Synthesis of Benzofurans and Indoles from Terminal Alkynes and Iodoaromatics Catalyzed by Recyclable Palladium Nanoparticles Immobilized on Siliceous Mesocellular Foam

Article abstract of DOI:10.1002/chem.201702614

Herein, we report on the utilization of a heterogeneous catalyst, consisting of Pd nanoparticles supported on a siliceous mesocellular foam (Pd⁰-AmP-MCF), for the synthesis of heterocycles. Reaction of o-iodophenols and protected o-iodoanilines with acetylenes in the presence of a Pd nanocatalyst produced 2-substituted benzofurans and indoles, respectively. In general, the catalytic protocol afforded the desired products in good to excellent yields under mild reaction conditions without the addition of ligands. Moreover, the structure of the reported Pd nanocatalyst was further elucidated with extended X-ray absorption fine-structure spectroscopy, and it was proven that the catalyst could be recycled multiple times without significant loss of activity.

Full text of DOI:10.1002/chem.201702614

A Journal of
Accepted Article
Title: Synthesis of Benzofurans and Indoles from Terminal Alkynes and
Iodoaromatics Catalyzed by Recyclable Palladium Nanoparticles
Immobilized on Siliceous Mesocellular Foam
Authors: Jan-E. Bäckvall, Alexandre Bruneau, Karl Gustafson, Ning
Yuan, Ingmar Persson, Xiaodong Zou, and Cheuk-Wai Tai
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To be cited as: Chem. Eur. J. 10.1002/chem.201702614
Link to VoR: http://dx.doi.org/10.1002/chem.201702614
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Synthesis of Benzofurans and Indoles from Terminal Alkynes and
Iodoaromatics Catalyzed by Recyclable Palladium Nanoparticles
Immobilized on Siliceous Mesocellular Foam
[
a]
[a]
[b,c]
b
[c]
Alexandre Bruneau, Karl P. J. Gustafson, Ning Yuan,
Cheuk-Wai Tai, Ingmar Persson,
[b]
[a]
Xiaodong Zou, and Jan-E. Bäckvall*
Dedication ((optional))
Abstract: Herein, we report on the utilization of a heterogeneous
catalyst, consisting of Pd nanoparticles supported on a siliceous
0
mesocellular foam (Pd –AmP–MCF), for the synthesis of
heterocycles. Reaction of o-iodophenols and protected o-
iodoanilines with acetylenes in the presence of a Pd nanocatalyst
produced 2-subsituted benzofurans and indoles, respectively. In
general, the catalytic protocol afforded the desired products in good
to excellent yields under mild reaction conditions without the addition
of ligands. Moreover, the structure of the reported Pd nanocatalyst
was further elucidated with Extended X-Ray Absorption Fine
Structure spectroscopy and it was proven that the catalyst can be
recycled multiple times without significant loss of activity.
Scheme 1. Straightforward synthesis of heterocycles from ortho halogenated
derivatives and terminal alkynes.
Many homogeneous systems have been reported for the
transformation in Scheme 1 since the pioneering work described
[
3a,3b,4]
by Castro et al.,
while less attention has been devoted to
[5]
the corresponding heterogeneous processes. By utilizing a
heterogeneous catalyst one can overcome the drawbacks of a
homogeneous protocol, such as the removal of trace metal
found in the product, purification, and recycling of the catalyst at
the end of the reaction. Over the past few years, metal
nanoparticles have found extensive use in the field of catalysis.
Their large surface-to-volume ratio and the high density of
unsaturated coordination sites have led to activities not seen for
Introduction
Indoles and benzofurans are common structural elements in
natural and synthetic products. These heterocycles possess
several unique properties, which explain their frequent
[
6]
[1]
occurrence in bioactive molecules as well as in materials.
Because of their importance, numerous synthetic methods have
been developed for the construction of these heterocycles
[7]
bulk metals.
[1f,2]
The Bäckvall group has previously reported on the preparation
and the application of a heterogeneous Pd catalyst, consisting of
palladium nanoparticles supported on a siliceous mesocellullar
during the past few decades.
Among these methods,
palladium-catalyzed protocols have been frequently used for the
[
1f,3]
formation of substituted benzofurans and indoles.
One of the
0
[8]
foam, Pd -AmP-MCF. This catalyst has been found to
effectively catalyze a wide range of transformations, exhibiting
most straightforward methods is based on the use of a terminal
alkyne and an ortho-halogenated phenol/aniline (Scheme 1).
[
9]
II
high recyclability and low metal leaching. The Pd pre-catalyst
0
for the Pd -AmP-MCF has also been shown to work well in
[
10]
several applications.
Herein, we report our results on
nanopalladium-catalyzed synthesis of benzofurans and indoles
II
and provide further characterization of both the Pd pre-catalyst
0
and the Pd -AmP-MCF catalyst with the aid of EXAFS. The
catalytic efficiency of Pd -AmP-MCF was demonstrated by the
synthesis of substituted heterocycles from terminal alkynes and
iodoaromatics under mild conditions.
[
a]
Dr. A. Bruneau, K. P. J. Gustafson, Prof. J.-E. Bäckvall
Department of Organic Chemistry and
Berzelii Centre EXSELENT on Porous Materials
Arrhenius Laboratory, Stockholm University
SE-106 91 Stockholm (Sweden)
0
E-mail: jeb@organ.su.se
[
b]
c]
N. Yuan, Prof. X. Zou
Berzelii Center EXSELENT on Porous Materials and
Department of Material and Environmental Chemistry (MMK)
Stockholm University, SE-106 91 Stockholm (Sweden)
N. Yuan, Prof. I. Persson
Results and Discussion
Characterization data of the catalysts
[
Department of Molecular Sciences
Swedish University of Agricultural Sciences
P.O. Box 7015, SE-750 07 Uppsala (Sweden)
The Pd nanoparticle catalyst was prepared as previously
[8b]
reported (Figure 1).
Pristine MCF was functionalized with
Supporting information for this article is given via a link at the end of
the document.
aminopropyl groups by refluxing it in toluene with 3-
aminopropyltrimethoxysilane to give the AmP-MCF. The amino
2 4
functionalized MCF was then impregnated with Li PdCl in a pH
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Chemistry - A European Journal
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II
9
LiOH solution to furnish the Pd -AmP-MCF precatalyst, which
a) Pd(II)@MCF
in the final step was reduced with NaBH to yield the palladium
4
0
[8b]
II
nanoparticle catalyst Pd -AmP-MCF. The Pd –AmP-MCF and
0
Pd –AmP-MCF catalysts have previously been characterized by
IR spectroscopy, elemental analysis (ICP-OES), scanning
electron microscopy (SEM), high-angle annular dark-field
scanning transmission electron spectroscopy (HAADF-STEM),
X-ray
photoelectron
spectroscopy
(XPS),
2
N -
absorption/desorption isotherms, high-resolution TEM and
[
8a,b,9]
convergent-beam electron diffraction (CBED).
From the
0
extensive analysis of the catalysts it was concluded that the Pd -
AmP-MCF consisted of predominantly crystalline palladium
nanoparticles within a porous siliceous network in the size range
b) Pd(0)@MCF
1
.5-2.6 nm with an overwhelming amount residing in the
II
0
oxidation state 0. Even though the Pd –AmP-MCF and Pd -
AmP-MCF have been thoroughly characterized, there is limited
[
9c]
knowledge concerning coordination to the Pd atoms.
Figure 2. Pd K-edge EXAFS spectra (left) and Fourier transforms (right) for (a)
II
0
Pd -AmP-MCF and (b) Pd -AmP-MCF. Solid black lines show the
experimental data and dashed red lines are the best fits. The Fourier
transforms data are without phase correction.
II
The Fourier transform data for the pre-catalyst Pd -AmP-MCF
(
Figure 2a) contains two major peaks corresponding to Pd–N
and Pd–Cl bonds individually. The best fit revealed that each Pd
atom in the pre-catalyst is bound to two nitrogen atoms with a
Pd-N distance of 2.02 Å and to two chloride atoms with a Pd-Cl
distance of 2.29 Å in a square planar geometry as a trans
complex. This indicates that the Pd precursor was embedded on
the amino groups in the framework in the form of a Pd(II)
complex. Meanwhile, a noticeable peak at ca. 3.0 Å (without
phase correction) with a shoulder is more likely attributed to
single scatterings than a multiple scatterings (MS), which means
that Pd(II) complex units were probably stacking together at
distances of 3.07 Å and 3.42 Å.
0
Figure 1. Preparation of the Pd nanoparticle catalyst Pd -AmP-MCF.
0
Concerning the catalyst Pd -AmP-MCF (Figure 2b), the
EXAFS spectrum changed significantly compared to that of the
pre-catalyst. In the Fourier transform, only one major peak
corresponding to Pd–Pd bonds appeared. The best fit showed
that on average each Pd atom is bound to approximately 6 Pd
atoms in the first coordination shell with a bond distance of 2.78
Å. The true coordination number of Pd-Pd in the first
coordination shell should be slightly higher than the value
achieved from the refinement because there is a small fraction
of Pd species other than nanoparticles. The peak for Pd–N
We have now used extended X-Ray Absorption Fine
Structure (EXAFS) spectroscopy to investigate the specific
II
coordination environment of the Pd atoms in Pd -AmP-MCF pre-
0
catalyst and Pd -AmP-MCF (Figure 2).
0
bonds in Pd -AmP-MCF became much less pronounced
compared to the pre-catalyst and It appeared with a shoulder,
which is due to the combination of the single scattering of Pd–N
bonds and side peaks from the strong Pd–Pd contribution. The
Pd–Cl bonds were not observed. The average number of Pd–N
bonds per Pd was determined to be about 0.7 with a distance of
2.0 Å. It should be noted that the peak assigned to the Pd–N
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bonds could in principle also be Pd–O bonds if the reduced Pd
atoms are partially oxidized in air or the remaining small amount
of Pd(II) species hydrolyzed into PdO. The significant decrease
of the Pd–N (or possibly the Pd–O) bonds and the inflated signal
for Pd–Pd bonds validates the efficient reduction of the pre-
acetylenes with different electronic properties were observed.
The slightly lower yield for the electron-rich acetylenes could be
ascribed to their increased relative rate for the homocoupling to
form diacetylenes. More sterically hindered acetylenes, such as
(2-MeO-phenyl)acetylene, were not tolerated, and gave compli-
cated mixtures. Alkynes bearing alkyl chains gave the desired
products 2d and 2f in satisfactory yields (63% and 62%). It is
worth noting that aza-benzofurans 2d and 2e were also obtained
in good yields (63 and 80%, respectively) starting from 2-
iodopyridin-3-ol. The synthesis of variously substituted
benzofurans 2f-k, with groups that can be functionalized
catalyst with NaBH
parameters of the fit can be found in the supporting information
Table S1). It should be noted that the number of Pd–N bonds
4
forming Pd nanoparticles. The detailed
(
per Pd and bond distances are average values.
Synthetic work
(
aldehyde, nitro, ester, phtalimide, alcohol, and sugar) was
Starting from reaction conditions similar to those of the classical
homogeneous Sonogashira reaction, the reaction of 2-
iodophenol (1a) with phenylacetylene was studied in the
compatible with the catalytic system; benzofurans 2f-k were
obtained in good yields ranging from 59 to 74%. In most case
small amounts of starting material were observed together with
presence of triphenylphosphine and copper(I) iodide (Scheme 2). dehalogenated and homocoupled aryliodides.
Table 2. Synthesis of substituted benzofurans
Scheme 2. Model reaction for the synthesis of benzofurans.
0
The catalytic system composed of 2.5 mol% of Pd -AmP-MCF, 5
mol% of CuI, 5 mol% of PPh
3
, 1.2 equiv. of NEt
3
in acetonitrile at
90 °C gave the expected benzofuran in high yield after 2h (85%,
Scheme 2, Table 1, entry 1).
[
a]
Table 1. Optimization of the reaction conditions
Alkyne
equiv.
PPh
(mol%)
3
CuI
(mol%)
T
(°C)
[b]
Entry
Time
NMR Yield
1
2
3
4
5
6
7
1,5
1,5
1,5
1,5
1,5
2
5
5
5
-
5
-
5
5
5
5
-
90
45
rt
90
90
60
70
2h
24h
24h
5h
6h30
20h
20h
85
63
48
66
48
75
5
5
[
c]
2
-
87 (85)
0
[
a] The reaction was carried out in acetonitrile using 2,5 mol% of Pd -AmP-
3
MCF an 1.2 equiv. of Et N. [b] 1,3,5-trimethoxybenzene used as internal
standard. [c] Average of 3 runs. Isolated yield in parenthesis.
0
Reaction conditions: 2-iodophenol (0.25 mmol), alkyne (0.5 mmol), Pd -AmP-
MCF (2.5 mol%), CuI (5 mol%), NEt
under Ar for 20h.
3
1.2 equiv., acetonitrile (1 mL) at 70 °C
Decreasing the temperature had a detremental effect on the
conversion but could be partially compensated by increasing the
reaction time (entries
3
2 and 3). Omitting PPh gave an
The catalytic system also proved to be compatible with aniline
derivatives, and N-tosyl-2-iodoaniline 3a afforded the
corresponding indole 4a in an excellent yield of 95% under
slightly milder conditions (40 °C, Scheme 3).
acceptable yield within 5h (66%, entry 4), while the removal of
CuI resulted in a considerable loss of the yield (48%, entry 5).
Using 2 equiv. of alkyne at 60 °C in the absence of phosphine
during 20h afforded the desired product in a good yield (75%,
entry 6), and increasing the temperature to 70 °C afforded 2a in
87% yield (entry 7).
With the optimal conditions in hand, we set out to evaluate the
scope of the catalytic system (Table 2). To our delight, variously
-substituted benzofurans could be obtained by using this
2
catalytic system. Electron-rich or electron-deficient aryl-
acetylenes can be used giving the corresponding benzofurans
0
Scheme 3. Indole synthesis using Pd -AmP-MCF.
2a-c in good yields. No clear trend in the reaction rate of
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With the excellent results observed for N-tosyl-2-iodoaniline
and phenylacetylene, we investigated the scope of the indole
synthesis (Table 3). A wide range of aromatic and aliphatic
alkynes (electron-rich and electron-deficient) were successfully
coupled with N-tosylated o-iodoanilines to give indoles 4a-4f in
very good to excellent yields (88 to 95%). In the case of aza-
indoles, indole 4g could be isolated in 95% yield. Gratifyingly,
complete chemoselectivity toward the iodine was observed
when using the 5-chloro-2-iodo derivative; only the expected
products 4e and 4h were isolated in high yields (95% and 90%
respectively). Free hydroxyl groups were tolerated (4h, 90%) as
well as more complex structures such as a carbohydrate and
steroid derivatives 4j and 4k (90 and 95% respectively). Mesyl
protected iodoaniline proved also to be compatible with the
reaction conditions and indole 4i was isolated in 91% yield.
Figure 3. Uncyclized products a. NMR Yields given. b. Isolated yield).
Interestingly, water proved to influence the outcome of the
reaction described above (Scheme 3), as shown in Figure 4.
When examining the acetonitrile used it contained approximately
one equivalent of water which also proved to be optimal for ob-
taining the maximum yield of the desired product 4a. A probable
explanation for the positive effect of small amounts of water is
that the rate of hydrolysis of the vinylpalladium intermediate (Int-
A, Scheme 5, see below), formed in the cyclization, is increased.
Table 3. Synthesis of substituted indoles
NMR Yield
100
95
90
85
80
75
70
92
86
86
82
76
0
1
2
3
4
5
equiv. of H O
2
Figure 4. Influence of water on the formation of the indole 4a. 1 equiv. is equal
to approximately 5000 ppm.
When examining the reaction for leaching, it was found that a
small but significant amount of palladium had leached, 25 ppm,
which is a minor fraction of the total palladium used (3-4 % of
the total palladium used). To gain further insight into the
mechanism concerning if the reaction is catalyzed by
homogenous or heterogeneous palladium, a hot filtration test
was preformed, which revealed that the reaction could be
partially catalyzed by homogenous species (31 % NMR yield
after 23 hours). Recycling of the catalyst is one of the most
important advantages associated with heterogeneous catalysis
0
Reaction conditions: 2-iodoaniline (0.25 mmol), alkyne (0.5 mmol), Pd -AmP-
MCF (2.5 mol%), CuI (5 mol%), NEt
3
1.2 equiv., acetonitrile (1 mL) at 40 °C
0
compared to homogeneous. The recyclability of Pd -AmP-MCF
was therefore investigated in the synthesis of the indole 4a from
under Ar for 20h. a. Reaction performed at 60 °C.
3a and phenylacetylene (Scheme 4).
The N-protecting group chosen for the iodoaniline was critical for
the cyclization step, since acetyl, trifluoroacetyl, and Boc-
protected iodoanilines as well as unprotected aniline did not lead
to the desired indole product but rather gave the Sonogashira
products 5a-d, in 65-95% yields (Figure 3).
Scheme 4. Conditions for investigation of the recyclability of the catalyst.
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It was observed that the catalyst could be used for at least 5
cycles with good results (Figure 5). Due to issues with re-
producing the results for the continued cycles, the recycling had
to be performed at 60 °C. The catalyst was recovered simply by
centrifugation and washing. TEM images of the recovered
catalyst showed that the mesoporous siliceous structure was
intact and that the palladium nanoparticles were still well
dispersed, some slight agglomerations of the nanoparticle were
observed (see Figure S1).
synthetize a range of benzofuran and indole derivatives. The
catalyst proved to be tolerant toward a wide range of functional
groups giving diversely substituted heterocycles under mild
reaction conditions in good to excellent yields. Interestingly, a
significant water effect was observed, having an optimum
around 1 equivalent of water. Furthermore, this Pd nanocatalyst
showed an excellent recyclability as it could be reused several
times with a constant activity.
1
00
Experimental Section
90
80
70
60
50
40
30
20
10
0
9
5
91
8
6
85
87
Standard procedure for the heterogeneous Pd-catalysed
synthesis of benzofurans
0
Into a sealable microwave tube were added Pd -AmP-MCF (2.5 mol%,
8
,3 mg), CuI (5 mol%, 2.4 mg), and the o-iodophenol 1a (0.25 mmol, 55
mg), and the reaction mixture was flushed with argon. Acetonitrile (1 mL,
containing 1 equiv. water or 4.5 l/mL), NEt (1.2 equiv., 42 l) and
3
alkyne phenylacetylene (2 equiv., 55 l) were successively added. The
tube was capped and the reaction was heated at 70C temperature
during 20h. The reaction vessel was cooled down and the reaction
mixture was diluted with ethyl acetate, concentrated in vacuo, and
1
2
3
4
5
Cycle
0
purified by silica gel chromatography using pentane to give 41 mg (85%
Figure 5. Representation of the recyclability of Pd -AmP-MCF as catalyst in
the synthesis of indole 4a.
1
yield) of benzofuran 2a as a colorless solid. H NMR (400 MHz, CDCl
3
) δ
7
7
.93 – 7.85 (m, 2H), 7.61 (dd, J = 7.6, 1.4 Hz, 1H), 7.58 – 7.53 (m, 1H),
.48 (dd, J = 8.5, 6.9 Hz, 2H), 7.41 – 7.35 (m, 1H), 7.31 (td, J = 7.7, 1.5
13
Hz, 1H), 7.28 – 7.22 (m, 1H), 7.06 (d, J = 1.0 Hz, 1H). C NMR (101
MHz, CDCl ) δ 156.4, 155.4, 131.0, 129.7, 129.2, 129.0, 125.4, 124.7,
23.4, 121.4, 111.6, 101.8.
A likely mechanism for the formation of the benzofurans and
indoles from the corresponding iodoaromatic compound and
terminal acetylenes is shown in Scheme 5. Sonogashira
coupling of the iodoaromatic compound (1a or 3a) and terminal
acetylene gives the acetylene compound A. A palladium-
catalyzed oxy- or amino-palladation gives the vinylpalladium
intermediate Int-A, which is hydrolyzed by water affording the
product 2 or 4.
3
1
See table 2 for deviating parameters.
Standard procedure for the heterogeneous Pd-catalysed
synthesis of indoles
0
Into a sealable microwave tube were added Pd -AmP-MCF (2.5 mol%,
8
(
,3 mg), CuI (5 mol%, 2.4 mg), and the N-tosyl protected o-iodoaniline 3a
0.25 mmol, 93 mg), and the reaction mixture was flushed with argon.
Acetonitrile (1 mL, containing 1 equiv. water or 4.5 l/mL), NEt (1.2
3
equiv., 42 l) and alkyne phenylacetylene (2 equiv., 55 l) were
successively added. The tube was capped and the reaction was heated
at 40C temperature during 20h. The reaction vessel was cooled down
and the reaction mixture was diluted with ethyl acetate, concentrated in
vacuo, and purified by silica gel chromatography using a EtOAc:pentane
(
gradient 0-10%) to give 83 mg (95% yield) of indole 4a as a colorless
1
solid.. H NMR (400 MHz, CDCl
3
) δ 8.33 (d, J = 8.4 Hz, 1H), 7.52 (dd, J =
.2, 2.5 Hz, 2H), 7.49 – 7.42 (m, 4H), 7.38 (ddd, J = 8.4, 7.2, 1.3 Hz, 1H),
.32 – 7.26 (m, 3H), 7.06 (d, J = 8.1 Hz, 2H), 6.56 (s, 1H), 2.30 (s,
7
7
3
1
1
13
3
H). C NMR (101 MHz, CDCl ) δ 144.6, 142.3, 138.4, 134.8, 132.6,
30.7, 130.5, 129.3, 128.8, 127.6, 126.9, 124.9, 124.4, 120.8, 116.8,
13.7, 21.7.
Scheme 5. Mechanism of Pd-catalyzed transformation of iodoaromatics to
benzofuranes and indoles.
See table 3 for deviating parameters.
Standard procedure for the recycling of the heterogeneous Pd
catalyst
Conclusions
0
We have developed a phosphine-free catalytic system using a
heterogeneous catalyst of Pd nanoparticles supported on
mesocellular foam (Pd –AmP–MCF) which allowed us to
Into a sealable microwave tube were added Pd -AmP-MCF (2.5 mol%,
24,9 mg), CuI (5 mol%, 7.2 mg), and the N-tosyl protected o-iodoaniline
a (0.75 mmol, 279 mg), and the reaction mixture was flushed with argon.
0
3
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Chemistry - A European Journal
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Acetonitrile (3 mL, containing 1 equiv. water or 13.5 l/mL), NEt
equiv., 126 l) and phenylacetylene (2 equiv., 165 l) were successively
added. The tube was capped and the reaction was heated at 60C during
3
(1.2
Institutes of Health, National Institute of General Medical
Sciences (including P41GM103393) . The contents of this
publication are solely the responsibility of the authors and do not
necessarily represent the official views of NIGMS or NIH. We
acknowledge the European Synchrotron Radiation Facility for
provision of synchrotron radiation facilities in using beamline
BM01B.
20h. After completion of the reaction, the reaction vessel was cooled
down and the crude material was transferred to a Falcon tube. The tube
was subsequently centrifuged for 10 min (4000 rpm). The supernatant
liquid was removed and the catalyst was washed with acetone (2x2 mL)
then water (2 mL) and acetone again (2 mL). The washing procedure
was as follows; addition of acetone or water (2 mL) and centrifugation for
Keywords: heterogeneous catalysis • palladium nanoparticles •
10 min followed by removal of supernatant liquid. The washed and dried
benzofurans
Structure
• indoles • Extended X-Ray Absorption Fine
catalyst was reused for the next reaction and NMR yields were measured
between each cycle from the supernatant liquid.
[
1]
a) M. Lounasmaa, A. Tolvanen, Nat. Prod. Rep. 2000, 17, 175-191; b)
V. Sharma, P. Kumar, D. Pathak, J. Heterocycl. Chem. 2010, 47, 491-
Extended X-Ray Absorption Fine Structure parameters
5
02; c) M. Watanabe, W.-T. Su, Y. J. Chang, T.-H. Chao, Y.-S. Wen, T.
II
0
EXAFS measurements on solid Pd -AmP-MCF and Pd -AmP-MCF were
J. Chow, Chem. Asian J. 2013, 8, 60-64; d) H. Khanam,
-
II
performed at the Pd K X-ray absorption edge. Data on Pd -AmP-MCF
were collected at the wiggler beam line 4-1 at the Stanford Synchrotron
Radiation Lightsource (SSRL), Stanford, USA, operating at 3.0 GeV and
97-300 mA (top-up mode), and on Pd -Amp-MCF at beamline BM01B at
Shamsuzzaman, Eur. J. Med. Chem. 2015, 97, 483-504; e) P. Han, X.
Gong, B. Lin, Z. Jia, S. Ye, Y. Sun, H. Yang, RSC Adv. 2015, 5, 50098-
50104; f) G. W. Gribble, in Indole Ring Synthesis, John Wiley & Sons,
Ltd, 2016, pp. 1-38.
0
2
the ESRF, Grenoble, France. The EXAFS stations were equipped with
Si[220] double crystal monochromators. Higher order harmonics were
reduced by detuning the second monochromator crystal to reflect 60 % of
maximum intensity at the end of the scans. Internal energy calibration
was made with a palladium metal foil assigned to 24350 eV (Pd K
[2]
a) M. G. Kadieva, É. T. Oganesyan, Chem. Heterocycl. Compd. (N. Y.,
NY, U. S.) 1997, 33, 1245-1258; b) G. R. Humphrey, J. T. Kuethe,
Chem. Rev. 2006, 106, 2875-2911; c) A. A. Abu-Hashem, H. A. R.
Hussein, A. S. Aly, M. A. Gouda, Synth. Commun. 2014, 44, 2285-2312.
(d) For a recent metal free one-pot synthesis of benzofuranes see: R.
Ghosh, E. Stridfeldt, B. Olofsson, Chem. Eur. J. 2014, 20, 8888-8892.
a) S. Cacchi, G. Fabrizi, Chem. Rev. 2011, 111, PR215-PR283; b) M.
Platon, R. Amardeil, L. Djakovitch, J.-C. Hierso, Chem. Soc. Rev. 2012,
[
11]
edge).
[
3]
All measurements were performed in transmission mode. Four scans
were averaged for each sample after energy calibration by means of the
41, 3929-3968; c) X.-F. Wu, Y. Li, in Transition Metal-Catalyzed
[
12]
Benzofuran Synthesis, Elsevier, 2017, pp. 3-27.
EXAFSPAK program package. The data treatment, including pre-edge
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Acknowledgements
Financial support from the Swedish Research Council (2016-
03897 and 349-2013-580 through the Röntgen Ångström
Cluster), the Berzelii Centre EXSELENT, the European
Research Council (ERC AdG 247014), and the Knut and Alice
Wallenberg Foundation is gratefully acknowledged. Use of the
Stanford Synchrotron Radiation Lightsource, SLAC National
Accelerator Laboratory, is supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences
under Contract No. DE-AC02-76SF00515. The SSRL Structural
Molecular Biology Program is supported by the DOE Office of
Biological and Environmental Research, and by the National
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Pd nanoparticles supported on siliceous mesocellular foam (Pd –AmP–MCF)
were found to efficiently catalyze the reaction of o-iodophenols and protected o-
iodoanilines with acetylenes to produce 2-subsituted benzofurans and indoles,
respectively. In general, the catalytic protocol afforded the desired products in good
to excellent yields under mild reaction conditions without the addition of ligands.
Synthesis of Benzofurans and
Indoles from Terminal Alkynes and
Iodoaromatics Catalyzed by
Recyclable Palladium Nanoparticles
Immobilized on Siliceous
Mesocellular Foam
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