A supported palladium nanocatalyst for copper free acyl Sonogashira reactions: One-pot multicomponent synthesis of N-containing heterocycles

Article abstract of DOI:10.1039/c1gc15869d

A convenient method has been developed for the synthesis of palladium nanoparticles (PdNPs) embedded into a polymer matrix, PPS [PPS = poly(1,4-phenylene sulfide)], via the thermolysis of palladium acetate. X-ray photoelectron spectroscopy (XPS) studies c

Full text of DOI:10.1039/c1gc15869d

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PAPER
A supported palladium nanocatalyst for copper free acyl Sonogashira
reactions: One-pot multicomponent synthesis of N-containing heterocycles†
Subhankar Santra,a Kalyan Dhara,a Priyadarshi Ranjan,a Parthasarathi Bera,b Jyotirmayee Dash*a and
Swadhin K. Mandal*a
Received 18th July 2011, Accepted 24th August 2011
DOI: 10.1039/c1gc15869d
A convenient method has been developed for the synthesis of palladium nanoparticles (PdNPs)
embedded into a polymer matrix, PPS [PPS = poly(1,4-phenylene sulfide)], via the thermolysis of
palladium acetate. X-ray photoelectron spectroscopy (XPS) studies confirmed the presence of
strong sulfur coordination of PPS to the palladium nanoclusters. These palladium nanoparticles
(PdNPs-PPS) have been used as an efficient heterogeneous nanocatalytic system for the copper
free acyl Sonogashira reaction. A wide range of ynones was synthesized in high yields under mild
reaction conditions. The catalyst was recovered and recycled up to four times. Transmission
electron microscopy (TEM) images revealed that the catalyst maintained nanospheric dimensions
for four consecutive catalytic cycles. This simple protocol was further explored in a one-pot
multicomponent synthesis of 2,4-disubstituted pyrimidines and a tetrahydro-b-carboline
derivative in improved yield.
Significant progress in this area has been achieved with a va-
Introduction
riety of homogeneous palladium catalysts.5,7,9,10 Homogeneous
The “ynones” are versatile building blocks for the synthesis
of pharmaceutically significant and biologically active N-hetero-
catalysis experiences the inherent problem of separating the
expensive catalyst from the product for re-use. In addition, the
cyclic compounds, such as pyrroles,1 pyrazoles,2 isoxazoles,3
homogeneous palladium catalysts tend to lose their catalytic
pyrimidines,4,5 quinolines,6 and tetrahydro-b-carbolines.7 Com-
mon methods for the preparation of ynones utilize the pal-
activity because of palladium metal aggregation and precipita-
tion. Besides, the use of a copper co-catalyst in Sonogashira
ladium catalyzed coupling of terminal alkynes with an acid
coupling reactions facilitates the homocoupling of alkynes,
chloride (acyl Sonogashira reaction)8,9 or with organic halides
which in turn makes the separation of the products more
in the presence of carbon monoxide (carbonylative Sonogashira
tedious. Considering both environmental and economic issues,
reaction).10–12 Most of these studies exploited well-defined
there is an urge to develop copper free palladium catalysts13
palladium catalysts except one reported by Liu et al. where
palladium nanoparticles impregnated with Fe3O4 nanoparticles
that can be easily recovered and efficiently reprocessed while
restoring the activity of the catalyst. The heterogenization of
have been used for carbonylative Sonogashira reactions at
the homogeneous palladium catalyst is an attractive solution
high temperature (130 ◦C) and pressure of carbon monoxide
to overcome such troubles by employing metal nanoparticles
(2 MPa).12 However, acyl Sonogashira reaction remains the more
on a solid support. Metal nanoparticles have attracted much
straightforward process for the generation of ynones avoiding
attention over the last decade due to their potential tunability
poisonous carbon monoxide gas and it can also be extended
in terms of size, shape and activity. This makes them par-
to design sequential reactions in a one-pot fashion leading to
ticularly interesting in a variety of applications, especially in
pharmaceutically important heterocycles.1–7
electronics,14 information technology,15 biomedical sciencess16
and catalysis.17 Many different materials have been employed
to support nanoparticles, such as mesoporous silicas, activated
carbons, bio-polymers etc.18 Moreover, the choice of support
aDepartment of Chemical Sciences, Indian Institute of Science Education
and Research, Kolkata, Mohanpur, 741252, India.
plays an important role in catalytic activity as well as in
multicomponent and domino syntheses since it can help in
holding the active catalyst in its surface by strong coordination
during catalysis. This particular property of the solid support
also encourages the development of reusable heterogeneous
nanocatalysts.
E-mail: jyotidash@iiserkol.ac.in,swadhin.mandal@iiserkol.ac.in;
Fax: (+)91-33-48092033; Tel: 91-9903676563
bSurface Engineering Division, National Aerospace Laboratories,
Bangalore, 560017, India
† Electronic supplementary information (ESI) available: Characteriza-
tion data, NMR spectra of compounds and size distribution histograms
of nanoparticles. See DOI: 10.1039/c1gc15869d
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As a part of our ongoing interest in developing palladium
nanocatalysts for different organic transformations,19 herein,
we demonstrate a simple method to synthesize palladium
nanoparticles stabilized in a polymer matrix, PPS [PPS = poly-
(1,4-phenylene sulfide)], avoiding the use of any external haz-
ardous reducing agents. The PdNPs-PPS showed efficient cat-
alytic behaviour towards the copper free Sonogashira reaction
of acid chlorides 1 with a variety of terminal alkynes 2 to give
the corresponding alkynones 3 in excellent yields under mild
conditions (Scheme 1). The catalyst was recovered and recycled
up to four times. Additionally, 2,4-disubstituted pyrimidines
4 and tetrahydro-b-carboline derivative 5 were synthesized in
high yields through multicomponent and sequential one-pot
processes (Scheme 1). To the best of our knowledge, this is
the first report emphasizing the synthesis of heterogeneous
and recyclable palladium nanoparticles for acyl Sonogashira
coupling reaction in copper free reaction conditions, leading to
a one-pot multicomponent synthesis of biologically important
N-heterocyclic ring systems with enhanced yields over those
reported earlier.
electron microscopy (TEM) imaging were prepared by placing
a 5 mL drop of an aqueous dispersion containing palladium
nanoparticles on a carbon-coated copper grid and allowing it
to dry in the air. EDX analysis on the TEM was performed
using an Oxford Instruments ISIS 300 system with a silicon
detector and beryllium window. ICP-AES data was recorded on
a Perkin Elmer Optima 7000 DV. All products were isolated by
chromatography on a silica gel (100–200 mesh) column using
the required ratio of hexane and ethyl acetate. The known
1
compounds were characterized by comparing their H and 13C
NMR spectra to the reported data and the relevant spectra
are presented in the Supporting Information.† The synthesis
of palladium nanoparticles, as well as all catalytic reactions, was
carried out under an atmosphere of dry nitrogen using standard
Schlenk line techniques.
Details of XPS study
XPS of PdNPs-PPS and PPS samples were recorded with a
Thermo Fisher Scientific Multilab 2000 (England) spectrometer
using non-monochromatic Mg-Ka radiation (1253.6 eV) run
at 15 kV and 10 mA as X-ray source. The binding energies
reported here were calculated with reference to the C1s peak at
284.5 eV with a precision of 0.1 eV. For XPS analysis, powder
samples were mounted on the sample holder and placed into
an ultrahigh vacuum (UHV) chamber at 10-9 Torr housing the
analyzer. Before placing the samples into the analyzing chamber,
theywere keptinthe preparationchamber at UHV (10-9 Torr) for
5 h in order to desorb any volatile species present on the surface.
All the spectra were obtained in the digital mode with Avantage
software on a personal computer with 30 eV pass energy
across the hemispheres of the electron analyzer and 0.05 eV
step increment. The experimental data were curve fitted with
Gaussian peaks after subtracting a linear background employ-
ing PeakFit v4.11 program. The spin–orbit doublet separation,
doublet intensities and full width at half maximum (FWHM)
of Pd(3d5/2,3/2) and S(2p3/2,1/2) peaks were fixed according to
the literature. The concentrations of different chemical states
were evaluated from the area of respective Gaussian peaks. The
relative surface concentration of several constituent elements
present in the sample could be obtained from area under the
peak, photoionization cross-section, inelastic mean free path
and analyzer detection efficiency.
Scheme 1 The PdNPs-PPS catalyzed acyl Sonogashira reaction and
one-pot synthesis of N-heterocycles.
Experimental
Materials and instruments
All starting materials were obtained from commercial suppliers
and used as received. Solvents were purified by standard
procedures. Experiments were generally carried out under a
Synthesis of PPS-supported palladium nanoparticles
(PdNPs-PPS)
1
dry nitrogen atmosphere. H NMR spectra were recorded at
500 MHz using Bruker AVANCE 500 MHz and JEOL 400 MHz
instruments at 278 K. Chemical shifts (d) are reported in ppm
downfield from tetramethylsilane with the solvent as the internal
reference (CDCl3: d 7.26 ppm). Data is reported as follows:
chemical shift, multiplicity (s = singlet, d = doublet, t = triplet,
q = quartet, p = pentet, br = broad, m = multiplet), coupling
constants (Hz), and integration. 13C NMR spectra were recorded
on either a JEOL-400 (100 MHz), or a Bruker AVANCE
500 MHz (125 MHz) with complete proton decoupling. Mass
spectra were recorded on a micromass Q-Tof microTM, Waters.
Powder X-ray diffraction (PXRD) data were collected on a
Rigaku Smart Lab diffractometer. TEM analyses were per-
formed in bright-field mode using a JEOL 1200 EX and/or
JEOL JEM 2010 electron microscope. Samples for transmission
Palladium acetate (100 mg, 0.45 mmol) and poly(1,4-phenylene
sulfide) (500 mg, 0.05 mmol, Mn ~10 000) were kept in a 100 mL
Schlenk flask under vacuum (~10-2 mbar) for 0.5 h. In terms of
the repeating 1,4-phenylene sulfide (PS) units, nearly 10 PS units
have been used per palladium center. Anhydrous toluene (35 mL)
was added under a nitrogen atmosphere. The mixture was heated
at 95 ◦C for 4 h, and then cooled to 25 ◦C under a nitrogen
atmosphere, then filtered through a 0.22 mm PTFE membrane
filter paper. The black residue was washed several times with
ethanol (15 mL ¥ 4) until the filtrate became colorless and finally
twice with acetone (15 mL ¥ 2). The palladium nanoparticles
(PdNPs-PPS) were finally dried under vacuum (~10-2 mbar) for
12 h before performing any catalytic reaction.
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Table 1 Binding energy, relative intensity, spin–orbit separation (D) and FWHM of different palladium and sulfur containing species observed in
PdNPs-PPS
Binding energies of
Pd(3d5/2) and S(2p3/2) (eV)
Relative intensities of
Pd and S species (%)
D of Pd(3d5/2,3/2) and
(S2p3/2,1/2) (eV)
FWHM of Pd(3d5/2
and S(2p3/2) (eV)
)
Element
Species
Pd
S
Pd0
335.1
336.7
162.3
163.7
166.6
67
33
55
25
20
5.20
5.25
1.18
1.16
1.18
1.63
2.93
1.97
1.97
2.01
Pd2+
Bound S
Unbound S
Oxidized S
Table 2 Acyl Sonogashira reaction catalyzed by PdNPs-PPSa
Entry
1
Acid chloride, 1
Alkyne, 2
Ynone, 3
Temp (◦C)
Yield (%)b
65
25
2
50
98
3
4
5
50
50
25
98
98
98
6
7
8
9
50
50
50
50
57
94
98
15
10
11
50
50
60
56
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Table 2 (Contd.)
Entry
12
Acid chloride, 1
Alkyne, 2
Ynone, 3
Temp (◦C)
50
Yield (%)b
55
13
14
50
50
64
74
15
50
92
16
17
50
25
54
96
a Reaction conditions: PdNPs-PPS (50 mg) was used together with acid chlorides 1 (3 mmol), terminal acetylenes 2 (2 mmol) and triethylamine
(3 mmol) in 10 mL toluene. b Isolated yields of the products after purification through column chromatography.
Catalytic acyl Sonogashira reaction
in a 50 mL Schlenk flask under vacuum (~10-2 mbar) for
0.5 h. The (TMS)-ynone moiety was synthesized using acid
chloride (1 mmol), trimethylsilylacetylene (1 mmol) and dry
triethylamine (1 mmol) in dry acetonitrile (~5 mL) following the
acyl Sonogashira procedure mentioned above. After completion
of the first step, the reaction mixture was cooled to 25 ◦C. Sodium
bicarbonate (3.5 mmol) was added to it followed by the addition
of guanidine hydrochloride (2.5 mmol) and methanol (5 mL)
at 25 ◦C. The reaction mixture was then refluxed for 14–16 h
until the ynone was completely consumed (monitored by TLC).
After completion it was cooled to 25 ◦C, filtered through a
0.22 mm PTFE membrane filter paper, washed several times
with ethyl acetate (15 mL ¥ 4). After complete evaporation of
the combined filtrate under reduced pressure, the residue was
then purified through column chromatography (silica gel 100–
200 mesh) using the appropriate ratio of hexane and ethyl acetate
(see ESI† for more details).
50 mg of the pre-synthesized PdNPs-PPS was kept in 50 mL
Schlenk flask under vacuum (~10-2 mbar) for 0.5 h. Dry toluene
(10 mL) was added to it followed by the addition of acid chloride
1 (3 mmol), alkyne 2 (2 mmol) and dry triethylamine (3 mmol)
consecutively under nitrogen atmosphere. Then the catalytic
reactions were performed under appropriate temperature as
mentioned in Table 2. After completion, the reaction mixture
was cooled to 25 ◦C, filtered through a 0.22 mm PTFE membrane
filter paper, and washed several times with ethyl acetate (15 mL
¥ 4). The combined filtrate was completely evaporated under re-
duced pressure free from any residual solvent and extracted with
ethyl acetate (20 mL ¥ 3). The ethyl acetate extract was washed
twice with water (20 mL ¥ 2), once with brine solution (20 mL),
dried over anhydrous sodium sulfate and concentrated under
reduced pressure. The residue was then purified through column
chromatography (silica gel 100–200 mesh) using the appropriate
ratio of hexane and ethyl acetate (see ESI† for more details).
Synthesis of tetrahydro-b-carboline system
Synthesis of pyrimidine system
Tetrahydro-b-carboline derivative 5 was synthesized following a
coupling–amination–azaannulation–Pictet–Spengler (CAAPS)
sequence. The key intermediate ynone was first synthesized as
previously described using 25 mg of the catalyst (PdNPs-PPS),
with thiophene-2-carbonyl chloride (0.11 mL, 1 mmol), phenyl
2,4-Disubstituted pyrimidines (4) were obtained in good
yields from trimethylsilyl-ynones [(TMS)-ynones] 3, applying
a one-pot three component synthesis by a coupling–addition–
cyclocondensation sequence. 25 mg of PdNPs-PPS was kept
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acetylene (0.11 mL, 1 mmol) and dry triethylamine (0.14 mL,
1 mmol) in dry toluene (5 mL) in a 50 mL Schlenk flask. To
generate the enaminone system, tryptamine (400 mg, 2.5 mmol)
was added at room temperature after completion of the first
catalytic step (monitored by TLC) and the reaction mixture was
heated at 100 ◦C for 16–20 h until the complete consumption of
ynone was observed (monitored by TLC). It was cooled to 25 ◦C
and methacryloyl chloride (0.49 mL, 5 mmol) was added. It was
then heated at 70 ◦C for 5 h. After cooling to room temperature,
the reaction mixture was diluted with methanol, stirred for
20 min, evaporated and purified by a column chromatography on
silica gel (100–200 mesh) using hexane and ethyl acetate in a 1 : 1
ratio to isolate the resultant tetrahydro-b-carboline derivative as
a white solid.
Results and discussion
Fig. 1 A TEM image of the PdNPs-PPS recorded on a carbon coated
copper grid; scale bar = 50 nm; inset shows the corresponding EDX
spectrum of PdNPs-PPS collected from TEM, confirming the presence
of palladium and sulfur.
The present strategy utilizes the synthesis of palladium nanopar-
ticles on a sulfur containing polymer solid support to grip the
metal nanoparticles strongly on its surface for carrying out
catalytic reactions. Palladium nanoparticles were synthesized
by thermolysis of Pd(OAc)2 after incorporation onto a polymer
matrix, (PPS) (Scheme 2).
Scheme 2 The synthesis of palladium nanoparticles: PdNPs-PPS (a
schematic representation).
Obare and co-workers reported a one-step procedure for
the size controlled synthesis of palladium nanoparticles using
thioethers as stabilizing ligands.20 We have utilized the thioether
linkage present in the polymer, PPS to carry out the syn-
thesis of monodisperse palladium nanoparticles by pyrolysis
of palladium acetate. A typical synthetic procedure involves
the pyrolysis of palladium acetate in the presence of PPS
(Mn ~ 10 000) in toluene at 95 ◦C for 4 h. The color was changed
from yellow to dark brown in 3–4 h indicating the formation
of palladium nanoparticles. The advantage of the very poor
solubility of PPS in many organic solvents (THF, toluene, EtOH,
MeOH, acetonitrile, dichloromethane, CHCl3, DMF, DMSO
etc) was utilized in designing palladium nanoparticle-based het-
erogeneous catalysts directed to the acyl Sonogashira reaction.
The palladium nanoparticles (PdNPs-PPS) were characterized
by transmission electronic microscopy (TEM), powder X-ray
diffraction (PXRD) and X-ray photoelectron spectroscopy
(XPS) studies.
TEM analysis reveals the presence of spherical nanoparticles
indicating isotropic growth20 (Fig. 1). The average diameter
of the nanoparticles was determined to be 6 nm (ESI†). The
presence of palladium and sulfur atoms was observed in the
EDX spectrum, collected from TEM (Fig. 1, inset). PXRD
measurement shows a broad peak at 40◦, characteristic of the
(111) peak for zerovalent Pd with a face-centered cubic (fcc)
structure (Fig. 2).20,21 The peak at 47◦ represent the (200) plane
of the fcc lattice. The size of nanoparticles was calculated from
the line broadening of the (111) reflection using the Scherrer
Fig. 2 The powder X-ray diffraction pattern of PdNPs-PPS having
strong reflections at 2q = 39.95◦ and 47.25◦, corresponding to (111) and
(200) planes of an fcc lattice, respectively.
formula and showed an average nanoparticle size of 4 nm, close
to that observed from TEM measurement.
To evaluate the oxidation state of the palladium in PdNPs-
PPS as well as to investigate the interaction of sulfur atoms
present in PPS with the palladium center we have carried out
XPS study in details. XPS of Pd(3d) core level in PdNPs-PPS
sample is shown in the Fig. 3. The Pd(3d5/2,3/2) peaks could be
resolved into two sets of spin–orbit doublet. Pd(3d5/2,3/2) peaks
were fitted by constraining the spin–orbit separation of 5.2 eV
and ratio of doublet intensities at 3 : 2. Accordingly, Pd(3d5/2,3/2
)
peaks observed at 335.1 and 340.3 eV could be assigned to
zerovalent Pd only, whereas peaks at 336.7 and 342.0 eV as
observed in the deconvoluted spectrum correspond to divalent
Pd(3d5/2,3/2) peaks of PdO. All these values for Pd metal and PdO
are close to the values accounted in the literature.22–24 Comparing
the intensity of Pd metal and PdO peaks in the deconvoluted
spectrum it may be noted that around 67% of the total Pd is in
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species weakly bound to PdO in the 164 eV region,25 but its
intensity would be very low, so formation of S→PdO species
might not be so significant. Thus, XPS discriminates the different
chemical environments of sulfur present in PdNPs-PPS and
demonstrates that sulfur bonded to Pd has a different binding
energy than unbound sulfur. Binding energies, relative intensities
and FWHM of several palladium and sulfur species with spin–
orbit separation are shown in Table 1.
We also assessed the relative surface concentration of Pd and S
from XPS data in PdNPs-PPS applying the following relation:29
CPd
CS
IPdsSlSDS
ISsPdlPdDPd
=
where C, I, s, l and D are the surface concentration, intensity,
photoionization cross-section, mean escape depth and analyzer
detection efficiency, respectively. Integrated intensities of Pd(3d)
and S(2p) peaks have been taken into account to estimate
the concentration; whereas photoionization cross-sections and
mean escape depths have been obtained from the literature.30,31
Analyzer detection efficiency, which is called geometric factor,
was taken as 1, because the maximum intensity in this spec-
trometer is obtained at 90◦. Hence, in PdNPs-PPS, the surface
concentration ratio of palladium and sulfur is 2.12 representing
that palladium is segregated on the surface of PdNPs-PPS
compared to the bulk. This fact could be greatly helpful during
the catalytic process as the substrate can locate more palladium
concentration, which is exposed on the surface of the catalyst.
Fig. 3 XPS of Pd(3d) core level in PdNPs-PPS.
the metallic state while the rest is present as PdO. Thus, XPS
analysis validates that PdNPs-PPS contains mainly zerovalent
Pd metal with a certain amount of PdO on the surface layer. This
patch of PdO could be formed during exposure of the sample to
the ambient conditions.25
XPS of S(2p) core level in PPS and PdNPs-PPS are shown in
Fig. 4. The spin–orbit doublet separation was held constant
at 1.2 eV and doublet intensities were fixed at a 2 : 1 ratio,
as given in the literature.24,26 It could be observed from XPS
that the envelope of S(2p) of PdNPs-PPS is broader than that
of PPS signifying the presence of different sulfur moieties in
PdNPs-PPS. S(2p3/2,1/2) peaks observed at 163.6 and 164.8 eV are
attributed to the sulfur present in PPS, as reported previously
(Fig. 4A).27 In contrast, the S(2p) core level region in PdNPs-
PPS could be resolved into three sets of spin–orbit doublets
with Full Width at Half Maximum (FWHM) of 2 eV and spin–
orbit splitting of 1.2 eV. Intense peaks of S(2p3/2,1/2) observed at
162.3 and 163.5 eV in PdNPs-PPS (Fig. 4B), which are absent
in free PPS, could be ascribed to the sulfur bound to the Pd
nanoparticle surfaces, indicating the presence of C–S→Pd type
bonds in PdNPs-PPS and this kind of linkage has been studied
by XPS previously.25,28 S(2p) peaks at 163.7 and 164.9 eV with
smaller intensities can be attributed to unbound sulfur present
on the PPS support. Higher binding energy peaks at 166.6 and
167.8 eV could be assigned to oxidized sulfur.22,25
PdNPs-PPS catalyzed acyl Sonogashira reaction
The benefits of having an exceedingly high surface-to-volume
ratio and heterogeneity have led many researchers to carry
out various organic transformations using metal nanoparticles
as catalysts anchored on a solid support.32 Our interest has
thus turned to synthesize ynones 3 employing a heterogeneous
palladium nanoparticles-catalyzed Sonogashira coupling reac-
tion of acid chlorides with terminal alkynes under copper and
phosphine free reaction conditions.
The catalytic efficiency of PdNPs-PPS towards the acyl Sono-
gashira reaction was first examined using commercially available
substrates, benzoyl chloride 1a and phenyl acetylene 2a at room
temperature using triethylamine in toluene. The desired product,
1,3-diphenylprop-2-yn-1-one (3aa) was obtained in good yield
(65%, entry 1, Table 2). To explore the generality and scope of the
acyl Sonogashira coupling reaction, a variety of aryl/aliphatic
acid chlorides was treated with different aromatic/aliphatic
terminal alkynes and the results are summarized in Table 2.
The coupling reactions occurred smoothly either at room
temperature or at a slightly elevated temperature (50 ◦C),
resulting in nearly quantitative yield of the corresponding
products with a number of substrates (Table 2). The coupling
reactions of phenyl acetylene 2a with naphthoyl chlorides, 1b–c
proceeded in almost quantitative yield (entries 2–3, Table 2). The
coupling of phenyl acetylene 2a with an a,b-unsaturated acid
chloride (cinnamoyl chloride, 1d) as well as with a heteroaryl
acid chloride (thiophene-2-carbonyl chloride, 1e) took place
smoothly, quantitatively yielding the resultant products (entries
4–5, Table 2). A good yield was obtained in the case of an
Fig. 4 XPS of S(2p) core level in (A) PPS and (B) in PdNPs-PPS.
From the intensity of S(2p) peaks of different sulfur species,
we can estimate that 55% of total sulfur is bound to Pd,
25% sulfur resides unbound and the remaining 20% sulfur
experiences oxidation. There could be a component of sulfur
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aliphatic acid chloride (pivaloyl chloride, 1f) when employed
as a substrate (entry 6, Table 2). Variation in the alkyne
moiety with substituted phenyl acetylenes, 2b–c also furnished
the desired products almost quantitatively with thiophene-2-
carbonyl chloride, 1e (entries 7–8, Table 2).
acetylene 2a. The separation of the heterogeneous nanocatalysts,
PdNPs-PPS, was simply performed by filtration. After each
catalytic cycle, the catalyst was recovered by filtering through
a 0.22 mm membrane filter paper, thoroughly washed with
polar organic solvents and finally dried under high vacuum
for 12 h before performing the next cycle with an equal
amount of substrate loading. The catalyst remained active up
to four consecutive cycles with gradual decrease of the catalytic
efficiency, as shown in Fig. 5. This gradual decrease of the
catalytic efficiency may be attributed to the fact that after each
catalytic run we progressively lose some amount of catalyst
during the catalyst recovery process and hence the effective
loading of the catalyst keeps on decreasing from one catalytic
cycle to the next. The TEM images recorded after the catalytic
runs revealed that the morphology of catalyst remains same even
after fourth catalytic cycle.
The catalytic performance was then studied using less acti-
vated aliphatic terminal alkynes, namely, 1-octyne 2d, 1-hexyne
2e and 1-decyne 2f. We found that the aliphatic alkynes can
be effortlessly activated by applying this particular catalytic
system without prior need of copper as a co-catalyst. New ynone
derivatives, 3dd to 3df have been synthesized in good yields by
the coupling of cinnamoyl chloride, 1d, with 2d, 2e, and 2f
(entries 10–12, Table 2). High yields of the desired products
were obtained from the coupling reactions of thiophene-2-
carbonyl chloride, 1e with 2d, 2e, and 2f (entries 13–15, Table
2). The high performance in the catalytic activity was monitored
for PdNPs-PPS nanocatalysts when an aliphatic acid chloride
(pivaloyl chloride, 1f) was allowed to couple with an aliphatic
alkyne (1-hexyne, 2e) at 50 ◦C to provide the corresponding
ynone, 3fe in 54% yield (entry 16, Table 2). The scope of the
ynone synthesis was further utilized by using 2-methoxybenzoyl
chloride, 1g as the coupling partner, resulting in the formation
of (2-methoxyaryl)-substituted ynone, 3ga (entry 17, Table 2) in
near quantitative yield. This particular ynone can be employed
as a starting material for the preparation of chromenones,
potential intermediates in the synthesis of bioactive molecules.33
Based on the literature reports on a copper free Sonogashira
reaction19,34 and its mechanism,35 we have proposed a plausible
mechanism depicted in Scheme 3. The catalytic cycle may
proceed with the oxidative addition of acid chloride 1 to the
Pd(0) center stabilized in PPS matrix to form intermediate (A).
Subsequent alkyne coordination in the presence of base NEt3
would form the palladium-alkyne complex (C) via the transient
complex (B). In the final step, the product 3 would form in a
reductive elimination with regeneration of the catalyst.
Fig. 5 A recyclability chart of the catalyst, PdNPs-PPS for four
consecutive catalytic runs. The 5th and 6th cycles were carried out using
1 : 1 fresh and recovered catalyst (after the 4th cycle).
TEM images of PdNPs-PPS showed no significant change in
size both before (Fig. 1) and after the first cycle of catalysis (Fig.
6A). The TEM image of the catalyst was also recorded after
completion of fourth catalytic run (Fig. 6B), which reveals that
the catalyst’s size remains unaltered even after fourth catalytic
cycle (ESI, Fig. S1†), which is quite different from the usual
nanocatalyst’s trend to form much larger and non-recyclable
aggregates during the reaction.36 This may be attributed to the
Scheme 3 The proposed mechanism where Pd(0)PPS represents
PdNPs-PPS.
Recyclability of the catalyst
A long catalyst lifetime and recyclability are highly enviable
for any catalytic system in favor of industrial applications.
The recycling efficiency of the catalyst was investigated for
the coupling of thiophene-2-carbonyl chloride 1e with phenyl
Fig. 6 TEM images of the PdNPs-PPS after the first catalytic cycle (A),
and the fourth catalytic cycle (B); scale bar = 50 nm (see also Fig. S1,
ESI†).
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Table 3 Screening of reaction conditions for the synthesis of (TMS)-
alkynonesa
stabilization of the palladium nanoparticles via the strong sulfur
coordination of PPS, which assists in holding the nanoparticles
in the polymer matrix even after several catalytic cycles. The
activity of the recovered catalyst after the fourth catalytic cycle
could also be enhanced after mixing the fresh catalyst with used
catalyst. The fresh catalyst, when mixed with recovered catalyst
after the fourth cycle, afforded the ynone derivative 3ea in 81%
and 63% yield in the fifth and sixth cycles, respectively (Fig. 5).
The controlled reactions were carried out to test if the
reaction is promoted by the supported Pd nanoparticles or if
PdNPs-PPS acts as reservoir of soluble Pd species. The catalytic
reactions were performed using the filtrates obtained after
stirring the PdNPs-PPS in dry toluene for 12 h in the absence
and in the presence of the required amount of triethylamine.
In both attempts there was no indication of product formation
after the addition of appropriate substrates. This result clearly
demonstrates that the PdNPs-PPS acts as a supported catalyst.
Further, the ICP-AES analysis of fresh catalyst yielded a metal
loading of 7.5 wt% in the PdNPs-PPS sample while the ICP-
AES study indicates 0.16 wt% of the palladium content in the
solution after catalysis indicating leaching of the catalyst by
about 2%.
Entry
CuI
Solvent (5 mL)
Temp (◦C)
Yield (%)b
1
2
3
4
5
6
7
—
toluenec
toluene
toluene
THFc
THF
THF
25
25
90
25
25
90
25
14
20
27
27
35
34
97
4 mol (%)
4 mol (%)
—
4 mol (%)
4 mol (%)
—
acetonitrile
a Reaction conditions: PdNPs-PPS (25 mg), thiophene-2-carbonyl chlo-
ride (1 mmol), trimethylsilyl acetylene (1 mmol), dry triethylamine
(1 mmol) in 5 mL of dry solvent was used. b Yield was calculated
after isolating the product through short column chromatography. c Dry
triethylamine was used in more than 1 equivalent (2 mmol).
2–3 and 5–6, Table 3) did not improve the yield of the resulting
ynone 3eg. However, reducing the amount of triethylamine to
a stoichiometric quantity and switching the solvent system to
acetonitrile allowed the isolation of ynone 3eg (entry 7, Table 3)
in a near quantitative yield (97%).
One-pot synthesis
One-pot multicomponent reactions are the most useful syn-
thetic methods to obtain improved yields of pharmaceutically
important compounds by avoiding the handling and isolation
of intermediates.37 Encouraged by the successful recyclability
result of the PdNPs-PPS nanocatalyst and the high yield of
the coupling products, we have further explored the one-
pot synthesis of pharmaceutically important heterocyclic ring
systems, such as 2,4-disubstituted pyrimidines and tetrahydro-b-
carboline derivatives. It was anticipated that the heterogeneous
nature of the catalyst will not affect the subsequent steps of the
one-pot reaction, which might result in enhanced yields of the
product.
This convenient access to (TMS)-ynone 3eg in high yield
under mild conditions motivated us to attempt the transfor-
mation of (TMS)-ynones into 2,4-disubstituted pyrimidines 4
by the consecutive addition of a nucleophile 6 in a coupling–
addition sequence. The nucleophilic additions proceed with the
concomitant loss of the trimethylsilyl group. Upon addition
of guanidinium salts in the presence of sodium carbonate
to the resulting (TMS)-ynones 3 in the one-pot process, the
pyrimidines 4 were obtained in modest to excellent yields
(Table 4). Naphthoyl chlorides 1b and 1c furnished the corre-
sponding pyrimidine products in 38% and 54% yields (entries 1–
2, Table 4). Pyrimidine derivative, 4-(thiophen-2-yl)pyrimidin-2-
amine (4c) was obtained in 72% yield from thiophene-2-carbonyl
chloride by using the one-pot three component coupling reaction
(entry 3, Table 4) while the corresponding oxygen analogue
1h endowed the resultant molecule 4d in 95% isolated yield
(entry 4, Table 4).
a) Synthesis of pyrimidines:. Pyrimidines symbolize an in-
tegral part of the purine nucleoside adenosine and pharmaceuti-
cally vital compounds having pronounced biological activities,38
such as KDR Kinase inhibitors, cholinesterase inhibitors,
mGlu4 receptor. Recently Mu¨ller and co-workers5 have reported
an acyl Sonogashira coupling using a well-defined palladium
catalyst, Pd(PPh3)2Cl2 and copper iodide (CuI) as co-catalyst
and further applied to a one-pot three component synthesis
of pyrimidine derivatives. We have explored the synthesis of
2,4-disubstituted pyrimidines 4 by a one-pot three component
coupling of acid chlorides, TMS-acetylene and guanidinium salt
6 in copper free reaction conditions.
For the synthesis of substituted pyrimidine derivatives we
carried out the coupling of TMS-acetylene 2g with thiophene-
2-carbonyl chloride 1e (Table 3). Under similar conditions to
those mentioned in Table 2, the desired product, 1-(thiophen-
2-yl)-3-(trimethylsilyl)prop-2-yn-1-one (3eg) was obtained in
14% isolated yield (entry 1, Table 3). We screened different
reaction conditions to improve the yield of the ynone 3eg, key
intermediates for the synthesis of 2,4-disubstituted pyrimidines.
We observed that reaction conditions using toluene or THF as
solvents (entries 1 and 4, Table 3), or by adding CuI (entries
b) Synthesis of
a
tetrahydro-b-carboline derivative:.
Tetrahydro-b-carbolines are widespread skeletons found in
alkaloids and pharmaceuticals, displaying a wide range of
biological activities,39 for example, antiaggregation, in vitro
trypanocidal activity, antimalarial, anticonvulsant, antitumor
and antiviral activities. Recently Mu¨ller and co-workers have
reported the synthesis of tetrahydro-b-carboline derivatives in
moderate yields (32–59%) using a one-pot four component
acyl Sonogashira coupling–amination–azaannulation–Pictet–
Spengler reaction based on the well-developed palladium
catalyst and with copper as a co-catalyst.7 We finally explored
the feasibility of such a multicomponent coupling reaction in
copper free reaction conditions. The indolo[2,3-a] quinolizin-
4-one 5 was obtained in 80% yield following the one-pot four
step sequence (Scheme 4). Thiophene-2-carbonyl chloride 1e
was treated with phenyl acetylene 2a using PdNPs-PPS and
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Table 4 Synthesis of 2,4-disubstituted pyrimidines in
a one-pot
studies revealed the bonding interaction of sulfur moieties with
palladium center in PdNPs-PPS. We outlined for the first time
that PPS supported palladium nanoparticles (PdNPs-PPS) can
be utilized as an efficient heterogeneous nanocatalyst for the acyl
Sonogashira coupling reaction under copper free mild reaction
conditions, resulting in a library of ynones in excellent yields. The
catalytic reaction was recycled up to four times consecutively to
test its recycling efficiency and observed no substantial change
in particle size, which may be attributed to the strong gripping
ability of polymer support through sulfur coordination. We fur-
ther explored the multicomponent synthesis of 2,4-disubstituted
pyrimidines and a tetrahydro-b-carboline derivative in one-pot
fashion with improved yields and efficiency than those reported
earlier. Overall, the acyl Sonogashira coupling using PdNPs-PPS
nanocatalysts and its further application to the multicomponent
synthesis of heterocycles provides a useful tool for organic
synthesis.
fashiona
Entry
1
Acid chloride, 1
Product, 4
Yield (%)b
38
2
54
3
4
72
95
Acknowledgements
We are thankful to DST, India for financial support. SS
thanks CSIR, India and KD thanks UGC, India for research
fellowships. We gratefully acknowledge Mann group, Bristol
University for using TEM and EDX analysis.
a Reaction conditions: 25 mg of the PdNPs-PPS was used together with
1 mmol each of the acid chloride, terminal alkyne and dry triethylamine
in 5 mL of dry acetonitrile; after completion of the first step, guanidine
hydrochloride (2.5 mmol) and Na2CO3 (3.5 mmol) were added into the
reaction mixture followed by the addition of 5 mL methanol. b Yields
after isolation of the product using column chromatography.
References
1 A. V. Kel’in, A. W. Sromek and V. Gevorgyan, J. Am. Chem. Soc.,
2001, 123, 2074.
2 (a) X-j. Wang, J. Tan and L. Zhang, Org. Lett., 2000, 2, 3107; (b) D.
B. Grotjahn, S. Van, D. Combs, D. A. Lev, C. Schneider, M. Rideout,
C. Meyer, G. Hernandez and L. Mejorado, J. Org. Chem., 2002, 67,
9200.
3 J. P. Waldo and R. C. Larock, Org. Lett., 2005, 7, 5203.
4 M. C. Bagley, D. D. Hughes and P. H. Taylor, Synlett, 2003, 259.
5 (a) A. S. Karpov and T. J. J. Mu¨ller, Org. Lett., 2003, 5, 3451; (b) A. S.
Karpov and T. J. J. Mu¨ller, Synthesis, 2003, 2815; (c) D. M. D’Souza
and T. J. J. Mu¨ller, Nat. Protoc., 2008, 3, 1660.
6 (a) M. C. Bagley, D. D. Hughes, R. Lloyd and V. E. C. Powers,
Tetrahedron Lett., 2001, 42, 6585; (b) A. Arcadi, F. Marinelli and E.
Rossi, Tetrahedron, 1999, 55, 13233.
7 (a) A. S. Karpov, T. Oeser and T. J. J. Mu¨ller, Chem. Commun., 2004,
1502; (b) A. S. Karpov, F. Rominger and T. J. J. Mu¨ller, Org. Biomol.
Chem., 2005, 3, 4382.
8 D. A. Alonso, C. Na´jera and Ma. C. Pacheco, J. Org. Chem., 2004,
69, 1615.
9 (a) L. Chen and C-J. Li, Org. Lett., 2004, 6, 3151; (b) R. J. Cox, D.
J. Ritson, T. A. Dane, J. Berge, J. P. H. Charmant and A. Kantacha,
Chem. Commun., 2005, 1037; (c) S. S. Palimkar, P. H. Kumar, N. R.
Jogdand, T. Daniel, R. J. Lahoti and K. V. Srinivasan, Tetrahedron
Lett., 2006, 47, 5527.
10 (a) M. S. M. Ahmed and A. Mori, Org. Lett., 2003, 5, 3057; (b) A.
Fusano, T. Fukuyama, S. Nishitani, T. Inouye and I. Ryu, Org. Lett.,
2010, 12, 2410.
Scheme
4
The synthesis of a tetrahydro-b-carboline derivative
through a one-pot four-component Sonogashira coupling–amination–
azaannulation–Pictet–Spengler sequence.
Et3N in toluene at 25 ◦C for 12 h to provide the corresponding
ynone 3ea, which was subsequently reacted with tryptamine 7
at 100 ◦C for 16 h. The resulting enaminone der◦ivative 9 was
further treated with methacrylol chloride 8 at 70 C for 5 h to
afford the desired product 5 in high isolated yield (80%) via the
Pictet–Spengler reaction of intermediate 10.
11 Md. T. Rahman, T. Fukuyama, N. Kamata, M. Sato and I. Ryu,
Chem. Commun., 2006, 2236.
12 J. Liu, X. Peng, Y. Zhao and C. Xia, Org. Lett., 2008, 10, 3933.
13 (a) D. A. Alonso, C. Na´jera and Ma. C. Pacheco, Tetrahedron Lett.,
2002, 43, 9365; (b) C. S. Consorti, F. R. Flores, F. Rominger and J.
Dupont, Adv. Synth. Catal., 2006, 348, 133; (c) P. R. Likhar, M. S.
Subhas, M. Roy, S. Roy and M. L. Kantam, Helv. Chim. Acta, 2008,
91, 259.
14 (a) H. Park, J. Park, A. K. L. Lim, E. H. Anderson, A. P. Alivisatos
and P. L. McEuen, Nature, 2000, 407, 57; (b) J. Nyga˚rd, D. H. Cobden
and P. E. Lindelof, Nature, 2000, 408, 342; (c) S. D. Franceschi and
L. Kouwenhoven, Nature, 2002, 417, 701.
15 X. Li, D. Hu, Y. Dang, H. Chen, M. C. Roco, C. A. Larson and J.
Chan, J. Nanopart. Res., 2009, 11, 529.
Conclusions
In summary, a convenient method for the synthesis of palladium
nanoparticles (PdNPs-PPS) embedded into a polymer matrix
via the thermolysis of palladium acetate is described. XPS
3246 | Green Chem., 2011, 13, 3238–3247
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
16 (a) N. L. Rosi and C. A. Mirkin, Chem. Rev., 2005, 105, 1547; (b) J.
Gao, H. Gu and B. Xu, Acc. Chem. Res., 2009, 42, 1097.
17 (a) M. Moreno-Man˜as and R. Pleixats, Acc. Chem. Res., 2003, 36,
638; (b) M-C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293;
(c) S. Schlo¨gl and S. B. A. Hamid, Angew. Chem., Int. Ed., 2004, 43,
1628; (d) L. C. Grabow and M. Mavrikakis, Angew. Chem., Int. Ed.,
2008, 47, 7390; (e) G. A. Somorjai, H. Frei and J. Y. Park, J. Am.
Chem. Soc., 2009, 131, 16589.
18 (a) W. C. Ketchie, M. Murayama and R. J. Davis, J. Catal., 2007, 250,
264; (b) P. Raveendran, J. Fu and S. L. Wallen, J. Am. Chem. Soc.,
2003, 125, 13940; (c) P. Raveendran, J. Fu and S. L. Wallen, Green
Chem., 2006, 8, 34.
19 S. Santra, P. Ranjan, S. K. Mandal and P. K. Ghorai, Inorg. Chim.
Acta, 2011, 372, 47.
20 M. Ganesan, R. G. Freemantle and S. O. Obare, Chem. Mater., 2007,
19, 3464.
21 (a) L. D’Souz and S. Sampath, Langmuir, 2000, 16, 8510; (b) S-W.
Kim, J. Park, Y. Jang, Y. Chung, S. Hwang and T. Hyeon, Nano Lett.,
2003, 3, 1289.
22 T. Wang, V. A. Kato and L. D. Schmidt, J. Catal., 1982, 78, 306.
23 K. Endo, H. Ohno, K. Matsuda and S. Asakura, Corros. Sci., 2003,
45, 1491.
Akiyama, H. Yoshida, T. Yoshida and S. Kobayashi, J. Am. Chem.
Soc., 2005, 127, 2125; (e) F. Cheng, S. M. Kelly, N. A. Young, C.
N. Hope, K. Beverley, M. G. Francesconi, S. Clark, J. S. Bradley
and F. Lefebvre, Chem. Mater., 2006, 18, 5996; (f) A. Dahan and M.
Portnoy, J. Am. Chem. Soc., 2007, 129, 5860; (g) F-Z. Su, Y-M. Liu,
L-C. Wang, Y. Cao, H-Y. He and K-N. Fan, Angew. Chem., Int. Ed.,
2007, 47, 334; (h) R. W. J. Scott, O. M. Wilson and R. M. Crooks,
Chem. Mater., 2004, 16, 5682.
33 N. Ahmed, C. Dubuc, J. Rousseau, F. Be´nard and J. E. Lier, Bioorg.
Med. Chem. Lett., 2007, 17, 3212.
34 (a) B. M. Choudary, S. Madhi, N. S. Chowdari, M. L. Kantam and
B. Sreedhar, J. Am. Chem. Soc., 2002, 124, 14127; (b) A. Koma´romi
and Z. Nova´k, Chem. Commun., 2008, 4968; (c) D. Rosario-Amorin,
M. Gaboyard, R. Cle´rac, S. Nlate and K. Heuze´, Dalton Trans., 2011,
40, 44.
35 (a) R. Chinchilla and C. Na´jera, Chem. Rev., 2007, 107, 874; (b) B. M.
Choudary, S. Madhi, M. L. Kantam, B. Sreedhar and Y. Iwasawa,
J. Am. Chem. Soc., 2004, 126, 2292; (c) A. Tougerti, S. Negri and
A. Jutand, Chem.–Eur. J., 2007, 13, 666; (d) M. B. Thathagar, P. J.
Kooyman, R. Boerleider, E. Jansen, C. J. Elsevier and G. Rothenberg,
Adv. Synth. Catal., 2005, 347, 1965; (e) Xiao-F. Wu, H. Neumann
and M. Beller, Chem.–Eur. J., 2010, 16, 12104.
24 C. D. Wagner, W. M. Riggs, L. E. Davis and J. F. Moulder, in Hand-
book of X-Ray Photoelectron Spectroscopy, ed. G. E. Muilenberg,
Perkin-Elmer, Eden Prairie, Minnesota, 1979.
25 B. Genorio, T. He, A. Meden, S. Polanc, J. Jamnik and J. M. Tour,
Langmuir, 2008, 24, 11523.
26 Bart-H. Huisman, F. C. J. M. Van Veggel and D. N. Reinhoudt, Pure
Appl. Chem., 1998, 70, 1985.
36 (a) R. G. Heidenreich, J. G. E. Krauter, J. Pietsch and K.
Ko¨hler, J. Mol. Catal. A: Chem., 2002, 182–183, 499; (b) F-X.
Felpin, E. Fouquet and C. Zakri, Adv. Synth. Catal., 2009, 351,
649.
37 For recent reviews, see: (a) D. E. Fogg and E. N. dos Santos, Coord.
Chem. Rev., 2004, 248, 2365; (b) J. C. Wasilke, S. J. Obrey, R. T. Baker
and G. C. Bazan, Chem. Rev., 2005, 105, 1001.
27 (a) J. A. Gardella Jr., S. A. Ferguson and R. L. Chin, Appl. Spectrosc.,
1986, 40, 224; (b) T. Sugama and N. R. Carciello, Int. J. Adhes. Adhes.,
1991, 11, 97; (c) J. J. Pireaux, J. Riga, P. Boulanger, P. Snauwaert, Y.
Novis, M. Chtaib, C. Gregoire, F. Fally, E. Beelen, R. Caudano and
J. Verbist, J. Electron Spectrosc. Relat. Phenom., 1990, 52, 423.
28 J. C. Love, D. B. Wolfe, R. Haasch, M. L. Chabinyc, K. E. Paul, G.
M. Whitesides and R. G. Nuzzo, J. Am. Chem. Soc., 2003, 125, 2597.
29 C. J. Powell and P. E. Larson, Appl. Surf. Sci., 1978, 1, 186.
30 J. H. Scofield, J. Electron Spectrosc. Relat. Phenom., 1976, 8, 129.
31 D. R. Penn, J. Electron Spectrosc. Relat. Phenom., 1976, 9, 29.
32 For some leading references see: (a) R. Narayanan and M. A. El-
Sayed, J. Catal., 2005, 234, 348; (b) L. Djakovitch and K. Koehler,
J. Am. Chem. Soc., 2001, 123, 5990; (c) J. C. Love, D. B. Wolfe, R.
Haasch, M. L. Chabinyc, K. E. Paul, G. M. Whitesides and R. G.
Nuzzo, J. Am. Chem. Soc., 2003, 125, 2597; (d) K. Okamoto, R.
38 (a) For reviews see: K. Undheim and T. Benneche, in Comprehensive
Heterocyclic Chemistry II, ed. A. R. Katritzky, C. W. Rees, E. F.
V. Scriven and A. McKillop, Pergamon, Oxford, 1996, Vol. 6, p
93; (b) A. W. Erian, Chem. Rev., 1993, 93, 1991; (c) T. Sasada, F.
Kobayashi, N. Sakai and T. Konakahara;, Org. Lett., 2009, 11, 2161;
(d) M. Movassaghi and M. D. Hill, J. Am. Chem. Soc., 2006, 128,
14254; (e) N. Sakai, A. Youichi, T. Sasada and T. Konakahara, Org.
Lett., 2005, 7, 4705; (f) H. Kakiya, K. Yagi, H. Shinokubo and K.
Oshima, J. Am. Chem. Soc., 2002, 124, 9032.
39 For a review see: (a) A. J. Kochanowska-Karamyan and M. T.
Hamann, Chem. Rev., 2010, 110, 4489; (b) B. Maiti, K. Chanda
and C.-M. Sun, Org. Lett., 2009, 11, 4826; (c) Y. Ohta, S. Oishi,
N. Fujii and H. Ohno, Org. Lett., 2009, 11, 1979; (d) S. Adam, X.
Pannecoucke, J.-C. Combret and J.-C. Quirion, J. Org. Chem., 2001,
66, 8744.
This journal is
The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 3238–3247 | 3247
©