Palladium bipyridyl complex anchored on nanosized MCM-41 as a highly efficient and recyclable catalyst for Heck reaction

Article abstract of DOI:10.1016/j.tetlet.2004.08.065

Palladium bipyridyl complex anchored inside the channels of nanosized MCM-41 silicas was found to be a highly efficient and recyclable catalyst for the Heck reaction with a turnover number up to 10⁶ for each cycle.

Full text of DOI:10.1016/j.tetlet.2004.08.065

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 7503–7506
Palladium bipyridyl complex anchored on nanosized MCM-41 as
a highly efficient and recyclable catalyst for Heck reaction
a
Fu-Yu Tsai,a Chen-Lin Wu,a Chung-Yuan Mou,a,b, Man-Chien Chao,
*
Hong-Ping Linc and Shiuh-Tzung Liua,*
aDepartment of Chemistry, National Taiwan University, Taipei 106, Taiwan
bCenter of Condensed Matter, National Taiwan University, Taipei 106, Taiwan
cDepartment of Chemistry, National Cheng-Kung University, Tainan 701, Taiwan
Received 20 May 2004; revised 15 July 2004; accepted 3 August 2004
Available online 27 August 2004
Abstract—Palladium bipyridyl complex anchored inside the channels of nanosized MCM-41 silicas was found to be a highly efficient
and recyclable catalyst for the Heck reaction with a turnover number up to 106 for each cycle.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Palladium catalyzed coupling of aryl halides and olefins,
known as Heck reaction, is an important reaction for
carbon–carbon bond formation.1,2 Although the homo-
geneous condition for this catalysis is well documented,
the search of new heterogeneous catalysts for both effi-
cient and recyclable purposes has received much atten-
catalyst for the Heck reaction, offering an opportunity
to meet the goal of green chemistry.
According to the procedure reported by Lin et al.,9 the
nanosized (70–120nm) mesoporous material MCM-41
possessing worm-hole type pore structure with a narrow
average pore diameter of ca. 2.9nm and surface area
of 888m2 gꢀ1 was used for loading the palladium
catalyst. The size of the MCM-41 materials is much less
than the normal micro-sized ones, facilitating easy trans-
port of the reactants and products. For the ligand part,
4,40-bis(bromomethyl)-2,20-bipyridine 1 was prepared
according to the reported method.10 Treating 1 with
3-aminopropyltriethoxysilane in the presence of Et3N
at 50ꢁC for 6h resulted in the formation of functional-
ized bipyridine 2.13 Because of the sensitivity of trieth-
oxysilyl groups toward moisture, 2 was subsequently
reacted with PdCl2(PhCN)2 in dried toluene to give the
desired species 3. The resulting palladium complex 3
was modified onto the freshly calcined nano-sized
MCM-41 material in toluene by refluxing for 48h, and
the unreacted silanol groups on the silica surface was
silylated by the treatment with chlorotrimethylsilane
(Scheme 1). The amount of Pd complex anchored on
the wall of MCM-41 was quantified to be 0.14mmol/g
by ICP mass spectrometer.
tion
recently.2c,d
Among
various
strategies,
incorporation of active metals or their complexes into
porous silica matrix is one of the promising
approaches.3 Ying and co-workers have reported the
preparation of a highly dispersed Pd species throughout
the entire MCM-41 for Heck reaction.4 Other studies
involving the palladium-amine5 and -imine/pyridine6
complexes anchored onto MCM-41 for this purpose
have appeared. Although most of the known hetero-
geneous catalysts have demonstrated the purpose of
recycle use, the catalytic turnover number (TON) for
a single run appeared to be a challenge as compared
to the homogeneous system. To our knowledge, the
TON of the heterogeneous catalysis is usually lower by
a factor between 101 and 102 than the corresponding
homogeneous one.3g,7,8 For the catalysis carried out
within a confined space, saturation of the product and
salt inside pore generally decreases the activity of cata-
lysts.6a In this context, we found that a bipyridine–palla-
dium complex grafted onto the surface of nanosized
MCM-41 performs as a high efficient and recyclable
From the N2 adsorption–desorption isotherm study, the
palladium-loaded MCM-41 possesses a BET (Brunner–
Emmett–Teller) surface area of 660m2 gꢀ1 with a narrow
pore size distribution centered at 2.5nm. The grafting of
*
Corresponding authors. Fax: +886 2 2366 0954 (C.-Y.M); e-mail
addresses: cymou@ntu.edu.tw; stliu@ntu.edu.tw
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.08.065

7504
F.-Y. Tsai et al. / Tetrahedron Letters 45 (2004) 7503–7506
Reaction of aryl iodides with methyl acrylate in the pres-
(EtO)3Si
(CH2)3
HN
Si(OEt)3
(CH2)3
NH
ence of Et3N yielded the corresponding cinnamate in
ꢁ80% yields (entries 1, 4). Replacing Et3N with Bu3N,
the products could be isolated almost quantitatively
(entries 2–3, 5–6), presumably due to the higher boiling
point of such base. However, the reaction of aryl bro-
mides with butyl acrylate or styrene did not proceed at
100ꢁC. It took place smoothly at an elevated tempera-
ture with excellent turnover numbers for activated aryl
bromide (entries 7–11). In the case of p-bromoacetophen-
one, the turnover frequency (TOF) can reach upto
6.6 · 104 (entry 7). As for bases, trialkylamine appeared
to be a better choice. The use of sodium acetate or potas-
sium carbonate provided only trace of product (entries
12–13) presumably due to the poor solubility of these
salts in the organic solvents. Reaction of bromobenzene
with butyl acrylate afforded the coupling product in good
yield, but poor result with styrene (entries 14, 15). Unfor-
tunately, this catalyst performance appeared to be poor
with the deactivated aryl bromides (entry 16).
Br
Br
(i)
N
N
N
N
1
2
(EtO)3Si
(CH2)3
HN
Si(OEt)3
(CH2)3
O
Si
O
O
O
O
O
NH
Si
(iii)
(CH2)3
NH
(ii)
(CH2)3
HN
N
N
Pd
Cl
Cl
N
N
Pd
Cl
Cl
3
4
Scheme 1. Preparation of Pd grafting nano-sized MCM-41: (i)
H2N(CH2)3Si(OEt)3, Et3N, THF; (ii) (CH3CN)2PdCl2; (iii) (a)
MCM-41, toluene, D; (b) Me3SiCl.
To demonstrate the good transport of molecules inside
the nanosized MCM-41, the identical palladium com-
plex was modified onto the wall of fibrous MCM-41 sili-
cas (pore size ꢁ2.7nm) for comparison.12 For the
fibrous silica consisted of long channels (in 103 nm
dimension), one would expect the difficulty in material
transporting inside the channels. In addition, the same
palladium complex catalyzed the coupling reaction
under homogeneous conditions was studied for compar-
ison. Figure 2 shows the difference of yield versus time in
the coupling reaction of phenyl iodide with acrylate
among these systems. It appears that nanosized catalyst
4 behaves in the same order of activity as the homogene-
ous one, but much better than the catalyst prepared via
the support of the fibrous MCM-41 silica. This result
clearly demonstrates very little diffusion interference
along the nanosized MCM-41 channels for transporting
molecules (i.e., the reactant molecules are easily access-
ing to the catalytic sites). On the other hand, the reac-
tion proceeded slowly in the fibrous MCM-41 system,
reflecting the difficult transport of substrates.
metal complex decreases the pore size and surface area
somewhat, but with enough space left for the reactant
molecules. One notices that there is a very large jump
of adsorption volume near saturation pressure. This re-
flects the large contribution to adsorption by the inter-
particle texture porosity, which shows these MCM-41
particles are indeed in nanosized dimension. The TEM
images of as calcined and grafted material are shown
in Figure 1 for comparison, indicating the preserva-
tion of nanoparticle morphology upon grafting of
complexes.
For the catalytic activity test, coupling reactions of var-
ious aryl iodides and bromides with acrylates and styr-
ene were investigated. In a typical experiment, freshly
distilled N-methylpyrrolidinone (NMP) solvent was
added to a degassed flask containing a mixture of aryl
halide, olefin, and triethylamine in a ratio of 1:1.5:1, fol-
lowed by the addition of the catalyst. The resulting mix-
ture was heated in an oil bath at the specified
temperature for a certain period of time. The catalyst
was recovered by centrifugation and washed sub-
sequently with THF and water twice for each washing.
The liquid portion was added to water, extracted with
ethyl acetate, and concentrated to yield the desired
For the recycling study, reaction was conducted by
using 100mg (4.76 · 10ꢀ2 mol% Pd) of catalyst in the
coupling of phenyl iodide with methyl acrylate ([sub-
strate]/[Pd] = 2100). After the first run of the reaction,
the catalyst was recovered by simple centrifugation
and then dried under vacuum overnight. Under the
same conditions, a complete conversion of the substrate
into the desired product by using the recovered catalyst
was observed within 5h. Furthermore, the catalyst
remained the same activity after four reuse runs, for
example, close to 100% conversion for each use, without
any saturation phenomenon during the reaction. As for
the metal leaching, we checked the solution of the reac-
tion mixture after the catalysis and found less than ppb
level of palladium in the liquid portion of the reaction
mixture. This observation also explains that the recycled
catalysts remain the high activity after several uses.11
1
product, which was either characterized by H NMR
spectroscopy or separated by column chromatography.
All of the results are summarized in Table 1.
In summary, we have prepared an extremely efficient
and recyclable palladium anchored nano-sized MCM-
Figure 1. TEM images of (a) as calcined; (b) Pd complex grafted
MCM-41 silicas.

F.-Y. Tsai et al. / Tetrahedron Letters 45 (2004) 7503–7506
Table 1. Results of the Heck reaction catalyzed by nanosized MCM-41-Pda
7505
Entry
Aryl halide
Olefin
Pd (mol%)
Base
T (ꢁC)
t (h)
Yield (%)b
TONd
1
C6H5I
C6H5I
C6H5I
Methyl acrylate
n-Butyl acrylate
Styrene
9.33 · 10ꢀ5
9.33 · 10ꢀ5
1.17 · 10ꢀ3
1.33 · 10ꢀ4
9.33 · 10ꢀ5
9.35 · 10ꢀ4
9.33 · 10ꢀ5
4.67 · 10ꢀ3
1.04 · 10ꢀ4
1.87 · 10ꢀ4
4.67 · 10ꢀ3
9.33 · 10ꢀ3
9.33 · 10ꢀ3
4.70 · 10ꢀ4
1.87 · 10ꢀ2
4.67 · 10ꢀ3
Et3N
100
100
100
100
100
100
170
170
170
170
170
170
170
170
170
170
96
96
72
96
96
72
16
72
24
16
72
48
48
48
72
96
83
8.90 · 105
1.05 · 106
8.12 · 104
6.09 · 105
1.04 · 106
1.04 · 105
1.05 · 106
1.61 · 104
8.17 · 105
4.97 · 105
1.54 · 104
—
2
Bu3N
Bu3N
Et3N
98
3
95c
81
4
p-MeC6H4I
p-MeC6H4I
p-MeC6H4I
p-CH3COC6H4Br
p-CH3COC6H4Br
p-BrC6H4CHO
p-BrC6H4NO2
p-BrC6H4NO2
p-BrC6H4NO2
p-BrC6H4NO2
C6H5Br
Methyl acrylate
n-Butyl acrylate
Styrene
5
Bu3N
Bu3N
Bu3N
Bu3N
Bu3N
Bu3N
Bu3N
NaOAc
K2CO3
Bu3N
Bu3N
Bu3N
97
6
97c
98
7
n-Butyl acrylate
Styrene
8
75c
9
n-Butyl acrylate
n-Butyl acrylate
Styrene
85
10
11
12
13
14
15
16
93
70c
Trace
Trace
72
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
Styrene
—
1.53 · 105
1.07 · 103
2.57 · 103
C6H5Br
p-BrC6H4OMe
20c
n-Butyl acrylate
12
a [RX]/[Pd] = 106; in N-methylpyrrolidinone.
b Isolated yields.
c trans:cis ꢁ 90:10.
d molmol of Pdꢀ1
.
2004.08.065. Figure of the N2 adsorption–desorption
isotherm for the unloaded MCM-41 and the MCM-41
sample after loaded with palladium catalyst.
110
100
90
80
70
60
50
40
30
20
10
0
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13. Bipyridinyl ligand 2: 1H NMR (C6D6) d: 0.67–0.73 (m,
4H), 1.14 (t, J = 7.0Hz, 18H), 1.63–1.68 (m, 4H), 2.42 (t,
J = 7.0Hz, 4H), 3.50 (s, 4H), 3.75 (q, J = 7.0Hz, 12H),
7.07 (d, J = 4.2Hz, 2H), 8.57 (d, J = 4.2Hz, 2H), 8.78 (s,
2H); 13C NMR d: 8.4 (2C), 18.6 (6C), 23.8 (2C), 52.4 (2C),
53.0 (2C), 58.4 (6C), 120.7 (2C), 123.1 (2C), 149.3 (2C),
151.2 (2C), 156.8 (2C). HRMS: m/z calcd for
C30H54N4O6Si2, 622.3582; found, 622.3539.