexation with 1,3,4–trisubstituted–1,2,3–triazole based ligands,
palladium(II) is the prime option owing to the success of palladium–based complexes as
exceptional catalysts for a variety of reactions such as oxidation, substitution and cross–
3

coupling reactions [25]. Despite the palladium(II) centre being the active species participating
in catalysis, the supporting ligand plays a pivotal role in the efficiency of the catalyst.
Evolution of such supporting ligands from phosphine–based compounds that were expensive
and air, temperature and moisture sensitive to NHC–based ligands that are relatively easier in
storage and handling and also more versatile in substitutions with a possibility of electronic
and steric property modifications has occurred [26, 27]. Additionally, the study conducted by
Xu et al. in 2010 revealed that abnormal NHC–palladium(II) complexes performed better
than normal NHC analogues in C–C cross–coupling reactions and another study conducted
by Bertrand et al. on a free 1,2,3–triazol–5–ylidene established that the donor properties
were superior to those of imidazol–2–ylidenes and 1,2,4–triazol–5–ylidenes and inferior to
those of imidazol–4–ylidenes rendering abnormal NHCs the more sought after ligand species
[28, 29].
Additionally, C–C bond forming reactions such as Suzuki–Miyaura cross coupling
and Heck–Mizoroki cross coupling have importance due to employment of biaryls and
functionalized olefins in pharmaceuticals, conducting polymers, molecular wires, liquid
crystals and in supramolecular chemistry [30, 31]. Recent research involving 1,2,3–triazol–5–
ylidene based palladium(II) complexes in Suzuki–Miyaura cross coupling and Heck–
Mizoroki cross coupling reactions demonstrate potential and motivate further research to
improve catalyst efficiency [32–36]. In this aspect, the present study investigates the base–
palladium–abnormal NHC–catalyzed Suzuki–Miyaura and Heck–Mizoroki cross–coupling
reactions under mild reaction conditions.
2. Experimental
2.1. Materials and methods
All experiments were conducted in normal laboratory conditions without precautionary
measures to eliminate air and moisture. 4–Bromoaniline, 4–chloroaniline, 4–toluidine,
4

sodium
azide,
sodium
nitrite,
magnesium
sulphate,
phenylacetylene,
bromotris(triphenylposphine) copper(I), methyl iodide, alkyl halides, phenyl boronic acid,
styrene, potassium carbonate, sodium carbonate, cesium carbonate, potassium hydroxide,
tripotassium phosphate, sodium tertiary butoxide, palladium(II) acetate, palladium(I) chloride
bis–acetonitrile, hexane, toluene, methanol, ethanol and tetrahydrofuran were procured from
commercial sources and used without purification or distillation. All reactions were carried
out in oven dried glassware and heating was accomplished by a silicone oil bath. Progression
of the reactions was monitored by thin–layer chromatography (TLC) performed on 0.25 mm
Merck TLC silica gel plates, using UV light as a visualizing agent. Purification of reaction
products was executed by column chromatography using silica gel 60 (230–400 mesh). The
melting points were determined using a Stuart Scientific (UK) instrument. Further, volatile
solvents were removed using a rotary evaporator attached to a dry diaphragm pump (10–15
1
mm Hg), followed by pumping to a constant weight with an oil pump (< 300 mTorr). H
NMR spectra were recorded on a Bruker AVANCE III 400 MHz spectrometer and are
reported relative to DMSO–d6 (2.50 ppm) or CDCl3 (δ 7.27). Coupling constants J are
13
reported in Hertz (Hz). Proton–decoupled C NMR spectra were recorded at 100 MHz and
reported relative to DMSO–d6 (39.51 ppm) or CDCl3 (δ 77.23 ppm). ATR–IR spectra were
recorded on a Bruker ECO–ATR spectrophotometer from 600–4000 cm–1. Elemental
analyses were performed using a Perkin Elmer 2400 Series II CHN/S microanalyzer. The
crystal data of 3b were collected using an Agilent (formerly Oxford Diffraction) SuperNova
A diffractometer that was fitted with an Atlas detector using monochromated Mo Kα
radiation (0.71073 Å). A complete set of data was collected assuming the Friedel pairs were
non–equivalent and an analytical absorption correction based on the shape of the crystal was
performed [37]. The structure was solved by direct methods using SHELXS–2014 and
refined by full–matrix least squares on F2 for all data using SHELXL– 2014 [38]. Hydrogens
5

were added at calculated positions and were refined using a riding model. Anisotropic
thermal displacement parameters were used for all non disordered non–hydrogen atoms. The
crystallographic data has been deposited in the Cambridge Crystallographic Data Center;
CCDC 1842020 contain the supplementary crystallographic data for 3b. The data can be
obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
2.2. General synthetic route to 4–substituted phenylazides (2a–c)
4–Substituted phenyl azides were synthesized following a reported procedure with minor
modifications [39]. 4–Substituted aniline (15.0 mmol) was taken in water (25 mL) to which
concentrated sulphuric acid (12.5 g) was added dropwise. The mixture was cooled in ice–cold
water and a mixture of sodium nitrite (2.625g, 38 mmol) in water (16 mL) was added.
Hexane (50 mL) was poured into the above mixture followed by a solution of sodium azide
(2.3 g, 35 mmol) in minimum water. The mixture was allowed to stir in ice–cold condition
for 3 h. The organic phase was then separated, dried over magnesium sulphate and filtered.
The solvent was removed in vacuo to obtain pure 2a–c.
2.2.1. Synthesis of 4–bromophenylazide (2a)
Compound 2a was synthesized according to the general procedure using 4–bromoaniline
(2.58 g, 15.0 mmol). Yield: 74%; Yellow liquid.
2.2.2. Synthesis of 4–chlorophenylazide (2b)
Compound 2b was synthesized according to the general procedure using 4–chloroaniline
(1.91 g, 15.0 mmol). Yield: 76%; Yellow liquid.
2.2.3. Synthesis of 4–methylphenylazide (2c)
Compound 2c was synthesized according to the general procedure using 4–toluidine (1.60 g,
15.0 mmol). Yield: 69%; Yellow liquid.
6

2.3. General synthetic route to access 1–(4–substitutedphenyl)–4–phenyl–1H–1,2,3–
triazoles (3a–c)
Compounds 3a–c were synthesized following the reported procedure with slight
modifications [40]. The previously synthesized 4–substituted phenylazides (2a–c) (1.0 mmol)
were treated with phenylacetylene (0.12 g, 1.2 mmol) in the presence of
bromotris(triphenylposphine)copper(I) catalyst (0.5 mol%) neat at room temperature for 1 h.
The reaction mixture was then washed with hexane (3 x 10 mL) to obtain the corresponding
1–(4–substitutedphenyl)–4–phenyl–1H–1,2,3–triazoles (3a–c).
2.3.1. Synthesis of 1–(4–bromophenyl)–4–phenyl–1H–1,2,3–triazole (3a)
Compound 3a was synthesized following the general procedure using the previously
synthesized compound 2a. Yield: 81%; Off white solid; M. P.: 156–158 ˚C; 1H NMR (δ ppm,
DMSO–d6, 400 MHz, ppm): 9.27 (s, 1H, C5), 7.91–7.89 (m, 4H, CHBr–Phenyl), 7.82–7.80 (m,
13
2H, CHPhenyl), 7.49–7.45 (m, 2H, CHPhenyl), 7.38–7.34 (m, 1H, CHPhenyl); C NMR (δ ppm
DMSO–d6, 100 MHz, proton decoupled, ppm): 142.9 (C5), 136.2, 133.2, 130.5, 129.4, 128.7,
125.8, 122.3, 121.7, 120.1 (C4 + CBr–Phenyl + CPhenyl); ATR–IR (cm–1): 3021.6 (w), 1610.5 (s),
1593.1 (m), 1228.7 (m), 1202.8 (m); Anal. Calc. for C14H10N3Br: C, 56.02; H, 3.36; N, 14.00.
Found: C, 55.99; H, 3.34; N, 14.11 %.
2.3.2. Synthesis of 1–(4–chlorophenyl)–4–phenyl–1H–1,2,3–triazole (3b)
Compound 3b was synthesized following the general procedure using the previously
synthesized 2b. Yield: 79 %; Off white solid; M. P.: 157–159 ˚C; 1H NMR (δ ppm, DMSO–
d6, 400 MHz, ppm): 9.48 (s, 1H, C5), 8.42–8.40 (m, 2H, CHCl–Phenyl), 8.23–8.21 (m, 2H,
CHCl–Phenyl), 7.91–7.90 (m, 2H, CHPhenyl), 7.47–7.43 (m, 2H, CHPhenyl), 7.36–7.35 (m, 1H,
CHPhenyl); 13C NMR (δ ppm DMSO–d6, 100 MHz, proton decoupled, ppm): 142.7 (C5),
135.9, 133.4, 130.1, 129.5, 126.0, 125.9, 122.8, 120.9, 120.5 (C4 + CCl–Phenyl + CPhenyl); ATR–
7

IR (cm–1): 3011.1 (w), 1636.4 (s), 1573.9 (m), 1232.6 (m), 1212.6 (m); Anal. Calc. for
C14H10N3Br: C, 65.76; H, 3.94; N, 16.43. Found: C, 65.67; H, 3.78; N, 16.36 %.
2.3.3. Synthesis of 1–(4–tolyl)–4–phenyl–1H–1,2,3–triazole (3c)
Compound 3c was synthesized following the general procedure using the previously
1
synthesized 2c. Yield: 77%; Off white solid; M. P.: 162–163 ˚C; H NMR (δ ppm, CDCl3,
400 MHz, ppm): 8.19 (s, 1H, C5), 7.92–7.90 (m, 2H, CHTolyl), 7.66–7.64 (m, 2H, CHTolyl),
7.46–7.42 (m, 2H, CHPhenyl), 7.37–7.24 (m, 3H, CHPhenyl), 2.41 (s, 3H, CH3Tolyl); 13C NMR (δ
ppm CDCl3, 100 MHz, proton decoupled, ppm): 138.4 (C5), 136.2, 135.3, 134.8, 130.4,
128.8, 128.3, 126.9, 125.8, 120.4 (C4 + CTolyl+ CPhenyl), 21.06 (CH3Tolyl); ATR–IR (cm–1):
3032.1 (w), 1650.8 (s), 1542.7 (m), 1213.3 (m), 1201.4 (m); Anal. Calc. for C15H13N3: C,
76.57; H, 5.57; N, 17.86. Found: C, 76.52; H, 5.61; N, 17.83 %.
2.4. General synthetic route to access 1–(4–substitutedphenyl)–3–methyl–4–phenyl 1,2,3–
triazolium iodide salts (4a–c)
The previously synthesized 1–(4–substituted–phenyl)–4–phenyl–1,2,3–triazoles (3a–c) (1.0
mmol) were treated with excess methyl iodide (10.0 mmol) in acetonitrile at 60 ˚C for 48 h
with vigorous stirring. The white precipitate obtained was filtered and washed with
acetonitrile (3 x 10 mL) and dried under vacuum to obtain the corresponding 1–(4–
substitutedphenyl)–3–methyl–4–phenyl–1,2,3–triazolium iodide salts (4a–c).
2.4.1. Synthesis of 1–(4–bromophenyl)–3–methyl–4–phenyl–1,2,3–triazolium iodide (4a)
Compound 4a was synthesized according to the general procedure. Yield: 62%; White solid;
1
M. P.: 160–163 ˚C; H NMR (δ ppm, DMSO–d6, 400 MHz, ppm): 9.84 (s, 1H, C5), 8.10–
7.90 (m, 4H, CHBr–Phenyl), 7.83–7.81 (m, 2H, CHPhenyl), 7.70–7.68 (m, 3H, CHPhenyl), 4.40 (s,
3H, CH3Methyl); 13C NMR (δ ppm DMSO–d6, 100 MHz, proton decoupled, ppm): 143.6 (C5),
134.4, 133.9, 132.2, 129.9, 129.8, 128.0, 125.5, 123.9, 122.8 (C4 + CBr–Phenyl + CPhenyl); ATR–
8

IR (cm–1): 3019.7 (w), 2782.1 (s), 1687.6 (s), 1559.2 (m), 1221.3 (m), 1198.9 (m); Anal.
Calc. for C15H13N3BrI: C, 40.75; H, 2.96; N, 9.50. Found: C, 40.58; H, 2.78; N, 9.65 %.
2.4.2. Synthesis of 1–(4–chlorophenyl)–3–methyl–4–phenyl–1,2,3–triazolium iodide (4b)
Compound 4b was synthesized according to the general procedure. Yield: 56%; White solid;
1
M. P.: 161–163 ˚C; H NMR (δ ppm, DMSO–d6, 400 MHz, ppm): 9.82 (s, 1H, C5), 8.09–
7.88 (m, 4H, CHCl–Phenyl), 7.80–7.77 (m, 2H, CHPhenyl), 7.70–7.64 (m, 3H, CHPhenyl), 4.42 (s,
3H, CH3Methyl); 13C NMR (δ ppm DMSO–d6, 100 MHz, proton decoupled, ppm): 142.9 (C5),
133.4, 132.7, 132.1, 129.8, 129.2, 128.5, 124.1, 123.1, 121.3 (C4 + CCl–Phenyl + CPhenyl); ATR–
IR (cm–1): 3028.5 (w), 2793.2 (s), 1671.4 (s), 1562.8 (m), 1234.9 (m), 1211.7 (m). Anal.
Calc. for C15H13N3ClI: C, 45.31; H, 3.35; N, 10.57. Found: C, 45.42; H, 3.41; N, 10.71 %.
2.4.3. Synthesis of 1–(4–tolyl)–3–methyl–4–phenyl–1,2,3–triazolium iodide (4c)
Compound 4c was synthesized according to the general procedure. Yield: 59%; White solid;
1
M. P.: 164–165 ˚C; H NMR (δ ppm, CDCl3, 400 MHz, ppm): 9.61 (s, 1H, C5), 8.00–7.98
(m, 2H, CHBr–Phenyl), 7.93–7.91 (m, 2H, CHBr–Phenyl), 7.47–7.44 (m, 3H, CHPhenyl), 7.31–7.25
(m, 2H, CHPhenyl), 4.39 (s, 3H, CH3Methyl), 2.38 (s, 3H, CHTolyl); 13C NMR (δ ppm CDCl3, 100
MHz, proton decoupled, ppm): 143.9 (C5), 142.5, 132.3, 131.8, 130.7, 130.0, 129.5, 127.0,
121.6, 121.4 (C4 + CTolyl + CPhenyl), 40.02 (CMethyl), 21.32 (CTolyl); ATR–IR (cm–1): 3025.2
(w), 2803.7 (s), 1652.1 (s), 1596.8 (m), 1236.5 (m), 1207.2 (m); Anal. Calc. for C16H16N3I: C,
50.94; H, 4.28; N, 11.14. Found: C, 50.88; H, 4.35; N, 11.07 %.
2.5. Application of in situ generated 1–(4–phenyl)–3–methyl–4–phenyl–1,2,3–triazole
based palladium(II) complexes in C–C cross coupling reactions
The synthesized 1–(4–phenyl)–3–methyl–4–phenyl–1,2,3–triazolium iodide salts were
treated with a palladium(II) sources in the presence of a base to catalyze C–C cross–coupling
reactions.
2.5.1. General procedure for Suzuki–Miyaura cross–coupling reactions
9

The synthesized 1–(4–substitutedphenyl)–3–methyl–4–phenyl–1,2,3–triazolium iodide salts
(4a–c) (1.0 mol %), palladium(II) acetate (0.5 equiv), aryl halide (1.0 mmol), phenylboronic
acid (1.1 mmol) and potassium carbonate (0.30 g, 2.2 mmol) were placed in a round
bottomed flask. To the reaction mixture, toluene (5 mL) was added and stirred at room
temperature for designated time. The progress of the reaction was monitored by TLC. After
completion of the reaction, water (10 mL) and ethyl acetate (10 mL) were added. The product
was extracted to the ethyl acetate layer using a separating funnel and dried over magnesium
sulphate. It was then concentrated in vacuo and the product was purified by column
chromatography using hexane and ethyl acetate as eluents to access the corresponding
products. As all the Suzuki–Miyaura cross–coupling products are known molecules, they
were confirmed by comparing the purity and R–factors on TLC with authentic samples
previously published [41].
2.5.2 General procedure for Heck–Mizoroki cross–coupling reactions
The synthesized 1–(4–substituted–phenyl)–3–methyl–4–phenyl–1,2,3–triazolium iodide salts
(4a–c) (1.0 mol %), palladium(II) acetate (0.5 equivalent), aryl halide (1.0 mmol), styrene
(1.2 mmol) and sodium carbonate (0.23 g, 2.2 mmol) were placed in a round bottomed flask.
To the reaction mixture, toluene (5 mL) was added and stirred at 80 °C for designated time.
The progress of the reaction was monitored by TLC. After completion of the reaction, water
(10 mL) and ethyl acetate (10 mL) were added. The product was extracted to the ethyl acetate
layer using a separating funnel and dried over magnesium sulphate. It was then concentrated
in vacuo and the product was purified by column chromatography using hexane and ethyl
acetate as eluents to access the corresponding products. The Heck–Mizoroki cross–coupled
products were confirmed by 1H NMR spectroscopy.
1
1. (E)–1,2–diphenylethene (Table 16, entries 1, 9 and 13): H NMR (400 MHz, CDCl3): δ
(ppm) = 7.51–7.48 (m, 4H), 7.42–7.40 (m, 4H), 7.34–7.21 (m, 4H).
10

1
2. (E)–1–methyl–4–styrylbenzene (Table 16, entries 2 and 11): H NMR (400 MHz, CDCl3):
δ (ppm) = 7.42–7.30 (m, 9H), 7.27–7.26 (m, 2H), 2.29 (s, 3H).
1
3. (E)–1–methoxy–4–styrylbenzene (Table 16, entries 3 and 12): H NMR (400 MHz,
CDCl3): δ (ppm) = 7.55–7.53 (m, 5H), 6.88–6.66 (m, 6H), 3.76 (s, 3H).
4. (E)–1–(4–styrylphenyl)ethanone (Table 16, entries 4 and 15): 1H NMR (400 MHz, CDCl3):
δ (ppm) = 7.42–7.40 (m, 4H), 7.34–7.31 (m, 5H), 7.27–7.26 (m, 2H), 2.56–2.55 (m, 3H).
1
5. (E)–4–styrylbenzenamine (Table 16, entries 5 and 14): H NMR (400 MHz, CDCl3): δ
(ppm) = 7.42–7.40 (m, 3H), 7.34–7.31 (m, 3H), 7.27–7.22 (m, 5H), 5.26–5.23 (m, 2H).
1
6. (E)–4–styrylphenol (Table 16, entries 6 and 10): H NMR (400 MHz, CDCl3): δ (ppm) =
7.34–7.30 (m, 5H), 6.73–6.69 (m, 6H), 4.84 (s, 1H).
7. (E)–1–(4–nitrostyryl)benzene (Table 16, entry 7): 1H NMR (400 MHz, CDCl3): δ (ppm) =
8.10–8.08 (m, 5H), 7.69–7.67 (m, 6H).
1
8. (E)–1–tert–butyl–4–styrylbenzene (Table 16, entry 8): H NMR (400 MHz, CDCl3): δ
(ppm) = 7.43–7.41 (m, 5H), 7.35–7.25 (m, 6H), 1.31 (s, 9H).
3. Results and discussion
3.1. Chemistry
1,3,4–Trisubstituted–1,2,3–triazole based palladium(II) complexes have attracted attention
owing to their superior catalytic potential in a vast array of coupling reactions. In the present
report, a set of three 1,3,4–trisubstituted–1,2,3–triazolium iodide salts (4a–c) are presented.
The 1,3,4–trisubstituted–1,2,3–triazolium iodide salts (4a–c) were synthesized in good yields
with excellent purity in a three step reaction sequence from economically available starting
materials. They were nonhygroscopic and can be stored at room temperature.
3.2. Synthesis
The reported 1,4–disubstituted–1,2,3–triazoles (3a–c) were synthesized according to the
reported protocol with minor modifications [31, 32]. Corresponding anilines (1a–c) were
11

treated with sodium azide and sodium nitrite in the presence of concentrated sulphuric acid in
ice–cold conditions to access the corresponding azides (2a–c) in pure form in yields ranging
from 69 to 76%. Compounds 2a–c were then treated with phenyl acetylene and
bromotris(triphenylphosphine)copper(I) catalyst in neat reaction condition at room
temperature to access pure 1,4–disubstituted–1,2,3–triazoles (3a–c) in yields ranging from 77
to 81%. The 1,4–disubstituted–1,2,3–triazoles (3a–c) were converted to the 1,3,4–
trisubstituted–1,2,3–triazolium iodide salts (4a–c) with 56–62% yields by treating them with
excess methyl iodide in acetonitrile at 60 0C for 48 h (Scheme 1).
3.3. Spectroscopic characterization
1
13
The 1,4–disubstituted–1,2,3–triazoles (3a–c) were characterized by H, C NMR, ATR–IR
1
spectroscopic techniques and elemental analysis. In the H NMR spectra of 3a–c, a distinct
peak was observed between δ = 9.48 and 8.19 ppm corresponding to the carbene carbon.
Peaks corresponding to the protons of the 4–substituted–phenyl ring appeared at δ = 8.42–
7.64 ppm, followed by the protons of 1–substituted–phenyl ring which appeared at δ = 7.91–
7.24. Further, the 13C NMR spectra of 3a–c revealed a distinct peak at δ = 142.9–138.4 ppm
attributed to carbene carbon. The remaining carbons were at δ = 136.2–120.1 ppm. Formation
of the compounds was confirmed by ATR–IR where the peaks corresponding to C–H, C=C
and C–N bonds were seen at 3032–3011, 1650–1610 and 1232–1201 cm–1 respectively.
Finally, the purities of 3a–c were confirmed by elemental analysis.
1
1,3,4–Trisubstituted–1,2,3–triazoliumioidide salts (4a–c) were also characterized by H and
13C NMR and ATR–IR spectroscopic techniques and elemental analysis. In 1H NMR spectra
of 4a–c, the peak attributed to carbene carbon was slightly deshielded as compared to the
corresponding 1,4–disubstituted–1,2,3–triazoles (3a–c) and observed between δ = 9.84–9.61
ppm. The peaks corresponding to protons of the 4–substituted–phenyl ring appeared slightly
12

deshielded at δ = 8.10–7.90 ppm, while those of 1–substituted–phenyl ring appeared at δ =
7.83–7.47 ppm. The methyl protons appeared at δ = 4.42–4.39 ppm. The 13C NMR spectra of
4a–c revealed carbene carbon peaks at δ = 143.9–142.9 ppm. Similar to 1,4–disubstituted–
1,2,3–triazoles (3a–c), the remaining carbons were observed at δ = 142.5–121.3 ppm.
However, Cmethyl peaks for 4a and 4b were masked by DMSO peaks but were observed at δ =
40.02 ppm for 4c. Also, the formation of the compounds was confirmed by ATR–IR where
the peaks corresponding to C–H, C=C and C–N bonds were seen at 3028–3019, 1687–
1652and 1236–1198 cm–1, respectively. Finally, the purities of 4a–c were confirmed by
elemental analyses.
3.4. Single crystal X–ray diffraction analysis
Single crystals suitable for X–ray diffraction analysis of 1–(4–chlorophenyl)–4–phenyl–
1,2,3–triazole (3b) were grown by slow evaporation of saturated DMSO solution at room
temperature. The crystal data and structure refinement details are given in Table 1. The
molecular structure of 3b is shown in Figure 1 and pertinent bond distances and angles are
tabulated in Table 2. Compound 3b crystallized in the triclinic space group P–1 with two
motifs in a unit cell. The asymmetric unit consists of one unit of (3b) and does not possess
any lattice water molecules or organic solvent molecules. The internal ring angle of the
1,2,3–triazole core at C5 carbon center N(1)–C(7)–C(8) is 104.71(11)0 and the bond angles
and distances are in agreement with those reported [30]. The molecules follow a linear
arrangement where the intermolecular hydrogen bonds between H(7)–N(3), H(7)–N(2),
H(2)–N(2) and Cl(1)–H(13) with distances 2.624 Å, 2.722 Å, 2.748 Å and 2.850 Å,
respectively, frame a sheet like appearance (Figure 2).
13

3.5. Catalytic activity
3.5.1. Suzuki–Miyaura cross–coupling reaction
The Suzuki–Miyaura cross–coupling is one of the most versatile and utilized reactions for
formation of carbon–carbon bonds, especially for the development of biaryl and heterobiaryl
derivatives [32]. In the present work, the catalytic potentials of 4a–c were assessed for
Suzuki–Miyaura cross–coupling reactions. The Suzuki–Miyaura cross–coupling between
bromobenzene and phenylboronic acid was chosen as the model reaction (Scheme 2). The
reaction conditions were optimized by varying different parameters such as solvent, base,
temperature, time, palladium(II) source and catalyst load (Tables 3–8) using 4a–c. Reactions
in toluene with potassium carbonate at room temperature for 1 h using 1 mol% of 4b and 0.5
equivalent palladium(II) acetate as catalyst resulted in good to excellent yields.
The catalytic activity of 4b was investigated in conjugation with palladium(II) acetate for
Suzuki–Miyaura cross–coupling under the optimized reaction conditions between various
aryl halides and phenylboronic acid (Table 9). Interestingly, the electron–donating or
electron–withdrawing groups of the aryl halides studied did not affect the efficiency of the
catalyst. However, significant changes in activity were observed when steric bulk of the aryl
halides was considered. Additionally, aryl iodides proved to be the best as compared with
aryl bromides and aryl chlorides due to the respective bond strengths. Blank experiments
were conducted in the absence of palladium(II) acetate which resulted in trace yields.
Selectivity of the catalytic system was examined by carrying out the reactions in the absence
of aryl halides that yielded trace amounts of homo–coupled biaryl compounds exhibiting high
selectivity.
Suzuki–Miyaura cross–coupling is the palladium–catalyzed cross–coupling between
organoboronic acid and an organo–halide first reported by A. Suzuki et al. in 1979. The first
14

step is the oxidative addition of palladium(0) to the organo–halide followed by reaction with
base to give an intermediate. Meanwhile, the boronic acid is converted to a boronate complex
upon action by the base. The intermediate species previously formed undergoes
transmetalation with the boronate complex to form an organo–palladium species. The desired
cross–coupled product is obtained by reductive elimination and the original palladium
catalyst is retained and the catalytic cycle is completed.
3.5.2. Heck–Mizoroki cross–coupling reaction
Encouraged by appreciable catalytic potential of the catalytic system toward Suzuki–Miyaura
cross–coupling reactions, Heck–Mizoroki cross–coupling reactions were also assessed
(Scheme 3). The reaction conditions were optimized by varying parameters such as solvent,
base, temperature, time, palladium(II) source and catalyst load (Tables 10–15). It was
observed that reactions in toluene with sodium carbonate for 5 h using 1 mol% of 4b and 0.5
equivalent palladium(II) acetate as catalyst at 80 0C resulted in good yields.
The catalytic potential of 4b was studied in conjunction with palladium(II) acetate for Heck–
Mizoroki cross–coupling reactions under the optimized reaction conditions between various
aryl halides and styrene (Table 16). Similar to Suzuki–Heck coupling, the electron–donating
or electron–withdrawing groups of the aryl halides studied did not affect the efficiency of the
catalyst and activity varied marginally when steric bulk of the aryl halides was considered.
Again, aryl iodides performed better than aryl bromides and chlorides. Blank experiments
conducted in the absence of palladium(II) acetate resulted in trace yields and that without aryl
halides yielded trace amounts of homo–coupled biaryl compounds. Hence, the selected
catalytic system possessed appreciable catalytic potential toward Heck–Mizoroki cross–
coupling reactions with desirable selectivity.
15

Heck–Mizoroki cross–coupling reaction is the palladium–catalyzed cross coupling between
an alkene and an organo–halide, first reported by T. Mizoroki et al. in 1971. The first step is
the oxidative addition of palladium(0) to the organo–halide. Upon insertion of the alkene,
palladium(II) forms a π complex. A β–hydride elimination follows to yield the desired cross–
coupled product and the reductive elimination step retains the original palladium catalyst and
the catalytic cycle is completed.
16

3.5.3. Comparison of catalytic activity
To understand the activity of 1,3,4–trisubstituted–1,2,3–triazoliumioidide salt (4b) with
palladium(II) acetate, the yields obtained were compared with the yields reported for the
Suzuki–Miyaura [42-46] and Heck–Mizoroki [42, 44, 47-50] cross–coupling reactions with
similar kind of catalysts and the results were comparable. Some of the key benefits of 4b with
palladium(II) acetate are: (i) economical, (ii) good yields and (iii) fewer synthetic steps.
Additionally, 4b can be prepared using inexpensive chemicals under aerobic conditions, and
does not require a sophisticated setup.
4. Conclusion
We report a series of 1,3,4–trisubstituted–1,2,3–triazoliumiodide salts (4a–c) that were
characterized by 1H and 13C NMR, ATR–IR and elemental analysis. 4a–c were evaluated for
catalytic potential for Suzuki–Miyaura and Heck–Mizoroki cross–coupling reactions by in
situ generation of the corresponding palladium(II) complexes. 4b was chosen as the best
candidate for both reactions after a thorough screening for optimized reaction conditions. The
catalytic system yielded excellent results for Suzuki–Miyaura cross–coupling reactions,
whereas those for Heck–Mizoroki cross–coupling reactions were moderate. However, the in
situ palladium(II) complex generation route omits an additional synthesis of the palladium(II)
complexes while still demonstrating satisfactory results. Additionally, the 1,3,4–
trisubstituted–1,2,3–triazolium iodide salts (4a–c) can be stored for future use which makes
the process more economical.
Supporting information
Additional Supporting Information may be found online in the supporting information tab for
this article.
Acknowledgement
17

Siddappa A. Patil thanks DST–SERB, India [SERB/F/1423/2017–18 (File No.
YSS/2015/000010)] and Jain University, India for financial support.
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20

Graphical abstract



Good yields
Economical
Reduction of
synthetic step
C6
C5
Cl1
N2
N3
C8
C1
C4
N1
C2
C3
C14
C9
C7
C13
C10
C12
C1
1
21

Scheme 1. The synthesis of 1,3,4–trisubstituted–1,2,3–triazolium iodide salts (4a–c).
22

Scheme 2. General reaction scheme for Suzuki–Miyaura cross–coupling reaction.
23

4a-c, Pd-source
Br
+
solvent, base
Scheme 3. General reaction scheme for Heck–Mizoroki cross–coupling reactions.
24

N3
N2
N1
C14
C13
C6
C8
C9
C1
C5
C7
C12
Cl1
C2
C10
C4
C11
C3
Figure 1. Molecular structure of 3b. Thermal ellipsoids are drawn at the 50% level.
25

Figure 2. Molecular interactions of 3b.
26

27

Table 1. Crystallographic data and the structure refinement details of 3b.
Identification code
Empirical formula
Formula weight
Temperature
3b
C14 H10 N3 Cl
255.70
100(2) K
Wavelength
1.54184 Å
Triclinic
Crystal system
Space group
P–1 (#2)
Unit cell dimensions
a = 5.65213(8) Å
b = 7.2020(1) Å
c = 14.5498(2) Å
α = 102.268(2)°.
β = 98.486(1)°.
γ = 90.119(1)°.
3
Volume
572.070(15) Å
2
Z
3
Density (calculated)
Absorption coefficient
F(000)
1.484 Mg/m
–1
2.805 mm
264
3
Crystal size
0.135 x 0.112 x 0.030 mm
6.293 to 76.939°.
Theta range for data
collection
Index ranges
–7<=h<=7, –9<=k<=9, –18<=l<=18
Reflections collected
Independent reflections
Completeness to theta =
67.684°
22625
2393 [R(int) = 0.0329]
100.0 %
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Gaussian
0.923 and 0.745
2
Full–matrix least–squares on F
2393 / 0 / 164
2
Goodness–of–fit on F
1.048
Final R indices [I>2sigma(I)] R1 = 0.0291, wR2 = 0.0697
R indices (all data)
R1 = 0.0327, wR2 = 0.0720
Extinction coefficient
Largest diff. peak and hole
0.0025(4)
–3
0.276 and –0.278 e.Å
Table 2. Pertinent bond distances and angles of 3b.
Bond distance (Å)
Bond angle ()
28

C(1)–N(1)
N(1)–C(7)
N(1)–N(2)
N(2)–N(3)
N(3)–C(8)
C(7)–C(8)
C(8)–C(9)
C(4)–Cl(1)
1.4279(16) C(7)–N(1)–N(2)
1.3534(17) C(7)–N(1)–C(1)
1.3550(15) N(2)–N(1)–C(1)
1.3119(16) N(3)–N(2)–N(1)
1.3718(17) N(2)–N(3)–C(8)
1.3753(18) N(1)–C(7)–C(8)
1.4683(18) N(3)–C(8)–C(7)
1.7402(13) N(3)–C(8)–C(9)
110.82(10)
129.44(11)
119.70(10)
107.25(10)
109.02(11)
104.71(11)
108.20(11)
121.68(12)
29