Novel and efficient polymer supported copper catalyst for heck reaction

Article abstract of DOI:10.1016/j.jorganchem.2020.121354

A novel polymer supported copper complex (m-PAN-Cu) is prepared by immobilizing Cuprous Iodide (CuI) on amidoxime modified Polyacrylonitrile (mPAN) and characterised by FTIR, XRD, EDX, ICP-MS and XPS analyses. This complex was further explored as a general and efficient heterogeneous catalyst for Heck coupling reaction of a series of aryl halides with olefins and afforded the corresponding coupling products in moderate to good yields. This catalyst offers easy preparation, good stability, excellent catalytic activity and reusability. This is the first study involving an amidoxime modified PAN supported copper catalyst towards greener and efficient Heck reaction.

Full text of DOI:10.1016/j.jorganchem.2020.121354

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Novel and efficient polymer supported copper catalyst for heck reaction
Pambingal Rajan Sruthi, Suresh Anjali, Nicy Varghese, Saithalavi Anas
PII:
S0022-328X(20)30256-4
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121354
JOM 121354
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 3 April 2020
Revised Date: 21 May 2020
Accepted Date: 25 May 2020
Please cite this article as: P.R. Sruthi, S. Anjali, N. Varghese, S. Anas, Novel and efficient polymer
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Novel and Efficient Polymer Supported Copper Catalyst for Heck Reaction
Pambingal Rajan Sruthia, Suresh Anjalib, Nicy Varghesea, Saithalavi Anas*a,c
aSchool of Chemical Sciences, Mahatma Gandhi University, Kottayam,Kerala, India-686560
bPost Graduate Department of Chemistry, Sacred Heart College, Thevara, Kerala, India-686101
cAdvanced Molecular Materials Research Centre, Mahatma Gandhi University, Kottayam, Kerala, India-686560
E-mail: anas@mgu.ac.in, anastvr@gmail.com
Tel: +91 481 2731036, Fax +91 481 2731002
Abstract: A novel polymer supported copper complex (m-PAN-Cu) is prepared by immobilising
Cuprous Iodide (CuI) on amidoxime modified Polyacrylonitrile (mPAN) and characterised by
FTIR, XRD, EDX, ICP-MS and XPS analyses. This complex was further explored as a general
and efficient heterogeneous catalyst for Heck coupling reaction of a series of aryl halides with
olefins and afforded the corresponding coupling products in moderate to good yields. This
catalyst offers easy preparation, good stability, excellent catalytic activity and reusability. This is
the first study involving an amidoxime modified PAN supported copper catalyst towards greener
and efficient Heck reaction.
Keywords: Green Chemistry, Heck Reaction, Heterogeneous Catalysis, Copper, Polymer
Support
Mizoroki-Heck reaction is an important carbon-carbon bond forming reaction in organic
synthesis and has been widely adopted as a key step towards the preparation of many complex
organic molecules.1 Classical Heck reaction involves the palladium catalyzed coupling between
an alkene and aryl halide under homogenous reaction conditions.2 The search for novel catalytic
systems involving other transition metals such as Ni,3 Co,4 Rh,5 Ir6 and Cu7 have also been
received much attention in recent years for substituting palladium catalysts. Among which, easily
available copper based catalytic systems are of special importance due to their cheaper price,
functional group tolerance and scalability for large scale procedures. However, only limited
1

reports are available on the copper catalyzed Heck type reaction and are usually performed under
increased catalyst loading, high temperature and longer reaction time. The copper catalyzed
Heck type reaction of aryl and vinyl iodides with olefins was first reported by Iyer and
coworkers in 1997, using a CuI/K2CO3/NMP catalyst system.7a After that, several catalytic
approaches under CuI/DABCO,7b copper bronze/Ionic Liquid,7c microwave7d and ligand free7e
conditions were developed for homogeneous copper catalyzed Heck type coupling reactions.
However, homogeneous catalytic processes usually suffer from difficulties such as poor
recyclability and reusability of the catalyst, possibility of metal contamination in the final
products etc. Thus, heterogenization of homogeneous catalytic processes have emerged as an
alternative to overcome these limitations.8 Naturally, some interesting and useful heterogeneous
palladium based catalytic systems were developed for Heck reaction9 while corresponding
copper catalyzed approaches are rare.10 Therefore the development of mild and efficient
heterogeneous copper catalytic systems for Heck coupling reaction remains as highly demanding
and challenging area of research.
Immobilization of homogeneous catalysts into insoluble polymeric supports by covalent
bonding or complexation/chelation enables the easy recovery and reuse of catalysts. In this
context, the applicability of synthetically modified Polyacrylonitrile (PAN) polymers11 in
heterogeneous catalysis as efficient support for various transition metal catalyzed coupling
reactions has been less investigated.12 Utilization of a preoxidated PAN supported copper
catalyst (Cu/Pre PAN fiber mat) for Heck coupling reaction was described by Shao et al.13
Recently our group has disclosed the amidoxime modified PAN supported Palladium (II)
complexes as efficient heterogeneous catalyst for Heck coupling reaction.14 In light of this report
as well as due to the lack of a general and efficient heterogeneous copper catalyzed
methodology, we have now developed a novel polymer supported copper catalyst for Heck
2

coupling reaction by immobilizing the metal salt on amidoxime modified PAN. The details of
the preparation, characterization of the supported copper complex and its catalytic activity,
stability and reusability studies for Heck type coupling reactions are presented.
Scheme 1. Synthesis of the polymer supported Cu catalyst
The preparation of the supported copper complex (m-PAN-Cu) through a two-step procedure
is illustrated in scheme 1. First step involves the synthetic modification of nitrile group in PAN 1
leading to poly acrylamidoxime 2 on reaction with hydroxyl amine hydrochloride. Next step is
chelation of CuI with 2 in acetonitrile to form the supported copper complex 3 (m-PAN-Cu). The
formation and structure of 3 was characterized using various spectroscopic techniques.
The synthetic modification of PAN and subsequent metal complexation was initially
evidenced from FTIR analyses and further studied by X-ray diffraction (XRD) analyses. Fig 1
gives the FTIR spectra of PAN, m-PAN and m-PAN-Cu. It is found that after modification
reaction, the intense peak due to CN group at 2245 cm-1 (Fig 1a) was disappeared and the
characteristic bands of amidoxime groups corresponding to OH/NH, C=N, C-N, N-O stretching
vibrations were observed at 3100 to 3300, 1646, 1389, and 918 cm1 respectively (Fig 1b). In the
case of m-PAN-Cu, the absorption bands of N-O, C=N vibrations became less intense and
shifted to lower frequency, accompanied with a weakening of OH/NH stretching (Fig 1c),
implying the complexation of copper salt with amidoxime groups. As shown in figure 2a, the
reflection planes of (100) at 2Ө =17.02 and (020) at 2Ө=28.88 were readily recognized from the
XRD pattern of PAN indicating partial crystalline nature. However, the XRD pattern of the m-
PAN exhibited amorphous characteristics after modification (Fig. 2b). But in the case of m-
3

PAN-Cu (Fig. 2c) three diffraction peaks at 2Ө= 25.02, 42.10 and 49.90 are attributed to the
(111), (220) and (311) planes indicating the presence of crystalline Cu(I) phase.
Fig. 1
Fig. 2
Figure 1 and 2: FTIR (left) and XRD (right) Spectra of (a) PAN, (b) m-PAN, (c) m-PAN Cu
Figure 3: EDX spectrum of m-PAN-Cu complex
Energy-Dispersive X-ray spectroscopy (EDX) analysis confirmed the presence of 10.32 wt%
of copper in m-PAN-Cu along with other expected elements like carbon, nitrogen, oxygen, and
iodine (Figure 3). Notably, the EDX results did not reveal any significant contamination from
other metal ions or from unwanted sources. Further, the ICP-MS analysis of the m-PAN-Cu
complex 3 provided the copper content in the complex as 0.0094 g per 1g. Moreover, XPS
studies were carried out to analyze the exact oxidation state of constituent elements and the
spectra is shown in figure 4. The less intense binding energy peaks of copper observed at 935.23
eV and 951.97 eV corresponding to Cu 2p3/2 and 2p1/2 respectively indicated the presence of
copper (I) species. The binding energy values of C1s, N1s, and O1s were detected at 286.88,
4

401.65 and 533.21 eV respectively. These observations in comparison with the literature values13
clearly revealed the presence of Cu(I) species, coordinated to the amidoxime group in the
complex.
Figure 4: (a) XPS data of m-PAN-Cu complex (b) High resolution analysis of Cu2P signal
After detailed characterization of the m-PAN-Cu, next we have investigated the catalytic
efficiency of this complex in promoting the Heck coupling reaction. Initial trail with 1 eq. of
iodo benzene (4a) and 1.2 eq. of methyl acrylate (5a) in presence of 2 mol% of m-PAN-Cu
catalyst, 3 eq. NaHCO3 base and NMP (2 mL) solvent at 140 °C for 24h under nitrogen
atmosphere gave the corresponding coupling product (6a) in 51 % isolated yield (Scheme 2).
I
O
O
m-PAN-Cu (2 mol%)
O
O
NaHCO3, NMP
140 °C, 24h,N2
51 %
5a
6a
4a
Scheme 2. Heck reaction using m-PAN-Cu catalyst
To probe the influence of various reaction parameters, the effect of catalyst loading, bases,
temperature and solvents on promoting the reaction was screened under the reaction condition
and the results are summarized in table 1. Firstly, the amount of catalyst was screened; and when
loading increased to 3 mol%, the yield was increased to 66% (Table 1, entry 1). Further increase
in catalyst loading (5 mol%) did not influenced much, while significant lowering in reaction
5

yield was resulted when catalyst loading was decreased to 1 mol% (Table 1, entry 2). Therefore
3% of catalyst was used for further studies. Other reaction parameters such as base and solvent
were optimized through a series of experiments (Table 1, entry 3-10). A considerable increase in
the reaction yield upto 81% was resulted by using K2CO3 as the base (Table 1, entry 3). Cs2CO3
failed to deliver the product (Table 1, entry 4), while, organic base DABCO gave only 33% yield
(Table 1, Entry 5). Interestingly, use of Et3N as base provided 6a in 78% yield (Table 1, Entry
6). Later, various solvents including N-methylpyrrolidone (NMP), DMF, DMSO, Dioxane and
H2O were explored by using K2CO3 as base (Table 1, Entry 7-10) and none of the solvent gave
better result than NMP.
Table 1: Optimization studies
Entry
Base
Solvent
NMP
NMP
NMP
NMP
NMP
NMP
DMF
DMSO
Dioxane
H2O
Yield (%)b
66(67)c
33d
1
NaHCO3
NaHCO3
K2CO3
Cs2CO3
DABCO
Et3N
2
3
82
4
trace
33
5
6
78
7
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
25
8
32
9
38
10
11
12
13
nd
nde
43f
80g
NMP
NMP
NMP
aReaction conditions: 4a (1 eq.), 5a (1.2 eq.), Base (3 eq.), solvent (2 ml), m-PAN-Cu (3 mol %), N2 atm,
140 °C, 24 h,
e
f
bIsolated yield, c5 mol% catalyst, d1mol% catalyst, without catalyst or Base, at 120 °C, g36h reaction
6

When control experiments were performed in the absence of catalyst or base no product
formation was observed (Table 1, Entry 11). This hinted that the presence of catalyst and base
are essential for promoting this reaction, under the given conditions. Finally, the effect of
temperature and reaction time were investigated. When the reaction was performed at lower
temperature (120 °C), only 43% product formation was observed (Table 1, Entry 12) andlonger
reaction time did not provide much improved results (Table 1, Entry 13).
Based on these initial screening studies an optimized reaction condition was established for
the Heck coupling reaction using m-PAN-Cu catalyst. Under this optimized reaction conditions,
we then examined a series of substrate scope studies to evaluate the generality of this
methodology and the results are summarized in Table 2. It was interesting to observe that the m-
PAN-Cu complex is capable of catalyzing Heck coupling reaction between a number of
substituted aromatic halides and olefins. When bromo benzene 4b and chloro benzene 4c are
treated with methyl acrylate 5a under the optimized reaction condition, product 6a was isolated
only in 36 % and 15% yields respectively (Table 2, entry 2-3). While, Iodo benzene 4a showed
good reactivity with other activated alkenes such as ethylacrylate 5b, tertiary butylacrylate 5c,
acrylic acid 5d, acrylamide 5e and styrene 5f and afforded the corresponding coupling products
in moderate to good yields (Table 2, entry 4-8). Similarly, functionalized aryl iodides containing
both electron withdrawing and electron donating groups underwent efficient coupling reaction
with alkenes. For example, aryl iodides with Nitro, Methoxy and Methyl substitution at para
positions(4d-f) reacted well with methylacrylate 5a, to yield the corresponding products (6g-i) in
moderate to good yields (entry 9,10 and 11). Relatively low reaction yields were observed, when
Iodo anisole 4d and 4-Iodonitrobenzene 4e were subjected to coupling with styrene 5f (entry 12
and 13). While, reaction of 2-iodo toluene 4g with methylacrylate 5a resulted in lower product
7

yield (34%, entry 14). Notably, in all the cases, complete trans selectivity was observed and the
formations of cis or homo coupling products were not observed.
Table 2: Substrate scope studies
Sl.
No
Aryl Halide
R1
Alkene
Yield
(%)a
Product
X
I
R2
1
H
CO2Me
CO2Me
CO2Me
CO2Et
82
36
15
73
64
52
57
45
61
82
46
44
23
34
4a
4b
4c
4a
4a
4a
4a
4a
4d
4e
4f
5a
5a
5a
5b
5c
5d
5e
5f
6a
6a
6a
6b
6c
6d
6e
6f
2
Br
Cl
I
H
3
H
4
H
t
5
I
H
CO2 Bu
6
I
H
CO2H
CONH2
Ph
7
I
H
8
I
H
9
I
4-OMe
4-NO2
4-Me
4-OMe
4-NO2
2-Me
CO2Me
CO2Me
CO2Me
Ph
5a
5a
5a
5f
6g
6h
6i
10
11
12
13
14
I
I
I
4d
4e
4g
6j
I
Ph
5f
6k
6l
I
CO2Me
5a
aIsolated yield
Finally, the recyclability and reusability of the m-PAN-Cu catalyst was tested for coupling
reaction between 4a and 5a under the optimized conditions. After completion of the reaction, the
catalyst 3 was recovered from the reaction mixture by simple filtration and washed with
ethylacetate. Then the recovered catalyst was directly used for the next cycle of reaction, without
any additional treatment. This process was repeated upto 5 cycles and observed that the catalyst
exhibits reasonably good activity even after recycling. These results are tabulated in Table 3.
8

Table 3. Reusability studies using m-PAN-Cu
1
2
3
4
5
Run
%Yield[a]
82 80 76 69 63
[a]Isolated yield
The information about the mechanism of Heck reaction under heterogeneous catalysis is
very limited. Earlier studies have evidenced that the actual active species in supported Heck
catalysis (especially with palladium), are the soluble metal species leached out from the starting
solid material and partially or completely re-precipitated onto the support at the end of the
reaction.12, 15a In a similar manner, a possible mechanism for the Heck reaction using the
supported copper catalyst is presented in Scheme 3. The catalytic cycle begin with the
dissolution of the active metal species Cu(I)Solv A from the support at higher temperature, which
then undergoes oxidative addition with aryl iodide 4a in solution to form intermediate B. This
step follows the migratory insertion of the B into the olefin via π-complexation leading to the
formation of intermediate complex C. The product 6a is formed from this complex C by the
βΗ elimination and the active the copper (I) complex is regenerated from H-Cu-I by the removal
of HI which is quenched by the base.7b, 15b
Scheme 3. Plausible reaction mechanism
9

In conclusion, we have developed a novel, stable and efficient polymer supported copper
catalyst for Heck coupling reaction of a series of substrates. The catalyst can be easily prepared
by chelating Cuprous Iodide (CuI) with amidoxime modified Polyacrylonitrile (PAN). Detailed
characterization of the catalyst was done using FTIR, XRD, EDX and XPS analyses, which
clearly indicated the formation of the polymer copper complex (m-PAN-Cu). This m-PAN-Cu
complex (3) was further explored as an effective heterogeneous catalyst for Heck coupling
reaction. The catalytic activity of 3 has been verified over a series of substrates and the
corresponding coupling products were isolated in moderate to good yields. This studies revealed
that the anchoring of the Copper salt on the modified polymeric support not only exhibits
improved catalytic activity, stability and product selectivity but also enables easy preparation,
recovery and reusability of the catalyst at least for 5 cycles. To the best of our knowledge this is
the first report of using amidoxime modified PAN supported copper catalyst for Heck type
coupling reaction. Therefore, despite of its low reactivity, the developed catalyst offers the
prospect for being an excellent greener alternative to conventional homogeneous catalytic
systems for Heck reaction.
PRS thank the Council of Scientific and Industrial Research (CSIR), New Delhi for Junior
Research Fellowship (Grant No. 09/499(0088)/2015-EMR-1). NV thanks Kerala State Council
for Science, Technology and Environment (KSCSTE) for Student Project Scheme (No.
739/SPS63/2018/KSCSTE). Authors are thankful to St. Thomas College (Pala), SAIF (CUSAT),
IIRBS and School of Environmental Sciences, Mahatma Gandhi University for Spectroscopic
Analyses. Authors also thank DST-PURSE. Govt. of India (SR/417 and 418/2017) for the
financial assistance.
Conflicts of interest
There are no conflicts to declare
10

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12

Highlights
•
•
•
•
A novel Copper complex (m-PAN-Cu) is prepared by immobilizing CuI on amidoxime
modified Polyacrylonitrile (PAN) and characterized.
m-PAN-Cu is explored as an efficient and greener heterogeneous catalyst for Heck
reaction.
First report of Heck reaction using a copper catalyst immobilized on amidoxime
functionalized PAN.
This catalyst offers easy preparation, good stability, excellent catalytic activity and
reusability.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: