Zhang et al.
A Recyclable Au(I) Catalyst for Selective Homocoupling of Arylboronic Acids
a
the catalytic site.10 In addtion, NMP dissolves in water
Table III. Selectivity studies of 3 in the presence of aryl halids.
3
forming a catalytic system, and the reactants can access
the surface of the nanoparticles easily, this also contributes
to the high catalytic activity. Decreasing the temperature
Ar −X +Ph −BꢀOHꢁ
2
−−−−−−−−−→ Ph–Ph +Ar–Ph
Cs CO ꢄH O/NMP
2
3
2
b
Yield(%)
ꢀ
ꢀ
from 95 C to 60 C leads to a low yield (Table I, entry
Entry
1
Ar–X
T (
ꢀ
C)/Time (h) Ph–Ph
Ar–Ph
9
→ 10). When the reaction temperature was elevated
110/24
110/24
110/24
99
99
16
not observed
ꢀ
to 110 C, the phenylboronic acid converted to biphenyl
almost quantitively (Table I, entry 9). Reaction conditions
of a 1:1 mixture of water and NMP, 110 C, and 110 C,
and Cs CO as the base were used for further studies.
Cl
NO2
ꢀ
ꢀ
2
3
not observed
trace
Br
I
COMe
CHO
2
3
2
.3. Homocoupling Reactions
Employing 3 as Catalyst
a
General conditions: Aryl halide (0.24 mmol, 1.0 equiv), phenylboronic acid
0.24 mmol, 1.0 equiv), Cs CO (0.31 mmol, 1.3 equiv), 0.1 ml of NMP, 0.1 ml
Under the optimized conditions, we carried out the
homocoupling reaction with a series of substrates, and
the results are summarized in Table II. The substrates
included electron-rich arylboronic acids, 5, 7, 9, 17,
(
2
3
b
of water, 0.68 mol% 3 (based on Au(I)). Yields were determined from isolated
samples of the reaction after 24 h.
1
1
9, 21, 23, electron-deficient arylboronic acids, 11, 13,
5, 25, and arylboronic acid, 27 with an o-substitute
4
-bromoacetophenone, and 4-iodobenzophenone) and the
results are shown in Table III. For 4-nitrochlorobenzene
and 4-bromoacetophenone, the reactions with catalyst 3
showed very high selectivity and nearly quantitive yields
(
1
group. For the electron-rich arylboronic acids, 5, 7, 9,
7, and 19, the desired homocoupling products were
1
obtained in high yields. For the electron-rich substrates 9-
phenanthrenylboronic acid, 21, and 1-pyrenylboronic acid,
99%) for the homocoupling products (see Table III, entries
and 2). However, the reaction time was longer than those
for reactions without the addition of aryl halides. Althouth
Delivered by Publishing Technology to:t hC eh bi ni pe hs ee n yU ln wi v ae sr s ai tl ys oo fo bH t oa inn ge dK oa sn gt he exclusive product
2
3, the reations proceeded smoothly, but gave phenan-
threne and pyrene respectively as the final products.
When arylboronic acids containing electron-withdrawing
IP: 220.142.68.132 On: Sat ,f o0 r54 M- i oa dr o 2b 0e 1n 6z o 1p 2h e: 4n 5o :n0e 8(Table III, entry 3), the yield was
groups, such as −Cl(11), −Br(13), and −F(15), were
Copyright: American Scientific Publishers
unexpected low (16%). These results clearly indicate that
can selectively catalyze the homocoupling of arylboronic
used as the substrate, the self-coupling reactions evolved
3
slowly, but good yields were still obtained with pro-
longed reaction times and elevated temperatures. However,
acids in the presence of aryl halides although the reaction
rate was slow. Thus, the catalyst 3 offers a new synthetic
method for the preparation of C -symmetric biaryls in the
precence of halogen groups via selective homocoupling of
halogenated aryl boronic acids. These are difficult to pre-
pare by tpyical Pd catalyzed Suzuki reactions.
3,5-bis(trifluoromethyl)benzeneboronic acid, 25, was inac-
ꢀ
2
tive even at higher temperatures (140 C), and only a trace
amount of the desired homocoupling product was observed
by TLC after 72 h (Table II, entry 12). Catalyst 3 is very
sensitive to steric hindrance. When o-methylphenylboronic
acid, 27, in which there is a methyl group on the ortho-
position, was used as reagent, only trace amounts of the
coupling product was observed even at 140 C after 72 h.
Whereas phenanthrylboronic acid, 21, and pyrenylboronic
acid, 23, were reverted to phenanthrene, 22, and pyrene,
2
.5. Comparison of Catalyst 3 with
ꢀ
Other Au Catalysts
To further evaluate its catalytic activity, 3 was compared
with other reported Au catalysts. Yields and turnover fre-
quence (TOF) for several compounds from the literature
as well as the results for catalyst 3, and the complex
1 are given in Table IV. Firstly, the Au(III) complex 1,
2-(2-pyridyl)phenyl dichloroaurum, was used directly as
the catalyst for two substrates under the optimized reac-
tion conditions (Table IV, entries 2 and 6). The kinetic
study of the catalytic process for homocoupling of 2 is
shown in Figure 3. Initially, the reaction proceeded slowly
to give the desired homocoupling product, however, the
conversion of arylboronic acid only reached about 20%
after 4 h, and did not increased further even after a pro-
longed reaction time. The maximum TOF for this Au(III)
2
4, in high yields because of the two bulky aromatic rings.
These results indicate that catalyst 3 favors electron-rich
arylboronic acids, and is unsuitable for arylboronic acids
having strongly electron-deficient groups, or steric hin-
dered arylboronic acids.
2
.4. Selectivity of Catalyst 3
Au(I) complexes are active catalysts for Suzuki cross-
coupling reactions of aryl halids and aryl boronic acids. So,
a series of reactions were set up to test the catalytic activity
of 3 for cross-coupling reactions. Three substrates known
to be active for cross-coupling reactions using other cata-
lysts were chosen to investigate (3, 4-nitrylchlorobenzene,
−1
complex 1 for phenylboronic acid was 0.8 h (Table IV,
J. Nanosci. Nanotechnol. 10, 5153–5160, 2010
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