A new hierarchically porous Pd@HSQ monolithic catalyst for Mizoroki-Heck cross-coupling reactions

Article abstract of DOI:10.1039/c3nj01433a

Pore architecture of catalyst supports is an important factor facilitating accessibility of reactants to catalytic sites. This holds the key to improving catalytic activities. Amongst various catalytic reactions, supported Pd nanoparticles-catalyzed C-C cross-coupling reactions have been attracting a great deal of attention in the last decade. Although various supports have been examined, applications of hierarchically porous monolithic materials have never been reported, mainly because of difficulties in multistep synthesis of catalysts. We herein report a novel on-site reduction-based methodology using hierarchically porous hydrogen silsesquioxane (HSQ) monoliths for one-step synthesis of Pd nanoparticles-embedded monoliths (Pd@HSQ). Characterization of these monoliths evidences the on-site reduction, i.e. formation of Pd nanoparticles and conversion of Si-H present in the monolith to Si-O～. Fast, quantitative reduction of Pd²⁺ to Pd(0) to form supported Pd nanoparticles is achieved with preservation of the porous structure of the original monolith, which makes this material attractive as a catalyst for C-C cross-coupling reactions. The obtained Pd@HSQ catalyst has been employed in the Mizoroki-Heck cross-coupling reaction. High accessibility of reactant molecules, undetectable leaching of Pd nanoparticles and easy separation of the monolith from liquid media provide high catalytic activity, reusability and easy handling.

Full text of DOI:10.1039/c3nj01433a

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A new hierarchically porous Pd@HSQ monolithic
catalyst for Mizoroki–Heck cross-coupling
reactions†
Cite this: New J. Chem., 2014,
38, 1144
Nirmalya Moitra,a Ayumi Matsushima,b Toshiyuki Kamei,b Kazuyoshi Kanamori,*a
Yumi H. Ikuhara,c Xiang Gao,c Kazuyuki Takeda,a Yang Zhu,a Kazuki Nakanishi*a
and Toyoshi Shimada*b
Pore architecture of catalyst supports is an important factor facilitating accessibility of reactants to catalytic
sites. This holds the key to improving catalytic activities. Amongst various catalytic reactions, supported
Pd nanoparticles-catalyzed C–C cross-coupling reactions have been attracting a great deal of attention
in the last decade. Although various supports have been examined, applications of hierarchically porous
monolithic materials have never been reported, mainly because of difficulties in multistep synthesis of
catalysts. We herein report a novel on-site reduction-based methodology using hierarchically porous
hydrogen silsesquioxane (HSQ) monoliths for one-step synthesis of Pd nanoparticles-embedded monoliths
(Pd@HSQ). Characterization of these monoliths evidences the on-site reduction, i.e. formation of Pd
nanoparticles and conversion of Si–H present in the monolith to Si–OB. Fast, quantitative reduction of
Pd2+ to Pd(0) to form supported Pd nanoparticles is achieved with preservation of the porous structure of
the original monolith, which makes this material attractive as a catalyst for C–C cross-coupling reactions.
The obtained Pd@HSQ catalyst has been employed in the Mizoroki–Heck cross-coupling reaction. High
accessibility of reactant molecules, undetectable leaching of Pd nanoparticles and easy separation of the
monolith from liquid media provide high catalytic activity, reusability and easy handling.
Received (in Porto Alegre, Brazil)
18th November 2013,
Accepted 2nd January 2014
DOI: 10.1039/c3nj01433a
www.rsc.org/njc
Such aggregation of nanoparticles can be prevented either
by using capping agents10 or immobilizing them on a solid
Introduction
The C–C bond formation reactions using Pd catalysts have become a support,11,12 while the latter offers more attractive advantages
standard repertoire1,2 in organic chemistry to synthesize compounds of higher reusability and recyclability,13 which impart a huge
for pharmacological,3–5 biological,6 or optical7 applications. impact from environmental and economic points of view.14
In particular, Pd nanoparticles as a heterogeneous catalyst A lot of research has therefore been devoted to engineering
have been attracting a great deal of attention due to their easier supported catalysts to achieve high catalytic activity, facile
separation from reaction systems, which is otherwise difficult separation, reusability and recyclability.15–18 Periodic mesoporous
when using Pd complexes or salts as homogeneous catalysts.8 materials,19–22 magnetic nanoparticles23 and porous polymers24–26
However, these nanoparticles generally undergo aggregation to have been studied extensively as supports. Among all the support
form bulky particles in solution, which decreases the surface morphologies, hierarchically porous monoliths containing well-
to volume ratio and results in reduced catalytic activities.9 defined three-dimensional open macro-mesoporous architecture,
which facilitates the transfer of reactant molecules to catalytic sites
and easy separation of the catalysts from reaction media, possess
a Department of Chemistry, Graduate School of Science, Kyoto University,
huge potential to act as catalyst supports.27 However, examples
Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
of such monolithic materials used as catalysts for C–C coupling
E-mail: kanamori@kuchem.kyoto-u.ac.jp, kazuki@kuchem.kyoto-u.ac.jp;
reactions are rare.28 To the best of our knowledge, only a
Fax: +81-75-753-7673; Tel: +81-75-753-7673
b Department of Chemical Engineering, Nara National College of Technology,
22 Yata-cho, Yamatokoriyama, Nara 639-1080, Japan.
few reports are known. One of which describes a three-step
methodology involving thiol functionalization on hierarchically
porous silica prepared via high internal phase emulsion (denoted
as Si(HIPE)), followed by introduction of Pd2+ and reduction of it by
sodium borohydride (NaBH4) to form Pd nanoparticle-immobilized
Si(HIPE).28 Another report uses ionic liquids to immobilize the
E-mail: shimada@chem.nara-k.ac.jp; Fax: +81-743-55-6454; Tel: +81-743-55-6454
c Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan
† Electronic supplementary information (ESI) available: Characterization of the
original HSQ monolith, FT-IR of materials, and NMR of isolated compounds. See
DOI: 10.1039/c3nj01433a
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PEO in the sol–gel system results in controlled phase separa-
tion tendency, i.e. controlled size of macroporous structures.
A scanning electron micrograph of a dried monolith used in this
study shows the presence of the macroporous co-continuous
structure (Fig. S1a, ESI†). The high connectivity of macropores is
critical for the facilitation of fluid transport, and this kind of
monolith is already applied to separation media.35,36 A sharp
size distribution of macropores with a most probable size of
1.6 mm was confirmed by mercury porosimetry (Fig. S1b, ESI†).
Mesopores (o2 nm, including ‘‘micropores’’) in the macropore
skeletons were characterized by nitrogen adsorption–desorption
measurements (Fig. S1c, ESI†). These small pores contribute to
high specific surface area (800 m2 gÀ1) of the HSQ monolith. The
significance of the HSQ monolith lies in the complete preserva-
tion of reactive Si–H bonds as evidenced by 29Si solid-state NMR
(Fig. S1d, ESI†). The NMR spectrum mostly consists of T3 with
only a small amount of T2 (Tn stands for the silicon species with
HSi(OSi)nX3Àn, where X = OH or OCH3, representative structures
shown in Fig. 2e), and no Q species (SiX4, where X = OSi, OH or
OCH3) are found. These abundant Si–H groups on the pore
surface can be used for reduction of Pd2+. Furthermore, the ratio
Fig. 1 (a) Schematic representation of on-site reduction of PdCl2 on a
HSQ monolith; simultaneous transformation of Pd2+ to Pd(0) nano-
particles and Si–H to Si–OB. (b, c) Digital camera images of the HSQ
monolith right after immersing in PdCl2 solution and after reacting for 3 h,
respectively.
Pd catalyst on Si(HIPE).29 In these examples, however, multi-
step catalyst synthesis limits their applicability.
In synthetic organic chemistry, hydrosilanes have been used
as efficient reducing agents because of their hydride donating
capacity.30 Their potential in reducing noble metal salts, however,
has long been overlooked, and only a few reports disclosed their
applicability.31–33 Very recently, we have reported the synthesis of
hierarchically porous hydrogen silsesquioxane monoliths (HSQ) via
a sol–gel reaction accompanied by phase separation.34 We herein
show that this solid-state analogue of hydrosilanes can reduce Pd2+
in aqueous media to Pd(0) via a host–guest redox mechanism on
the pore surfaces (on-site reduction, a scheme shown in Fig. 1a).
Characterization of the as-synthesized catalyst-embedded monolith
provides clear evidence of on-site reduction and embedment of
Pd(0) nanoparticles confined in the pores.
Applications of this monolith to the Mizoroki–Heck cross-
coupling reaction unveil high catalytic activity and easy separation
of the catalyst. A variety of substrates were examined in order to test
the catalytic and substrate tolerances. Furthermore, high enough
reusability of the catalyst was confirmed without decreasing the
catalytic activity. These features are made possible by facile access
of reactant molecules to catalytic sites, easy separation from the
system, and undetectable leaching of Pd nanoparticles.
Fig. 2 Characterizations of the obtained Pd@HSQ monolith. (a) SEM image
of Pd@HSQ, showing the preservation of macroporous co-continuous
structure after on-site reduction. (b) XRD pattern after the on-site reduction,
showing the presence of Pd(0) by its corresponding Bragg diffractions.
(c) Nitrogen adsorption–desorption isotherms, showing the decrease in
specific surface area (800 to 420 m2 gÀ1) from HSQ to Pd@HSQ. (d) Another
evidence of the on-site reduction by 29Si solid-state NMR, showing the
appearances of Q3 and Q4 signals in Pd@HSQ due to transformation
of O1.5Si–H to O1.5Si–OH and O1.5Si–O–SiO1.5 moieties. (e) Schematic
representation of T and Q centers comprising HSQ and Pd@HSQ. (f) Digital
camera image of the as-dried Pd@HSQ monolith.
Results and discussion
HSQ monolith as the host
As reported previously,34 the hierarchically macro-mesoporous
HSQ monolith was prepared by a dilute acid-catalyzed sol–gel
reaction of trimethoxysilane (HTMS) accompanied by phase
separation in the presence of poly(ethylene oxide) (PEO). The
acidic condition is advantageous to preserving the reactive Si–H
groups as described later, and a change in the concentration of
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of T3 in the original monolith is sufficiently high (87%, corresponds
to 96% crosslinking density), which promises mechanical stability
of this material.
Host–guest redox for Pd@HSQ synthesis
The obtained HSQ monolith with the preserved Si–H moieties
was subjected to reduction of Pd2+ (as PdCl2) to Pd(0) by
immersing the monolith in an aqueous solution containing a
desired concentration of PdCl2. An instant color change in
the monolith and hydrogen evolution indicate the successful
proceeding of on-site reduction. The oxidation of Si–H to
Si–OB occurs simultaneously with reduction of Pd2+ and hydrogen
formation (eqn (1) and Fig. S2, ESI†).
2O1.5Si–H + Pd2+ + 2H2O - 2O1.5Si–OH + Pd(0) + H2 + 2H+
(1)
Fig. 3 Characterizations of Pd(0) nanoparticles in Pd@HSQ by HAADF-STEM.
(a) Low-magnification image showing the presence of nanoparticles embedded
in the macropore skeletons of the HSQ monolith. (b–d) Small nanoparticles
confined in the meso- and micropores of the Pd@HSQ monolith.
Termination of the reaction after 3 h was assumed by
disappearance of the solution color and the end of the hydrogen
evolution (Fig. 1b and c). This monolith obtained through the
reduction of Pd2+ will be referred hereafter as Pd@HSQ. Even
after the reduction, preservation of macroporous co-continuous
structure, which is important for fast transfer of reactants, was
revealed by the SEM image of Pd@HSQ (Fig. 2a). The formation
of Pd(0) particles is confirmed by XRD by appearance of sharp
peaks corresponding to metallic Pd(0) (Fig. 2b). An average
particle size of 20 nm was deduced from the XRD by applying
the Scherrer equation. Although specific surface area and pore
size decreased in Pd@HSQ (Fig. 2c) because of partial blocking
of meso- and micropores by as-formed Pd nanoparticles, the
decrease is not detrimental to its application as a catalyst as
shown later. The oxidation of Si–H to Si–OB can be confirmed
using 29Si solid-state NMR on Pd@HSQ by appearance of the
Q signals (Fig. 2d). Disappearance of T2 and a decrease in T3
(87 to 61%) with appearance of Q3 (22%) and Q4 (17%) because
of the oxidation of (SiO)3SiH to (SiO)3SiOH and condensation of
as-formed Q3 silanols, respectively, were detected. These results
provide strong evidence of the on-site reduction via host–guest
redox chemistry in the porous monolith.
Interestingly, changes in base and solvent from TEA to K2CO3 and
from acetonitrile to DMF, respectively, improved the catalytic
performance of Pd@HSQ. The turnover frequency (TOF, the mole
amount of converted reactant molecules in 1 h per mole of the
catalyst) value was 1.0 Â 10 hÀ1 at a reaction temperature of 100 1C
(entry 2). A decrease in catalyst loading to 0.5 mol% under identical
reaction conditions yields a slightly higher TOF value for the
Pd@HSQ catalyst (entry 3). Reaction temperature plays a crucial
role in increasing the reactivity of the catalyst in most of the cases.
Indeed, an increase in reaction temperature from 100 to 130 1C
while maintaining other reaction parameters constant results in a
jump of the TOF value from 1.4 Â 10 to 1.0 Â 102 hÀ1
.
We then examined versatility of the Pd@HSQ catalyst in cross-
coupling of various aryl halides and n-butyl acrylate. Results are
summarized in Table 2; all the reactions were performed under
optimized reaction conditions to achieve the maximum catalytic
activity. The reaction rate and catalytic activities are not influenced
by electronic properties of the substituents on the aromatic ring
of the starting material. Highly electron donating substituents
such as 4-methoxy (entry 7) and 4-amino (entry 8) do not
change the catalytic activity. Similar results were obtained with
electron withdrawing substituents such as 4-nitro (entry 10)
and 4-acetate (entry 11). Functional groups with weak donating/
withdrawing properties like 4-bromo (entry 12), 4-ethyl acetate
(entry 13), 3-methoxy (entry 9) and 3-methyl (entry 6) groups
The formation of the supported Pd(0) nanoparticles in
Pd@HSQ is also confirmed by HAADF-STEM measurements of
the Pd@HSQ monolith as shown in Fig. 3. A broad size distribution
from 50 to 1 nm for supported Pd nanoparticles was observed.
The as-formed nanoparticles are embedded in the monolithic
support by physical confinement.
Catalytic performances
The Pd@HSQ monolith was then evaluated as a catalyst for the naturally do not change the reactivity.
Mizoroki–Heck coupling reaction between 4-iodotoluene and
In order to investigate effects of the kind of halogen sub-
n-butyl acrylate (Table 1, 1H and 13C NMR spectra of all the stituents on the coupling reaction, we also tested bromo- and
products are given in the ESI†). In order to achieve the highest chloro-derivative aromatics with n-butyl acrylate. A lower catalytic
catalytic activity, conversion and yield, the reaction conditions performance was obtained for the coupling reaction of 4-bromo-
were initially optimized by means of changing catalyst loading, toluene (entry 14) compared to 4-iodotoluene (entry 4). Interestingly,
base, solvent and reaction temperature. The catalytic activity a slight increase in catalytic activity was observed while performing
in the coupling reaction was low in the presence of 2 mol% the reaction with highly deactivated 4-nitrobromobenzene (entry 15).
catalyst and triethylamine (TEA) as base at 100 1C (entry 1). A further increase in electronegativity of the halogen substituent
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Table 1 Conditions of the Mizoroki–Heck cross-coupling reaction of 4-iodotoluene and n-butyl acrylate
Entry
Pd mol% in Pd@HSQ
Base
Solvent
Temperature/1C
Reaction time/h
Conversion/%
Yield/%
TOF/hÀ1
1
2
3
4
2.0
1.0
0.5
0.5
TEA, 2 eq.
CH3CN
DMF
DMF
100
100
100
130
72
10
14
2
73
100
100
100
69
98
98
99
5.1 Â 10À1
1.0 Â 10
1.4 Â 10
1.0 Â 102
K2CO3, 2 eq.
K2CO3, 2 eq.
K2CO3, 2 eq.
DMF
Table 2 The Mizoroki–Heck cross-coupling reaction of various aryl
halides and n-butyl acrylate catalyzed by Pd@HSQ
Reaction
time/h
Entry
R
X
Conversion/% Yield/% TOF/hÀ1
5
6
7
8
H
I
I
I
I
I
I
I
I
I
2
2
2
2
2
2
2
2
2
100
100
100
100
100
100
100
100
100
16
99
99
98
99
99
98
98
97
97
15
80
0
1.0 Â 102
1.0 Â 102
1.0 Â 102
1.0 Â 102
1.0 Â 102
1.0 Â 102
1.0 Â 102
1.0 Â 102
1.0 Â 102
6.7 Â 10À1
1.2 Â 10
0
3-Me
4-MeO
4-NH2
3-MeO
4-NO2
4-Ac
9
10
11
12
13
14
15
16
4-Br
4-COOEt
4-Me
4-NO2
4-Me
Br 48
Br 14
Cl 48
Fig. 4 Reusability test results of the Pd@HSQ catalyst. (a) Conversion
versus cycle number of the Pd@HSQ catalyst for 10 consecutive cycles.
(b) SEM image of the Pd@HSQ catalyst after 10-time reuse. (c) XRD patterns
of the Pd@HSQ catalyst before and after 10-time reuse. (d) Nitrogen
adsorption–desorption isotherms of the Pd@HSQ catalyst before and after
10-time reuse.
82
0
from bromo to chloro (entry 16) inhibits the proceeding of C–C
coupling due to lower reactivity, demonstrating the limitation
of the Pd@HSQ catalyst.37–39
slight increase in specific weight due to the oxidation of the
HSQ support in the course of reusing (Fig. 4d and Fig. S3, ESI†).
All of these test results clearly demonstrate an excellent
catalytic performance in terms of activity, stability, reusability
and robustness. The new Pd@HSQ monoliths can be prepared
and used by the facile processes and can be used many times
without deteriorating catalytic activity and the porous structure.
It can be concluded that the newly developed Pd@HSQ catalyst
is one of the ideal catalysts for C–C cross-coupling reactions.
In addition to the catalytic properties investigated above,
environmental advantages such as recyclability/reusability have
to be taken into consideration to develop a practical catalyst
material. Reusability of the Pd@HSQ was therefore investigated
using the previously mentioned model reaction. As the catalyst
is embedded in the monolith, recovery tests were performed just by
taking out the Pd@HSQ from the reaction mixture, washing with
solvents and drying at 40 1C, and then the monolith was subjected
to the next cycle. The reusability tests for ten times did not show
any indication of a decrease in the catalytic activity; the TOF values
of 1.0 Â 102 hÀ1 were almost unchanged at all cycle numbers
(Fig. 4a). Moreover, elemental analysis of the product showed no
Conclusions
indication of leaching of Pd, which is particularly important for A single-step three-hour process for synthesizing Pd nanoparticles-
preserving the catalytic activity of Pd@HSQ. A detailed character- embedded HSQ monoliths using an on-site reduction methodology
ization of the recovered catalyst after ten repetitions was performed. is demonstrated. This methodology is based on host–guest redox
Preservation of macroporous co-continuous structure was observed consisting of reduction of Pd2+ to Pd(0) and oxidation of Si–H to
in the SEM image of the Pd@HSQ repetitively used 10 times Si–OB in aqueous solution. The nanoparticles reduced in hier-
(Fig. 4b). Preservation of Pd nanoparticles in their complete loading archically porous HSQ are simultaneously immobilized (confined)
and entirety as well as unchanged average particle size was in the HSQ host with preservation of its microstructure, making
demonstrated by XRD measurements; diffraction patterns it a suitable material for catalytic applications. Quantitative
including the peak heights and widths are almost the same reduction of Pd2+ to Pd(0) is advantageous to change the
before and after the 10-time reuse (Fig. 4c). A slight decrease of catalyst loading just by treating with a different amount of
specific surface area after reusing for 10 times is attributed to a metal salt. The average Pd particle size of 20 nm is confirmed
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by XRD and HAADF-STEM, and the access of reactants to the room temperature and the monolithic catalyst was taken out.
catalytic sites is facilitated through its open pore architecture. The reaction was quenched by addition of water, followed by
The as-synthesized Pd@HSQ monolith was then subjected to the extraction with diethyl ether. The organic layer was dried over
Mizoroki–Heck cross-coupling reaction, in which diverse kinds MgSO4 and evaporated to obtain the crude product, which was
of aromatic halides were tested to examine the applicability of then purified by silica gel column chromatography.
the catalyst. Regardless of the substituent groups in aromatic
Characterization
iodides, very high catalytic activity is confirmed in the model
reactions. A decrease in catalytic activity was, however, observed
with increasing electronegativity of the halogen substituent.
Facile recovery and excellent reusability of the Pd@HSQ
catalyst are confirmed by undetectable leaching of the catalyst
from the monolith and unchanged catalytic activity even after 10
repetitive uses. This feature is made possible by multiple factors
such as the facile access of reactants to the catalytic sites, physical
confinement of the nanoparticles in the meso- and micropores and
the preservation of meso- and microporosity of the Pd@HSQ
even after 10-time reuse. With conceivable scalability, these
results suggest the high potential of the Pd@HSQ catalyst in
various organic reactions even on industrial scales.
Microstructures of the fractured surfaces of the samples were
observed under a scanning electron microscope (JSM-6060S,
JEOL Ltd. (Japan)). Macropore size and volume in the samples
were characterized by using a mercury porosimeter (Pore Master
60-GT, Quantachrome Instruments (USA)). The meso- and micro-
pores present in the samples were analyzed by nitrogen adsorp-
tion–desorption measurements (BELSORP-mini II, BEL Japan,
Inc. (Japan)). The samples were degassed at 200 1C for 6 h before
each measurement. The solid-state 29Si single pulse NMR experi-
ment was performed on an OPENCORE NMR40,41 at 299.52 MHz
1
(for H) with a contact time of 10 ms using a 5 mm probe at
5 kHz. The X-ray diffraction patterns of the Pd@HSQ were
recorded using a powder X-ray diffractometer (RINT Ultima III,
Rigaku Corp. (Japan)) and using Cu Ka (l = 0.154 nm) as an
incident beam. To characterize the size of the Pd nanoparticles
and their spatial distributions within the monoliths, high angle
annular dark-field (HAADF)-scanning transmission electron
microscopy (STEM) was employed. The composite powder samples
were placed on a Cu-grid after grinding the bulk samples and were
examined by Z-contrast HAADF-STEM. The high-resolution TEM
JEM-2100F (JEOL Ltd. (Japan)) was operated at 200 kV with an STEM
unit equipped with a spherical-aberration corrector (CEOS GmbH
(Germany)), which provides a minimum probe of about 0.1 nm in
diameter. During HAADF-STEM imaging, a probe convergence angle
of 25 mrad and an annular dark-field detector with an inner
angle higher than 52 mrad were used. Liquid-state 1H and
13C NMR measurements were performed using an ECX-400
spectrometer (JEOL Ltd. (Japan)) in CDCl3.
Experimental
Synthesis of the HSQ monolith
The HSQ monolith with well-defined macroporous structure
was synthesized by the previously published procedure with
some modifications. In short, to 25 mL of a 1 : 1 (in volume)
mixture of MeOH and 0.05 M HNO3 aq., 1.05 g of PEO with
number average molecular weight of 35 000 was added and
stirred at room temperature for 30 min to dissolve completely.
To this mixture was added 10.50 mL of HTMS and stirred for
2 min for hydrolysis. After stopping stirring, the resultant
transparent solution was transferred to a cylindrical tube with
a diameter of 5.5 mm and a length of 225 mm, and left at room
temperature for gelation. Gelation occurred in 15 min, and the
as-formed gel was aged at room temperature for 24 h. The
column-shaped gel was taken out from the tube and washed
with methanol (3 times, 8 h every time). It was then dried at
40 1C for 12 h in air.
Acknowledgements
The present work was financially supported by the Advanced Low
Carbon Technology Research and Development Program (ALCA)
from the Japan Science and Technology Agency (JST), and by a
research grant from The Kyoto Technoscience Center.
Synthesis of Pd@HSQ
Typically, 50 mg (B35 mm length and 5.5 mm diameter) of
as-synthesized HSQ was immersed in 5 mL of distilled water,
and 0.2 mL of 50 mM PdCl2 (0.01 mmol Pd) solution was added
to this mixture. The solution was allowed to stand for 3 h at
room temperature. The monolith was then taken out of the
solution and solvent-exchanged with methanol followed by
drying at 40 1C for 12 h.
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The as-synthesized Pd@HSQ containing different amounts
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was dipped in a solution containing aromatic halide (2 mmol),
n-butyl acrylate (3 mmol) and a base (TEA or K2CO3, 4 mmol) in
DMF. The mixture was allowed to stand at desired temperature.
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