Palladium-poly(3-aminoquinoline) hollow-sphere composite: Application in sonogashira coupling reactions

Article abstract of DOI:10.1002/cctc.201300131

We report on the use of palladium acetate for the synthesis of a palladium-based polymer composite material as a catalyst for Sonogashira cross-coupling reactions for aryl and heteroaryl of iodides and bromides.

Full text of DOI:10.1002/cctc.201300131

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DOI: 10.1002/cctc.201300131
Palladium–Poly(3-aminoquinoline) Hollow-Sphere
Composite: Application in Sonogashira Coupling Reactions
Rafique Ul Islam,[a] Sanjit K. Mahato,[a] Sudheesh K. Shukla,[b] Michael J. Witcomb,[c] and
Kaushik Mallick*[a]
We report on the use of palladium acetate for the synthesis of
a palladium-based polymer composite material as a catalyst for
Sonogashira cross-coupling reactions for aryl and heteroaryl of
iodides and bromides.
Introduction
In situ polymerization and composite formation (IPCF) types of
reactions[1–5] for the synthesis of metal–polymer composite ma-
terial have potential advantages in the field of ‘synthetic mate-
rial science’ because these reactions produce both the polymer
and the nanoparticles simultaneously and thus facilitates an in-
timate contact between the particles and the polymer through
functionalization. Such a composite material with a higher
degree of nanolevel interaction could be expected to demon-
strate improved properties. The incorporation of metal nano-
particles into functional polymers provides enhanced catalytic
performance for both the ‘host’ and the ‘guest’.[6] Polymer-en-
capsulated palladium nanoparticles show a remarkable catalyt-
ic efficiency for the gas phase hydrogenation reaction. Poly-
mer–palladium composite materials synthesized by using the
IPCF approach have been reported to demonstrate excellent
catalytic activity for both Heck and Suzuki types of carbon–
carbon coupling reactions.[7,8]
Palladium-catalyzed Suzuki, Stille, Hiyama, Negishi, Kumada,
Heck and Sonogashira coupling reactions are among the most
powerful carbon–carbon bond-forming reactions.[14,15] The pal-
ladium-catalyzed Sonogashira reaction is one of the most fre-
quently used synthetic tools for the construction of new
C(sp)ÀC(sp2) bonds in organic synthesis.[16–18] The palladium-
catalyzed Sonogashira cross-coupling reaction between aryl
halides and terminal acetylenes serves as a powerful route to
prepare various target compounds with applications ranging
from natural products[19,20] to organic electronic materials.[21,22]
Since the discovery of the Sonogashira reaction, several
modifications have been reported, such as the variation of li-
gands, palladium sources, solvents and bases, to improve the
coupling reactions. In the original protocol for the Sonogashira
reaction, Cu+ was recommended as a co-catalyst to facilitate
the coupling reaction efficiently. However, it can also induce
a Glaser-type oxidative homocoupling[23] of the terminal acety-
lene as a byproduct. To avoid the formation of this undesired
byproduct, several Cu-free methods of the palladium-catalyzed
Sonogashira coupling have been developed.[24–28]
Palladium-based catalysts, particularly palladium nanoparti-
cles, have drawn enormous attention because of their versatile
role in organic synthesis.[9,10] The use of palladium nanoparti-
cles in catalysis is not only industrially important[11,12] but also
scientifically interesting in terms of the sensitive relationship
between catalytic activity, nanoparticle size and shape, and the
nature of the surrounding media.[13]
We report here on the synthesis of a palladium–polymer
supramolecular system by using the IPCF approach. We paid
special attention to characterize the polymer product by using
optical techniques. The subsequent spectral data confirmed
the formation of the polymer, which is also supported by the
microscopic characterization. The supramolecular material was
used as a catalyst for the Sonogashira coupling reaction pro-
ducing a high turnover frequency (TOF) value under Cu-free
and ligand-free conditions.
[a] Dr. R. Ul Islam, Dr. S. K. Mahato, Dr. K. Mallick
Department of Chemistry
University of Johannesburg
P.O. Box 524, Auckland Park 2006 (South Africa)
Fax: (+27)11-559-2819
E-mail: kaushikm@uj.ac.za
Results and Discussion
[b] S. K. Shukla
The SEM image in Figure 1a reveals that the product was com-
posed of microspheres. A higher-magnification SEM image in
Figure 1b shows the spheres demonstrating a wide range of
sizes. The image also reveals that the surface of the spheres is
not smooth. The TEM image in Figure 2a show highly populat-
ed 2 D structure of the spheres. A few broken spheres can be
seen in Figure 2b (marked by arrows), which indicate the low
mechanical stability of the polymer spheres. The higher-mag-
Department of Applied Chemistry
University of Johannesburg
Doornfontein 2028 (South Africa)
[c] Prof. M. J. Witcomb
DST/NRF Centre of Excellence in Strong Materials
University of the Witwatersrand
Private Bag 3, WITS 2050 (South Africa)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300131.
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Figure 3. Raman spectrum of the product in the range of 1700–1100 cmÀ1
indicates the formation of the polymeric product of 3-aminoquinoline with
both the quinoid and the benzenoid ring structure.
Figure 1. a) SEM image of poly(3-aminoquinoline) microspheres. b) Higher-
magnification SEM image of spheres with various sizes. Scale bars=10 mm
(a) and 5 mm (b).
1334 and 1376 cmÀ1 correspond to the CÀN·+ stretching
mode. The band at 1463 cmÀ1 corresponds to the C=N stretch-
ing mode of the quinoid units.[29] A shoulderlike appearance at
1510 cmÀ1 results from the NÀH bending deformation band,
whereas two other similar features at 1550 and 1572 cmÀ1
result from the C=C and CÀC stretching vibrations, respective-
ly. The peak at 1621 cmÀ1 is derived from the CÀC deformation
vibration for the benzenoid ring, which is characteristic for
semiquinone rings.[29,30] The peaks at 1590 and 1175 cmÀ1
result from the CÀC and CÀH benzene deformation modes, re-
spectively, which indicate the presence of quinoid rings.[31]
The UV/Vis spectrum of the product material shows two
prominent absorption peaks at 320 and 600 nm (Figure 4). The
Figure 2. a) TEM image of high-density poly(3-aminoquinoline) micro-
spheres. b) TEM image of a few broken polymer hollow spheres (marked by
arrow). c, d) Higher-magnification TEM images of the spheres with the dark
outer shell and the light core, which indicate the hollow spherical structure
of the polymer. The images also reveal the variation in the shell wall thick-
ness and sphere diameter. Scale bars=2 mm (a), 1 mm (b), 200 nm (c), and
100 nm (d).
Figure 4. The UV/Vis spectrum of the product shows two prominent absorp-
tion bands at 320 and 600 nm, which indicates the p–p transition of the
*
benzenoid rings and the benzenoid to quinoid excitonic transition, respec-
tively. A shoulderlike feature is seen at approximately 450 nm (indicated by
a circle), which is due to a polaron–bipolaron transition in the product
material.
nification TEM images in Figure 2c and d provide information
regarding the internal structure of the microspheres. Both
images show a dark outer shell and a light core of the circles,
which indicate the hollow spherical nature of the polymer. No-
tably, the nanoparticles are absent.
absorption peak at 320 nm is due to the p–p transition of the
*
benzenoid rings. The absorption peak at 600 nm is due to the
transition from a localized benzenoid highest occupied molec-
ular orbital to a quinoid lowest unoccupied molecular orbi-
tal,[32] that is, a benzenoid to quinoid excitonic transition.[33]
The position of the excitonic peak can shift towards a higher-
wavelength region because it is sensitive to the nature of the
ions present,[34] to the solvent[35] and to the chemical struc-
The Raman spectrum of the material within the range 1100–
1650 cmÀ1 is shown in Figure 3. The spectrum is well character-
ized by the prominent bands above 1000 cmÀ1 (in-plane
modes). In the range 1300–1400 cmÀ1, two bands appearing at
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ture.[36] A shoulderlike feature can be seen at approximately
450 nm (indicated by a circle), which could be due to the pres-
ence of a polaron–bipolaron transition in the product material.
The Raman spectrum and the UV/Vis spectrum indicate the for-
mation of the polymeric product of aminoquinoline.
tioned at 335.7 eV, which indicates the presence of Pd0,[40]
whereas the peak at approximately 337.75 eV can be assigned
to the Pd2+ state.[41]
The experimentally obtained characteristic peaks corre-
sponding to Pd 3d5/2 and Pd 3d3/2 states from the spin–orbital
splitting are shown in Figure 5. Herein, we used only the bind-
ing energy value of the Pd 3d5/2 line to determine the oxida-
tion state of palladium. The Pd–PAQ sample shows a broad
spectrum within the range 334.5–339.0 eV (Figure 5). After de-
convolution, two separate peaks appeared at 336.47 and
337.37 eV (Figure 6, inset). As mentioned earlier, the peak at
During the addition of palladium acetate to 3-aminoquino-
line, a radical cation is formed, which is accompanied by the
release of an electron. This step is the initiation process of the
polymerization reaction. Palladium acetate is an oxidizing
agent and can initiate a polymerization process for the mono-
mers of aniline or aniline derivatives,[1,37] and a similar kind of
such a mechanism is followed in the present reaction. Spectro-
scopic analysis confirmed that the product has only a head-to-
tail arrangement, not a head-to-head arrangement. The head-
to-head coupling occurs only under neutral or basic pH condi-
tions. In contrast, 3-aminoquinoline oxidation products ob-
tained in an acidic medium (presence of acetate ions) have
predominantly a head-to-tail arrangement.[2]
During polymerization, each step involves the release of an
electron and that electron is then used to reduce the Pd2+ ion.
In general, the released electron reduces Pd2+ to Pd0, which ul-
timately forms the palladium nanoparticles[1,2,38]; however, in
our case we found evidence for the formation of Pd+ rather
than the Pd0 state. The most sensible explanation for this is
that the Pd2+ ionic state first forms Pd+ species, which then co-
ordinates with the lone pair of the nitrogen of quinoline and
forms a NÀPd+ bond. The nitrogen center of aminoquinoline
features a basic lone pair of electrons, and this lone pair is not
part of the aromatic ring, chemistry similar to that of pyridine.
Here, poly(3-aminoquinoline) (PAQ) plays the role of a macroli-
gand that can coordinate with Pd+. X-ray photoelectron spec-
troscopy (XPS) analysis has confirmed the formation of the Pd+
-like species (Figure 5). The TEM images showed no evidence
Figure 6. Comparative kinetic study of the Sonogashira reaction involving
&
*
a) 4-iodonitrobenzene ( ) and b) 4-iodoaniline ( ) with phenyl acetylene cat-
alyzed by the palladium–poly(3-aminoquinoline) hybrid catalyst (0.03 mol%
of palladium) at 808C.
337.37 eV is due to the presence of unreacted Pd2+ whereas
the new peak that appeared at 336.47 eV is due to the pres-
ence of Pd+ in the sample.[41,42] We found that not all Pd2+
was converted to Pd+ and unreacted Pd2+ species could coor-
dinate with chain ‘N’ of the polymer.[43]
XPS analysis indicates that 1.64 wt% of palladium was pres-
ent on the surface of the polymer matrix. TEM analysis failed
to reveal any evidence of the formation of metallic palladium
in the sample.
Study of catalytic property
The catalytic property of the palladium–polymer complex for
a carbon–carbon bond formation reaction was the subject of
interest in our investigation. Of the various carbon–carbon
bond formation protocols, Sonogashira cross-coupling is one
of the important reactions from a standpoint of its versatile ap-
plications for the preparation of arylalkynes or conjugated
enynes and it is crucial in the synthesis of numerous biological-
ly active compounds.[44]
Figure 5. XPS spectrum of the palladium–poly(3-aminoquinoline) composite
material.
for the formation of palladium nanoparticles. Therefore, to ex-
plain our data, the partial reduction of Pd2+ acetate to Pd+
species becomes an attractive proposal. This would lead to
similar, though not necessarily identical, Pd+ carbonyl carboxyl-
ate complex,[39] which can be envisaged as a model for such
Pd+ intermediates.
The original protocol of the Sonogashira reaction, a dimetal-
lic-mediated process, which is typically performed in the pres-
ence of catalytic amounts of a Pd2+ complex with Cu+ in an
amine solvent, has been repeatedly modified and improved
over time to overcome several significant limitations. The most
important improvement concerned the elimination of Cu+,
To identify the chemical state of palladium in the product,
XPS measurements were done. The peak corresponding to the
Pd 3d5/2 state resulting from the spin–orbital splitting is posi-
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which is used as a co-catalyst, because it can induce homocou-
donating groups, such as ÀCH3, ÀOCH3, ÀOH and ÀNH2, in the
iodobenzene ring also lowered the TOF values to 1738, 1547,
1380 and 1214 hÀ1 with a yield of 73, 65, 58 and 51%, respec-
tively (entries 3–6, row a), under the same catalytic conditions.
Comparable yield and TOF values were observed for both io-
dobenzene and bromobenzene on coupling with phenyl acety-
lene. For the coupling reactions of 4-nitro and 4-carboxy bro-
mobenzene with phenyl acetylene catalyzed by 0.03 mol% of
palladium, the yield of the products was 90 and 84% with the
TOF value of 2142 and 2000 hÀ1, respectively (entries 1 and 2).
A decrease in both the yield and TOF values was recorded
when electron-donating groups, such as ÀCH3, ÀOCH3, ÀOH
and ÀNH2, were attached to the bromobenzene ring (en-
tries 3–6, row a, values within parentheses). The TOF values
were found to be 1714, 1451, 1380 and 1190 hÀ1 with a yield
of 72, 61, 58 and 50%, respectively, under the same catalytic
conditions. An increase in the catalytic concentration
(0.05 mol% palladium) improved the yield percentage, where-
as TOF values decreased in all the cases (Table 2, entries 4–6,
row b). From the above study, we found a high catalytic activi-
ty of Pd–PAQ for both the deactivated and activated aryl
iodide and bromides with excellent yields when reacted with
phenyl acetylene. The activity of aryl halides decreased in the
order of I>Br>Cl, and the electron-deficient aryl halide was
generally more active than the electron-rich one.[47,48] The cou-
pling of 4-nitrochlorobenzene with phenyl acetylene catalyzed
by 0.05 mol% of palladium gave a yield of 30% with TOF
428 hÀ1 from the coupled product at 808C for 10 h. With the
high palladium concentration (0.10 mol% of palladium),
pling reactions of terminal alkynes (Glaser-type reactions).[22]
A
continuation of our work towards the synthesis and catalytic
study of a palladium–polymer composite for Heck and Suzuki
reactions[7,8,45,46] prompted us to study the Sonogashira reac-
tion in a Cu-free situation. By using palladium supported on
a PAQ catalyst, we have performed the Sonogashira coupling
reaction under Cu-free conditions with excellent yields.
Table 1. Solvent–base optimization study for the Sonogashira coupling
reaction between iodobenzene and phenyl acetylene.[a]
Entry
Base
Yield
[%] [T/P]
1
2
3
4
5
C5H10NH
Et3N
K2CO3
Na2CO3
K3PO4
75/63
80/65
58/51
55/52
55/45
[a] All reactions were carried out with iodobenzene (1.0 mmol), phenyl
acetylene (1.5 mmol), base (1.5 mmol) and solvent, and toluene (T) and
propanol (P) (5.0 mL).
A solvent–base optimization study was performed initially
for the coupling of iodobenzene with phenyl acetylene cata-
lyzed by Pd–PAQ in the presence of toluene and propanol as
solvents and Et3N, C5H10NH, K2CO3, Na2CO3 and K3PO4 as bases.
We found that amine bases (triethylamine and piperi-
a higher yield (50%) was achieved with a TOF value of 357 hÀ1
;
however, with an increase in temperature (1008C), the TOF
dine) were effective for the reaction but inorganic
bases (K2CO3, Na2CO3 and K3PO4) were slow for this
system, and toluene produced better results than
propanol (Table 1).
Table 2. Sonogashira coupling reaction between phenyl acetylene and iodo-/bromo-
benzene derivatives.[a]
We have investigated the coupling of aryl halides
with phenyl acetylene in the presence of the Pd–PAQ
catalyst (Table 2). Notably, the values within the pa-
rentheses are for aryl bromides whereas the values
outside the parentheses are for aryl iodides. The high
catalytic activity of Pd–PAQ for both the deactivated
and activated aryl halides was observed with the for-
mation of the corresponding diphenylethyne com-
pounds with excellent yields. The electron-deficient
aryl halide was generally more active than the elec-
tron-rich one.[47,48] Aryl halide with electron-withdraw-
ing groups (Table 2, entries 1 and 2) showed higher
reactivity than those possessing electron-donating
groups (Table 2, entries 3–6). The TOF was as high as
2190 hÀ1 for the coupling of 4-nitroiodobenzene with
phenyl acetylene (entry 1) and 0.03 mol% of palladi-
um. A slight decrease in the TOF value to 2119 hÀ1
was recorded when a comparatively weak electron-
withdrawing group (ÀCOOH) was attached at the
para position of iodobenzene (entry 2) under the
same catalytic conditions. The presence of electron-
Entry
Aryl halide
(X=I/Br)
Product
Yield
[%]
TOF[b]
[hÀ1
]
1
2
3
4
5
6
92 (90)
89 (84)
73 (72)
2190 (2142) (a)
2119 (2000) (a)
1738 (1714) (a)
65 (61)
71 (70)
58 (58)
71 (65)
51 (50)
68 (52)
1574 (1451) (a)
1014 (1000) (b)
1380 (1380) (a)
1014 (928) (b)
1214 (1190) (a)
971 (742) (b)
[a] Reaction conditions: aryl benzene (1.0 mmol), phenyl acetylene (1.5 mmol), triethyl-
amine (1.5 mmol), toluene (5 mL). Both iodobenzene and bromobenzene were used
for the coupling reaction with phenyl acetylene. The values within the parentheses
are for bromobenzene derivatives. Row a: The values were obtained with a catalytic
concentration of 0.03 mol% of palladium. Row b: The values were obtained with a cat-
alytic concentration of 0.05 mol% of palladium; [b] TOF=turnover frequency.
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value increased to 464 hÀ1 and yield to 65% within the same
time period.
tBuOK, tBuONa, MeONa, Et3N, K2CO3, Na2CO3 and K3PO4 as
bases with the Pd–PAQ catalyst for the above reaction. We
found amine (triethylamine) was effective as a base and 2-
propanol produced better results as a solvent for the coupling
reactions with heterocyclic compounds (Table 3). The results of
the coupling reactions of heterocycle-containing substrates
with phenyl acetylene are summarized in Table 4. The TOF
value was 888 hÀ1 for the coupling of 3-bromopyridine and
The coupling reaction of phenyl acetylene with the electron-
deficient 4-nitroiodobenzene (Table 2, entry 1) and electron-
rich 4-iodo aniline (Table 2, entry 6, row a) was chosen for the
investigation of the kinetics in the presence of the Pd–PAQ cat-
alyst. To achieve maximum yields of the coupling product, a rel-
atively longer time span (10 h) was chosen for a comparative
study of the above reactions. Both the reactions were per-
formed at 808C in the presence of 0.03 mol% of palladium.
From the kinetic study (Figure 6), aryl iodide with an electron-
withdrawing group (ÀNO2) showed faster conversion than for
aryl iodide with an electron-donating group (ÀNH2). At the be-
ginning of the coupling reaction, the conversion of both 4-ni-
troiodobenzene and 4-iodoaniline was slow and the rates of
the reactions were comparable; however, with time, the reac-
tion between phenyl acetylene and 4-nitroiodobenzene was
faster and an 80% of the conversion was achieved within 6 h
whereas only 40% of the conversion was achieved for 4-iodoa-
niline. After that time, a fatigue nature was observed for both
the reactions; this is probably due to the deposition of a solid
product on the palladium particles, which hinders the access
of the reactants to the catalyst surface.
Table 3. Solvent–base optimization study for the Sonogashira coupling
reaction between 3-bromopyridine and phenyl acetylene.[a]
Entry
Base
Yield
[%] [IP/T]
1
2
3
4
5
6
7
tBuOK
tBuONa
MeONa
Et3N
K2CO3
Na2CO3
K3PO4
66/59
64/58
59/53
87/71
53/52
51/49
57/46
1H and 13C NMR spectroscopic signatures confirm the forma-
tion of cross-coupled products. The presence of the methoxy
[a] All reactions were performed with 3-bromopyridine (1.0 mmol), phenyl
acetylene (1.5 mmol), base (1.5 mmol) and solvent, and 2-propanol (IP)
and toluene (T) (5.0 mL).
1
(ÀOCH3) group in the H NMR and 13C NMR spectra at 3.8 and
at 55.26 ppm clearly indicates that the formation of a cross-
coupled product give a homo-coupled product of phenyl acet-
ylene, at which these peaks should have been absent. These
data were found to be similar to earlier reported data.[49,50]
.
Table 4. Coupling reaction between phenyl acetylene and heterocyclic
compounds with iodo/bromo derivatives.[a]
Furthermore, the formation of the Glaser-type product should
give bisphenyl acetylene, and in the NMR spectra the nitro
(ÀNO2),[51a] methyl (ÀCH3)[51b] or methoxy (ÀOCH3)[51c] substitu-
ents should be absent; however, in our cases we obtained
only the Sonogashira cross-coupling products, which was con-
firmed by the NMR spectra of the compounds. In contrast,
a Glaser-type product is bisphenyl acetylene and thus the
peaks in the 13C NMR for acetylene ‘C’ appear at approximately
74 and 81 ppm, which was not found in our case.[50]
Entry
Heterocycle substrate
(X=I/Br)
Product
Yield
[%]
TOF[b]
[hÀ1
898
867
908
]
1
2
3
4
88
85
89
87
Although considerable effort has been made to develop the
active catalysts for the Sonogashira coupling reaction, most of
them have involved the simple aryl halide system. In addition
to aromatic systems, we have examined the behavior of the
Pd–PAQ catalyst for the Sonogashira coupling reaction with
heterocyclic compounds. The study of the Sonogashira cou-
pling reactions using heterocycle-containing substrates has
been less popular, and thus the references are less common.
Such methods involved a palladium catalyst in the presence of
phosphine ligands. A Cu-free Sonogashira coupling of hetero-
cycles with the palladium catalyst along with a phosphine
ligand has been reported.[52,53] Herein, we have investigated
the coupling of heterocycle-containing substrates with phenyl
acetylene in the absence of both Cu+ and a phosphine ligand
using the Pd–PAQ catalyst.
888
531
52
73*
5
6
425*
50
69*
510
402*
[a] Reaction conditions: heterocyclic compounds (1.0 mmol), phenyl acet-
ylene (1.5 mmol), triethylamine (1.5 mmol), 2-propanol (5 mL). Both iodo
and bromo heterocyclic compounds were used for the coupling reaction
with phenyl acetylene in the presence of 0.10 mol% of palladium (entries
1–6). An increased catalyst concentration (0.175 mol% of palladium) im-
proved the yields for both iodo- (73%) and bromo- (69%) quinolone-
based coupled product, and the TOF value has been decreased for both
the cases (entries 5 and 6, values with asterisk); [b] TOF=turnover
frequency.
As a model reaction, the coupling between 3-bromopyridine
and phenyl acetylene was chosen for a solvent–base optimiza-
tion study. Toluene and 2-propanol were used as solvents and
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phenyl acetylene (entry 4) with 0.10 mol% of palladium as the
catalyst in the presence of triethylamine as a base and 2-prop-
ity for both the deactivated and activated aryl iodide and bro-
mides was observed with the formation of the corresponding
coupling compounds in excellent yields. Aryl iodides and bro-
mides with electron-withdrawing groups (entries 1 and 2) dem-
onstrate higher reactivity compared to those possessing elec-
tron-donating groups (entries 3–6). The yield and the TOF
values within the parentheses are for bromo derivatives. The
reaction of p-iodonitrobenzene and p-bromonitrobenzene with
2-ethynylpyridine produced the coupling product with a yield
of 88 and 84% and the TOF value of 1197 and 1142 hÀ1, re-
spectively (entry 1). The coupling of 2-ethynylpyridine with aryl
halide possessing the activating group showed a decrease in
TOF values (entries 3–5); however, in the presence of the ÀNH2
group attached to both iodo- or bromo-aryl compounds, the
yield and TOF values decreased significantly (entry 6), which is
due to the fact that the amine functionality in the 4-bromoani-
line (or 4-iodoaniline) could coordinate to the matrix of the
catalyst.
anol as a solvent. A slightly improved value of TOF (908 hÀ1
)
was achieved with the coupling of 3-iodopyridine and phenyl
acetylene (entry 3) under the similar catalytic conditions. The
reaction between 2-iodothiophene and phenyl acetylene
(entry 1) produced a coupling product with a yield of 88% and
a TOF value of 898 hÀ1. The expected trend has also been ob-
served in the case of 2-bromothiophene (entry 2). A relatively
low yield of 52% (TOF=531 hÀ1) and 50% (TOF=510 hÀ1) was
achieved, respectively, with the coupling of 3-iodo- and bro-
moquinoline with phenyl acetylene (entries 5 and 6, value
without asterisk) in the presence of 0.10 mol% of palladium.
To improve the yield of the quinolone-based coupled product,
we increased the catalyst concentration. A substantial improve-
ment was achieved in terms of the yields for both iodo- (73%)
and bromo- (69%) quinolone-based coupled products (en-
tries 5 and 6, with asterisk) at 1208C; however, the TOF value
decreased for both with the increase in the palladium catalyst
concentration to 0.175 mol%. Importantly, there was no in-
crease in their yield with the increased temperature.
We have also studied the Sonogashira coupling reaction be-
tween heterocyclic acetylene and heterocyclic halide systems
in the presence of triethylamine and 2-propanol. The results
from the coupling reaction of iodo- and bromo-substituted
heterocyclic compounds with 2-ethynylpyridine are summar-
ized in Table 6. A similar amount of the coupled product was
In addition to the coupling between aryl and heterocyclic
iodo and bromo systems with phenylacetylene, we have stud-
ied the coupling behavior of heterocyclic acetylene, 2-ethynyl-
pyridine, with aryl iodide and bromide systems. We again
found that triethylamine and 2-propanol were effective, re-
spectively, as a base and as a solvent for the coupling
reactions.
Table 6. Sonogashira coupling reaction between 2-ethynylpyridine and
heterocyclic compounds of iodo/bromo derivatives.[a]
The results from the coupling of iodo- and bromoaryls with
2-ethynylpyridine are shown in Table 5. The high catalytic activ-
Entry Heterocycle substrate Product
Yield
[%]
TOF[b]
[hÀ1
(X=I/Br)
]
Table 5. Sonogashira coupling reaction between 2-ethynylpyridine and
iodo-/bromobenzene derivatives.[a]
1
2
88 (85) 524 (509)
89 (87) 529 (517)
3
52 (50) 310 (298)
Entry Aryl halide
Product
Yield
[%]
TOF[b]
[hÀ1
(X=I/Br)
]
[a] Reaction conditions: heterocyclic compounds (1.0 mmol), 2-ethynyl-
pyridine (1.5 mmol), triethylamine (1.5 mmol), 2-propanol (5 mL). Both
iodo and bromo derivatives were used for the coupling reaction with 2-
ethynylpyridine in the presence of 0.175 mol% of palladium. The values
within the parentheses are for bromo derivatives; [b] TOF=turnover
frequency.
1
2
3
4
5
6
88 (84) 1197 (1142)
85 (83) 1156 (1129)
71 (70) 965 (952)
62 (59) 843 (802)
62 (55) 843 (748)
49 (48) 666 (653)
achieved for thiophene- and pyridine-based heterocyclic com-
pounds (entries 1 and 2). The values within the parentheses
are for bromo derivatives. The TOF value of 524 and 509 hÀ1
was achieved, respectively, for the coupling product of 2-iodo-
and 2-bromothiophene with 2-ethynylpyridine in the presence
of 0.175 mol% of palladium at 1208C for 12 h. Slightly higher
TOF values of 529 and 517 hÀ1 were achieved, respectively, for
the coupling product of 2-iodo- and bromo-pyridine with 2-
ethynylpyridine under the same catalytic concentration and
temperature conditions, whereas a relatively low TOF values of
[a] Reaction conditions: aryl benzene (1.0 mmol), 2-ethynylpyridine
(1.5 mmol), triethylamine (1.5 mmol), 2-propanol (5 mL). Both iodoben-
zene and bromobenzene derivatives were used for the coupling reaction
with 2-ethynylpyridine in the presence of 0.075 mol% of palladium. The
values within the parentheses are for bromobenzene derivatives;
[b] TOF=turnover frequency.
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310 and 298 hÀ1 were achieved, respectively, for the coupled
product of 3-iodo- and bromoquinoline with 2-ethynylpyridine.
The original recommendation for the Sonogashira coupling
reaction involved a phosphine-based palladium catalyst, a co-
catalyst Cu+, and an amine base. A Cu-free version of the So-
nogashira reaction has been developed to avoid the formation
of homocoupling products as mentioned earlier. Herein, we
avoid the use of expensive phosphine ligand and Cu+. For the
Sonogashira coupling reactions, Pd0 is the active catalyst spe-
cies that can be of colloidal nature and stabilized by the li-
gands or polymer present in the system. As mentioned earlier,
the XPS analysis did not show any evidence of Pd0 species in
the composite material, which was confirmed by TEM analysis.
In the Pd–PAQ composite material, two species of palladium
(Pd+ and Pd2+) ions were present in the sample, as confirmed
by using XPS analysis. In addition to the acetylene compounds,
the amine bases may act as reducing agents and enable the
reduction of Pd2+ to Pd0.[54–56] Thus, it is expected that Pd+
could also be reduced to Pd0 under the same reaction condi-
tions. The proposed mechanism of the Sonogashira coupling
reaction is illustrated in Scheme 1. The active palladium cata-
um during the recovery process was unknown. We also did
comparative TEM analysis of the catalyst during recyclability
tests of the catalyst. We collected the sample for TEM analysis
at the early stage of the reaction and found that the nanoparti-
cles were 5–7 nm in size (Figure 7a), whereas at the end of the
third cycle, Ostwald ripening of the particles resulted in a less
Figure 7. TEM image of the palladium nanoparticles (5–7 nm in size) in
a sample that was collected a) at an early stage of the reaction and b) at the
end of the third cycle, which shows that Ostwald ripening resulted in a less
dense distribution of larger palladium particles. Scale bars=20 nm (a) and
0.5 mm (b).
dense distribution of palladium particles, which were up to
120 nm in size (Figure 7b). The enlargement of the palladium
particles could be another reason for the deactivation of the
catalyst after the end of the third cycle of the reaction. Impor-
tantly, the inductively coupled plasma mass spectrometry anal-
ysis did not support any kind of leaching of palladium during
the recovery process of the catalyst.
Conclusions
A new kind of metal ion-incorporated polymer composite ma-
terial has been reported here. This palladium ion-based poly-
meric material has potential application in carbon–carbon cou-
pling reactions. The composite material served as an efficient
and versatile catalyst for Sonogashira coupling reactions for
both aryl and heteroaryl systems under a phosphine-free con-
dition and in the absence of Cu+, which is environmentally
and economically beneficial. The composite catalyst was found
to be stable and could be kept for several years without nota-
ble deactivation because the catalytically active species are co-
ordinated and encapsulated by the polymer. We believe that
this polymer-based nanocomposite material is a promising
candidate for other kinds of carbon–carbon coupling reactions.
The combination of easy synthesis route, wide range of appli-
cation and a long expiry period could make the hybrid materi-
al attractive for industrial applications.
Scheme 1.
Proposed mechanism of the Sonogashira coupling reaction.
lyst reacts with the aryl or heteroaryl halide to produce the
aryl or heteroaryl–palladium halide complex [R1PdIIX] (A), a pro-
cess of oxidative addition; the complex then couples with
alkyne to form an intermediate (B), which finally forms the
coupling product C through the reductive elimination of Pd0.
The recyclability of the Pd–PAQ composite material as a cata-
lyst was also investigated for the coupling of 2-iodothiophene
with 2-ethynylpyridine in the presence of 0.175 mol% of palla-
dium. After the first cycle, the product was extracted with
ethyl acetate and the residual catalyst was used twice under
the same reaction conditions. The results indicated that the
used material was also active as a catalyst without a significant
loss of catalytic performance. At the end of the third cycle,
a yield of 71% was achieved and the TEM images of the used
catalyst showed palladium nanoparticles with a wide size dis-
tribution. The decrease in the yield was mainly due to the loss
of catalyst during the washing process. In every cycle, the
fresh reaction mixture had a lower amount of palladium in the
catalyst, which thus affects the yield; however, it may not have
affected the TOF value, which is difficult to calculate at that
stage of the process because the amount of the loss of palladi-
Experimental Section
TEM was performed at an accelerated voltage of 197 kV by use of
a Philips CM200 transmission electron microscope equipped with
a LaB6 source. An ultrathin window energy-dispersive X-ray spec-
trometer and a Gatan imaging filter attached to the transmission
electron microscope were used to determine the chemical compo-
sition of the samples.
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b) Phenyl-p-tolylacetylene: 1H NMR (400 MHz, CDCl3): d=7.52 (m, 2H),
7.42–7.44 (d, J=7.6 Hz, 2H), 7.32 (m, 3H), 7.14–7.14 (d, J=6 Hz, 2H),
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Published online on May 21, 2013
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