Effect of stabilizing ligands bearing ferrocene moieties on the gold nanoparticle-catalyzed reactions of arylboronic acids

Article abstract of DOI:10.1016/j.tetlet.2008.10.040

The homocoupling reaction of phenylboronic acid and demetalation reaction of ferrocenylboronic acid was inhibited and highly active, respectively, in the presence of gold nanoparticles stabilized by ligands containing ferrocene moieties.

Full text of DOI:10.1016/j.tetlet.2008.10.040

Tetrahedron Letters 49 (2008) 7299–7302
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Effect of stabilizing ligands bearing ferrocene moieties on the gold
nanoparticle-catalyzed reactions of arylboronic acids
a
b
b
a
Laksamee Chaicharoenwimolkul , Ampaporn Munmai , Sanoe Chairam , Udomchai Tewasekson ,
a
c
a,d,
*
Sarawut Sapudom , Yuthana Lakliang , Ekasith Somsook
NANOCAST Laboratory, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University,
72 Rama VI Road Rachathewi, Bangkok 10400, Thailand
Institute for Innovation and Development of Learning Process, Mahidol University, 272 Rama VI Road Rachathewi, Bangkok 10400, Thailand
Samsen Wittayalai School, 132/11 Rama VI Road Phayathai, Bangkok 10400, Thailand
a
2
b
c
d
Center for Alternative Energy, Faculty of Science, Mahidol University, 272 Rama VI Road, Rachathewi, Bangkok 10400, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
The homocoupling reaction of phenylboronic acid and demetalation reaction of ferrocenylboronic acid
was inhibited and highly active, respectively, in the presence of gold nanoparticles stabilized by ligands
containing ferrocene moieties.
Received 30 July 2008
Revised 2 October 2008
Accepted 8 October 2008
Available online 14 October 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Nanoparticles (NPs) in catalysis have gained significant interest
due to the mimicking of metal surface activation on nanoscale
leading to the improvement of the efficiency and selectivity of
N
N
N
S
N
Fe
1
,2
2-phenyl
benzothiazole (1)
reactions and also the recovery and recyclability of the catalysts.
S
Fe
Gold nanoparticles (AuNPs) have been used as catalysts in many
organic transformations3 due to their remarkable catalytic activ-
ities especially in the homocouplings of boronic acids.6–8 Most of
the active catalysts are based on NPs stabilized by ligands with
2
-ferrocenyl
benzothiazole (2)
–5
Fe
O
C
1
,2-diferrocenyl
benzimidazole (3)
OH
9
7
electron transfer properties or
p
-conjugated polymers that exhi-
o-ferrocenylbenzoic acid (4)
m-ferrocenylbenzoic acid (5)
p-ferrocenylbenzoic acid (6)
bit highly reversible redox behavior. Since the discovery of ferro-
cene in the 1950s, this fascinating molecule, which demonstrates
reversible one-electron redox reactions, has been an attractive
reagent for many chemical applications.10
Fe
Figure 1. Stabilizing ligands in the formation of AuNPs.
Improvement of the efficiency and selectivity of reactions cata-
lyzed by nanocatalysts can be achieved by particle size optimiza-
tion and surface modification of the nanoparticles, the latter can
be carried out simply by tuning the structures of the stabilizing li-
The solution became red-purple indicating the formation of
AuNPs; a TEM image of the AuNPs is shown in Figure 2.
Au(III) ions can be reduced to Au(0) by ferrocene-attached
ligand 2 giving a product with a ferricinium moiety which can be
observed by EPR spectroscopy as a singlet paramagnetic signal at
g = 2.01. The first-order characteristic curve for ligand 2 (Fig. 3)
was observed in which the rate constant was determined by fitting
to a single exponential with two adjustable parameters: the rate
constant and the EPR intensity at infinite time. It should be noted
that no EPR signal was observed for a mixed solution of Au(III) and
ligand 1.
From Table 1, the rates of the electron transfer were dependent
on the number and position of the ferrocene moieties, binding
atoms, and functional groups. AuNPs with ligand 3 were probably
destabilized either by the number of ferrocene moieties or by
weaker interactions between the AuNPs and nitrogen atoms of
the benzimidazole moiety resulting in a lower rate of ferricinium
1
1–15
gands.
In this study, the effect of ligands containing ferrocene
moieties on the activities of AuNPs-catalyzed reactions of aryl
boronic acids was investigated. Two substrates, phenylboronic acid
(A) and ferrocenylboronic acid (B), were chosen for the reactions
catalyzed by AuNPs, and two groups of stabilizing ligands, phenyl
1
6
17
18 19
20
(
1
) and ferrocenyl (2, 3, 4, 5, and 6 ) groups, were utilized,
Figure 1.
Here, AuNPs were prepared by a simple reduction method with
stabilizing ligands consisting of a ferrocene moiety. HAuCl
4 2
Á3H O
in THF (1 mM, 1.5 mL) and ligand 2 in THF (1 mM, 1.5 mL) were
mixed without any other reducing reagent and stirred vigorously.
*
Corresponding author. Tel.: +66 2 201 5123; fax: +66 2 354 7151.
E-mail address: scess@mahidol.ac.th (E. Somsook).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.10.040

7
300
L. Chaicharoenwimolkul et al. / Tetrahedron Letters 49 (2008) 7299–7302
Table 2
Homocoupling reactions of A catalyzed by Au(III) and AuNPs stabilized by ligands
a
1–6
AuNPs:Ligand
K CO (3 eq)
B(OH)2
+
OH
2
3
H O, air, RT
2
A
C
D
Entry
AuNPs:Ligand
Au(III) (%)
% Yieldb
A
C
D
c
1
AuNPs:1
AuNPs:2
AuNPs:3
AuNPs:4
AuNPs:5
AuNPs:6
Au(III)
4
4
4
1
1
1
4
50
80
79
86
73
72
54
33
14
19
8
16
19
37
17
6
2
6
11
9
c
2
c
3
d
4
d
5
d
6
c
7
9
a
2 3 2
Conditions: phenylboronic acid (0.82 mmol), K CO (2.5 mmol), 5 mL of H O,
and 5 mL of THF, stirred for 24 h.
b
Estimated from 1H NMR analysis.
[Au(III)] and [Ligand] = 3.13 mM.
[Au(III)] and [Ligand] = 0.83 mM.
c
Figure 2. The TEM micrograph of gold nanoparticles stabilized by ligand 2. The
scale bar is 20 nm.
d
Table 3
Demetalation reactions of B catalyzed by Au(III) and AuNPs stabilized by ligands
a
1–6
O
B(OH)2
AuNPs:Ligand
K CO (3 eq)
Fe
+
Fe
F
2
3
H O, air, RT
2
E
B
O
Entry
AuNPs:Ligand
Au(III) (%)
% Yieldb
3350
3425
3500
3575
3650
B
E
F
c
Field [G]
1
AuNPs:1
AuNPs:2
AuNPs:3
AuNPs:4
AuNPs:5
AuNPs:6
Au(III)
7
7
7
2
2
2
7
—
—
—
—
—
—
—
47
53
–
–
–
–
c
2
>99
>99
>99
>99
>99
34
c
Figure 3. The kinetic EPR spectra of the ferricinium moiety generated from the
reduction of Au(III) and ligand 2 were collected every 5 min for 100 min.
3
d
4
d
5
d
6
–
66
c
7
Table 1
a
The observed rate constants of ferricinium formation during the preparation of AuNPs
as observed by EPR spectroscopy
Conditions: ferrocenylboronic acid (0.44 mmol), K
O, and 5 mL of THF, stirred for 24 h.
2 3
CO (1.3 mmol), 5 mL of
H
2
b
Estimated from 1H NMR analysis.
4
À1
Entry
Ligand
k  10 (s
)
c
[
Au(III)] and [Ligand] = 3.13 mM.
[Au(III)] and [Ligand] = 0.83 mM.
d
1
2
3
4
5
2-Ferrocenylbenzothiazole (2)
1,2-Diferrocenylbenzimidazole (3)
o-Ferrocenylbenzoic acid (4)
m-Ferrocenylbenzoic acid (5)
p-Ferrocenylbenzoic acid (6)
4.51
0.17
2.89
9.53
6.04
properties of ferrocene moieties were proposed here to study their
activities in AuNPs-catalyzed reactions of arylboronic acids, A
(Table 2) and B (Table 3), in a mixed solvent of THF/water under
formation and slower electron transfer rate. Ligands 4–6 were
selected to compare with the thiazole group 2 to study the binding
of the carboxylic group to AuNPs. Ligand 5 showed an excellent
rate of ferricinium formation due to the meta-effect resulting from
the electron withdrawing nature of the carboxylic acid while a
slower rate was observed for ligand 4, probably due to intramole-
cular interactions between the ferricinium and carboxylic groups.
The higher rate of ferricinium formation using the meta- and
para-isomers of ferrocenylbenzoic than that with the thiazole
group indicated the presence of stronger interactions between
the carboxylate groups and AuNPs probably resulting from electro-
aerobic conditions for 24 h.
The results of the homocoupling of A showed similar conver-
sions (ꢀ50%) for entries 1 and 7 (Table 2) indicating the low cata-
lytic activity of ligand 1 while low conversions (14–28%) were
observed for entries 2–6 (Table 2). It was surprising to find that
the addition of stabilizing ligands containing ferrocene moieties
inhibited the homocoupling of A even at a low %loading of Au(III).
In contrast to A, the AuNPs-catalyzed demetalation reactions of
B showed excellent catalytic activities with complete conversions.
Biferrocene and ferrocenyl alcohol were not observed but an
2
2–24
unexpected dicyclopentadienone
(E) and ferrocene (F) were
1
,2
static effects.
obtained as decomposition products. Moreover, the catalysts
containing ferrocene moieties (Table 3, entries 2–6) showed
excellent selectivity giving product E, exclusively.
The C–C bond formation catalyzed by Au catalysts has been well
exploited in synthetic chemistry.21 The excellent electron transfer

L. Chaicharoenwimolkul et al. / Tetrahedron Letters 49 (2008) 7299–7302
7301
Table 4
Homocoupling reactions of A catalyzed by AuNPs stabilized by polyaniline and its
a
derivatives (7–10)
Entry
AuNPs:Polymer
Au(III) %
% Yieldb
A
C
D
1
2
3
4
5
6
7
8
AuNPs:7M
AuNPs:8M
AuNPs:9M
AuNPs:10M
4
4
4
4
4
4
4
4
90
2
—
7
3
61
60
53
7
5
12
5
37
40
45
—
—
4
2
c
AuNPs:7N
93
95
84
95
c
AuNPs:8N
c
AuNPs:9N
c
AuNPs:10N
–
a
Conditions: phenylboronic acid (0.82 mmol), Au(III) (0.03 mmol), polymer
CO (2.5 mmol), 5 mL of H O, and 5 mL of THF, stirred for 24 h.
Estimated from 1H NMR analysis.
(
12.34 mg), K
2
3
2
b
c
N = ferricinium-doped polymer (50 mol % of monomer).
Scheme 1. Proposed mechanisms for the demetalation reaction of B catalyzed by
AuNPs. Q = Fc –O–O–Fc .
+
+
were also used for the stabilization of AuNPs. The result of the
reactions of A catalyzed by AuNPs:polymers (7–10) are shown in
Table 4.
The mechanisms for the reactions of B catalyzed by AuNPs are
The ferricinium-doped polymers (Table 4, entries 5–8) gave
very low conversions (5–16%). However, it was unexpected that
the polyaniline (entry 1) would show a similar result. The form
of polyaniline (7M) used in this experiment was as the emeraldine
7
modified from the literature and proposed as shown in Scheme
1
2
. Initially, the oxidation by O occurred on the surface of the AuN-
Ps leading to a superoxo species. Subsequently, transmetalation of
B produced ferrocenyl and boron peroxyl adsorbed on AuNPs in
which ferrocene and boron peroxide may be partly released and
then circulated back to the initial step. Next, the oxidation and
2
6
salt with a positive charge on the backbone of the polymer. The
lower positive charge density on the backbone of polymers (8M–
9
M) was evident from the smaller EPR intensities compared to
2
nd-transmetalation occurred sequentially providing two ferroce-
polyaniline (7M). Hence, it is proposed here that cationic species
destabilized the transition state during homocoupling of A. The
nyl and a superoxo group on the AuNPs. Two ferricinium species
were next formed and then bridged by the superoxo species to pro-
1
,2
electrostatic interactions
between ferricinium or cations and
duce the species Q which was further oxidized to form dicyclopen-
the peroxo species might inhibit the transmetalation of A.
In summary, we have demonstrated the catalytic activities of
AuNPs stabilized by ligands containing ferrocene moieties in the
homocoupling of A and demetalation of B.
tadienone (E) as reported.2
2–24
It should be noted that this is a rare
example of the demetalation of ferrocene derivatives catalyzed by
gold nanocatalysts in which a bicyclic ketone can be synthesized
quantitatively on large scale with a good yield for potential appli-
cations in host-guest chemistry.
General procedure for reactions of boronic acid: Arylboronic acid
(
2 3 2
1 equiv) and K CO (3 equiv) were mixed in 5 mL of H O. Gold
The low conversions for the homocouplings of A inhibited by
AuNPs:2–6 were further investigated in the reactions catalyzed
by AuNPs stabilized by polyanilines2 and its derivatives (7M–
nanoparticles in 5 mL of THF were added, and the reaction mixture
was stirred vigorously at room temperature under aerobic condi-
tions for 24 h. The reaction was quenched with 1 M HCl solution
and the products were extracted with EtOAc (20 mL Â 3). The com-
5
1
0M), the structures of the polymers are shown inFigure 4. In addi-
tion, ferricinium-doped polyaniline and its derivatives (7N–10N)
2 4
bined organic layer was dried over Na SO and evaporated in va-
1
cuo. The resulting mixture was analyzed by H NMR spectroscopy.
H
N
H
N
H
N
H
N
Acknowledgments
We acknowledge the support from the Thailand Research Fund,
the Commission on Higher Education (RMU4980050), and Center
of Excellence for Innovation in Chemistry (PERCH-CIC).
X
Polyaniline (PANI) (7M)
Fc
H
H
N
H
N
N
N
N
Supplementary data
X
Y
Experimental methods, kinetic curves, and NMR spectra of 3
and dicyclopentadienone (E) are available. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2008.10.040.
Poly(aniline-co-m-ferrocenylaniline) (8M)
Fc
Fc
Fc
Fc
H
N
H
N
N
N
X
X
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