Mesoporous silica MCM-41 supported N-heterocyclic carbene-Pd complex for heck and sonogashira coupling reactions

Article abstract of DOI:10.1002/jccs.201400196

Heterocyclic carbene-Pd complex was anchored onto the mesoporous silica MCM-41 which exhibits high catalytic activity in Heck reaction under phosphine free reaction conditions for the reaction of iodo/bromoarenes with olefinic compounds such as butyl acry

Full text of DOI:10.1002/jccs.201400196

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Mesoporous Silica MCM-41 Supported N-Heterocyclic Carbene-Pd Complex for Heck
and Sonogashira Coupling Reactions
Shaheen M. Sarkar, Mashitah Mohd Yusoff and Md. Lutfor Rahman*
Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Gambang, Kuantan, Malaysia
(Received: Apr. 29, 2014; Accepted: Jul. 15, 2014; Published Online: ??; DOI: 10.1002/jccs.201400196)
Heterocyclic carbene-Pd complex was anchored onto the mesoporous silica MCM-41 which exhibits high
catalytic activity in Heck reaction under phosphine free reaction conditions for the reaction of iodo/bro-
moarenes with olefinic compounds such as butyl acrylate, isopropyl acrylate and styrene. This catalytic
system also showed high activity for Sonogashira coupling reaction of various aryl halides under copper,
phosphine and solvent-free reaction conditions. The air and thermally stable catalyst were reused several
times without significant loss of its activity. High efficiency of the catalyst along with its recycling ability
and the rather low Pd-loading demonstrated in both Heck and Sonogashira coupling reactions are the mer-
its of the presented catalyst system.
Keywords: Heck reaction; Mesoporous MCM-41; NHC-Pd catalyst; Sonogashira reaction.
INTRODUCTION
Palladium catalysts are among the most popular tran-
a recyclable catalyst.18
However, these systems do not reach the excellent ac-
tivities that are observed with homogeneous catalyst and
convert the interesting and broadly available aryl bromide
system. In our earlier study we reported MCM-41 (Mobil
Composition of Matter) supported N-heterocyclic carbene
palladium (NHC-Pd) catalyzed Suzuki-Miyaura cross cou-
pling reaction of aryl halide and boronic acid in aqueous
N,N¢-dimethylformamide (DMF) solution.19 In this report,
considerable effors have been made for this reaction in or-
der to achieve a high degree of efficiency under mild reac-
tion conditions. Herein, our recent finding is the develop-
ment of highly effective heterogeneous MCM-41-sup-
ported NHC-Pd catalyst for Heck and Sonogashira cou-
pling reactions of activated and deactivated aryl iodide and
bromide under mild reaction conditions.
sition metal catalysts widely used in numerous organic syn-
theses and synthetic transformations.1-4 The traditional
coupling reactions are mostly catalyzed by phosphine-
based palladium complexes in homogeneous solution.5,6
One practical limit to perform homogeneous catalysis reac-
tions is the difficulty of separation of the product from cata-
lyst and reuse of the catalyst. In this study, heterogeneous
Pd-catalyst is emerging as an alternative to the homoge-
neous one that suffers various limitations both commer-
cially and environmentally. Heterogeneous catalysts have
some advantages such as heat stability and ease of separa-
tion from the product when compared with homogeneous
catalysts. One common strategy to prepare a heterogeneous
catalyst consists of anchoring the conveniently modified
transition metal complex onto an insoluble support pro-
vided the anchoring procedure and maintains the intrinsic
activity and selectivity of the catalytic center.7,8
EXPERIMENTAL
General Information: All manipulations were performed
under atmospheric conditions otherwise noted. Reagents and sol-
vents were obtained from commercial suppliers and used without
further purification. Water was deionized with a Millipore system
as a Milli-Q grade. Pd(OAc)2 was purchased from Aldrich chemi-
cal industries, Ltd. Proton nuclear magnetic resonance (1H NMR,
500/400 MHz) and carbon nuclear magnetic resonance (13C
NMR, 125 MHz) spectra were measured with a JEOL JNM ECA-
From these perspectives, heterogeneous catalysis
seems particularly well suited since the palladium metal
immobilized on a support could be easily removed by fil-
tration leaving products virtually free of palladium resi-
dues. Anumber of supports such as zeolite,9 metal oxides,10
silica-starch,11 polymers,12,13 carbon nanofiber,14 clay,15
montmorillonite,16 silica and magnetic mesoporous silica17
have been used. Alternatively, molecular complexes bound
to a ligand anchored on a support have also been studied as
1
500/400 spectrometer. The H NMR chemical shifts were re-
ported relative to tetramethylsilane (TMS, 0.00 ppm). The 13C
* Corresponding author. Tel: +6 016 9399011; Fax: +6 09 5492766; Email: lutfor73@gmail.com
J. Chin. Chem. Soc. 2014, 61, 000-000
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1

Article
Sarkar et al.
NMR chemical shifts were reported relative to deuterated chloro-
form (CDCl3 77.0 ppm). Elemental analyses were performed on a
Yanaco CHN Corder MT-6 elemental analyzer by the chemical
analysis team in Rikagaku Kenkyãjo (RIKEN), Wako, Japan. In-
ductively coupled plasma atomic emission spectrometry (ICP-
AES) was performed on a Shimadzu ICPS-8100 equipment by the
chemical analysis team in RIKEN Wako, Japan. The gas chroma-
tography-mass spectrometry (GC-MS) was measured by an
Agilent 7860A/JEOL JMS-T100GC equipped with a capillary
column (DB-Wax, 0.25 mm i.d. × 30 m or HP-1, 0.32 mm i.d. × 30
m). Thin layer chromatography (TLC) analysis was performed on
Merck silica gel 60 F254. Column chromatography was carried out
on silica gel (Wakogel C-300).
110 °C, a fine brown color powder was collected by filtration. The
powder was successively washed by methylene chloride and
dried under vacuum at 60 °C to give MCM-41-supported NHC-
Pd/MCM-41. Elemental analysis and weight gain showed that
0.22 mmol of Pd was anchored onto 1.0 g of the catalyst NHC-
Pd/MCM-41 (see ESI).
General procedure for the Heck reaction: All reactions
were carried out in a 4 mL glass vial equipped with a Teflon screw
cap. A mixture of aryl halide (1 mmol), olefin (1.2 mol equiv),
Na2CO3 (2 mol equiv), and the NHC-Pd/MCM-41 (2.27 mg, 0.05
mol%) in N,N¢-dimethylacetamide: water (DMF: H2O, 2 + 2 mL)
was stirred at 130 °C for an appropriate time, monitoring periodi-
cally by GC analysis. After completion of the reaction, it was cold
at room temperature and diluted with ethyl acetate (EtOAc). The
immobilized NHC-Pd/MCM-41 was separated by filtration and
washed by EtOAc. The organic layer was washed by H2O, brine
and dried over MgSO4 and evaporated under reduced pressure.
The residue was purified by short column chromatography on sil-
ica gel eluted with n-hexane/EtOAc afforded the desired coupled
products in 92-97% yields (Tables 3 and 4).
Preparation of the mesoporous MCM-41 silica: The
MCM-41 was prepared according to the reported method20 using
cetyltrimethylammonium bromide as a surfactant and tetraeth-
oxysilane as a silicate source. The MCM-41 was characterized by
X-ray diffraction (XRD) and high resolution transmission elec-
tron microscopy (HRTEM) analyses.
Preparation of 1-methyl-3-[(triethoxysilyl)propyl]-imi-
dazolium chloride 1: A 50 mL round bottomed flux was charged
with (3-chloropropyl)triethoxysilane (1 g, 4.166 mmol) and 1-
methyl imidazole (342 mg, 4.16 mmol) in toluene and the reac-
tion mixture was heated at 70 °C for 3 h. The solvent was evapo-
rated and the obtained ionic liquid was washed several times by
diethylether and dried under reduced pressure to give the 1-
methyl-3-[(triethoxysilyl)propyl]-imidazolium chloride in 95%
yield.21,22 1H NMR (300 MHz, CDCl3): d 0.57 (t, J = 8.0 Hz, 2 H),
1.16 (t, J = 6.9 Hz, 9 H), 1.99 (p, J = 7.6 Hz, 2 H), 3.81 (q, J = 6.9
Hz, 6 H), 4.12 (s, 3 H), 4.20 (t, J = 7.3 Hz, 2 H), 7.35 (s, 1 H), 7.62
(s, 1 H), 10.60 (s, 1 H) ppm; 13C NMR (75 MHz, CDCl3): d 6.82,
18.12, 23.92, 36.13, 51.31, 58.12, 121.92, 124.12, 138.13.
Grafting of the NHC precursor onto mesoporous MCM-
41 2: MCM-41 (1 g) was suspended into a stirred solution of N-1-
(3-triethoxysilylpropyl)-3-methylimidazolium chloride 1 (0.22 g,
0.68 mmol) in toluene. The mixture was stirred at 105 °C for 12 h
and filtrated off on a glass filter and washed with methylene chlo-
ride. After drying under vacuum at 60 °C, MCM-41-supported
N-heterocyclic carbine 2 (1.16 g) was obtained. Elemental analy-
sis and weight gain showed that 0.75 mmol of triethoxysilyl-
propylimidazolium chloride 1 was grafted onto the 1.0 g of
MCM-41 silica 2 (see ESI).
1
(E)-3-Phenylacrylic acid n-butyl ester 3a24: H NMR
(500 MHz, CDCl3) d = 7.70 (d, J = 16.0 Hz, 1 H), 7.53-7.51 (m, 2
H), 7.39-7.36 (m, 3 H), 6.46 (d, J = 16.05 Hz, 1 H), 4.21 (t, J = 6.9
Hz, 2 H), 1.72-1.67 (m, 2 H), 1.47-1.40 (sextet, J = 7.45 Hz, 2 H),
0.96 (t, J = 7.45 Hz, 3 H). 13C NMR (125 MHz, CDCl3) d: 167.07,
144.51, 134.45, 130.16, 1128.84, 128.02, 118.28, 64.39, 30.75,
19.17, 13.72. EI-MS m/z = 204 (M+). (E)-3-(4´-tolyl)acrylic acid
n-butyl ester 3b24: 1H NMR (500 MHz, CDCl3) d = 7.67 (d, J =
16.05 Hz, 1 H), 7.43 (d, J = 8.10 Hz, 2 H), 7.19 (d, J = 8.05 Hz, 2
H), 6.40 (d, J = 16.0 Hz, 1 H), 4.20 (t, J = 6.3 Hz, 2 H), 2.36 (s, 3
H), 1.71-1.65 (m, 2 H), 1.47-1.40 (m, 2 H), 0.94 (t, J = 7.45 Hz, 3
H). 13C NMR (125 MHz, CDCl3) d: 167.28, 144.52 140.58,
131.73, 129.57, 128.02, 117.18, 64.32, 30.77, 21.42, 19.18,
13.73. EI-MS m/z = 218 (M+). (E)-3-(4´-Methoxyphenyl)acrylic
acid n-butyl ester 3c24: 1H NMR (500 MHz, CDCl3) d = 7.65 (d, J
= 16.0 Hz, 1 H), 7.48 (d, J = 8.6 Hz, 2 H), 6.90 (d, J = 8.5 Hz, 2 H),
6.32 (d, J = 16.0 Hz, 1 H), 4.19 (t, J = 7.0 Hz, 2 H), 3.83 (s, 3 H),
1.71-1.65 (m, 2 H), 1.45-1.41 (sextet, J = 7.5 Hz, 2 H), 0.96 (t, J =
7.5 Hz, 3 H). 13C NMR (125 MHz, CDCl3) d: 167.42, 161.29,
144.17, 129.66, 127.20, 115.76, 114.28, 64.23, 55.33, 30.79,
19.18, 13.73. EI-MS m/z = 234 (M+). (E)-3-(4´-Nitrophenyl)-
acrylic acid n-butyl ester 3d24: 1H NMR (500 MHz, CDCl3) d =
8.25 (d, J = 8.55 Hz, 2 H), 7.72-7.66 (m, 3 H), 6.58 (d, J = 16.0 Hz,
1 H), 4.24 (t, J = 6.3 Hz, 2 H), 1.73-1.67 (m, 2 H), 1.48-1.42 (m, 2
H), 0.97 (t, J = 7.7 Hz, 3 H). 13C NMR (125 MHz, CDCl3) d:
166.10, 148.47, 141.56, 140.59, 128.60, 124.14, 122.61, 64.90,
30.68, 19.15, 13.70. EI-MS m/z = 249 (M+). (E)-3-(4-Acetyl-
Preparation of the MCM-41-supported NHC-Pd/MCM-
41: Pd(OAc)2 (0.94 mg, 0.42 mmol) was added to a stirred solu-
tion of MCM-41-supported NHC precursor 2 (1 g, 0.75 mmol) in
N,N¢-dimethyl sulfoxide (DMSO, 10 mL).23 The mixture was
stirred for 2.5 h at 65 °C and after stirring for an additional 1 h at
2
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Mesoporous Silica MCM-41 Supported Pd Complex for Heck Reaction
1
phenyl)acrylic acid n-butyl ester 3e24: H NMR (500 MHz,
d: 136.62, 131.19, 128.78, 128.27, 127.11, 126.76, 126.56,
125.63 (q, J = 3.57 Hz, C-F), 123.12. EI-MS m/z = 248 (M+).
CDCl3) d = 7.97 (d, J = 8.0 Hz, 2 H), 7.71 (d, J = 16.5 Hz, 1 H),
7.621 (d, J = 8.0 Hz, 2 H), 6.54 (d, J = 16.05 Hz, 1 H), 4.22 (t, J =
6.9 Hz, 2 H), 2.61 (s, 3 H), 1.72-1.67 (m, 2 H), 1.46-1.42 (m, 2 H),
0.97 (t, J = 7.45 Hz, 3 H). 13C NMR (125 MHz, CDCl3) d: 197.28,
166.55, 142.93, 138.79, 137.94, 128.82, 128.08, 120.82, 64.65,
30.71, 26.64, 19.15, 13.70. EI-MS m/z = 246 (M+). (E)-3-(4´-
Trifluoromethylphenyl)acrylic acid n-butyl ester 3f24: 1H
NMR (500 MHz, CDCl3) d = 7.70 (d, J = 15.0 Hz, 1 H), 7.65-7.61
(m, 4 H), 6.52 (d, J = 16.0 Hz, 1 H), 4.23 (t, J = 6.7 Hz, 2 H),
1.72-1.64 (m, 2 H), 1.48-1.40 (sextet, J = 7.5 Hz, 2 H), 0.97 (t, J =
7.45 Hz, 3 H). 13C NMR (125 MHz, CDCl3) d: 166.48, 142.63,
137.83, 131.81 (q, J = 23.2 Hz), 128.13, 125.86 (q, J = 3.57 Hz),
124.89, 122.72, 120.86, 64.69, 30.71, 19.15, 13.69. EI-MS m/z =
272 (M+). n-Butyl trans-3-(3-pyridinyl)acrylate 3g25: 1H NMR
(500 MHz, CDCl3) d = 8.75 (s, 1 H), 8.61-8.60 (m, 1 H), 7.84 (d, J
= 7.5 Hz, 1 H), 7.68 (d, J = 16.1 Hz, 1 H), 7.34-7.32 (m, 1 H), 6.53
(d, J = 16.1 Hz, 1 H), 4.23 (t, J = 7.0 Hz, 2 H), 1.72-1.67 (quint, J =
6.5 Hz, 2 H), 1.47-1.40 (sextet, J = 7.5 Hz, 2 H), 0.97 (t, J = 7.5
Hz, 3 H). 13C NMR (125 MHz, CDCl3) 166.37, 150.93, 149.68,
140.78, 134.16, 130.21, 123.69, 120.48, 64.68, 30.69, 19.15,
13.70. EI-MS m/z = 205 (M+). (E)-N-Isopropyl-cinnamamide
3h25: 1H NMR (500 MHz, CDCl3) d = 7.62 (d, J = 15.5 Hz, 1 H),
7.48-7.47 (m, 2 H), 7.36-7.33 (m, 3 H), 6.39 (d, J = 16.6 Hz, 1 H),
1
(E)-4-Methylstilbene 3m28: H NMR (500 MHz, CDCl3) d =
7.51 (d, J = 8.0 Hz, 2 H), 7.42 (d, J = 7.5 Hz, 2 H), 7.34 (t, J = 8.0
Hz, 2 H), 7.25-7.22 (m, 1 H), 7.16 (d, J = 8.0 Hz, 2 H), 7.07 (d, J =
4.05 Hz, 2 H), 2.36 (s, 3 H).13C NMR (125 MHz, CDCl3) d:
137.51, 134.54, 129.38, 128.64, 127.69, 127.39, 126.37, 21.23.
EI-MS m/z = 194 (M+). (E)-4-Methoxystilbene 3n28: H NMR
1
(500 MHz, CDCl3) d = 7.49-7.44 (m, 4 H), 7.34 (t, J = 7.5 Hz, 2
H), 7.26-7.21 (m, 1 H), 7.08 (d, J =16.0 Hz, 1 H), 6.99 (d, J = 16.5
Hz, 1 H), 6.90 (d, J = 5.0 Hz, 2 H), 3.82 (s, 3 H). 13C NMR (125
MHz, CDCl3) d: 159.30, 137.64, 130.14, 128.63, 128.20, 127.70,
127.20, 126.61, 126.23, 114.13, 55.31. EI-MS m/z = 210 (M+).
(E)-4-Styrylbenzonitrile 3o28: 1H NMR (500 MHz, CDCl3) d =
7.63 (d, J = 8.15 Hz, 2 H), 7.57 (d, J = 6.5 Hz, 2 H), 7.53 (d, J =
7.45 Hz, 2 H), 7.38 (m, 2 H), 7.31 (t, J = 7.45 Hz, 1 H), 7.23 (t, J =
16.6 Hz, 1 H), 7.10 (d, J = 16.6 Hz, 1 H). 13C NMR (125 MHz,
CDCl3) d: 141.81, 136.26, 132.46, 132.38, 128.84, 128.62,
126.89, 126.83, 126.70, 119.00, 110.56. EI-MS m/z = 205 (M+).
General procedure for Sonogashira reaction: Aryl ha-
lide (1 mmol), phenylacetylene (1.2 mol equiv), piperidine (2 mol
equiv) and the NHC-Pd/MCM-41 (1 mg, 0.02 mol%) was stirred
at 85 °C for an appropriate time monitoring periodically by GC
analysis. The reaction mixture was cold at room temperature and
diluted with EtOAc and the immobilized Pd catalyst was sepa-
rated by filtration. The organic layer was washed by H2O and
dried over MgSO4. Solvent was evaporated under reduced pres-
sure and the residue was purified by short column chromatogra-
phy on silica gel eluted with n-hexane/EtOAc to afford the corre-
sponding coupling products in up to 95% yields (Table 5).
5.62 (brs, 1 H), 4.26-4.19 (m, 1 H), 1.22 (d, J = 6.3 Hz, 6 H). 13
C
NMR (125 MHz, CDCl3) d: 164.98, 140.63, 134.91, 129.48,
128.74, 127.68, 121.06, 41.54, 22.81. EI-MS m/z = 189 (M+).
(E)-N-isopropyl-3-(p-tolyl)acrylamide 3i26: 1H NMR (500
MHz, CDCl3) d = 7.48 (d, J = 15.5 Hz, 1 H), 7.39 (d, J = 8.0 Hz, 2
H), 7.16 (d, J = 7.45 Hz, 2 H) 6.32 (d, J = 15.45 Hz, 1 H), 5.46
(brs, 1 H), 4.25-4.19 (m, 1 H), 2.35 (s, 3 H), 1.22 (d, J = 6.3 Hz, 6
H). 13C NMR (125 MHz, CDCl3) d: 165.15, 140.63, 139.77,
132.15, 129.48, 127.66, 119.97, 41.50, 22.85, 21.36. EI-MS m/z =
203 (M+). (E)-N-isopropyl-3-(4-methoxyphenyl)acrylamide
Diphenylacetylene 4a29: H NMR (CDCl3, 400 MHz) d:
1
7.52-7.47 (m, 4H), 7.35-7.28 (m, 6H). 13C NMR (CDCl3, 125
MHz) d: 131.5, 128.1, 128.1, 123.2, 89.6. EI-MS m/z = 178 (M+).
4-Methoxyphenyl-phenylacetylene 4b30: 1H NMR (CDCl3, 400
MHz) d: 7.51-7.47 (m, 4 H), 7.35-7.31 (m, 3 H), 6.85 (dd, J =
8.78, 2.2 Hz, 2 H), 3.83 (s, 3 H).13C NMR (CDCl3, 125 MHz) d:
159.3, 133.2, 131.2, 128.4, 127.8, 123.6, 115.4, 114.0, 89.4, 88.1,
1
3j27: H NMR (500 MHz, CDCl3) d = 7.58 (d, J = 16.05 Hz, 1
H),7.43 (d, J = 8.6 Hz, 2 H), 6.86 (d, J = 8.6 Hz, 2 H), 6.26 (d, J =
16. 0 Hz, 1 H), 5.56 (brs, 1 H), 4.25-4.18 (m, 1 H), 3.81 (s, 3 H),
1.22 (d, J = 6.85 Hz, 6 H). 13C NMR (125 MHz, CDCl3) d: 165.31,
160.72, 140.22, 129.20, 127.62, 118.69, 114.17, 55.30, 41.46,
22.84. EI-MS m/z = 219 (M+). (E)-Stilbene 3k28: 1H NMR (500
MHz, CDCl3) d = 7.51 (d, J = 7.4 Hz, 4 H), 7.36-7.33 (m, 4 H),
7.27-7.25 (m, 2 H), 7.10 (d, J = 11.5 Hz, 2 H). 13C NMR (125
MHz, CDCl3) d: 137.38, 128.65, 127.6, 126.49. EI-MS m/z = 180
(M+). (E)-1-Styryl-4-(trifluoromethyl)benzene 3l 28: 1H NMR
(500 MHz, CDCl3) d = 7.60 (m, 4 H), 7.54 (d, J = 7.7 Hz, 2 H),
7.38 (t, J = 7.4 Hz, 2 H), 7.30 (t, J = 7.5 Hz, 1 H), 7.21 (d, J = 16.5
Hz, 1 H), 7.13 (d, J = 16.5 Hz, 1 H). 13C NMR (125 MHz, CDCl3)
55.2. EI-MS m/z = 208 (M+). Phenyl-p-tolyacetylene 4c31: H
1
NMR (CDCl3, 400 MHz) d: 7.51-7.49 (m, 2 H), 7.42 (d, J = 7.89
Hz, 2 H), 7.32-7.26 (m, 3 H), 7.12 (d, J = 7.88 Hz, 2 H), 2.32 (s,
3H). 13C NMR (CDCl3, 125 MHz) d: 138.2, 131.4, 129.2, 128.3,
128.3, 123.4, 120.3, 89.7, 88.6, 21.5. EI-MS m/z = 192 (M+).
4-Acetylphenyl-phenylacetylene 4d32: H NMR (CDCl3, 400
1
MHz) d: 7.92 (d, J = 8.49Hz, 2 H), 7.60 (d, J = 8.49 Hz, 2 H),
7.55-7.53 (m, 2H), 7.37-7.34 (m, 3 H), 2.59 (s, 3H). 13C NMR
(CDCl3, 125 MHz) d: 197.2, 136.3, 131.9, 131.7, 128.9, 128.6,
128.1, 128.1, 122.5, 92.8, 88.4, 26.7. EI-MS m/z = 220 (M+).
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Article
Sarkar et al.
4-Nitrophenyl-phenylacetylene 4e33: H NMR (CDCl3, 400
MHz) d: 8.20 (d, J = 8.92 Hz, 2 H), 7.65 (d, J = 8.90 Hz, 2 H),
7.57-7.54 (m, 2 H), 7.41-7.37 (m, 3H). 13C NMR (CDCl3, 125
MHz) d: 147.1, 132.1, 131.9, 130.3, 129.3, 128.6, 123.8, 122.1,
94.8, 87.6. EI-MS m/z = 223 (M+). 4-Cyanophenyl-phenyl-
acetylene 4f34: 1H NMR (CDCl3, 400 MHz) d: 7.66-7.59 (m, 4 H),
7.57-7.54 (m, 2 H), 7.41-7.38 (m, 3 H). 13C NMR (CDCl3, 125
MHz) d: 132.1, 131.7, 129.2, 128.6, 128.3, 122.3, 118.6, 111.5,
93.8, 87.8. EI-MS m/z = 203 (M+). 4-Trifluoromethylphenyl-
phenylacetylene 4g35: 1H NMR (CDCl3, 400 MHz) d: 7.64-7.61
(m, 4 H), 7.58-7.55 (m, 2 H), 7.38-7.35 (m, 3 H). 13C NMR
(CDCl3, 125 MHz) d: 131.9, 131.8, 130.0 (J = 54.66 Hz), 128.9,
128.5, 127.2 (J = 2.0 Hz), 125.4 (J = 6.16 Hz), 123.8, 122.6, 91.8,
87.7. EI-MS m/z = 246 (M+).
Pd/MCM-41.
1
Both images showed that the hexagonal symmetry of
the pore arrays is conserved after the immobilization of
NHC-Pd complex onto the MCM-41 and there is no change
after the reuse of the NHC-Pd/MCM-41. This mesoporous
character is also confirmed by XRD (see ESI Fig. 1c). Ap-
parently, there is less change of the lattice parameters upon
the immobilizing process. However, the surface area, pore
volume and pore size decreased due to the grafting of or-
ganic moieties onto the mesoporous silica MCM-41 (Table
1).
Initial screening of catalytic performance in the Heck
reaction
The heterogeneous MCM-41 supported NHC-Pd/
MCM-41 was tested for its activity in the Heck coupling re-
action. The coupling of iodobenzene with butyl acrylate in
the presence of Na2CO3 with 1.5 mol% of NHC-Pd/MCM-
41 was initially chosen as a model reaction (Table 2, entry
1). The catalyst showed high activity and provided corre-
sponding coupling product in quantitative yield within 1 h
reaction time. The rate of coupling is dependent on a vari-
ety of parameters such as solvent, base and catalyst load-
ing. Thus, the reaction conditions were systematically opti-
mized, and the results are presented in Table 2. The catalyst
loading could be decreased even further, 1 to 0.01 mol%,
when 1 to 5 h reaction time was required to complete the
reaction (entries 2-6).
RESULTS AND DISCUSSION
Preparation and characterization of the NHC-Pd/
MCM-41
According to our previous work,19 we have synthe-
sized MCM-41-supported NHC-Pd/MCM-41 complex
which processes a three methylene spacer between the ni-
trogen atom of NHC and MCM-41 silica backbone. The at-
tachment of NHC-Pd complex onto MCM-41 was carried
out in two step reactions (see ESI Scheme 1).
In the first step, ionic liquid 1 was heated with MCM-
41 silica for 12 h in refluxing toluene. The incorporation of
the NHC precursor was confirmed by both nitrogen analy-
sis and weight gain provided MCM-41-supported NHC
precursor 2 with loading ratio of 0.75 mmol/g. The MCM-
41 supported NHC precursor 2 was then treated with
Pd(OAc)2 (84 mg, 0.375 mmol) in DMSO at 65 °C for 2.5 h
and 110 °C for 1 h. The resultant brown color MCM-41-
supported NHC-Pd/MCM-41 was filtrated on a glass filter
and washed by methylene chloride and dried under vacuum
at 60 °C to give MCM-41-supported NHC-Pd/MCM-41.
Elemental analysis and weight gain showed that 0.22 mmol
of Pd was anchored onto 1.0 g of the NHC-Pd/MCM-41.
The amount (0.22 mmol/g) of palladium in the catalyst
NHC-Pd/MCM-41 was further determined by ICP-AES
analysis. D. H. Lee et al. reported that N-heterocyclic
carbene can form a rigid and stable complex with Pd(II),
and NHC-Pd catalyst contains only Pd(II) with no Pd(0). It
was also confirmed that the palladium was immobilized
specifically to the imidazolium ligand in the forms of
Pd(NHC)OAcCl (major) and Pd(NHC)Cl2(minor).36 The
HRTEM image (Fig. 1a) obtained after the modification of
the parent MCM-41 and after reuse (Fig. 1b) of the NHC-
It was found that the best system for the reaction was
DMA: H2O in combination with Na2CO3 at 130 °C which
delivered a 98% yield of the product within 2 h when 0.05
mol% of NHC-Pd/MCM-41 was used (entry 5). The cata-
lyst loading could be decreased even further, 0.01 mol%,
when longer reaction time is required to complete the reac-
tion. The high conversion could be still maintained at 0.01
mol% of ultra-low Pd catalyst loading (entry 6). Further
optimization of the reaction condition was not attempted
Fig. 1. (a) HRTEM image of MCM-41-supported
NHC-Pd/MCM-41 before use and (b) after use
of NHC-Pd/MCM-41.
4
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Mesoporous Silica MCM-41 Supported Pd Complex for Heck Reaction
Table 1. Physical property of MCM-41-supported NHC-Pd/
MCM-41
Table 3. NHC-Pd/MCM-41 catalyzed Heck reaction of aryl
iodides with olefinsa
Surface
area
Pore
Pore
Sample
Pd loading
diameter volume
MCM-41
890 m2/g 2.2 nm 0.78 m3/g
-
NHC-Pd/MCM-41 554 m2/g 1.82 nm 0.45 m3/g 0.22 mmol/g
Table 2. Heck coupling of iodobenzene with butyl acrylatea
O
O
NHC-Pd/MCM-41
OBu
I
+
OBu
130 °C, base
solvent (1:1)
3a
Catalyst
Yield
(%)b
Entry Solvent (1:1)
Base
Time (h)
(mol%)
1
DMA:H2O
DMA:H2O
DMA:H2O
DMA:H2O
DMA:H2O
DMA:H2O
DMA:H2O
DMA:H2O
DMA:H2O
DMF:H2O
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
1.5
1.0
0.3
0.1
0.05
0.01
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1
1
100
98(95)c
2
3
1.5
1.5
2
97
4
97
5
98(96)c
88 (85)c
93
6
5
7
1
8
NaOMe
K2PO4
1
94
9
1
95
10
11
12
13
Na2CO3
1
92
DMSO:H2O Na2CO3
NMP:H2O Na2CO3
Toluene:H2O Na2CO3
1
94
1
95
3.5
82
a All reactions were carried using 1 mmol of aryl iodide, 1.2 mol
equiv of olefin, 0.05 mol% of NHC-Pd/MCM-41, 2 mol equiv of
Na2CO3, in 4 mL of DMA/H2O (1:1) at 130 °C for 2 h.
a Reactions were carried using 1 mmol of iodobenzene, 1.2 mol
equiv of butyl acrylate, a catalytic amount of NHC-Pd/MCM-41
and 2 mol equiv of base in 4 mL of solvents. b GC yield
determined using n-dodecane as an internal standard. c Isolated
yield.
iodobenzene with butyl acrylate (3d-f). The deactivated
aryl iodides possessing electron-donating group showed a
slight drop in reactivity compared to the activated aryl io-
dides possessing electron-withdrawing group. The cou-
pling of 3-iodopyridine with butyl acrylate was also excel-
lent (3g). The high turnover frequency (TOF, 970 h-1) value
was obtained in the coupling of 4-iodoacetophenone and
butyl acrylate (3e). The Heck coupling reaction of aryl io-
dides was also efficiently promoted to afford the corre-
sponding products when isopropyl acrylamide was used as
a coupling partner (3h-j). The bulky coupling partner sty-
rene was also testified in this coupling reaction. All acti-
vated and deactivated aryl iodides provided the corre-
sponding coupling products with excellent yields (3k-o).
In addition, all the reactions provided approximately
similar efficiency with regard to the various olefins. To
give an idea of a recent similar work, Yao and co-workers
obtained good yields in the Heck coupling of aryl halides
and olefins using polymer-supported diphenylphosphine-
due to the less amount of catalyst is required.
When the reaction was conducted in aqueous solution
of DMF, DMSO, N-methyl-2-pyrrolidone (NMP) instead
of DMA, in the presence of different bases, similar results
were obtained under the same reaction conditions (entries
7-12). A longer reaction time is required when aqueous
non-polar solvent (toluene: H2O) was used (entry 13). The
presence of water appears to be necessary due to the low
solubility of bases in organic solvents.
With these results in hand, several Heck coupling of
aryl iodides with different acrylates were then tested, and
the results are summarized in Table 3. Iodobenzene react
with butyl acrylate, to give the corresponding coupling
product 3a in 96% isolated yield. Excellent catalytic activ-
ity were observed in the couplings of deactivated 4-iodo-
toluene, 4-iodoanisole (3b, c) as well as activated 4-iodo-
nitrobenzene, 4-iodoacetophenone and 4-trifluoromethyl-
J. Chin. Chem. Soc. 2014, 61, 000-000
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5

Article
Sarkar et al.
Table 4. NHC-Pd/MCM-41 catalyzed Heck reaction of aryl
bromides with olefinsa
Pd complex. However, the catalytic system required longer
reaction time and special equipment due to the phosphine-
based ligands.37
The development of effective catalysts for the car-
bon-carbon bond formation of olefins with aryl bromides is
highly important, since these are interesting substrates for
industrial applications. Using diol-functionalized imida-
zolium ionic liquids along with PdCl2, Cai et al.38 observed
moderate yields in the Heck reaction of aryl bromides with
various alkenes but the reaction rates were quite slow. By
observing the promising catalytic activity of NHC-Pd/
MCM-41 for various aryl iodides, the catalyst was investi-
gated for the coupling of aryl bromides and olefins. It was
found that the catalyst showed outstanding activity in the
coupling of bromobenzene, 4-bromotoluene, 4-nitro bro-
mobenzene and 4-bromoacetophenone with butyl acrylate
(Table 4, 3a, b, d, e). A catalyst loading of 0.05 mol% was
sufficient to achieve the high conversion within 2 h.
The Heck reaction of aryl bromides with isopropyl
acrylamide and styrene were also efficiently promoted to
give the corresponding coupling products in up to 95%
yield (3i-k, m-o).
a All reactions were carried using 1 mmol of aryl bromide, 1.2
mol equiv of olefin, 0.05 mol% of NHC-Pd/MCM-41, 2 mol
equiv of Na2CO3, in 4 mL of DMA/H2O (1:1) at 130 °C for 2 h.
Table 5. NHC-Pd/MCM-41 catalyzed Sonogashira reaction of
aryl halidesa
Heterogeneous Sonogashira cross-coupling reaction
Sonogashira cross-coupling of an aryl halides and a
terminal alkynes are useful tool for the synthesis of aryl-
substituted acetylene compounds.39 This method has been
widely used for the synthesis of natural products,40 biologi-
cally active molecules41 nonlinear optical materials and
molecular electronics42 dendrimeric and polymeric materi-
als43 macrocycles with acetylene links44 and polyalkynyl-
ated molecules.45 With the heterogeneous Pd catalyst in
hand, we investigated its catalytic activity towards Sonoga-
shira coupling reaction of various aryl iodides with phenyl-
acetylene in the presence of piperidine. The results are
summarized in Table 5. The supported Pd catalyst pro-
moted the Sonogashira coupling reaction using only 0.02
mol% of NHC-Pd/MCM-41 under solvent-free conditions
(Table 5). The high TOF (4d, 2375 h-1) was obtained when
4-iodoacetophenone was as an aryl halide. It is worth to
note that the catalytic activities in the coupling of bromo-
benzene and substituted bromobenzenes were also excel-
lent. The results came as a bit of a surprise since most of the
reactions were complete within a very short time with
nearly 100% conversion and 100% selectivity.
a All reactions were carried using 1 mmol of aryl halide, 1.2 mol
equiv of phenyl acetylene, 0.02 mol% of NHC-Pd/MCM-41, 2
mol equiv of piperidine under solvent free condition at 85 °C for
2 h.
usability of our Pd catalyst. Catalyst was reused five times
in the reactions of iodobenzene with acrylate and phenyl-
acetylene. The NHC-Pd/MCM-41 was recovered and re-
used by the following steps: the reaction mixture was
cooled to room temperature and diluted with EtOAc and
filtered. The solid catalyst was washed with dichloro-
methane and dried at 80 °C under vacuum, and then used in
Recycling of the catalyst
The recycling of the catalyst is an important issue in
heterogeneous catalysis. Again, we turn our attention to re-
6
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J. Chin. Chem. Soc. 2014, 61, 000-000

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Mesoporous Silica MCM-41 Supported Pd Complex for Heck Reaction
Table 6. Heck and Sonogashira reaction with recycle catalyst
NHC-Pd/MCM-41a
ing of Pd from the complex into the solution was lower than
0.20 ppm, which suggests the heterogeneous nature of the
prepared NHC-Pd/MCM-41 catalyst. (b) The reaction was
carried out in a similar manner as the general procedure for
Heck reaction (Table 3, 3a). After 50% conversion of the
starting material the reaction mixture was filtered off and
the aqueous solution was heated under identical reaction
conditions for an additional 1 h and analyzed for further
conversion. No starting material was converted to the cor-
responding product after removal of the catalyst (Fig. 2).
This experiment also indicated that the Heck reactions we
investigated followed a heterogeneous pathway.
Reaction
Run
Yield (%)
1
2
3
4
96
96
Heck
O
94
OBu
92
3a
91 (<0.2 ppm Pd was
5
leached out)
1
2
3
4
5
94
93
93
90
88
Sonogashira
Ph
4a
a Reactions were carried out according to Table 3 and 5.
CONCLUSIONS
In conclusion, we have synthesized MCM-41-sup-
ported NHC-Pd catalyst and it has been applied to the Heck
and Sonogashira coupling reactions of activated and deac-
tivated aryl iodides and bromides. The NHC-Pd catalyst
promoted both of these reactions with high catalytic activ-
ity. The heterogeneity studies disclosed that the reaction
occurs at the heterogeneous pathway. The catalyst was re-
covered from the reaction by simple filtration and used for
five times without significant loss of activity. This catalyst
can be regarded as an important step toward a simple sys-
tem with the potential for commercial exploitation of heter-
ogeneous catalysts in the coupling reactions.
the next run without changing the reaction conditions. Af-
ter carrying out the reaction, the catalyst had consistent cat-
alytic activity as shown in Table 6. Only slight loss of cata-
lytic activity was observed under the same reaction condi-
tions as for initial run. The slight loss of catalyst activity af-
ter cycles case may be due to the loss of palladium from the
support during the reaction. Thus, it is reasonable to be-
lieve that the immobilized catalyst can be repeatedly used
for large-scale production without significant loss of its
catalytic activity.
Heterogeneity test
In order to check the heterogeneity (Table 3, 3a) of
the catalyst, we considered two experiments: (a) to deter-
mine the Pd content in the solution, a part of the filtrate was
placed in a 50 mL crucible after removing the solvent it was
calcined at 600 °C for 3 h. The residue was dissolved in 20
mL aqua-regia and diluted to 50 mL. The Pd leaching in so-
lution was determined by inductively coupled plasma mass
spectrometry (ICP-MS) analysis. It revealed that the leach-
ACKNOWLEDGEMENT
This work is supported by University Malaysia Pahang,
fund no. RDU 140343. The authors are grateful to Profes-
sor Myung-Jong Jin, Inha University, South Korea and
Professor Yasuhiro Uozumi, RIKEN, Wako, Japan for giv-
ing us valuable guidelines in this work.
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