A Bio-Inspired Magnetically Recoverable Palladium Nanocatalyst for the Ullmann Coupling reaction of Aryl halides and Arylboronic acids In Aqueous Media

Article abstract of DOI:10.1002/aoc.5570

Palladium nanoparticles supported on polydopamine-coated iron oxide nanoparticles (Pd/Fe₃O₄@PDA) were found to catalyze the Ullmann homocoupling of a wide variety of aryl halides, arylboronic acids and aryldiazonium salts in aqueous media in the presence of randomly methylated β-cyclodextrin (RM-β-CD). The synthesized nanoparticles were characterized by techniques such as TEM, SEM, EDX, XPS, ICP-AES and XRD. The synthesized catalyst can be easily recovered magnetically and reused up to five cycles without any significant loss of activity. This is the first report demonstrating the use of magnetically recoverable catalyst for Ullmann homocoupling reactions of aryl halides, arylboronic acids and aryldiazonium salts in water.

Full text of DOI:10.1002/aoc.5570

Received: 12 August 2019
DOI: 10.1002/aoc.5570
Revised: 6 January 2020
Accepted: 13 January 2020
F U L L P A P E R
A Bio-Inspired Magnetically Recoverable Palladium
Nanocatalyst for the Ullmann Coupling reaction of Aryl
halides and Arylboronic acids In Aqueous Media
Abhishek V. Dubey
| A. Vijay Kumar
Department of Chemistry, Institute of
Abstract
Chemical Technology, Matunga, Mumbai,
Maharashtra, India
Palladium nanoparticles supported on polydopamine-coated iron oxide
nanoparticles (Pd/Fe O @PDA) were found to catalyze the Ullmann homo-
3
4
Correspondence
coupling of a wide variety of aryl halides, arylboronic acids and aryldiazonium
A. Vijay Kumar, Department of
Chemistry, Institute of Chemical
Technology, Matunga. Mumbai,
Maharashtra, India – 400019.
Email: vijayakki@gmail.com
salts in aqueous media in the presence of randomly methylated β-cyclodextrin
(RM-β-CD). The synthesized nanoparticles were characterized by techniques
such as TEM, SEM, EDX, XPS, ICP-AES and XRD. The synthesized catalyst
can be easily recovered magnetically and reused up to five cycles without any
significant loss of activity. This is the first report demonstrating the use of mag-
netically recoverable catalyst for Ullmann homocoupling reactions of aryl
halides, arylboronic acids and aryldiazonium salts in water.
Funding information
DST-SERB, Grant/Award Number:
YSS/2015/002064; University Grant
Commission (UGC)
K E Y W O R D S
biomimetic catalysis, Cyclodextrins, Homocoupling, Polydopamine, water
[
4,31]
1
| INTRODUCTION
desirable
as most of these methods are homogeneous
protocols and suffer from limited utility in practical pro-
cesses because of challenges associated with catalyst non-
reusability, products free of metal contaminations and
requisite of ligands. Furthermore, these protocols are
mainly demonstrated in organic solvents and very few
In recent years, the aryl-aryl cross/homo-coupling reac-
tions have emerged as one the most important synthetic
transformations for the synthesis of symmetrical and
[1–5]
unsymmetrical biaryls,
which are building blocks for
[32]
the synthesis of a wide range of pharmaceuticals, medici-
nally important molecules, polymers, alkaloids, and other
are reported to work under aqueous conditions.
Hence, from a sustainable chemistry viewpoint, the
development of heterogeneous based catalysts for
Ullmann homocoupling protocols in a cheap and benign
solvent media like water are highly desirable.
Although, the heterogeneous nanocatalysts regime of
catalysts are categorically superior over the homogenous
systems for most organic transformations, it is often diffi-
[6–8]
advanced materials.
In this context, over the decades,
the Ullmann homocoupling of aryl halides and
arylboronic acids has been widely utilized for the prepa-
ration of symmetrical biaryls. However, the use of stoi-
chiometric amount of copper, high reaction temperature
conditions renders these methods impractical for the syn-
thesis of biaryls bearing thermosensitive functional
cult to isolate and separate them through filtration or
[9]
[33,34]
groups. Till date, substantial work has been introduced
centrifugation methods.
To overcome these issues,
[
10–17]
for the preparation of symmetrical biaryls with Pd,
incorporation of recoverable magnetic nanoparticles
(MNPs) as supports appear to be the most logical solu-
tion, which in addition also increases surface area of
nanocatalysts. Among MNPs, Fe O -based magnetic
[18–21]
[22]
[23,24]
[25]
[26]
Cu,
in catalytic amounts.
and efficient methods is of great interest and highly
Ru,
Au,
Fe
and Ni-based catalysts
[
27–30]
Yet, the development of new
3
4
Appl Organometal Chem. 2020;e5570.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5570

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DUBEY AND KUMAR
[54,55]
nanocatalysts are most preferred, 35–37] on account of
their non-toxic nature and easy preparation methods.
[
various synthetic organic transformations
and
[
38]
[56–59]
designing of sustainable catalytic systems,
herein
Often, these iron oxide nanoparticles are fabricated with
silica, dopamine, carbons and polymers, such capping
plays a dual role of preventing the nanoparticle from
we demonstrate a biomimetic magnetically retrievable
Pd/Fe O @PDA nanocatalyst for homo-coupling and
3
4
cross-coupling reactions of arylboronic acids and aryl
halides for construction of symmetrical-unsymmetrical
biaryl compounds utilizing water as solvent. Scheme 1
outlines various palladium based catalytic approaches for
homocoupling reactions.
[
39–41]
aggregation and enhancing overall catalytic activity.
[42–44]
Polydopamine
is one such analogous and promising
biomimetic polymeric material, which has high affinity
for iron oxide surface and is easily prepared by self-
polymerization of dopamine (the well-known neurotrans-
mitter) at ambient conditions.
The other important aspect is solvent, as
2 | RESULTS AND DISCUSSION
[45–47]
solvents
play a major role in the execution of pro-
cesses in chemical industry, they have a profound impact
on cost, safety and health issues. Henceforth, from green
chemistry perspective, development of new methodolo-
gies in a benign solvent like water can be a promising
alternative. However, the low solubility of various
organic substrates in aqueous medium invalidates the
preferences and makes it hard to channel the full utility.
In such cases, the utilization of supramolecular based
Initially, the synthesis of the Pd/Fe O @PDA catalyst
3
4
was carried out by an in-situ coating of Fe O
3
4
nanoparticles (50–100 nm) with polydopamine via self-
polymerization of dopamine precursor in air under basic
pH (8.4) conditions. In the subsequent step, palladium
nanoparticles were immobilized on polymer-coated iron
oxide nanoparticle. The as-made catalyst was further
completely characterized by techniques such as TEM,
SEM, XPS, XRD, and ICP-AES for establishing the struc-
ture and morphological changes.
[48–50]
catalysts such as cyclodextrins (CDs)
that emulate
the solubility issues and favor the reaction progress are
necessary for achieving significant product conversions.
Also owing to the unique properties such as host-guest
inclusion complexes with various organic molecules, the
use of hydrophobic cavity and hydrophilic exterior of
CDs highly qualify them as benign mass transfer
reagents, which has been widely explored over the past
years for various reactions, such as oxidations, reduc-
tions, catalysis by CD-metal complex catalysis, biphasic
The X-ray diffraction patterns of the obtained for
PdCl , Fe O , Polydopamine (PDA), Fe O @PDA, native
2
3
4
3 4
and recycled Pd/Fe O @PDA are presented in Figure S5
3
4
(refer to ESI). The characteristic diffraction peaks at 2θ of
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
30.2 , 35.5 , 37.3 , 43.3 , 53.8 , 57.2 , and 62.8 corre-
spond to the diffraction of (220), (311), (222), (400), (422),
(511), and (440) of the Fe O , which were indexed to
3
4
Fe O4 (JCPDS 65–3107). All these diffraction peaks
3
[51–53]
reactions and epoxide ring opening
match with the cubic structure of Fe O4 (JCPDS
3
Therefore, with such a view, in continuation to our
interest in the development of metal supported on
functionalized and polymer-coated iron oxides for
65–3107). Polydopamine being more amorphous struc-
ture did not show characteristic peaks. The fifth recycled
Pd/Fe O @PDA catalyst spectra shows the same
3
4
SCHEME 1 Different Catalytic Approaches for Homocoupling Reactions

DUBEY AND KUMAR
3 of 10
characteristic peaks as the native one, this evidently indi-
cates that crystallography of the catalyst remains intact
even after multiple cycles of usage.
found to be most effective to afford the product yield 92%
and was continued for further study (Table 1, entry 8).
The drastic decrease in homocoupled product was
observed when the equivalents of base was reduced
(Table 1, entry 9), which clearly shows the quantitative
To determine the surface morphology of the
Pd/Fe O @PDA catalyst we performed the SEM and
3
4
[
61]
TEM analysis; for elemental composition and mapping
EDX analysis was also carried out. In Figure S1 and S3
role of bases in typical coupling mechanism.
The reac-
tion did not go to completion and afforded less yield
when 0.15 mmol of glucose was used, this might be due
to the inability in reducing metal further back to its lower
oxidation state which is mainly responsible for catalytic
cycle. On the other hand, more dehalogenated product
was observed when an excess of glucose used, and its
absence gave poor yields of desired product (Table 1,
entries 10 and 11). The blank homocoupling reaction
without use of any catalyst showed no progress (Table 1,
entry 12). A decrease in the amount of Pd/Fe O @PDA
(ESI), SEM images show equal distribution of globular
types layers stacked on each other, with a uniform parti-
cle distribution. The EDX analysis Figure S2 and S4 (ESI)
suggest that about 6.81 wt% of palladium is supported on
the Pd/Fe O @PDA catalyst. Further detailed study of
3
4
surface morphology was investigated by using TEM anal-
ysis, Figure 1 and 2 shows a uniform polydopamine coat-
ing (10–100 nm) loaded with evenly distributed Pd
nanoparticles of approximate size (2–5 nm).
3
4
The amount of Pd metal loaded on Fe O @PDA as
catalyst resulted in lowering of product yield and no
results was obtained under base-free conditions (Table 1,
entries 13 and 19). Varying the stoichiometry and use of
other cyclodextrin derivatives did not improve the prod-
uct yield (Table 1, entries 14–18). A reaction without use
of Cyclodextrin shows poor yield of homocoupled prod-
uct (Table 1, entry 15). Cyclodextrin possess a distin-
guished hydrophilic outer surface and a hydrophobic
inner cavity. The cavity is mainly accountable for the spe-
cific reversible binding of molecules (inclusion phenome-
non) enhancing the interaction of reactant and catalyst,
which is a driving force for reactions. Also, role of CD
may also be to increases the solubility of the sub-
3
4
revealed by ICP-AES analysis was found to be 6.92 wt%.
The XPS survey spectrum of the native and recycled
Pd/Fe O @PDA catalysts showed Pd 3d core level lines,
3
4
the binding energy peak of Pd 3d5/2 and 3d3/2 in the fresh
and five-time reused catalyst is found to be
35.31/340.51 eV and 335.6/340.6 (Figure 3 and 4), indi-
3
cating that the loaded palladium is in its zero-oxidation
state. The peaks at 338.7 eV and 343.4 eV in figure 4 can
be assigned to Pd (II) species i.e. PdO which is in agree-
[60]
ment with literature values.
The low intensities of
these peaks in the spectra indicate that the major species
present is Pd (0).
[
62,63]
In an attempt to ascribe catalyst efficiency for homo-
coupling reactions, initially, we started our investigation
using 4-iodoanisole as a substrate with the combination
of Pd/Fe O @PDA and randomly methylated (RM)-β-
strate.
This could be the explanation for improve in
yield with the usage of Cyclodextrin in reaction. We per-
formed a test with Fe O @PDA to understand the real
3
4
catalyst, however no sign of homocoupled product
observe which clearly shows role of Pd is important for
reaction.
With the optimized reaction conditions in our hand,
we studied the substrate scope by using various
substituted aryl halides. Initially, iodobenzene was tried,
as they are the model substrates and provide access to
basic information to understand the electronic effect in
3
4
cyclodextrin as a mass transfer reagent, Na CO as base,
2
3
and glucose as a reductant under aqueous conditions at
ꢀ
1
00 C. The reaction was allowed to proceed for 24 hr,
the homocoupled product was obtained in 52% yield
Table 1, entry 1) with dehalogenated side products. With
this preliminary result, next we screened various other
(
bases (Table 1, entry 2–8), among which Cs CO was
2
3
FIGURE 1 TEM images of
native Pd/Fe @PDA
nanoparticles (a and b)
3 4
O

4
of 10
DUBEY AND KUMAR
FIGURE 2 TEM images of
recycled Pd/Fe O @PDA
3 4
nanoparticles (c and d)
a
TABLE 1 Optimization of reaction conditions
Yield
b
Entry Base
Na CO
CO
KOH
NaOMe
Et
Pyridine
Cs CO
Reductant Cyclodextrin [%]
1
2
3
4
5
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
3
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
--
RM-β-CD
RM-β-CD
RM-β-CD
RM-β-CD
RM-β-CD
RM-β-CD
RM-β-CD
RM-β-CD
RM-β-CD
RM-β-CD
RM-β-CD
RM-β-CD
52
K
2
3
64
24
72
39
32
92
3
N
3 4
FIGURE 3 XPS analysis of Pd in native Pd/Fe O @PDA
2
3
c
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
57
d
e
0
1
2
3
4
5
6
7
8
9
0
1
62 , 71
26
f
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
NR
g
68
h
RM-β-CD
--
74
19
β-CD
61
α-CD
27
γ-CD
32
--
RM-β-CD
γ-CD
NR
i
Cs
2
CO
3
3
32
j
Cs
2
CO
RM-β-CD
NR
a
Reaction conditions: Pd/Fe3O4@PDA (50 mg), 4-iodoanisole (1 mmol),
base (1.0 mmol), glucose (0.25 mmol), RM-β-CD (100 mg) and solvent (4
b
c
d
e
ml), Isolated yield, 0.5 mmol of Cs
2
CO
3
, 0.15 mmol of glucose, 0.5 mmol
f
g
h
i
of glucose, without catalyst, Catalyst 25 mg, 50 mg of RM-β-CD, Pd/C as
j
FIGURE 4 XPS analysis of recycled Pd/Fe
3
O
4
@PDA
3 4
catalyst, Fe O @PDA as catalyst, RM = Randomly methylated

DUBEY AND KUMAR
5 of 10
any coupling mechanism. All halobenzenes viz. bromo
and chlorobenzenes homocoupled effectively, to afford
the corresponding biphenyl products in good to excellent
yields (Table 2). A variety of substituted aryl halide bear-
ing functional groups, such as -OCH , -CH , -F, -NO and
Heterocyclic halides which are challenging substrates
in coupling reactions also afforded moderate yields of
product (Table 2, 2i). The Ullmann coupling of the aryl
chlorides wasn't that promising, due to the sluggish reac-
tivity of C-Cl bond. The results as summarized in Table 2,
clearly shows that the coupling reaction proceeds via oxi-
dative insertion of aryl halide to metal, which followed
the general trend of aryl iodide > aryl bromide > aryl
chloride.
Having completed the homocoupling reactions, we
further extended the study to explore the versatility of the
catalyst we further tested it for cross-coupling reactions
of two different aryl halides. The cross-coupling of aryl
halides is interesting and less explored owing to several
issues pertaining to selectivity if formation of cross-
coupled products. Therefore, with such a view, we choose
4-iodoanisole and 4-iodotoluene to check for the effi-
ciency of the developed catalyst (Table 3).
3
3
2
-CN were tolerated to afford the products in moderate to
excellent yields (Table 2, 2a-2 h, 92–68%).
The electron-donating group attached on the phe-
nyl moiety furnished the products in good yield com-
pared to their electron withdrawing counterparts
(
Table 2, 2b and 2f). The Ullmann reaction of bro-
moarene and chloroarene bearing electron-withdrawing
substituent afforded dehalogenated by-product
Table 2, 2d, 2 g and 2 h). Additionally, the products/
(
reactants containing cyano which are susceptible to
undergo hydrolysis and oxidation did not show any
formation of hydrolyzed (or) oxidized products
(Table 2, 2d). The ortho-substituted halide substrates
showed slightly lower conversion, might be due to ste-
Interestingly, the cross-coupled product in case of
these substrates was found to be 48% when one equiva-
lent of the coupling partners each. The later was
improved to 72% when one of the coupling partner
ric hindrance (Table 2, 2f). Presence of -OCH group
3
at meta position to a small degree acts as a deactivator
for oxidative addition reactions (Table 2, 2e).
a
TABLE 2 Ullmann coupling of various aryl halides using Pd/Fe
3
O
4
@PDA
a
3 4
Reaction conditions: Pd/Fe O @PDA (50 mg), aryl halide (1 mmol), base (1.0 mmol), glucose (0.25 mmol), RM-β-CD (100 mg) and solvent (4 ml), aryl
o
b
bromide and chloride at 120 C, RM = Randomly methylated. Isolated Yield.

6
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DUBEY AND KUMAR
a
TABLE 3 Optimization of cross-coupling reaction in aryl halide
a
3 4
Reaction conditions: Pd/Fe O @PDA (50 mg), 4-iodoanisole (0.5 mmol), 4-iodotoluene (0.5/1.0 mmol), base (1.0 mmol), glucose (0.25 mmol), RM-β-CD (100
b
mg) and solvent (4 ml). RM = Randomly methylated. Isolated Yield
4
-iodotoluene was employed 2 equivalents. Other sub-
and ester to afford products in good to excellent yields
(Table 5, 2a, 2b, 2c, 2 h, 2 m and 2n). Moreover, poor
yields were observed with the nitro-substituted
arylboronic acid, this could be due to the influence of
substituents on the transition state formed during the
progress of the reaction (Table 5, 2 g).
Unlike homocoupling, the cross-coupling of
arylboronic acids furnished product in low yield
(Table 6), also the electron withdrawing counterpart did
not give satisfactory result similar to aryl halide cross-
coupling reactions (Table 6, 2 k and 2 l). Following this,
looking at popularity of aryldiazonium salts as an electro-
phile in various organic transformations, we also decided
to test it for homocoupling reactions (Table 7). Unfortu-
nately, the catalyst showed very poor activity toward
4-methoxy and 4-nitro diazonium salts. We could only
strates with different substituent such as nitro were also
tested for conversion but afforded the cross-coupled prod-
ucts in only moderate yields (Table 4, 2 k and 2 l).
Having completed the substrate scope of the homo-
coupling and cross-coupling of aryl halides, we further
extended the study for alternate organic substrates such
as arylboronic acids and aryldiazonium salts. We started
our investigation using arylboronic acids as a model sub-
strate, under the optimized reaction conditions. The best
result was obtained with water as the solvent, RM-β- CD
as phase transfer reagent, 50 mg of (6.92 wt%)
Pd/Fe O @PDA of catalyst, K CO as base at room tem-
3
4
2
3
perature under air atmosphere. Interestingly, we were
able to homocouple several arylboronic acids bearing var-
ious functional groups such as methoxy, methyl, fluoro
a
3 4
TABLE 4 Ullmann cross-coupling of various aryl halides using Pd/Fe O @PDA
a
3 4
Reaction conditions: Pd/Fe O @PDA (50 mg), aryl halide A (0.5 mmol), aryl halide B (1.0 mmol), base (1.0 mmol), glucose (0.25 mmol), RM-β-CD (100 mg)
o
b
and solvent (4 ml), aryl bromide at 120 C, RM = Randomly methylated. Isolated Yield.

DUBEY AND KUMAR
7 of 10
a
3 4
TABLE 5 Ullmann coupling of various arylboronic acids using Pd/Fe O @PDA
a
3 4
Reaction conditions: Pd/Fe O @PDA (50 mg), arylboronic acid (1.0 mmol), base (1.0 mmol), RM-β-CD (100 mg) and solvent (4 ml) under air atmosphere. RM
b
c
o
=
Randomly methylated. Isolated yield. at 60 C.
a
3 4
TABLE 6 Ullmann cross-coupling of various arylboronic acids using Pd/Fe O @PDA
a
3 4
Reaction conditions: Pd/Fe O @PDA (50 mg), arylboronic acid A (0.5 mmol), arylboronic acid B (1.0 mmol), base (1.0 mmol), RM-β-CD (100 mg) and solvent
b
(
4 ml) under air atmosphere. RM = Randomly methylated. Isolated yield.
TABLE 7 Ullmann homocoupling of various aryldiazonium
salts using Pd/Fe O @PDA
3 4
get 38% yield in case of former whereas the later having
electron withdrawing substituent gave 90% yields of the
desired homocoupled product (Table 7, 2b and 2 g).
Separation, Recovery and Recycling of the catalyst is
most important in sustainable organic transformation,
which is typically accomplished by traditional and
tedious methods of filtration or centrifugation with
reduced efficiency. In our catalytic system, due to the
magnetic nature of nano-Fe O , catalyst was recovered
a
3
4
easily using an external laboratory magnet and the
coupled product is extracted from aqueous layer using
ethyl acetate. The same catalyst was used for next
five cycles (Scheme 2). Although, the yields of coupled
product were slightly decreasing in the further next
a
3 4
Reaction conditions: Pd/Fe O @PDA (50 mg), aryldiazonium salt (1
b
mmol), methanol (3 ml) under air atmosphere. Isolated yield.

8
of 10
DUBEY AND KUMAR
SCHEME 2 Recyclability of
Pd/Fe @PDA nanocatalyst
3 4
O
cycles, still we obtained good yield even after five-time
use of the catalyst. Catalyst recovery was excellent with
concerns in heterogeneous catalysis. This case was inves-
tigated by removing the catalyst from the reaction mix-
ture after the first and fifth cycle, subjecting the solution
to ICP analysis. The Pd leaching was found to be negligi-
ble 1.4 and 7.2 ppm respectively. To study in detail about
9
8% amount of catalyst recovered after five cycles.
Thereafter, we examined the leaching of palladium
metal from the support, which is one of the major
SCHEME 3 Plausible Mechanistic
Pathway

DUBEY AND KUMAR
9 of 10
the heterogeneity of the catalyst, we did a hot filtration
test. For this, the homo-coupling reaction between
The yields of the products in all transformations were
found to be good to excellent. Furthermore, catalyst could
be easily recovered using a magnet, recycled and reused
up to five cycles. Use of RM-β-CD was found to increase
the yields of coupled products and hence improved the
catalyst performance in water.
4
-iodoanisole was allowed for the initial 2 hr, which
accomplished the desired coupled product in 58% yield,
subsequently Pd/Fe O @PDA catalyst was separated
3
4
magnetically. The collected filtrate was then taken in
ꢀ
another flask and reaction continued at 100 C for next
2
4 hr, which showed no increase in the coupled product
ACKNOWLEDGMENTS
yield. Another hot filtration test was performed under
the same optimized conditions (reaction conversion after
AVD is grateful to University Grant Commission
(UGC) for the research fellowship. AVK is thankful to
DST-SERB funding (YSS/2015/002064). SAIF-IITB and
National Centre for Nanoscience and Nanotechnology
(NCNNUM), University of Mumbai are acknowledged
for providing the analysis. Department of Chemistry, ICT
for the XRD and SEM analysis.
2
hr ~ 57%) wherein the catalyst was separated by centri-
fugation (3000 rpm) and filtrated solution was continued
for reaction to take place. This also showed no further
increase in coupled product yield the and hence confirm
catalyst's heterogeneous property.
A plausible reaction mechanism for the Ullmann type
homocoupling of aryl halide catalyzed by the
Pd/Fe O @PDA was proposed in Scheme 3. The first step
CONFLICT OF INTEREST
The authors declare no conflict of interest.
3
4
involves the reactant-CD inclusion complex interaction
with the Pd NPs. Followed by oxidative addition of Pd
ORCID
(0) onto aryl halide produces arylpalladium halide inter-
Abhishek V. Dubey https://orcid.org/0000-0001-8291-
mediate (A). Here onwards the reaction may go through
two pathway, Path 1 involve the addition of the another
equivalent of aryl halide to the intermediate A under the
basic medium which results into the formation of
diarylpalladium complex (C) with subsequent liberation
of halide ions, followed by the elimination of Ullmann
products facilitated by glucose molecules thus
regeneating the Pd (0) species. Aternatively, the reaction
may traverse through Path by the reaction of aryl halide
with the intermediate A, forming the Ullmann products
and Pd (II)X . The Pd (II) X so formed may get reduced
9
261
A. Vijay Kumar https://orcid.org/0000-0001-9753-0590
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2
2
by glucose to complete the cycle. During the reduction
[
[57]
process, glucose gets oxidized to gluconic acid.
More-
over, any chance of formation of acidic species from the
halide ion remnants is not possible under the basic aque-
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[
[
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3
| CONCLUSIONS
2
[
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How to cite this article: Dubey AV, Kumar AV.
A Bio-Inspired Magnetically Recoverable
Palladium Nanocatalyst for the Ullmann Coupling
reaction of Aryl halides and Arylboronic acids In
Aqueous Media. Appl Organometal Chem. 2020;
e5570. https://doi.org/10.1002/aoc.5570
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