Palladium-catalysed room-temperature Suzuki–Miyaura coupling in water extract of pomegranate ash, a bio-derived sustainable and renewable medium

Article abstract of DOI:10.1002/aoc.5126

An agro waste-derived, ‘water extract of pomegranate ash’ (WEPA), has been utilized for the first time as a renewable medium for Pd(OAc)₂-catalysed Suzuki–Miyaura cross-coupling at room temperature. This method offers a simple and sustainable synthesis of biaryls from aryl halides and arylboronic acids under ligand- and external base-free aerobic and ambient conditions. This method has been found effective for both activated and unactivated aryl halides in the production of biaryls with moderate to nearly quantitative yields. The protocol shows high chemoselectivity over identical/similar reactive sites in aryl halides (i.e. selectivity over identical halogens or different halogens of aryl halides). This method exhibits high regioselectivity, i.e. the selective reactivity of a halogen over other identical halogens at different positions on the aromatic nucleus. Therefore, we disclose here a clean, benign, substantial chemo- and regioselective and highly economic alternative method for the palladium-assisted synthesis of biaryls using an agro waste-derived medium.

Full text of DOI:10.1002/aoc.5126

Received: 8 May 2019
DOI: 10.1002/aoc.5126
Revised: 19 June 2019
Accepted: 27 June 2019
C O M M U N I C A T I O N
Palladium‐catalysed room‐temperature Suzuki–Miyaura
coupling in water extract of pomegranate ash, a bio‐derived
sustainable and renewable medium
Rama Moorhy Appa | S. Siva Prasad | Jangam Lakshmidevi |
Bandameeda Ramesh Naidu | Manchala Narasimhulu | Katta Venkateswarlu
Laboratory for Synthetic and Natural
An agro waste‐derived, ‘water extract of pomegranate ash’ (WEPA), has been
Products Chemistry, Department of
Chemistry, Yogi Vemana University,
utilized for the first time as a renewable medium for Pd(OAc)2‐catalysed
Kadapa 516005, India
Suzuki–Miyaura cross‐coupling at room temperature. This method offers a
simple and sustainable synthesis of biaryls from aryl halides and arylboronic
acids under ligand‐ and external base‐free aerobic and ambient conditions.
This method has been found effective for both activated and unactivated aryl
halides in the production of biaryls with moderate to nearly quantitative yields.
The protocol shows high chemoselectivity over identical/similar reactive sites
in aryl halides (i.e. selectivity over identical halogens or different halogens of
aryl halides). This method exhibits high regioselectivity, i.e. the selective reac-
tivity of a halogen over other identical halogens at different positions on the
aromatic nucleus. Therefore, we disclose here a clean, benign, substantial
chemo‐ and regioselective and highly economic alternative method for the
palladium‐assisted synthesis of biaryls using an agro waste‐derived medium.
Correspondence
Katta Venkateswarlu, Department of
Chemistry, Yogi Vemana University,
Kadapa 516005, India.
Email: kvenkat@yogivemanauniversity.
ac.in
Funding information
Council of Scientific and Industrial
Research, Grant/Award Number:
02(0196)/14/EMR‐II; Science and Engi-
neering Research Board, Grant/Award
Number: INSPIRE Fellowship Code:
IF150772
KEYWORDS
aryl halides, arylboronic acids, sustainable media, Suzuki–Miyaura cross‐coupling, WEPA
1 | INTRODUCTION
productive in synthesizing biaryls and offers several
advantages like ready availability of substrates, use of safe
Suzuki–Miyaura cross‐coupling (SMC) serves as a unique
and versatile tool in making carbon–carbon bonds, espe-
cially in biaryl synthesis.[1–5] Biaryls and their heteroatom
analogues are among the most important structural fea-
tures of biologically active natural products, pharmaceuti-
cals and agrochemicals.[5–8] These motifs also serve as
ligands in catalysts, functional materials, conductive poly-
mers and theoretically interesting molecules.[9–12] Due to
the wide‐ranging usefulness of biaryls, transition‐metal‐
mediated methods such as SMC,[13,14] Stille,[15,16]
Negishi,[17] Kumada[18] and Hiyama[19,20] reactions are
realized by cross‐coupling of haloarenes with organome-
tallic partners. Among these protocols, SMC is the most
and high‐stability arylboronic acids, large functional
group tolerance and ease of handling.[21,22]
Palladium catalysts with appropriate ligands and bases
in organic or biphasic media assist as common systems for
SMC reactions.[23] The frequently employed ligands for
this purpose include phosphines,[24,25] palladacycles,[26,27]
sterically crowded N‐heterocyclic carbenes,[28,29]
imines[30,31] and oximes.[32,33] These ligands are highly
active and enhance the rate of oxidative addition (usually
the rate‐determining step) in catalytic cycles.[34] Further-
more, the use of base is obligatory; these play a key role
in the reaction mechanism by increasing the reactivity
of organoborate and metal halide in the transmetallation
Appl Organometal Chem. 2019;e5126.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5126

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APPA ET AL.
step.[35,36] However, most of these catalysts are expensive,
are air‐ and moisture‐sensitive and are to be used in inert
atmosphere in organic solvents.[37,38] Although some
researchers have developed water‐ and (or) air‐stable
ligands, their commercial unavailability, prolonged reac-
tion times and high cost restrict their widespread applica-
bility.[39,40] During the past decade several attempts have
been made in the development of ligand‐free cross‐
coupling reactions.[41–46] Nevertheless, most of these are
operated in harsh conditions[41] or require organic sol-
vents[42,43] and additives such as surfactants[44,45] and
polymers.[46]
WEPA (2.5 ml) was initially used as reaction medium in
the coupling reaction of 4‐bromoanisole (1a; 1 mmol)
and 4‐chlorophenylboronic acid (2a; 1.10 mmol) in the
presence of 0.5 mol% of Pd(OAc)2. To our delight the
reaction proceeded at room temperature (r.t.) to produce
the biphenyl 3a in 58% isolated yield (Table 1, entry 1).
Then the reaction was studied with 3.0 and 3.5 ml of
WEPA and it was observed that 3.0 ml of WEPA is suffi-
cient to mediate the reaction (Table 1, entries 2 and 3).
An increase in the catalyst (Pd(OAc)2) load to 1.0 mol%
enhanced the yield to 92% in 15 min (Table 1, entry 4).
Towards further optimization of the reaction conditions,
we studied the same reaction with some other palladium
catalysts such as PdCl2, Pd(PPh3)2Cl2, Pd(PPh3)4 and
Pd(dba)3 and 3a was formed in yields of 74, 80, 88 and
10% (Table 1, entries 5–8), indicating that Pd(PPh3)2Cl2
and Pd(PPh3)4 are also suitable for the present reaction,
but Pd(OAc)2 was found to be the best. With the addition
of 0.5 ml of ethanol to the reaction mixture containing
Pd(OAc)2, the yield of the product was enhanced to 98%
in 10 min (Table 1, entry 9). This may be due to the com-
plete solubility of the substrates in ethanolic WEPA.
Additional increase of Pd(OAc)2 to 1.5 mol% did not show
any significance difference (Table 1, entry 10). The
absence of Pd(OAc)2 and WEPA was also studied inde-
pendently, and no reaction was observed (Table 1, entries
11 and 12), indicating that the use of Pd(OAc)2 and
WEPA was essential for these experimental conditions.
To understand the effectiveness of WEPA mediation in
SMC, we investigated this method with some reported
agro‐waste‐derived basic media such as water extract of
banana peel ash (WEB),[47] water extract of papaya bark
ash (WEPBA)[53] and water extract of rice straw ash
(WERSA)[54] along with a new base, water extract of
sweet lemon ash (WESLA). The formation of 3a in 82,
68, 52 and 76% yields in 20, 30, 80 and 20 min (Table 1,
entries 13–16) was identified, which indicates WEPA is
the best among these media. This may be due to the pres-
ence of a large quantity of potassium and potassium may
exist in the form of carbonates or oxides.[63] The XPS data
for ash of sweet lemon peel (provided in the supporting
information) also suggest the presence of potassium,
magnesium and calcium with a ratio of 50.2:12.4:37.4
along with carbon, chlorine and oxygen. Furthermore,
the inorganic bases K2CO3 (3 eq.) and Cs2CO3 (3 eq.)
and organic bases triethylamine (3 eq.) and 1,8‐
diazabicyclo[5.4.0]undec‐7‐ene (DBU; 2 eq.) were also
examined in a mixture of pure water (3 ml) and ethanol
(0.5 ml) and it was observed that these reactions are not
at all effective under these conditions (Table 1, entries
17–20) and these require ligands or phase transfer cata-
lysts or organic solvents as that are frequently used in
SMC. On the other hand, WEPA and other agro‐waste
Although recently some efficient methodologies have
been developed for SMC by avoiding external base and
ligands,[47–54] there is still a demand to develop new syn-
thetic protocols since unexpected results can be seen in
SMC by a small change of reaction conditions like tem-
perature, ligand, base, nature of substrates, solvent,
etc.[55] Typical trends in substrate selectivity and reactiv-
ity may be broken.[55] Well‐known ligands which show
significant role in SMC often do not work in the simplest
cases. Even though several hundreds of studies have
appeared in the literature, it remains unanswered, even
for a simple case, which procedure or catalyst is the best
one.[55] On the other hand, several new methodologies
have still appeared using several modified conditions
including ligands, catalysts, solvent, additives, etc.[23–
33,56–61]
In this connection and in continuation of our
recent research on the development of benign methodol-
ogies for C–C coupling,[60–63] we report here an effective
method for SMC using a natural feedstock, water extract
of pomegranate ash (WEPA),[63] as a green and sustain-
able (basic) medium. Indeed, this method with inherent
safeness and renewability can further strengthen the
scope of the use of naturally derived (basic) media for
C–C cross‐couplings, especially in SMC.
2 | RESULTS AND DISCUSSION
WEPA was prepared from pomegranate skins according
to our recent report on WEPA‐mediated Ullman coupling
of aryl halides.[63] WEPA was been found to have a pH of
between 11.7 and 12.1 which is slightly greater than the
pH of aqueous K2CO3 solution.[64] The ash and its water
extract were subjected to X‐ray photoelectron spectros-
copy (XPS) and energy‐dispersive X‐ray spectroscopy
(EDS) analyses, and found to consist of a high amount
of potassium and considerable amount of calcium and
magnesium (K:Ca:Mg ratio of 86.9:7.6:5.5) along with car-
bon, chlorine and oxygen.[63] The basic nature of the
extract has been assumed because of the presence of
metal (such as K, Mg and Ca) oxides and carbonates.

APPA ET AL.
3 of 10
TABLE 1 Optimization of reaction conditions for SMCa
Entry
Catalyst (mol%)
Media (ml) + base (eq.)
Time (min)
Isolated yield (%)
1
Pd(OAc)2 (0.5)
Pd(OAc)2 (0.5)
Pd(OAc)2 (0.5)
Pd(OAc)2 (1.0)
PdCl2 (1.0)
WEPA (2.5)
60
30
58
2
WEPA (3)
85
3
WEPA (3.5)
30
84
4
WEPA (3)
15
92
5
WEPA (3)
30
74
6
Pd(PPh3)2Cl2 (1.0)
Pd(PPh3)4 (1.0)
Pd(dba)3 (1.0)
Pd(OAc)2 (1.0)
Pd(OAc)2 (1.5)
—
WEPA (3)
30
80
7
WEPA (3)
15
88
8
WEPA (3)
30
10
9
WEPA (3) + EtOH (0.5)
WEPA (3) + EtOH (0.5)
WEPA (3) + EtOH (0.5)
Ethanol (3)
10
98
10
11
12
13
14
15
16
17
18
19
20
10
97
720
720
20
No reaction
Pd(OAc)2 (1.0)
Pd(OAc)2 (1.0)
Pd(OAc)2 (1.0)
Pd(OAc)2 (1.0)
Pd(OAc)2 (1.0)
Pd(OAc)2 (1.0)
Pd(OAc)2 (1.0)
Pd(OAc)2 (1.0)
Pd(OAc)2 (1.0)
No reaction
WEB (3) + EtOH (0.5)
WEPBA (3) + EtOH (0.5)
WERSA (3) + EtOH (0.5)
WESLA (3) + EtOH (0.5)
Water (3) + EtOH (0.5) + K2CO3 (3)
Water (3) + EtOH (0.5) + Cs2CO3 (3)
Water (3) + EtOH (0.5) + triethylamine (3)
Water (3) + EtOH (0.5) + DBU (2)
82
30
68
80
52
20
76
720
720
720
720
<5
Trace
Trace
No reaction
aReaction conditions: 1a (1.0 mmol), 2a (1.10 mmol), r.t. and in open air.
extracts with complex bases and other chemical constitu-
ents made the SMC reaction possible in the absence of
added base and ligand system, the reasons for which are
obscure at this stage.
conveniently into the corresponding products (Table 2,
entries 1–19, 24–29 and 37). It is important to note that
the position and electronic properties of aryl bromides
have a minor influence on the coupling products.
Towards the generalization of this method, we exam-
ined a series of differently functionalized aryl bromides
and arylboronic acids with the optimized reaction condi-
tions described above. The results indicated that the aryl
bromides containing neutral or electron‐donating groups,
such as bromo, hydrogen, methoxy, amino, methyl and
dimethoxy, are coupled with arlylboronic acids to provide
high to nearly quantitative yields (87–99%) of biaryls in
5–15 min (Table 2, entries 1–14, 18, 24 and 26). Aryl bro-
mides containing electron‐withdrawing groups such as
cyano, nitro, chloro, formyl and phenyl also produced
the corresponding products in excellent yields (87–96%)
in 10–20 min (Table 2, entries 15–17, 25, 27, 36 and 37).
The functional groups of arylboronic acids such as chloro
(2a), nitro (2c), ester (2d and 2i), methanesulfonyl (2e),
dimethyl (2f), fluoro (2g), acetyl (2h), hydroxyl (2j and
2m), formyl (2k) and methoxy (2l) were transformed
Furthermore, we studied the scope of this reaction
with the most reactive aryl halides such as aryl iodides
and found that biaryls were formed in excellent yields
(94–99%) in just 5 min (Table 2, entries 20–23, 30 and
31). Aryl iodides with electron‐releasing groups such as
methoxy (1j) and methyl (1k), neutral hydrogen (1l)
and electron‐withdrawing groups such as chloro (1q)
were also transformed conveniently into coupling prod-
ucts (3a, 3b, 3m, 3p, 3s and 3z). Arylboronic acids with
different functionalities such as formyl, hydrogen, nitro,
methoxy and chloro showed no influence on the yields
of 3 in the case of aryl iodides.
It is worth mentioning here is that this method
shows its applicability to several challenging substrates
(Table 2, entries 24–43). In this connection polycyclic aryl
bromides
such
as
2‐bromonaphthalene
(1i),
9‐bromoanthracene (1o) and 9‐bromophenanthrene (1p)

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APPA ET AL.
TABLE 2 Substrate screening for WEPA‐mediated SMCa
Entry Aryl halide (1)
Arylboronic acid (2)
Time (min) Biaryl (3)
Yield (%)b
1
2
3
4‐Bromoanisole (1a)
4‐Bromoanisole (1a)
4‐Bromoanisole (1a)
Phenylboronic acid (2b)
5
3b
3c
3d
99
98
92
3‐Nitrophenylboronic acid (2c)
10
15
4‐(tert‐Butoxycarbonyl)phenylboronic acid
(2d)
4
4‐Bromoanisole (1a)
4‐Bromoanisole (1a)
4‐Bromoanisole (1a)
4‐Bromoanisole (1a)
4‐Bromoanisole (1a)
3‐Bromoanisole (1b)
3‐Bromoanisole (1b)
3‐Bromoanisole (1b)
4‐Bromotoluene (1c)
4‐Bromoveratrole (1d)
4‐Bromoanisole (1a)
3‐(Methanesulfonyl)phenylboronic acid (2e)
3,4‐Dimethylphenylboronic acid (2f)
3‐Fluorophenylboronic acid (2g)
4‐Acetylphenylboronic acid (2h)
3‐(Methoxycarbonyl)phenylboronic acid (2i)
Phenylboronic acid (2b)
15
15
15
15
15
10
10
15
15
10
10
3e
3f
92
94
95
95
93
96
95
87
93
93
97
5
6
3g
3h
3i
7
8
9
3j
10
11
12
13
14
4‐Chlorophenylboronic acid (2a)
4‐Hydroxyphenylboronic acid (2j)
3‐Nitrophenylboronic acid (2c)
3k
3l
3m
3n
3o
4‐Formylphenylboronic acid (2k)
4‐Formylphenylboronic acid (2k)
(Continues)

APPA ET AL.
5 of 10
TABLE 2 (Continued)
Entry Aryl halide (1)
Arylboronic acid (2)
4‐Chlorophenylboronic acid (2a)
Time (min) Biaryl (3)
Yield (%)b
15
16
17
18
19
20
21
22
23
4‐Bromobenzonitrile (1e)
15
15
10
5
3p
3q
3r
92
95
96
99
94
94
99
94
99
1‐Bromo‐4‐nitrobenzene (1f) 3‐Nitrophenylboronic acid (2c)
4‐Bromobenzaldehyde (1g)
Bromobenzene (1h)
2‐Bromonaphthalene (1i)
4‐Iodoanisole (1j)
4‐Formylphenylboronic acid (2k)
Phenylboronic acid (2b)
3s
3‐Nitrophenylboronic acid (2c)
4‐Formylphenylboronic acid (2k)
Phenylboronic acid (2b)
10
5
3 t
3p
3b
3 m
3s
4‐Iodoanisole (1j)
5
4‐Iodotoluene (1k)
Iodobenzene (1l)
3‐Nitrophenylboronic acid (2c)
Phenylboronic acid (2b)
5
5
Challenging examples:
24
25
4‐Bromoaniline (1m)
Phenylboronic acid (2b)
15
15
3u
3a
95
92
1‐Bromo‐4‐chlorobenzene
(1n)
4‐Methoxyphenylboronic acid (2l)
26
4‐Bromoanisole (1a)
2‐Methoxyphenylboronic acid (2m)
15
3v
91
(Continues)

6 of 10
APPA ET AL.
TABLE 2 (Continued)
Entry Aryl halide (1)
Arylboronic acid (2)
Time (min) Biaryl (3)
Yield (%)b
27
28
29
30
31
32
33
34
35c
36
37
38
39
41
4‐Bromobenzonitrile (1e)
2‐Methoxyphenylboronic acid (2m)
Phenylboronic acid (2b)
15
10
15
5
3w
93
9‐Bromoanthracene (1o)
3x
95
9‐Bromophenanthrene (1p)
3‐(Methoxycarbonyl)phenylboronic acid (2i)
3y
92
1‐Chloro‐4‐iodobenzene (1q) 4‐Methoxyphenylboronic acid (2l)
1‐Chloro‐4‐iodobenzene (1q) 4‐Chlorophenylboronic acid (2a)
3a
95
5
3z
95
1,4‐Dibromobenzene (1r)
1,3‐Dibromobenzene (1s)
1‐Bromo‐2‐iodobenzene (1t)
1‐Bromo‐2‐iodobenzene (1t)
2‐Bromobiphenyl (3ac)
Phenylboronic acid (2b)
Phenylboronic acid (2b)
Phenylboronic acid (2b)
Phenylboronic acid (2b)
Phenylboronic acid (2b)
3‐Nitrophenylboronic acid (2c)
Phenylboronic acid (2b)
Phenylboronic acid (2b)
Phenylboronic acid (2b)
15
15
15
20
20
15
15
20
15
3aa
89
3ab
86
3ac
90
3ac + 3ad
3ad
24 and 68
87
2‐Bromonitrobenzene (1u)
1,2‐Dibromobenzene (1v)
2,4‐Dibromoaniline (1w)
1,3,5‐Tribromobenzene (1x)
3ae
93
3ac + 3ad
3af
74 and 13
88
3ag + 3ah + 3ai 33, 39 and 17
(Continues)

APPA ET AL.
7 of 10
TABLE 2 (Continued)
Entry Aryl halide (1)
Arylboronic acid (2)
Time (min) Biaryl (3)
Yield (%)b
42
1,3‐Dibromo‐5‐
Phenylboronic acid (2b)
15
3ag + 3ah + 3ai 65, 18 and trace
iodobenzene (1y)
43d
1,3‐Dibromo‐5‐
Phenylboronic acid (2b)
15
3ag + 3ah + 3ai 12, 21 and 51
iodobenzene (1y)
aReaction conditions: 1 (1.0 mmol), 2 (1.10 mmol), WEPA (3 ml), EtOH (0.5 ml) and Pd(OAc)2 (0.1 mol%) at r.t. and in open air.
bIsolated yield.
cReaction conditions: 1t (2.1 mmol), 2b (1.0 mmol), WEPA (5 ml), EtOH (0.8 ml) and Pd(OAc)2 (0.1 mol%) at r.t. and in open air.
dReaction conditions: 1x (3.1 mmol), 2b (1.0 mmol), WEPA (6 ml), EtOH (1 ml) and Pd(OAc)2 (0.1 mol%) at r.t. and in open air.
were found to produce coupled products with 92–95%
yields in 10–15 min. However, in these cases the forma-
tion of low yields of products or protodebromination is
frequently encountered.[65,66] The selectivity of halogen
was found in the presence of free‐amino function (which
may otherwise result in a Chan–Lam coupling product[67]
or inactivation of the catalyst[68]) (Table 2, entries 24 and
39). A very high degree of chemoselectivity was observed
when the substrates contained mixed halogens (chlorine–
bromine (1n), chlorine–iodine (1q) and bromine–iodine
(1t and 1y)) and identical bromines (1r, 1s and 1v). The
selective reactivity of bromine or iodine was found in
the presence of chlorine (Table 2, entries 25, 30 and 31).
When we used 1,4‐dibromobenzene (1r), 1,3‐
dibromobenzene (1s) and 1,2‐dibromobenzene (1v) to
react individually with phenylboronic acid (2b), we found
the formation of chemoselective products in the case of 1r
and 1s (Table 2, entries 32 and 33), but the formation of

8 of 10
APPA ET AL.
chemoselective product 2‐bromobiphenyl (3ac) in 74%
yield along with o‐terphenyl (3ad) in 13% yield was
observed with 1v (Table 2, entry 38). To our delight the
reaction of 1‐bromo‐2‐iodobenzene (1t; 1 eq.) with 2b
(1 eq.) takes place only at iodo group to give 3ac in 90%
yield (Table 2, entry 34) while 2.1 eq. of 2b gives 3ad as
major product (68%) along with 3ac (24%) (Table 2, entry
35). Significant regioselectivity was also observed in the
reaction of 2,4‐dibromoaniline (1w) with 2b, which forms
only 4‐amino‐3‐bromobiphenyl (3af) in 88% yield
(Table 2, entry 39). In this case the reaction of bromine
at the 4‐position was only observed instead of bromine
at the 2‐position of 1w. We also studied the SMC reaction
of symmetric 1,3,5‐tribromobenzene (1x; 1 eq.) with 2b
(1 eq.) and found the formation of single‐SMC, double‐
SMC and triple‐SMC products (3ag–3ai) in 33, 39 and
17% yields (Table 2, entry 41). Negligible yield of triple‐
SMC product 3ai was observed when 1,3‐dibromo‐5‐
iodobenzene (1y) was used as substrate (Table 2, entry
42) instead of 1x. The single‐SMC product 3ag was the
major one (65%) and double‐SMC product 3ah was the
minor one (18%) in this case. No trace amount of product
with iodo was observed in this reaction, which indicates
that this reaction is also chemoselective. However, the
reaction of 1y (1 eq.) with excess of 2b (3.1 eq.) forms
3ag, 3ah and 3ai in 12, 21 and 51% yield (Table 2,
entry 43).
intermediate C contributes to further transmetallation
(IV) with the ionic species E (species E may be formed
by a reaction of arylboronic acid (2) with the base of
WEPA) to form diarylpalladium(II) intermediate D.
Finally, the reductive elimination (V) of D produces the
biaryls (3) and active Pd(0) species (A).
3 | EXPERIMENTAL
3.1 | Materials and methods
The substrates used for the current protocol were obtained
from Spectrochem, TCI Chemicals, Sigma‐Aldrich, Merck
and Alfa Aesar and were used (unless and otherwise men-
tioned) with no further purification. WEPA and WESLA
were prepared by the procedures mentioned at Sections
3.2 and 3.3. All the reactions were performed under ambi-
ent conditions in open air and the reactions were moni-
tored for their progress using TLC (Merck silica gel
60F254). Silica gel‐packed glass columns were employed
for the purification of biaryls using various ratios of n‐
hexane and ethyl acetate as eluent. An Avance
1
400/100 MHz spectrometer was used to record H NMR
and 13C NMR spectra at ambient temperature with
tetramethylsilane as an internal standard. A Thermo
LCQ Advantage Max‐IT system was used for recording
the molecular weight of the biaryls. Scanning electron
microscopy (SEM) images and EDS data were recorded
using a JEOL JSM‐IT‐500 instrument. Particle size of palla-
dium was measured using a Zetasizer Nano series Nano‐
S90. The pH was measured using an ELICO pH meter.
Finally, we carried out a reaction of 4‐chloroanisole
(1z) with 2b and found the reaction to form a very low
yield (<15%) of coupling product (3b) in 12 h under the
current conditions. This may be due to the stronger C─Cl
bond when compared with C─Br and C─I. Thus all the
above results indicate that our palladium–WEPA system
is quite significant for achieving biaryls from activated
and deactivated aryl bromides/iodides at r.t. in aerobic
conditions.
3.2 | Procedure for preparation of WEPA
A plausible mechanism of WEPA‐mediated SMC is
illustrated in Scheme 1 and is similar to the routine
base‐mediated couplings[69,70] but base of WEPA replaces
the conventional base. The complex ingredients and base
systems of WEPA and other agro‐waste extracts are sup-
posed to be responsible for the effectiveness of the SMCs.
Initially, the reduction (I) of Pd(II) of Pd(OAc)2 takes
place to form Pd(0) species (A) in WEPA. The formation
of A was evidenced by EDS spectral analysis (provided
in supporting information) of the residual solid that was
obtained by phase separation after the reaction of 1a with
2a. Also observed is the formation of very fine particles of
Pd(0) with size ranges of 4–6 and 27–34 nm by particle
size analysis. Species A formed may participate in oxida-
tive addition (II) with aryl halides (1) to result in the
intermediate B and it is involved in transmetallation
(III) with the base of WEPA to form C. The Pd(II)
WEPA was prepared according to our recent report on
the Ullman coupling of aryl halides.[63]
3.3 | Procedure for preparation of WESLA
We adopted a similar procedure for the preparation of
WESLA to that used in the case of WEPA.[63] Sweet
lemon peels were washed with distilled water to remove
impurities and were cut into small pieces. These pieces
after shade‐drying for 4 days were turned into ash by
burning. An amount of 10 g of ash was suspended in
100 ml of deionized water and the resultant suspension
was heated at 80°C for 50 min with vigorous stirring.
The mixture was then cooled to r.t. and filtered to afford
WESLA.

APPA ET AL.
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