Well-Defined N-Heterocyclic Carbene-Palladium Complexes as Efficient Catalysts for Domino Sonogashira Coupling/Cyclization Reaction and C-H bond Arylation of Benzothiazole

Article abstract of DOI:10.1002/aoc.4936

Well-defined and air-stable PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed palladium bis-N-heterocyclic carbene complexes have been developed for the domino Sonogashira coupling/cyclization reaction of 2-iodophenol with a variety of terminal alkynes and C-H bond arylation of benzothiazole with aryl iodides. The PEPPSI themed palladium complexes, 2a and 2b were synthesized in good yields from the reaction of corresponding imidazolium salts with PdCl₂ and K₂CO₃ in pyridine. The new air-stable palladium-NHC complexes were characterized by NMR spectroscopy, X-ray crystallography, elemental analysis, and mass spectroscopy studies. The PEPPSI themed palladium(II) bis-N-heterocyclic carbene complexes 2a and 2b exhibited excellent catalytic activities for domino Sonogashira coupling/cyclization reaction of 2-iodophenol with terminal alkynes yielding benzofuran derivatives. In addition, the palladium complexes, 2a and 2b successfully catalyzed the direct C-H bond arylation of benzothiazole with aryl iodides as coupling partners in presence of CuI as co-catalyst.

Full text of DOI:10.1002/aoc.4936

Received: 23 January 2019
DOI: 10.1002/aoc.4936
Revised: 14 March 2019
Accepted: 16 March 2019
F U L L P A P E R
Well‐Defined N‐Heterocyclic Carbene‐Palladium Complexes
as Efficient Catalysts for Domino Sonogashira
Coupling/Cyclization Reaction and C‐H bond Arylation of
Benzothiazole
Seema Yadav1 | Ajeet Singh2
Chandrakanta Dash1
| Isha Mishra1 | Sriparna Ray3 | Shaikh M. Mobin2
|
1 Department of Chemistry, School of
Chemical Sciences and Pharmacy, Central
University of Rajasthan, Bandarsindri,
India
2 Department of Chemistry, Indian
Institute of Technology (IIT) Indore,
Simrol, Khandwa Road, Indore 453552,
India
Well‐defined and air‐stable PEPPSI (Pyridine Enhanced Precatalyst Preparation
Stabilization and Initiation) themed palladium bis‐N‐heterocyclic carbene com-
plexes have been developed for the domino Sonogashira coupling/cyclization
reaction of 2‐iodophenol with a variety of terminal alkynes and C‐H bond
arylation of benzothiazole with aryl iodides. The PEPPSI themed palladium
complexes, 2a and 2b were synthesized in good yields from the reaction of
corresponding imidazolium salts with PdCl2 and K2CO3 in pyridine. The new
air‐stable palladium‐NHC complexes were characterized by NMR spectroscopy,
X‐ray crystallography, elemental analysis, and mass spectroscopy studies.
The PEPPSI themed palladium(II) bis‐N‐heterocyclic carbene complexes 2a
and 2b exhibited excellent catalytic activities for domino Sonogashira
coupling/cyclization reaction of 2‐iodophenol with terminal alkynes yielding
benzofuran derivatives. In addition, the palladium complexes, 2a and 2b success-
fully catalyzed the direct C‐H bond arylation of benzothiazole with aryl iodides
as coupling partners in presence of CuI as co‐catalyst.
3 Department of Chemistry, School of
Basic Sciences, Manipal University Jaipur,
Jaipur, Rajasthan 303007, India
Correspondence
Chandrakanta Dash, Department of
Chemistry, School of Chemical Sciences
and Pharmacy, Central University of
Rajasthan, Bandarsindri, India.
Email: ckdash@curaj.ac.in
Funding information
Science and Engineering Research Board,
Grant/Award Number: YSS/2014/000054;
DST‐FIST, Grant/Award Number: SR/
FST/CSI‐257/2014(c); UGC‐BSR Start‐Up
research grant, Grant/Award Number:
F4.4‐5/2006(BSR
KEYWORDS
benzofuran derivatives, C‐H arylation, N‐heterocyclic carbene, palladium, PEPPSI
1 | INTRODUCTION
catalytic activities towards various carbon–carbon and
carbon‐heteroatom coupling reactions,[2] since its first
N‐heterocyclic carbene (NHC) based palladium complex
catalyzed C‐C and C‐X (X = heteroatom) bond forming
reactions is one of the most important classes of organome-
tallic reactions for making building blocks of bioactive
molecules.[1] Among the various N‐heterocyclic carbene
palladium complexes, PEPPSI (Pyridine Enhanced
Precatalyst Preparation Stabilization and Initiation)
themed Pd‐NHC complexes have shown remarkable
report by Michael G. Organ in 2006.[3] In view of their
increasing popularity in catalytic C‐C and C‐X bond
forming reactions in air, developing well‐defined
efficient catalysts with low palladium loadings remains a
significant challenge. As one of our objective is to design
new well‐defined catalysts for carbon–carbon and
carbon‐heteroatom bond forming reactions,[4] we became
interested in exploring the chemistry of well‐defined bis‐
Appl Organometal Chem. 2019;e4936.
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https://doi.org/10.1002/aoc.4936

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YADAV ET AL.
NHC based homodinuclear transition metal complexes
for the most challenging tandem and C‐H bond
functionalization reactions.
Benzofuran is an important framework for many
natural products and pharmaceutical candidates, which
exhibit a wide range of biological activities.[5] Various
approaches have been developed for making the
benzofuran framework.[6] Palladium catalyzed domino
Sonogashira coupling reaction and intramolecular
cyclization is one of the well‐developed methods.[7]
Though various reports have been published on this type
of reactions by transition metal catalysts, there are
few instances of using well‐defined catalysts in the
FIGURE 1 Palladium complexes of bis‐NHC ligands
b, 7c,
8
literature.6a,
Ghosh and co‐workers recently
reported PEPPSI themed well‐defined 1,2,4‐triazole
derived NHC based palladium complexes for one‐pot tan-
dem Heck alkynylation/cyclization sequence (also known
as domino Sonogashira coupling/cyclization reaction) for
preparing biologically relevant benzofuran compounds.[9]
The binuclear palladium complexes were observed as
more efficient catalysts than their monometallic counter-
parts due to the presence of two separate independent
catalytic sites without displaying any cooperativity effect
in the catalytic cycle. In this context, we became inter-
ested in exploring the catalytic activity of imidazole‐based
PEPPSI themed binuclear palladium complexes for
domino Sonogashira coupling/cyclization reaction of 2‐
iodophenol with terminal alkynes.
their catalytic applications for domino Sonogashira
coupling/cyclization reaction sequence to afford biologi-
cally relevant benzofuran framework under copper free
condition. Furthermore, we also explored the catalytic
activity of the palladium complexes, 2a and 2b in C‐H bond
arylation of benzothiazole by using aryl iodides.
2 | RESULTS AND DISCUSSION
2.1 | Synthesis and characterization of
PEPPSI themed Pd‐NHC complexes
The transition‐metal‐catalyzed direct C‐H bond
arylation of benzothiazoles[10] is one of the most conve-
nient processes to synthesize the building blocks of several
biological and pharmaceutical compounds like thioflavin‐
T, TEI‐6720, etc.[11] A variety of ligand supported transi-
tion metal catalysts have been reported for the selective
C‐2 arylation of benzothiazoles with aryl halides to achieve
2‐arylated benzothiazole derivatives.[12] Surprisingly,
PEPPSI themed palladium precatalysts for the arylation
of benzothiazole unit with aryl halides were not reported
till date, whereas the arylation of other heterocyclic com-
pounds like imidazoles, 2‐substituted thiophene deriva-
tives, and 1‐methylpyrrole‐2‐carboxaldehyde with aryl
halides have been studied extensively.2n, p, 13 Furthermore,
mechanistic details of palladium catalyzed arylation of
benzothiazoles by Punji and co‐workers revealed the
importance of σ‐donor ligands on palladium center, which
will enhance the oxidative addition of aryl halide electro-
philes and stabilized the Pd (IV) intermediate.[14] Based
on the above findings we have explored the catalytic activ-
ity of our palladium complexes (having strong σ‐donor
ligands such as N‐heterocyclic carbene) for C‐H bond
arylation of benzothiazole at low catalyst loading.
PEPPSI‐themed palladium(II) N‐heterocyclic carbene
complexes, 2a & 2b were prepared using the methodology
developed by Organ and co‐workers.[3] The bis‐NHC
ligand[15] (1a or 1b) was reacted with PdCl2 in pyridine
and K2CO3 (5 equiv.) at 120 °C for 24 hrs (Scheme 1).
After purification, the complexes were obtained as a pale
1
yellow powder in 69–78% yield. H‐NMR spectra of 2a
and 2b showed the loss of the imidazolium proton
(NCHN) peak. This suggests the formation of the
palladium‐NHC complexes. The 13C{1H} spectra of palla-
dium complexes showed a peak at around 149.8 ppm
(2a) and 150.6 ppm (2b), and this peak corresponds to
the Pd‐Ccarbene carbon peak.
The molecular structure of PEPPSI themed palladium
(II) complexes, 2a and 2b was determined by X‐ray dif-
fraction studies (Figures 2 and 3). The structure of the
palladium complexes 2a and 2b revealed the bimetallic
complexes and its metal centers displaying square‐planar
geometry as expected in d8 configuration. The molecular
structures of 2a and 2b were depicted in Figures 2 and 3
respectively. The average Pd‐Ccarbene bond distances in
2a [1.964(6) Å] and 2b [1.963(6) Å] are slightly smaller
than the sum of their individual covalent radii of
palladium and carbon atoms (2.12 Å),[16] as well as also
comparable to the reported complexes namely, [1,1′‐
Here in, we describe the synthesis of binuclear PEPPSI‐
themed palladium complexes, 2a and 2b (Figure 1) and

YADAV ET AL.
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SCHEME 1 Synthesis of PEPPSI
themed palladium complexes
FIGURE 2 Molecular structure of 2a, ellipsoids are shown at 30% probability level
diisopropyl‐4,4′‐ethylenedi‐1,2,4‐triazoline‐5,5′‐diylidene]
Pd2Br4(NC5H5)2 [av.1.954(6) Å], and [1,1′‐diisopropyl‐
4,4′‐propylenedi‐1,2,4‐triazoline‐5,5′‐diylidene]Pd2Br4(N
C5H5)2 [av.1.964(7) Å].[9] The average Pd‐Npyridine bond
distances in 2a [2.111(5) Å] and 2b [2.100(6) Å] are also
comparable to the reported PEPPSI themed complexes
namely, [1,1′‐diisopropyl‐4,4′‐ethylenedi‐1,2,4‐triazoline‐
5,5′‐diylidene]Pd2Br4(NC5H5)2 [av.2.083(5) Å], and [1,1′‐
diisopropyl‐4,4′‐propylenedi‐1,2,4‐triazoline‐5,5′‐diyliden
e]Pd2Br4(NC5H5)2 [av.2.080(6) Å].[9] Selected bond dis-
tances and bond angles are shown in Table 1.
complexes 2a and 2b for domino Sonogashira
coupling/cyclization reaction. To optimize the reaction
conditions, we first screened various bases and
solvents using 2a as a catalyst for domino Sonogashira
coupling/cyclization reaction of 2‐iodophenol and
phenylacetylene. We found that the use of K3PO4 and
DMSO provided the best condition at 90 °C for com-
plete conversion with 0.001 mmol (0.2 mol %) catalyst
loading in 16 hrs in air. In all other conditions, the
cyclized product 3 was obtained along with a by‐product
4 (Glaser coupling)[17] as shown in Table 2.
Using the optimized condition (entry 4, Table 2), several
substituted benzofuran derivatives were synthesized in
excellent yield from commercially available different
substituted terminal alkynes and 2‐iodophenol (Table 3).
Only the reaction between 2‐iodophenol and propargyl
alcohol gave lower yields up to 63%. The two PEPPSI
themed palladium complexes were equally active for
the domino Sonogashira coupling/cyclization reaction
2.2 | Domino Sonogashira
coupling/cyclization reaction using
Pd‐NHC complexes
2‐iodophenol and phenylacetylene were used as a model
substrate to evaluate the catalytic activity of palladium

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YADAV ET AL.
FIGURE 3 wMolecular structure of 2b,
ellipsoids are shown at 30% probability
level
TABLE 1 Selected bond lengths (Ǻ) & bond angles (deg) in 2a and 2b
Bond lengths:
2a
1.971(6), 1.958(6)
2b
Pd‐Ccarbene
1.949(6), 1.977(6)
2.115(6), 2.085(6)
Pd‐Npyridine
Pd‐Cl1
2.107(5), 2.115(5)
2.2932(16), 2.3012(17)
2.2964(18), 2.2975(17)
2.3038(17), 2.3195(19)
2.3073(18), 2.286(2)
Pd‐Cl2
Bond angles:
Ccarbene‐Pd‐Npyridine
Cl1‐Pd‐Cl2
176.2(2), 177.1(2)
177.99(7), 177.91(7)
88.72(16), 87.67(18)
90.74(16), 90.40(18)
179.7(2), 178.2(3)
174.36(7), 175.76(10)
88.24(17), 93.08(19)
88.32(17), 87.15(19)
Ccarbene‐Pd‐Cl1
Ccarbene‐Pd‐Cl2
sequence with 0.2 mol% of catalyst loading. Lower yield
was observed by decreasing the catalyst loading from
0.2 mol% to 0.1 mol%. The catalyst 2a also showed com-
plete conversion with a low catalyst loading of 0.05 mol%
but a relatively higher temperature of 130 °C.
The catalytic activities of 2a and 2b complexes are com-
parable to the other reported catalysts, both well‐defined
and in‐situ palladium catalysts for domino Sonogashira
coupling/cyclization reaction (Supporting information,
Table S7). To our knowledge, only two reports have been
published on well‐defined PEPPSI themed palladium‐
NHC catalysts for domino Sonogashira coupling/
cyclization reaction. In 2009, Peris et al. reported a series
of NHC‐Pd‐pyridine complexes (NHC = 1,2,4‐trimethyl
triazolyldiylidene, 1,3‐dimethylimidazolylidene, and 1,4‐
coupling/cyclization reaction.7c It has been mentioned
that the bimetallic palladium PEPPSI themed precatalyst
having 1,2,4‐trimethyltriazolyldiylidene based N‐hetero-
cyclic carbene ligand showed excellent conversion with
1 mol% of catalyst loading in 8 hrs of reaction time at
80 °C. Ghosh et al. also reported the same tandem reaction
by a series of PEPPSI themed precatalysts and mixed
phosphine‐NHC based palladium catalysts.[9] Interest-
ingly, the mixed phosphine‐NHC palladium catalysts
showed the better result and product conversion up to
81% in 4 hrs of reaction time with 1 mol% catalyst loading
at temperature 80 °C. In this context, our PEPPSI themed
precatalysts 2a and 2b, show up to 85% conversion of
benzofuran product using 0.2 mol% (0.001 mmol) catalyst
loading in 4 hrs at 90 °C. To avoid separation and isolation
of desired products by column chromatography, the
dimethyltriazolylidene)
for
domino
Sonogashira

YADAV ET AL.
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TABLE 2 Optimization of the reaction conditions for synthesis of 2‐arylbenzofurana
% Yieldb
3
Catalyst Loading
(mol %)
Time
(h)
Entry
Solvent
Base
4
1
2a (0.2)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
Et3N
16
16
4
16
11
6
2
2a (0.2)
2a (0.2)
2a (0.2)
2a (0.05)
2a (0.1)
2a (0.2)
2a (0.2)
2a (0.2)
2a (0.2)
2a (0.2)
2a (0.2)
PdCl2 (0.4)
‐
KOH
43
3
K3PO4
K3PO4
K3PO4
K3PO4
Cs2CO3
Cs2CO3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
85
>99(97)c
0
4
16
16
16
16
16
16
16
16
16
16
16
16
0
5
10
0
6
47
0
7
95
5
8
72
15
13
16
12
nr
0
9
DMF
84
10
11
12
13
14
15e
1,4‐Dioxane
EtOH
15
31
Toluene
DMSO
DMSO
DMSO
nr
60
nrd
nr
2
2a (0.05)
98
aReaction conditions:‐2‐iodophenol (0.50 mmol), phenylacetylene (0.60 mmol), 0.2 mol % (0.001 mmol) of catalyst, Base (1.00 mmol), and solvent (3 ml).
bThe yields (%) were determined by GC using 1,3,5‐trimethoxybenzene as an internal standard.
cIsolated yield.
dWithout catalyst.
eAt higher temperature 130 °C, nr = No reaction.
TABLE 3 Selected results for domino Sonogashira coupling/cyclization reaction of 2‐iodophenol and terminal alkynes catalyzed by 2a or 2ba
aReaction conditions: 2‐Iodophenol (0.50 mmol), alkyne (0.60 mmol), 0.001 mmol (0.2 mol %) of 2a or 2b, K3PO4 (1.00 mmol), DMSO (3 ml), 90 °C, 16 hrs.
Yields refer to isolated products.
reaction was continued up to 16 hrs for complete conver-
sion of the benzofuran derivatives. As for the other transi-
tion metal based well‐defined catalysts, a copper (II) pincer
complex catalyzed domino Sonogashira coupling/
cyclization reaction with low catalyst loading 0.15 mol %
yielded good to excellent product conversion, but at high
temperature (130 °C).6a The catalytic activities of 2a and
2b complexes are among the best results reported to date

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YADAV ET AL.
for domino Sonogashira coupling/cyclization reaction in
presence of relatively weak base K3PO4.
–NO2 and –CF3 on aryl iodides, were well tolerated,
producing the corresponding arylated benzothiazole
product in good to excellent yields (Table 5). However,
in the case of aryl bromides such as 4‐bromoace-
tophenone, 4‐bromoanisole and 4‐bromotoluene, the
arylated products were obtained in a much lower yield
of 8–20%.
2.3 | C‐H bond arylation of benzothiazole
using Pd‐NHC complexes
The catalytic activity comparison of 2a and 2b com-
plexes with the other reported catalysts is notable. To
our knowledge, PEPPSI themed palladium‐NHC
Binuclear PEPPSI‐themed palladium complexes, 2a and
2b were also applied for C‐H bond arylation of
benzothiazole with various aryl iodides. Benzothiazole
and 4‐iodoanisole were taken as model reactants for
optimization of reaction conditions in various base, sol-
vent and effect of additives with catalyst loading 1 mol
% (0.005 mmol) of 2a (Table 4). The reaction of
benzothiazole with 4‐iodoanisole in the presence of
LiOtBu as the base at 120 °C in DMF, yielding the
desired arylated product in 18 hrs of the reaction time
was the best condition. Separate experiments were per-
formed in the absence of palladium complex and also
with CuI. No arylated product was observed in both
the control experiments.
TABLE 5 C‐H bond arylation of benzothiazole catalyzed by 2a
and 2ba
After having optimized the protocol in hand, we
explored the substrate scope of aryl iodides for direct
arylation of benzothiazole. Electron donating groups
such as –CH3, –OMe, and electron withdrawing groups
aReaction conditions: Benzothiazole (0.50 mmol), aryl iodide (0.60 mmol),
0.005 mmol (1 mol %) of 2a or 2b, CuI (0.01 mmol) LiOtBu (1.00 mmol),
DMF (3 ml), 120 °C, 18 hrs. Yields refer to isolated products.
TABLE 4 Optimization of Pd catalyzed C‐H bond arylation of benzothiazolea
Catalyst
Loading
(mol%)
Yieldb
(%)
Entry
Additive
solvent
base
1
2a (1.0)
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
‐
DMF
K3PO4
65
80
2
2a (1.0)
2a (1.0)
2a (1.0)
2a (1.0)
2a (1.0)
2a (1.0)
2a (1.0)
‐
DMF
Cs2CO3
LiOtBu
Na2CO3
LiOtBu
LiOtBu
K3PO4
3
DMF
DMF
94(91)c
4
59
5
1,4‐dioxane
toluene
DMAc
DMF
87
6
62
7
47
8
KOtBu
LiOtBu
LiOtBu
LiOtBu
61
9
DMF
nrd
nre
52
10
11
2a (1.0)
PdCl2 (2.0)
DMF
CuI
DMF
aReaction conditions: Benzothiazole (0.50 mmol), 4‐iodoanisole (0.60 mmol), 0.005 mmol (1 mol %) of 2a catalyst, CuI (0.01 mmol), base (2 mmol) and solvent
(3 ml), 18 hr, 120 °C.
bThe yields (%) were determined by NMR using 1,3,5‐trimethoxybenzene as an internal standard.
cIsolated yield.
dWithout catalyst..
eWithout CuI. nr = No reaction.

YADAV ET AL.
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catalyzed direct arylation of benzothiazole unit by aryl
iodides was not reported. Huang et al. reported an effi-
cient Pd/Cu cocatalytic system for the arylation of
azoles by aryl halides and aryl triflates, with low cata-
lyst loadings 0.25 mol% of palladium complex,
deuterated solvent.[19] 1H NMR peaks are labeled as sin-
glet (s), doublet (d), triplet (t), multiplet (m), and septet
(sept). Elemental Analysis was carried out on Thermo
Quest FLASH 2000 SERIES (CHNS) Elemental Analyzer.
Mass spectrometry measurements were done on a
Micromass Q‐Tof spectrometer. All catalytic reactions
were monitored on a Thermo Fisher Trace 1300 series
GC system by GC‐FID with a TG‐IMS column of 30 m
length, 0.25 mm diameter and 0.25 μm film thickness.
X‐ray diffraction data for compounds 2a and 2b were col-
lected on an Oxford Diffraction XCALIBUR‐S diffractom-
eter. The structures were solved using direct methods and
standard difference map techniques and were refined by
full‐matrix least‐squares procedures on F2 with
SHELXTL (Version 6.10).[20] CCDC‐1877336 (2a) and
CCDC‐1877335 (2b) contain the supplementary crystallo-
graphic data for this paper. The data can be obtained free
of charge at www.ccdc.cam.ac.uk/conts/retrieving.html.
dichlorobis
(chloro‐di‐tert‐butylphosphine)palladium
and 1 mol% Cu (Xantphos)I complex.12a It was observed
that phenyl iodide reacts slowly compared to other cou-
pling partners such as activated aryl chlorides and phe-
nyl triflates. Punji et al. reported the phosphine‐free
(quinolinyl)amido‐based pincer palladium complexes
for C‐H bond arylation of benzothiazoles at 0.5 mol %
catalyst loading in DMSO in the presence of K2CO3 as
the base in 20–89% yield.10c Several phosphine based
well‐defined catalysts also have been reported in the lit-
erature which shows excellent conversions for the
arylation of azoles with aryl iodides by using 0.25–
0.5 mol% of the catalyst loading (Supporting Informa-
tion, Table S8).12c, d, 18
4.2 | Materials
3 | CONCLUSIONS
NHC ligands were synthesized using literature
procedures.[15] PdCl2, K2CO3, K3PO4, pyridine, LiOtBu,
CuI, 2‐iodophenol, phenylacetylene, 4‐ethynyltoluene, 3‐
ethynyltoluene, 4‐ethylphenylacetylene, 4‐tert‐butylphe
nylacetylene, 4‐ethynylanisole, propargyl alcohol,
benzothiazole, iodobenzene, 4‐iodotoluene, 2‐iodoto
luene, 4‐iodoanisole, 1‐iodo‐3‐nitrobenzene, and 4‐iodo
benzotrifluoride were purchased from commercial sup-
pliers and used as received.
In summary, we have synthesized a new series of well‐
defined and air‐stable PEPPSI themed palladium(II) bis‐
N‐heterocyclic carbene complexes. All the PEPPSI
themed palladium complexes were fully characterized by
NMR spectroscopy, elemental analysis, Mass spectros-
copy, and X‐ray crystallography. The palladium com-
plexes, 2a and 2b have been employed as precatalysts in
domino Sonogashira coupling/cyclization reaction as well
as C‐H bond arylation of benzothiazole. The cata-
lysts were found to be extremely effective for domino
Sonogashira/cyclization reaction at low palladium load-
ings of 0.2 mol %. For C‐H bond arylation of benzo-
thiazole using aryl iodides as coupling partner, the
catalysts show good to excellent conversion with 1 mol
% of catalyst loading. This work further illustrated the
potential of the PEPPSI themed palladium complexes as
an effective catalyst for C‐C and C‐heteroatom bond
forming reactions.
4.3 | Syntheses
4.3.1 | Synthesis of Pd‐PEPPSI precatalyst
(2a)
A 100 ml round bottom flask was charged with 1a
ligand (0.170 g, 0.281 mmol), PdCl2 (0.100 g,
0.563 mmol) and K2CO3 (0.388 g, 2.81 mmol) and
5 ml of pyridine. The mixture was heated at 120 °C
for 24 hrs. The reaction mixture was cooled down to
room temperature and CuSO4 solution was added (to
remove unbound pyridine), and the organic component
was extracted with ethyl acetate (3 x 20 ml). The
organic layer was dried over anhydrous MgSO4, and
the solvent was removed in a rotary evaporator. The
resulting crude product was then purified by column
chromatography (95:5, CH2Cl2:CH3OH) to afford the
4 | EXPERIMENTAL SECTION
4.1 | General procedures
All manipulations were carried out using standard
Schlenk techniques. Solvents were purified and degassed
by standard procedures. 1H and 13C{1H} NMR spectra
were recorded in CDCl3 on a Bruker 500 MHz NMR spec-
trometer. Proton and carbon chemical shifts are reported
in ppm and referenced using the residual proton
(7.26 ppm) and carbon signals (77.16 ppm) of the
1
desired product 2a as a yellow solid (0.229 g, 78%). H
NMR (500 MHz, CDCl3): δ 8.72 (m, 4H, NC5H5), 7.65
(t, 2H, JHH = 7.5 Hz, NC5H5), 7.46 (d, 2H, JHH = 7.5 Hz),
7.34 (t, 2H, JHH = 7.5 Hz), 7.27–7.21 (m, 7H), 6.99 (s,

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YADAV ET AL.
4H), 6.77 (s, 2H), 4.72 (t, 4H, JHH = 6.5 Hz, CH2), 4.07
(s, 2H, CH2Ph), 3.58 (t, 4H, JHH = 6.5 Hz, CH2), 2.35(s,
6H, p‐CH3C6H2), 2.22 (s, 12H, o‐CH3C6H2). 13C{1H}
NMR (125 MHz, CDCl3): δ 151.4, 149.8, 139.3, 139.2,
137.9, 136.4, 135.0, 129.4, 129.3, 128.6, 127.5, 124.4,
123.6, 123.5, 60.6, 55.5, 49.7, 21.3, 19.0 ppm. MS (ESI):
m/z 849.6 ([M – (py2 + Cl)]+), Calcd: m/z 850.0. Anal.
Calcd for C45H51Cl4N7Pd2•0.5 NC5H5: C, 52.62; H,
4.97; N, 9.69; Found: C, 52.67; H, 5.18; N, 8.69.
organic layer was further extracted with EtOAc (ca. 3 x
20 ml). The organic layers were combined and vacuum
dried to obtain a crude product that was subsequently
purified by column chromatography. In the case of com-
plete conversion, the crude product was taken directly
as it is for NMR characterization.
4.4.1 | 2‐Phenylbenzo[b]furan (3aa)7a, 9
4.3.2 | Synthesis of Pd‐PEPPSI precatalyst
1H NMR (500 MHz, CDCl3):
δ 7.89 (d, 2H,
(2b)
JHH = 7.5 Hz), 7.60 (d, 1H, JHH = 7.5 Hz), 7.55 (d,
1H, JHH = 8.0 Hz), 7.47 (t, 2H, JHH = 8.0 Hz), 7.37 (t,
1H, JHH = 8.0 Hz), 7.31–7.24 (m, 2H), 7.05 (s, 1H).
13C{1H} NMR (125 MHz, CDCl3): δ 156.0, 155.0, 130.6,
129.3, 128.9, 128.7, 125.1, 124.4, 123.1, 121.0, 111.3,
101.4 ppm.
A 100 ml round bottom flask was charged with 1b
ligand (0.194 g, 0.281 mmol), PdCl2 (0.100 g,
0.564 mmol) and K2CO3 (0.388 g, 2.81 mmol) and
3 mL of pyridine. The mixture was heated at 120 °C
for 24 hrs. The reaction mixture was cooled down to
room temperature and CuSO4 solution was added (to
remove unbound pyridine), and the organic component
was extracted with ethyl acetate (3 x 20 ml). The
organic layer was dried over anhydrous MgSO4, and
the solvent was removed in a rotary evaporator. The
resulting crude product was then purified by column
chromatography (95:5, CH2Cl2:CH3OH) to afford the
4.4.2 | 2‐(p‐tolyl)benzofuran (3ab)[21]
1H NMR (500 MHz, CDCl3): δ 7.84–7.82 (m, 2H), 7.64–
7.58 (m, 2H), 7.34–7.30 (m, 4H), 7.03 (s, 1H), 2.46 (s, 3H).
13C{1H} NMR (125 MHz, CDCl3): δ 156.3, 154.9, 138.7,
129.6, 129.3, 127.9, 125.0, 124.1, 123.0, 120.9, 111.2,
100.7, 21.5 ppm.
1
desired product 2b as a yellow solid (0.219 g, 69%). H
NMR (CDCl3, 500 MHz): δ 8.71 (m, 4H, NC5H5), 7.66
(t, 2H, JHH = 7.5 Hz, NC5H5), 7.48 (t, 4H, JHH = 8 Hz),
7.35–7.29 (m, 8H), 7.24–7.22 (m, 5H), 6.85–6.84 (m, 2H),
4.73 (t, 4H, JHH = 6.5 Hz, CH2), 4.07 (s, 2H, CH2Ph),
3.63 (t, 4H, JHH = 6.5 Hz, CH2), 2.87 (sept, 4H,
JHH = 7.0 Hz, CH (CH3)2), 1.36 (d, 12H, JHH = 7.0 Hz,
CH (CH3)2), 1.01 (d, 12H, JHH = 7.0 Hz, CH (CH3)2).
13C{1H} NMR (125 MHz, CDCl3): δ 151.3, 150.6, 147.0,
137.8, 134.6, 130.3, 129.3, 128.6, 127.4, 125.1, 124.4,
124.0, 123.0, 60.7, 55.7, 49.8, 28.5, 26.5, 23.1 ppm. MS
(ESI): m/z 935.7 ([M – (py2 + Cl)]+), Calcd: m/z 934.1.
Anal. Calcd for C51H63Cl4N7Pd2•0.5NC5H5: C, 55.00;
H, 5.65; N, 8.99; Found: C, 55.13; H, 5.28; N, 8.77.
4.4.3 | 2‐m‐Tolylbenzofuran (3 ac)7b
1H NMR (500 MHz, CDCl3): δ 7.74 (s, 1H), 7.71–7.70
(m, 1H), 7.62–7.60 (m, 1H), 7.57–7.55 (m, 1H), 7.38–7.36
(m, 1H), 7.35–7.29 (m, 1H), 7.28–7.25 (m, 1H), 7.24–7.19
(m, 1H), 7.03 (s, 1H), 2.46 (s, 3H). 13C{1H} NMR
(125 MHz, CDCl3): δ 156.2, 154.9, 138.5, 130.5, 129.5,
129.4, 128.8, 125.6, 124.3, 123.0, 122.2, 121.0, 111.2,
101.3, 21.6 ppm.
4.4 | General procedure for domino
Sonogashira coupling/cyclization reaction
4.4.4 | 2‐(4‐Ethylphenyl)benzofuran
(3ad)8b
In a typical run, performed in air, a 25 ml of round bot-
tom flask was charged with a mixture of 2‐iodophenol,
terminal alkyne, and K3PO4 in a molar ratio 1:1.2:2. A
palladium complex (2a or 2b, 0.001 mmol) was added to
the mixture, followed by DMSO (ca. 3 ml) as solvent,
and then the reaction mixture was heated at 90 °C for
16 hrs. The reaction mixture was cooled to room temper-
ature, and water (ca. 20 ml) was added. The resulting
mixture was extracted with EtOAc (ca. 50 ml). The
1H NMR (500 MHz, CDCl3):
δ 7.86 (d, 2H,
JHH = 8.0 Hz), 7.63 (d, 1H, JHH = 7.5 Hz), 7.59 (d, 1H,
JHH = 8.0 Hz), 7.35–7.34 (m, 3H), 7.33–7.28 (m, 1H),
7.03 (s, 1H), 2.75 (q, 2H, JHH = 11.2 Hz), 1.34 (t, 3H,
JHH = 8.0 Hz). 13C{1H} NMR (125 MHz, CDCl3): δ

YADAV ET AL.
8 of 10
156.2, 154.8, 145, 129.4, 128.3, 128.0, 125.0, 124.0, 122.9,
120.8, 111.1, 100.6, 28.8, 15.5 ppm.
with EtOAc (ca. 3 x 30 ml). The organic layers were com-
bined and vacuum dried to obtain a crude product
1
(Supporting Information Figure S38 for H NMR data of
crude product) that was subsequently purified by column
chromatography and the product was obtained as
colourless solid (0.883 g, 98%).
4.4.5 | 2‐(4‐tert‐Butylphenyl)benzofuran
(3ae)6a
1H NMR (500 MHz, CDCl3):
δ 7.81 (d, 2H,
4.6 | General procedure for C‐H bond
JHH = 8.0 Hz), 7.58 (d, 1H, JHH = 7.5 Hz), 7.53 (d, 1H,
JHH = 8.0 Hz), 7.48 (d, 2H, JHH = 8.0 Hz), 7.29–7.22 (m,
2H), 6.99 (s, 1H), 1.37 (s, 9H). 13C{1H} NMR (125 MHz,
CDCl3): δ 156.3, 154.9, 151.9, 129.5, 127.8, 125.8, 124.8,
124.1, 122.9, 120.8, 111.2, 100.8, 34.9, 31.4 ppm.
arylation of benzothiazole
In a typical run, a 25 ml of round bottom flask was
charged with a mixture of benzothiazole (0.50 mmol),
aryl iodides (0.60 mmol) and LiOtBu (1.00 mmol) in
DMF (3 ml). Palladium complex (2a or 2b, 0.005 mmol)
and CuI (0.01 mmol) were added to the mixture, and then
the reaction mixture was heated at 120 °C for 18 hrs in an
inert atmosphere. The reaction mixture was cooled to
room temperature, and water (ca. 20 ml) was added.
The resulting mixture was extracted with EtOAc (ca.
50 ml). The organic layer was further extracted with
EtOAc (ca. 3 x 20 ml). The organic layers were combined
and vacuum dried to obtain a crude product that was sub-
sequently purified by column chromatography.
4.4.6 | 2‐(4‐Methoxyphenyl)benzofuran
(3af)7a, 9
1H NMR (500 MHz, CDCl3):
δ 7.83 (d, 2H,
JHH = 8.0 Hz), 7.59 (d, 1H, JHH = 7.0 Hz), 7.53 (d, 1H,
JHH = 8.0 Hz), 7.30–7.23 (m, 2H), 7.01 (d, 2H,
JHH = 8.0 Hz), 6.92 (s, 1H), 3.89 (s, 3H). 13C{1H} NMR
(125 MHz, CDCl3): δ 160.1, 156.2, 154.8, 129.6, 126.5,
123.9, 123.5, 123.0, 120.7, 114.4, 111.1, 99.8, 55.5 ppm.
4.6.1 | 2‐phenylbenzo[d]thiazole (5aa)[23]
4.4.7 | Benzofuran‐2‐ylmethanol (3ag)[9,22]
1H NMR (500 MHz, CDCl3): δ 8.12–8.09 (m, 3H), 7.92
(d, 1H, JHH = 8.0 Hz), 7.53–7.50 (m, 4H), 7.42–7.40 (m,
1H). 13C{1H} NMR (125 MHz, CDCl3): δ 168.2, 154.3,
135.2, 133.7, 131.1, 129.2, 127.7, 126.5, 125.3, 123.4,
121.8 ppm.
1H NMR (500 MHz, CDCl3):
δ 7.55 (d, 1H,
JHH = 7.5 Hz), 7.47 (d, 1H, JHH = 8.0 Hz), 7.31–7.22 (m,
2H), 6.66 (s, 1H), 4.78 (s, 2H). 13C{1H} NMR (125 MHz,
CDCl3): δ 156.5, 155.2, 128.2, 124.5, 122.9, 121.3, 111.4,
104.3, 58.3 ppm.
4.6.2 | 2‐p‐tolylbenzo[d]thiazole (5ab)[24]
4.5 | Synthesis of 2‐phenylbenzo[b]furan
from 2‐iodophenol and phenylacetylene in
gram scale
A
mixture of 2‐iodophenol (1.02 g, 4.63 mmol),
phenylacetylene (0.568 g, 5.56 mmol) and K3PO4 (1.96 g,
9.27 mmol) was taken in a 50 ml round bottom flask. Pal-
ladium complex 2a (0.0097 g, 0.009 mmol) was added to
the mixture, followed by DMSO (6 ml) as solvent. The
reaction mixture was heated at 90 °C for 16 hrs. The reac-
tion mixture was cooled to room temperature, and water
(ca. 100 ml) was added. The organic layer was extracted
1H NMR (500 MHz, CDCl3): δ 8.07 (d, 1H, JHH = 8.0 Hz),
8.00 (d, 2H, JHH = 8.0 Hz), 7.90 (d, 1H, JHH = 8.0 Hz), 7.49
(t, 1H, JHH = 7.5 Hz), 7.38 (t, 1H, JHH = 7.5 Hz), 7.31 (d,
2H, JHH = 7.5 Hz), 2.44 (s, 3H). 13C{1H} NMR (125 MHz,
CDCl3): δ 168.4, 154.3, 141.6, 135.1, 131.1, 129.9, 127.6,
126.4, 125.1, 123.2, 121.7, 21.7 ppm.

9 of 10
YADAV ET AL.
ACKNOWLEDGEMENT
4.6.3 | 2‐(2‐Methylphenyl)benzothiazole
(5 ac)10a
We are grateful for the financial support of the Science
and Engineering Research Board, Grant/Award Number:
YSS/2014/000054 and UGC‐BSR Start‐Up research grant
(Grant No. F4.4‐5/2006(BSR)). SY thanks Central Univer-
sity of Rajasthan for the research fellowship. Authors are
thankful to DST‐FIST (Grant No. SR/FST/CSI‐257/
2014(c)) for providing instrumental facilities.
1H NMR (500 MHz, CDCl3): δ 8.09 (d, 1H, JHH = 8.0 Hz),
7.96–7.87 (m, 3H), 7.50 (t, 1H, JHH = 8.0 Hz), 7.40–7.37 (m,
2H), 7.31 (d, 1H, JHH = 7.5 Hz), 2.46 (s, 3H). 13C{1H} NMR
(125 MHz, CDCl3): δ 168.5, 154.3, 139.0, 135.2, 133.7, 131.9,
129.1, 128.1, 126.4, 125.2, 125.0, 123.3, 121.7, 21.5 ppm.
ORCID
Ajeet Singh https://orcid.org/0000-0002-2665-5123
Shaikh M. Mobin https://orcid.org/0000-0003-1940-3822
4.6.4 | 4‐Methoxyphenylbenzo[d]thiazole
Chandrakanta Dash
https://orcid.org/0000-0002-6945-
(5ad)[23]
1366
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4.6.5 | 2‐(3‐Nitrophenyl)benzothiazole
(5ae)[25]
1H NMR (500 MHz, CDCl3): δ 8.93 (s, 1H), 8.42 (d,
1H, JHH = 7.5 Hz), 8.33 (d, 1H, JHH = 7.5 Hz), 8.12 (d,
1H, JHH = 8.0 Hz), 7.95 (t, 1H, JHH = 8.0 Hz), 7.69 (t, 1H,
JHH = 8.0 Hz), 7.55 (t, 1H, JHH = 7.5 Hz), 7.46 (t, 1H,
JHH = 7.5 Hz). 13C{1H} NMR (125 MHz, CDCl3): δ 165.0,
154.0, 148.8, 135.4, 135.3, 133.1, 130.2, 127.0, 126.2, 125.3,
123.8, 122.4, 122.0 ppm.
4.6.6 | 2‐(4‐(trifluoromethyl)phenyl)
benzo[d]thiazole (5af)[26]
1H NMR (500 MHz, CDCl3):
δ 8.22 (d, 2H,
JHH = 8.0 Hz), 8.12 (d, 1H, JHH = 8.0 Hz), 7.95 (d,
1H, JHH = 8.0 Hz), 7.77 (d, 2H, JHH = 8.0 Hz), 7.54 (t,
1H, JHH = 7.5 Hz), 7.45 (t, 1H, JHH = 7.5 Hz). 13C{1H}
NMR (125 MHz, CDCl3): δ 166.2, 154.1, 136.9, 135.3,
132.7, 132.4, 127.9, 126.8, 126.1, 125.9, 123.7, 121.9. 19F
NMR (470 MHz, CDCl3) δ −62.86 ppm.
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SUPPORTING INFORMATION
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