Extended lifetimes of gold(III) chloride catalysts using copper(II) chloride and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)

Article abstract of DOI:10.1002/adsc.201000859

The turnovers of a gold(III) chloride catalyst were increased by 3,300% with the addition of several equivalents of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and catalytic amounts of copper(II) chloride. A three-component coupling reaction between pipe

Full text of DOI:10.1002/adsc.201000859

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DOI: 10.1002/adsc.201000859
Extended Lifetimes of Gold(III) Chloride Catalysts using
G
Copper(II) Chloride and 2,2,6,6-Tetramethylpiperidine 1-Oxyl
(TEMPO)
Tyler A. Graf,a Thomas K. Anderson,a and Ned B. Bowdena,*
a
University of Iowa, Department of Chemistry, W425 Chemistry Building, Iowa City, IA 52242, USA
Fax : (+1)-319-335-1270; e-mail: ned-bowden@uiowa.edu
Received: November 16, 2010; Revised: February 18, 2011; Published online: April 28, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000859.
Abstract: The turnovers of a gold
(III) chloride cat-
counterions that greatly increased the enantioselectiv-
ities of gold catalysts.[33–38] As the reactions and appli-
cations of gold catalysts increase, the need to decrease
the loadings of gold and increase their turnovers has
become an important consideration requiring further
development. For instance, both AuCl3 ($28,800 per
mole at 99% purity from Sigma–Aldrich) and AuCl
($29,800 per mole at 99.9% purity from Sigma Al-
drich) are expensive compounds so efforts to decrease
the amounts required for catalysis are important. In
this paper, we show unprecedented stabilization of
AuCl3 using a combination of 2,2,6,6-tetramethyl-1-pi-
peridine 1-oxyl (TEMPO) and low levels of CuCl2
alyst were increased by 3,300% with the addition of
several equivalents of 2,2,6,6-tetramethylpiperidine
1-oxyl (TEMPO) and catalytic amounts of cop-
ACHTUNGTRENNUNGper(II) chloride. A three-component coupling reac-
tion between piperidine, phenylacetylene, and ben-
zaldehyde yielded a propargylic amine in quantita-
tive conversions and isolated yields when goldACTHNUGTRNEUNG(III)
chloride was added in catalytic amounts, but the
gold catalyst decomposed and had little to no reac-
tivity when a second set of piperidine, phenylacety-
lene, and benzaldehyde was added after the reac-
tion was complete. Thus, only one cycle was possi-
ble with gold
TEMPO and copper(II) chloride to reactions with
gold(III) chloride maintained the catalytic activity
of gold for up to 33 cycles. This result demonstrates
a new way to greatly increase the turnovers of a
G
that allowed a Au
times.
Homogeneous gold-catalyzed reactions are typical-
ly run at loadings of one to five mole percent catalyst
because it is readily reduced to colloidal Au(0) which
decreases its turnovers (Figure 1,a).[26,39–41] Initial
strategies to increase the lifetime of Au catalysts in-
ACHTUNGTRNE(NUNG III) catalyst to be recycled up to 33
A
goldACHTUNGTRENNUNG(III) chloride catalyst with the addition of inex-
pensive, commercially available reagents.
Keywords: catalyst recycling; oxidation; redox
chemistry; TEMPO
Previously thought to be catalytically inactive, homo-
geneous gold catalysts have received increasing atten-
tion in recent years due to the discovery of new reac-
tions that they catalyze and successful efforts to yield
enantiomerically enriched products.[1–13] These cata-
lysts complete numerous different types of reactions
including those requiring Lewis acid activation of al-
À
kynes and alkenes, insertion into the C H bond of al-
kynes, oxidation of alkenes or alcohols, and reductive
silylation of alcohols.[14–32] Asymmetric versions of ho-
mogeneous gold catalysts have been reported includ-
Figure 1. a) The decomposition of Au(I) to yield colloidal
Au(0). b) The three-component reaction that was studied in
_
this article. c) The structure of AuACTHNUTRGNENUG(CN)Cl2. d) The structure
ing recent work with chiral ligands or with chiral of TEMPO.
Adv. Synth. Catal. 2011, 353, 1033 – 1038
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Tyler A. Graf et al.
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volved reduction in the concentration of gold species
because the rate of decomposition is proportional to
the square of the concentration of the gold species.
Unfortunately, this method also slowed the desired re-
action and led to incomplete reactions. In more
recent work, bulky ligands were employed to stabilize
the catalyst by preventing gold atoms from coming
into contact. N-Heterocyclic carbene ligands stabi-
lized Au catalysts from decomposition and increased
catalytic activity via steric effects and significant elec-
tron donation by the carbene.[42–45] In a recent paper
_
by Che et al., a pyridine-based ligand [AuACTHNURGTNEUNG(CN)Cl2]
bonded to gold salts increased the lifetime of the cata-
lyst in the reaction shown in Figure 1,b.[46] When
AuCl3 was used as the catalyst without additional
ligand, it decomposed after one cycle and no further
activity was observed. In reactions with the ligand
shown in Figure 1,c, the catalyst was recycled up to
ten cycles. The recycling reactions were run for 24 h
1
after which the conversion was checked by H NMR
spectroscopy and
a new set of substrates was
added.[46] Although 812 out of a possible 1,000 turn-
overs were reached through ten cycles, the reaction
did not go to completion after the second cycle and
the conversion on the last cycle was only 60%. Cata-
lyst decomposition limited the turnovers and conver-
sions that could be obtained.
Figure 2. a) Three reactions that were completed at 5 mol%
loadings of AuCl3, at 5 mol% AuCl3 and 0.25 mol% CuCl2,
and at 5 mol% AuCl3, 0.25 mol% CuCl2, and 60 mol%
TEMPO. b) This figure shows the same three reactions as in
a) but up to 33 cycles for the reaction with AuCl3, CuCl2,
We discovered that although this reaction was suc-
cessful at loadings of 0.1 to 5 mol% AuCl3, the cata- and TEMPO. The dip at the thirteenth cycle corresponds to
when the reaction slowed and each cycle time was increased
to 24 h at the 15th cycle. At cycle 20 the reaction time was
increased to 36 h.
lyst decomposed during the reaction. In a typical re-
action, the substrates were allowed to react for 12 h
during which a dark solid precipitate was observed to
form along the glass walls and in the bottom of the
reaction flask. This precipitate was believed to be col-
loidal Au(0) that is commonly formed in Au-catalyzed was 100%, after two cycles it was 200%, and after
reactions. When more of the alkyne, aldehyde, and three cycles it was 300%.
amine were added at the end of the reaction, no fur-
When no TEMPO or CuCl2 were added with
ther conversion was observed due to the decomposi- 5 mol% AuCl3, the reaction went to quantitative con-
tion of the Au catalyst. Although the reactions were version after one cycle and then no further turnover
completed faster with higher loadings of AuCl3, the was observed upon the addition of more reagent
decomposition of the catalyst was also accelerated.
(Figure 2). When 0.25 mol% of CuCl2 was added
We discovered that when CuCl2 and TEMPO were (Au:Cu ratio of 20:1) the reaction yielded quantita-
added to 5 mol% loadings of AuCl3 catalyst, the tive turnovers for four cycles and then the catalyst
number of cycles where the catalyst was active greatly was decomposed by the end of the 7th cycle for a total
increased (Figure 2). In these and all subsequent reac- conversion of 510%. When 60 mol% TEMPO and
tions, the catalysts were added at the beginning of the 0.25 mol% CuCl2 were added with AuCl3, the reaction
first cycle with the amine, aldehyde, and alkyne and yielded quantitative conversions through 33 cycles.
the reaction was heated to 708C. After 12 h an aliquot Thus, the number of cycles and turnovers of the
of the organics was removed to check the conversion AuCl3 catalyst were increased by a factor of 33 with
by 1H NMR spectroscopy and an additional set of catalytic amounts of TEMPO and CuCl2.
amine, aldehyde, and alkyne was added. The reaction
A study of the amount of CuCl2 and TEMPO
was heated to 708C and the conversion was checked needed to increase the lifetime of the AuCl3 catalyst
1
by H NMR spectroscopy after 12 h and another new at 5 mol% loadings was undertaken (Table 1). The ad-
set of amine, aldehyde, and alkyne was added. Each dition of CuCl2 with no TEMPO added increased the
cycle corresponded to the addition of one set of the number of cycles and overall conversions of the
amine, aldehyde, and alkyne and 12 h of heating at AuCl3 catalyst even at AuCl3:CuCl2 loadings as low as
708C. Thus, the highest conversion after one cycle 40:1. When the AuCl3:CuCl2 loading was increased to
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Extended Lifetimes of GoldACTHNUTRGNEUG(N III) Chloride Catalysts using Copper(II) Chloride
Table 1. Effect of CuCl2 and TEMPO on the lifetimes of
AuCl3 catalysts.
(entry 5 in Table 2), which demonstrated that copper
was needed to increase the lifetime of the catalyst. In
summary, at 5 mol% of AuCl3 the addition of CuCl2
increased the lifetime of the Au catalyst and the addi-
tion of both CuCl2 and TEMPO lead to a greater in-
crease in the lifetime of the catalyst.
Entry AuCl3
CuCl2
Au:Cu TEMPO Con-
Cy-
(mol%) (mol%)
(mol%)
version[a] cles
[%]
1
2
3
4
5
6
7
8
9
5
5
5
5
5
5
5
5
5
0.25
20:1
40:1
80:1
40:1
80:1
120:1 60
200:1 60
80:1
80:1
0
0
0
60
60
510
280
100
360
370
290
100
100
90
8
4
2
4
4
4
2
4
4
It is always desirable to use less of an expensive
catalyst such as AuCl3, so the three-component reac-
tion was studied at 1 and 0.1 mol% loadings of AuCl3.
Reactions with less than 0.1 mol% of AuCl3 did not
go to completion so they were not studied. Control
experiments with only 1 or 0.1 mol% AuCl3 and no
CuCl2 or TEMPO were carried out for five cycles (en-
tries 1 and 7 in Table 3). These reactions had quantita-
tive conversions after the first cycle, but they only
reached 140–150% conversion after five cycles. These
reactions demonstrated that the Au catalyst decom-
posed rapidly after the first cycle. A control reaction
was completed with an Au:TEMPO ratio of 1:12 and
the conversion was 200% after five cycles which dem-
onstrated that TEMPO by itself had little effect on
the lifetime of the Au catalyst (entry 2 in Table 3).
Reactions completed at one mol% AuCl3 had high
conversions for ten cycles when CuCl2 was added at
mole ratios of Au:Cu of 2:1 and 20:1 (entries 3 to 6 in
Table 3). These reactions were not complete after ten
cycles, but when the reactions were allowed to pro-
ceed for an additional 36 to 60 h after the tenth cycle
0.125
0.0625
0.125
0.0625
0.042
0.025
0.0625
0.0625
15
5
[a]
Each reaction run at 708C for 12 h. After 12 h the con-
version was checked by H NMR spectroscopy and a new
1
set of substrates were added. The maximum conversion
after cycle “n” would be 100 multiplied by n.
Table 2. Control reactions to study how O2 and different
amounts of Au, Cu, and TEMPO affected this reaction.
Entry AuCl3
CuCl2
TEMPO
Conversion Cycles
[%]
(mol%) (mol%) (mol%)
1[a]
2
3
4
5
5
0
0
0
5
0.25
0.25
0
0.25
0
0
0
130
60
60
95
0
0
0
110
2
1
1
1
2
Table 3. Reactions at 1 and 0.1 mol% AuCl3.
[a]
Reaction completed under an atmosphere of O2.
Entry[a] AuCl3
(mol%) (mol%) (mol%)
CuCl2
TEMPO Conversion Cycles
[%]
80:1, the reaction stopped after the first cycle when
no TEMPO was present. In contrast, when 60 mol%
TEMPO was added, the catalyst was active over four
cycles at AuCl3:CuCl2 loadings as low as 120:1.
1
2
3
4
5
6
7
8
1
1
1
1
1
1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0
0
0.5
0.05
0.5
0.05
0
0.1
0.02
0.005
0.1
0.02
0.005
0
140
200
5
5
12
24
12
0
0
0
24
24
24
0
0
0
930 (980)[b] 10
910 (990)[c] 10
850 (980)[d] 10
840 (980)[d] 10
A series of control experiments was completed to
further study this reaction (Table 2). In entry 1 of
Table 2 an experiment to probe whether O2 could act
as the oxidant rather than TEMPO is described. This
reaction had Au and Cu loadings identical to entry 1
of Table 1 that resulted in a conversion of 510% after
eight cycles when the reaction was run under an at-
mosphere of N2. When the same reaction was com-
pleted under an atmosphere of O2 the conversion
reached 95% after one cycle but no further conver-
sion was observed. An atmosphere of O2 poisoned
the AuCl3 such that the copper had no noticeable
effect on the lifetime of the Au catalyst.
Copper was a poor catalyst for this reaction and at
loadings used here no conversion was observed by
1H NMR spectroscopy in experiments without AuCl3
but with CuCl2, TEMPO, or a combination of
TEMPO and CuCl2 (entries 2–4 in Table 2). In addi-
tion, the control experiment with AuCl3 and TEMPO
showed only modest conversion after the first cycle
150
400
400
330
820
400
320
5
5
5
5
10
5
5
9
10
11
12
13
[a]
[b]
[c]
[d]
Reactions at 1 mol% AuCl3 were run for 12 h for each
cycle and reactions at 0.1 mol% AuCl3 were run for 24 h
for each cycle.
The number in parenthesis is the conversion after the re-
action was allowed to run for 48 h after the end of the
tenth cycle.
The number in parenthesis is the conversion after the re-
action was allowed to run for 36 h after the end of the
tenth cycle.
The number in parenthesis is the conversion after the re-
action was allowed to run for 60 h after the end of the
tenth cycle.
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all of the reactions went to quantitative conversions. effect. In the reaction with a CuCl2 loading of
The additional time was necessary due to dilution of 0.005 mol% the conversions was 330%, but without
the reagents and catalysts at later cycles. No organic CuCl2 the conversion was only 150%. Clearly, a small
solvent was used in these reactions so the product amount of CuCl2 had pronounced effects on the turn-
acted as a solvent for subsequent cycles and diluted overs and conversions.
the catalyst and reagents. Interestingly, the presence
This reaction was complicated to study because Au,
of TEMPO led to only a small increase in the conver- Cu, and TEMPO each existed in three oxidation
sion at the end of the tenth cycle but no difference in states and the reaction was biphasic with different sol-
conversion when the reactions were allowed to go to ubilities for each oxidation state of the molecules in
completion after the tenth cycle.
water and the organic phases. In addition, the amines
The same result was seen for reactions completed used in the reaction could coordinate to Au and Cu
with 0.1 mol% AuCl3 (entries 8 to 13 in Table 3). In and change their oxidation potentials.[47,48] Because of
these reactions the presence of 24 mol% TEMPO had these difficulties, it was not possible to completely dis-
no observable effect on the conversions after 5 cycles. entangle the exact mechanism responsible for the
Surprisingly, very low concentrations of CuCl2 had an long lifetime of the Au catalyst, but a proposed mech-
anism is outlined in Figure 3.
In the original mechanism proposed by Li for this
AuCl3 coupling reaction, the active Au catalyst was
Au(I).[49] A limitation of Au(I) catalysis is the well
known reduction of Au(I) to colloidal Au(0) by the
reaction shown in Figure 1a. We believe that Cu(II)
oxidized Au(0) to Au(I) which increased the number
of turnovers. The Cu(I) was oxidized back to Cu(II)
by O2 or TEMPO. These reactions were run under
conditions where most, but not all, of the O2 was re-
moved from the reaction vessel so low levels of O2
were present. This mechanism is consistent with the
observation that CuCl2 was necessary to observe an
increase in turnover of the AuCl3 catalyst.
To further investigate this effect, two different sets
of three component coupling reactions were studied
(Figure 4). These reactions were completed at load-
ings of 1 mol% AuCl3 with and without 1 mol% of
CuCl2. In the control experiments with AuCl3 and
without CuCl2, the reactions went to conversions of
80% for the reaction with morpholine and 140% for
the reaction with m-bromobenzaldehyde over three
Figure 3. a) and b) Two proposed cycles for how Au, Cu,
and TEMPO interact to increase the lifetime of the Au cata-
lyst where both TEMPO and O2 oxidize Cu(I) to Cu(II).
Figure 4. a) and b) Three component reactions that were investigated. c) A hydroarylation reaction that was studied for the
effect of added CuCl2.
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Extended Lifetimes of GoldACTHNUTRGNEUG(N III) Chloride Catalysts using Copper(II) Chloride
Table 4. Effect of AuCl3 and CuCl2 on the hydroarylation re-
actions shown in Figure 4c.
completed. Impressively, the lifetime of the AuCl3 cat-
alyst at a loading of 5 mol% increased from 12 h to
over 792 h when 0.25 mol% CuCl2 and 60 mol%
TEMPO were added. In fact, limitations in determin-
ing the conversions rather than catalyst decomposi-
tion prevented further study of this reaction.
This research represents one approach to lowering
the costs of homogeneous Au catalysts for use in aca-
demic or industrial work. A low loading of AuCl3 cat-
alyst is desired because of its high cost, but low load-
ings result in slow reactions or low conversions to
product. What is desired is to use a high loading of
AuCl3 so the reactions are rapid, but the AuCl3 cata-
lysts should be recycled to keep the costs low.
Common approaches to solving this problem have
been to develop new ligands to accelerate the reac-
tion rate or to find new, cheaper catalysts with higher
Entry[a[
R
AuCl3
(mol%)
CuCl2
(mol%)
Conversion[b]
[%]
1
2
3
4
5
6
H
H
H
CH3
CH3
CH3
1
1
0
1
1
0
1
0
1
1
0
1
34
16
0
79
50
0
[a]
Reactions were completed for 24 h and no further turn-
over was observed for longer reaction times.
The conversions were measured by H NMR spectrosco-
[b]
1
py at the completion of the reaction.
cycles. When AuCl3 and CuCl2 were added to the re- reactivities. Another solution – the one described in
actions they went to 460 and 470% conversions for this paper – was to find a method to prevent the Au
five cycles. The increase in conversions by the addi- catalyst from decomposing such that it could be used
tion of CuCl2 was clearly not substrate dependent but in high concentrations and be recycled. In contrast to
successful for three different examples of the three the high cost of AuCl3, the cost for CuCl2 ($18 per
component coupling reaction.
mole for 97% purity from Sigma–Aldrich) and
A second reaction was also studied to investigate TEMPO ($1,030 per mole for 98% purity from
whether CuCl2 would increase the turnovers in a reac- Sigma–Aldrich) are modest and can be added to in-
tion where the Au is proposed to act as a Lewis acid. crease the conversions and lifetimes of the AuCl3 cat-
In the three component reaction shown in Figure 1b, alyst. Because CuCl2 and TEMPO do not require any
À
the Au catalyst was proposed to insert into the C H ligands on the AuCl3 to be effective, this method may
bond of the acetylene and then react with the imine prove to be applicable to more reactions and will be
formed by the condensation of the aldehyde and studied for such activity.
amine.[38] In the reaction shown in Figure 4c, the Au
is proposed to act as a Lewis acid to activate mesity-
lene for nucleophilic attack by phenylacetylene.[50]
This reaction has been reported by Sommer to pro-
Experimental Section
ceed with AuCl3 as the catalyst for a variety of sub-
strates, but our work demonstrated that it does not
proceed with CuCl2 which was only partially soluble
in the solvent (Table 4).[50]
A detailed description for each reaction is given in the Sup-
porting Information.
Multiple reactions with phenylacetylene, mesity-
lene, and 1 mol% AuCl3 were completed, and highest
conversion was only 20% which was consistent with
the report by Sommer. When CuCl2 was added with
AuCl3, the highest conversion increased to 36%.
More surprising results were found when 4-ethynylto-
luene was used as the substrate. The reaction with
only AuCl3 reached 50% conversion but the reaction
Typical Procedure
This is the procedure for entry 1 in Table 1 and illustrates
how these reactions were completed. Benzaldehyde (100 mL,
1.0 mmol), piperidine (106 mL, 1.1 mmol), and phenylacety-
lene (160 mL, 1.5 mmol) were added to a 10-mL Schlenk
tube. A solution of CuCl2 was made by dissolving 25 mg of
CuCl2 in 7.4 mL of H2O. 100 mL (0.34 mg, 2.5ꢁ10À3 mmol)
with AuCl3 and CuCl2 reached 79% conversion. Al- of this solution were added to the organics. AuCl3 (15 mg,
0.05 mmol) was taken from a glove box and added to the
Schlenk tube in 2 mL of H2O under flowing nitrogen. The
reaction mixture was then heated at 708C and stirred in an
oil bath. After 12 h the reaction was briefly cooled to ambi-
ent temperatures and a small aliquot was removed from the
though the effect of the addition of CuCl2 was much
more modest than in the three-component coupling
reaction, these experiments demonstrated that CuCl2
increased the conversions in a second reaction where
the Au catalyst has a significantly different role in the
catalytic cycle.
This work demonstrated a remarkable stabilization
of AuCl3 catalysts using catalytic amounts of CuCl2.
The effect was robust at loadings of AuCl3 that
1
organic phase to check the conversion by H NMR spectros-
copy. Additional benzaldehyde (100 mL, 1.0 mmol), piperi-
dine (106 mL, 1.1 mmol), and phenylacetylene (160 mL,
1.5 mmol) were added at this time. Heating was resumed for
an additional 12 h. This process was repeated for a total of
ranged by 50 fold and allowed up to 33 cycles to be eight cycles.
Adv. Synth. Catal. 2011, 353, 1033 – 1038
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Tyler A. Graf et al.
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