Oxime palladacycle in PEG as a highly efficient and recyclable catalytic system for phenoxycarbonylation of aryl iodides with phenols

Article abstract of DOI:10.1002/aoc.4741

In this report, we have developed a sustainable protocol for the synthesis of aromatic esters by a carbonylative method using di-μ-chlorobis [5-hydroxy-2-[1-(hydroxyimino-?N) ethyl] phenyl-?C] palladium (II) dimer (1) catalyst in PEG-400 as a greener and recyclable solvent. The reaction is carried out at room temperature using CO in a balloon. Good to excellent yield of various esters can be synthesize using this protocol. Direct insertion of CO moiety leads to the high atom and step economy. Compared to previous protocol this phosphine free approach for the synthesis of aromatic esters provides high Turnover Number (TON) and Turnover Frequency (TOF). Developed approach has an alternative route for use of conventional palladium precursor with high conversion and selectivity. The catalyst system and product can easily be separated using diethyl ether as a solvent. The Pd/PEG-400 system could be reused up to a fifth consecutive cycle without any loss of its activity and selectivity.

Full text of DOI:10.1002/aoc.4741

Received: 31 July 2018
DOI: 10.1002/aoc.4741
Revised: 31 October 2018
Accepted: 5 November 2018
F U L L P A P E R
Oxime palladacycle in PEG as a highly efficient and
recyclable catalytic system for phenoxycarbonylation of aryl
iodides with phenols
Vinayak V. Gaikwad | Bhalchandra M. Bhanage
Department of Chemistry, Institute of
In this report, we have developed a sustainable protocol for the synthesis of
aromatic esters by a carbonylative method using di‐μ‐chlorobis [5‐hydroxy‐2‐
[1‐(hydroxyimino‐ĸN) ethyl] phenyl‐ĸC] palladium (II) dimer (1) catalyst in
PEG‐400 as a greener and recyclable solvent. The reaction is carried out at
room temperature using CO in a balloon. Good to excellent yield of various
esters can be synthesize using this protocol. Direct insertion of CO moiety leads
to the high atom and step economy. Compared to previous protocol this phos-
phine free approach for the synthesis of aromatic esters provides high Turnover
Number (TON) and Turnover Frequency (TOF). Developed approach has an
alternative route for use of conventional palladium precursor with high conver-
sion and selectivity. The catalyst system and product can easily be separated
using diethyl ether as a solvent. The Pd/PEG‐400 system could be reused up
to a fifth consecutive cycle without any loss of its activity and selectivity.
Chemical Technology, N. Parekh Marg,
Matunga, Mumbai 400 019, Maharashtra,
India
Correspondence
Bhalchandra M. Bhanage, Department of
Chemistry, Institute of Chemical
Technology, N. Parekh Marg, Matunga,
Mumbai 400 019, Maharashtra, India.
Email: bm.bhanage@ictmumbai.edu.in;
bm.bhanage@gmail.com
KEYWORDS
palladacycle, PEG‐400, phenoxycarbonylation, recyclable, TON and TOF
1 | INTRODUCTION
has been emerging most potent and convenient metho-
dology for the synthesis of carboxylic derivative pioneered
Aromatic esters consisting structural motifs are found in
many agrochemicals, pharmaceuticals (Figure 1), ionic
crystal, photosensitizer, and natural product.[1–3] The
application of these aromatic esters has been well
explored in the industry and fine chemicals. They also
act as an intermediate for the synthesis of multistep
organic molecules.[4]
Several attempts have been made for the synthesis
aromatic esters by a non‐classical carbonylative approach
such as the (i) reaction between carboxylic acid with
phenol or alcohol using “lewis” acid or another strong
acid such as hydrochloric acid, sulphuric acid[5–10] (ii)
acylation of alcohol/phenol with an acyl chloride or acid
anhydride[11–18] (iii) alkylation of carboxylate salts of
alkali/alkaline earth metal, silver and copper.[19] Alterna-
tively, palladium catalyzed carbonylation of aryl halide
by Heck in 1970.[20] Afterward, this method is widely used
for various research groups for synthesis of aromatic esters
by homogeneous precursors.[21–24] Although, these homo-
geneously catalyzed reactions have high efficiency,
recycling of this catalyst still poses a great challenge.
Therefore, it is advantageous to develop a heterogeneous
catalytic system that can easily be separated from the
reaction mixture and can be recycled several times. In this
regard, several groups disclosed the use of palladium
supported on triphenylphosphine functionalized porous
organic polymer,[25] Charcoal,[26] Imm‐IL,[27,28] silica sup-
port[29] and PdCl2(phen)@Y[30] for phenoxycarbonylation.
Various CO surrogates[31–36] have been utilized for the
synthesis of aromatic esters. Nonconventional techniques
such as microwaves assisted methods were also employed
for effective transformation for the synthesis of aromatic
Appl Organometal Chem. 2019;e4741.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4741

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GAIKWAD AND BHANAGE
water as a solvent is very advantageous, the low solubility
of an organic substrate in water led to handling issues
thereby further restricting its applicability. Similarly, the
use of ionic liquid[38,39] itself has its own disadvantages
such as tedious synthesis process, laborious workup, and
toxicity. Recently polyethylene glycol (PEG) has been
emerges as an ideal solvent in organic synthesis due to
its resilience of degradation and less possibility of
formation of a side product. It also has the ability to
coordinate with metal and form a stable complex with
them.[40–43]
To the best of our knowledge and literature survey,
we observed that the phenoxycarbonylation was achieved
with high catalyst loading which makes overall process
economically unfavorable. Oxime palladacycle catalyst
plays an important role in homogeneous catalysis. Due
to its high thermal stability and high activity at very low
concentration, the oxime palladacycle catalyst was used
for replacements of a conventional palladium catalyst.
In 2000, Najera's group successfully applied oxime
palladacycles for C‐C bond formation reactions.[44] In
2002 Sugi and co‐workers reported oxime palladium for
the synthesis of the ester from aryl halides.[45] Recently
our group reported a phosphine based palladacycle
catalyst for carbonylative Suzuki‐miyaura coupling[46]
and Oxime palladacycle for the carbonylative sonogashira
FIGURE 1 Examples of pharmaceutically active phenyl esters
compounds.[34,37] However, these methods suffer from
one or more disadvantages such as the use of an excess
starting material, use of the hazardous metal/substrate,
limited substrate scope, use of a dry solvent, high
temperature requirement which leads to economic and
environmental concerns. Comparative study of our proto-
col with previous protocol shown in Scheme 1.
From a green chemistry point of view it is advisable to
develop a system which allows easy separation of product,
recycling of expensive transition metal and minimizing
contamination of the environment. Although the use of
SCHEME 1 Comparative study of the
previous report with the current protocol

GAIKWAD AND BHANAGE
3 of 8
with high TOF and TON.[47] Although palladacycle cata-
lyst shows very high catalytic activity, recycling of these
precious catalysts has not been attempted till date. Hence
it is desirable to develop a catalytic system which works
at the benign condition with high activity and provision
of their recycling. Considering the above problem herein,
we report the oxime palladacycle as a homogeneous and
recyclable catalyst for phenoxycarbonylation in PEG as
a green solvent. This reaction works under mild reaction
condition with CO (balloon).
TABLE 1 Screening of optimal reaction conditiona
Entry
[Pd]
Base
Solvent
Conversionb (%)
Yieldb (%)
1
PdCl2
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
PEG‐400
CH3CN
48
56
63
15
‐‐
45
53
60
12
‐‐
2
Pd (OAc)2
PdCl2(PPh3)2
Pd (PPh3)4
Pd/C (10 mol %)
Pd (3)
3
4
5
6c
7d
8
78
84
98
86
55
65
50
54
40
35
18
10
25
82
75
83
70
80
63
75
84
97
83
50
62
47
53
31
25
10
8
Pd (2)
Pd (1)
Pd (1)
9
10
11
12
13
14
15
16
17
18
19e
20
21f
22g
23h
24i
Pd (1)
NaHCO3
Cs2CO3
NaOH
Pd (1)
Pd (1)
Pd (1)
KOH
Pd (1)
NEt3
Pd (1)
DBU
Pd (1)
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Pd (1)
Toluene
DMF
Pd (1)
23
78
69
82
63
78
60
Pd (1)
PEG‐400/H2O
PEG‐600
PEG‐400
PEG‐400
PEG‐400
PEG‐400
Pd (1)
Pd (1)
Pd (1)
Pd (1)
Pd (1)
aReaction conditions: 3a (0.5 mmol), 4a (0.7 mmol), ‘Pd’ (0.6 mol%), Base (1 mmol), CO (balloon), Solvent (3 ml),
bConversion & Yield determined by GC‐GCMS,
cPd (3) chloro (2‐dicyclohexylphosphino‐2′,6′‐diisopropoxy‐1,1′‐biphenyl)[2‐(2′‐amino‐1,1′‐biphenyl)]palladium (II), RuPhos‐Pd‐G2.
dPd (2) Oxime palladacycles‐di‐m‐chlorobis [5‐chloro‐2‐[(4‐ chlorophenyl)(hydroxyimino‐kN) methyl] phenyl‐kC] palladium (II) dimer.
ePEG‐400 (3 ml)/H2O (1 ml),
f[Pd] (0.4 mol%),
g[Pd] (0.2 mol%),
h18h,
i12 h.

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GAIKWAD AND BHANAGE
15% and 0% conversion of 3a by screening of PdCl2, Pd
(OAc)2, PdCl2(PPh3)2 Pd (PPh3)4 and 10% Pd/C as a
catalyst with Na2CO3 as a base at 24 h respectively
(Table 1, entry 1–5). Interestingly, we obtained the higher
conversion of starting material by a screening of various
palladacycle compared with conventional Pd precursor
(Table 1, entry 6–8, structures of palladacycle 1, 2 and 3
shown in Figure 2). Among these, Oxime palladacycle
(1) shows better catalytic activity for ester synthesis with
good conversion (98%) (Table 1, entry 8). Further, in base
study experiments we employed various inorganic bases
(Table 1, entry 9–13) such as K2CO3 (86%), NaHCO3
(55%), Cs2CO3 (65%), NaOH (50%), KOH (54%) and
organic bases (Table 1, entry 14 and 15) such as NEt3
(40%), and DBU (35%). However, the use of Na2CO3
provide superior conversion of 3a and it has used for
further optimization study. Next, we compared the
various conventional organic solvents with PEG‐400 for
phenoxycarbonylation reaction (Table 1, entry 16–20).
These results showed that considerable effects were found
when PEG replaced with other organic solvents. Polar
aprotic solvents such as acetonitrile and toluene offered
very low conversion of 3a (Table 1, entry 16 and 17).
Other hands, non‐polar aprotic solvent like toluene pro-
vide only 10% conversion of 3a (Table 1, entry 18). The
mixture of PEG‐400 and water not suitable and we
observed decreasing the conversion of starting material
(82%) as compared to pure PEG‐400 system (Table 1,
entry 19). Next, we obtained 75% conversion of 3a by
using PEG‐600 as a solvent (Table 1, entry 20). After
investigating the above result, we concluded that
PEG‐400 effective solvent for phenoxycarbonylation with
the superior conversion. Finally, we studied the effect of
catalyst loading on the model reaction (Table 1, entry
21–22). Catalyst loading study showed that when decreas-
ing catalyst loading from 0.6 mol% to 0.4 mol% and
2 | EXPERIMENTAL
2.1 | General
All chemicals are purchased from Sigma Aldrich, Alfa
Asear, S.D. fine and used without further purification.
The reaction progress identified by using GC–MS and
thin layer chromatography (using Merck silica gel
60 F254 plates) with 254 nm UV lamp. The mass of
product was detected by GC–MS‐QP 2010 instrument
(Rtx‐17, 30 m × 25 mm ID, the film thickness
(df) = 0.25 μm) (column flow 2 ml min−1, 100 °C to
240 °C at 10 °C min−1 rise). Purification of the product
was done by using column chromatography on a silica
gel 100–200 mesh. Isolated products were characterized
by 1H NMR and 13C NMR. 1H NMR spectra were
recorded on 400 MHz spectrometers in a CDCl3 solvent
with tetramethylsilane (TMS) as an internal standard.
13C NMR spectra were recorded on 101 MHz spectrome-
ter in CDCl3 as a solvent. The chemical shift was recorded
in ppm (part per million) with respect to TMS and
coupling constant were recorded in Hertz. The splitting
pattern of recorded spectra was described as s (singlet),
d (doublet), t (triplet) and m (multiplate).
The experimental procedure for phenoxycarbonylation
reaction to the synthesis of aromatic esters using Oxime
palladacycle catalyst.
In 50 ml round bottom flask equipped with magnetic
stirring bar was charged with iodobenzene (0.5 mmol),
phenol (0.7 mmol), “1” (0.6 mol %), Na2CO3 (1 mmol)
and PEG‐400 (3 mL). The round bottom flask was flushed
three times with CO (g) and the mixture was slowly
stirred at r.t. for 24 hr under CO (balloon). After,
completion of the reaction the reaction mixture was
extracted with diethyl ether (15 ml X 4 times) and the
remaining residue was used for next run by charging with
iodobenzene, phenol, and base without further addition
of catalyst and PEG. The layer of diethyl ether was
concentrated under vacuum pressure and it purified with
column chromatography using petroleum ether and ethyl
acetate as eluents to give the pure product.
3 | RESULTS AND DISCUSSION
We started our reaction optimization by choosing the
carbonylative coupling between iodobenzene (3a) with
phenol (4a) leads to synthesis of phenyl benzoate as a
model reaction. Various reactions parameter are tested
on the model reaction and the obtained results are
summarized in Table 1. Initially, we investigate various
palladium precursors to find the maximum conversion
of starting material. We have obtained 48%, 56%, 63%,
FIGURE 2 Structures of Palladacycle 1, 2 and 3

GAIKWAD AND BHANAGE
5 of 8
0.2 mol% the conversion also decreasing from 98% to 83%
and 70% respectively. Next, we decrease the reaction
duration from 24 hr to 18 hr and 12 hr the conversion
also decreases from 98% to 80% and 63%, respectively
(Table 1, entry 23–24).
However, optimized reaction condition in hand next
we explored the detailed substrate scope of the range of
aryl iodide with substituted phenol for the synthesis of
aromatic esters under developed condition (Table 2). Ini-
tially, we carried out the reaction between different aryl
iodide with phenol for phenoxycarbonyation reaction.
Simple iodobenzene carbonylative coupled with phenol
to give 97% yield of phenyl benzoate (6a). Next, Aryl
iodide bearing electron donating groups such as p‐CH3,
p‐OMe could be smoothly coupled with phenol to give
corresponding esters 6b and 6c with 93% and 91% yield
respectively. Further, we tried to carry out the reaction
between electron withdrawing groups such as p‐CN,
m‐NO2 on aryl iodide carbonylative coupled with phenol
and they provided a very good yield of corresponding
esters 6d and 6e with a high TON and TOF. Further,
o‐cresol, p‐cresol, and m‐cresol successfully converted to
corresponding esters 6f, 6 g and 6 h by carbonylative
coupling with iodobenzene under developed condition.
O‐cresol provided slightly less yield (83%) as compared
with p‐cresol (90%) and m‐cresol (88%) due to sterically
hindrance of a methyl group at ortho position and
catalytic TON and TOF of the order of 122 to 132 and 5
to 5.2 could be achieved. Next, the phenol bearing
methoxy groups at o and m position were tested with
iodobenzene and they are smoothly converted into
respected esters 6i and 6j with 81% and 85% yield.
TABLE 2 Scope of the Oxime palladacycle for the synthesis of aromatic estersa
aReaction conditions: 4 (0.5 mmol), 5 (0.7 mmol), ‘Pd, 1’ (0.6 mol%), Na2CO3 (1 mmol), CO (balloon), PEG‐400 (3 ml), 24 h, TOF (h−1). Conversion & Yield
determined by GC‐GCMS.

6 of 8
GAIKWAD AND BHANAGE
Subsequently,
1‐iodonaphthalene
and
p‐cresol
3.24 and PdCl2(PPh3)2 shows that 88.25 and 3.67 as a
TON and TOF respectively (Table 3, entry 2 and 3). Next,
we use of Pd (PPh3)4 could effect only 15% conversion of
starting material with 12% yields and catalytic activity
showing 17.64 TON and 0.7 TOF (Table 3, entry 4). Next,
we tested a series of palladacycle 1, 2 and 3 for this reac-
tion. We were delighted to observe that higher conversion
and yield with a high TON and TOF of the expected prod-
uct as compared with conventional palladium precursors.
Initially, we tested palladacycle 3 and 2 offered 78% and
85% conversion of starting material with 75% and 84%
yield of phenyl benzoate, respectively. The catalytic
activity of palladacycle 3 and 2 showing in terms TON
was 110, 123.52 and TOF was 4.58, 5.14 respectively
(Table 3, entry 5 and 6). Next, we obtained the higher
conversion and yield by using oxime palladacycle 1 with
a higher TON (142.64) and TOF (5.94) compared to other
studied catalyst (Table 3, entry 7).
To reduce the cost of the experiment, next we
investigate the recyclability of oxime palladacycle (1)
for phenoxycarbonylation reaction. The carbonylative
coupling of iodobenzene and phenol for the synthesis of
phenyl benzoate was chosen as a model reaction for
recycling study (Table No. 4). After completion of the first
run, the PEG‐400 layer was extracted with diethyl ether
(3 X 15 ml) and applied for the second run after charging
with iodobenzene and phenol. Surprisingly, we observed
that the system Pd/PEG‐400 can recycle up to five
consecutive cycles with a significant loss of activity and
selectivity with maintaining constant TON and
TOF. (Details recycle procedure of solvent and catalyst
are shown in ESI)
carbonylative coupled and transformed into ester 6 k
with 82% yield. Next, we were tried to synthesize various
halogenated esters which are the very important back-
bone in pharmaceutically active drug and also in medici-
nal chemistry.[48,49] Chlorinated ester
6
l
were
synthesized by carbonylative coupling with iodobenzene
as an electrophilic partner in 83% yield. Further, 2, 4 di‐
fluorophenyl esters 6 m and 6n were successfully
synthesized by carbonylative coupling with correspond-
ing aryl iodide in 80%, 78% yields respectively. Substituted
aromatic esters such as 6o and 6p were synthesized by
coupling of 4‐CN iodobenzene with p‐cresol and
4‐iodotoulene with guaiacol (2‐methoxyphenol) in 75%
and 83% yield. Further, we were synthesized various
biphenyl esters by carbonylative coupling with various
aryl iodide. Biphenyl esters 6q, 6r, and 6 s were
synthesized in 73%, 70% and 72% yields respectively by
carbonylative coupling with corresponding aryl
iodides. (1H and 13C of compounds are shown in ESI.)
Next, we compared the catalytic activity of oxime
palladacycle with conventional palladium precursor and
observed results are mentioned in Table 3. The
carbonylative coupling between iodobenzene (3a) with
phenol (4a) for the synthesis of phenyl benzoate choose
as a model reaction. Initially, we checked various conven-
tional palladium precursors such as PdCl2, Pd (OAc)2,
PdCl2(PPh3)2 and Pd (PPh3)4 for this reaction (Table 3,
entries 1–4). We observed that PdCl2 offered 48%
conversion and 45% yield. The catalytic activity showing
in terms of TON and TOF are 66.17 and 2.75 respectively
(Table 3, entry 1).
Next, we used Pd (OAc)2 and PdCl2(PPh3)2 for this
reaction showing 56% and 63% conversion of starting
material with 53% and 60% yields, respectively. The
catalytic activity of Pd (OAc)2 showed that 77.94 and
The plausible reaction mechanism of oxime
palladacycle (1) catalyzed phenoxycarbonylation for ester
synthesis is shown in Scheme 2. Initially, Pd (0) insert
between aryl iodide via breaking of C‐I bond to the for-
mation of species B.[50] Then CO (g) coordinate with aryl
palladium species (B) to give C. Next, phenol reacts with
species C to form D. Simultaneous, reductive elimination
of palladium from D to offered phenyl benzoate product.
TABLE 3 Comparative study of Oxime palladacycle (1) with
conventional palladium precursora
Pd
precursor
Conv.
(%)b
Yield
(%)b
TOF
(h−1
)
Entry
TON
TABLE 4 Recycle study of oxime palladacycle (1) in PEG‐400a
1
2
3
4
5
6
7
PdCl2
48
56
63
15
78
85
98
45
53
60
12
75
84
97
66.17
77.94
88.25
17.64
110
2.75
3.24
3.67
0.7
Run
Conv. (%)a
Yield (%)a
TON
TOF (h−1
)
Pd (OAc)2
PdCl2(PPh3)2
Pd (PPh3)4
Pd (3)
1
2
3
4
5
98
96
96
93
91
97
95
94
92
90
142.64
139.70
138.23
135.29
132.35
5.94
5.82
5.75
5.63
5.51
4.58
5.14
5.94
Pd (2)
123.52
142.64
Pd (1)
aReaction conditions: 3a (0.5 mmol), 4a (0.7 mmol), ‘Pd’ (0.6 mol%), Na2CO3
(1 mmol), CO (balloon), PEG‐400 (3 ml),
Reaction Conditions: 3a (0.5 mmol), 4a (0.7 mmol), ‘1’ (0.6 mol%), Base
(1 mmol), CO (balloon), PEG‐400 (3 ml), 24 hr.
bConversion & Yield determined by GC‐GCMS (See in references 50 to55).
aConversion & Yield determined by GC‐GCMS.

GAIKWAD AND BHANAGE
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Comparative study of palladacycle (1) with conventional
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Bhalchandra M. Bhanage
9538-3339
https://orcid.org/0000-0001-
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