MizorokiHeck cross-coupling reaction of haloarenes mediated by a well-controlled modified polyacrylamide brush grafted silica/pd nanoparticle system

Article abstract of DOI:10.1246/bcsj.20160374

Modified polyacrylamide brushes including phosphinite functionality grafted onto silica supported Pd nanoparticles were synthesized via RAFT polymerization technique in a controlled manner with elucidated graft density and chain length. Proper activity of the catalyst was indicated in the MizorokiHeck coupling reaction of various haloarenes with olefins. Different aryl iodides with electron rich and electron deficient substituent and also ortho and heterocyclic substrate showed good reactivity to generate the corresponding coupled products in good to excellent yields using low Pd loading. The turnover number (TON) for this catalyst can be reduced up to 9.5×103. Simple filtration, appropriate reusability and negligible palladium leaching of this catalyst are among other advantages.

Full text of DOI:10.1246/bcsj.20160374

Mizoroki-Heck Cross-Coupling Reaction of Haloarenes Mediated by
a Well-Controlled Modified Polyacrylamide Brush Grafted Silica/Pd
Nanoparticle System
Soheila Ghasemi* and Saiede Karim
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 7194684795, Iran
E-mail: ghasemis@shirazu.ac.ir
Received: November 13, 2016; Accepted: January 25, 2017; Web Released: March 21, 2017
Soheila Ghasemi
Soheila Ghasemi received her B.Sc., M.Sc. and Ph.D. from Shiraz University in 2003, 2006, and 2010,
respectively. She was appointed as an Assistant Professor of the Department of Chemistry of Shiraz
University in 2012. Her current research interest is in the areas of polymer supported catalysts, reagents and
scavengers, controlled polymer and nanocomposite synthesis and improvement of mechanical properties of
industrial polymers.
Abstract
materials are not only the sum of the individual contributions of
both phases, but have synergetic properties of both parts. In this
regard, polymer brushes, polymer tethered to inorganic, metal
or carbon-based surfaces, have special importance in numerous
fields of application.26,27 Polymer grafting on inorganic sur-
faces such as Fe3O4,28-30 SiO2,31 Au,32 ZnO33 or alumina34 has
been developed extensively due to the ability to modify the
surface properties and has found uses in applied and basic inter-
facial studies. Of these, silica particles are perfect substrates for
surface modification due to superior properties involving high
surface area, mechanical strength and permeability, chemical
and thermal stability and fairly low refractive index. To achieve
a well-defined structure of polymer grafted surfaces, with con-
trol over the polymer density, chain length and topology, so-
called “living” polymerizations can be applied.35-37 The most
common techniques for generating living polymers grafted
onto surfaces are the “grafting to”,38,39 “grafting from”40,41 and
“grafting through”42 processes. Figure 1 illustrates the repre-
sentative difference of these techniques. However, to the best
of our knowledge there are no reports in the literature of Pd
nanoparticle supported well-controlled modified polyacryl-
amide brush grafted silica as a heterogeneous catalyst for use
Modified polyacrylamide brushes including phosphinite
functionality grafted onto silica supported Pd nanoparticles
were synthesized via RAFT polymerization technique in a con-
trolled manner with elucidated graft density and chain length.
Proper activity of the catalyst was indicated in the Mizoroki-
Heck coupling reaction of various haloarenes with olefins.
Different aryl iodides with electron rich and electron deficient
substituent and also ortho and heterocyclic substrate showed
good reactivity to generate the corresponding coupled products
in good to excellent yields using low Pd loading. The turnover
number (TON) for this catalyst can be reduced up to 9.5 © 103.
Simple filtration, appropriate reusability and negligible palla-
dium leaching of this catalyst are among other advantages.
1. Introduction
Undoubtedly, the Heck reaction is one of the most beneficial
and extensively used reactions for C-C bond generation.1,2
Aryl-, alkyl- or vinylation are performed through the reaction
between numerous olefins and aryl, benzyl, vinyl or either allyl
halides, acetates or triflates using Pd catalyst and an appropri-
ate base under diverse conditions. A considerable number of
reviews are allocated to this subject due to the great impact
and empirical importance in the production of pharmaceuticals
and fine chemicals.3-6 Principally, the Pd catalyzed Heck reac-
tion is performed homogeneously either in organic medium,7-9
water10,11 or ionic liquids.12-14 The desirability of recovery and
reuse of the precious Pd catalyst incline to favor heterogeneous
catalysis systems.15-17 The supported catalysts are based on
(bio)organic,18-22 inorganic23,24 and hybrid structures.25 Hybrid
monomer
initiator
"grafting from"
"grafting through"
"grafting to"
Figure 1. The most common techniques of polymer
grafting.
Bull. Chem. Soc. Jpn. 2017, 90, 485–490 | doi:10.1246/bcsj.20160374
© 2017 The Chemical Society of Japan | 485

in cross-coupling reactions. In continuation of our research on
immobilized Pd catalysts based on polymer grafted silica,43-46
at present, we report the synthesis and applications of a well-
defined modified polyacrylamide incorporating phosphinite
group/Pd nanoparticles via RAFT process through “grafting
to” approach in the Heck coupling reaction. Applying con-
trolled RAFT polymerization for producing grafted polymers
enable us to improve catalyst activity with control over branch
density and molecular weight of the polymers.
(1.2 mmol) was added to a reaction flask containing aryl iodide
(1.0 mmol), K2CO3 (2.0 mmol) and Pd complex (0.001-0.002
mmol) in DMF (3 mL). The reaction mixture was maintained at
85 °C for an appropriate time while being stirred. The progress
of the reaction was followed by TLC on silica gel until no
traces of starting aryl iodide were observed. Upon completion
of the reaction, the slurry was filtered and 50 mL of water
added to the filtrate and it was then extracted with 15 mL of
CH2Cl2 three times. Drying over Mg2SO4, evaporating the
solvent and passing through a short silica gel column using n-
hexane/ethyl acetate as eluent provided the pure product. The
coupling product was characterized by comparing the spectro-
2. Experimental Section
General Remarks. Aminopropyl silica gel (09297 Sigma-
Aldrich) with an average particle size of 15-35 ¯m and a
1
scopic data (FT-IR, H NMR and 13C NMR) with those of an
¹1
loading of 0.95 mmol g of NH2 group used in this study. All
authentic sample.
other chemicals were of reagent grade and provide from Acros,
Merck Millipore or Sigma-Aldrich companies and used as
received without additional purification. The progress of the
reactions was followed by thin layer chromatography (poly-
grams SILG/UV 254 plate of silica gel) or by gas chromatog-
raphy (GC-14A Shimadzu gas chromatograph with a flame
ionization detector equipped with DC-200 stationary phase
packed glass column and N2 carrier gas). FT-IR spectra were
recorded on a Shimadzu FTIR-8300 spectrophotometer, and the
samples were ground with KBr and the mixture was then
pressed into a pellet for IR measurement. NMR analyses were
performed on a Bruker Avance DPX instrument (250 MHz).
Pd content and leaching test were carried out on samples by
ICP-OES analysis (Varian, Vista-Pro with CCD Simultaneous
spectrometer).
Reuse of the Pd Catalyst. Recycling of the catalyst was
carried out for the reaction between iodobenzene and n-butyl
acrylate according to the procedure elucidated in the previous
section. The catalyst was recovered by cooling the mixture to
room temperature after completion of the reaction, filtering off
the slurry, washing the solid with DMF, H2O and acetone thor-
oughly and consequently vacuum drying. The resulting catalyst
was reused in repeating runs with a new amount of reagents
without any pre-treatment.
Heterogeneity Tests: Experimental Evidence for Pd
Redeposition and Leaching. A mixture of reactants including
iodobenzene (1.0 mmol), n-butyl acrylate (1.2 mmol), K2CO3
(2.0 mmol) and Pd complex (0.5 mol %) in 3 mL of DMF in a
round-bottomed flasks was stirred for 15 min at 85 °C. At this
time (25% conversion of iodobenzene), the catalyst was filtered
off from the hot solution and the residue was heated for another
24 h. At last, product formation was evaluated using GC
analysis.
Synthesis of Pd Nanoparticles Supported on Modified
Polyacrylamide Grafted Silica (SiO2-g-Modified PAAm/Pd)
Catalyst.
SiO2-g-PAAm was obtained by reaction of
aminopropylsilica, dicyclohexylcarbodiimid (DCC), 4-dimeth-
ylaminopyridine (DMAP) and polyacrylamide with an acidic
chain end, which is prepared by RAFT technique (PAAm-
CTA). The reaction of this product with ethanolamine and
subsequent reaction with chlorodiphenylphosphine produced
the corresponding polymeric phosphinite ligand which pro-
vided SiO2-g-modified PAAm/Pd catalyst by treating with
Pd(OAc)2. (Scheme 1) (Detailed synthesis of the catalyst is
provided in the supplementary information).
3. Results and Discussion
In this study, we have introduced Pd nanoparticles supported
on modified polyacrylamide containing phosphinite ligand
grafted silica as an efficient catalyst for Heck coupling reaction.
Initially, controlled synthesis of polyacrylamide with acidic
functionality at the chain ends and degree of polymerization
(DP) of 100 and 250 is accomplished via RAFT polymerization.
Organic-inorganic hybrid supports (SiO2-g-PAAm) were syn-
thesized through “grafting to” strategy. The DP and graft density
of theses polymers are represented in Table 1. Then, hydroxyl-
modified polyacrylamide was obtained by transamidation reac-
tion of SiO2-g-PAAm with ethanolamine which was converted
to the phosphinite group through reaction with ClPPh2. Iodo-
metric titration afforded the phosphorous content of the ligands
to be 4.6 mmol/g for the polymer with DP = 100 and 4.56
mmol/g for the polymer with DP = 250. The corresponding
phosphinite ligand benefits from both the π-accepting capac-
ity and good donor strength of phosphorus compounds.47-49
Mizoroki-Heck Cross-Coupling Reaction: General Pro-
cedure. An olefinic compound (n-butyl acrylate or styrene)
S
S
CH3
CH3
CH2 CH
Dry Dioxane/ 80 oC
HO2C
O
C12H25
AIBN
+
HO2C
S
n
S C12H25
+
S
S
H3C
freeze-evacuate-thaw
cycle
NH2
CH3
O
NH2
AAm
CTA
n=100, 250
S
PAAm-CTA
NH2
SiO2
DCC, DMAP
O
C
CH3
CH2 CH
NH
n=100, 250
S
n
S C12H25
SiO2
CH3
"Grafting to"
approach
O
NH2
SiO2-g-PAAm
S
O
C
CH3
CH2 CH
1) NH2CH2CH2OH
NH
S
S
C12H25
OPPh2
SiO2
n
2) ClPPh2/Et3N/THF
CH3
O
NH
Table 1. DP and graft density for PAAm with DP of 100
SiO2-g-modified PAAm
and 250
O
C
CH3
Pd(OAc)2/ DMF/ 100 oC
Pd (0)
NH
OPPh2
SiO2
Graft density
PAAm
n
CH3
SiO2-g-modified PAAm/Pd catalyst
Entry
DPn,th
DPn,NMR
¹1
(¯mol g
)
PAAm (DP = 100)
PAAm (DP = 250)
1.00 © 102
2.50 © 102
1 © 102
3 © 102
19
13
Scheme 1. Synthetic design for the production of SiO2-g-
modified PAAm/Pd catalyst.
486 | Bull. Chem. Soc. Jpn. 2017, 90, 485–490 | doi:10.1246/bcsj.20160374
© 2017 The Chemical Society of Japan

Table 2. Heck reaction of iodobenzene with n-butyl acrylate
using Pd catalysts
Table 3. Optimization of base, solvent and Pd content for
the Heck reaction of iodobenzene with n-butyl acrylatea)
K2CO3 (2.0 Eq.)
I
O
CO2Bun
I
Bun
NMP
CO2Bun
+
O
Pd catalyst
Bun
O
+
Pd Cat. (0.5 mol%)
100 oC
base/solvent/ 85 o
C
O
butyl cinnamate
(1.0 Eq.)
(1.2 Eq.)
Pd
(mol %)
Time
(h)
Conversion
Entry
Catalyst
Time (h)
Conversion (%)a)
Entry Base
Solvent
(%)b)
1
2
DP = 100
DP = 250
1
5
100
95
1
K2CO3
DMSO
NMP
DMF
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.25
0.1
0.05
0.01
5
1
100
100
100
50
80
80
2
3
4
5
6
7c)
8
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
a) Conversion based on iodobenzene.
0.75
24
24
24
24
4
THF
CH3CN
MeOH
DMF
Finally, complexing this ligand with Pd(OAc)2 produces the
heterogeneous Pd catalyst as illustrated in Scheme 1. ICP
analysis revealed that the catalysts with DP = 100 and 250
contained an average of 0.55 and 0.50 mmol/g of Pd content
respectively. The formation of metallic Pd0 was disclosed using
XRD and XPS. Furthermore, SEM and TEM images of the
catalyst manifest quasi-spherical morphology of particles and
fine distribution of PdNPs in less than 10 nm size respectively
(see supporting information). Comparative evaluation of cata-
lytic activity of Pd catalysts with DP = 100 and 250 were inves-
tigated in the Heck coupling reaction of iodobenzene with n-
butyl acrylate in NMP in the presence of K2CO3 and 0.5 mol %
of Pd catalysts at 100 °C. The results are summarized in Table 2.
The catalyst with DP = 100 displayed superior catalytic activ-
ity, probably due to the optimum graft density and chain length
of this catalyst which allowed substrates to access the catalytic
sites. Accordingly, we selected this catalyst for more explora-
tion in the Heck reaction.
20
80
NaOAc DMF
9
NEt3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
3
100
100d)
45
100
100
100
100
10
KOH
®
10
24
1
2
3
11
12
13
14
15
K2CO3
K2CO3
K2CO3
K2CO3
4
a) Reactions were run using 1.0 Eq. iodobenzene, 1.2 Eq. n-
butyl acrylate, 2.0 Eq. of various bases and 0.005-0.0001 Eq.
Pd catalyst in different solvents (5 mL) at 85 °C. b) Based on
iodobenezene conversion. c) Reaction performed at room
temperature. d) Biphenyl is also produced as a by-product.
X
O
0.1 mol% Pd cat.
K2CO3,DMF
Bun
O
Bun
O
+
85 oC, 24h
O
Optimization of Reaction Condition for Heck Reaction.
To determine the capability of this Pd catalyst, it was exam-
ined in the Heck coupling reaction. To optimize the reaction
conditions, initially, the coupling of iodobenzene with n-butyl
acrylate as model coupling partners was studied. The influence
of different solvents on the Heck coupling reaction of the
model substrates was investigated over the catalyst as presented
in Table 3. Among the solvents used, DMF, NMP and DMSO
showed high catalytic activity (Entries 1-3). In spite of the fact
that DMSO needed longer reaction time for complete conver-
sion of iodobenzene (Entry 1). Other solvents such as CH3CN,
THF and MeOH were probed and proved to be inferior (Entries
4-6). Performing the reaction at room temperature gave only
20% coupled product after 24 h (Entry 7), while increasing the
temperature up to 85 °C produced the coupled product with
100% conversion of iodobenzene in less than 1 hour (Entry 3).
Increasing the temperature above 85 °C caused byproduct for-
mation. The influence of different bases for the model reac-
tion was further investigated. The utilization of a base in the
Mizoroki-Heck reaction is required for neutralizing hydrogen
halides and in preventing homo-coupling product formation.
Among bases tested, K2CO3 was the best choice. It gave quan-
titative conversion of iodobenzene without considerable forma-
tion of homo-coupling product in DMF (Entry 3). Other bases
such as Et3N, KOH and NaOAc were tested and required
longer time or provided lower yield (Entries 8-10). In the
absence of a base, only 45% of coupling product was produced
after 24 h (Entry 11). Furthermore, we examined the effect of
catalyst loading on the coupling reaction of iodobenzene with
X= Br (75%), X= Cl (38%)
GC-based yields
X= Cl, Br
Scheme 2. Heck reaction of bromobenzene and chloroben-
zene with n-butyl acrylate using SiO2-g-modified PAAm/
Pd catalyst under optimized reaction conditions for 24 h.
n-butyl acrylate (Entries 12-15). The loading of the catalyst can
be even reduced down to 0.01 mol % of Pd by prolonging the
reaction time to 4 hours in order to get complete conversion
(Entry 15). The turn over number (TON = mmole of product/
mmole of Pd catalyst) is calculated for the least amount of
catalyst (0.01 mol %) and is 9.5 © 103 as a factor to show the
high efficiency of the catalyst. In all, we chose DMF as a
solvent and K2CO3 as a base using 0.1 mol % of catalyst at
85 °C as the best option for this reaction (Table 3, Entry 13).
Additionally, bromobenzene and chlorobenzene produced the
coupled product with 75% and 38% reaction yields after 24 h
under optimized reaction conditions, respectively (Scheme 2).
The Mizoroki-Heck Coupling Reaction of Aryl Iodides
with Olefins. The generality of this reaction system was
shown when the other coupling reactions were carried out using
different aryl iodides and styrene or n-butyl acrylate in the
presence of catalyst (Scheme 3 and Table 4).
The aryl iodides with electron-rich and electron-poor sub-
stituent reacted with n-butyl acrylate very well to generate the
corresponding cross-coupling products in good to excellent
yields. Electron withdrawing groups decrease the reaction time,
but electron donor groups increase it. Therefore, p-nitro
Bull. Chem. Soc. Jpn. 2017, 90, 485–490 | doi:10.1246/bcsj.20160374
© 2017 The Chemical Society of Japan | 487

R2
reductive elimination
I
Ar-I
K2CO3, DMF
Pd Cat., 85 o
KHCO3 + KI
(4)
Pd0 Ln
R2
oxidative addition
+
C
K2CO3
R1
R1
(1)
L= SiO2 grafted phosphinite
functionalized polyacrylamide
R1= CH3, OCH3, Br, Cl, OH, NO2
R2= CO2Bun, Ph
L
L
II
II
I
Pd Ar
L
I
Pd
L
H
Scheme 3. Heck reaction of aryl iodides with olefins using
SiO2-g-modified PAAm/Pd catalyst.
Ar
R
coordination
R
Heck reaction catalytic cycle
Table 4. Reaction of n-butyl acrylate or styrene with
different aryl iodides in the presence of Pd catalysta)
Ar
I
H
I
L
Pd
L
L
Pd
L
Ar
0.1-0.2 mol%Pd catalyst
R
Ar
I
+
R
Ar
DMF/K2CO3/85 o
C
R
R
π
complex
π
complex
R
H
Entry
Ar
R
Time (h)
Yield (%)b)
Ar
H
migratory insertion
II
β
- H elimination
L
Pd
I
(2)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
C6H5
CO2Bun
CO2Bun
CO2Bun
CO2Bun
CO2Bun
CO2Bun
CO2Bun
CO2Bun
Ph
Ph
Ph
Ph
Ph
2
9
3
1
11
7
1
7
3
12
4
98
93
97
98
91
55
96
63
95
94
98
98
93
60
97
67
(3)
L
2-Me-C6H4
4-OMe-C6H4
4-Br-C6H4
4-Cl-C6H4
4-OH-C6H4
4-NO2-C6H4
4-CH3-C6H4
C6H5
2-Me-C6H4
4-OMe-C6H4
4-Br-C6H4
4-Cl-C6H4
4-OH-C6H4
4-NO2-C6H4
4-CH3-C6H4
Pd−σ intermediate
Scheme 5. Mechanism depiction of the Heck coupling
reaction in the presence of SiO2-g-modified PAAm/Pd
catalyst.
100
100
100
100
98
98
95
95
95
95
2
15
10
2
4
4
4
4
4
3.5
3.5
3.5
2.5
2
Ph
Ph
Ph
1
2
3
4
5
6
7
8
9
10
Run
Conversion (%)
Time (h)
9
Figure 2. Reusability of the SiO2-g-modified PAAm/Pd
catalyst. Reaction condition: Molar ratios of iodobenzene:
n-butyl acrylate: K2CO3: Pd catalyst = 1.0:1.2:2.0:0.005 in
DMF at 85 °C.
a) All reactions were performed using 1.0 Eq. iodoarene, 1.2
Eq. n-butyl acrylate or styrene, 2.0 Eq. K2CO3 and 0.001 Eq.
(when R = CO2Bun) or 0.002 Eq. (when R = Ph) Pd catalyst
in 5 mL DMF at 85 °C for appropriate time. b) Yields refer
to the isolated products based on the aryl iodide used and
the coupling products were characterized by comparing the
The mechanism for the Heck catalytic cycle has been
thoroughly studied. The first step (1) is an oxidative addition in
which Pd(0) inserts into the aryl halide bond. In step (2), the
alkene inserts itself in the Pd-carbon bond in a syn addition
step. Step (3) is a ¢-hydride elimination with the formation of
alkene. The Pd(0) compound is regenerated by the reductive
elimination of the Pd(II) compound in the final reductive
elimination step (4) (Scheme 5).
1
spectroscopic data (FT-IR, H NMR and 13C NMR) with those
of the authentic samples.
O
O
Bun
Bun
S
Pd catalyst
O
O
+
I
or
8 h, 72%, 0.1 mol% Pd cat.
S
DMF/K2CO3/85 o
C
or
S
The Recyclability and Heterogeneity Test of the Catalyst.
Heck coupling reaction of iodobenzene with n-butyl acrylate
was used as a model reaction to inspect the recyclability of the
Pd catalyst. The recovered catalyst was used up to 10 runs
without significant alteration in its efficacy as indicated in
Figure 2. This sustainable recyclability may be the result of Pd
nanoparticles’s protection from aggregation due to the poly-
meric ligand system. Furthermore, in order to estimate the
quantity of Pd leaching, crude reaction mixture was analyzed
by ICP analysis in ten repeating cycles and showed negligible
amount of palladium. Analysis of the reaction mixture after the
first run and likewise after the 10 runs of recycling indicates a
leaching of 0.3% and 4.8% of the palladium respectively,
which justifies the decrease in the yield after run 10. Loading
of fresh catalyst and recycled one demonstrated insignificant
variation. Pd content (mmol g¹1) for fresh catalyst was 0.55
while for the recycled catalyst after 10 run was 0.51. Addi-
10 h, 75%, 0.2 mol% Pd cat.
Scheme 4. Heck coupling of 2-iodothiophene using SiO2-g-
modified PAAm/Pd catalyst.
iodobenzene reacts faster than p-methyl iodobenzene (Entries
7, 8 and Entries 15, 16). This cross-coupling reaction was also
tolerant for substrate with ortho substitution, which is sterically
hindered, and led to good yields (Entries 2, 10). Furthermore,
2-iodothiophene as a heterocyclic aromatic iodide was tolerated
and produces the coupling product with n-butyl acrylate and
styrene (Scheme 4). In comparison between the reactions of
styrene with n-butyl acrylate, the reaction time with styrene
was higher and needed higher Pd loading (Entries 1-8 with
Entries 9-16). Finally, these results revealed that the catalyst
acted efficiently in this transformation.
488 | Bull. Chem. Soc. Jpn. 2017, 90, 485–490 | doi:10.1246/bcsj.20160374
© 2017 The Chemical Society of Japan

100
90
80
70
60
50
40
30
20
10
0
catalyst especially size and chemical oxidation state of the
catalyst remained unchanged. The TEM image of the recycled
catalyst exhibited even distribution of Pd and no agglomeration
even after 10 repeated cycles of the Heck reaction.
no filtration
hotfiltration
Shiraz University Research Council is acknowledged for
partial supporting of the work [grant number 164090]. Valuable
comments and cooperation of Prof. B. Tamami from Shiraz
University is greatly appreciated by the authors.
hot filtration of the catalyst
0
2
4
6
8
10
12
Time (h)
Figure 3. Hot filtration experiment for the reaction of
iodobenzene and with n-butyl acrylate using Pd catalyst.
Supporting Information
Supplementary material (synthetic procedure and character-
ization data of SiO2-g-modified PAAm/Pd catalyst and original
1H and 13C NMR spectra of coupling products) are available on
http://dx.doi.org/10.1246/bcsj.20160374 as PDF file.
(111)
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flask was stirred at 85 °C up to 25% conversion of starting
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this moment (after 15 min), the catalyst was filtered from the
hot solution and the filtrate was further heated under preceding
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reaction (Figure 3).
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state of PdNPs and also shape and morphology of the particles
appear almost unchanged (Figure 4). TEM image specify no
aggregate of PdNPs in the recovered catalyst.
9
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