A polystyrene supported [PdCl-(SeCSe)] complex: A novel, reusable and robust heterogeneous catalyst for the Sonogashira synthesis of 1,2-disubstituted alkynes and 1,3-enynes

Article abstract of DOI:10.1039/c8nj01042k

3,5-Bis((phenylselanyl)methyl)phenol has been synthesized as a novel SeCSe pincer type pre-ligand from commercially available 5-hydroxy isophthalic acid through a multistep synthesis. Then, a polystyrene-supported SeCSe pincer ligand and its corresponding palladium complex were prepared and characterized using different techniques including FT-IR, SEM/EDX, TG/DTG, and XPS. The amount of palladium incorporated into the polymer matrix as determined by ICP analysis was obtained to be 3.64 wt% (0.34 mmol g^-1). The PS[PdCl-SeCSe] catalytic activity has been evaluated in the Sonogashira coupling of diverse aryl halides with aromatic and aliphatic terminal alkynes. It was also noted that less reactive and inexpensive aryl bromides and aryl chlorides have been successfully cross-coupled with terminal alkynes using low catalyst loading. Moreover, the developed methodology is effective for the stereoselective preparation of substituted 1,3-enynes.

Full text of DOI:10.1039/c8nj01042k

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Polystyrene supported [PdCl-(SeCSe)] complex: A novel, reusable
and robust heterogeneous catalyst for the Sonogashira synthesis
of 1,2-disubstituted alkynes and 1,3-enynes
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Elmira Mohammadi, Barahman Movassagh*
www.rsc.org/
3
,5-Bis((phenylselanyl)methyl)phenol has been synthesized as a novel SeCSe pincer type pre-ligand from commercially
available 5-hydroxy isophthalic acid through a multistep synthesis. Then, polystyrene-supported SeCSe pincer ligand and
its corresponding palladium complex was prepared and characterized using different techniques including FT-IR, SEM/EDX,
TG/DTG, XPS. The amount of palladium incorporated into polymer matrix as determined by ICP analysis was obtained to
-1
be 3.64 wt% (0.34 mmolg ). The PS[PdCl-SeCSe] catalytic activity has been evaluated in the Sonogashira coupling of
diverse aryl halides with aromatic and aliphatic terminal alkynes. It was also noted that less reactive and inexpensive aryl
bromides and aryl chlorides have been successfully cross-coupled with terminal alkynes using low catalyst loading.
Moreover, the developed methodology is effective for the stereoselective preparation of substituted 1,3-enynes.
ligand and its corresponding Pd(II) complex were developed by Yao
1
7
Introduction
in 2004. This Pd(II)-SeCSe pincer complex has shown promising
catalytic activities in various organic transformations such as Heck
1
7
cross-coupling reaction,
allylation of aldehydes with
Since the discovery of the first metal-pincer organometallic
18
19
allyltributyltin, direct boronation of allyl alcohols, and highly
1
-4
complexes by Shaw and co-workers, the chemistry of this class of
compounds has been widely investigated. Up to now, various metal
complexes bearing ECE-pincer ligands (E = donor atom of the side
20
regio- and stereoselective coupling of allyl alcohols and aldehydes.
A Pd(II) complex of SeNSe pincer ligand has been designed and
efficiently utilized in the Heck reaction of iodo- and bromoarenes
5
, 6
arms, C = ipso anionic carbon atom) have been developed. These
21
with styrene and n-butyl acrylate. All of the above mentioned Se
monoanionic, tridentate ligands usually form very robust and stable
containing pincer complexes are homogeneous catalysts. Despite
the superior catalytic features of homogeneous catalysts such as
the reaction conditions, selectivity, well-defined active site, and
mechanistic insight, heterogeneous catalysts are widely used in
industry. The major drawbacks of homogeneous catalysts are
difficult separation of products from the catalyst, expensive
recycling processes, the loss of valuable metals, and contamination
of the final products with ligands and metals residues. On the other
hand, heterogeneous catalysts are well known for their recyclability
and easy separation from final product. With these objectives, the
development of heterogeneous catalysts has received much
attention in the catalysis field. Furthermore, during the last decade,
many heterogeneous catalysts designed have similar or common
properties with their homogeneous analogues. Regarding to the
growing interest in heterogeneous catalysis as well as
organometallic complexes of pincer ligands, developing
heterogeneous pincer complexes has attracted considerable
attention. However, designing appropriate pincer scaffold for
immobilizing it onto a solid support is always a challenging aspect in
this field. In the structure of prototypical aryl pincer ligands, para
position to anionic carbon center is a suitable site for attaching the
6
, 7
complexes with metals. However, the binding modes in this type
of organometallic complexes are strongly connected to the
coordination properties and steric hindrance of donor atoms
8
-13
3
flanking to the metal center.
Typical η-coordinated ECE pincer
ligands form two five-membered metallacycles, that result in strong
5
,
metal-ligand interactions and prevent ligand exchange processes.
1
4
Therefore, according to their structure properties, this kind of
organometallic species are very air-, moisture- and thermally stable.
Among versatile prototypical aryl pincer ligands, those
incorporating selenium as donor atom in the side arms have been
known as privileged pincer platform. Electronically, selenium is
known as strong electron-donor, which in turn has led to the
development of many transition metal complexes with
1
5, 16
organoselenium ligands.
Furthermore, air- and moisture-
stability of organoselenium ligands are the most important facets of
these class of compounds ligands. However, the first SeCSe pincer
Department of Chemistry, K. N. Toosi University of Technology, P.O. Box 16315-
1
618, Tehran, Iran, Email: bmovassagh1178@yahoo.com; movassagh@kntu.ac.ir
Electronic Supplementary Informaꢀon (ESI) available: [details of any
†
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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pincer ligand to a solid support. There are some examples of inductively coupled plasma (ICP) instrument. Thermogravimetry
2
2-27
supported pincer complexes reported by researchers.
analysis and differential thermogravimetry analysis were carried
To the best of our knowledge, there is only one example of out using a thermal gravimetric analysis instrument (NETZSCH STA
28
heterogeneous selenium pincer ligand anchored to PEG. This 409 PC/PG) under nitrogen atmosphere. X-ray photoelectron
heterogeneous catalyst delivered Heck cross-coupling reaction, but spectroscopy (XPS) was performed with Physical Electronics PHI-
in the term of reusability it encountered prolonged reaction times 5400. The surface morphology of the polystyrene supported species
as well as decreasing product yields. For this reason, designing a was analyzed by scanning electron microscope (Vega, TESCAN-
heterogeneous catalyst containing Se pincer type ligand is an area Model) equipped with energy dispersive X-ray (EDX) facility.
of interest.
Dimethyl 5-hydroxyisophthalate (1b)
Among
organometallic
complexes,
organopalladium(II)
5
-hydroxyisophthalic acid (1a) (3.5 g, 19 mmol) and concentrated
SO (1 mL) was dissolved in MeOH (35 mL). The solution was
complexes are of particular interest, because of their wide catalytic
H
2
4
2
9, 30
applications in modern organic synthesis.
Nowadays, cross-
warmed to 65 °C and refluxed for 24 h; then, 20 mL of water was
added to the solution at room temperature and the resulting white
solid was filtered off and washed with water. The final product was
air dried and obtained as a white solid (3.8 g, 97%).
coupling reactions are considered as pivotal synthetic tools for
many organic transformations, which mainly catalyzed by palladium
2
9-35
containing organometallic complexes.
important cross-coupling reactions is the Sonogashira coupling
reaction of aryl and alkenyl halides with terminal alkynes, affording 3,5-Di(hydroxymethyl)phenol (1c)
One of the most
1
,2-disubstituted alkynes and conjugated enynes via single step C-C To a suspension of LiAlH
4
(1.357 g, 36 mmol) in dry THF (50 mL) at
3
6-40
bond formation reaction;
internal alkynes and 1,3-enynes are 0 °C, dimethyl 5-hydroxyisophthalate (3 g, 14 mmol) (1b) was slowly
41-43
valuable precursors for the preparation of heterocycles,
added. The mixture was gently heated to 50 °C and stirred
The overnight under argon atmosphere. The suspension was cooled to
Sonogashira coupling reaction usually proceeds in the presence of room temperature and THF was evaporated. The resulting paste
4
3, 44
46-48
pharmaceuticals,
and biologically active compounds.
49-51
an additive, for instance, Cu(I)X as co-catalyst.
2 4
However, during was cooled down to 0 °C and 20% aq (v/v) H SO (10 mL) was added
the last decades many additive-free protocols for the Sonogashira dropwise to neutralize unreacted metal hydride; then the residue
5
2-55
cross-coupling reaction have been reported.
Moreover, many was partitioned between EtOAc (50 mL) and water (50 mL). The
heterogeneous-catalyzed coupling of terminal alkynes and aryl aqueous layer was extracted with EtOAc (4 × 20 mL) and the
5
6-61
halides have been developed,
the use of heterogeneous [PdCl-(SeCSe)] pincer complex as catalyst evaporated under reduced pressure and the resulting viscose
in the Sonogashira reaction. The focus of this research work lies at yellow oil was subjected to column chromatography (SiO , eluent:
but there is no report concerning combined organic layers were dried with MgSO . The solvent
4
2
designing a heterogeneous catalyst by combining the characteristic n-hexane : EtOAc = 1:4) to afford (1c) as colorless viscose oil in 72%
properties of SeCSe pincer complexes and insoluble Merrifield resin. yield (1.55 g).
Owing to our ongoing interest in investigation of heterogeneous
3
,5-Di(bromomethyl)phenol (1d)
In a 100mL round-bottom flask, 3,5-di(hydroxymethyl)phenol
1.540 g, 10 mmol) was dissolved in dry THF (35 mL) and the
6
2-68
catalysis in cross-coupling reactions,
we intend to utilize the
heterogeneous [PdCl-(SeCSe)] complex as an efficient catalyst in
copper- and additive-free Sonogashira reaction. We found that
immobilized Pd-SeCSe pincer complex can serve as robust, air- and
heat-stable, as well as reusable precatalyst in the Sonogashira
cross-coupling reaction of aryl halides and β-bromostyrene with
terminal alkynes.
(
solution cooled to 0 °C. The solution of phosphorus tribromide (3
mL, 30 mmol) in dry THF (5 mL) was slowly added over 30 min;
then, the mixture was stirred for 72 h at 40 °C under argon
atmosphere. After cooling to room temperature, the mixture was
washed with saturated aq solution of NaHCO
EtOAc (3 × 15 mL). The combined organic layers were dried over
MgSO and concentrated over reduced pressure. The crude mixture
was purified by column chromatography (SiO , eluent: n-hexane :
EtOAc = 4:1) to give (1d) in pure form (2.2 g, 78%).
3
and extracted with
4
2
Experimental
3
,5-Bis((phenylselanyl)methyl)phenol (1e)
Diphenyl diselenide (2.7 g, 8.6 mmol) was dissolved in EtOH (70
mL); then a solution of NaBH (0.654 g, 17.2 mmol) in aq 20% NaOH
General remarks
All materials were commercial grade and purchased from Merck,
Aldrich, and Fluka. Chloromethylated polystyrene resin cross-linked
with 2% DVB (200-400 mesh, 2.8-3.2 mmol/g of Cl) was a product of
Acros chemicals. The NBP test procedure was applied according to
the literature [69]. FTIR spectra were recorded using an ABB
4
(
5 mL) was added to the above solution and stirred under argon
atmosphere for 10 min. During this period, the yellow color of
solution turned to colorless, indicating the reduction of Ph Se .
2
2
1
13
Then, (1d) (2 g, 7.2 mmol) was dissolved in EtOH (30 mL) and then
0% aq NaOH (20 mL) was added to this solution, and the formation
Bomem model FTLA 2000 spectrophotometer. H (300 MHz) and
C
2
NMR (75 MHz) spectra of the final Sonogashira products were
recorded on Bruker AQS-300 Avance spectrometer. The metal
content of the catalyst was determined with Varian Vista-MPX
of the corresponding phenolate salt was monitored by thin layer
chromatography, TLC. Next, this mixture was transferred to the
2
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PhSeNa solution and stirred at 40 °C for 15 h under Ar atmosphere,
after which the EtOH was evaporated and EtOAc (15 mL) was added
to the crude mixture. Then, the mixture was treated with 20% aq
H SO (20 mL) and extracted with EtOAc (2 × 15 mL). Organic layers
2
4
4
were collected and dried over MgSO . For purification of the
resulting mixture, silica gel column chromatography was applied (n-
hexane : EtOAc = 4:1), which gave the (1e) in 52% yield as yellow oil
(1.62 g).
Synthesis of polystyrene supported-SeCSe pincer ligand (1f)
In a 50 mL round-bottom flask, chloromethylated polystyrene resin
(0.6 g, ~ 1.8 mmol Cl) was allowed to swell in DMSO (15 mL) for 24 h
at room temperature. Then, to the above flask was added
successively 3,5-bis((phenylselanyl)methyl)phenol (1.5 g, 3.5 mmol),
K
2
3
CO (1.5 g, 11 mmol), and tetra-n-butylammonium iodide (TBAI)
(0.133 g, 0.36 mmol). The mixture was stirred at 60 °C under argon
atmosphere. After 72 h, the functionalized polymer beads were
filtered and washed with DMSO (10 mL), H O (20 mL), and MeOH
2
Scheme 1. Synthetic pathway of PS-[PdCl(SeCSe)].
(20 mL) and dried in an oven at 80 °C for 2 h (1.052 g of 1f was
recovered).
quantitative yield (97%), which, subsequently reduced by lithium
Synthesis of PS-[PdCl-(SeCSe)] (1g)
aluminum hydride (LAH) to form 3,5-(bishydroxymethyl)phenol (1c).
The PS-SeCSe resin beads (0.88 g) were swelled for 1 h in dry THF The 3,5-bis(bromomethyl)phenol (1d) was synthesized in 78% yield
25 mL); then PdCl (0.177 g, 1 mmol) and Et N (0.3 mL, 2 mmol) by treatment of diol (1c) with phosphorous tribromide (PBr ) in THF
(
2
3
3
were added to the suspension and stirred for 15 h at 50 °C. The at room temperature. The selenoether moiety was introduced by
resulting polymeric [PdCl-(SeCSe)] was filtered off and the filtrate stirring of diphenyl diselenide with NaBH4 in EtOH and aq 20%
‒
was washed with THF (10 mL), CH
3
CN (10 mL), and MeOH (10 mL) NaOH to generate PhSe anion which then reacted with the
and dried in an oven at 70 °C for 2 h (0.933 g of 1g was obtained).
dibromide (1d) to form 3,5-bis((phenylselanyl)methyl)phenol (1e) in
2% yield. Accordingly, these multistep syntheses have led us to the
5
General procedure for the Sonogashira coupling of aryl halides or
β-bromostyrene with terminal alkynes
SeCSe pincer pre-ligand with p-hydroxy functional group in 30%
overall yield starting from 5-hydroxyisophthalic acid. To the best of
our knowledge, this is the first time that (1e) has been synthesized,
for which full characterization is given (data are available in ESI).
A test tube was charged with aryl halide or β-bromostyrene (1
equiv.) and TBAF (2 equiv) and PS-[PdCl-(SeCSe)] (0.3 mol%) and
NMP (1 mL). To the above mixture was added alkyne (1.5 equiv.)
and heated to 70 °C under aerobic conditions. The progress of the
reaction was monitored by TLC. Upon completion of the reaction,
catalyst was filtered off and water was added to the reaction
mixture (5 mL), the residue was extracted with EtOAc (2 × 5 mL) and
The Next step, involved the attachment of SeCSe pincer type pre-
ligand (1e) to choloromethylated polystyrene beads (CMPS,
Merrifield resin). To accomplish this goal, (1e) was covalently
immobilized onto CMPS beads, by the nucleophilic substitution of
washed with H O (2 × 5 mL). The combined organic layers were
2
the –CH Cl groups on polystyrene (PS) with the hydroxyl group of
2
4
dried, using MgSO and concentrated under reduced pressure. The
the SeCSe pincer pre-ligand in basic media.The functionalization of
crude mixture was then purified by preparative thin layer
2 3
the polymer beads with pincer ligand was performed using K CO as
chromatography (SiO , eluent: n-hexane).
2
the base and DMSO as solvent at 60 °C for 72 h under Ar
atmosphere (Scheme 1). Palladation of the polymer supported free
SeCSe pincer ligand was achieved in refluxing THF during 17 h to
afford polystyrene functionalized with [PdCl-(SeCSe)] complex. In
the next step, the as-prepared heterogeneous pincer catalyst was
characterized using various identification techniques. In order to
Results and Discussion
Synthesis of building blocks
To facilitate anchoring of the SeCSe pincer ligand onto a solid confirm that CMPS resin beads functionalized with SeCSe pincer
support, we decided to synthesize a pincer ligand bearing an OH ligand and its corresponding Pd(II) complex, FT-IR analysis was first
moiety as the binding site. The synthetic pathway toward the key used at different stages of preparation (Figure 1). The IR spectrum
intermediate 3,5-bis((phenylselanyl)methyl)phenol (1e) is outlined of the starting polymer showed two sharp characteristic bands at
-
1
-1
in Scheme 1. To this end, ligand 1e was prepared in four steps 1262 cm (due to bending –CH Cl) and 670 cm (C-Cl stretching)
2
69, 70
starting from commercially available 5-hydroxyisophthalic acid (1a). (Figure 1a).
However, these two bands disappeared after the
Esterification of 1a gave the corresponding diester (1b) in almost
introduction of SeCSe pincer type ligand to the polymer matrix
(
Figure 1b). Moreover, the FT-IR spectrum of PS-SeCSe exhibited
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ligand and Pd(II) catalyst (Figure 3). It can be easily observed from
the SEM images, that comparing to the polymer supported SeCSe
ligand (Figure 3a), some morphological changes occurred on the
surface of the PS-[PdCl-(SeCSe)] catalyst (Figure 3b). Accordingly, it
can be demonstrated that Pd(II) has been coordinated to the ligand.
In addition, elemental mapping images of the as-synthesized PS-
(
a)
[
PdCl-(SeCSe)] revealed the uniform distribution of Pd and Se on
(
b)
(
a)
(b)
(
c)
Figure 1. FT-IR spectra of chloromethylated polystyrene resin
a) PS-SeCSe (b), and PS-[PdCl-(SeCSe)] (c).
(
-
1
two new bands at 1225 and 1100 cm , which can be attributed to
C-O-C vibrations (Figure 1b).
(c)
(d)
7
1
To prove that 3,5-bis((phenylselany)methyl)phenol molecules
were bounded to the polymer matrix via covalent bonding and no
adsorption occurred during the functionalization step, 4-(4´-
nitrobenzyl)pyridine (NBP) test was performed as a colorimetric
test. The NBP test was first reported by Luis et al., providing useful
information to evaluate that active C-Cl bond present in the
7
2
polymer structure. As depicted in Figure 2a, under NBP test
conditions, the color of CMPS beads turned to deep purple due to
the presence of C-Cl bond. On the other hand, when this test was
applied to PS-SeCSe specie, the color of the polymer beads didn’t
change, confirming that the SeCSe pincer ligand molecules have
been attached to the Merrifield resin in high yield and via covalent
bonding (Figure 2b). Furthermore, the content of palladium
incorporated into the polymer matrix for PS-[PdCl-(SeCSe) was
determined by inductively coupled plasma (ICP) and found to be
(e)
-1
3.64 wt % (0.34 mmolg ).
(
f)
(
a)
(b)
Figure 2. The NBP test result for CMPS (a), and polystyrene
supported SeCSe pincer ligand (b).
Figure 3. SEM images of polystyrene supported SeCSe pincer ligand
The scanning electron microscopy (SEM) images were taken to 1f (a) and PS-[PdCl-(SeCSe)] 1g (b), elemental mapping of Se (c), and
investigate the morphological features of the polymer supported Pd (b), EDX spectrum of 1f (e) and 1g (f).
4
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the surface of the polymer (Figure 3c and 3d). Moreover, to thermal analysis (TG-DTG). The experiment was carried out under
ascertain the presence of the selenium and palladium atoms in the nitrogen atmosphere and the results are depicted in Figure 6. TGA
polymeric SeCSe pincer ligand and Pd(II) catalyst, energy dispersive profile of 1g indicates that thermal degradation of the
X-ray spectroscopy (EDX) was also used (Figure 3). As shown in heterogeneous catalyst occurred after 225 °C, which implies the
Figure 3e, the EDX spectrum proved the presence of the selenium thermal stability of the PS-[PdCl-(SeCSe)] on heating. Additionally,
and the absence of the chlorine atom, confirming the attachment of from the DTG curve it can be outlined that the decomposition of
the pincer ligand to the polymer resin beads in high yield. The the organic structure mainly occurred in one step from 320-480 °C.
signals of the palladium and chlorine ions in the EDX spectrum of All the above results, suggest that polymer supported [PdCl-
the final catalyst, also infers the coordination of the palladium (SeCSe)] complex is thermally stable and can be utilized in broad
metal to the polymer anchored SeCSe ligand (Figure 3f).
range of temperatures.
Transmission electron microscopic (TEM) image of polystyrene
supported [PdCl-SeCSe] complex was taken, which clearly exhibited
Pd nanoparticles on the surface of the polymer (Figure 4).
Furthemore, the spherical structure of the polystyrene beads has
remained intact during functionalization steps.
Figure 6. TG-DTG curves of PS-[PdCl-(SeCSe)].
Further evidence for the functionalization of polystyrene
resin beads came from the X-ray photoelectron spectroscopy
(
XPS), that revealed the electronic properties and chemical
Figure 4. TEM image of PS-[PdCl-(SeCSe)] 1g.
state of the species formed on the surface. The full range XPS
spectrum of PS-[PdCl-SeCSe] has been shown in Figure 7a. The
peaks at 56.1, 198.4, 284.6, 337.5, and 532.3 eV are attributed
to Se 3d, Cl 2p, C 1s, Pd 3d, and O 1s, respectively. Peaks at
Additionally, X-ray diffraction measurement was carried out to
determine the structure of polystyrene supported [PdCl-(SeCSe)]
resin (Figure 5). It has been reported that the broad peak at 2θ
values between 10° and 30° are attributed to the amorphous
structure of the polystyrene [73]. Moreover, it has been observed
that PdNPs show a lattice expansion in the XRD measurement [74].
The diffraction angle of Pd nanoparticles in the present catalytic
system has been identified at 2θ value of 39.56°, which is smaller
than that of the palladium black 40.080° [74].
2
83.8, 284.9, 285.7, and 286.7 eV in the C 1s spectra can be
assigned to binding energies of C–Pd, C=C, C–C, and C–O bonds
7
5-77
(
Figure 7b).
the peaks of Pd(II) 3d5/2 and 3d3/2 at 337.6 and 343.3 eV,
respectively (Figure 7c). The shift of Pd(II) 3d5/2 of PdCl
binding energy peak from 338.4 eV to 337.6 eV confirms the
Additionally, the spectrum of Pd(II) 3d shows
2
7
8
formation of [PdCl-(SeCSe)] pincer complex. Figure 7d,
Furthermore, thermal stability as well as thermal behavior of exhibits Se 3d XPS spectrum of PS-[PdCl-SeCSe)] complex, in
the catalyst was investigated by thermogravimetric and differential which peaks of Se 3d5/2 and 3d3/2 at 55.5 and 57.1 eV are
clearly observed. Analytical interpretation of peak areas of
Pd(II) and Se ions, perfectly confirmed the molar ratio of Se/Pd
to be 2:1.
The Catalytic activity of the PS-[PdCl-SeCSe]
To evaluate the activity of the chloromethylated
polystyrene resin supported SeCSe-Pd(II) pincer complex, it
was used in the Sonogashira cross-coupling reaction. Initial
studies were performed upon the cross-coupling reaction of
phenylacetylene (1.5 equiv.) with iodobenzene (1.0 equiv.) as a
2
θ(°)
model reaction, to optimize the reaction parameters including
the solvent, base, catalyst loading and the reaction
Figure 5. X-ray diffraction pattern of fresh PS-[PdCl-(SeCSe)] 1g.
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temperature. The results are summarized in Table 1. The effect of
different bases on the model reaction under aerial conditions was
first investigated. It was found that the highest yield of the control
reaction was achieved when tetra-n-butylammonium fluoride
(a)
(
TBAF) was used as the base (Table 1, entry 6). Moreover, in the
presence of tetra-n-butylammonium bromide (TBAB) the cross-
coupling adduct was obtained in 63% yield (Table 1, entry 5). The
lower yield of the latter reaction may be due to the lower basicity of
‒
the Br anion comparing to the fluoride ion. However, other organic
as well as inorganic bases were less effective in this organic
transformation. The influence of solvent on the cross-coupling
reaction of phenylacetylene and phenyl iodide was also surveyed.
Among, the solvents tested N-methylpyrolidone (NMP) was the
most suitable reaction medium for the formation of 1,2-
diphenylethyne (Table 1, entry 6). DMF was inferior and produced
the desired product in 61% yield (Table 1, entry 9). When THF and
acetonitrile were used as reaction solvents, the corresponding
product was obtained in 46% and 53% yields, respectively (Table 1,
entries 10 and 11). No conversion of starting material was observed
when reaction proceeded in water, which may be due to the poor
solubility of the reactants (Table 1, entry 14).
(
b)
Table 1. Optimization of the reaction parameters for the
a
Sonogashira coupling.
(
c)
b
Entry
Solvent
Base
Catalyst
Temp.
(°C)
Time
(h)
Yield
(%)
(mol% of
Pd)
1
2
3
4
5
6
7
8
9
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
DMF
THF
KOH
0.5
50
50
50
50
50
50
50
50
50
50
50
50
50
50
25
70
70
70
70
2
2
41
61
NaOH
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.7
0.3
0.2
K
2
CO
3
2
56
Cs
2
CO
3
2
52
TBAB
TBAF
2
63
2
86
Et
3
N
2
22
DIPEA
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
2
18
2
61
(d)
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
2
46
CH
3
CN
2
53
Toluene
EtOH
2
trace
13
2
H
2
O
2
trace
81
NMP
NMP
NMP
NMP
NMP
2
0.5
0.5
0.5
0.5
91
92
91
81
a
b
Reaction conditions: phenylacetylene (1.5 equiv.), iodobenzene (1 equiv.),
base (2 equiv.), solvent (1 mL), cat. PS-[PdCl-(SeCSe)].
Isolate yields.
The scope of the Sonogahsira cross-coupling reaction under
above-derived optimized reaction conditions was investigated
using 0.3 mol% of PS-[PdCl-(SeCSe)] catalyst in NMP as the
Figure 7. XPS spectrum of as-synthesized PS-[PdCl-(SeCSe)] (a)
full range, (b) C 1s, (c) Pd 3d, and (d) Se 3d.
6
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reaction medium and 2.0 equiv. of TBAF as the base at 70 ºC. when aryl chlorides with electron-withdrawing groups such as 4-
Wide array of terminal acetylene and aryl halides with nitrochlorobenzene and 4-Chlorobenzonitrile were used (Table 2,
different electronic and steric properties were surveyed and entries 17 and 18) after a prolonged reaction time, 24 h.
the results are outlined in Table 2. As expected, aryl iodides Predictably, comparing to phenylacetylene, electron-donating
gave better results than aryl bromides and aryl chlorides, groups on the alkyne substrate such as p-Me and p-MeO decreased
which reflects the ease of oxidation addition of Pd(0) to the C- the yield and prolonged the time of the reaction due to the lower
79
X bond. As illustrated in Table 2, the electron rich and acidity of these reactants (Table 2, entries 1, 5 and 8, also entries 11
electron poor aryl iodides reacted with aromatic as well as and 15). Additionally, the protocol was extended to the Sonogashira
aliphatic terminal alkynes to afford the corresponding cross-coupling reaction of 1-hexyne and 1-octyne
and
products in 61-95% yields within 0.25-3 h. It was observed that iodobenzene, which it was observed that the reaction proceeded
aryl iodides bearing electron-withdrawing groups performed slowly (3 h) and afforded the corresponding products in 66% and
the reactions in shorter reaction times with higher yields 61% yields, respectively (Table 2, entry 9 and 10).
(Table 2, entries 4 and 6). Comparing to iodobenzene, aryl
iodide having a substituent in ortho position suffered from
The above results encouraged us to extend the scope of
slower conversion rate (1 h) and lower yield (71%) due to the the Sonogashira cross-coupling reaction to the synthesis of
steric hindrance effect (Table 2, entry 3). Importantly, under 1,3-enynes. To this end, β-bromostyrene cross-coupled with
the optimum reaction conditions aryl bromides cross-coupled aromatic as well as aliphatic terminal alkynes (Table 3).
with terminal alkynes to obtain the final products in good to Delightfully, terminal alkynes effectively reacted with β-
excellent yields (62%-91%) (Table 2, entries 11-15) without bromostyrene to obtain the corresponding 1,3-enyne products
using without using any additive. However, electron-deficient in 61%-86% yields with high to excellent stereoselectivity. 1-
aryl bromides exhibited better reactivities than their electron Octyne, as aliphatic alkyne, also cross-coupled with β-
rich counterparts (Table 2, entries 12 and 13 comparing to bromostyrene, affording the final adduct within 8 h in 61%
entries 10 and 11).
yield with E:Z ratio of 100:0 (Table 3, entry 4).
Table 2. Sonogashira cross-coupling reaction of aryl halides with
terminal alkynes.
a
a
Table 3. Sonogashira coupling of β-bromostyrene.
1
b
Entry
Aryl halide
R
Time Yield
h) (%)
91
1
b
(
Entry
R
Time (h)
Yield (%)
E:Z (%)
93:7
1
2
3
4
5
6
7
8
9
Iodobenzene
1-Iodo-4-methoxybenzene
1-Iodo-2-methoxybenzene
1-Iodo-4-nitrobenzene
Iodobenzene
C
C
C
C
6
6
6
6
H
5
0.5
0.5
1
1
2
3
4
C
6
H
5
2
4
4
8
86
72
65
61
H
5
89
71
95
84
88
81
79
66
61
88
72
91
88
62
41
56
53
23
4-MeC
4-MeOC
n-C
6
H
4
92:8
H
5
6
H
4
93:7
H
5
0.25
1
6
H
13
100:0
4-MeC
4-MeC
4-MeC
6
6
6
H
4
a
b
Reaction conditions: alkyne (2 equiv.), β-bromostyrene (1 equiv.), TBAF
2 equiv.), PS-(PdCl-(SeCSe)] catalyst (0.3 mol%), NMP (1 mL).
Isolated yields.
4-Iodobenzonitrile
H
4
0.75
1
(
1-Iodo-4-methoxybenzene
Iodobenzene
H
4
4-MeOC
n-C
n- C
6
H
4
1.5
3
Iodobenzene
4
H
9
Beside environmental aspects, the recyclability of the
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
Iodobenzene
6
H
13
3
heterogeneous catalysts, is a very important issue from industrial
and commercial points of view. Hence, the recovery and reusability
of the PS-[PdCl-(SeCSe)] catalyst have been investigated in the
reaction of iodobenzene and phenylacetylene using optimal
reaction conditions. The results of recyclability test have been
depicted in Figure 8. For performing the recyclability test, upon
completion of the reaction (monitored by thin layer
chromatography, TLC, the catalyst was removed from the reaction
mixture by filtration. The filtrate was then thoroughly rinsed with
water, hot ethanol and dichloromethane. The catalyst was
subsequently dried in an oven at 70 °C for 1 h. As expected, the
catalyst showed good reusability and retained its activity after 6
successive runs. Furthermore, hot filtration test was performed for
the model reaction. When the catalyst was removed from the
Bromobenzene
C
C
C
C
6
6
6
6
H
5
4
1-Bromo-4-methoxybenzene
1-Bromo-4-nitrobenzene
1-(4-Bromophenyl)ethan-1-one
Bromobenzene
H
5
6
H
5
2
H
5
2
4-MeOC
6
H
4
6
Chlorobenzene
C
C
C
C
6
6
6
6
H
5
24
24
24
24
1-Chloro-4-nitrobenzene
4-Chlorobenzonitrile
1-Chloro-4-methoxybenzene
H
5
H
5
H
5
a
b
Reaction conditions: alkyne (2 equiv.), aryl halide (1 equiv.), TBAF (2
equiv.), PS-[PdCl-(SeCSe)] catalyst, (0.3 mol%), NMP (1 mL).
Isolated yields.
Then, we tried to examine the reactivity of the PS-[PdCl-(SeCSe)]
catalyst in the cross-coupling reaction of less reactive aryl chlorides;
it was observed that acceptable yields of desired products obtained
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reaction after 15 min, the yield of the reaction was found to be exhibited high catalytic activity in this reaction, affording
2%. Then, the reaction was allowed to continue for 45 min and the diverse range of 1,2-disubsituted acetylenes in moderate to
4
desired product was obtained in 42.6% yield after this period, which excellent yields. Interestingly, inexpensive aryl bromides and
demonstrated that the amount of Pd leached to the solution is aryl chlorides proceeded the reaction in acceptable yields
negligible. Additionally, at the end of the first and the seventh without using any additive. Furthermore, β-bromostyrene
cycles, the amount of palladium released to the solution was cross-coupled successfully with aromatic and aliphatic terminal
determined by ICP analysis and found to be 0.16% and 1.1%, alkynes in the presence of the above catalyst to produce
respectively. Meanwhile, after the end of the first cycle the XRD substituted 1,3-enynes. The developed approach was
pattern of this heterogeneous catalytic system has been also stereoselective in the latter reaction and the corresponding
studied, indicating that the structure of the polymer remain intact products obtained with high E:Z ratios. This heterogeneous
during the course of the reaction (Figure 9). Also, the XPS spectra catalyst was easily recovered by simple filtration and reused
of the polystyrene supported [PdCl-(SeCSe)] complex after the first up to 6 consecutive runs without significant loss of its catalytic
reaction run clearly confirms the presence of the Pd(II) in the final activity.
catalyst (See ESI, Figure S12 and S13).
Acknowledgements
The authors are gratefully acknowledged research council of K.
N. Toosi University of Technology for its financial support in
this work.
Notes and references
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8
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Graphical Abstract
PS-[PdCl-(SeCSe)] has been introduced as a novel and heterogeneous catalyst, which exhibits outstanding catalytic activity in
the Sonogashira cross-coupling reactions.