Synthesis and architecture of polystyrene-supported Schiff base-palladium complex: Catalytic features and functions in diaryl urea preparation in conjunction with Suzuki-Miyaura cross-coupling reaction by reductive carbonylation

Article abstract of DOI:10.1016/j.jorganchem.2018.09.017

This work represents an efficient and unique phosphine-free approach for the polystyrene embedded Schiff-base palladium catalyzed diaryl urea synthesis and Suzuki-Miyaura cross-coupling reaction by reductive carbonylation process. The careful instrumental investigations with FE-SEM, TEM, EDAX, TGA, UV–Vis, FTIR, AAS, and elemental analysis precisely characterized the developed heterogeneous catalyst. Reaction parameters, like catalytic natures, starting materials, reaction environment, and solvent were examined sequentially. The present work has been adequately addressed to account for the generation and characterization of a new polymer bound Pd-catalyst and using it in the synthesis of diaryl ureas and diaryl ketones, with no substantial decay of catalytic activity.

Full text of DOI:10.1016/j.jorganchem.2018.09.017

Accepted Manuscript
Synthesis and architecture of polystyrene-supported Schiff base-palladium complex:
Catalytic features and functions in diaryl urea preparation in conjunction with Suzuki-
Miyaura cross-coupling reaction by reductive carbonylation
Priyanka Basu, Sk Riyajuddin, Tusar Kanto Dey, Aniruddha Ghosh, Kaushik Ghosh,
Sk Manirul Islam
PII:
S0022-328X(18)30622-3
10.1016/j.jorganchem.2018.09.017
JOM 20574
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 14 July 2018
Revised Date: 17 September 2018
Accepted Date: 20 September 2018
Please cite this article as: P. Basu, S. Riyajuddin, T.K. Dey, A. Ghosh, K. Ghosh, S.M. Islam, Synthesis
and architecture of polystyrene-supported Schiff base-palladium complex: Catalytic features and
functions in diaryl urea preparation in conjunction with Suzuki-Miyaura cross-coupling reaction by
reductive carbonylation, Journal of Organometallic Chemistry (2018), doi: https://doi.org/10.1016/
j.jorganchem.2018.09.017.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Synthesis and architecture of polystyrene-supported Schiff base-Palladium
complex: Catalytic features and functions in diaryl urea preparation in
conjunction with Suzuki-Miyaura cross-coupling reaction by reductive
carbonylation
Priyanka Basua, Sk Riyajuddinb, Tusar Kanto Deya¥, Aniruddha Ghosha¥, Kaushik Ghoshb*
and Sk. Manirul Islama*
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia 741235, West Bengal, India
bInstitute of Nano Science and Technology, Mohali, Punjab-160062, India
¥These authors contributed equally.

ACCEPTED MANUSCRIPT
Synthesis and architecture of polystyrene-supported Schiff base-Palladium
complex: Catalytic features and functions in diaryl urea preparation in
conjunction with Suzuki-Miyaura cross-coupling reaction by reductive
carbonylation
Priyanka Basua, Sk Riyajuddinb, Tusar Kanto Deya¥, Aniruddha Ghosha¥, Kaushik Ghoshb*
and Sk. Manirul Islama*
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia 741235, West Bengal, India
bInstitute of Nano Science and Technology, Mohali, Punjab-160062, India
¥These authors contributed equally.
Abstract
This work represents an efficient and unique phosphine-free approach for the polystyrene
embedded Schiff-base palladium catalyzed diaryl urea synthesis and Suzuki-Miyaura cross-
coupling reaction by reductive carbonylation process. The careful instrumental investigations
with FE-SEM, EDAX, TGA, UV-Vis, FTIR, AAS, and elemental analysis precisely
characterized the developed heterogeneous catalyst. Reaction parameters, like catalytic
natures, starting materials, reaction environment, and solvent were examined sequentially.
The present work has been adequately addressed to account for the generation and
characterization of a new polymer bound Pd-catalyst and using it in the synthesis of diaryl
ureas and diaryl ketones, with no substantial decay of catalytic activity.
Keywords: Suzuki-Miyaura cross-coupling; Diaryl urea; Biaryl ketone; Palladium;
Salicylaldehyde; Schiff base.
______________________________________________________________________
* Author to whom correspondence should be addressed.
Tel.: + 91 33 2582 8750; Fax: +91 33 2582 8282.
E-mail address: manir65@rediffmail.com (S.M. Islam), kaushik@inst.ac.in (K. Ghosh)
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Introduction
The increasing worldwide demand for bulk chemicals and materials required strong
investigation to discover novel manufacturing procedures from unconventional resources. As
necessary raw materials for the chemical manufacturing plant, the C1 compounds: CO and
CO2 can be obtained on a huge scale including renewable. Specifically, CO is a most
versatile and cost-effective chemical building block and is easily produced from naturally
occurring gas, coal, biomass and so on [1].
Transition metal catalyzed carbonyl conversions have been a centre of attraction among
organic chemists for developing environmentally sustaining protocols for synthesizing a
variety of medicinal and industrial compounds [2,3]. N,N′-diaryl ureas are one such
established biologically active molecules of enormous significance predominantly in
medicinal fields [4]. In addition it has several impacts on materials science [5] and catalytic
fields [6,7]. Simultaneously, urea has been used as a precursor for synthesizing amines,
isocyanates, carbamates and imides and so on [2,3]. The diphenyl urea production from a
combination of aniline and nitrobenzene under polystyrene attached Ru(II) catalysis is one of
the established methods for oxidative >C=O conversions [8]. At present, the commercial
synthesis of N,N′-diaryl urea involves the extremely lethal raw substance phosgene, from
which corrosive HCl is liberated, as a by-product. Incidentally, the N,N′-diaryl urea could be
prepared in an alternate process, involving the reaction of amine and nitrobenzene with CO in
presence of palladium catalyst [9]. Immobilization of palladium metal on the various polymer
supports has drawn great attention to the research scientists for its successful functions in
multiple and distinct carbonylation reactions [10]. Present approach demonstrates how the
surface group of polymer support influences on the stability and the catalytic function of PS-
Pd-Schiff base complex. It proved successful for disubstituted urea synthesis from a wide
extent of substituted aryl amines and nitrobenzenes. A rational illustration on the
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heterogeneous Pd-catalyzed synthesis of N,N′-diaryl urea by the use of CO is inadequate. The
designing of simple, generous and efficient supported catalyst is the main target for many
researchers due to its great potential in catalyzing organic transformations [11].
Palladium catalysts with varieties of ligands [12-14] attached onto polystyrene surface has
attracted huge concentration for making C-C bond in organic synthesis. Pd catalyzed
carbonylative Suzuki-Miyaura is one such type of C-C coupling reaction and a momentous
system for the insertion of CO in both academia and industries. The Pd catalyst containing
phosphine-based ligands reported for S-M coupling reaction have adverse hazardous effects,
air-sensitivity and high economic resist their widespread applications in catalytic chemistry.
Currently multiple examples of phosphine free ligands such as, macromolecular polymers
[15,16], mesoporous molecular sieve [17,18], graphene oxide [19,20] are employed to assist
the palladium incorporated Schiff base catalyst.
Solid supported Pd complex (such as polymers, graphene, silicate and zeolite) improves
facile isolation process alongside enormous catalytic efficiency in Suzuki cross-coupling for
synthesizing biaryl ketones [21]. Sufficient attempts have been reached for diaryl ketone
preparation concerning their huge involvement in cosmetics [22], biological sensitive
compounds [23, 24], pharmaceuticals [25, 26, 27], and in other functional materials [28-30].
The anti-inflammatory drug, S-ketoprofen was derived from diaryl ketone [28,29]. Another
derivative of diaryl ketone is fenofibric acid, a regulating agent for lipid substance and is
utilized for the medication of high cholesterol in addition to blood triglyceride levels [31].
Evidently from earlier reports the designed polystyrene grafted Pd Schiff base complex has
not been employed as a catalyst for carbonylative Suzuki-Miyaura cross-coupling reaction
along with diaryl urea synthesis. The current work elucidates the efficient, innovative
synthetic routes for the production of diaryl urea from polymer supported heterogeneous Pd
catalyzed conversion of aryl amine and nitrobenzene using economically cheaper reagents.
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Experimental Section
Materials
5.5 % crosslinked polystyrene, conc. nitric acid were collected from Sigma-Alrich Co.; U.S.A
and used without purification. Predistillation was performed for the liquid substrates and
dried using specific process. Palladium acetate, salicylaldehyde, nitrobenzene, amine
derivatives, phenyl boronic acid and iodobenzene derivatives were purchased from Alfa-
Aesar. DMF, ferric chloride, anisole, potassium carbonate were acquired from Merck and
used without refinement.
Physical and spectroscopic instruments
FTIR spectra for the respective samples were obtained by placing KBr disc inside a Perkine
Elmer FTIR 783 spectrophotometer within range of 400 to 4000 cm-1. The Mettler Toledo
TGA/DTA 851e equipment was helpful to analyze the thermogravimetry of the samples. The
UV-Vis band of solid samples were obtained by Shimadzu UV-2401PC doubled beam
spectrophotometer associated with an integrating sphere counterpart. The surface morphology
of the samples was specifically observed with SEM, ZEISS EVO40, England instrument
having EDX advantage. The morphological analysis of our synthesized Pd(PS-SAL) complex
catalyst was carried out by transmission electron microscopy (TEM) (JEOL JEM 2100)
operated at 200 kV. Atomic absorption spectrophotometer (Model Varian AA240) was used
to realize the % content of palladium in the palladium loaded material. High pressure
reactions were performed with the help of a high pressure reactor containing a magnetic
stirrer attached with an autoclave (AmAr Equipment, model no.SE-1TAP-II).
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Fig. 1 High Pressure Reactor.
Preparation of catalyst [Pd(PS-SAL)]
The desired catalyst was prepared cautiously by following two-step strategies. Initially the
preparatory method of p-aminopolystyrene was performed by means of the previously reports
[32]. Secondly, 2 g of p-aminopolystyrene was taken in a 250 mL round bottom flux and
swirled for 5 min with 10 mL methanol to prepare polymer anchored salicylaldehyde
complex. Methanolic solution (10 mL) of salicylaldehyde (0.8 mL) was added immediately
after to it and kept for reflux for 24 h. The colour of the solution was changed from pale
yellow to green. Then greenish type solid solution was filtered and washed with methanol.
Freshly prepared polymer anchored Schiff base complex (500 mg) was taken in 100 mL rb
and settled it with refluxing condition after 10 min mixing of 0.1 g of palladium acetate in 10
mL acetic acid. The reflux was continued for 10 h. The resultant mixture was filtered, washed
concomitantly with methanol for 8 to 10 times to remove impure component absolutely and
dried in vacuum. Preparation scheme of the polymer supported Schiff base-palladium
compound is outlined in Scheme 1.
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Scheme 1. Synthesis of polymer anchored Schiff base-palladium catalyst Pd(PS-SAL).
General procedure for synthesis of diaryl ureas
Each of derivatives of amine (6 mmol) was introduced with 6 mL DMF, 6 mL methanol and
2.5 mmol (0.405 g) of ferric chloride and followed by nitrobenzene 3 mmol in a 100 mL
glass-lined stainless steel autoclave fitted with a magnetic stirrer (Fig. 1). Then 20 mg (0.013
mmol based on Pd active centre) of polystyrene anchored salicylaldehyde palladium catalyst
was added by exerting 80 psi of carbon monoxide. The reaction temperature was fixed at 75
oC. The used catalyst was collected after 6 h completion of reaction. Later, the rest of the
mixture was allowed to work up for 3 times using ethyl acetate and organic part was dried
over sodium sulfate. The solid product was analyzed by proton NMR spectroscopy
(supporting information).
Procedure of carbonylative Suzuki-Miyaura cross coupling catalysed by Pd(PS-SAL)
In a 100 mL, autoclave equipped with magnetic stirrer (Fig. 1), phenyl boronic acid (1
mmol), iodobenzene (1 mmol), potassium carbonate (3 equiv.), anisole (5 mL) and Pd(PS-
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SAL) catalyst (15 mg, 0.01 mmol based on Pd active centre) were added. Then it was purged
three times with carbon monoxide (65 psi) and after closing the autoclave tightly, it was
heated at 90 oC. The reaction was continued for 8 h, cooled at room temperature then Pd(PS-
SAL) catalyst was removed from the reaction system and washed down with methanol. The
organic content was dried out absolutely with sodium sulfate after work up it with ethyl
acetate (3×10 mL) followed by water. The unstable components were allowed to evaporate
1
through a rotary evaporator. The solid product was analyzed by H NMR spectroscopy
(supporting information).
Characterization of catalyst
In this context it is noteworthy to mention that the prepared polystyrene embedded Pd
catalyst has some boundaries to investigate and characterize only to their physicochemical
properties, SEM, TEM, EDX, TGA-DTA, FTIR, and UV-Vis spectral data. The complete
integration of the organic substructure in the substance was established by elemental analysis.
Scanning electron micrographs (SEM) and energy dispersive X-ray analyses (EDX)
Fig. 2 SEM images of (A) polymer-supported Schiff base ligand (PS-SAL) and (B) the fresh
palladium catalyst Pd(PS-SAL).
SEM pictures in Fig. 2 represent the polymer bound Schiff-base ligand (A) and the
coordinated Pd-complex on tailored polystyrene (B). The observed SEM images demonstrate
the morphological and characteristic distinct vision between the metal free PS-SAL ligand
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and the metal loaded catalyst. The presence of Pd caused changes, demonstrated by
disappearing of the extent of porosity and roughness of the -OH functionalized polymeric
material surface.
Fig. 3 EDX spectrum of (A) PS-SAL and (B) Pd(PS-SAL).
Moreover the EDX analysis supports the metal attachment on the polymer matrix (Fig. 3).
The interaction of the Pd metal onto the polystyrene anchored ligand causes a small extent of
smoothening on the surface layer, which probably due to metal ion and ligand interaction and
orienting a specific geometry of the complex. From this spectroscopic analysis (EDX
analysis) of the metal complex confirms the metal attachment along with C and O and
recommends the construction of the metal complex Pd(PS-SAL) containing polystyrene
anchored ligand. SEM and EDX analysis confirmed the palladium attachment onto the
polymer matrix.
Transmission electron microscopy (TEM)
Image 4(a) and 4(b) represents the TEM images of nitro and amino polystyrene respectively.
These two images suggest that there is no distinct change in morphology when nitro
polystyrene converts to amino polystyrene. From TEM image 4(c) of PS-SAL ligand, the
black spot strike the attachment of salicyladehyde on the surface of amino polystyrene. Image
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4(d) and 4(e) are the TEM images of Pd(PS-SAL) complex catalyst in two different
magnification, where black spherical balls are spread over the whole specimen. These black
balls confirm coordination of Pd with polymer supported ligand.
Fig. 4 TEM images of (a) p-nitro polystyrene; (b) p-amino polystyrene; (c) polymer-
supported Schiff base ligand (PS-SAL); (d) and (e) polymer anchored Schiff base-palladium
complex catalyst Pd(PS-SAL) in two different magnification.
Thermogravimetric analysis
Thermogravimetric analysis (TGA) was carried out to predict the thermal stability of p-
nitropolystyrene, p-aminopolystyrene, polymer surrounded Schiff base ligand and the Pd-
complex (Fig. 5) respectively. Thermal stability analyses of the synthesized materials were
conducted in N2 atmosphere with 10 oC/minute heating rate at the temperature measured from
o
30-600 C. p-Nitropolystyrene starts to decompose in the range of 290-300oC, where p-
o
aminopolystyrene starts to decompose from 313 C (Fig. 5A). The weight reduction of
o
o
polymeric ligand varies from 320 C to 450 C, where the maximum weight loss of metal
o
o
loaded polymeric catalyst arises from 340 C to 458 C (Fig. 5B). The loss of weight in this
region comes out due to the disintegration of the component in the polymeric matrix. Hence
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the prepared polymeric supported catalyst is capable to execute its catalytic activities up to
340 oC without any loss of weight.
(A)
(B)
Fig. 5 TGA plot of (A) p-nitropolystyrene and p-aminopolystyrene, (B) PS-SAL and Pd(PS-
SAL).
FTIR spectroscopy
The attachment of salicylaldehyde and palladium metal onto the amino functionalized
polymeric support was confirmed by IR spectral bands (Fig. 6).
Fig. 6 FTIR spectra of PS-SAL and Pd(PS-SAL).
The formation of p-nitro polystyrene and p-amino polystyrene were recognised by FTIR
spectroscopy (supporting information). Usually the Schiff base type ligands and its metal
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coordinated complexes are fundamentally recognized by the FTIR analysis due to different
stretching vibrations of PS-SAL appeared at the expected positions. The range of 1600-1500
cm−1 corresponds to the υ(C-C) and υ(C=N) stretch of aromatic rings. C-O stretching furnishes its
characteristic vibrations at 1319 cm-1. Formation of the PS-SAL is indicated from strong IR
band appeared at 1604 cm-1 due to the C=N azomethyne nitrogen stretching. The C=C
stretching frequencies for the polymer supported free ligand was appeared around 1504 cm-1
accordingly [33,34]. The stretching vibration at 3424 cm-1 has been assigned to the
corresponding hydrogen bonded phenolic -OH group present in the polymer bound Schiff
base ligand.
The C=N and C=C stretching frequencies for the polymer supported Pd-Schiff base complex
were observed at 1600 cm-1 and 1502 cm-1 respectively. The Pd(PS-SAL) complex shows
asymmetric and symmetric stretching vibrations of corresponding COO for bridging
bidentate acetate groups at 1597 cm-1 and 1454 cm-1 respectively. The band appeared near
647 cm-1 is assigned for the stretching vibration of Pd-N present in the metal incorporated
polymeric complex. Pd-O stretching was also inferred by the observed weak absorption peak
at 539 cm-1. The polymer bound palladium-Schiff base complex Pd(PS-SAL) exhibits a broad
band at 3412 cm-1 correspond to the stretching vibrations of deprotonated phenolic -OH group
coordinated with palladium. In the polymer-anchored Pd-complex, both υAr–O–C and υC=N
experience a small shift highlighting the Pd is chelated with the nitrogen and oxygen atom.
Chemical analysis
After metal ion incorporation the resulting material in solution phase was analyzed with AAS
instrument which offered a view about the complexation of palladium metal onto the polymer
anchored Schiff base. Present of metal in the polymer anchored Schiff-base Pd catalyst was
ascertained quantitatively with AAS expressing 7.05% Pd loading in the polymer matrix.
However, the elemental composition of Pd(PS-SAL) revealed Schiff base complex formation.
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The elemental analysis of the polymer embedded Schiff base ligand and its palladium
complex (Table 1) substantiate to the prediction of the complex as proposed. This implies
that polystyrene was embedded on the surface of the Pd-Schiff base complex. The ICP-AES
was also performed to analyze the entire palladium content of the synthesized Pd(PS-SAL)
catalyst. Elemental analysis of catalyst by ICP spectroscopy provided a palladium loading of
7.03 wt% (Table 1). Table 1 presents the C, H and N elemental analysis data of PS-SAL and
Pd(PS-SAL) catalyst. The comparative analytical data provided from elemental experiment
(C, H, N), ICP-AES and EDX analysis facilitates to determine the composition of ligand and
Pd supported material consistent with the proposed structure.
Table 1. Elemental analysis of the polymer anchored ligand and polymer anchored palladium
catalyst
Compound
PS-SAL
C %
79.78
72.12
H%
5.11
4.8
N%
7.15
5.6
Pd %
-
Pd(PS-SAL)
7.05a
7.03b
adetermined by AAS, bdetermined by ICP-AES
Results and discussion
Activity of catalyst
Activity of polymer supported palladium Schiff-base catalyst has a vital role in various
industrial applications. The possible activity of the synthesized palladium catalyst was
investigated by altering different experimental conditions like temperature, pressure, solvent,
amount of co-solvent and catalyst used.
Scheme 2. Carbonylation reaction of nitrobenzene and phenylmethanamine in the presence of
Catalyst.
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Effect of solvent in the presence of methanol
Fig. 7 Effect of solvent on polymer anchored Schiff base-palladium catalyzed 1-benzyl-3-
phenylurea production. Reaction condition: nitrobenzene (3 mmol), benzyl amine (6 mmol),
Catalyst (20 mg, 0.013 mmol based on Pd active centre), FeCl3 (2.5 mmol), MeOH (6 mL),
temperature (75 oC), time (6 h), PCO=80 psi.
The formation of carbonylative product 1-benzyl-3-phenylurea was developed with the
catalyst Pd(PS-SAL) and co-solvent methanol. Optimizing the reaction condition with
various solvent the most suitable one was chosen for 1-benzyl-3-phenylurea formation. The
polar aprotic solvent dimethyl formamide (DMF) is a solvent of highest priority with respect
to the yield of 1-benzyl-3-phenylurea production as rationalized from above Fig. 7. The polar
aprotic solvent like DMSO and acetonitrile were also effective but provided low yield for 1-
benzyl-3-phenylurea production. Non polar solvents like hexane, toluene contribute an
insignificant yield to the product formation. The 95% yield was observed for using DMF
solvent. So, aprotic solvent with polar nature has been found more appropriate than only
aprotic one for 1-benzyl-3-phenylurea formations.
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Effect of temperature and pressure on 1-benzyl-3-phenylurea production
(A)
(B)
Fig. 8 Effect of (A) temperature and (B) pressure for 1-benzyl-3-phenylurea production
catalysed by Pd(PS-SAL). Reaction condition: nitrobenzene (3 mmol), benzylamine (6
mmol), Catalyst (20 mg), FeCl3 (2.5 mmol), DMF (6 mL), methanol (6 mL), time (6 h).
The yield of 1-benzyl-3-phenylurea was varies with carbon monoxide pressure and also with
temperature on the carbonylation of nitrobenzene and benzylamine. Moreover the
optimization of temperature and pressure on 1-benzyl-3-phenylurea production was studied.
The temperature of reaction medium was increased gradually from 40 oC to 70 oC and at final
temperature 1-benzyl-3-phenylurea yield was 83%. In addition the present reaction was set to
o
undergo at 75 C and the significant yield obtained 95% (Fig. 8A). Even in the presence of
methanol no yield of product isolated although the reaction proceed in room temperature. At
low carbon monoxide pressure (< 30 psi) no trace of product was found since the amount of
reactants remains unchanged. Whereas an outstanding performance was present at 80 psi of
CO pressure providing 95% yield (Fig. 8B). As the reaction pressure was boosted from 30 psi
to 85 psi progressively increases of % yield of 1-benzyl-3-phenylurea from 40% to 95%. At
comparatively low temperature and CO pressure no development of product detected but
highest (95%) yield was obtained at 75 oC and 80 psi pressure.
Table 2. Optimization of amount of catalyst for 1-benzyl-3-phenylurea productiona
Entry
Time (h)
Amount of catalyst (mg) Yield (%)b
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1.
2.
3.
4.
5.
6.
7.
2
20
20
20
20
20
15
25
24
45
72
95
91
76
95
3.5
5
6
6.5
6
6
aReaction condition: nitrobenzene (3 mmol), phenylmethanamine (6 mmol), FeCl3 (2.5
b
mmol), DMF (6 mL), MeOH (6 mL), temperature (75 oC), PCO (80 psi). Isolated yield.
The amount of catalyst also performs as crucial function in current reaction for carbonylative
diaryl urea production (Table 2). In above table, the yield of carbonylation reaction was
moderate when reaction was simulated in presence of 15 mg of catalyst for 6 h. Increasing
catalyst amount from 15 mg to 20 mg for 6 h reaction percent production of product increase
but enhancement of time with same amount of catalyst (20 mg) reduce the yield percentage.
No yield of product was obtained without addition of Pd(PS-SAL) catalyst. The sample
carbonylative reaction was elaborated for 2 to 6 h. When the reaction was progress for 5 h
provide moderate yield 72% whereas 2 h reaction offer a lowest yield of product. A rise in
the catalyst amount (25 mg) showed no advancement in the model reaction. A preeminent
result was found when the model reaction was optimized with 20 mg of catalyst and 6 h.
Table 3. Optimization of co-catalyst for 1-benzyl-3-phenylurea synthesisa
Entry
1.
Co-catalyst
Yield (%)b
-
-
2.
SnCl4
KOH
91
30
3.
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4.
5.
6.
7.
Et3N
Pyridine
25
23
95
92
FeCl3
p-toluene sulphonic acid
aReaction condition: nitrobenzene (3 mmol), phenylmethanamine (6 mmol), DMF (6 mL),
MeOH (6 mL), temperature (75 oC), PCO (80 psi). bIsolated yield.
Exploration of optimized condition for co-catalyst having acidic and basic nature was done.
The acidic catalyst like SnCl4, FeCl3, p-toluene sulphonic acid (PTSA) was improved the
yield of the product maintaining the other optimized condition fixed (Table 3). The mixture
of nitrobenzene and benzylamine may undergo carbonylation in the presence of Lewis acidic
co-catalyst FeCl3 to afford the corresponding phenyl urea, with almost quantitative yield. But
the basic co-catalyst like Et3N, pyridine provide worst yield of product. Without co-catalyst
no existence of product established even at 80 psi CO pressure, 6 mL MeOH and other
necessary controlled conditions. The percent yield of product in model reaction will decrease
if moisture is in the reaction system. Finally, an excellent yield (95%) was observed. The %
yield of product for the reaction between nitrobenzene and benzylamine with co-catalyst was
highest for FeCl3. Iron co-catalysis in the above system offers a satisfactory high yield due to
development of the catalytic cycles associated with activation of the CO molecule [35].
Synthesis of diaryl urea production using different derivatives
The carbonylation of nitrobenzene together with a number of different substituted amines
underwent spontaneously under gentle conditions, affording the carbonylative products i.e.
diaryl urea in moderate to high yields is outlined in Table 4.
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Table 4. Reductive carbonylation of nitrobenzene with different derivative of amines by
polymer anchored Schiff base-palladium complexa
Conv.
(%)b
Yield
(%)c
TOF
(h-1)
Entry
Amines
Product
1.
98
95
37.69
2.
17
Trace
6.53
3.
4.
5.
6.
96
99
95
90
96
91
36.92
38.07
36.53
98
40
97
32d
98
37.69
15.38
38.46
7.
8.
100
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96
16
94
36.92
6.15
9.
Tracee
10.
aReaction condition: nitrobenzene (3 mmol), amine (6 mmol), FeCl3 (2.5 mmol), DMF (6
mL), MeOH (6 mL), Catalyst (20 mg, 0.013 mmol based on Pd active centre), temperature
o
b
c
d
(75 C), time (6 h), 80 psi CO pressure. GC conversion. Isolated yield. nitromethane used
instead of nitrobenzene. ewithout Pd(PS-SAL) catalyst but FeCl3 present.
Under optimized reaction system, the reaction of nitrobenzene and the derivatives of aniline
were observed as represented in Table 4. The effectiveness of catalyst was examined with
cyclohexylamine, p-toluidine, aniline, benzylamine, 4-methoxyaniline etc. and among their
corresponding products 1-(4-methoxyphenyl)-3-phenylurea grant for the highest yield and
better conversion (Table 4, Entry 8). Disappointingly, o-nitroaniline provides a negligible
yield of urea and lower conversion of amine signifying that -NO2, an electron withdrawing
group strictly weaken the -NH2 group activity (Table 4, Entry 2). When aliphatic nitro
compound are used instead of aromatic nitro compound the yield is too low with respect to
other derivatives (Table 4, Entry 7). But when propylamine treated with nitrobenzene the
yield is interestingly high than that of methylamine product (Table 4, Entry 5,9). This is due
to more inductive effect of propylamine. Urea of cyclohexylamine, p-toluidine, and aniline
led to 96%, 97%, 90% yield respectively with a satisfactory conversion (Table 4, Entries
3,4,6). Hence the group that has electron donating capability give better yield than others.
Benzylamine also provide a better result than other aliphatic amines (Table 4, Entry 1).
However, without Pd(PS-SAL) catalyst and presence of ferric chloride afford a trace amount
of reaction product. FeCl3 provides the definite work in the reaction of which it enhances the
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yield of the reaction but sufficient amount of Pd(PS-SAL) catalyst ensure better yield of the
product.
Mechanism of diphenyl urea synthesis in presence of Pd(PS-SAL) catalyst
Scheme 3. Plausible mechanistic pathway for polymer supported palladium catalysed
diphenyl urea synthesis.
The possible mechanism is suggested on the basic assumption that a Pd-CONHAr complex is
formed during carbonylation (Path A, Scheme 3). There are lot of literature examples which
proposed the same intermediate [36,37]. Nitrobenzene engaged itself in the mechanism as an
oxidant. Nitrobenzene undergoes react with CO molecule and itself reduced to aniline. In
this way the carboxylation cycle commences as depicted in Scheme 3. The reaction forwards
with involving the catalyst proceeding through carbonylation of aniline-Pd intermediate to
generate Pd-carbamoyl complex followed by withdrawal of oxygen atom from PhNO2 by CO
molecule. The simultaneous reduction process of PhNO2 to PhNH2 was experienced by the
uptake of the H-atom released from aniline molecule [38]. Several authors reported a nitrene
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mechanism followed for rationalizing the product urea [39,40]. Path A, describes a
carbamoyl intermediate is generated during the carbonylation reaction between PhNH2 and
CO. Next it is proficient to reacts with aniline and nitrobenzene with the subsequent
formation of diphenyl urea. Nitrobenzene in this reaction treats as an oxidant through
drawing H atom from the carbamoyl complex [36,41].
Likewise additional mechanistic pathway can be considered in Scheme 3 based on the
Pd-nitrene intermediate as explored in Path B. First, from aniline as the substrate, the
feasible Pd-nitrene species is formed simultaneously with the release of CO2 where aniline is
also formed in the reaction system from nitrobenzene reduction. The nitrobenzene reduction
by the CO in the presence of Pd(PS-SAL) catalyst occurs at a faster rate. The insertion of CO
is next into Pd-nitrene species took place to produce Pd-coordinated isocyanate. Ultimately,
amine attacks as a nucleophile to the isocyanate escalated by the palladium complex
consequences to the yield of expected unsymmetrical ureas alongside regeneration of Pd
catalyst [42]. As represented in Scheme 3, PhNCO[Pd] complex is known to be formed by
carbonylation of PhN[Pd] (a nitrene complex), which has been generated by stepwise
deoxygenation of nitrobenzene with CO (Path B). The phenyl isocyanate complex is able to
react with aniline and gives rise to the targeted N,N'-diphenyl urea [36,43]. The
transformation to isocyanate is very rapid since isolation of this intermediate is beyond
feasibility [38].
Catalytic Activity of Carbonylative S-M cross coupling reaction
Scheme 4. Carbonylative Suzuki-Miyaura cross coupling reaction of iodobenzene and
phenylboronic acid.
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Effect of pressure for diphenyl ketone synthesis
Fig. 9 Pressure effect for diphenyl ketone formation by polystyrene supported Pd catalyst.
Reaction condition: Iodobenzene (1 mmol), phenyl boronic acid (1 mmol), K2CO3 (3 equiv.),
o
anisole (5 mL), temperature (90 C), time (8 h), catalyst (15 mg, 0.01 mmol based on Pd
active centre).
Effect of carbon monoxide pressure on carbonylative Suzuki-Miyaura cross coupling
reaction of iodobenzene and phenylboronic acid (a model reaction) illustrated in Fig. 9.
Increase of CO pressure from 65 psi, promotion of product yield was not observed which
might cause that rising of CO concentration perform a petite role in the cross-coupling
reaction. Investigation of pressure over the carbonylative S-M cross-coupling reaction was
done from 29 to 98 psi. However, enhance of CO pressure from 29 to 65 psi a continuous
increasing of product yield in the model reaction from 41% to 96% respectively. Moreover,
maintaining all the optimized condition we find 96% yield at 65 psi carbon monoxide
pressure.
Effect of amount of catalyst on cross-coupling product diphenyl ketone synthesis
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Fig. 10 Effect of amount of catalyst on diphenyl ketone formation by Pd catalyst. Reaction
condition: Iodobenzene (1 mmol), phenylboronic acid (1 mmol), K2CO3 (3 equiv.), anisole (5
mL), PCO=65 psi, temperature (90 oC), time (8h).
The amount of catalyst is another essential part in model reaction for carbonylative cross-
couping reaction present in Fig. 10. Elaboration of optimized condition with the quantity of
catalyst was done from 1 mg to 25 mg. The low yield of carbonylative product was observed
in the model reaction containing low amount of catalyst. The captivating outcome was
achieved (96% yield) when 15 mg of catalyst was used. Without catalyst no product was
obtained. Increase of catalyst amount in the sample reaction, no reasonable yield of
carbonylative product.
Table 5. Benzophenone synthesis using different solvent and basea
Entry
1.
Base
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
-
Solvent
EtOH
Yield (%)b
0
37
96
0
2.
DMSO
Anisole
-
3.
4.
5.
Anisole
Anisole
Anisole
Anisole
Anisole
Toluene
H2O
65
0
6.
7.
Et3N
trace
42
51
69
0
8.
KOH
9.
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
10.
11.
12.
13.
14.
15.
DMF
62
72c
88d
93
Anisole
Anisole
Anisole
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aReaction condition: Iodobenzene (1 mmol), phenylboronic acid (1 mmol), PCO=65 psi,
temperature =90 oC, ctemperature =80 oC, dtemperature =100 oC, time (8 h), catalyst (15 mg,
0.01 mmol based on Pd active centre). bIsolated yield.
Maintaining the other optimized condition unaltered for the reaction between phenylboronic
acid and iodobenzenes in the presence of polystyrene supported palladium catalyst was also
selected for optimization with various base and solvent. The impact of different
organic/inorganic bases and protic/aprotic solvents are represented in Table 5, Entries 1-14.
According to the observation it is proved that polar aprotic solvent is more reliable for better
yield formation than polar protic solvent. When the model Suzuki-Miyaura cross coupling
reaction was optimized with ethanol, water (polar protic) (Table 5, Entries 1,11) no desired
product was found. But for nonpolar protic solvent toluene product contribution was 69%
(Table 5, Entry 10). However, the percentage yield of cross-coupling reaction product was
moderately good for DMF, DMSO, anisole (polar aprotic) (Table 5, Entries 2,12,13).
Afterwards a satisfactory result was obtained where anisole was used as a solvent. The
maximum yield 96% (Table 5, Entry 3) was obtained for polar aprotic solvent anisole
keeping the other optimized parameter same.
As base also acts a reasonable role in the model reaction has endured a variety of organic and
inorganic bases (Table 5, Entries 1-14). The result of optimizing base parameter indicates that
bases of inorganic origin like K2CO3, Cs2CO3, K3PO4 provide a better result than organic
bases like Et3N. The excellent report was inferred for K2CO3 base while the reaction was
executed under 15 mg of Pd catalyst and 65 psi of CO pressure. But temperature has an effect
on reaction condition when other conditions are optimized. When sample reaction
temperature was reached to its upper limit 90 oC excellent yield of product (Table 5, Entry 3)
o
but extending this temperature to 100 C lower the yield of carbonylative product. So, it is
concluded that to achieve a marvelous result in terms of % of yield a base K2CO3, solvent
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o
anisole and heating under 90 C temperature is necessary with other optimized parameter.
Effect of time on diphenyl ketone synthesis
Fig. 11 Effect of time on carbonylative Suzuki-Miyaura reaction catalysed by polystyrene
supported Pd catalyst. Reaction condition: Iodobenzene (1 mmol), phenylboronic acid (1
o
mmol), K2CO3 (3 equiv.), anisole (5 mL), PCO=65 psi, temperature (90 C), catalyst (15 mg,
0.01 mmol based on Pd active centre).
After accomplishment of optimized CO pressure and amount of catalyst now investigate
about the effect of time in the model reaction (Fig. 11). The elevation in reaction period from
3 h to 8 h results constant increase of yield of carbonylative product from 36% to 96%. A
moderate yield 73% obtained when the reaction time was 6 h. Maximum yield obtained when
o
the same was performed with 15 mg of catalyst, 80 psi of CO pressure, 90 C temperature
and surely 8 h certain reaction time.
Synthesis of diaryl ketone by carbonylative Suzuki-Miyaura reaction
The scope of S-M cross-coupling reaction was investigated by using aryl boronic acids and
an extensive class of aryl iodides as starting materials.
Table 6. Carbonylative S-M cross-coupling reaction of aryl iodide with aryl boronic acid
catalyzed by polymer anchored Schiff base-palladium catalysta
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Aryl boronic
acid
Conv. Yield TOF
Entry Aryl iodide
Product
(%)b
(%)c
(h-1)
1.
98
96
12.25
2.
93
87
11.62
3.
4.
95
96
94
93
11.87
12
5.
75
73
9.38
aReaction condition: aryl iodide (1 mmol), aryl boronic acid (1 mmol), K2CO3 (3 equiv.),
o
anisole (5 mL), PCO=65 psi ,temperature (90 C), time (8 h), catalyst (15 mg, 0.01 mmol
based on Pd active centre). bGC conversion. cIsolated yield.
With established optimization conditions we expand the cross-coupling with various aryl
boronic acid and aryl iodide (Table 6) in the presence of CO have been expanded. Both aryl
boronic acids and aryl iodides containing electron releasing and attracting groups are
efficiently employed for carbonylative cross-coupling reaction with polystyrene supported Pd
catalyst having moderate to excellent yield 73-96%. Phenyl boronic acid without any
substituted group is finely tolerated and provide corresponding carbonylative product as
maximum yield with 96% and the % conversion is also excellent. Likewise, 4-chloro
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substituted iodide provide almost 93% yield efficiently with polymer supported palladium
catalyst. Iodide containing para substitution having electron releasing -CH3 group in phenyl
ring affording corresponding product yield 94% whereas ortho substituted product give only
73% of yield with relatively low conversion. This low yield happen due to bulky nature of -
CH3 group which may creat steric hinderance. Relatively low yield (87%) of isolated product
was obtained for the corresponding reaction of iodobenzene and 3-cyanophenylboronic acid
with palladium catalyst.
Mechanism of carbonylative S-M cross-coupling reaction in presence of heterogeneous
palladium catalyst
Scheme 5. Plausible mechanism for the carbonylative S-M cross-coupling reaction catalyzed
by polymer anchored Schiff base-palladium complex.
The possible mechanistic pathway for cross coupling suggests the catalytically active Pd(PS-
SAL) species coordinates to the π-system in the electrophile, followed by the migration of Pd
metal to an initially possible species (I). The next step in the carbonylation reaction is the
coordination of CO to palladium leading to an intermediate (II) (Scheme 5) followed by the
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migration of organyl group to the CO carbon that creates an acylpalladium complex (III)
[44].
The aryl iodide undergoes oxidative addition to the Pd center to form the aryl palladium
intermediate (I). The inserted CO molecule coordinates with the aryl palladium species and
later an internal migration process leads to the formation of acyl palladium species. The
arylboronic acid undergoes transmetallation with the Pd center in the existence of K2CO3 to
give PhCO-Pd-Ph species. The subsequent transmetallation between PhB(OH)2 and PhCO-
Pd-L offering the PhCO-Pd-Ph molecule, which immediately experiences an elimination to
generate the diaryl ketone exclusively accompanied by the release of Pd(PS-SAL) catalyst
[45,46].
The step following insertion in carbonylative cross-coupling is defined as transmetallation
(Scheme 5) and transports the organonucleophile to the Pd center. Attack of base (here
K2CO3) to aryl boronic acid followed by -Ph group migration delivers the intermediate (III)
which undergoes the reductive elimination, to produce the ultimate unsymmetrical diaryl
ketones by regenerating the Pd-catalyst (Scheme 5) [47-49].
Heterogeneity test
Heterogeneity test was performed to prove whether Pd(PS-SAL) truly act as heterogeneous or
not in carbonylation reaction of nitrobenzene and benzylamine as a model reaction. The
reaction was conceded for 4 h firstly then the catalyst was removed. The yield of 1-benzyl-3-
phenylurea was 57%. The reaction was performed for another 2 h in absence of catalyst but
there was no satisfactory enhancement in the yield of product (Fig. 12).
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Fig. 12 Heterogeneity test of polymer supported schiff base-Pd catalyst.
The yield of product remains same. The reaction mixture was passed through a filter which
upon AAS analyses of the liquid phase confirmed the absence of Pd. These inferred that Pd is
not being leached out from the Pd(PS-SAL) catalyst throughout the reaction. Moreover, when
a blank reaction (absent of catalyst) was performed no yield of corresponding 1-benzyl-3-
phenylurea observed. This outcome suggested the catalytic character of Pd is midpoint in this
carbonylation reaction and was heterogeneous in nature.
Recyclability of catalyst
Heterogeneous catalyst has an important character shown through its recyclability and
stability. It is pertinent to remove the separation problems and disadvantages through the
immobilization via covalent bond with the support to make it heterogeneous. The system
becomes more convenient, lucrative over the commonly used incorporation process in terms
of diminishing catalyst leaching and next to enhancing stability of Pd-catalyst. The leaching
of Pd metal in each run has been given in the following Table 7 indicating that very small
amount (almost negligible) of palladium metal was leached out even after 5 times reaction.
28