Recyclable Pd/CuFe2O4 nanowires: A highly active catalyst for C-C couplings and synthesis of benzofuran derivatives

Article abstract of DOI:10.1039/c8ra03697g

Pd/CuFe₂O₄ nanowire-catalyzed cross coupling transformations are described. Notably, these reactions showed excellent functional group tolerance. Further, the protocol is applied to a one-pot synthesis of benzofurans via a Sonogashira coupling and intramolecular etherification sequence. The catalyst was reused and found to maintain its activity and stability.

Full text of DOI:10.1039/c8ra03697g

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Recyclable Pd/CuFe2O4 nanowires: a highly active
catalyst for C–C couplings and synthesis of
benzofuran derivatives†
Cite this: RSC Adv., 2018, 8, 21030
*
Bhairi Lakshminarayana, Jhonti Chakraborty, G. Satyanarayana
*
and Ch. Subrahmanyam
Received 30th April 2018
Accepted 1st June 2018
Pd/CuFe2O4 nanowire-catalyzed cross coupling transformations are described. Notably, these reactions
showed excellent functional group tolerance. Further, the protocol is applied to a one-pot synthesis of
benzofurans via a Sonogashira coupling and intramolecular etherification sequence. The catalyst was
reused and found to maintain its activity and stability.
DOI: 10.1039/c8ra03697g
rsc.li/rsc-advances
category of the mixed metal ferrites for catalysis in organic
reactions.16,17 In this context, we designed CuFe2O4 support for
Introduction
organic transformation reactions (C–C couplings).
The ability to form various C–C bonds under transition metal
catalysis is important in organic synthesis. Particularly, one-pot
construction of heterocyclic core structures of biological rele-
vance is indispensable.1 In this context, Pd is one of the most
widely used transition metals for a variety of coupling trans-
formations. When compared with homogeneous counterparts,
heterogeneous catalysts enable the formation of less contami-
nated products and promote green chemistry.1–3 In the recent
past, heterogeneous catalysis, for the formation of C–C, C–O,
C–N and C–S bonds, became the central part of synthetic
chemistry. Amongst them all, carbon–carbon (C–C) bond
forming reactions for the preparation of carbon scaffolds are of
high importance from the organic synthetic viewpoint.
Supported nanocatalysts have become an integral part of
heterogeneous catalysis for various organic transformations
and industrial applications.4–7 Especially, metal oxide supported
heterogeneous nanocatalysts are preferred because of their
activity, better selectivity, and high stability over conventional
metal based catalyst frameworks.8 Unsupported nanocatalysts
face problems due to issues related recovery and recyclability
from the reaction mixture.9 This problem can be resolved by
immobilizing the active species on a support having high
surface area materials such as nano-metal oxides,10 carbon
nano-materials,11,12 polymers13 etc. Among all the supporters,
nano-metal oxide supported nanocatalysts are easily separable
from the reaction mixture.14,15 Especially, the substitution of the
copper ion into the ferrite lattice constitutes a well-developed
The biaryls and diarylacetylenes are useful scaffolds that
constitute pharmaceuticals, agrochemicals and biologically
active compounds.17,18 The Suzuki–Miyaura cross-coupling
reactions of aryl halides with aryl boronic acids and Sonoga-
shira coupling reactions of aryl halides with arylacetylenes are
considered to be important carbon–carbon bond forming
strategies, for constructing biaryls and internal acetylenes,
respectively.19–21 However, most of these reactions made use of
toxic ligands in conjunction with Pd-salts or Pd nanoparticles
and Pd/Cu nanoparticles.22 Benzofuran derivatives are an
important class of heterocyclic compounds, due to their
miscellaneous biological prole, such as analgesia, antitumor,
antimicrobial, kinase inhibitor and antihyperglycemic activi-
ties.23–28 Recently, the research group of Mariusz Jan Bosiak
showed the utility of benzofurans in photovoltaic and opto-
electronic properties, wherein benzofurans absorb and emit the
light.29–32 While, one-pot synthesis of benzofurans, under
homogeneous catalysis, is well established. Recently, the
synthetic community turned their interest to develop new
methods for the synthesis of benzofurans by means of hetero-
geneous catalytic conversion.33–39 Very recently, there have been
various reports described that the catalytic activity of Pd nano-
particles can be retained, the stability of the Pd nanoparticles
also enhanced to some extent by using magnetic nanomaterials
as a supporter for immobilization of Pd nanoparticles.40–46
Recently, we have developed Pd/CuFe2O4 nanowires and
showed its synthetic utility toward Heck couplings.47 Herein, we
present the applicability of catalyst for various C–C bond
forming reactions (Suzuki, Sonogashira). Further, this catalyst
applied for the one-pot synthesis of benzofurans via intermo-
lecular Sonogashira followed by intramolecular cyclization.
Initially, a comparative study of as prepared catalyst with other
recently reported Pd supported catalysts for Suzuki coupling
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi,
Sangareddy, 502285, Telangana, India. E-mail: gvsatya@iith.ac.in; csubbu@iith.ac.
in; Tel: +91(40) 23016054
† Electronic supplementary information (ESI) available: Details of catalyst
synthesis, characterization and experimental studies. 1H, 13C-NMR spectra of all
isolated products. See DOI: 10.1039/c8ra03697g
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Table 1 Comparison of the catalytic activity of Pd/CuFe2O4 nanowires with other reported recent catalysts for Suzuki couplings
Entry
Catalyst
Conditions
Yield (%)
Ref.
1
2
3
4
5
6
7
8
PdO/GO
K2CO3, DMSO, 120 ꢀC, 10 min
88
96
95
95
96
93
93
94
95
94
98
48
49
50
51
52
53
54
55
Pd-SBT@MCM
GO/Fe2O4/Pd composite
Pd@CQD@Fe3O4
Pd/C@Fe2O4
K2CO3, PEG, 80 ꢀC, 175 min
K2CO3, H2O/EtOH, 80 ꢀC, 20 min
t-BuOK, EtOH : H2O (1 : 1), 120 ꢀC, 60 min
K2CO3, H2O, 100 ꢀC, 30 min
Pd@PVP
K2PO4, H2O/EtOH (1 : 3), 80 ꢀC, 3–18 h
CaO, H2O/EtOH (1 : 1), 85 ꢀC, 20 min
K2CO3, TBAB (10%), dioxane : H2O (1 : 1), 90 ꢀC, 4 h
Na2CO3, NMP : H2O, 120 ꢀC, 4 h
Fe3O4@SiO2–Pd
Pd/Celite-PANI
Pd/TiO2
HMMS-salpr-Pd
Pd/CuFe2O4 nanowires
9
10
11
56
57
this work
K2CO3, H2O/EtOH (1 : 1), 70 ꢀC, 60 min
K2CO3, DMSO, 120 ꢀC, 10 min
Table 2 Optimization studies for the formation of biphenyl 3haa
mole%), K2CO3 (2 equiv.) and in DMSO (1 mL) at 120 ꢀC for 2 h.
Gratifyingly, the reaction was quite successful and furnished
the biaryl product 3ha, in near quantitative yield (Table 2, entry
1). On the other hand, other solvents, such as DMF, DMA,
toluene, 1,4-dioxane, THF and acetonitrile were inferior (Table
2, entries 2 to 7). While the reaction gave fair, moderate and
poor yields of the product 3ha with other bases, such as Cs2CO3,
Entry
Base
Solvent
Yieldb 3aa(%) K3PO4, and NaOH respectively (Table 2, entries 8 to 10).
Further to optimize the mol% of the Pd/CuFe2O4 catalyst, for
the formation of 3ha, the reaction was performed between 4-
methoxy iodobenzene 1h and phenylboronic acid 2a with 0, 1, 2,
4, 6, 8, and 10 mol% of Pd in Pd/CuFe2O4, under standard
conditions. Among all, it was observed that the reaction yields
were more or less same with $4 mol% of Pd in Pd/CuFe2O4
(Fig. 1). Thus it was concluded that the 4 mol% of Pd was an
optimal load to drive the reaction.
1
2
3
4
5
6
7
8
9
10
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
K3PO4
NaOH
DMSO
DMF
DMA
Toluene
1,4-Dioxane
THF
Acetonitrile
DMSO
98
95
67
52
76
72
63
67
40
31
DMSO
DMSO
With these best conditions in hand, the scope was extended
for the Suzuki–Miyaura coupling with various iodoarenes 1a–l
and arylboronic acids 2a–d. Gratifyingly, the reaction was found
to be amenable and afforded the corresponding biaryls 3aa–lb,
in fair to near quantitative yields (Table 3). Interestingly, the
reaction was successful with simple iodobenzene 1a and
amenable with abroad range of functional groups (COOMe,
NO2, OMe, F & Cl) on aromatic ring 1b–j, which reveals a wide
functional group tolerance of this reaction. Notably, hetero-
aromatic iodide 1l, gave the products 3lb, in excellent yield.
Signicantly, protecting group free iodopyridine 1k was also
tolerable and afforded the products 3ka–kc (Table 3). Interest-
ingly, the reaction was also amenable to simple arylboronic
acids 2 bearing broad range of functional moieties on the
aromatic ring [i.e. OMe, F and Cl, (Table 3)]. To demonstrate the
utility of the strategy, the Suzuki coupling reaction was explored
with bromoarenes 1s–u with simple boronic acid 2a, under
established conditions. Gratifyingly, biphenyls 3aa, 3da and 3ha
were isolated in 82%, 70% and 68% yields (Table 3), respec-
tively. Which ascertains the usefulness of Pd(0)/CuFe2O4
a
Reaction conditions: aryl iodides 1h (0.25 mmol), arylboronic acid 2a
(0.5 mmol), Pd/CuFe2O4 (4 mol%), base (0.5 mmol) and solvent (1 mL) at
120 ꢀC. b Isolated yields of product 3ha.
reactions was shown in Table 1. As shown in Table 1, most of
the Pd supported nanocatalysts produces the yields 88–96%
(Table 1, entries 1–10). Whereas, with our catalyst Pd/CuFe2O4
nanowires we got near quantitative yields (98%) (Table 2, entry
11).
Results & discussion
To begin with, the optimization of Suzuki–Miyaura coupling
was planned. For this purpose, 4-methoxy iodobenzene 1h and
phenylboronic acid 2a were chosen as the model substrates.
Recently, we reported that Pd/CuFe2O4 nanowires showed the
best catalytic activity for Heck couplings.47 Thus, the reaction
was carried out between 4-methoxy iodobenzene 1h and phe-
nylboronic acid 2a in the presence of Pd/CuFe2O4 nanowires (4
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Fig. 1 Optimization of loading the catalyst for Suzuki-coupling reaction.
nanowires catalyst. Whereas the coupling with chlorobenzene reaction with base gives the intermediate B, which on coupling
1v and meta-chloronitrobenzene 1w as reacting partners with with the activated boronic acid derivative C (produced by reac-
the boronic acid 2a, furnished the desired biaryls 3aa and 3da in tion of the boronic acid 2a with K2CO3 base), gives trans-
poor yields (25% & 27% respectively, Table 3). It is noteworthy to metalation complex D. Finally, the reductive elimination of D
mention that in case of 1v and 1w, the homo coupling of affords the coupled product 3ha restores the active Pd(0)/
boronic acid 2a was also noticed. In addition, the chloroarenes CuFe2O4 nanowires, thus completes the catalytic cycle.
1v and 1w were not completely consumed even aer 24 h.
Aer successful synthesis of biaryls using Suzuki–Miyaura
The recovery of the catalyst was done by centrifugation and coupling, in order to check the efficiency of the Pd/CuFe2O4
washing with ethyl acetate and acetone followed by drying in nanowires catalyst, we aimed at Sonogashira coupling between
a hot air oven at 60 ꢀC for 12 h. The recovered Pd/CuFe2O4 iodoarenes and arylacetylenes. Therefore, initially, the reaction
nanowires catalyst was then subjected to the next catalytic was explored between 2-amino iodobenzene 1m and phenyl-
cycles. It is worth mentioning that the catalyst retains its acetylene 4a, under established conditions as above (see; Table
activity, which is evident with nearly no loss of activity even aer 2, entry 1). The reaction was quite successful and furnished the
h reaction cycle (Fig. 2). The marginal loss of activity aer the 2-amino diphenylacetylene product 5ma, in good yield (Table 4,
h cycle (<3%) may be due to loss of some amount of the entry 1). On the other hand, the reaction was found to be infe-
catalyst during the recovery of Pd/CuFe2O4 nanowires. The rior with the other solvents, such as DMF, DMA, toluene, 1,4-
catalyst was recycled ve times without an appreciable change dioxane, THF and acetonitrile (Table 4, entries 2 to 7). While the
in the product 3ha yield, under the established conditions at reaction with bases Cs2CO3 and KOH gave moderate yields of
120 ꢀC. Thus, based on the above results it is conrmed that Pd/ the product 5ma (Table 4, entries 8 and 9).
CuFe2O4 nanowires catalyst is stable enough and can be reused.
With the above optimized conditions, for Sonogashira
coupling, to demonstrate the utility of the strategy, the activity
of Pd/CuFe2O4 catalyst was assessed for the reaction between
iodoarenes 1a–q and phenylacetylene 4a. The results of the
catalytic reactions are as depicted in Table 5, which showed
broad substrate scope and delivered the corresponding internal
acetylenes 5aa–qa, in fair to very good yields. For example,
iodoarenes bearing electron-withdrawing groups (meta-COOMe,
para-COOMe, meta-NO2) were found to be smooth and afforded
the products (5ba, 5ca & 5da) moderate yields (Table 5). In
addition, the reaction was quite successful with electron
releasing groups as well (5fa, 5ga & 5ha, Table 5). Notably, the
reaction was tolerable to protecting group free NH2 moiety on
pyridine ring (5ka, Table 5). To our delight, 2-iodobenzylalcohol
The relation between catalytic
properties and structure of Pd/
CuFe2O4 nanowires
The mechanism for the relation between catalytic properties
(Suzuki coupling reaction) and structure of Pd/CuFe2O4 nano-
wires is shown in Scheme 1. The rst step would be the oxidative
addition of Pd(0)/CuFe2O4 nanowires to the 4-methoxy iodo-
benzene 1h to form Pd(II) species A. In most cases the oxidative
addition is the rate determining step of a catalytic cycle. During
this step, the Pd is oxidized from Pd(0) to Pd(II). Further, the
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Table 3 Synthesis of biphenyls with aryl iodide and arylboronic acida,b
a
Reaction conditions: aryl iodides (0.25 mmol), arylboronic acid (0.5 mmol), Pd/CuFe2O4 (4 mol%), K2CO3 (0.5 mmol) and DMSO (1 mL) at 120 ꢀC.
b
Isolated yields of chromatographically pure products 3aa–lb. c Isolated yields of products when bromoarenes 1s–u were used. d Isolated yields of
products when chloroarenes 1v–w were used.
Fig. 2 Recyclability of the catalyst Pd/CuFe2O4 nanowires in Suzuki reaction.
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Scheme 1 The mechanism for the relation between catalytic properties (Suzuki coupling reaction) and structure of Pd/CuFe2O4 nanowires.
2n coupled with phenylacetylene 4a and gave the product 5na, and selective and gave the product 5jb, in which relatively more
in very good yield (Table 5). Further, we have explored Sono- reactive iodo substituent was selectively involved in the
gashira coupling between bromobenzene 1s and phenyl- coupling reaction. It is worth mentioning that the reaction with
acetylene 4a. Notably, the reaction was smooth and yielded ethyl propiolate as coupling partner, did not lead to any product
diphenylacetylene 5aa, in yields 68%. Even the reaction was also and only starting materials were recovered. This may be due to
successful meta-bromonitrobenzene 1t and para-methoxy- the fact that less nucleophilicity of acetylenic b-carbon of ethyl
bromobenzene 1u with 4a. While, the reaction is somewhat propiolate.
sluggish with chloroarenes. Therefore, the present catalyst
Benzofurans are ubiquitous oxygen-containing heterocyclic
seems to be active enough and could promote the reactions with motifs that constitute many natural products, pharmaceuticals,
bromoarenes as well. To further check the selectivity of the biologically important compounds and organic materials. In
process, it was planned to test the substituent susceptibility. this context, many synthetic strategies have been established for
Thus, the reaction was conducted with 1-chloro-4-iodobenzene their synthesis. Notably, transition metal-catalyzed (Pd,58–66
1j in the presence of phenylacetylene 4a, in very good yields Rh,67 Ru,68–71 Ir,72 Au73 and Cu74–76) annulations proved to be
(Table 5). To our delight, the reaction was found to be smooth powerful strategies for the synthesis of benzofurans. Quite
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Table 4 Optimization studies for the formation of 2-(phenyl ethynyl) Table 6 Synthesis of benzofurans 7ra–ri from 2-iodophenols 1n–r
aniline 5maa
and aryl alkynes 4a–ia,b
Entry
Base
Solvent
Yield (5ma)b
Time (h)
1
2
3
4
5
6
7
8
9
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
KOH
DMSO
DMF
DMA
Toluene
1,4-Dioxane
THF
CH3CN
DMSO
DMSO
75
60
35
12
12
24
48
24
24
24
12
12
c
—
36
30
32
55
48
a
Reaction conditions: aryl iodides 1m (0.5 mmol), phenylacetylene 4a (1
mmol), Pd/CuFe2O4 (4 mol%), base (1 mmol) and solvent (1 mL) at
c
120 ꢀC. b Isolated yields of product 5ma. No progress was observed.
interestingly, when 2-iodophenols 1n–r were treated with
terminal acetylenes 4a–i, benzofurans were obtained as the
products (Table 6). For example, the reaction was amenable to
different arylacetylenes (4a, 4c & 4d) and furnished the corre-
sponding benzofurans 7ra–rd (Table 6). Notably, the reaction
was also successful with terminal alkyl acetylenes 4e–h and
a
Reaction conditions: 2-iodophenols 1n–r (0.5 mmol), aryl alkynes 4a–i
(0.5 mmol), Pd/CuFe2O4 nanowires (4 mol%), K2CO3 (1 mmol) and
DMSO (1 mL) at 120 ꢀC. b Isolated yields of product 7na–ri.
Table 5 The Pd/CuFe2O4 nanowires catalyzed Sonogashira coupling reaction between iodoarenes 1a–q and phenylacetylene 4aa,b
a
Reaction conditions: aryl iodides 1a–q (0.5 mmol), phenylacetylene 4a (1 mmol), Pd/CuFe2O4 (4 mol%), K2CO3 (1 mmol) and DMSO (1 mL) at
b
c
d
120 ꢀC. Isolated yields of product 5aa–5qa. Isolated yields of products when bromoarenes 1s–u were used. Isolated yields of products when
chloroarenes 1v–w were used.
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Table 7 Attempting for Sonogashira reaction between iodoarenes 1m-n and ethyl propiolate 4ba,b
a
Reaction conditions: aryl iodides 1m & 1n (0.5 mmol), ethyl propiolate 4b (1 mmol), Pd/CuFe2O4 (4 mol%), K2CO3 (1 mmol) and DMSO (1 mL) at
120 ꢀC. b Isolated yields of products 6.
Table 8 Screening conditions for the formation of 6nb+6nb0 from iodophenol 1n and ethyl propiolate 4ba,b
Yield (%)
Entry
Catalyst (Pd/CuFe2O4)
Base (K2CO3)
Solvent (DMSO)
6nb (E)
6nb0 (Z)
1
2
3
4
5
6
7
4 mol%
—
—
4 mol%
—
4 mol%
—
2 eq.
2 eq.
2 eq.
2 eq.
—
1 mL
1 mL
—
57
32
35
53
—
—
—
39
16
18
35
—
—
—
—
1 mL
1 mL
—
—
—
a
Reaction conditions: aryl iodides 1n (0.5 mmol), ethyl propiolate 4b (1 mmol), Pd/CuFe2O4 (4 mol%), K2CO3 (1 mmol) and DMSO (1 mL) at 120 ꢀC.
Isolated yields of products 6.
b
afforded the benzofurans 7re–rh, in very good to excellent yields
To understand the nature of the reaction and whether or not
(Table 6). Signicantly, the reaction was tolerable to the alde- the palladium is necessary to drive this Michael addition reac-
hyde functionality on the aromatic ring of 2-iodophenol deriv- tion, the reaction was performed under varying conditions, as
ative 1r. Signicantly, 3-hydroxy-2-iodo-4-methoxybenzaldehyde illustrated in Table 8. The reaction under standard conditions,
1r coupled with 1,3-diethynylbenzene 4i, in which only one furnished the product 6nb+6nb0 (57 + 39) (Table 8, entry 1). The
acetylene group was reacted and gave 7ri as an exclusive product reaction was also successful without Pd-catalyst, but with the
(Table 6).
base and solvent, albeit there was a drop in the yields [6nb+6nb0
Moreover, to check the scope and generality of Sonogashira (32 + 16), Table 8, entry 2]. In addition, the reaction was smooth,
coupling, 2-iodophenol was treated with ethyl propiolate 4b. under neat conditions with base and without catalyst [6nb+6nb0
Surprisingly, no Sonogashira product was formed; instead, (35 + 18), Table 8, entry 3]. While the yield of the product was
Michael addition product 6nb was obtained as major diaste- good with both catalyst and base and without solvent [6nb+6nb0
reomer via nucleophilic attack of the hydroxyl group of phenol (53+35), Table 8, entry 4]. Only with solvent and without base
(Table 7). This sort of nucleophilic 1,4-addition of phenolic and catalyst, no progress was noted (Table 8, entry 5). Similarly,
hydroxyl across the activated triple/double bonds is already with the catalyst and solvent, without base, no progress was
established in the literature.77–79 Whereas, when 2-iodoaniline seen (Table 8, entry 6). The reaction did not proceed without the
was reacted with ethyl propiolate 4b, we could not observe any catalyst, base and solvent (Table 8, entry 7).
required product. The reaction was inconclusive from the TLC.
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Table 9 Attempting for Sonogashira reaction between iodoarenes and ethyl propiolate 4ba,b
a
Reaction conditions: aryl iodides 1n–q (0.5 mmol), ethyl propiolate 4b (1 mmol), Pd/CuFe2O4 (4 mol%), K2CO3 (1 mmol) and DMSO (1 mL) at
120 ꢀC. b Isolated yields of products 6. c Only single E-isomer 6qb was formed.
To further conrm this sort of reactivity, the reaction was
explored with different halophenols (1n, 1o, 1p & 1q) with ethyl
Acknowledgements
propiolate 4b, under standard reaction conditions. As anticipated,
furnished the corresponding arylvinyl ethers as Z and E diaste-
reomeric mixture, as minor and major isomers, respectively (Table
9). Surprisingly, when 2-iodobenzylalcohol 1q was used as the
nucleophile, exclusively, gave the E isomer in 72% yield (Table 9).
B. L. Narayana would like to thanks to University Grant
Commission (UGC), New Delhi, for awarding Junior & Senior
Research Fellowship (JRF & SRF).
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