A recyclable polystyrene-supported gold(I) catalyst

Article abstract of DOI:10.1002/adsc.201100133

A polystyrene-(benzotriazole)(triphenylphosphine)gold(I) [Btz·Au(I)PPh₃] resin, representing the first polystyrene-immobilized homogeneous gold(I) complex, was conveniently prepared, and showed remarkable catalytic activities and recyclability in three model transformations, including the tandem 3,3-rearrangement and Nazarov reaction of an enynyl acetate, the cyclization of a 1,6-enyne, and the rearrangement of an alkyne-furan. Copyright

Full text of DOI:10.1002/adsc.201100133

COMMUNICATIONS
DOI: 10.1002/adsc.201100133
A Recyclable Polystyrene-Supported Gold(I) Catalyst
a
a,
Wenjie Cao and Biao Yu *
a
State Key Laboratory of Bio-organic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21-6416-6128; e-mail: byu@mail.sioc.ac.cn
Received: February 28, 2011; Revised: April 13, 2011; Published online: August 4, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100133.
Abstract: A polystyrene-(benzotriazole)(triphenyl-
gold(I) catalysts. The vast research efforts on the het-
erogeneous gold-catalyzed synthesis have thus far
been dedicated to the gold particles supported on in-
organic materials (mainly metal oxides) in which both
the zerovalent gold clusters and the cationic gold(I)
phosphine)gold(I) [Btz·Au(I)PPh ] resin, represent-
3
ing the first polystyrene-immobilized homogeneous
gold(I) complex, was conveniently prepared, and
showed remarkable catalytic activities and recycla-
bility in three model transformations, including the
tandem 3,3-rearrangement and Nazarov reaction of
an enynyl acetate, the cyclization of a 1,6-enyne,
and the rearrangement of an alkyne-furan.
[4]
and gold ACHUTNRGNENUG( III) species are involved in the reactions.
Herein we report a polystyrene-supported gold(I)
species which is highly effective and recyclable for the
gold(I)-catalyzed reactions.
Shi and co-workers have recently developed tri-
Keywords: benzotriazole; gold; heterogeneous cat-
alysis; polymer-supported gold(I) catalyst; polystyr-
ene
ACHTUNGTNERNGNUa zole-Au(I) complexes, such as Ph PAu·BtzBn (A;
3
1
BtzBn=N -benzylbenzotriazole), as a new class of
gold(I) catalysts with improved thermal stability and
[
5]
reactivity. Accordingly, we prepared conveniently a
polystyrene-supported benzotriazole-Au(I) complex 3
as shown in Scheme 1.
The divinylbenzene (DVB) cross-linked polystyr-
Cationic gold(I) complexes have recently emerged as enes (PS) are still the most popular polymeric materi-
powerful homogeneous catalysts for the electrophilic als used in organic synthesis due to their low cost,
activation of carbon-carbon multiple bonds towards ready availability, mechanical robustness, chemical in-
[
3]
nucleophiles to promote a number of unprecedented ertness, and facile functionalization; we thus firstly
[
1,2]
chemical transformations.
Immobilization of the chose Merrifield resin 1 (chloromethyl-polystyrene
gold(I) complexes onto solid supports would facilitate resin purchased from TCI, with 1% DVB, 200-400
ꢀ
1
the recovery and reuse of these precious catalysts in mesh, and 1.2 mmol·g as Cl) as the solid support.
practical synthesis. While polymer-supported transi- Treatment of resin 1 with a solution of benzotriazole
tion metal catalysts have become a topic of intensive (1.4 g, 12 mmol) in toluene (20 mL) under reflux for
[3]
[6]
research, such an interest has not been extended to 48 h, followed by precipitation from diethyl ether
Scheme 1. Preparation of a polystyrene-supported gold(I) catalyst 3.
Adv. Synth. Catal. 2011, 353, 1903 – 1907
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1903

Wenjie Cao and Biao Yu
COMMUNICATIONS
Table 1. XPS analysis of the PS-triazole resin 2 and the PS-
supported gold(I) complex 3.
Table 2. The transformation of enynyl acetate 4 to cyclopen-
tenone 5 catalyzed by the PS-supported gold(I) complex 3
and the recycling experiments.
[a]
[a]
Resins C
Au
P
N
Cl
Sb
F
[%]
[%]
[%] [%]
[%]
[%]
[%]
2
3
89.69
82.67 0.79
2.55
0.80 2.11
0.19
0.14
0.42
3.07
[
a]
Surface concentrations are shown as atomic%.
[b]
Cycle
Time [min]
Yield [%]
and drying under vacuum, provided the desired ben-
zotriazole-functionalized resin 2 as a pale yellow
powder. The elemental analysis of resin 2 revealed
that the wt% contents of nitrogen and chloride were
1
2
3
4
5
6
7
8
9
30
35
70
80
94
93
93
93
94
95
92
94
93
94
93
95
94
92
93
90
4
3
.65%
and
<0.5%,
respectively,
indicating
120
120
150
150
180
180
210
240
270
300
ꢀ
1
ꢀ1
.32 mmol·g as N and less than 0.14 mmol·g as Cl
in the resin. These data were in good accordance with
those implied from the X-ray photoelectron spectros-
copy (XPS) analysis (Table 1), in that the wt% con- 10
tents of nitrogen and chloride were calculated to be 11
~
3.0% and ~0.5%, respectively. Prolongation of the 12
1
1
1
3
4
5
reaction time did not increase considerably the load-
ing of benzotriazole any more.
To a suspension of resin 2 in anhydrous CH Cl ,
2
2
[a]
PPh AuSbF (freshly prepared as a solution in CH Cl
Reaction conditions: 4 (0.3 mmol), HOTf (0.03 mmol),
and the PS-supported gold(I) complex 3 (36 mg, ca.
0.018 mmol Au) in wet CH
3
6
2
2
[
7]
from PPh AuCl and AgSbF ) was then added at
3
6
room temperature, and the resulting mixture was
stirred overnight. The precipitate was filtered, washed
2 2
Cl (6 mL, 0.05M) at room
temperature.
Isolated yield.
[b]
with CH Cl2 and diethyl ether, and dried under
2
vacuum to afford the PS-supported gold(I) catalyst 3
as a gray powder. According to the weight gain of the
resin before and after the reaction, the wt% content catalyze the reaction to completion, although the re-
of Au was calculated to be 10.3%. This result was action rate slowed down gradually (from 30 min at
th
supported by the XPS analysis of resin 3, which re- the first cycle to 300 min at the 15 cycle).
vealed the atomic% content of Au to be 0.79%, equal
The reactivity/stability of the triazole-Au(I) com-
to a wt% of 11.1%. The Au atomic% (0.79%) was plexes (e.g., A) has been attributed to the dynamic
also in a satisfactory agreement with the stoichiomet- coordination of the triazole with Au(I) cation; the
[5]
ric proportions of atomic% of P, N, Sb, and F based real catalytic species is still supposedly Ph PAu(I).
3
on the structure of the complex 3.
After 15 cycles of reaction 4!5, the Au atomic% in
To evaluate the catalytic activity and recyclability the recovered PS-supported gold(I) complex 3 had
of the PS-supported gold(I) complex 3, three well dropped to 0.30% (from the original 0.79%) as mea-
documented homogeneous gold(I)-catalyzed transfor- sured by XPS, indicating a lose of ~6% Au per cycle.
[
8]
[9]
[5b,10]
mations, i.e., 4!5, 6!7, and 8!9,
amined as model reactions with 3 as a heterogeneous role in the transformation, the resin 3 was filtered off
catalyst. when half the amount of 4 was converted into 5, the
were ex- To examine if the leached Au played the catalytic
The transformation of enynyl acetates to cyclopen- transformation of the remaining 4 in the filtrate was
tenones (e.g., 4!5) via tandem 3,3-rearrangement hardly observable on TLC, indicating that the PS-sup-
and Nazarov reaction was disclosed by Zhang and ported gold(I) complex 3 played the decisive role in
Wang with Ph PAuSbF as the most effective cata- the reaction.
3
6
[8]
lyst. With the present PS-supported gold(I) complex
The morphology of the recovered catalyst 3 (after
th
3
1
as catalyst (ca. 6 mol% Au) in the presence of 15 use in reaction 4!5) together with the original 3
0 mol% HOTf in wet CH Cl at room temperature, and the PS-triazole resin 2 were observed by scanning
2
2
enynyl acetate 4 was converted to cyclopentenone 5 electron microscopy (SEM). As shown in Figure 1,
in excellent yields (Table 2). The filtered catalyst 3, the PS-triazole resin 2 remained largely spherical as
after being washed with CH Cl , was used directly for the underivatized PS resin, the spherical beads fell
2
2
the next reaction cycle. It was remarkable that after apart after formation of the gold(I) complex in 3, and
14 reaction cycles, the catalyst 3 was still effective to the particles in 3 aggregated after several cycles of re-
1904
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1903 – 1907

A Recyclable Polystyrene-Supported Gold(I) Catalyst
Figure 1. SEM of the PS-triazole resin 2 (left), the PS-supported gold(I) catalyst 3 (middle), and the recovered catalyst 3
th
after 15 use in reaction 4!5 (right).
action. The aggregation of the PS-supported gold(I) Table 4. Synthesis of phenol 9 via rearrangement of alkyne-
complex might also account partly for the degradation furan 8 catalyzed by the PS-supported gold(I) complex 3
[a]
and the recycling experiments.
of the catalytic activity.
We then examined the utility of the present PS-sup-
ported gold(I) complex 3 for the cyclization of 1,6-
enyne 6 (to give 7), a transformation well documented
by Echavarren et al. with Ph PAuSbF as an effective
3
6
[9]
catalyst. As shown in Table 3, complex 3 catalyzed
this reaction effectively, and could be reused after Cycle
simple filtration and washing with the solvent. At the
[
b]
Time [min]
Yield [%]
1
5
30
180
2160
2160
88
86
82
85
80
first three cycles, the reaction proceeded smoothly,
leading to product 7 in >90% yield within 90 min,
however, the reaction with the recycled catalyst 3
2
3
4
5
became considerably more sluggish thereafter.
Lastly, the PS-supported gold(I) complex 3 was ap-
plied to the Hashmi phenol synthesis via alkyne-furan
[a]
Reaction conditions: 8 (0.2 mmol) and the PS-supported
gold(I) complex 3 (24 mg, ca. 0.012 mmol Au) in dry ni-
tromethane (10 mL, 0.02M) at room temperature under
an argon atmosphere.
[
5b,10]
arrangement (i.e., 8!9).
As the results in Table 4
show, complex 3 was also demonstrated to be effec-
tive and reusable in this transformation. The reaction
[b]
Isolated yield.
Table 3. The cyclization of 1,6-enyne 6 catalyzed by the PS-
supported gold(I) complex 3 and the recycling experiment-
s.
rate became considerably slower after each cycliza-
tion of the catalyst, nevertheless, within the tested 5
cycles, the yields of the phenol product 9 could be
maintained at >80%.
[
a]
In summary, we have immobilized, for the first
time, a homogeneous gold(I) catalyst onto a polystyr-
ene support, the resulting PS-benzotriazole-gold(I)
complex 3 showed comparable catalytic activities as
those of the homogenous gold(I) catalysts and recy-
clability at least in the tested model transformations,
including the tandem 3,3-rearrangement and Nazarov
reaction of enynyl acetate 4, the cyclization of 1,6-
enyne 6, and the rearrangement of alkyne-furan 8.
These preliminary results reveal the potential of the
solid-supported gold(I) catalysts in future practical
applications of the flourishing gold(I)-catalyzed chem-
ical transformations.
[
b]
Cycle
Time [min]
Yield [%]
1
2
3
4
5
6
30
60
90
180
420
1440
93
93
90
81
75
70
[
a]
Reaction conditions: 6 (0.3 mmol) and the PS-supported
gold(I) complex 3 (36 mg, ca. 0.018 mmol Au) in dry
CH Cl2 (3 mL, 0.1M) at room temperature under an
2
argon atmosphere.
Isolated yield.
[
b]
Adv. Synth. Catal. 2011, 353, 1903 – 1907
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1905

Wenjie Cao and Biao Yu
COMMUNICATIONS
Experimental Section
Synthesis of Phenol 9 via Rearrangement of Alkyne-
furan 8 Catalyzed by the PS-Supported Gold(I)
Complex 3 and the Recycling Experiments
Preparation of the PS-benzotriazole-Gold(I)
Complex 3
[12]
To a solution of 8 (60 mg, 0.2 mmol) in dry nitromethane
To a solution of benzotriazole (1.4 g, 12 mmol) in toluene
(10 mL, 0.02M) was added the PS-supported gold(I) com-
plex 3 (24 mg, 0.012 mmol Au) at room temperatur under
an argon atmosphere. The stirring was continued until TLC
showed the completion of the reaction. The solid was fil-
(
20 mL) were added Merrifield resin 1 (chloromethyl-poly-
ꢀ
1
styrene resin, 1% DVB, 200–400 mesh, 1.2 mmol·g as Cl,
.0 g). After being refluxed for 48 h, the mixture was poured
2
into ethyl ether (200 mL). The precipitate was filtered,
washed with ethyl ether several times, and then dried under
oil pump vacuum for 12 h to provide the PS-benzotriazole
resin 2 as a pale yellow powder; yield: 2.2 g.
tered, washed with dry CH Cl several times, then dried
2 2
under oil pump vacuum for 12 h, and used for the next reac-
tion cycle under similar conditions. The organic layers were
combined and concentrated. The residue was purified by
silica gel column chromatography (petroleum ether/ethyl
acetate/dichloromethane=5/1/1) to provide phenol 9.
To a suspension of the resin 2 (0.6 g) in anhydrous CH Cl
2
2
(
(
10 mL), PPh AuSbF [freshly prepared as a CH Cl solution
3 6 2 2
[13]
10 mL) by mixing PPh AuCl
(0.5 g, 1.0 mmol with
3
AgSbF (0.35 g, 1.0 mmol) followed by filtration through a
6
[7]
celite plug to remove precipitated AgCl] was added at
room temperature. After being stirred overnight, the precip-
itate was filtered, washed with CH Cl and ethyl ether three
Acknowledgements
2
2
times in turn, and dried under oil pump vacuum for 12 h to
give the desired PS-supported gold (10.3 wt%) catalyst 3 as
a gray powder; yield: 0.94 g.
This work was supported by the National Natural Science
Foundation of China (20932009 and 20921091) and the Min-
istry of Science and Technology of China (2010CB529706).
Transformation of Enynyl Acetate 4 to
Cyclopentenone 5 Catalyzed by the PS-Supported
Gold(I) Complex 3 and the Recycling Experiments
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Adv. Synth. Catal. 2011, 353, 1903 – 1907
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1907