Recyclable solid ruthenium catalysts supported on metal oxides for the addition of carboxylic acids to terminal alkynes

Article abstract of DOI:10.1002/adsc.201000431

Ceria-supported ruthenium catalysts (Ru/CeO₂) were found to be quite effective for the addition of various carboxylic acids to terminal alkynes, which gave the corresponding enol esters in moderate to high yields. The major products of the reac

Full text of DOI:10.1002/adsc.201000431

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DOI: 10.1002/adsc.201000431
Recyclable Solid Ruthenium Catalysts Supported on Metal
Oxides for the Addition of Carboxylic Acids to Terminal Alkynes
Masami Nishiumi,a Hiroki Miura,a Kenji Wada,a,* Saburo Hosokawa,a
and Masashi Inouea
a
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Kyoto
615-8510, Japan
Fax : (+81)-75-383-2479; phone: (+81)-75-383-2482; e-mail: wadaken@scl.kyoto-u.ac.jp
Received: June 2, 2010; Revised: September 8, 2010; Published online: November 17, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000431.
Abstract: Ceria-supported ruthenium catalysts (Ru/ showed activity comparable to that of the ceria-sup-
CeO2) were found to be quite effective for the addi- ported catalyst. These catalysts were recyclable with-
tion of various carboxylic acids to terminal alkynes, out a significant loss of activity, and the leaching of
which gave the corresponding enol esters in moder- ruthenium species into the liquid phase was negligi-
ate to high yields. The major products of the reaction ble after cooling the reaction mixture, which indi-
were E-isomers of anti-Markovnikov adducts. cates marked superiority of the present solid oxide
Among the ceria-supported ruthenium catalysts ex- catalysts to conventional homogeneous catalysts.
amined, those prepared using ruthenium precursors
with chloride ligands showed high activities. The zir- Keywords: alkynes; carboxylic acids; ceria; enol
conia-supported ruthenium catalyst (Ru/ZrO2) esters; ruthenium; solid catalysts
Introduction
ports on the application of solid oxide-supported
ruthenium catalysts.
Homogeneous catalysts play a significant role in or-
Ceria characteristically exhibits oxygen-storage ca-
ganic syntheses, and can be used to achieve excellent pacity and strongly interacts with supported precious
yields and/or control the selectivity through the metal species,[22] and ceria-supported catalysts are
choice of suitable ligands and additives. However, used for various reactions.[23] Among them, Ru/CeO2
they are often associated with practical and environ- is known to be quite effective as a catalyst for the
mental disadvantages, such as the difficulty of the sep- liquid-phase oxidation of alcohols[24] and wet oxida-
aration and recovery of the catalysts from the reac- tive processes for the treatment of water pollution.[25]
tion mixture as well as their relatively low thermal Very recently, we reported that Ru/CeO2 acts as an
and/or chemical stability. The use of heterogeneous effective heterogeneous catalyst for transfer-allylation
catalysts, particularly solid oxide catalysts, is an attrac- from homoallyl alcohols to aldehydes,[26] the nitrogen-
À
tive alternative to overcome these drawbacks.[1,2]
directed arylation of stable aromatic C H bonds with
On the other hand, enol esters are important reac- aryl halides,[27a] and the addition of aromatic C H
À
tants and useful intermediates for many organic reac- bonds to vinylsilanes.[27b] These reactions take place
À
À
tions because of their numerous industrial applica- via cleavage of a non-strained C C or C H bond.
tions,[3–10] and the addition of carboxylic acids to al- These results suggest that an Ru/CeO2 catalyst may
kynes has been shown to be an atom-economical be a good alternative to homogeneous ruthenium cat-
route to enol esters.[11,12] Low-valent ruthenium com- alysts.
plexes have been reported to act as excellent homoge-
We report here the first example for solid oxide-
neous catalysts for this reaction.[13–20] Whereas the ac- supported ruthenium catalysts that are effective for
tivities of ruthenium complex catalysts immobilized the synthesis of enol esters by the addition of carbox-
onto phosphine-modified resin[21a] as well as related ylic acids to terminal alkynes. Simple catalysts, Ru/
Pd[21b] and Au[21c] catalysts for analogous reactions CeO2 and Ru/ZrO2, showed excellent activities and
have been reported, there have been no previous re- could be used for the reactions of a variety of sub-
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strates. These catalysts were recyclable without a sig- neous ruthenium carbonyl catalyst.[14a] Under the
nificant loss of activity, and the leaching of ruthenium present conditions, the reaction catalyzed by
species was not observed after cooling the reaction Ru3(CO)12 afforded the enol esters in a total yield of
mixture. The effects of ruthenium precursors were ex- 77% with 73% selectivity for 3aa (entry 9). [RuCl
2ACHTUNGTRENNUNG(p-
amined and the catalysts were characterized by spec- cymene)]2, the precursor of the supported Ru cata-
troscopic techniques.
lysts, showed a lower activity (entry 10), whereas sev-
eral homogeneous ruthenium complex catalysts bear-
ing phosphorus ligands[15–20,28] have been reported to
show excellent activities at lower temperatures.
Results and Discussion
The activity of the catalysts was significantly affect-
Ru/CeO2 catalysts were prepared by the impregnation ed by the catalyst support (entries 4–8). The ZrO2-
of [RuCl2A(p-cymene)]2 on cerium oxide followed by supported catalyst showed activity comparable to that
CHTUNGTRENNUNG
calcination at 4008C. The reaction of benzoic acid 1a of the CeO2-supported catalyst. The other ruthenium
with ethynylbenzene 2a at 1308C for 12 h in the pres- catalysts supported on Al2O3, TiO2, SiO2, and MgO
ence of a catalytic amount of Ru/CeO2 afforded ad- were not effective. Note that a similar trend was ob-
ducts in an excellent total yield of 96% [Eq. (1), served in the transfer-allylation of homoallylic alco-
entry 1 in Table 1). The major product was the E- hols, the chelation-assisted direct arylation of aromat-
À
À
isomer of the anti-Markovnikov adducts (3aa), with ic C H bonds, and the addition of C H bonds to vi-
74% selectivity, and was obtained together with the nylsilanes.[26,27]
Z-isomer 4aa and a small amount of the Markovnikov
As shown in Table 2, the activity of Ru/CeO2 cata-
adduct 5aa. There was no sign of dimers or oligomers lysts was markedly affected by the ruthenium precur-
of 2a. The reaction at 1208C afforded the adducts in a sors used for their preparation. In general, catalysts
lower total yield (entry 2). The reaction proceeded prepared from precursors with chloride ligands
even in the open air, although the yields of the prod- showed high activities. However, the ruthenium pre-
ucts were moderate (entry 3). The activity of the pres- cursors did not influence the selectivity. Among the
ent solid catalyst is comparable to that of a homoge- precursors examined, [RuCl2ACTHUNTRGNEUNG(p-cymene)]2 was found
to be most suitable. The Ru/CeO2 catalyst prepared
from Ru3(CO)12 which has no chloride ligands showed
low activity, but the addition of ammonium chloride
(10 equivalents to Ru) to the reaction mixture gave a
higher activity than the catalyst without NH4Cl (en-
tries 5 and 6). This result clearly indicates that the
chloride species has a promoting effect. Catalysts pre-
Table 1. Activity of ruthenium catalysts for the addition of
benzoic acid 1a to ethynylbenzene 2a.
pared from [RuCl
lowing study.
2ACHTUNGERTN(NUNG p-cymene)]2 were used in the fol-
The effects of ruthenium precursors on the surface
composition of the fresh catalysts were examined by
Table 2. Effect of ruthenium precursors of Ru/CeO2 cata-
Entry Catalyst[a]
Total yield Selectivity [%][c]
AHCTUNGTRENNUNG
lysts.[a]
[%][b]
3aa:4aa:5aa
Entry Precursor
Total yield Selectivity [%][c]
1
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/ZrO2
Ru/Al2O3
Ru/TiO2
Ru/SiO2
96
69
46
94
8
7
trace
trace
77
74:21:5
74:20:6
65:33:2
73:21:6
75:25:0
71:29:0
–
[%][b]
3aa:4aa:5aa
2[d]
3[e]
4
1[d]
2
3
4
5
N
E
74:21:5
73:22:5
72:22:6
70:25:5
64:29:7
65:26:9
5
6
7
ACHTUNGTRENNUNG
8
9
Ru/MgO
Ru3(CO)12
–
6
[f]
73:19:8
52:43:5
[a]
[f]
Reaction conditions: 1a 1.0 mmol, 2a 1.3 mmol, mesity-
lene 1.0 mL, 2.0 wt% Ru/CeO2 catalyst (125 mg;
0.025 mmol as Ru), at 1308C for 12 h.
Total yield of 3aa, 4aa, and 5aa determined by GLC.
Molar ratio of isomers determined by GLC.
Entry 1 in Table 1.
10
G
N
54
[a]
[b]
[c]
[d]
[e]
[f]
Supported Ru catalyst 125 mg.
[b]
[c]
[d]
[e]
Total yield of 3aa, 4aa, and 5aa determined by GLC.
Molar ratio of isomers determined by GLC.
At 1208C.
Reaction under the air atmosphere.
Homogeneous catalyst.
NH4Cl (10 equiv. to Ru) was added to the reaction mix-
ture.
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Recyclable Solid Ruthenium Catalysts Supported on Metal Oxides
Table 3. XPS analysis of Ru (2.0 wt%)/CeO2 catalysts prepared from various precursors.[a]
Precursors
[RuCl2A(p-cymene)]2
C [%]
Ru [%]
Cl [%]
Ce [%]
O [%]
Ru 3d5/2 [eV]
G
G
12.00
17.39
17.50
20.13
17.50
1.37
1.21
1.29
1.36
1.34
3.71
3.70
3.34
trace
trace
28.77
26.34
25.73
24.80
26.41
54.15
51.36
52.14
53.71
54.75
281.93
281.63
281.73
281.61
281.53
RuCl3·nH2O
[RuCl2(CO)3]2
Ru3(CO)12
Ru(acac)3
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a]
Surface concentrations are shown as atomic%, measured at À1008C.
XPS. As shown in Table 3, residual chloride species after the reaction had proceeded at 1308C for 12 h by
were observed on the surface of the catalysts pre- ICP-AES. These results indicate that the reaction ba-
pared using Cl-containing ruthenium complexes, sically requires the presence of the solid catalyst. The
while there were no significant differences in the ceria support might act as a macroACTHNUTRGENNUGliCAHUTGTNRENgNUG and that stabiliz-
atomic ratios of the surface ruthenium species. The es and/or holds the catalytically active ruthenium spe-
Ru 3d5/2 binding energies of these catalysts indicated cies. However, the contribution of soluble ruthenium
the presence of surface Ru(IV) species,[29] but the species or nano-sized ruthenium colloids, which were
electronic effects of Cl species were not distinctly rec- generated via “release and capture”[30,31] and passed
ognized.
through the filter, could not be completely ruled out.
It is important to investigate whether the reaction A further mechanistic study which includes observa-
actually proceeds on the surface of the solid cata- tion of the catalytically active species is now in prog-
lyst.[30,31] To examine the contribution of ruthenium ress.
species in solution generated by the so-called “release
The Ru/CeO2 catalyst could be used with a variety
and capture” manner, the effect of removal of the cat- of substrates. The results of the reaction of various
alysts by hot filtration through a PTFE filter (pore carboxylic acids 1b–i with 2a at 1308C for 12 h are
size 0.47 mm) was examined. Figure 1 shows the time shown in Eq. (2) and Table 4. The reaction with aro-
course of the reaction at 1308C. Formation of the matic carboxylic acids with both electron-donating
enol esters was greatly suppressed, but not completely and electron-withdrawing substituents (1b–e) pro-
stopped by removal of the solid catalyst. Distinct in- ceeded smoothly to afford adducts in moderate to
duction periods were not recognized. Note that ruthe- high yields. A bulky substrate (1f) and carboxylic
nium species was not detected in the cold filtrate
Table 4. Scope of substrates (carboxylic acids).[a]
Entry
1
R1
Total yield Selectivity [%][c]
[%][b]
3(E):4(Z):5
1
2
3
4
5
6
7
8
1b 4-CH3C6H4
1c 3-CH3OC6H4
1d 3-NO2C6H4
1e 4-CF3C6H4
58
90
87
82
79:17:4
76:18:6
48:34:18
66:24:10
71:27:2
56:38:6
72:22:6
75:21:4
1f 2,4,6-(CH3)3C6H2 92
1g 2-thienyl
1h 3-furyl
1i n-C7H15
81
76
40
[a]
Figure 1. Effects of catalyst removal by hot filtration on the
reaction of 1a with 2a at 1308C. The reaction conditions
were identical to those for entry 1 in Table 1.
Ru/CeO2 catalyst 125 mg.
Isolated yield.
Molar ratio of isomers determined by H NMR.
[b]
[c]
1
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Table 5. Scope of substrates (alkynes).[a]
Entry
2
R2
R3
Total yield [%][b]
Selectivity [%][c] 3(E):4(Z):5
1
2
3
4
5
6
7
2b
2c
2d
2e
2f
2g
2h
4-CH3C6H4
4-CH3OC6H4
3-thienyl
H
H
H
H
H
n-C3H7
Ph
83
72
78
24
trace
0
67:23:10
44:17:39
59:17:24
43:0:57
–
–
–
2-pyridyl
A
n-C3H7
Ph
0
[a]
Ru/CeO2 catalyst 125 mg.
Isolated yield.
Molar ratio of isomers determined by H NMR.
[b]
[c]
1
acids with thiophene and furan rings (1g and 1h) were
also applicable. The reaction with an aliphatic carbox-
ylic acid (1i) also gave the desired adduct in a moder-
ate yield. In all of the cases examined, the major
products were E-isomers of anti-Markovnikov ad-
ducts (3).
The reactions with 1a and various alkynes were ex-
amined [Eq. (3)]. As shown in Table 5, the reactions
of terminal alkynes with phenyl groups (2b and 2c)
and that with a thiophene ring (2d) afforded the ad-
ducts in high yields, while the reaction with 2-ethynyl-
pyridine (2e) resulted in a low yield of enol esters.
The selectivity greatly depended on the alkynes. High
selectivity of a Markovnikov adduct (5) was observed
with an alkyne bearing a pyridyl group (2e). On the
other hand, the reactions of a terminal alkyne with a
trimethylsilyl substituent (2f) and internal alkynes (2g
and 2h) did not proceed under the present conditions.
To date, several mechanisms have been proposed
for
homogeneously
catalyzed
reactions
(Scheme 1).[11,20b] One plausible mechanism involves
the formation of vinylidene intermediates (cycle
B),[18b,19b] while another probable cycle is likely to
start with the coordination of an alkyne to a rutheni-
um species, followed by the addition of a carboxylate
nucleophile to the h2-coordinated alkyne (cycle A).[18b]
Scheme 1. Proposed catalytic mechanisms for the addition
of carboxylic acids to terminal alkynes. Cycle A: Markovni-
kov addition; Cycle B: anti-Markovnikov addition.
À ꢀ À
We examined the reaction of 1a with Ph C C D (2a-
d1), and the deuterium shifting from the terminal to
the 2-position was found to be predominant, which would promote the formation of a h2-alkyne inter-
supported the formation of the ruthenium vinylidene mediate, which leads to a Markovnikov product
species as an intermediate (cycle B). In entry 4 of through cycle A.
Table 5 (the reaction of 1a with 2e), the strong coordi-
One major advantage of heterogeneous catalysts is
nation of a pyridyl group to a ruthenium center their reusability. Therefore, the changes in the activity
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Recyclable Solid Ruthenium Catalysts Supported on Metal Oxides
Table 6. Activities of fresh and reused Ru/CeO2 catalysts.[a]
Treatment of recovered catalyst
Catalyst
Total yield [%][b]
Selectivity [%][c] 3aa:4aa:5aa
Washing+drying+calcination
Fresh[d]
2nd use
3rd use
4th use
4th use[e]
2nd use
3rd use
4th use
96
93
72
71
82
79
43
28
74:21:5
75:20:5
72:21:7
72:23:5
73:22:5
72:23:5
70:23:7
71:22:7
Washing+drying
[a]
Reaction conditions: 1a 1.0 mmol, 2a 1.3 mmol, mesitylene 1.0 mL, Ru/CeO2 catalyst 0.025 mmol as Ru (loading level
2.0 wt%) prepared from [RuCl2A(p-cymene)]2, at 1308C for 12 h.
Total yield of 3aa, 4aa, and 5aa determined by GLC.
Molar ratio of isomers determined by GLC.
Entry 1 in Table 1.
CTHUNGTRENNUNG
[b]
[c]
[d]
[e]
Prolonged run for 18 h.
of Ru/CeO2 catalyst over the course of repeated uses
were examined, as shown in Table 6. After the first
run, which gave the adducts in 96% yield, the catalyst
was recovered from the reaction mixture by centrifu-
gation followed by washing three times with diethyl
ether (10 mL) at room temperature, and then calcined
at 4008C for 30 min in air. The recovered Ru/CeO2
catalyst gave the adducts in 93% yield in the 2nd use,
although the yields decreased to some extent in the
3rd and 4th uses. Reuse of the catalyst without calcina-
tion resulted in a significant decrease in catalytic ac-
tivity. The severe deposition of organic species on the
surface would suppress the activity of the catalyst (see
below). In addition, re-deposited ruthenium species
after cooling via release and capture cycles would be
less or no longer active, which could be one reason
for the poor reusability of the catalysts.
Figure 2. H2 TPR profiles of (a) fresh and (b) used Ru
(2.0 wt%)/CeO2 catalyst prepared from [RuCl2A(p-cymene)]2.
The used catalyst was calcined in air at 4008C for 30 min.
CTHUNGTRENNUNG
To examine the properties of the Ru/CeO2 catalyst
before and after the reaction, H2 temperature-pro-
grammed reduction (TPR) measurements were per-
formed, as shown in Figure 2. In the profile of the diffuse reflectance infrared Fourier transform spec-
fresh catalyst, two peaks appeared at a low-tempera- troscopy (DRIFTS), and TG. According to the XPS
ture range, one at ca. 508C and one at 758C. The re- results shown in Table 7, a large part of the surface of
duction of ruthenium species on other supports such the used catalyst was covered by carbonaceous mate-
as titania occurred at a temperature higher than rials, probably because of the deposition of benzoate
2008C, and a temperature higher than 1008C was re- species on the surface during the catalytic run, as sug-
quired for the reduction of bulk RuO2.[26,32] These re- gested by DRIFTS (see Supporting Information). TG
sults indicate that the surface ruthenium species on analysis of the catalyst after the first use showed ca. 4
ceria is easily reduced. The used catalyst after the cal- wt% weight decrease due to the combustion of organ-
cination showed a profile similar to that of the fresh ic species adsorbed on the surface. Unfortunately,
catalyst, suggesting that the original surface rutheni- severe overlapping with C 1s peaks disturbed the pre-
um species are regenerated by calcination of the used cise measurement of the Ru 3d5/2 binding energy of
catalyst. Note that characterization of the fresh cata- the used catalyst. Most of the carbonaceous species
lysts was performed by nitrogen gas adsorption and was removed by calcination at 4008C for 30 min in
XRD, and the results are summarized in Supporting air. The surface atomic ratio of Cl greatly decreased
Information.
after the 1st use, indicating that some of the chloride
The surface states of the Ru/CeO2 catalyst before species were leached from the surface during the cata-
and after the reaction were also examined by XPS, lytic run. A slight decrease in the surface ruthenium
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Table 7. XPS analysis of the Ru (2.0 wt%)/CeO2 catalyst prepared from [RuCl2ACHTNUTRGNEUNG
(p-cymene)]2.[a]
Remarks
C [%]
Ru [%]
Cl [%]
Ce [%]
O [%]
Ru 3d5/2 [eV]
fresh catalyst
12.00
40.62
12.35
1.37
1.02
1.10
3.71
1.90
2.84
28.77
15.81
24.12
54.15
40.65
59.60
281.93
–
281.23
after 1st use
after 1st use, calcined
[a]
Surface concentrations are shown as atomic%, measured at À1008C.
ratio of the used catalyst after calcination can be at-
As demonstrated above, the catalytic activity of
tributed to the sintering of ruthenium species during Ru/CeO2 gradually decreased during repeated uses.
the regeneration procedure, since the leaching of In the present case, the gradual loss of surface chlo-
ruthenium species was not observed after cold filtra- ride species shown in the XPS study would not be a
tion of the catalyst (see above).
major reason: The addition of NH4Cl (1 to 4 equiva-
Figure 3 shows DRIFT spectra of the Ru/CeO2 cat- lents to Ru species) did not improve the activity of re-
alyst in the range of 800 to 1100 cmÀ1 before and after cycled catalysts, although the addition of NH4Cl sig-
the catalytic run (entry 1 of Table 1). The spectrum of nificantly promoted the reaction by the catalysts pre-
the fresh catalyst showed a distinct band at 984 cmÀ1 pared using chlorine-free Ru sources (see Table 2, en-
that could be assigned to a ruthenium oxo (Ru=O) tries 5 and 6). The slight sintering of ruthenium spe-
species.[32] Although such a distinct peak was not rec- cies as recognized by the XPS study may have
ognized for Ru/ZrO2, the formation of similar ruthe- contributed to this result.
nium-oxygen species has also been proposed for the
The combined results of the spectroscopic study
zirconia-supported catalyst.[33] There were no signs of suggest the formation of well-dispersed Ru(IV) oxo
oxo species for ruthenium catalysts supported on species on the surface of the fresh Ru/CeO2 catalyst.
Al2O3, TiO2, SiO2, or MgO. The characteristic band at A previous XANES study showed the formation of
984 cmÀ1 of Ru/CeO2 disappeared after the reaction five-coordinate Ru(IV) species on the surface of Ru/
[Figure 3 (c)], suggesting that the ruthenium oxo spe- CeO2 prepared by a similar method.[32] Based on the
cies were transformed to other catalytically active results of DRIFTS and an H2-TPR study, these oxo
species during the reaction. The band at 980 cmÀ1 re- species are thought to be transformed to reduced
appeared in the spectrum of the used catalyst after active ruthenium species during the catalytic runs.
calcination, indicating that ruthenium oxo species The absence of distinct induction periods (see
were regenerated [Figure 3 (d)].
Figure 1) suggests that the generation of active spe-
cies might occur at the very initial stage of the reac-
tion. Although the nature of the true catalytically
active species is not yet clear, the absence of leaching
of ruthenium species after the reaction and the recy-
clability of the catalyst indicate that the present
oxide-supported catalysts are promising alternatives
to conventional homogeneous catalysts.
Conclusions
Solid oxide-supported ruthenium catalysts that are ef-
fective for the addition of a variety of carboxylic acids
to terminal alkynes have been developed for the first
time: Ru/CeO2 and Ru/ZrO2 showed excellent activi-
ties. A variety of carboxylic acids and terminal al-
kynes could be used with this catalyst. The present
Ru/CeO2 catalyst was recyclable, and the leaching of
ruthenium species was not observed after cooling the
reaction mixture. These features are quite attractive
and advantageous from synthetic, industrial, and envi-
ronmental points of view. Further studies to improve
the regio- and stereoselectivities of the recyclable
solid ruthenium catalysts are currently in progress.
Figure 3. DRIFT spectra of (a) CeO2, (b) fresh Ru/CeO2
catalyst prepared from [RuCl
2ACHTUNGRTEN(NUNG p-cymene)]2, (c) used cata-
lyst, and (d) used catalyst after calcination.
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Recyclable Solid Ruthenium Catalysts Supported on Metal Oxides
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Experimental Section
Preparation of the Ru/CeO2 Catalysts
Supported catalysts were prepared by the impregnation
method through the use of various supports and ruthenium
complexes. A typical procedure is as follows: 1.0 g of CeO2
was added to a solution of [RuCl2ACTHNUTRGNE(UNG p-cymene)]2 (62 mg,
0.10 mmol, 20 mg as Ru) in 10 mL of methanol at 508C.
After impregnation, the resulting light yellow powder was
calcined in air at 4008C for 30 min to afford Ru (2.0 wt%)/
CeO2 as a dark brown powder.
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Re-
search (No. 21360393) from the Ministry of Education,
Sports, Culture, Science and Technology, Japan.
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