Oxidative homo-coupling of potassium aryltrifluoroborates catalyzed by gold nanocluster under aerobic conditions

Article abstract of DOI:10.1016/j.jorganchem.2006.04.054

Gold(0) nanoclusters, stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP-n), catalyzed the oxidative homo-coupling reaction of potassium aryltrifluoroborate in water under air. Catalytic activity was dependent on the size of clusters. The smallest cluster Au:PVP-1 (d_av = 1.3 ± 0.3 nm) gave the highest activity, while Au:PVP-7 (d_av = 9.5 ± 1.0 nm) did not catalyze the homo-coupling. The catalyst was reusable for several times. Positively charged surface on the Au cluster, generated by the adsorption of molecular oxygen, would be the active site of the catalysis.

Full text of DOI:10.1016/j.jorganchem.2006.04.054

Journal of Organometallic Chemistry 692 (2007) 368–374
www.elsevier.com/locate/jorganchem
Oxidative homo-coupling of potassium aryltrifluoroborates catalyzed
by gold nanocluster under aerobic conditions
a,
a
a,b
*
Hidehiro Sakurai , Hironori Tsunoyama , Tatsuya Tsukuda
a
Research Center for Molecular-scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan
b
CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan
Received 20 February 2006; accepted 18 April 2006
Available online 6 September 2006
Abstract
Gold(0) nanoclusters, stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP-n), catalyzed the oxidative homo-coupling reaction of
potassium aryltrifluoroborate in water under air. Catalytic activity was dependent on the size of clusters. The smallest cluster
Au:PVP-1 (dav = 1.3 ± 0.3 nm) gave the highest activity, while Au:PVP-7 (dav = 9.5 ± 1.0 nm) did not catalyze the homo-coupling.
The catalyst was reusable for several times. Positively charged surface on the Au cluster, generated by the adsorption of molecular oxy-
gen, would be the active site of the catalysis.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Gold nanocluster; Aerobic oxidation; Aryltrifluoroborate; Homo-coupling
1
. Introduction
ronic acids [4], oxidation of benzylic alcohols [5], and gen-
eration of H O in the presence of ammonium formate [6]
2
2
Since Haruta and co-workers reported that supported
gold nanoclusters (NCs) exhibited high activity toward
in water under ambient conditions. We have also demon-
strated that the catalytic activity was strongly dependent
on the size of the clusters, that is, the smaller cluster
showed superior activity.
the oxidation of CO to CO [1], heterogeneous catalyst sys-
2
tem consisting of Au-NC has been extensively studied from
both scientific and practical points of view [2]. In contrast,
potentiality of colloidal Au-NCs stabilized by organic com-
pounds/polymers as quasi-homogenious catalyst directly
applicable to precise organic synthesis, still remains undev-
eloped subject [2,3]. Previously, we have reported prepara-
tive methods of Au nanoclusters with various sizes ranging
from 1.3 ± 0.3 nm to 9.5 ± 1.0 nm, stabilized by water-sol-
uble polymer, poly(N-vinyl-2-pyrrolidone) (Au:PVP-n)
using rapid reduction [4] and seed-mediated growth [5,6].
It was found that Au:PVP-n worked as an excellent
quasi-homogenious catalyst which promoted the aerobic
oxidations: oxidative homo-coupling reaction of arylbo-
To the best of our knowledge, the oxidative homo-cou-
pling of organoboron compounds [4] is the first and only
example of the Au-NC catalyzed carbon–carbon bond
forming reaction. The homo-coupling of organoboron
compounds has been recognized as one of the convenient
methods especially for the preparation of symmetrical biar-
yls [7,8]. In most cases Pd(II) catalyst system [7–9] has been
utilized, therefore Suzuki–Miyaura type cross-coupling
reaction should often compete against the homo-coupling
in the presence of organohalide compounds. Some metallic
oxidants such as Cu(II) [9a,10] or Mn(III) [11] have also
been utilized. After our report, Au(III) supported on
CeO2 [12] and Au(III) Schiff base-complexes [13] were
reported to promote the homo-coupling selectively without
formation of Suzuki–Miyaura coupling product. These
results indicate that Au catalyst would possess the charac-
*
Corresponding author. Fax: +81 564 59 5527.
E-mail address: hsakurai@ims.ac.jp (H. Sakurai).
0
022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.04.054

H. Sakurai et al. / Journal of Organometallic Chemistry 692 (2007) 368–374
369
teristic feature for the specific promotion of homo-cou-
1 atom% Au:PVP-1
300 mol% K CO
H O, air, 300 K, 24 h
2
PhB(OH)2
Ph-Ph
+
Ph-OH
pling. In this paper we report selective homo-coupling reac-
tion of potassium aryltrifluoroborates [14] catalyzed by
Au:PVP in water under ambient conditions.
2
3
Scheme 3.
2
. Results and discussion
ꢀ
2
.1. Preparation of Au:PVP-n
nucleophilic attack of HOO to form the hydroperoxyb-
ꢀ
orate intermediate (R BOOH) and 1,2-migration of
3
Preparation of Au:PVP-n of various sizes was per-
organic group (R) to give the C–O bond product [15].
In order to achieve the selective reaction for the biphenyl
formation, generation of such a peroxyborate intermedi-
ate should be suppressed.
Potassium aryltrifluoroborates (ArBF K) are stable
3
salts and has been utilized for the Suzuki–Miyaura cou-
pling reactions [16]. In some cases the cross-coupling reac-
formed according to the reported procedure. Rapid
reduction of HAuCl4 solution by NaBH4 at 273 K in
the presence of PVP (K-30) afforded the smallest cluster
Au:PVP-1 with good reproducibility (Scheme 1) [4]. The
larger clusters Au:PVP-n (n = 2–7) were prepared by the
seed-mediated growth method (Scheme 2) [5,6]. It is note-
worthy that Au(III) is not reduced to Au(0) by Na SO
tion using ArBF
proceed under the neutral condition [17]. The fact indicated
that the homo-coupling of ArBF K might also occur under
neutral condition by virtue of slow ligand transfer between
K did not require the addition of base but
2
3
3
in the absence of Au-NC. The average diameters of
Au:PVP-n used in this paper are listed in Table 1.
3
ꢀ
ꢀ
2.2. Au:PVP-1 catalyzed homo-coupling reation of
PhB(OH) and PhBF K
F
and HOO . The results of the reaction of PhBF K
3
with Au:PVP-1 under the various conditions are listed as
Table 2.
2
3
As reported in 2004, Au:PVP-1 catalyzed the aerobic
As shown in entries 1 and 2, the reaction was not influ-
oxidation of PhB(OH)2 in aqueous K CO solution
enced by the pH of the solution starting from PhB(OH)
giving both biphenyl and phenol in approximately 3:1
ratio. The reaction using PhBF K was found to be depen-
dent on the pH of the solution (entries 3–7). When PhBF
was treated with 1 atom% of Au:PVP-1 in K CO solution
2
,
2
3
under aerobic conditions, giving two products, biphenyl
Ph-Ph) and phenol (Ph-OH) (Scheme 3) [4]. The latter
(
3
product may not have been produced by direct Au-cata-
K
3
lyzed processes but by the oxidation of PhB(OH) with
2
3
2
H O co-produced in the coupling reaction. Time-depen-
dent product ratio was monitored in the reaction of
at 320 K under air, both biphenyl and phenol are obtained
in approximately 3:1 ratio, indicating rapid ligand
2
2
ꢀ
ꢀ
PhB(OH) and the results are shown in Fig. 1. Phenol
exchange between F and HOO under the conditions.
As expected, in weakly basic to neutral solutions (pH
9.18–6.86), biphenyl was yielded as a sole product and phe-
2
was not detected at the early stage of the reaction, while
the yield of phenol was drastically increased after the
formation of Ph-Ph. Indeed, phenol can be selectively
formed in the presence of appropriate reductant such
as HCO NH [6]. It is well known that the conversion
1
nol was not detected by H NMR spectroscopy. In contrast,
no oxidation occurred under the slightly acidic (pH 4.01)
conditions (entry 7).
Next, reaction rate was monitored at various tempera-
ture under pH 6.86 solution condition (entries 8–11). At
2
4
ꢀ
of C–B bond to C–O bond by H O /OH involves the
2
2
300 K the reaction was slightly slower than that from
PhB(OH) . The reaction did not completed and phenol
2
was also produced in 2% yield after 24 h. At 310 K,
320 K, and 370 K, the reaction proceeded quantitatively
within 16 h, 10 h, and 1 h, respectively. Since the average
size of Au:PVP-1 (1.3 ± 0.3 nm) is estimated to be 55mer
NaBH4
H O, 273 K
HAuCl4 + PVP (K = 30)
Au:PVP-1
Au:PVP-n
2
Scheme 1.
[
>
4], TOF of the reaction at 370 K is calculated as
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
50 h atom
and >2750 h cluster . As described
Na SO
above, selective and quantitative formation of biphenyl
was realized in the Au:PVP-1 catalyzed aerobic oxidation
reaction starting from PhBF K in place of PhB(OH)
2
3
Au:PVP-1 + 1 ~ 9 amounts HAuCl
4
H O, 273 K
2
3
2
Scheme 2.
under the neutral condition (see Scheme 4).
Table 1
Average size of Au:PVP-n
n
1
2
3
4
5
6
7
Average diameter (nm)
1.3 ± 0.3
2.3 ± 0.4
3.3 ± 0.5
4.3 ± 0.6
4.7 ± 0.5
5.9 ± 0.5
9.5 ± 1.0

370
H. Sakurai et al. / Journal of Organometallic Chemistry 692 (2007) 368–374
2
Fig. 1. Time-dependent product ratio of the Au:PVP-1 catalyzed homo-coupling of PhB(OH) .
Table 2
Optimization of the Au:PVP-1 catalyzed homo-coupling of phenylboron derivatives
Entry
Organoboron
Solution
CO aq.
pH 6.86 buffer
CO aq.
pH 9.18 buffer
pH 7.41 buffer
pH 6.86 buffer
pH 4.01 buffer
pH 6.86 buffer
pH 6.86 buffer
pH 6.86 buffer
pH 6.86 buffer
Temperature (K)
Time (h)
Biphenyl (%)
Phenol (%)
1
2
3
4
5
6
7
8
9
PhB(OH)
PhB(OH)
2
K
2
3
320
320
320
320
320
320
320
300
310
320
370
24
24
24
24
24
24
24
24
16
10
1
72
75
76
>99
>99
>99
–
27
24
24
–
–
–
–
2
–
–
2
PhBF
PhBF
PhBF
PhBF
PhBF
PhBF
PhBF
PhBF
PhBF
3
3
3
3
3
3
3
3
3
K
K
K
K
K
K
K
K
K
K
2
3
85
>99
>99
>99
1
1
0
1
–
1
atom% Au:PVP-1
1 atom% Au:PVP-1
PhB(OH) or PhBF K
Ph-Ph
+
Ph-OH
ArBF
3
K
Ar-Ar
+
Ar-OH
2
3
conditions, H O, air
pH=6.86 buffer
air, 320 K, 24 h
2
Scheme 4.
Scheme 5.
Table 3
Substituent effect
2
.3. Substituent effect
Entry
Ar
3
ArBF K (%)
Ar-Ar (%)
Ar-OH (%)
Substituent effect on aryl group of borate was examined
under the optimized reaction conditions at 320 K for 24 h
1
2
3
4
5
6
C
p-CH
m-CH
o-CH
p-CH
p-FC H
6
H
5
–
–
–
>99
>99
95
14
89
>99
98
>99
–
–
3
C
6
H
4
3
C
6
H
4
4
(
Table 3).
a
3
C
OC
6
H
4
80
–
11
–
Steric effect was monitored using tolyl-substituted
3
6
H
4
–
–
–
–
borates as shown in entries 2–4. As reported previously,
remarkable effect was observed in the reaction of ortho-,
meta-, and para-tolylboronic acid. That is, o-
CH C H B(OH) was not oxidized under Au:PVP-1 cata-
6
4
b
7
8
p-BrC
p-CF
6
H
4
1
3
C
6
H
4
–
a
2
Recovered as ArB(OH) .
3
6
4
2
b
Other products derived by the reaction in which bromo group par-
lyzed conditions, yielding the corresponding bitolyl in only
% after 24 h. The result was in sharp contrast with the fact
ticipates are not detected.
4
that Pd(II) or Au(III) ions do catalyze the homo-coupling
of o-CH C H B(OH) [4]. Similar phenomena were also
acid in the reaction of o-CH C H BF K even after 24 h at
3
6
4
2
3
6
4
3
observed using tolyl-substituted trifuoroborates. Only
4% of bitolyl was produced with 80% recovery of boronic
320 K. As for meta or para derivatives, quantitative oxida-
tion proceeded under the same conditions.
1

H. Sakurai et al. / Journal of Organometallic Chemistry 692 (2007) 368–374
371
1
atom% Au:PVP-1
1 atom% Au:PVP-n
BF K
Ph
PhBF K
3
3
Ph-Ph
Ph
Ph
pH=6.86 buffer
air, 320 K, 24 h
pH=6.86 buffer, air
conditions
67%
Scheme 7.
Scheme 6.
Electronic effect was monitored using various para-
substituted substrates, but no apparent tendency was found
Table 4
Size effect of Au cluster
(
entries 5–8). Yield of ArOH was increased in the electron
Entry
Catalyst
Yield
(300 K,
15 h)
Yield
(320 K,
15 h)
(%)
Yield
(320 K,
24 h) (%)
donative p-CH OC H BF K reaction. As shown in entries
7
reaction of more electron accepting CF -substituted
3
6
4
3
and 8, however, quantitative yield was obtained in the
(
%)
3
borate, while 1% of ArOH was detected in the less accept-
ing Br-substituted borate.
1
2
3
4
5
6
7
a
Au:PVP-1 (1.3 ± 0.3 nm)
Au:PVP-2 (2.3 ± 0.4 nm)
Au:PVP-3 (3.3 ± 0.5 nm)
Au:PVP-4 (4.3 ± 0.6 nm)
Au:PVP-5 (4.7 ± 0.5 nm)
Au:PVP-6 (5.9 ± 0.6 nm)
Au:PVP-7 (9.5 ± 1.0 nm)
32
16
9
Trace
Trace
–
>99
50
39
23
17
10
–
>99
>99
>99
89
It should be pointed out that bromo group on arene was
tolerant under the reaction conditions as shown in entry 7.
Haroarenes were not capable for oxidative addition to
Au(0) cluster, generating Ar-Au intermediate [18]. Such a
resistant character of Au nanocluster toward the oxidative
addition of haloarenes is in sharp contrast with the reactiv-
ity of Pd:PVP nanocluster, which readily catalyzes Suzuki–
Miyaura type cross-coupling reaction [19,20] (see Scheme
66
a
34
–
–
Phenol was also detected in 3% yield.
At 320 K, the reaction was completed within 10 h using
Au:PVP-1 (see Table 2 entry 10), while the yields were
diminished to 50%, 39%, 23%, 17%, and 10%, respectively,
using Au:PVP-2–6 catalysts even after 15 h stirring. In par-
ticular, Au:PVP-7 (dav = 9.5 ± 1.0 nm) no longer possessed
the catalytic activity. However, from the practical view-
point, Au:PVP-2 (dav = 2.3 ± 0.4 nm) and Au:PVP-3
5
).
The Au:PVP-1 catalyzed reaction condition was also
applicable to the alkenylborate as a substrate. The
reaction of potassium 2-(E)-phenylethenyltrifluoroborate
under the above conditions afforded the homo-coupling
product, 1,4-(E,E)-diphenylbutadiene, in 67% yield
(dav = 3.3 ± 0.5 nm) were also applicable to accomplish
(
Scheme 6).
the reaction quantitatively by prolonging the reaction time
to 24 h at 320 K, although Au:PVP-1 (dav = 1.3 ± 0.3 nm)
obviously showed the highest performance (see Scheme 7).
2
.4. Size effect of Au cluster
We have already demonstrated remarkable differences
2
.5. Reusability
dependent on the cluster size in catalytic activity toward
homo-coupling of PhB(OH)2 [4], oxidation of benzylic
alcohols [5], and generation of H O [6]. Unfortunately it
Au:PVP catalyst was able to be recovered by centrifugal
2
2
ultrafiltration. Reusability of Au:PVP-1 was tested by the
reaction of PhBF K (Scheme 8). Reaction conditions were
set up at 320 K for 24 h on the basis of the result in Section
was unable to investigate quantitative kinetic studies in
the present reaction due to the size growth of the clusters
during the reaction as shown in the following section.
Therefore, the size effect was evaluated by comparing the
yields of biphenyl in the 1 atom% Au:PVP-n catalyzed
3
2.4, and the results with TEM images of the recovered
nanoclusters are shown in Table 5 and Fig. 2. Apparent
size growth was observed after the first cycle from
reaction of PhBF K at three different reaction conditions,
3
1.3 ± 0.3 nm to 2.3 ± 0.7 nm. In contrast, average size of
1
4
5 h at 300 K, 15 h at 320 K, and 24 h at 320 K (Table
). No side reaction such as a formation of PhOH was
observed in most of the reactions except for Au:PVP-6 cat-
alyst for 24 h at 320 K.
1
atom% reused Au:PVP-1
PhBF K
Ph-Ph
3
Biphenyl was obtained when Au:PVP-1–3 were used as a
catalyst at 300 K for 15 h in 32%, 16%, and 9% yield,
respectively. Normalization by the surface areas of the cor-
responding nanoclusters by assuming spherical shapes with
the diameters gave the relative reaction rate as PVP-
pH=6.86 buffer
air, 320 K, 24 h
Scheme 8.
1
:PVP-2:PVP-3 = 1:0.88:0.71. Only trace amounts of
Table 5
Reusability of Au:PVP-1
1
biphenyl (lower than 1%) were detected by H NMR under
the Au:PVP-4 or -5 catalyzed reaction conditions. No
biphenyl formation was observed using Au:PVP-6 or -7.
These results indicated that the smaller clusters showed
superior reactivity to the larger clusters.
Yield (%)
d (nm)
First cycle
Second cycle
Third cycle
>99
>99
98
1.3 ± 0.3 ! 2.3 ± 0.7
2.3 ± 0.7 ! 2.3 ± 0.7
2.3 ± 0.7 ! 2.4 ± 0.7

372
H. Sakurai et al. / Journal of Organometallic Chemistry 692 (2007) 368–374
Fig. 2. Typical TEM images and cluster size distributions of Au:PVP-1 (a) and recycled catalyst after (b) first, (c) second, and (d) third cycles.
the cluster did not change appreciably after second cycle;
.3 ± 0.7 nm and 2.4 ± 0.7 nm in the second and third
cycles, respectively. Similar phenomenon was also obtained
the contrary, it has been proposed that atomic Pd species
leached out from the clusters are involved as the real catalyst
in Pd(0) nanocluster-catalyzed reaction [23].
2
in the PhB(OH) homo-coupling reaction [4]. As described
It is known that nanosized gold cluster readily adsorbs
2
in Section 2.4, gold clusters of 2–3 nm in size still possess
enough catalytic activity toward homo-coupling of
O , generating superoxo complex-like intermediate [24].
2
Therefore, unique character of Au:PVP as an aerobic oxi-
dation catalyst is reasonably explained in such a way that
positively charged site on surface of Au cluster (A) would
be generated by O2 adsorption (Scheme 9). Positively
charged Au site is likely to show a similar reactivity as ionic
Au such as Au(III) and to undergo the attack of nucleo-
PhBF K so that the reaction proceeded quantitatively at
3
3
20 K for 24 h. In fact, biphenyl was obtained quantita-
tively both in the first and second cycle, and in 98% yield
in the third cycle. The reason why the activity of the third
cycle catalyst was slightly diminished was probably due to
the formation of gigantic clusters over 6 nm, which are
inert in the reaction. In conclusion, Au:PVP-1 was found
to be applicable as a reusable catalyst at least for several
times.
phile such as PhBF K, generating Au–C bond (B) as equal
3
as transmetallation. On the other hand, larger clusters such
as Au:PVP-7, which scarcely adsorb O , may not undergo
2
nucleophilic attack.
Several reaction pathways are plausible once the Au–C
intermediate B is generated and the schematic mechanisms
are drawn in Scheme 9. Pathway (a) involves second nucle-
ophilic attack (transmetallation) on the same site according
to the hypothesis that the site where aryl group attaches is
still positively charged due to the coordination of peroxo
group. Pathway (b) is based on the assumption that
multi-active sites exist on the same cluster. After multiple
organic groups adsorb onto the same cluster, the organic
groups migrate on the surface of Au cluster. Collision of
each group at the same site (C) affords spontaneously
reductive elimination. At present other mechanisms such
as intermolecular ligand transfer (c) [25] or radical coupling
(d) [11,26] cannot be excluded. Further study should be
required to clarify the mechanism.
2
.6. Possible mechanisms
As described above, Au:PVP showed excellent catalytic
activities toward the aerobic oxidations. After our report,
some heterogeneous and homogeneous Au(III) catalysts
were found to promote aerobic oxidations such as homo-
coupling of ArB(OH) [12,13] or benzylic alcohols [21] in a
2
similar fashion. Very recently, Corma et al. reported that
Au(I) Schiff base complexes do catalyze Suzuki–Miyaura
coupling predominantly whereas Au(III) complexes with
same ligand catalyze only homo-coupling [22]. These facts
imply that the reactivity of Au:PVP clusters rather resembles
to that of Au(III) species. However, as confirmed in the pre-
vious study, gold in Au:PVP is in essentially zero valence
state and contamination of ionic species such as AuX is
n
not detected by XPS and other measurements [4]. Remark-
able interference by ortho substitution (Section 2.3) also
indicates that active catalytic site would not be atomic/ionic
gold, which would possibly be leached out through the reac-
tion, but the two dimensional surface of Au(0) cluster. On
3. Summary
In summary, we have demonstrated that Au:PVP-1
works as a selective, quantitative, and reusable catalyst
for the homo-coupling reaction of potassium aryltrifluo-

H. Sakurai et al. / Journal of Organometallic Chemistry 692 (2007) 368–374
373
Ph
Ph Ph
free radical formation
(d)
δ-
F
2
B
O
PhBF3
O
O
O2
O
δ+
Ph
Au
Au
Au
A
B
(
c)
(b)
(a)
2
F B
O
O
Ph-Ph
B'
Ph
multiple-site
adsorption
Au
transmetallation
PhBF3
reductive
elimination
intermolecuar
ligand exchange
O2
F2B
F2B
O
O
Ph
O
O
Ph
BF2
Ph
Au
Au
δ+
C
O
O
δ-
Ph
migration
on cluster surface
transmetallation
Au
2
x
BF2
O
Ph
PhBF3
O
Scheme 9. Possible mechanisms.
roborates in neutral aqueous media under ambient condi-
tion. The reaction can be carried out at reflux conditions
and the estimated TOF at 370 K is more than
a JEOL Lambda 400 (400 MHz) spectrometer. UV–Vis
spectra were obtained by using a Hitachi U-2010 spectro-
photometer. Transmission electron microscopy (TEM)
images were recorded with electron microscopes operated
at 100 kV (Hitachi H-7500). Preparative thin layer chroma-
tography (PTLC) was conducted on silica gel (Wako gel B-
5F).
ꢀ
1
ꢀ1
2
750 h cluster . Characteristic feature of Au:PVP cata-
lyst is that it behaves as if it is positively charged, not as
zero-valent metals. For example, haloarenes do not oxida-
tively add to Au:PVP, which realizes the selective homo-
coupling with prevention of competitive Suzuki–Miyaura
type cross-coupling. Further improvement by changing
the stabilizing polymer from PVP to proper polymers/mol-
ecules would lead us to develop more precise and robust
catalyst for synthetic organic chemistry.
Preparation of Au:PVP-n was followed by the reported
procedure [4–6].
4.2. General procedure of Au:PVP-n catalyzed homo-
coupling reaction of aryltrifluoroborate
4
. Experimental
All the reaction was carried out using an EYELA PPS-
2
510. A test tube (B = 30 mm) was placed with ArBF K
3
4
.1. General
(0.25 mmol) and 3 mL of pH 6.86 phosphate buffer (Wako
Pure Chemical Industries). The aqueous solution of
Au:PVP-n (0.5 mM, 5 mL, 1 atom%) was added and the
reaction mixture was stirred (1300 ± 5 rpm) at a given tem-
perature. The reaction was quenched by adding 5 mL of
AcOEt and the aqueous phase was extracted twice with
10 mL of AcOEt. The combined organic phase was dried
Potassium aryltrifluoroborates were prepared according
to the literature procedure [14]. Other reagents and solvents
were purchased from commercial sources and were further
purified by the standard methods, if necessary. The water is
1
of a Milli-Q grade. The H NMR spectra were recorded on

374
H. Sakurai et al. / Journal of Organometallic Chemistry 692 (2007) 368–374
over Na SO , and evaporated in vacuo. Products were
(c) J.P. Perrish, Y.C. Jung, R.J. Floyd, K.W. Jung, Tetrahedron
Lett. 43 (2002) 7899;
2
4
1
characterized by H NMR. If necessary, further purifica-
tion was performed by PTLC.
(
d) H. Yoshida, Y. Yamaryo, J. Ohshita, A. Kunai, Tetrahedron
Lett. 44 (2003) 1541;
e) S. Punna, D.D. D ´ı az, M.G. Finn, Synlett (2004) 2351.
1
All products are known compounds and their H NMR
(
spectra are in good accordance with those of authentic
samples.
[10] A.S. Demir, O¨ . Reis, M. Emrullahoglu, J. Org. Chem. 68 (2003)
10130.
¨
[
[
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.3. Recycle experiment
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All the reaction was carried out according to the stan-
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dard procedure described in Section 4.2. After quenching
and extraction with AcOEt, the aqueous phase was filtered
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and deionized three times (10 mL H O) by using centrifu-
2
gal ultrafiltration (Vivascience, Vivaspin 20) featuring a
membrane with a cutoff molecular-weight of 10 kDa. The
deionized hydrosol of Au:PVP was diluted to 5 mL and
was used for the next cycle. Yields of the products were
determined according to the standard procedure above.
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3
Acknowledgments
was treated with Au:PVP-1, cross-coupling product, m-hydroxybi-
phenyl, was not detected but homo-coupling product, biphenyl, was
obtained as a sole product. Details of such competitive reactions will
be described elsewhere.
The present work was partially supported by Grant-in-
Aid for Scientific Research on Priority Areas (No.
[
19] (a) Y. Li, X.M. Hong, D.M. Collard, M.A. El-Sayed, Org. Lett. 2
1
6033236, ‘‘Reaction Control of Dynamic Complexes’’)
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2000) 2385;
from Ministry of Education, Culture, Sports, Science and
Technology, Japan. T.T thanks a CREST program spon-
sored by JST for financial support.
(b) Y. Li, E. Boone, M.A. El-Sayed, Langmuir 18 (2002) 4921;
(c) R. Narayanan, M.A. El-Sayed, J. Am. Chem. Soc. 125 (2002)
8
340;
(
d) R. Narayanan, M.A. El-Sayed, J. Phys. Chem. B 108 (2004) 8572.
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