Agro-Waste Generated Pd/CAP-Ash Catalyzed Ligand-Free Approach for Suzuki–Miyaura Coupling Reaction

Article abstract of DOI:10.1007/s10562-021-03597-6

Abstract: We converted agro-waste Custard Apple Peels (CAP) to ash via thermal treatment, on which Pd(OAc)2 was immobilized easily that produced a low-cost, highly efficient Pd/CAP-ash catalyst. The prepared catalyst was fully characterized by using FT-IR, SEM, EDX, XRF, DSC-TGA, BET, HR-TEM, and XPS techniques. The Pd/CAP-ash catalyst was conveniently applied for the Suzuki–Miyaura coupling reaction under external base free and ligand-free conditions in an aqueous-organic solvent to produce biphenyls in good to excellent yields. The main attraction of our protocol an application of palladium-supported agro-waste material which is easily recoverable and recyclable provides mono and bis-coupled derivatives in a short reaction time. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-021-03597-6

Catalysis Letters
https://doi.org/10.1007/s10562-021-03597-6
Agro‑Waste Generated Pd/CAP‑Ash Catalyzed Ligand‑Free Approach
for Suzuki–Miyaura Coupling Reaction
Rupesh C. Patil1 · Ashutosh A. Jagdale1 · Uttam P. Patil1 · Jeevan S. Ghodake2 · Sawanta S. Mali3 · Chang K. Hong3 ·
Suresh S. Patil1
Received: 12 August 2020 / Accepted: 6 March 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
We converted agro-waste Custard Apple Peels (CAP) to ash via thermal treatment, on which Pd(OAc)2 was immobilized
easily that produced a low-cost, highly efcient Pd/CAP-ash catalyst. The prepared catalyst was fully characterized by using
FT-IR, SEM, EDX, XRF, DSC-TGA, BET, HR-TEM, and XPS techniques. The Pd/CAP-ash catalyst was conveniently
applied for the Suzuki–Miyaura coupling reaction under external base free and ligand-free conditions in an aqueous-organic
solvent to produce biphenyls in good to excellent yields. The main attraction of our protocol an application of palladium-
supported agro-waste material which is easily recoverable and recyclable provides mono and bis-coupled derivatives in a
short reaction time.
Graphic Abstract
Keywords Agro-waste · Custard apple peels · Palladium · Suzuki–Miyaura coupling
* Suresh S. Patil
1 Introduction
sanyujapatil@yahoo.com
1
Synthetic Research Laboratory, PG Department
of Chemistry, PDVP College, (afliated to Shivaji
University, Kolhapur), Tasgaon, Sangli, MS 416312, India
Palladium being the most specifc transition metal has one
of the most versatile catalytic properties. Salts of Pd have
already proved that they are highly efcient catalysts in mak-
ing new carbon–carbon (Csp2–Csp2) bonds, which seemed
to be very challenging scenario in the past. Eventually it was
discovered that Pd salts have tremendous scope in synthetic
chemistry as its advantages were revealed one by one. In
2
3
Department of Physics, PDVP College, (afliated to Shivaji
University, Kolhapur), Tasgaon, Sangli, MS 416312, India
Polymer Energy Materials Laboratory, School of Advanced
Chemical Engineering, Chonnam National University,
Gwangju 500-757, South Korea
Vol.:(0123456789)
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R. C. Patil et al.
combination with organoboranes, organostannanes, alkenes,
acrylates Pd salts have succeeded in designing many of the
vital organic molecules. It was observed that almost any two
organic molecules can be coupled by Pd (salts) with the help
of appropriate ligands. So it can be said that ligand help to
enhance activity of Pd (salts) to great extent. This has lead to
establish coupling reactions like Suzuki, Heck, Sonogashira
etc. as milestones in the feld of synthetic organic chemistry
[1, 2].
and unsymmetrical biaryls in high yields. Synthesis of biaryl
motifs has attracted considerable interest, as they are important
building blocks for various natural products, herbicides, phar-
maceuticals including antibiotic vancomycin and anticancer
drug gossypol [43–45].
The survey of literature reveals that the careful selection
of ligand can promote both frst and last steps of the cata-
lytic cycle [46]. Numerous electron-rich phosphine-based
ligands [47–50], sterically crowded NHC [51], oximes [52,
53], imines [54], and palladacycles [55, 56] were developed
and employed as catalyst precursors under both conventional
organic or biphasic media. Although, these ligand assisting
systems bear excellent activity, the principle drawbacks associ-
ated with them are availability, stability, air/moisture sensitiv-
ity and tedious preparation steps. In the most crucial stage of
the mechanism, the transmetallation, transfer of the organo
boron species to the palladium in the presence of external
base occurs. However, added base often competes with many
functional groups, and also causes protodeboronation with
electron-defcient aryl boronic acids [57–59].
In the late of 20th century it was realized that the ligands
used in combination with Pd salts have many disadvanta-
geous characteristics like toxicity, highly expensive, avail-
ability, stability and tedious preparation methods and also
the contamination of metal and ligands in the fnal products.
Therefore it seemed to be an urgent need to develop ligand
free methodologies in order to cut short human impact on
ecology [3–5].
In 1989, Bumagin and et al. for the frst time realized
a palladium catalyzed ligand free Suzuki reaction of aryl
halides in aqueous condition under an argon atmosphere [6].
During the last decade several groups described biphasic
protocols for SM couplings commonly based on water solu-
ble catalysts, including contributions by Beller [7], Plenio
[8], Miyaura [9], Shaughnessy [10], Genet [11], Leadbeater
[12] and Buchwald [13]. Lipshutz et al. demonstrated ambi-
ent temperature couplings in water utilizing nonionic micelle
forming additives [14], phase-transfer catalysis [15] or
water:organic solvents have been employed [16, 17]. Alter-
native approaches include heating by microwave irradiation
[18, 19], or utilization of supported catalysts [20–23]. Yet,
despite a variety of advantageous features, water mediated
cross-coupling protocols typically necessitate co-solvents,
high catalyst loading [24–26], elevated temperatures [27]
and/or tedious product work-up by column chromatography
[28].
The immobilization methods to deposit the palladium metal
on suitable solid supports [60], polymers [61], metal oxides
[62, 63], clay [64] carbon nanofber [65], montmorillonite [66]
magnetic mesoporous silica [67, 68], and zeolite [69] have
been reported for coupling reactions. However, as most of
the aryl halides are hydrophobic in nature, the use of pure
water medium for Suzuki–Miyaura coupling reaction often
fails to furnish utmost efciency [70]. Indeed, several reports
mentioned the use of organic-water system, which aids in the
complete dissolution of the organic substrates, and also in the
stabilization of the metal. As a part of our ongoing eforts to
develop sustainable protocols [71–73], we decided to revisit
the Suzuki–Miyaura coupling reaction with the aim to make
a protocol environmentally benign with eco-friendly catalytic
system.
Thus, with simple and economical methodologies, the
exercise of “ligand-free” catalytic system, moderate basic-
ity and wide compatibility has become the need of current
manufacturing process to establish greener and sustainable
strategy [29–32]. In fulfllment to the green chemistry prin-
ciples, we decided to study in detail the use of an agro-waste
stuf that signifcantly fulflls the desired expectation of safe
reaction strategy. The Suzuki–Miyaura coupling reaction is
highly dependent on the presence of external base and sol-
vent [33–35].
Herewith, we present a novel and highly green
Suzuki–Miyaura coupling reactions using Pd/CAP-ash which
was prepared from Pd(OAc)2 and dead Custard apple peels
that is thrown to rot/deteriorate/decay. CAP-ash obtained from
agro-wastes is a better choice in certain conditions because
of characteristics: they are low‐cost materials and obtained
from renewable resources. A large amount of solid wastes of
peel, stones or seeds are formed in the processing of fruits like
Custard apple which can be applied in organic transforma-
tion. CAP is a highly abundant agro-waste materials across the
world and therefore its application in synthetic chemistry will
attract tremendous interest from the point of environmental
issues.
In present-scenario researchers are interested in develop-
ment of green protocols as sustainability has become a prior
issue in every area of human activity [36, 37]. Among the prin-
ciples of Green Chemistry, the use of naturally sourced green
catalyst is a key aspect since it minimizes toxicity, hazard,
pollution and waste treatment issues [38–42]. Suzuki–Miyaura
coupling reaction involves the selective cross-coupling
between aryl halide and aryl boronic acids to give symmetrical
1 3

Agro-Waste Generated Pd/CAP-Ash Catalyzed Ligand-Free Approach for Suzuki–Miyaura Coupling…
into small pieces (Fig. 1b) and crushed using mortar and
2 Experimental Section
pestle. After crushing, the raw powder (100 g) was heated
at the rate 5 °C/min in Mufe furnace up to 900 °C and
this temperature was maintained for 1 h. After Mufe fur-
nace treatment, most of the organic materials were burnt
out and got transformed into white soft ash (8.5 g) denomi-
nated as CAP-ash. This CAP-ash (Fig. 1c) obtained was
further utilized as a support for Pd metal to produce a low
cost catalyst.
2.1 General
Except CAP-ash, all chemicals used in study were pur-
chased from Sigma-Aldrich and used without additional
drying and purifcation. Proton and carbon-13 NMR spectra
were recorded with a Avance-300 instrument in CDCl3 and
DMSO-d6 as a solvent with tetramethylsilane (TMS) as an
internal standard. Chemical shifts (δ) are reported in ppm
and coupling constants (J) are expressed in Hz. Mass spectra
analyses were recorded on Agilent gas chromatograph mass
spectrometer. Fourier transform infrared (FT-IR) spectra
were recorded on a Perkin-Elmer-Spectrum-100 spectropho-
tometer (KBr). Scanning electron microscope (SEM) images
were obtained on FEI, NOVA, NanoSem 450 equipment.
Energy dispersive X-ray spectroscopy (EDX) analysis was
carried on Quanta 200 3D, FEI Scanning electron micro-
scope. Measurement temp. 25 °C with step size 0.0170 [2°
Th] and Diferential Scanning Calorimetry-Thermogravimet-
ric analysis (DSC-TGA) was performed on TA SDT Q600
V20.9 Build 20 instrument in nitrogen and at a heating rate
of 10 °C min−1. X-ray fuorescence (XRF) analysis was per-
formed on X-ray fuorescence spectrometer (Pananlytical,
USA). N2-adsorption-desorption isotherms were obtained
with a BELSORP surface area and porosity analyzer. High
resolution transmission electron microscopy (HRTEM)
images and selected area electron difraction (SAED) pattern
were obtained by TECNAI F20 Philips operated at 200 kV.
The surface elemental studies were analyzed using an X-ray
photoelectron spectrometer (XPS) (VG Multilab 2000-SSK,
USA, Kα) measurement were reported with a multi-channel
detector, which can endure high photonic energies from 0.1
to 3 KeV. The progress as well as completion of reaction
was monitored by TLC on Merck silica gel (60 F254) plates.
2.3 Palladium Support in the CAP‑Ash
To the 10 mL of Pd(OAc)2 solution in acetone (0.1 g/L),
500 mg of CAP-ash was added and the resulting mix-
ture was stirred constantly at room temperature for 24 h.
Finally, the resulting mixture was fltered and oven dried
at 80 °C for 2 h and stored in a desiccator. The resulting
catalyst (Fig. 1d) so obtained was nominated as Pd/CAP-
ash and utilized for Suzuki–Miyaura coupling reaction.
2.4 General Experimental Procedure for Suzuki–
Miyaura Coupling Reaction
All the Suzuki–Miyaura coupling reactions were carried
out under air atmosphere in dried glassware. In a 25 mL
round bottom fask equipped with a magnetic stirrer, aryl
bromides (1.0 mmol), arylboronic acids (1.2 mmol), Pd/
CAP-ash (5 wt%) and water:ethanol (3 mL) were placed.
The resulting reaction mixtures were stirred at room tem-
perature for appropriate time. The progress of reactions
was monitored by TLC. After completion of reactions,
the reaction mixtures were extracted with ethyl acetate
(2 × 10 mL). The organic layers were washed with brine
(2 × 10 mL). The combined organic layer was collected,
dried over Na2SO4 and concentrated in vacuo. The residue
was purifed by silica gel column chromatography using
n-hexane:EtOAc (9:1 v/v) to give the corresponding biaryl
compound. The desired products were characterized by
2.2 Preparation of CAP‑Ash
During this study, Custard apple (Annona squamosa)
peels were obtained from nearby agricultural farm. Ini-
tially agro-waste fruit peels (Fig. 1a) were washed with
distilled water and sun-dried. They were manually broken
1
comparing H, 13C NMR spectral data with authentic
samples.
Fig. 1 Preparation of Pd/CAP-ash a dry custard apple peels, b small spices, c CAP-ash, d Pd/CAP-ash
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R. C. Patil et al.
3.1.1 FT‑IR Analysis
2.5 General Experimental Procedure for Double
Suzuki–Miyaura Coupling Reaction
FT-IR analysis performed as preliminary tool (Fig. 2) to
obtain information about the existence of functional groups
in Pd/CAP-ash. It shows that both CAP-ash and Pd/CAP-ash
have bands attributed at 618, 1058, 1443, 1612, 2927, 3381
cm−1 and 616, 1047, 1392, 2357, 2924 cm−1 respectively.
A sharp stretching band is observed at 3381 cm−1 in CAP-
ash indicates the presence of hydroxyl group (-OH) due to
trapping of moisture, which is not observed in IR spectrum
of Pd/CAP-ash indicates the formation of metal oxides. The
observation of absorption band at 1392, 1047, 616 cm−1 are
characteristic peaks of metal oxides in Pd/CAP-ash supports
metal oxides formation.
In a 25 mL round bottom fask equipped with a magnetic
stirrer, aryl dibromides (1.0 mmol), arylboronic acids (2.4
mmol), Pd/CAP-ash (5 wt%) and H2O:DMF (3 mL) were
placed. The resulting reaction mixtures were stirred at room
temperature for appropriate time. The progress of reactions
was monitored by TLC. After completion of reactions, the
reaction mixtures were extracted with ethyl acetate (2 × 10
mL). The organic layers were washed with brine (2 × 10
mL). The combined organic layer was collected, dried over
Na2SO4 and concentrated in vacuo. The residue was purifed
by silica gel column chromatography using n-hexane:EtOAc
(9:1 v/v) to give the corresponding biaryl compound.
3.1.2 SEM Analysis
SEM micrographs (Fig. 3) of CAP-ash (Fig. 3a and b)
observed the porous nature of catalyst, with the smaller
particle size, smooth and soft surface area which upon Pd
loading efectively applied for catalyzing the reaction. Pd
metal can easily be spotted on the ash surface. (Fig. 3c and
d) Impregnation of Pd on porous surface of the CAP-ash is
confrmed by following SEM analysis.
3 Results and Discussion
3.1 Characterization of Pd/CAP‑Ash
Herein, we report our preliminary research on the combus-
tion of agro-waste CAP to ash, which can be used as pal-
ladium support CAP-ash to produce good catalytic system.
The Pd/CAP-ash was analyzed by FT-IR, SEM, EDX, XRF,
DSC-TGA, BET, HR-TEM and XPS technique. Moreover,
an attempt was also made to investigate the catalytic prop-
erty of the Pd/CAP-ash employing Suzuki–Miyaura cou-
pling reaction.
3.1.3 EDX Analysis
The presence of elements as based on the EDX analysis
of the CAP-ash is shown in Fig. 4a. The report reveals a
Fig. 2 FT-IR of a CAP-ash, b
Pd/CAP-ash
1 3

Agro-Waste Generated Pd/CAP-Ash Catalyzed Ligand-Free Approach for Suzuki–Miyaura Coupling…
Fig. 3 SEM images for CAP-ash (a), (b) and Pd/CAP-ash (c), (d)
distribution of the oxides of K, Ca, Na, Al and Mg. This
distribution is comparably higher than that of the waste bio-
mass such as banana peels [74], rice straw [75], wheat straw
[76] and Eichhornia crassipes [77]. EDX spectrum of Pd/
CAP-ash (Fig. 4b) distinctively shows presence of Pd metal
which supports the distribution of Pd metal on ash surface.
Three distinct stages of weight loss were observed in custard
apple peels powder (Fig. 5a), initially weight percent loss of
12.05% below 184.73 °C that can be attributed to the evolu-
tion of moisture. The key weight loss of 37.50% up to 362.29
°C is attributed to combined weight loss for decomposition
of organic compounds. The sharp exothermic peak exhibited
by DSC at the temperature 508.58 °C could be assigned to
the decomposition of carbohydrates. The third weight loss
of 44.67% corresponds to the decomposition of metal car-
bonates up to 536.82 °C. Last two combined weight loss of
around 82.17% at a temperature between 184.73 to 536.82
°C is due to hemicellulose, cellulose, and pectin degradation.
At the end 4.79% weight of residue remains after 988.79 ºC
was due to the metal oxides. As the weight remains constant
after 900 °C, this temperature suitable for complete conver-
sion of peels powder to ash. TGA profle of Pd/CAP-ash
shows that the catalyst is stable up to 399.73 °C (Fig. 5b).
3.1.4 XRF Analysis
To investigate the particular chemical compositions of the
inorganic phase of fresh CAP-ash, XRF analysis was per-
formed and it presented that K2O as the predominant com-
ponent (% mass fraction 40.5%) while CaO, MgO, Al2O3,
P2O5, SiO2 have mass fractions 4.35%, 6.73%, 0.53%,
11.62%, 0.0011% respectively. Fe2O3, Na2O constituted the
remaining ash.
3.1.5 DSC‑TGA Analysis
3.1.6 BET Analysis
The thermogravimetric analysis (TGA) of custard apple
peels powder and newly synthesized Pd/CAP-ash catalyst
was carried out over the temperature range of 0–1000 °C
at a heating rate of 10 °C/min under nitrogen atmosphere.
We investigated the suitable ashing temperature with weight
change of parent custard apple peels powder by DSC-TGA.
The catalytic activity of Pd/CAP-ash was determined by
using BET surface area analysis. The surface area, pore vol-
ume and average particle size of prepared Pd/CAP-ash cata-
lyst was determined using Brunauer–Emmett–Teller (BET)
method. The surface area of the catalyst was found to be 4.04
1 3

R. C. Patil et al.
Fig. 4 a EDX spectrum of the fresh CAP-ash. b EDX spectrum of Pd/CAP-ash
m2g−1 while the pore volume and the pore radius are 0.044
ccg−1 and 29.05 nm respectively which is presented in Fig. 6
The pore-size distribution curve depicts mesopores of very
uniform sizes. The isotherms of prepared Pd/CAP-ash were
type III isotherm with a H4-type hysteresis loop.
spherical Pd nanoparticles exhibit dimensions from 24 to 31
nm. The selected area electron difraction (SAED) (Fig. 7d)
pattern of the same confrms the crystallinity which was
revealed from the presence of difraction rings.
3.1.8 XPS Analysis
3.1.7 HR‑TEM
Chemical oxidation states of prepared catalyst was deter-
mined by X-ray photoelectron spectroscopy (XPS) tech-
nique. From XPS spectra (Fig. 8a) it is observed that peaks
due to K, Ca, Mg, Na, C, O, and Pd displayed in the bind-
ing energy region from 0 to 1100 eV which confrms the
The structure of the prepared catalyst was also inspected
by high resolution transmission electron microscopy (HR-
TEM). TEM images (Fig. 7a–c) disclosed the uniform
distribution of Pd nanoparticles on CAP-ash. The roughly
1 3

Agro-Waste Generated Pd/CAP-Ash Catalyzed Ligand-Free Approach for Suzuki–Miyaura Coupling…
Fig. 5 DSC-TGA curves of
a parent custard apple peels
powder, b Pd/CAP-ash
successful attachment of Pd on the surface of CAP-ash.
Another interesting observation deriving from the XPS data
(Fig. 8b) was the determination of the oxidation state of the
catalytic activity site and the high-resolution Pd 3d XPS
signals.
The characteristic peaks at 336.96 and 340.70 eV can
be indexed to the Pd0 state and the other set of peaks at
337.95 and 342.02 eV are related to the Pd+2 oxidation
state [78, 79]. The XPS spectrum clearly shows that Pd0
1 3

R. C. Patil et al.
Fig. 6 BET isotherm and pore-
size distribution curve of Pd/
CAP-ash
Fig. 7 a–c HR-TEM images and d SAED pattern of Pd/CAP-ash
1 3

Agro-Waste Generated Pd/CAP-Ash Catalyzed Ligand-Free Approach for Suzuki–Miyaura Coupling…
Fig. 8 a Full-range XPS spectra
of Pd/CAP-ash, b Pd 3d
is also present along with Pd+2 oxidation state supports
are summarized in table 1. Various solvents, catalyst amount
the reduction of Pd+2 to Pd0 by metal oxides of CAP-ash.
and temperature conditions were screened in order to estab-
lish ideal coupling parameters.
3.2 Optimization of Reaction
Initially, 25 mL round bottom fask charged with 4-bro-
mobenzophenone (1 mmol), phenyl boronic acid (1.2
mmol), Pd/CAP-ash (1 wt%) in water (3 mL) and the reac-
tion mixture was stirred at room temperature, after 2 h low
yield (30%) of corresponding product was observed on TLC
To optimize the reaction conditions 4-bromobenzophenone
was stirred with phenylboronic acid in presence of Pd/CAP-
ash as a model reaction (Scheme 1) and results of the study
1 3

R. C. Patil et al.
Scheme 1 Suzuki–Miyaura
reaction of 4-bromobenzophe-
none and phenylboronic acid
O
O
B(OH)2
Reaction Conditions
+
Br
1m
3q
2a
Table 1 Screening of amount
of Pd/CAP-ash, solvent and
temperature for Suzuki–
Entry
Pd/CAP-ash (wt %)
Solvent (3 mL)
Temp. (°C)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
1
2
3
5
10
5
5
5
5
5
5
5
H2O
H2O
H2O
H2O
H2O
H2O
MeOH
EtOH
DMF
DCM
THF
H2O:EtOH (1:1)
EtOH
H2O:EtOH (1:1)
–
Rt
Rt
Rt
Rt
Rt
80
Rt
Rt
Rt
Rt
Rt
Rt
Rt
Rt
Rt
120
120
60
30
30
30
30
30
30
30
30
10
10
60
60
30
36
41
43
Miyaura coupling reactiona
43
87 (77, 83)c
87
91
88
43
42
98
98d
Nr
Nr
9
10
11
12
13
14
15
Extract
–
5
aReaction conditions: 4-bromobenzophenone (1 mmol), phenylboronic acid (1.2 mmol), Pd/CAP-ash (wt%)
and solvent (mL) at room temperature
bIsolated yields
cYields of product at 60 and 100 °C temp
dYields of product using CAP-ash water extract (3 mL) and Pd(OAc)2 (0.5 mol%)
(Table 1, entry 1). On increasing catalytic amount (1 to 5
wt%), no signifcant improvement in the result was obtained
after prolonged reaction time (Table 1, entries 2 to 5).
We continued our eforts for improvement in the result
when model reactants were allowed to react at elevated tem-
perature (80 °C), in presence of 3 mL water, after 30 min,
surprisingly the product was obtained in 87% yield (Table 1,
entry 6). Again, we were not satisfed with these results;
therefore, we continued our eforts for better conversion of
reactant into product. For this, we decided to examine the
solvent system for improvement in the yield of the product.
At frst, to optimize a suitable reaction medium, the model
reaction was performed in presence of diferent organic sol-
vents such as MeOH, EtOH, DMF, DCM and THF as well
as in H2O:EtOH using catalytic amount of Pd/CAP-ash (5
wt%) at ambient temperature. To our outmost satisfaction,
when H2O:EtOH (3 mL, 1:1, v/v) was used as a solvent, a
considerable enhancement in the conversion of reactants into
the product was observed after 10 min with 98% product
yield (Table 1, entry 12).
satisfactorily and it resulted into lower yield even after 30
min. (Table 1, entries 10-11). Furthermore, we also per-
formed the model reaction directly in presence of water
extract of CAP-ash (3 mL) in ethanol (3 mL), the result was
equally good (Table 1, entry 13), but separation of catalytic
residue from the product became a tedious. However, reac-
tions were not performed at room temperature as well as
at elevated temperature under catalyst-free and solvent-free
conditions may be due to loss of homogeneity of reactant
and catalyst (Table 1, entry 14, 15).
Furthermore, to investigate the scope and limitation of
the current protocol, with high yielding conditions in hand,
several test reactions were carried out using electronically
diverse aryl bromide and boronic acid under the optimized
reaction conditions (Table 2).
It was noticed that aryl bromides with electron‐with-
drawing substituents, like nitro, cyano, carbonyl groups at
para-position, (Table 2, entry 3c–i) aforded the correspond-
ing coupled products in excellent yield while in the case of
meta-substituted electron-withdrawing groups the yield was
very moderate (Table 2, entry 3j). It is important to men-
tion that, generally, aryl bromides with electron withdrawing
Moreover, when THF and DCM were used as solvents,
product formation started but the reaction could not proceed
1 3

Agro-Waste Generated Pd/CAP-Ash Catalyzed Ligand-Free Approach for Suzuki–Miyaura Coupling…
Table 2 Suzuki–Miyaura
Pd/CAP-ash, water: ethanol
X
(HO)2B
coupling of aryl bromides and
arylboronic acids in Pd/CAP-
ash (3a–t)a
R1
R1
R2
R2
RT/ Stirr open air
2a-c
3a-t
1a-n
2ꢀ1
2ꢀ1
3ab (10c min, 96d %) 3b (10 min, 90 %)
3c (5 min, 97 %)
3d (5 min, 94 %)
2
2
1&
1&
3e (5 min, 95 %)
3f (5 min, 92 %)
3g (10 min, 93 %)
3h (10 min, 90 %)
2
+
+ꢁ&2
+ꢀ1
2ꢀ1
3i (5 min, 95 %)
3j (15 min, 88 %)
3k (5 min, 93 %)
3l (5 min, 94 %)
1
+
0H2
3m (10 min, 92 %)
3n (15 min, 85 %)
3o (10 min, 82 %)
3s (10 min, 90 %)
3p (20 min, 78 %)
2
2
3q (10 min, 98 %)
3r (15 min, 96 %)
3t (15 min, 88 %)
aReaction conditions: aryl bromide (1.0 mmol), arylboronic acid (1.2 mmol), Pd/CAP-ash (5 wt
%), H2O: EtOH (3 mL), room temperature in air, b entry, c time, d isolated yields
groups at the para position are much more reactive than aryl
bromides bearing electron‐donating groups. However, the
reaction of aryl bromides with electron‐donating groups,
such as methoxy and amine, gave the coupling products with
slightly lower yields (Table 2, entry 3k-l). No signifcant
diferences were observed in the yield with para as well as
meta substituted electron‐rich aryl bromides (Table 2, entry
3m). Further, to investigate the efects of the new catalytic
system with sterically hindered substrate, we performed the
reaction between 9-bromoanthracene and phenylboronic
acid under similar reaction conditions. It was observed that
9-bromoanthracene gave superior product formation within
shorter reaction time (Table 2, entry 3n). No signifcant dif-
ferences were observed in the yield when phenylboronic
acid was replaced by naphthylboronic acid and 4-ethylphe-
nylboronic acid (Table 2, entry 3a–b, 3q–t). The coupling
with 5-bromo-1H-indole and phenyl boronic acid also occurs
efciently with low yield (Table 2, entry 3p).
Encouraged by these tempting results for coupling of
aryl bromides with boronic acids for making biphenyls, we
then decided to check the efciency of Pd/CAP-ash cata-
lytic system for double Suzuki–Miyaura coupling reaction
under optimized reaction conditions (Scheme 2). Unfortu-
nately after 1h very poor yield of the product was observed
Scheme 2 Double Suzuki–
B(OH)2
Miyaura reaction of 4, 4′-dibro-
Reaction Conditions
Br
Br
+
2
mobiphenyl and phenylboronic
acid
3v
1p
2a
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R. C. Patil et al.
on TLC when 1:2 molar quantities of 4, 4′-dibromobiphe-
nyl reacted with phenyl boronic acid (Table 3, entry 1). A
very few methods are reported in the literature for double
Suzuki–Miyaura reaction by the coupling of aryl dibromides
with boronic acids employing various solvent systems such
as DMF [80], DMSO [81], 1,4-Dioxane [82] and THF [83].
Therefore, we decided to optimize another solvent system
for double Suzuki–Miyaura coupling reaction by keeping the
same amount of catalyst.
was observed in a H2O:DMF (4:1) (Table 3, entry 8), it was
chosen as reaction solvent for further study.
From results demonstrated in Table 4 (entry 3u–w), it was
observed that the double coupling also worked efciently
under this catalytic system with quantitative yields within
stipulated time period.
3.3 Reusability of Catalyst
For optimization study, the reaction between 4, 4′-dibro-
mobiphenyl (1.0 mmol), phenyl boronic acid (2.4 mmol) in
presence of Pd/CAP-ash (5 wt%) was performed in difer-
ent organic solvents such as EtOH, CH3CN, Toluene, DMF
and results are summarized in Table 3. In case of EtOH,
CH3CN, and Toluene good to moderate yields of products
were recorded (Table 3, entries 2–4) while DMF as a sol-
vent gave quantitative yield after 60 min. Furthermore, it
was observed that addition of water in DMF had a dramatic
efect on reaction. When the reaction was carried out in
H2O:DMF solvent, the time required for reaction was dras-
tically reduced to 15 min with excellent yield of the product
(table 3, entries 6–8). This efect is probably due to the fne
dispersibility of CAP-ash in water and more solubility of
organic substrates in DMF. As the drastic rate enhancement
For any catalytic process, the reusability of catalyst is one
of the most important aspects for possible application on
commercial and industrial scale. In order to check recycla-
bility of the catalytic system investigation of the reaction of
4-bromobenzophenone with phenyl boronic acid in presence
of Pd/CAP-ash was done. After the frst cycle, the reaction
mixture was extracted with ethyl acetate (2 × 10 mL). The
clearly separated organic fraction containing product has
been removed from the system and the Pd/CAP-ash and
aqueous layer remaining in the reactor were directly used for
the next reaction run followed by addition of fresh reactants
and ethanol. After 3 consecutive run the catalytic activity
signifcantly decreases with a lower yield of cross-coupled
product (Fig 9). The decrease in the yield of the model reac-
tion on recycling was accounted to the fair dispersion of fne
palladium particles into organic portion during the work-up
process.
Table 3 Efect of solvent on double Suzuki–Miyaura coupling
reactiona
The catalytic activity of Pd/CAP-ash is currently not well
understood. Although we did not investigate the reaction
mechanism, a probable mechanistic rationale portraying the
sequence of events for this cross coupling reaction is sche-
matically shown by a conceptual picture (Scheme 3).
A carbon-carbon bond formation mechanism is very
straight forward. The frst step being oxidative addition of
aryl halides to palladium metal which is supported on CAP-
ash. This results in the formation of frst organopalladium
species. The organopalladium species on reaction with metal
hydroxides present in CAP-ash are subjected to transmetal-
lation with boronate complex. This leads to formation of
second organopalladium species where both the aryl rings
are attached to same metal. Finally this organopalladium
Entry
Solvent (3 mL)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
H2O:EtOH
EtOH
CH3CN
Toluene
DMF
H2O:DMF(1:1)
H2ODMF(2:1)
H2O:DMF(4:1)
60
60
60
60
60
15
15
15
41
82
80
78
90
92
93
96
aReaction conditions: 4, 4′ dibromobiphenyl (1.0 mmol), phenylbo-
ronic acid (2.4 mmol), Pd/CAP-ash (5 wt %) and solvent (3 mL) at
room temperature
bIsolated yields
Table 4 The Suzuki–Miyaura
coupling of various aryl
1
dibromides and arylboronic
acids in Pd/CAP-ash (3u-w)a
3ub (15c min, 92d %)
3v (10 min, 96 %)
3w (10 min, 90 %)
aReaction conditions: aryl dibromide (1.0 mmol), arylboronic acid (2.4 mmol), Pd/CAP-ash (5
wt %), H2O: DMF (3 mL), room temperature in air, b entry, c time, d isolated yields
1 3

Agro-Waste Generated Pd/CAP-Ash Catalyzed Ligand-Free Approach for Suzuki–Miyaura Coupling…
Fig. 9 Procedure for reusability of catalytic system
Scheme 3 Plausible mechanism of Suzuki–Miyaura coupling reactions in the presence of Pd/CAP-ash
species upon reductive elimination gives the desired biaryl
derivatives.
high Pd loading, longer reaction time, use of base for the
reaction. Pd/CAP-ash (Table 5, entry 8) is superior in
terms of external base-free, signifcantly avoids the neces-
sity of an added ligand, non-toxicity, the low cost and eco-
friendly reaction protocol that generates the product in
very good yields.
The catalytic activity of Pd/CAP-ash was examined
by comparing it with several reported catalysts (Table 5,
entry 1–7) for Suzuki–Miyaura coupling reaction between
4-bromoanisole and phenylboronic acid. The comparison
reveals that the most of the reported Pd catalysts acquired
1 3

R. C. Patil et al.
Table 5 Comparison of Pd/CAP-ash catalyst with some previously reported catalysts for synthesis of 3k
Entry
Catalyst
Water extract of agro
Co-solvent (mL)
Temp. (ºC)
Time
Yield (%)
Ref
waste ash (mL)
1
2
3
Pd (OAc)2
(1 mol %)
Rice Straw (3 mL)
–
RT
RT
RT
2 h
88
99
88
[84]
[85]
[86]
Pd (OAc)2
Banana peels
(3 mL)
–
5 min
3 h
(0.5 mol %)
Pd (OAc)2
Water Hyacinth
–
(1 mol %)
(6 mL)
4
5
Pd (OAc)2 (0.5 mol %)
Pd(OAc)2 (1.0 mol %)
Papaya bark (2 mL)
WEPA (3 ml)
EtOH (2 mL)
RT
RT
5 min
5 min
97
99
[87]
[88]
EtOH
(0.5 mL)
6
Pd/RHA
K2CO3
EtOH (3 mL)
100
24 h
70
[89]
(0.5 mol% of Pd)
7
8
Pd/CCSF (0.5 mol %)
K2CO3
–
EtOH
H2O:EtOH (3 mL)
100
RT
3 h
5 min
99
93
[90]
Present work
Pd/CAP- ash
(5 wt %)
Declarations
4 Conclusion
Conflict of interest The authors declare that they have no confict of
In conclusion, we have developed cost-efective and a
convenient greener protocol for Pd/CAP-ash catalyzed
Suzuki–Miyaura coupling reactions under ligand free
conditions in open air at room temperature. The absence
of any additive/promoter, use of CAP-ash in aqueous
organic solvent, wider substrate scope makes this proto-
col attractive pathway for Suzuki–Miyaura coupling reac-
tions. The protocol greatly enhances the synthetic utility
of biaryl products and adds a new dimension to chemistry
of cross coupling reaction. The present protocol ofers
many advantages including high conversion in short reac-
tion time, avoiding use of expensive and environmentally
unfavorable organic ligands and external base, wasteful
procedure using waste bio-mass for catalyst preparation,
easily separated from organic product and recycling of
catalyst system which carries a dramatic impact on the
sustainability of the process. Further eforts to utilize pre-
sent catalytic systems in other synthetic reactions are in
progress in our laboratory, and we expect the methodology
presented will fnd great utility in academic, research and
industrial applications in the near future.
interest.
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1 3