Organic transformations catalyzed by palladium nanoparticles on carbon nanomaterials

Article abstract of DOI:10.1007/s12039-018-1449-9

Abstract: An efficient C–C bond coupling reactions (Suzuki–Miyaura and Glaser) catalyzed by PdO/GO nano-catalyst is presented. In addition, PdO/MWCNT nano-catalyst-mediated domino one-pot synthesis of 2-alkyl/2-aryl benzofurans has been accomplished from 2-iodophenols and terminal alkynes. The formation of benzofurans proceeds through intermolecular Sonogashira reaction followed by intramolecular nucleophilic addition of internal hydroxyl group onto the acetylenic bond. The catalyst PdO/GO has been reused successfully, with nearly no loss of activity up to 5 cycles. Graphical Abstract: : Synopsis: An efficient PdO/GO and PdO/MWCNT nanocatalysts have been developed for C–C coupling reactions like Suzuki and Glaser coupling reactions, multi-catalytic one-pot synthesis of 2-aryl, 2-alkyl benzofurans staring from 2-iodophenols and terminal alkynes. The protocol involves formation of benzofuran derivatives, i.e., Sonogashira reaction followed by 2-ethynyl phenol cyclization. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-018-1449-9

J. Chem. Sci. (2018) 130:47
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1449-9
REGULAR ARTICLE
Organic transformations catalyzed by palladium nanoparticles on
carbon nanomaterials
BHAIRI LAKSHMINARAYANA, LODI MAHENDAR, JHONTI CHAKRABORTY,
GEDU SATYANARAYANA∗ and CH SUBRAHMANYAM∗
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Medak District, Sangareddy,
Telangana 502 285, India
E-mail: gvsatya@iith.ac.in; csubbu@iith.ac.in
MS received 8 August 2017; revised 16 March 2018; accepted 16 March 2018
Abstract. An efficient C–C bond coupling reactions (Suzuki–Miyaura and Glaser) catalyzed by PdO/GO
nano-catalyst is presented. In addition, PdO/MWCNT nano-catalyst-mediated domino one-pot synthesis of
2-alkyl/2-aryl benzofurans has been accomplished from 2-iodophenols and terminal alkynes. The formation
of benzofurans proceeds through intermolecular Sonogashira reaction followed by intramolecular nucleophilic
addition of internal hydroxyl group onto the acetylenic bond. The catalyst PdO/GO has been reused successfully,
with nearly no loss of activity up to 5 cycles.
Keywords. Graphene oxide (GO); multi-walled carbon nanotubes (MWCNT); C–C coupling reaction;
benzofuran derivatives.
1. Introduction
Until now, homogenous [Pd]-catalysis has been
broadly explored for organic coupling transformations.
In modern organic synthesis, the development of envi-
ronmentally benign organic transformations is highly
desirable.14–17 Recently, we have reported the syntheses
and characterization of PdO nanoparticles impregnated
on various nano-carbon supports (SWCNT, MWCNT,
CNF, GO and RGO).18,19 Also, the efficiency of these
ligand-free catalysts has been demonstrated with the
Heck reaction between iodoarenes and olefins. Among
them, PdO/GO catalyst was found to be the best, due
to a high degree of surface-bound oxygenated moieties.
The combined effects of high surface area, mesoporous
From a green chemistry point of view, heterogeneous
catalysis holds paramount advantages over its homoge-
neous counterparts such as, easy separation, reusability
and stability of the catalyst. However, dispersion of the
chosen catalytic material on a suitable support is still
a challenge to the scientific community. This problem
is even more complex in nanomaterial catalysts, due
to aggregation problems. In this context, hybrid car-
bon materials such as graphene oxide (GO), reduced
graphene oxide (RGO), multi-walled carbon nanotubes
(MWCNT), single-walled carbon nanotubes (SWCNT),
and carbon nanofiber (CNF) have been tested as sup-
ports.1–4 Palladium is one of the most widely used
metals for organic transformations. However, unsup-
ported [Pd]-nanoparticles agglomerate or sinter upon
heating.5–8 Carbon-supported [Pd]-based nanoparticles
proved to be highly active catalytic systems and fur-
nished the coupling products in high yields with good
selectivity. These advantageous features are attributed
to the electronic, mechanical and thermal properties of
the carbon support.9–13
π
nature, -interactions, optimum electron conductivity
of GO favored the activity of PdO/GO nanohybrids,
for Heck cross-coupling reactions. Encouraged by these
initial results, we are reporting the extension of the
scope of this versatile system for other organic coupling
transformations. This communication deals with the
applications of ligand-free PdO/GO, for C-C bond form-
ing Suzuki–Miyaura and Glaser coupling reactions,
respectively. In addition, ligand-free heterogeneous
PdO/MWCNT-catalyzed domino one-pot synthesis of
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1449-9) contains
supplementary material, which is available to authorized users.

47 Page 2 of 8
J. Chem. Sci. (2018) 130:47
2-arylbenzofurans through intermolecular Sonogashira 2.3 General procedure 2 (for the synthesis of
coupling followed intermolecular nucleophilic addition symmetrical 1,3-diynes 5)
to the acetylenic bond, is also described.
In an oven-dried Schlenk tube terminal alkynes 4 (0.5 mmol),
Pd/GO nano catalyst (5 mol%), K2CO3 (1 mmol) and solvent
(DMSO) (1.0 mL) were added. The resulting reaction mix-
ture was stirred at 120 ◦C for 2 h. The progress of the reaction
was monitored by TLC. After completion of reaction, the
2. Experimental
reaction mixture was allowed to cool to room temperature,
2.1 General considerations
then diluted with (∼ 10 mL) ethyl acetate and NH4Cl solu-
1
tion (∼ 10 mL) was added fallowed by extraction with ethyl
IR spectra were recorded on a FTIR spectrophotometer. H
acetate. The organic layers were dried with Na2SO4 and con-
centrated in reduced vacuum. Purification of the residue by
silica gel column chromatography using distilled petroleum
ether/ethyl acetate as the eluent furnished the cinnamates and
stilbenes.
NMR spectra were recorded on 400 MHz spectrometer at
295 K in CDCl ; chemical shifts ( ppm) and coupling
δ
3
constants (Hz) are reported in standard fashion with ref-
erence to either internal standard tetramethylsilane (TMS)
(
= 0.00ppm) or CHCl (
= 7.25 ppm). 13 C NMR
δ
H
δ
H
3
spectra were recorded on 100 MHz spectrometer at RT in
CDCl ; chemical shifts ( ppm) are reported relative to CHCl
δ
3
[
=377.00ppm(central line of triplet)]. In the 13C NMR, the
δ
C
2.4 General procedure 3 (for the synthesis of
benzofurans 7)
nature of carbons (C, CH, CH2, and CH3) was determined by
recording the DEPT-135 spectra and is given in parentheses
and noted as s=singlet (for C), d = doublet (for CH), t=triplet
In an oven-dried Schlenk tube 2-iodophenol 6 (0.5 mmol), ter-
minal alkyne 4 (0.5 mmol), PdO/MWCNT (5 mol%), K2CO3
(1 mmol) and solvent (DMSO) (1.0 mL) were added. The
resulting reaction mixture was stirred at 120 ◦C for 24 h. The
progress of the reaction was monitored by TLC. After com-
pletion of reaction, the reaction mixture was allowed to cool
to room temperature, then diluted with (10 mL) ethyl acetate
and NH4Cl (10 mL) was added fallowed by extraction with
ethyl acetate. The organic layers were dried (Na2SO4) and
concentrated in vacuum. Purification of the residue by sil-
ica gel column chromatography using petroleum ether/ethyl
acetate as the eluent furnished the benzofuran 7.
1
(for CH2) and q = quartet (for CH3). In the H NMR, the
following abbreviations were used throughout: s = singlet,
d = doublet, t = triplet, q = quartet, qui = quintet,
m = multiplet and br s. = broad singlet. The assignment
1
of signals was confirmed by H, 13C CPD (carbon proton
decoupled), and DEPT spectra. High-resolution mass spec-
tra (HR-MS) were recorded using Q-TOF multimode source.
Melting points were determined on an electrothermal melt-
ing point apparatus and are uncorrected. Iodophenols, PdO on
MWCNT, terminal alkynes and K2CO3 were commercially
available (local made) used without further purification.
Reactions were monitored by TLC on silica gel using a
combination of petroleum ether and ethyl acetate as elu-
ents. Solvents were distilled prio◦r to use; petroleum ether
with a boiling range of 40 to 60 C was used. Acme’s sil-
ica gel (60–120 mesh) was used for column chromatography
(approximately 20 g per one gram of crude material).
2.5 Characterization data
2.2 General procedure 1 (for the synthesis of
biphenyls 3)
In an oven-dried Schlenk tube iodoarenes 1 (0.25 mmol), aryl-
This compound was prepared according to the GP and iso-
boroic acids 2 (0.5 mmol), PdO/GO nano catalyst (5 mol%), lated as colorless liquid 94% yield (105 mg). [TLC (petroleum
K2CO3 (0.5 mmol) and solvent (DMSO) (1.0 mL) were ether/ethyl acetate 9:1, Rf(6a) = 0.50, Rf(4 h) = 0.60, UV
◦
added. The resulting reaction mixture was stirred at 120 C detection]. 1H NMR (CDCl3, 400 MHz): δ = 7.59 (d, J =
for 5–30 min. The progress of the reaction was monitored 7.3 Hz, 1H), 7.53 (d, J = 7.3 Hz, 1H), 7.46 (d, J = 8.8 Hz,
by TLC. After completion of reaction, the reaction mixture 1H), 7.42 (s, 1H), 7.36 (dd, J = 7.8 and 7.8 Hz, 1H), 7.29
was allowed to cool to room temperature, then diluted with (dd, J = 8.3 and 7.3 Hz, 1H), 7.23 (d, J = 7.3 Hz, 1H), 7.03
(approximately 10 mL) ethyl acetate and NH4Cl solution (s, 1H), 6.91 (dd, J = 7.3 and 2.0 Hz, 1H), 3.89 (s, 3H) ppm.
(approximately10mL)wasaddedfollowedbyextractionwith 13C NMR (CDCl3, 100 MHz): δ = 159.9 (Cq), 155.7 (Cq),
ethyl acetate. The organic layers were dried with Na2SO4 and 154.8 (Cq), 131.7 (Cq), 129.8 (CH), 129.1 (Cq), 124.3 (CH),
concentratedinreducedvacuum. Purificationoftheresidueby 122.9 (CH), 120.9 (CH), 117.5 (CH), 114.4 (CH), 111.2 (CH),
silica gel column chromatography using distilled petroleum 110.1 (CH), 101.6 (CH), 55.3 (CH3) ppm. IR (MIR-ATR,
ether/ethyl acetate as the eluent furnished the biphenyls.
4000−600 cm−1): νmax = 2921, 2851, 1482, 1460, 1380,

J. Chem. Sci. (2018) 130:47
Page 3 of 8 47
1123, 1109, 1011, 928 cm−1. HR-MS (ESI+) m/z calculated
for [C15H12O2]+ = [M]+: 224.0832; found: 224.0840.
This compound was prepared according to the GP and iso-
lated as brown color viscous liquid 89% yield (116 mg).
[TLC (petroleum ether/ethyl acetate 8:2, Rf(6b) = 0.50,
Rf(4i) = 0.60, UV detection]. 1H NMR (CDCl3, 400 MHz):
δ = 10.01 (s, 1H), 7.62 (d, J = 8.3 Hz, 1H), 7.14 (s, 1H), 6.82
(d, J = 8.3 Hz, 1H), 4.08 (s, 3H), 2.81 (t, J = 7.3 Hz, 2H),
1.85–1.70 (m, 2H), 1.45–1.15 (m, 6H), 0.87 (t, J = 7.3 Hz,
3H) ppm. 13C NMR (CDCl3, 100 MHz): δ = 190.8 (Cq),
163.4 (Cq), 149.4 (Cq), 143.6 (Cq), 131.3 (CH), 129.8 (Cq),
122.6 (Cq), 105.0 (CH), 102.6 (CH), 56.3 (CH3), 31.5 (CH2),
28.9 (CH2), 28.5 (CH2), 27.6 (CH2), 22.5 (CH2), 14.0 (CH3)
ppm. IR (MIR-ATR, 4000–600 cm−1): νmax = 2930, 2843,
1560, 1481, 1456, 1375, 1187, 1160, 1009, 929 cm−1. HR-
MS(ESI+)m/zcalculatedfor [C16H20NaO3]+ = [M+Na]+:
283.1305; found: 283.1308.
This compound was prepared according to the GP and iso-
lated as pale yellow color viscous liquid 90% yield (127 mg).
[TLC (petroleum ether/ethyl acetate 8:2, Rf(6b) = 0.50,
Rf(4 h) = 0.60, UV detection]. 1H NMR (CDCl3, 400 MHz):
δ = 10.03 (s, 1H), 7.78 (s, 1H),7.65 (d, J = 8.3 Hz, 1H), 7.53
(d, J = 7.8 Hz, 1H), 7.44 (dd, J = 2.4 and 2.0 Hz, 1H), 7.36
(dd, J = 8.3 and 7.8 Hz, 1H), 6.93 (dd, J = 8.8 and 3.2 Hz,
1H), 6.88 (d, J = 8.3 Hz, 1H), 4.11 (s, 3H), 3.88 (s, 3H) ppm.
13C NMR (CDCl3, 100 MHz): δ = 190.8 (Cq), 159.9 (Cq),
158.8 (Cq), 149.8 (Cq), 144.0 (Cq), 132.1 (CH), 130.9 (Cq),
129.9 (CH), 129.8 (Cq), 122.9 (Cq), 118.0 (CH), 115.5 (CH),
110.3 (CH), 106.1 (CH), 102.2 (CH), 56.4 (CH3), 55.4 (CH3)
ppm. IR (MIR-ATR, 4000–600 cm−1): νmax = 2922, 2850,
1478, 1380, 1201, 1165, 1011, 920 cm−1. HR-MS (ESI+) m/z
calculated for [C17H14NaO4]+ = [M + Na]+: 305.0784;
found: 305.0794.
This compound was prepared according to the GP and iso-
lated as brown color viscous liquid 86% yield (118 mg):
[TLC (petroleum ether/ethyl acetate 9:1, Rf(6b) = 0.40,
Rf(4j) = 0.70, UV detection]. 1H NMR (CDCl3, 400 MHz):
δ = 10.02 (s, 1H), 7.63 (d, J = 8.3 Hz, 1H), 7.15 (s,
1H), 6.84 (d, J = 8.3 Hz, 1H), 4.09 (s, 3H), 2.83 (t,
J = 7.8 Hz, 2H), 1.85–1.70 (m, 2H), 1.45–1.15 (m, 8H),
0.88 (t, J = 7.8 Hz, 3H) ppm. 13C NMR (CDCl3, 100 MHz):
δ = 190.8 (Cq), 163.4 (Cq), 149.4 (Cq), 143.7 (Cq), 131.3
(CH), 129.9 (Cq), 122.6 (Cq), 105.0 (CH), 102.6 (CH), 56.3
(CH3), 31.7 (CH2), 29.1 (CH2), 29.0 (CH2), 28.5 (CH2), 27.6
(CH2),22.6 (CH2), 14.0 (CH3) ppm. IR (MIR-ATR, 4000–
600 cm−1): νmax = 2927, 2850, 1560, 1480, 1471, 1380,
1190, 1165, 1010, 926 cm−1. HR-MS (ESI+) m/z calcu-
lated for [C17H22NaO3]+ = [M + Na]+: 297.1461; found:
297.1465.
This compound was prepared according to the GP and iso-
latedasbrowncolorsolid91%yield(128mg). M.p. 80–82◦C;
[TLC (petroleum ether/ethyl acetate 8:2, Rf(6b) = 0.40,
Rf(4 h) = 0.50, UV detection]. 1H NMR (CDCl3, 400 MHz):
δ = 10.04 (s, 1H), 7.87 (d, J = 8.8 Hz, 1H), 7.65 (s, 1H),
7.63 (d, J = 8.3 Hz, 1H), 6.97 (d, J = 8.8 Hz, 1H),6.85
(d, J = 8.3 Hz, 1H), 4.12 (s, 3H), 3.86 (s, 3H) ppm. 13C
NMR (CDCl3, 100 MHz): δ = 190.8 (Cq), 160.6 (Cq), 159.3
(Cq), 149.6 (Cq), 143.8 (Cq), 131.8 (CH), 130.4 (Cq), 127.0
(CH), 122.7 (Cq), 122.5 (Cq), 114.3 (CH), 105.8 (CH), 100.4
(CH), 56.4 (CH3), 55.4 (CH3) ppm.IR (MIR-ATR, 4000–
600 cm−1): νmax = 2921, 2850, 1563, 1480, 1465, 1381,
1121, 1103, 1012, 928 cm−1. HR-MS (ESI+) m/z calcu-
lated for [C17H14NaO4]+ = [M + Na]+: 305.0784; found:
305.0788.
This compound was prepared according to the GP and iso-
lated as brown color viscous liquid 90% yield (129 mg):
[TLC (petroleum ether/ethyl acetate 8:2, Rf(6b) = 0.50,
Rf(4 k) = 0.60, UV detection]. 1H NMR (CDCl3, 400 MHz):
δ = 10.01 (s, 1H), 7.62 (d, J = 8.3 Hz, 1H), 7.14 (s,
1H), 6.83 (d, J = 8.3 Hz, 1H), 4.08 (s, 3H), 2.81 (t,
J = 7.3 Hz, 2H), 1.85–1.70 (m, 2H), 1.45–1.15 (m, 10H),
0.87 (t, J = 7.3 Hz, 3H) ppm. 13C NMR (CDCl3, 100 MHz):
δ = 190.8 (Cq), 163.4 (Cq), 149.4 (Cq), 143.7 (Cq), 131.3
(CH), 129.9 (Cq), 122.6 (Cq), 105.1 (CH), 102.6 (CH), 56.3

47 Page 4 of 8
J. Chem. Sci. (2018) 130:47
(CH3), 31.8 (CH2), 29.3 (CH2), 29.2 (CH2), 29.1 (CH2), 28.5
(CH2), 27.6 (CH2), 22.6 (CH2), 14.1 (CH3) ppm. IR (MIR-
ATR, 4000–600 cm−1): νmax = 2920, 2865, 1560, 1484,
1463, 1380, 1132, 1009, 926 cm−1. HR-MS (ESI+) m/z cal-
culated for [C18H25O3]+ = [M + H]+: 289.1798; found:
289.1801.
Table 1. Optimization studies for the formation
of biphenyl 3aa.a
Entry Base
Solvent
Yield 3aa(%)b
1
2
3
4
5
6
7
8
9
K2CO3 DMSO
90
78
55
43
70
64
50
72
65
K2CO3
K2CO3
DMF
DMA
K2CO3 toluene
K2CO3 1,4-dioxane
K2CO3
THF
This compound was prepared according to the GP and
isolated as pale yellow color liquid 90% yield (142 mg).
[TLC (petroleum ether/ethyl acetate 8:2, Rf(6b) = 0.50,
Rf(4 l) = 0.60, UV detection]. 1H NMR (CDCl3, 400 MHz):
δ = 10.01 (s, 1H), 7.62 (d, J = 7.8 Hz, 1H), 7.14 (s, 1H), 6.83
(d, J = 8.3 Hz, 1H), 4.08 (s, 3H), 2.82 (t, J = 7.3 Hz, 2H),
1.85–1.70 (m, 2H), 1.45–1.15 (m, 14H), 0.87 (t, J = 7.3 Hz,
3H) ppm. 13C NMR (CDCl3, 100 MHz): δ = 190.8 (Cq),
163.4 (Cq), 149.4 (Cq), 143.7 (Cq), 131.3 (CH), 129.9 (Cq),
122.6 (Cq), 105.1 (CH), 102.6 (CH), 56.3 (CH3), 31.9 (CH2),
29.6 (CH2), 29.5 (CH2), 29.3 (2C, CH2), 29.2 (CH2), 28.5
(CH2), 27.6 (CH2), 22.7 (CH2), 14.1 (CH3) ppm. IR (MIR-
ATR, 4000–600 cm−1): νmax = 2921, 2851, 1530, 1483,
1460, 1321, 1180, 1163, 1107, 1009, 920 cm−1. HR-MS
(ESI+) m/z calculated for [C20H28NaO3]+ = [M + Na]+:
339.1931; found: 339.1930.
K2CO3 acetonitrile
Na2CO3 DMSO
K3PO4
DMSO
aReaction Conditions: aryl iodides 1a
(0.25 mmol), aryl boronic acid 2a (0.5 mmol),
PdO/GO (5 mol%), K2CO3 (0.5 mmol) and
DMSO (1 mL) at 120 ◦C. bIsolated yields of
product 3aa.
Gratifyingly, the reaction was found to be amenable and
afforded the corresponding biaryls 3aa-gc, in very good
to excellent yields (Table 2). Interestingly, the reaction
was successful with simple iodobenzene 1a and also
with different functional (CF3, F, Me and OMe) groups
on aromatic ring 1b-g (Table 2). Notably, the reaction
was also amenable with simple arylboronic acid 2a as
well as electron-rich 2b and electron deficient 2c aro-
matic moieties (Table 2). However, in an analogy to our
3. Results and Discussion
The first part of the report deals with the optimization previous report,18 the reaction with bromoarenes did not
of conditions for Suzuki–Miyaura coupling. Recently, show progress. This may due to the fact the iodoarenes
we reported that PdO/GO shows the best activity for are relatively more reactive than that of bromo ones.
Heck coupling, where it showed the best efficacy, turn
It is worth noting that the catalyst retains its activity,
over number (TON) and turn over frequency (TOF).18 which is evident with nearly no loss of activity until
Thus, the reaction was explored between iodobenzene the fifth cycle (Figure 1). This was done by recovering
1a and phenylboronic acid 2a in the presence PdO/GO the catalyst by centrifugation and washing with ethyl
of K2CO3 base in DMSO as solvent at 120 ◦C. Interest- acetate and acetone followed by drying in a hot air oven
ingly, the reaction was quite successful and furnished the at 60 ◦C for 12 h. The recovered PdO/GO catalyst was
biaryl product 3aa, in excellent yield (Table 1, entry 1). then subjected to the next catalytic cycle. The marginal
On the other hand, the reaction was found to be inferior loss of activity after the fifth cycle (< 3%) may be due to
with the other solvents, such as DMF, DMA, toluene, loss of some amount of the catalyst during the recovery
1,4-dioxane, THF and acetonitrile (Table 1, entries 2–7). ofPdO/GO. Thecatalystwasrecycledfivetimeswithout
Also, the reaction gave fair to good yields of the prod- appreciable change in the product 3aa yield under the
◦
uct 3aa with the bases Na2CO3 and K3PO4, respectively established conditions at 120 C. Thus, based on the
(Table 1, entries 8 to 9).
above results it was confirmed that PdO/GO catalyst is
Table 1 summarizes the optimization conditions of stable enough and can be reused.
the present study. With these optimized conditions in
After successful synthesis of biaryls using Suzuki–
hand, to extend the scope and generality of the method, Miyaura coupling, we attempted Glaser coupling. Sym-
the Suzuki–Miyaura coupling was explored between metrical 1,3-diynes comprise an important class of com-
various iodoarenes 1a-g and arylboronic acids 2a-c. pounds used for the accomplishment of π-conjugated

J. Chem. Sci. (2018) 130:47
Page 5 of 8 47
Table 2. Synthesis of biphenyls 3aa-gc with aryl iodides 1a-g and arylboronic acids 2a-c.a,b
OH
B
R2
I
PdO/GO
OH
R1
R2
R1
K2CO3, DMSO
+
120 °C, time
1a-g
2a-c
3aa-gc
OMe
OMe
F
OMe
F3C
Cl
3aa (5 min, 90%)
3ab (5 min, 90%)
3ac (5 min, 90%)
3ba (10 min, 85%)
OMe
3cb (5 min, 91%)
F
MeO
OMe
MeO
3fa (10 min, 88%)
MeO
OMe
3dc (10 min, 86%)
3da (10 min, 91%)
3ea (60 min, 90%)
3fb (10 min, 94%)
F
OMe
F
F
MeO
MeO
3fc (5 min, 94%)
F
F
3ec (30 min, 81%)
3gb (10 min, 80%)
3gc (5 min, 85%)
aReaction Conditions: iodobenzene 1a-g (0.25 mmol), arylboronic acid 2a-c (0.5 mmol), PdO/GO (5 mol%),
K2CO3 (0.5 mmol) and DMSO (1 mL) at 120 ◦C. bIsolated yields of product 3aa-gc.
have been developed. Though Glaser coupling was well
established under [Cu]-catalysis, there are a few reports
by using other transition-metal-catalysts, such as Pd-
Cu,32,33 Pd-Ag,34 Pd,25,35 Ni,36 Au,37,38 etc. Herein, we
intended to check the applicability of present PdO/GO
catalyst for Glaser coupling. Therefore, the reaction was
performed with terminal acetylenes 4a-g under estab-
lished reactions of Suzuki–Miyaura coupling. As seen
from Table 3, the reaction was quite successful and fur-
nished the Glaser coupling products 5a-g, in fair to good
yields (Table 3).
To further demonstrate the applicability of PdO/GO
catalyst, one-pot synthesis of benzofurans was tested.
Benzofurans are ubiquitous oxygen-containing hete-
rocyclic motifs that constitute many natural products,
pharmaceuticals, biologicallyimportantcompoundsand
organic materials. In this context, many synthetic strate-
gies have been established for their synthesis. Notably,
Figure 1. Recyclability of the catalyst in Suzuki reaction.
transition metal-catalyzed (Pd,39–47 Rh,48 Ru,48–52 Ir,53
Au54 and Cu55–57) annulations proved to be powerful
strategies for the synthesis of benzofurans. With this
background, PdO/GO catalyst was employed for the
one-pot synthesis of 2-substituted benzofurans starting
from 2-iodophenol 6a and terminal acetylene 4a, under
standard conditions. However, the reaction furnished
the simple Glaser product through homo-coupling and
PdO/MWCNT catalyst is the best catalyst and fur-
nished the benzofuran 7aa, in excellent yield (Table 4).
polymeric materials, supramolecular structures, natural
products and pharmaceuticals.20–25
The first synthesis of symmetric 1,3-diynes through
homo-coupling of terminal alkynes was reported by Carl
Glaser in 1869 in the presence of Cu(I) salt and oxygen
in air.26,27 Subsequently, Eglinton28 and Hay29,30 modi-
fied the method to improve the versatility of the method.
Thereafter, some interesting [Cu]-catalyzed approaches

47 Page 6 of 8
J. Chem. Sci. (2018) 130:47
4. Conclusions
Table 3. Synthesis of Symmetrical 1,3-diynes5a-g
from terminal alkynes4a-g.a,b
In summary, an efficient PdO/GO nano-catalyst
catalyzed Suzuki–Miyaura and Glaser coupling reac-
tions have been demonstrated, for the synthesis of
biaryls and 1,3-diynes. Also, domino one-pot synthe-
sis of 2-alkyl/2-aryl benzofurans has been accomplished
starting from 2-iodophenols and terminal alkynes, in
the presence of PdO/MWCNT nano-catalyst. The for-
mation of benzofurans proceeds through intermolecular
Sonogashira reaction followed by intramolecular nucle-
ophilic addition of internal hydroxyl group onto the
acetylene bond. Catalyst retains its activity even after
5 cycles.
Supplementary Information (SI)
1H, 13C NMR spectras of all isolated products, XRD pattern,
SEM images of as-synthesized catalysts are given. Supple-
mentary Information is available at www.ias.ac.in/chemsci.
aReaction Conditions: aryl alkynes 4a-g (0.5 mmol),
PdO/GO (5 mol%), K2CO3 (0.10 mmol) and DMSO
(1 mL) at 120 ◦C. bIsolated yields of product 5a-g.
Acknowledgements
Table 4. Synthesis of benzofurans 7aa-bl from
2-iodophenols 6a-b and aryl alkynes 4a-l.a,b
B.L. thanks the University of Grant Commission (UGC), New
Delhi, for the award research fellowship.
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