Xantphos-coordinated palladium dithiolates: Highly efficient catalyst for decarboxylative Sonogashira reaction into corresponding alkynes

Article abstract of DOI:10.1002/aoc.6328

This work reports Xantphos-coordinated palladium dithiolate complexes as catalysts for decarboxylative Sonogashira coupling reaction of phenyl propiolic acid and 2-butynoic acid with various iodoarenes. These palladium aryl dithiolate complexes were synthesized and characterized by ¹H and ³¹P nuclear magnetic resonance (NMR) spectroscopy, melting point, and elemental analysis (CHNS). Synthetic utility for the reported protocol is explored for the effect of various functional groups on the yield of corresponding heteroaryl alkynes. The current protocol showed excellent catalytic activity towards decarboxylative alkynylation reaction with high turn-over number (TON) up to 10⁵ and turn-over frequency (TOF) up to 10⁴ h^?1. The catalyst could be recycled up to six recycles without losing its catalytic activity. The in situ generation of palladium nanoparticles (PdNPs) was observed after the third recycle, and the amount was significant after the sixth recycle, which were confirmed and characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray (EDX) analysis and high-resolution transmission electron microscopy (HR-TEM). The catalytic activity of the reaction is attributed to the formation of PdNPs.

Full text of DOI:10.1002/aoc.6328

Received: 19 February 2021
DOI: 10.1002/aoc.6328
Revised: 28 April 2021
Accepted: 14 May 2021
F U L L P A P E R
Xantphos-coordinated palladium dithiolates: Highly
efficient catalyst for decarboxylative Sonogashira reaction
into corresponding alkynes
1
2
2,3
Manjunath S. Lokolkar
| Pravin A. Mane
| Sandip Dey
|
1
Bhalchandra M. Bhanage
1
Department of Chemistry, Institute of
This work reports Xantphos-coordinated palladium dithiolate complexes as
catalysts for decarboxylative Sonogashira coupling reaction of phenyl propiolic
acid and 2-butynoic acid with various iodoarenes. These palladium aryl
Chemical Technology, Mumbai, India
2
Chemistry Division, Bhabha Atomic
Research Centre, Mumbai, India
3
Training School Complex, Homi Bhabha
1
31
dithiolate complexes were synthesized and characterized by H and
P
National Institute, Mumbai, India
nuclear magnetic resonance (NMR) spectroscopy, melting point, and elemen-
tal analysis (CHNS). Synthetic utility for the reported protocol is explored for
the effect of various functional groups on the yield of corresponding heteroaryl
alkynes. The current protocol showed excellent catalytic activity towards
decarboxylative alkynylation reaction with high turn-over number (TON) up
Correspondence
Sandip Dey, Chemistry Division, Bhabha
Atomic Research Centre, Mumbai 400085,
India.
Email: dsandip@barc.gov.in
5
4
ꢀ1
Bhalchandra M. Bhanage, Department of
Chemistry, Institute of Chemical
Technology, Nathalal Parekh Marg,
Matunga, Mumbai 400019, India.
Email: bm.bhanage@ictmumbai.edu.in;
bm.bhanage@gmail.com
to 10 and turn-over frequency (TOF) up to 10 h . The catalyst could be
recycled up to six recycles without losing its catalytic activity. The in situ gen-
eration of palladium nanoparticles (PdNPs) was observed after the third recy-
cle, and the amount was significant after the sixth recycle, which were
confirmed and characterized by powder X-ray diffraction (XRD), scanning
electron microscope (SEM), and energy-dispersive X-ray (EDX) analysis and
high-resolution transmission electron microscopy (HR-TEM). The catalytic
activity of the reaction is attributed to the formation of PdNPs.
Funding information
Department of Atomic Energy, Mumbai
University Collaborative Scheme;
Department of Atomic Energy,
Board of Research in Nuclear Sciences,
Grant/Award Number: 37
K E Y W O R D S
(
2)/14/32/2018-BRNS/37238
decarboxylative Sonogashira reaction, dithiolate, recyclable, TON and TOF, Xantphos
1
| INTRODUCTION
solvent. However, the use of copper salts facilitates the
Sonogashira reaction. But it leads to the formation of
Substituted alkynes are fundamental structural motifs,
which serve as versatile building blocks in the synthesis
of pharmaceuticals, natural products, agrochemicals, and
the homocoupling reaction of terminal alkynes through a
[
5]
Cu-mediated Hay/Glaser reaction.
Furthermore, the
presence of an excess amount of amine, either as a base
[1,2]
[6,7]
functional materials.
Over the past three decades, the
or solvent, is not environmentally friendly.
To sur-
Sonogashira cross-coupling reaction plays the most effi-
mount the impediments, numerous kinds of Sonogashira
reactions have been reported to enhance the protocol's
catalytic efficiency involving amine and copper-free sys-
tems in the chemical literature.
Alkynyl carboxylic acids represent a potential candi-
date for terminal alkynes as they are readily available,
2
cient and reliable approach for Csp–Csp bond formation
[3,4]
in the synthesis of substituted alkynes.
Generally, the
[
8,9]
catalytic system for these reactions includes phosphine
ligated palladium complexes and copper salts as
cocatalyst with an excess amount of amine as a base or
Appl Organomet Chem. 2021;e6328.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
1 of 14
https://doi.org/10.1002/aoc.6328

2
of 14
LOKOLKAR ET AL.
[
10,11]
stable, and easy to handle and store.
coworkers employed the first
Moon and
decarboxylative
Pd(II) complexes can be constructed. Recently, we have
synthesized the supramolecular Pd dithiolate complexes
capping with diphosphine ligands wide-bite angles.
[36]
Sonogashira coupling methodology between alkynyl car-
boxylic acids with aryl halides in 2008 by using Pd(dba)
However, owing to the larger bite angle, Xantphos-
capped Pd dithiolate complexes exhibited better catalytic
3
[12]
catalyst and phosphine ligand.
Since then, several cat-
[
36]
[37]
alytic systems have been achieved to yield internal
activity in Suzuki
and Heck
cross-coupling reac-
[11,13–18]
[38,39]
alkynes.
tions compared with dppe analogue
and offered a
The Pd-catalyzed cross-coupling reactions are being
known as an attractive and versatile tool for organic
transformations. The tolerance of many functionalities
present on the substrates under mild reaction conditions,
and owing to the facile oxidation state of Pd(0) to
Pd(II) or Pd(II) to Pd(IV), palladium became a center of
high turn-over number (TON). Besides, these stable and
highly soluble complexes provided excellent catalytic
[40,41]
activity in carbonylative Suzuki–Miyaura reactions
[42]
as well as in amino and phenoxycarbonylation.
How-
ever, organochalcogen ligated palladium catalysts have
2
rarely been reported for Csp–Csp cross-coupling
[
19,20]
[43,44]
attraction in the group of transition metal catalysts.
reaction,
in particular for decarboxylative
Phosphines and their improved systems attracted
significant attention in the early development of
Sonogashira cross-coupling reaction.
Encouraged by the above results, we utilized
Xantphos-coordinated Pd dithiolate complexes (Scheme 2)
as catalysts to explore decarboxylative Sonogashira cou-
pling reactions between alkynyl carboxylic acids and aryl
halides with Cu-free conditions. The various reaction
parameters were screened to get optimized reaction condi-
tions followed by different aryl halide coupling partners at
lower catalyst loading. The comparison between catalytic
activity of synthesized macrocycles with conventional
Pd catalysts was studied. The reusability of these
catalysts, the formation and characterization of palladium
nanoparticles (PdNPs), and their active participation
have been discussed here. The compositions of
catalysts Pd(1) {[Pd(Xantphos)(1,4-SC H SH)] (OTf) (I)
[21,22]
homogeneous Pd catalysts.
Subsequently, numerous
research works initiated to develop palladium complexes
based on N-containing ligands, N-heterocyclic carbenes,
imines, oximes, and palladacycles have arisen as highly
effective catalysts in C–C bond forming reactions
[23–26]
(Scheme 1).
Palladium complexes with S and Se-based ligands are
recognized as valuable phosphine-free complexes owing
to the family of moisture-insensitive, air-stable, and effi-
[
27–30]
cient catalysts.
Similarly to the previous misbelief,
[
31]
sulfur-based ligands are known as catalyst promoters
rather than acting as catalytic poison.
[32]
Due to the
electron-donating property of sulfur and the strong Pd–S
bond in palladium thiolate complexes holding improved
stability, easier handling and preparation shows better
and vigorous catalytic activity than amine, thioether,
6
4
2
2
and [Pd (Xantphos) (1,4-SC H S)] (OTf) (II)} and Pd(2)
2
2
6
4
2
4
0
[Pd (Xantphos) (4,4 -SC H S)] (OTf) (II) are shown in
Scheme 2.
2
2
12
8
2
4
[
36]
[33]
carbene, selenium-ligand-based complexes.
Numerous
Pd catalysts derived from hybrid thiolate (PS, NS) and
carbene-thiolate-based ligands are appreciated in C–C
2 | EXPERIMENTAL
[34,35]
cross-coupling reactions.
However, thiolate ligands being less explored as to
their tendency to form insoluble polymeric complexes.
Moreover, with the appropriate design of thiolate moiety
and its coligands, soluble discrete molecules of
2.1 | Decarboxylative Sonogashira
reaction
To the oven-dried round bottom (RB) flask, aryl halide
(
0.6 mmol), phenyl propiolic acid (0.5 mmol), K CO (1.2
2 3
equiv), palladium complex (0.1 mol%), and 3 ml of sol-
vent were added, and the content was stirred on an oil
ꢁ
bath for 10 h at 100 C. Afterwards, the reaction was
cooled. The reaction mixture was diluted with distilled
water and solvent ethyl acetate (3 ꢂ 15 ml) used to
extract the product. The solvent of the extract was evapo-
rated under reduced pressure to obtain a crude product.
The purification was carried out by column chromatogra-
phy (100–200 mesh size, silica gel) using petroleum
ether/ethyl acetate as the mobile phase. Then the prod-
1
13
ucts were analyzed by GC–MS and H and C nuclear
magnetic resonance (NMR) spectroscopy.
SCHEME 1 Decarboxylative coupling

LOKOLKAR ET AL.
3 of 14
SCHEME 2 Xantphos-coordinated
palladium dithiolate complexes applied as
catalysts in decarboxylative Sonogashira
a
reaction. Bite angle and flexibility range in
[36]
parentheses (in degrees), Mane et al.
0
2.2 | Catalyst recycling
only complex II exists for the catalyst Pd(2) with 4,4 -
biphenyldithiolate.
The recyclability of any catalyst plays a crucial role.
Hence, the developed catalyst Pd(2) was evaluated for
recyclability with optimal reaction conditions. To the
oven-dried 50-ml RB flask, starting materials, catalyst,
and later the solvent were added. After completion of the
reaction, it was allowed to cool. The solvent was evapo-
rated under vacuum, and the product was extracted in
ethyl acetate. The aqueous layer was centrifuged and
decanted; the obtained catalyst was washed with distilled
water, followed by n-hexane and diethyl ether, and dried.
Then it was used for the next consecutive recycle run by
charging the flask with starting materials. However, the
catalyst could be reused up to the sixth cycle. After
the sixth cycle, the black residue obtained was analyzed
with powder X-ray diffraction (XRD), scanning electron
microscope (SEM), and transmission electron microscopy
Initially, we performed the reaction between phenyl
propiolic acid 1a and iodobenzene 2a to optimize the
reaction conditions, and the effects of various parameters
such as base, time, temperature, and solvents have been
analyzed. The obtained results are shown in Table 1. In
the progress of the reaction, solvent plays a crucial role.
We have explored the effect of various solvents to get
maximum conversion of starting materials on the model
reaction. Initially, we have used K CO as a base and
Pd(2) catalyst, and the content was heated at 120 C for
12 h with different solvents. The polar aprotic solvents
such as acetonitrile (45%), dimethylacetamide (60%),
dimethylformamide (78%), and dimethyl sulfoxide
(DMSO) (92%) provided good to the excellent conversion
of 2a (Table 1, entries 1, 3, 2, and 4, respectively). How-
ever, the use of 1,4-dioxane as a solvent provided only
26% of conversion of 2a, and no conversion of starting
materials was obtained by using toluene as a solvent
2
3
ꢁ
(TEM) technique.
(
Table 1, entries 6 and 5). Excellent conversion (92%) of
3
| RESULTS AND DISCUSSION
2a was achieved using DMSO as polar aprotic solvent;
hence, it was chosen for further optimization studies.
A base screening study with organic and inorganic
The catalysts Pd(1) and Pd(2) complexes were prepared
and analyzed according to the reported method.
[
36]
In
bases revealed that K CO3 (92%) provided excellent
2
case of Pd(1) of phenylene-1,4-dithiolate, the complexes I
conversion compared with other bases such as Na CO
2
3
and II exist in equal proportions in the solution, as
shown by the P NMR spectrum (Figure S1). However,
(60%), Et N (46%), DIPEA (15%), and DBU (88%)
(Table 1, entries 7–11).
3
31

4
of 14
LOKOLKAR ET AL.
TABLE 1 Optimization of reaction parameters for decarboxylative Sonogashira coupling reaction
ꢁ
a
a
Entry
Solvent
Base
Time (h)
Temp ( C)
Conversion (%)
Yield (%)
Effect of solvent
1
ACN
K
2
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
12
12
12
12
12
12
120
120
120
120
120
120
45
78
60
92
NR
26
42
75
56
90
NR
23
2
DMF
3
DMA
4
DMSO
5
Toluene
1,4-Dioxane
6
Effect of base
7
8
9
DMSO
DMSO
DMSO
DMSO
DMSO
Na
2
CO
3
12
12
12
12
12
120
120
120
120
120
60
92
46
15
88
57
90
42
11
86
2 3
K CO
3
Et N
1
1
0
1
DIPEA
DBU
Effect of temperature
12
13
14
15
DMSO
DMSO
DMSO
DMSO
K
2
K
2
K
2
K
2
CO
CO
CO
CO
3
3
3
3
12
12
12
12
60
40
76
92
92
36
73
90
90
80
100
120
Effect of time
1
1
1
1
2
2
2
6
7
8
9
0
1
2
DMSO
DMSO
DMSO
DMSO
DMSO
EtOH
K
2
K
2
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
6
100
100
100
100
100
100
100
65
79
92
92
-
62
75
90
90
-
8
10
12
12
10
10
b
65
28
60
25
MeOH
Note: Reaction condition: 1a (0.5 mmol), 2a (0.6 mmol), base (1.2 equiv), catalyst (0.1 mol%), solvent (3 ml).
a
Conversion and yield of product determined by GC and product identified by GC–MS.
In the absence of Pd(2) catalyst.
b
Further, the effect of temperature and time was
screened on the model reaction using K CO as a base
changes were noted in the conversion of 2a (Table 1,
entries 14 and 15). However, further decreasing the reac-
2
3
ꢁ
ꢁ
and DMSO as a solvent. As we decreased the reaction
tion temperature from 100 C to 60 C, the conversion of
ꢁ
ꢁ
temperature from 120 C to 100 C, no considerable
2a was lowered to 40% (Table 1, entries 12 and 14).

LOKOLKAR ET AL.
5 of 14
ꢁ
It was found that 100 C is the optimal reaction tem-
perature for maximum conversion. However, when the
reaction time was increased from 10 to 12 h, there was
no change in the conversion of 2a (Table 1, entries
(entries 4 and 5) or the 4-iodo anisole containing electron-
donating group yielded 87% and 85% (entries 6 and 7)
when catalyzed by catalysts Pd(2) and Pd(1), respectively.
Both the catalysts are found to be stable enough in
the solid state, which was evaluated from the ther-
1
8 and 19). Thus, based on these results, 10-h duration
ꢁ
and 100 C temperature are sufficient to complete the
reaction with excellent conversion and hence, the yield of
the product. Further, there was no conversion of 2a
observed, and the reaction does not proceed in the
absence of palladium catalyst, which shows the catalyst's
requirement. Thus, the optimized reaction parameters for
the synthesis of substituted alkynes through
decarboxylative Sonogashira reaction are 1a (0.5 mmol),
mogravimetric
analysis.
The
thermogravimetric
curves reveal that the thermal stability and the decompo-
ꢁ
sition phase start after 320 C for catalyst Pd(2) and after
ꢁ
[36]
305 C for catalyst Pd(1)
(Figures S2 and S3). However,
under the optimal reaction conditions, these catalysts
show superior activity as compared with traditional Pd
catalysts, such as Pd(OAc) , PdCl (PPh ) , and PdCl
2
2
2
3 2
(Table 3).
2
a (0.6 mmol), Pd (0.1 mol%), K CO (1.2 equiv), and
Subsequently, the consequence of catalyst loading
was investigated on the present protocol (Table 4). The
effect of catalyst loading experiments on the yield of
substituted alkynes was explored using the coupling reac-
tion of 4-iodonitrobenzene with phenyl propiolic acid by
using tetranuclear complex Pd(2). When decreasing the
catalyst loading of Pd(2) from 1 to 0.01 mol% of Pd,
the yield of the product was achieved to be 87% (Table 4,
entry 3). Further reduction of catalyst loading to
0.001 mol% of Pd, 68% of the considerable yield of the
product was obtained, and TON was found to be
2
3
ꢁ
DMSO (3 ml) at 100 C for 10 h. Later, the effect of protic
solvents like methanol and ethanol was investigated on
optimal reaction, and their results are shown in Table 1
(entries 21 and 22).
Subsequently, we investigated the substrate scope
study concerning the various aryl iodides and alkynyl
carboxylic acids (with R = Ph and Me) under the opti-
mum reaction conditions. At the beginning of the study,
we performed the cross-coupling reaction of different aryl
iodides with phenyl propiolic acid for the decarboxylative
alkynylation process, and the obtained results are shown
in Table 2. The simple iodobenzene reacted smoothly
with phenyl propiolic acid to give biphenyl acetylene
with 90% yield (3a). Then the aryl iodides with electron-
4
5
6.8 ꢂ 10 . The maximum TON 4.6 ꢂ 10 and turn-over
4
ꢀ1
frequency (TOF) 4.6 ꢂ 10 h
could be achieved by
using 0.0001 mol% of Pd, yielding 46% of the respective
product (Table 4, entry 5). The recyclability of the com-
plex Pd(2) was examined; the reaction between
4-iodonitrobenzene and phenyl propiolic acid to yield the
corresponding alkyne was chosen as a model reaction.
After completion of the zero cycle of the reaction, the sol-
vent was evaporated under vacuum, and extraction of the
coupling product with ethyl acetate. The catalyst was sep-
arated, washed, and dried in the oven, and it was used to
check the recyclability with optimal reaction conditions.
The flask containing the Pd catalyst was charged with the
required quantities of starting materials for the first cycle
of the reaction, which provided 91% of the coupling prod-
uct. Similarly, 89% and 87% of the product were obtained
for the second and third cycles, respectively (Table S1).
The catalyst effectively works by providing good conver-
sion of the product. However, the catalyst can be recycled
up to six successive runs with a slight decrease in its cata-
lytic activity. After the third recycle, black particles were
isolated and characterized by PXRD pattern.
donating functionality such as p-CH and p-OMe well
3
coupled with phenyl propiolic acid to give corresponding
internal alkynes 3b and 3c providing 89% and 87% yields,
respectively (entries 2 and 3).
Next, the electron-deficient functional groups such as
p-NO , p-CN, p-acetyl, m-NO , and m-CHO on aryl
iodides showed improved conversion and provided cou-
pling products in significant yields of the corresponding
internal alkynes about 76% to 92% (entries 4–8); this
may be achieved due to the electron-withdrawing
nature of functionality. Furthermore, heteroaryl iodides
having N, O, and S on the aryl group were also success-
fully coupled to give respective products (entries 9–11).
Similarly, various aryl iodides, coupled with 2-butynoic
acid, also gave a good yield of respective products 78%
2
2
(
3l), 74% (3m), and 75% (3n) (entries 12–14).
However, the reaction of aryl bromides with phenyl
propiolic acid and 2-butynoic acid provided only traces
of the coupled product (entries 15–17) under the
same reaction conditions. The comparison study of the
Xantphos-coordinated Pd catalysts, Pd(1) and Pd(2), with
conventional palladium precursors, has been investigated.
Both the catalysts show comparable activity as
evident from Table 3. 4-Nitro iodobenzene containing
electron-withdrawing group gave 92% and 90% yield
The formation of PdNPs was characterized by intense
ꢁ
ꢁ
ꢁ
ꢁ
peaks found at 2θ ꢃ 28.8 , 40 , 46.5 , and 68.2 . Simi-
larly, after the sixth recycle, the obtained black residue
was analyzed by PXRD pattern. The more intense peaks
ꢁ
ꢁ
ꢁ
ꢁ
were observed at 2θ ꢃ 28.8 , 40 , 46.5 , and 68.2
(Figure S5). They were attributed to the planes (111),
(200), and (220) of the cubic face-centered structure of

6
of 14
LOKOLKAR ET AL.
TABLE 2 Substrate study for substituted alkynes in the Pd-catalyzed decarboxylative Sonogashira reaction
a
Entry
Propiolic acid
Aryl halide
Product
Yield (%)
1
90
2
1a
89
3
1a
87
4
1a
92

LOKOLKAR ET AL.
7 of 14
TABLE 2 (Continued)
a
Entry
Propiolic acid
1a
Aryl halide
Product
Yield (%)
5
88
6
1a
86
7
1a
76
(
Continues)

8
of 14
LOKOLKAR ET AL.
TABLE 2 (Continued)
a
Entry
Propiolic acid
1a
Aryl halide
Product
Yield (%)
8
80
9
1a
82
1
0
1a
76
1
1
1a
84

LOKOLKAR ET AL.
9 of 14
TABLE 2 (Continued)
a
Entry
12
Propiolic acid
Aryl halide
Product
Yield (%)
78
1
3
1b
1b
1b
74
1
4
75
1
5
Trace
(
Continues)

10 of 14
LOKOLKAR ET AL.
TABLE 2 (Continued)
a
Entry
16
Propiolic acid
1b
Aryl halide
Product
Yield (%)
Trace
1
7
1a
Trace
Note: Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), K
2
CO
3
(1.2 equiv), catalyst (0.1 mol%), DMSO (3 ml).
a
Isolated yield of the product.
[
45]
[47]
PdNPs.
The formation of PdNPs serves as a reservoir
coupling reaction. The hot filtration test
was carried
[46]
of active catalytic species
for high activity of the
out (Sr. No. 3 in the supporting information) on model
reaction using the catalyst Pd(2). The hot filtration test
was performed for the third cycle, which showed PdNPs,
the filtrate exhibited incomplete conversion (28%) reac-
tion indicating that some Pd catalyst is decomposed to
NPs. The reaction in the third cycle happens through
“cocktail” reaction system. Moreover, the filtrate was
subjected to ICP-AES (inductively coupled plasma-atomic
emission spectroscopy) analysis to check the palladium
leaching. No detectable amount of palladium (below
0.01 ppm) was found, signifying negligible amount of cat-
alyst remained in the solution. Similar experiment was
done at zero cycle. Interestingly, the filtrate after separa-
tion can show the maximum conversion of 2a, signaling
the presence of soluble Pd species, in turn, demonstrating
homogeneous pathway for the reaction. Further, the
decarboxylative Sonogashira reaction. The morphology of
the PdNPs was revealed by SEM (Figure 1a,b), which
shows the spherical shape of the product. The HR-TEM
analysis of PdNPs (Figure 1c,d) reveals that PdNPs are
well dispersed and spherical.
However, the average size of these NPs are ranging
from 5 to 10 nm. The EDAX (energy-dispersive X-ray
analysis) spectrum shows the presence of major and
minor constituent elements as Pd and S in a 9:1 ratio of
atom% (Figure S4). The S of the thiolate group may be
present as capping group of PdNPs, which are generated
after the loss of thiolate ligands from the dithiolate com-
plex of Pd(II).
The hot filtration test and Hg poison test were exam-
ined to investigate the nature of catalysis process in the

LOKOLKAR ET AL.
11 of 14
TABLE 3 Comparative study of Pd complexes 1 and 2 with commercial palladium precursors
a
ꢀ1
Entry
2a
Pd catalyst
Yield (%)
TON
400
620
180
920
900
870
850
TOF (h
40
)
0
1
2
3
4
5
6
7
4-Iodonitrobenzene (2a )
Pd(OAc)
2
40
62
18
92
90
87
85
0
2a
PdCl
PdCl
2
2
62
0
2a
(PPh
3
)
2
18
0
2a
Pd(2)
Pd(1)
Pd(2)
Pd(1)
92
0
2a
90
0
0
4-Iodoanisole (2a )
87
0
0
2a
85
Note: Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), K
2
CO
3
(1.2 equiv), catalyst (0.1 mol%), DMSO (3 ml).
a
Yield of product calculated by GC and GC–MS.
TABLE 4 Effect of catalyst loading for decarboxylative Sonogashira reaction
a
a
ꢀ1
Entry
Pd (mol%)
1
Conv. (%)
Yield (%)
TON
92
TOF (h )
1
2
3
4
5
95
95
89
72
46
92
92
87
68
46
9.2
0.1
920
92
0.01
8700
68,000
460,000
870
0.001
0.0001
6800
46,000
Note: Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), K
2
CO
3
(1.2 equiv), DMSO (3 ml).
a
Conversion and yield were determined based on GC–GC–MS.
[
48]
mercury poison test
(Sr. No. 4 in the supporting infor-
Pd(2) catalyst suppresses the coupling of phenyl propiolic
acid and iodobenzene. It implies that the catalytic process
was quenched in the presence of Hg(0), indicating the
heterogeneous pathway is involved in the reaction.
mation) was examined on the optimal reaction using an
excess of the elemental Hg (Pd:Hg = 1:400). The reaction
of Hg with PdNPs separated after the third recycle using

12 of 14
LOKOLKAR ET AL.
FIGURE 1 The PdNPs after the sixth
recycle run of Pd complex (2): (a and b) FE-SEM
images; (c and d) high-resolution TEM images
4
| CONCLUSION
In summary, Xantphos-coordinated Pd complexes of
dithiolate ligands employed as highly stable and efficient
catalysts for the synthesis of substituted alkynes through
the decarboxylative Sonogashira reaction with various
aryl iodides. This current protocol works well without the
use of external ligand and copper as cocatalyst. Ther-
mally stable, moisture, and air-insensitive complexes are
effective catalysts for decarboxylative Sonogashira cou-
pling reactions. Catalyst tolerates both electron-rich and
-
poor substituents. The catalyst remains highly active
5
with low concentration yielding high TON (4.6 ꢂ 10 )
4
ꢀ1
and TOF up to 10 h . Comparative study of Pd com-
plexes with conventional Pd precursors shows that Pd(2)
maintains superior catalytic activity at low catalyst load-
ing (0.1 mol%) with higher conversion of the product.
The developed catalyst could be recycled for six cycles.
The in situ formation of PdNPs during the catalysis has
been confirmed. The activation of decarboxylative
Sonogashira coupling by these NPs independently sup-
ports their role in catalysis.
FIGURE 2 Recycle study of catalyst Pd(2)
However, another experiment similar to the above
procedure employing Pd(2) catalyst was performed.
0
The reaction of Pd complex [Pd (Xantphos) (4,4 -
2
2
SC H S)] (OTf) with excess of Hg at zero cycle does not
12
8
2
4
cease the coupling reaction. The later result exhibits the
homogeneous nature of the catalysis process, which may
proceed via the Pd(0)/Pd(II) mechanism. However, the
formation PdNPs at subsequent cycles suggest the pres-
ence of “cocktail”-type catalytic system, which can
undergo both homogeneous and heterogeneous catalysis
4.1 | Electronic supplementary
information
All the details of general materials, the procedure for cat-
alysts preparation, and their thorough data of
[49,50]
processes, as discussed by others (Figure 2).

LOKOLKAR ET AL.
13 of 14
characterization are summarized, procedure for the hot
filtration test and Hg poison test, the curves of ther-
mogravimetric analysis of catalysts Pd(1) and Pd(2),
EDAX spectrum of PdNPs, the table data of recyclability,
PXRD pattern of catalyst Pd(2), and for all the
[9] J. Ruiz, N. Cutillas, F. Lopez, G. Lopez, D. Bautista, Organo-
metallics 2006, 25, 5768.
10] K. Park, J. M. You, S. Jeon, S. Lee, European J. Org Chem.
[
[
[
2
013, 2013, 1973.
11] D. Zhao, C. Gao, X. Su, Y. He, J. You, Y. Xue, Chem. Commun.
010, 46, 9049.
2
1
decarboxylative Sonogashira coupling products H and
12] J. Moon, M. Jeong, H. Nam, J. Ju, J. H. Moon, H. M. Jung, S.
Lee, Org. Lett. 2008, 10, 945.
13] J. Moon, M. Jang, S. Lee, J. Org. Chem. 2009, 74, 1403.
14] W. W. Zhang, X. G. Zhang, J. H. Li, J. Org. Chem. 2010, 75,
1
3
C NMR spectra and GC–MS (gas chromatography-mass
[
[
spectrometers).
5
259.
15] A. Pyo, J. D. Kim, H. C. Choi, S. Lee, J. Organomet. Chem.
013, 724, 271.
ACKNOWLEDGMENTS
The authors (M.S.L) are grateful to the Department of
Atomic Energy, Board of Research in Nuclear Sciences
[
[
2
16] P. Prabhala, H. M. Savanur, R. G. Kalkhambkar, K. K. Laali,
(DAE-BRNS), Government of India, for providing a
Eur. J. Org. Chem. 2019, 2019, 2061.
Junior Research Fellowship (JRF) Collaborative Scheme.
The author (P.M.) is thankful to the Department of
Atomic Energy, Mumbai University Collaborative
Scheme for providing a senior research fellowship. We
are also grateful to the DAE-BRNS for funding this work
[17] H. Kim, P. H. Lee, Adv. Synth. Catal. 2009, 351, 2827.
[18] C. Maaliki, Y. Chevalier, E. Thiery, J. Thibonnet, Tetrahedron
Lett. 2016, 57, 3358.
[19] J. Magano, J. R. Dunetz, Chem. Rev. 2011, 111, 2177.
[20] G. Yin, X. Mu, G. Liu, Acc. Chem. Res. 2016, 49, 2413.
[21] C. A. Fleckenstein, H. Plenio, Chem. Soc. Rev. 2010, 39, 694.
[22] A. Fihri, P. Meunier, J. C. Hierso, Coord. Chem. Rev. 2007, 251,
(Sanction No. 37(2)/14/32/2018-BRNS/37238).
2017.
AUTHOR CONTRIBUTIONS
Manjunath Lokolkar: Conceptualization;
[
23] F. Churruca, R. SanMartin, B. Ines, I. Tellitu, E. Dominguez,
data
Adv. Synth. Catal. 2006, 348, 1836.
curation; formal analysis; investigation; methodology;
visualization. Pravin Mane: Data curation; formal anal-
ysis; methodology. Sandip Dey: Conceptualization; data
curation; investigation; project administration; resources;
supervision. BM Bhanage: Conceptualization; data
curation; funding acquisition; investigation; project
administration; resources; supervision.
[24] O. Kuhl, Chem. Soc. Rev. 2007, 36, 592.
[
[
25] W. A. Herrmann, Angew. Chem., Int. Ed. 2002, 41, 1290.
26] J. Dupont, C. S. Consorti, J. Spencer, Chem. Rev. 2005, 105,
2527.
[27] A. Kumar, M. Agarwal, A. K. Singh, R. J. Butcher, Inorg. Chim.
Acta 2009, 362, 3208.
[28] Q. Yao, E. P. Kinney, C. Zheng, Org. Lett. 2004, 6, 2997.
[29] G. K. Rao, A. Kumar, J. Ahmed, A. K. Singh, Chem. Commun.
010, 46, 5954.
2
[
30] G. K. Rao, A. Kumar, B. Kumar, D. Kumar, A. K. Singh,
Dalton Trans. 2012, 41, 1931.
31] R. Vanjari, T. Guntreddi, S. Kumar, K. N. Singh, Chem.
Commun. 2015, 51, 366.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are avail-
able in the supporting information of this article.
[
[32] J. K. Dunleavy, Platinum Met. Rev. 2006, 50, 110.
[33] D. Yuan, H. V. Huynh, Organometallics 2010, 29, 6020.
[34] D. K. Paluru, S. Dey, K. R. Chaudhari, M. V. Khedkar, B. M.
Bhanage, V. K. Jain, Tetrahedron Lett. 2014, 55, 2953.
ORCID
Manjunath S. Lokolkar https://orcid.org/0000-0003-
3
514-6686
Pravin A. Mane https://orcid.org/0000-0002-0999-9362
Sandip Dey https://orcid.org/0000-0001-9198-8808
Bhalchandra M. Bhanage https://orcid.org/0000-0001-
[35] A. Dervisi, D. Koursarou, L. L. Ooi, P. N. Horton, M. B.
Hursthouse, Dalton Trans. 2006, 48, 5717.
[
[
[
36] P. A. Mane, S. Dey, A. K. Pathak, M. Kumar, N. Bhuvanesh,
Inorg. Chem. 2019, 58, 2965.
37] P. A. Mane, S. Neogy, S. Dey, Appl. Organomet. Chem. 2020,
9
538-3339
34, 5405.
REFERENCES
38] K. V. Vivekananda, S. Dey, D. K. Maity, N. Bhuvanesh, V. K.
[
[
1] J. He, K. Yang, J. Zhao, S. Cao, Org. Lett. 2019, 21, 9714.
2] Q. Jiang, H. Li, X. Zhang, B. Xu, W. Su, Org. Lett. 2018, 20,
Jain, Inorg. Chem. 2015, 54, 10153.
[39] P. A. Mane, S. Dey, K. V. Vivekananda, Tetrahedron Lett. 2017,
58, 25.
2
424.
[
[
[
3] H. Doucet, J. C. Hierso, Angew. Chem., Int. Ed. 2007, 46, 834.
4] R. Chinchilla, C. Najera, Chem. Rev. 2007, 107, 874.
5] G. Evano, N. Blanchard, M. Toumi, Chem. Rev. 2008, 108,
[40] V. V. Gaikwad, P. A. Mane, S. Dey, B. M. Bhanage, Appl.
Organomet. Chem. 2020, 34, 5255.
[41] V. V. Gaikwad, P. A. Mane, S. Dey, B. M. Bhanage,
ChemistrySelect 2019, 4, 8269.
3
054.
[
[
[
6] K. Sonogashira, J. Organomet. Chem. 2002, 653, 46.
7] L. Yin, J. Liebscher, Chem. Rev. 2007, 107, 133.
8] V. P. Bohm, W. A. Herrmann, Eur. J. Org. Chem. 2000, 2000,
[42] V. V. Gaikwad, P. A. Mane, S. Dey, D. Patel, B. M. Bhanage,
Mol. Catal. 2020, 482, 110672.
[43] S. Kumar, F. Saleem, M. K. Mishra, A. K. Singh, New J. Chem.
2017, 41, 2745.
3
679.

14 of 14
LOKOLKAR ET AL.
[
44] R. Bhaskar, A. K. Sharma, M. K. Yadav, A. K. Singh, Dalton
Trans. 2017, 46, 15235.
45] M. Nasrollahzadeh, New J. Chem. 2014, 38, 5544.
46] D. Astruc, Inorg. Chem. 2007, 46, 1884.
47] A. Biffis, P. Centomo, A. Del Zotto, M. Zecca, Chem. Rev. 2018,
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
[
[
[
1
18, 2249.
[
48] V. M. Chernyshev, A. V. Astakhov, I. E. Chikunov, R. V.
How to cite this article: M. S. Lokolkar, P.
A. Mane, S. Dey, B. M. Bhanage, Appl Organomet
Chem 2021, e6328. https://doi.org/10.1002/aoc.
Tyurin, D. B. Eremin, G. S. Ranny, V. P. Ananikov, ACS Catal.
2
019, 9, 2984.
[49] D. Eremin, V. P. Ananikov, Coord. Chem. Rev. 2017, 346, 2.
50] V. P. Ananikov, I. P. Beletskaya, Organometallics 2012, 31,
6328
[
1
595.