Pd-N-heterocyclic carbene (NHC) organic silica: synthesis and application in carbon-carbon coupling reactions

Article abstract of DOI:10.1016/j.tet.2008.02.098

The first Pd-N-heterocyclic carbene (NHC) complex in the form of organic silica was prepared using sol-gel method and its application in Heck and Suzuki reactions was demonstrated. These C-C coupling reactions proceeded efficiently under the influence of microwave irradiation, with excellent yield, without any change in catalytic activity for at least five reaction cycles, with negligible Pd concentration in the end product.

Full text of DOI:10.1016/j.tet.2008.02.098

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 4637e4643
www.elsevier.com/locate/tet
PdeN-heterocyclic carbene (NHC) organic silica: synthesis and
application in carbonecarbon coupling reactions
*
Vivek Polshettiwar, Rajender S. Varma
Sustainable Technology Division, National Risk Management Research Laboratory, U.S. Environmental Protection Agency,
MS 443, Cincinnati, OH 45268, USA
Received 23 January 2008; received in revised form 19 February 2008; accepted 28 February 2008
Available online 4 March 2008
Abstract
The first PdeN-heterocyclic carbene (NHC) complex in the form of organic silica was prepared using solegel method and its application in
Heck and Suzuki reactions was demonstrated. These CeC coupling reactions proceeded efficiently under the influence of microwave irradiation,
with excellent yield, without any change in catalytic activity for at least five reaction cycles, with negligible Pd concentration in the end product.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Organic silica; Heterogeneous catalysis; PdeNHC; Suzuki reaction; Heck reaction; Microwave irradiation
1. Introduction
process to obtain solegel catalyst consists of the synthesis of
an inorganic network by hydrolysis and polycondensation of
tetraethyl orthosilicate (TEOS) in the presence of the catalytic
species. Most of encapsulated-silica catalyst contains 90% in-
organic part from TEOS and only 10% the catalytic part. Also,
not all catalytic sites are available for reaction, since most of
them are in the walls of silica and only those, which are in
the pores are available for catalysis. Consequently there is need
to design and synthesize pure organic silica, which does not use
any inorganic solegel precursor, so that most of its sites are cat-
alytically active. In continuation of our work on supported
catalysis,6,8 pursued under the green chemistry program,9 we
propose a new concept of catalyst heterogenisation, by synthe-
sizing the first example of PdeN-heterocyclic carbene (Pde
NHC) complex in the form of organic silica and its application
in CeC coupling reactions, which could shed new light on tran-
sition-metal catalysis.
Palladium catalyzed carbonecarbon cross-coupling reac-
tions exemplify one of the important processes in organic
chemistry.1 The Heck2 and Suzuki3 reactions are among the
most widely used reactions for the formation of carbonecarbon
bonds. These reactions are generally catalyzed by soluble Pd
complexes with various ligands.4 However, the efficient separa-
tion and subsequent recycling of homogeneous transition-metal
catalysts remains a scientific challenge and an aspect of eco-
nomical and ecological relevance. Heterogeneous Pd-catalyst
systems were found to be highly effective to overcome some
of these issues.5 The majority of the novel heterogenised cata-
lysts are based on silica supports, primarily because silica dis-
plays some advantageous properties, such as excellent stability
(chemical and thermal), good accessibility, porosity and the fact
that organic groups can be robustly anchored to the surface to
provide catalytic centers.6,7
The common structural feature of these materials is the
entrapment of the dopant (catalytic) molecule in the pores of
silica, a phenomenon, which imparts unique chemical and
physical properties to resulting hybrid silica. In general, the
2. Results and discussion
The first step in the accomplishment of this goal is the syn-
thesis of an organometallic precursor that has multiple polycon-
densable organosiloxane functionalized donor ligands. The
PdeN-heterocyclic carbene complex IV was chosen as it has
* Corresponding author. Tel.: þ513 487 2701; fax: þ513 569 7677.
E-mail address: varma.rajender@epa.gov (R.S. Varma).
0040-4020/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.02.098

4638
V. Polshettiwar, R.S. Varma / Tetrahedron 64 (2008) 4637e4643
O
Si
O
Si
O
O
O
O
O
O
I, MW
100 °C
NaH
THF, RT
O
Si
I
O
NH
O
N
Si
I
N
N
N
N
O
II
I
III
Pd(OAc)2
THF
O
Si
O
O
O
Si
O
O
O
O
O
O
N
N
Si
Si
O
O
N
N
Sol-Gel
0.5 N NH4F, THF
I
Pd I
I Pd I
N
N
N
N
O
O
O
O
Si
Si
O
O
Si
Si
O
O
O
O
O
O
IV
Pd-NHC Organic Silica (V)
Scheme 1. Preparation of PdeN-heterocyclic carbene (NHC) organic silica.
been demonstrated to be an excellent catalyst for CeC coupling
reactions.10
The organic silica V was also characterized by solid state
MAS NMR spectroscopy. The 29Si CP-MAS spectra displayed
two characteristic signals, of these, the signal at ꢁ59 ppm cor-
responded to T2 and ꢁ66 ppm to T3 sites for RSiO3 substruc-
tures of different condensation degree. In order to identify
organic functionalities attached to the silica support, 13C CP-
MAS experiments were performed. The spectrum showed
signals at 166.1, 128.9, 57.8, 53.3, 24.6, 18.4, and 11.1 ppm,
similar to the chemical shifts found in the liquid 13C NMR
spectrum of the silylated organic precursor molecule IV.
We explored this PdeNHC organic silica V as catalyst in
Heck reactions (Scheme 2). In view of the emerging MW chem-
istry interest in heterogeneous cross-coupling reactions,13 we
used MW irradiation as a heating source for this reaction. Initial
trials were performed to optimize reaction conditions using
iodobenzene and methyl acrylate as a substrate. After screening
a range of usual inorganic and organic bases and exploring the
scope of various solvents, we found that this catalyst is most
efficient for Heck reaction in presence of triethylamine as a
base and DMF as a solvent at 120 ꢀC. Using these optimized
reaction conditions, the scope of this catalyst was studied for
Heck reaction of various aryl halides and alkenes and results
are summarized in Table 1.
The PdeNHC organic silica V was prepared in THF as
shown in Scheme 1. The reaction of imidazole with 3-iodo-
propyl triethoxysilane (I) generated silylated imidazole II,
that on reaction with 3-iodopropyl triethoxysilane (I) provided
ionic imidazole species III, which on treatment with Pd(OAc)2
in THF under reflux condition afforded the complex IV. Micro-
wave (MW) irradiation was ideally suited for the conversion of
II to ionic liquid III, which normally requires an extended pe-
riod of 12 h for the completion of the reaction. This may be due
to selective absorption of microwaves by reactants and polar
intermediates, which accelerate the reaction rate.11 The in situ
13C NMR analysis of the complex IV showed two peaks at
162.9 and 173.1 ppm concerning two regioisomers of IV, which
clearly proved the formation of NHCePd carbene complex.10f
We wanted to use this precursor IV directly, without using
any inorganic solegel precursor TEOS, in the synthesis of
organic silica under the classical aqueous basic solegel condi-
tions; however, this complex is not stable under these condi-
tions. Alternatively, we adapted the condensation reaction
under the mild conditions in THF using ammonium fluoride
as a catalyst.12 Various combinations of THF, NH4F, and H2O
were explored and the optimum results were obtained with
5 mL THF, 0.6 mL 1 N NH4F, 0.6 mL H2O for 0.003 mol of
IV, which led to the formation of shining orange colored
PdeNHC organic silica material V. Thus, we achieved our goal
of organic silica synthesis without using any inorganic solegel
precursor.
X
Pd-NHC
Organic Silica
+
R
Y
Et3N, DMF, 120 °C
R
Y
X - I, Br
In order to determine the properties of this organic silica ma-
terial, it was characterized by various techniques. TGA analysis
of organic silica V showed a weight reduction at 180 ꢀC, fol-
lowed by loss of 30% weight at 350 ꢀC, which corresponds to
organic part of the complex. The catalyst V exhibited 5 m2/g
of surface area by BrunauereEmmetteTeller (BET) measure-
ment. The Pd content in the organic silica was 4% based on
inductively coupled plasma atomic emission spectroscopy
(ICP-AES) analysis.
Y - CO2CH3, C6H5
R - H, Me, OMe, COMe,
CHO.
Scheme 2. Heck reaction using PdeN-heterocyclic carbene organic silica.
Aromatic iodo and bromo compounds reacted efficiently
with alkenes to give coupling products in good to excellent
yield. The reactions with 2-iodothiophene were successfully

V. Polshettiwar, R.S. Varma / Tetrahedron 64 (2008) 4637e4643
4639
Table 1
Heck reaction of various aryl halides using PdeNHC organic silicaa
Entry
Aryl halide
Alkene
Product
Yieldb (%)
95
TOF
OMe
OMe
1
C6H5I
10,270
10,378
9729
O
O
O
O
O
MeO
Me
OMe
OMe
OMe
2
3
4
5
4-OMeC6H4I
4-MeC6H4I
OMe
96
90
87
88
O
OMe
OMe
O
MeO
4-OMeC6H4Br
4-OMeC6H4Br
9405
O
O
9513
OMe
OMe
OHC
OMe
OMe
OMe
6
7
4-CHOC6H4Br
4-CHOC6H4Br
88
89
9513
9621
O
O
O
CHO
OMe
MeOC
8
9
4-COMeC6H4Br
4-COMeC6H4Br
85
84
9189
9081
O
COMe
O
10
11
2-Iodothiophene
2-Iodothiophene
90
88
9729
9513
OMe
S
S
O2N
12
2-NO2C6H4Br
78
8432
a
Reaction was carried out by irradiating 1 mmol of aryl halide, 1.2 mmol of alkene, 1.5 mmol of triethylamine, and 0.1 g of organic silica (0.037 mol % Pd)
catalyst in 1 mL of DMF by microwaves for 0.25 h at 120 ꢀC.
b
GC yields.
achieved (entries 10 and 11), which provide a useful strategy
to introduce unsaturated group on the thiophene ring.
TheutilityofthisPdeNHCorganicsilicaVwasthenexplored
for Suzuki reaction using a variety of substrates (Scheme 3).
Firstly, the reaction conditions were optimized using iodoben-
zene as a substrate. We found that this catalyst is most efficient
for Suzuki reaction in presence of K2CO3 as a base and DMF/
water (1:2) as a solvent. Using these optimized reaction con-
ditions, the efficiency of this catalyst was studied for Suzuki
reaction of various aryl halides and the results are summarized
in Table 2.
Aryl iodide (entries 1e3) and aryl bromide (entries 4e11)
with various functional groups efficiently reacted with boronic
acid to yield Suzuki products in good to excellent yields. 2-Io-
dothiophene underwent smooth reaction with various boronic
acids (entries 12e14), providing a useful way for the synthesis
of aryl substituted thiophene heterocycles. High turnover fre-
quencies (TOF) are achieved for iodo- and bromoarenes,
which are comparable with reported catalyst systems.14e21
For practical applications of heterogeneous systems, the
lifetime of the catalyst and its level of reusability are very
X
B(OH)2
Pd-NHC
R2
Organic Silica
+
K2CO3, DMF:H2O, 100 °C
R1
R1
R2
Where, X- I, Br
R1-H, Me, OMe, COMe, CHO
R2-H, Cl, F, napthyl
Scheme 3. Suzuki reaction using PdeN-heterocyclic carbene organic silica.

4640
V. Polshettiwar, R.S. Varma / Tetrahedron 64 (2008) 4637e4643
Table 2
Suzuki reaction of various aryl halides using PdeNHC organic silicaa
Entry
1
Aryl halide
C6H5I
Boronic acid
Product
Yieldb (%)
97
TOF
16,385
B(OH)2
B(OH)2
OMe
OMe
2
3
4
5
6
7
8
9
4-OMeC6H4I
4-MeC6H4I
96
94
90
90
91
90
90
88
16,216
15,878
15,202
15,202
15,371
15,202
15,202
14,864
Cl
Cl
B(OH)2
B(OH)2
Cl
4-OMeC6H4Br
4-OMeC6H4Br
4-CHOC6H4Br
4-CHOC6H4Br
4-CHOC6H4Br
4-COMeC6H4Br
B(OH)2
B(OH)2
Cl
OMe
CHO
Cl
F
B(OH)2
Cl
F
CHO
B(OH)2
B(OH)2
CHO
COMe
10
11
12
13
14
4-COMeC6H4Br
4-COMeC6H4Br
2-Iodothiophene
2-Iodothiophene
2-Iodothiophene
87
88
90
91
91
14,695
14,864
15,202
15,202
15,202
Cl
F
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
Cl
COMe
COMe
F
S
Cl
F
S
Cl
F
S
NO2
15
2-NO2C6H4Br
84
14,189
B(OH)2
a
Reaction was carried out by irradiating 1 mmol of aryl halide, 1.2 mmol of boronic acid, 1.5 mmol of K2CO3, and 0.1 g of organic silica (0.037 mol % Pd)
catalyst in 1 mL of DMF/H2O (1:2) by microwaves for 0.16 h at 100 ꢀC.
b
GC yields.
important factors. To clarify this aspect, we established a set of
experiments using the recycled catalyst for Heck reaction of
iodotoluene and methyl acrylate. The reactions were con-
ducted under similar conditions (120 ꢀC/30 min) in DMF.
After the completion of the first reaction to afford the corre-
sponding methyl cinnamate in 90% yield, the catalyst was
recovered by filtration, washed with dichloromethane and ace-
tone, and finally dried at 100 ꢀC for 1 h. A fresh reaction was
then performed under same conditions. Although the color of
silica changed from orange to blackish brown, the PdeN-het-
erocyclic carbene complex in the form of organic silica could
be used for at least five times without any change in activity.
Similar study for Suzuki reaction also showed excellent
recyclability of catalyst.
Heterogeneity and Pd-leaching of this catalyst for the Heck
reaction of iodotoluene and methyl acrylate were also exam-
ined by the ‘hot filtration’ test.14 Filtration using Whatman
filter paper after 10 min (35% conversion) followed by an

V. Polshettiwar, R.S. Varma / Tetrahedron 64 (2008) 4637e4643
4641
additional 20 min of reaction time gave a final conversion of
4.2. Synthesis of N-3-propyl imidazole triethoxysilane (II)
90% for the catalyst-containing portion, and 59% conversion
for the filtered portion. Though these results do not rule out
the possibility that the organic silica might have been involved
in the reaction in a heterogeneous way, it is evident that the
catalyst released some Pd species into the solution. Pd-leach-
ing was also studied by Atomic Absorption Spectroscopy
(AAS) analysis of the organic part after completion of reaction
(before column chromatography), which showed negligible
6 ppm Pd-leaching. These two divergent results indicate that
after the completion of reaction (iodotoluene completely dis-
appears), the leached palladium goes back to the surface of the
support and gets redeposited6,15 thereby resulting in negligible
Pd content in the final product.
Sodium hydride 2.4 g (0.1 mol) was suspended in 100 mL of
dry THF under inert atmosphere. To this suspension, 4.8 g of
imidazole (0.07 mol) in 50 mL THF was added dropwise over
30 min and stirred for 30 more minutes at room temperature.
Freshly distilled triethoxysilane propyl iodide (I) (23.3 g, 0.07
moles) was then added dropwise over 30 min and the reaction
mixture was stirred for 2 days at room temperature. After com-
pletion of reaction, THF was evaporated and ether was added
to residual and then filtered. After removal of ether from the
filtrate, the crude N-3-propyl imidazole triethoxysilane was
purified under vacuum distillation (10 mm/190 ꢀC) to yield
12.5 g of pure product (65%).
Our results are consistent with previously reported studies
using supported Pd-catalysts,16 some pincer complexes,17 pal-
ladacycles,18 and simple Pd compounds,19 which were also
demonstrated as Pd reservoirs. The other two similar reports
on PdeN-heterocyclic carbene complex functionalized silica by
Aksin et al. 20 showed considerable Pd-leaching whereas Kar-
imi and Enders 21 indicate no Pd-leaching. In fact, to date there
is no 100% leach proof Pd-catalyst and hence the most impor-
tant criteria in choosing the catalyst is the Pd-metal recovery. It
would be preferable to use a more accessible, low cost silica
catalyst provided that the process works efficiently and that
the catalyst leaves no remnants of metal within the end product
since metal contamination is highly regulated by pharmaceuti-
cal industry. All above conditions are well satisfied by the
PdeNHC organic silica catalyst (V) used in present studies,
with negligible Pd concentration within end product after
completion of reaction.
FTIR (KBr): 3110, 2974, 2929, 2888, 1506, 1391, 1228,
1
1104, 1978, 957, 783, 664 cmꢁ1; H NMR (CDCl3): d 0.42
(2H, t, J¼8.0 Hz), 1.08 (9H, t, J¼6.8 Hz), 1.74 (2H, m), 3.66
(6H, q, J¼6.8 Hz), 3.80 (2H, t, J¼7.0 Hz), 6.78 (1H, s), 6.90
(1H, s), 7.34 (1H, s) ppm.
13C NMR (CDCl3): d 7.2, 18.1, 24.8, 49.1, 58.4, 118.7,
129.1, 137.1 ppm.
MS: (Mþ) 272. Anal. Calcd for C12H24N2O3Si: C, 52.91;
H, 8.88; N, 10.28. Found: C, 52.85; H, 8.90; N, 10.26%.
4.3. Synthesis of 1,3-N,N-bis(3-(triethoxysilyl)propyl)-
imidazolium iodide (III)
N-3-Propyl imidazole triethoxysilane (II) (2.72 g, 0.01 mol)
and 3-iodopropyl triethoxysilane (4.3 g, 0.011 mol) were added
to 1 mL of acetonitrile in a 10 mL crimp-sealed thick-walled
glass tube equipped with a pressure sensor and a magnetic stir-
rer and the reaction tube was placed inside the cavity of a CEM
Discover focused microwave synthesis system at 80 ꢀC for
30 min. After completion of reaction, solvent was evaporated
and product was washed with ether to remove excess of 3-iodo-
propyl triethoxysilane, to yield 5.1 g (85%) of the pure solid
product (decompositiond250 ꢀC).
3. Conclusions
In conclusion, we have developed a new concept for the
design and synthesis of highly active and recyclable heteroge-
nised PdeNHC catalysts in the form of organic silica, which
does not use any inorganic solegel precursor and most of its
sites are catalytically active. This work could shed new light on
transition-metal catalysis.
FTIR (KBr): 3070, 2972, 2922, 2885, 1562, 1445, 1294,
1
1163, 1076, 956, 783, 639 cmꢁ1; H NMR (CDCl3): d 0.48
(4H, t, J¼8.0 Hz), 1.07 (t, 18H, J¼7.2 Hz), 1.89 (4H, m),
3.67 (12H, q, J¼7.2 Hz), 4.24 (4H, t, J¼7.2 Hz), 7.33 (2H, s),
9.85 (1H, s) ppm.
4. Experimental
13C NMR (CDCl3): d 6.9, 18.2, 24.3, 51.7, 58.5, 122.4,
135.9 ppm.
4.1. General
MS: (Mþ) 604. Anal. Calcd for C21H45N2O6Si2: C, 52.79;
H, 9.49; N, 5.86. Found: C, 52.81; H, 9.46; N, 5.85%.
All the solvents and reagents were purchased at the highest
commercial quality and used without further purification, un-
less otherwise stated. Thin layer chromatography (TLC) (silica
gel; 20% ethyl acetate/hexane) and gas chromatography (GC)
were used to monitor the reactions. The crude products were
identified by GCeMS qualitative analysis using a GC system
with a Mass selective detector. The identities were further con-
firmed for representative compounds by 1H and 13C NMR spec-
tra that were recorded in deuterated chloroform-d (CDCl3) with
tetramethylsilane (TMS) as internal reference using a 300 MHz
NMR spectrometer. CEM Discover focused microwave synthe-
sis system was used to carry out Heck and Suzuki reactions.
4.4. Synthesis of PdeNHC complex (IV)
To a solution of 1 g of Pd(OAc)2 (0.0045 mol) in 100 mL dry
THF, 5.4 g of 1,3-N,N-bis(3-(triethoxysilyl)propyl)-imidazo-
lium iodide (III) (0.009 mol) was added and reaction mixture
was stirred at room temperature for 1.5 h and then refluxed
for 3 h. The formation of Pdecarbene complex was confirmed
by in situ NMR. After completion of reaction, THF was evap-
orated to yield the crude oily product, which can be solidified

4642
V. Polshettiwar, R.S. Varma / Tetrahedron 64 (2008) 4637e4643
after tituration by hexane and yielded 2.9 g of orange colored
PdeNHC complex (73%).
FTIR (KBr): 3121, 2974, 2929, 2885, 1458, 1079, 949, 794,
Education through an interagency agreement between the U.S.
Department of Energy and the U.S. Environmental Protection
Agency.
713 cmꢁ1
.
1H NMR (CDCl3): d 0.6 (8H, m), 1.16 (t, 36H, J¼7.2 Hz),
2.1 (8H, m), 3.77 (24H, q, J¼7.2 Hz), 4.3 (8H, m), 7.87 (4H,
s) ppm.
Supplementary data
Supplementary data associated with this article can be
found in the online version, at doi:10.1016/j.tet.2008.02.098.
13C NMR (CDCl3): d 7.6, 18.3, 23.5, 40.7, 53.3, 58.4,
120.9, 162.9 173.1 ppm.
MS (FABþ): (Mþ) 1315. Anal. Calcd for C42H90I2N4O12-
PdSi4: C, 38.34; H, 6.89; N, 4.26. Found: C, 38.31; H, 6.92;
N, 4.24%.
References and notes
1. (a) Yin, L.; Liebsher, J. Chem. Rev. 2007, 107, 133e173; (b) Tsuji, J.
Palladium Reagents and Catalysts; John Wiley and Sons: 2004; (c)
Heck, R. F. Palladium Reagents in Organic Synthesis; Academic: London,
1985.
4.5. Synthesis of PdeNHC organic silica (V)
2. (a) Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971, 44, 581;
(b) Heck, R. F.; Nolly, J. P. J. Org. Chem. 1972, 37, 2320e2322; (c)
Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009e3066;
(d) Trzeciak, A. M.; Ziolkowski, J. J. Coord. Chem. Rev. 2005, 249,
2308e2322.
The oily Pdecarbene complex (IV) after evaporation of
THF (in above procedure) was dissolved in 4 mL of THF and
1.5 mL of 0.5 N NH4F was added to this solution. Gelification
of mixture occurred at room temperature within 10 min, which
was allowed to stand for 24 h. The solid orange colored or-
ganic silica (V) was then washed with ethanol and dried under
vacuum at 80 ꢀC.
3. (a) Miyaura, N.; Yamada, K.; Suzuki, A. Tetrahedron Lett. 1979, 36,
3437e3440; (b) Bellina, F.; Carpita, A.; Rossi, R. Synthesis 2004, 15,
2419e2440; (c) Miyaura, N. Top. Curr. Chem. 2002, 219, 11e59.
4. (a) Kotha, S.; Lahiri, K.; Kashinath, D. Tetrahedron 2002, 58, 9633e
9695; (b) Herrmann, W. A.; Cornils, B. Angew. Chem., Int. Ed. 1997,
36, 1048e1067.
Yield: 2.4 g.
FTIR (KBr): 3120, 2930, 2863, 1126, 1001, 876, 691 cmꢁ1
.
5. Phan, N. T. S.; Sluys, M. V. D.; Jones, C. W. Adv. Synth. Catal. 2006, 348,
609e679.
29Si CP-MAS: d ꢁ59 (T2), ꢁ66 (T3) ppm. 13C CP-MAS:
d 11.1, 18.4, 24.6, 53.3, 57.8, 128.9, 166.1 ppm.
BET: porosity 5 m2/g.
6. Polshettiwar, V.; Molnar, A. Tetrahedron 2007, 63, 6949e6976.
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4.6. Typical experimental procedure for Heck reaction
The aryl halide (1 mmol), alkene (1.2 mmol), triethylamine
(1.5 mmol), and 0.1 g of PdeNHC silica (V) (0.004 g of Pd)
were added to 1 mL DMF in a 10 mL crimp-sealed thick-walled
glass tube equipped with a pressure sensor and a magnetic stir-
rer. The reaction tube was then placed inside the cavity of a
microwave oven, operated at 120ꢂ5 ꢀC (temperature monitored
by a built-in infrared sensor), power 40e140 W and pressure
40e70 psi for 30 min. After completion of the reaction, the re-
action mixture was filtered and crude was subjected to column
chromatography to afford pure Heck product.
9. (a) Polshettiwar, V.; Varma, R. S. J. Org. Chem. 2007, 72, 7420e7422; (b)
Polshettiwar, V.; Varma, R. S. Tetrahedron Lett. 2007, 48, 5649e5652; (c)
Polshettiwar, V.; Varma, R. S. Tetrahedron Lett. 2007, 48, 7343e7346; (d)
Ju, Y.; Kumar, D.; Varma, R. S. J. Org. Chem. 2006, 71, 6697e6700; (e)
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R. S. Org. Lett. 2005, 7, 2409e2411; (g) Wei, W.; Keh, C. C. K.; Li, C.-J.;
Varma, R. S. Clean Tech. Environ. Policy 2005, 7, 62e69; (h) Kumar, D.;
Chandra Sekhar, K. V. G.; Dhillon, H.; Rao, V. S.; Varma, R. S. Green
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4.7. Typical experimental procedure for Suzuki reaction
The aryl halide (1 mmol), boronic acid (1.2 mmol), K2CO3
(1.5 mmol), and 0.1 g of PdeNHC silica (V) (0.004 g of Pd)
were added to 1.5 mL DMF/H2O (1:2) in a 10 mL crimp-
sealed thick-walled glass tube equipped with a pressure sensor
and a magnetic stirrer and similar procedure as describe above
was followed at 100 ꢀC for 10 min.
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He is currently supported by the Postgraduate Research Pro-
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administered by the Oak Ridge Institute for Science and

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