A heterogeneous Ru/CeO2 catalyst effective for transfer-allylation from homoallyl alcohols to aldehydes

Article abstract of DOI:10.1039/b901830a

A simple heterogeneous Ru/CeO₂ catalyst was found to be effective for transfer-allylation from homoallyl alcohols to aldehydes, followed by isomerization to give the saturated ketones in high yields.

Full text of DOI:10.1039/b901830a

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A heterogeneous Ru/CeO2 catalyst effective for transfer-allylation from
homoallyl alcohols to aldehydesw
Hiroki Miura, Kenji Wada,* Saburo Hosokawa, Masahiro Sai, Teruyuki Kondo
and Masashi Inoue
Received (in Cambridge, UK) 28th January 2009, Accepted 27th May 2009
First published as an Advance Article on the web 9th June 2009
DOI: 10.1039/b901830a
A simple heterogeneous Ru/CeO2 catalyst was found to be
effective for transfer-allylation from homoallyl alcohols to
aldehydes, followed by isomerization to give the saturated
ketones in high yields.
aldehydes. The desired ketone, 4a, was obtained in the best
yield in the reaction of 0.50 mmol of 2a with 1.0 mmol of 1a.
Note that the reaction with double the amount of substrate
successfully proceeded in the presence of a Ru(1.0 wt%)/CeO2
catalyst (0.025 mmol as Ru, 250 mg), whereas the reaction
with more than 1.0 mmol of 2a did not afford 4a in the
presence of Ru(2.0 wt%)/CeO2 (0.025 mmol as Ru, 125 mg)
(see the ESIw). The effect of the catalyst support on the
performance was examined. Among the supported Ru catalysts
examined, only the Ru/CeO2 catalyst showed any activity
(entries 3, 7–10).9 As shown in entries 11-14, other transition
metals supported on CeO2 did not exhibit any catalytic
activity either. Note that Lewis acidic TiO2 (ST-01)10 and/or
Ru/TiO2 catalyst gave 1,1,3-trimethyl-1H-indene, quantitatively,
via the intramolecular Friedel–Crafts-type reaction of 1a
The development of heterogeneous catalysts that show high
catalytic activities and selectivities towards organic synthesis is
of great significance from both industrial and environmental
points of view.1,2 Heterogeneous catalysts, especially solid
oxide-based ones, could overcome the disadvantages of
homogeneous catalysts, such as the difficulty of recovering
the catalysts after the reaction. Recently, a few inorganic solid
catalysts2,3 were found to be effective for several carbon–
carbon bond-forming reactions. Although recent progress on
homogeneous catalysts has realized selective cleavage of
carbon–carbon bonds under mild and neutral conditions,4
for example, transfer-allylation reactions,5,6 there is no
reported example of solid oxide catalysts that are effective
for these carbon–carbon bond cleaving reactions. We report
here the first heterogeneous Ru/CeO2 catalyst7 effective for
transfer-allylation from homoallyl alcohols to aldehydes,
followed by isomerization to give the corresponding saturated
ketones in high yields, which proceeds via a different mechanism
from that of Lewis acid-mediated reactions.8 The present solid
oxide-catalysed reaction proceeds without additives such as
phosphines, bases and CO.5,6
Table
transfer-allylation from 1a to 2a
1
The effect of solid oxide-supported catalysts on the
Entry
Catalysta
Yield of 4a (%)b
The treatment of homoallyl alcohol 1a and benzaldehyde
(2a) at 170 1C under an argon atmosphere in the presence of
Ru(2.0 wt%)/CeO2 (0.025 mmol as Ru)z gave saturated
ketone 4a in a good yield. As summarized in Table 1, the
reaction was almost complete within 6 h (entries 3 and 4). No
catalytic activity was observed below 150 1C (entry 5). The
addition of phosphines to this Ru/CeO2 catalyst completely
suppressed the reaction, and no 4a was obtained (entry 6),
although the addition of phosphines or the use of a phosphine
complex was indispensable in the previous homogeneous
systems.5,6b,c Strong coordination of PPh3 to the surface’s
catalytic active sites might hamper the reaction. The reaction
was sensitive to the molar ratio of homoallyl alcohols and
1
2
3
None
CeO2
0
0
69
63
Trace
Trace
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/CeO2
Ru/SiO2
Ru/Al2O3
Ru/TiO2
Ru/MgO
Fe/CeO2
Cu/CeO2
Rh/CeO2
Pd/CeO2
Ru3(CO)12
4c
5d
6e
7
0
0
8
9
0f
0
10
11
12
13
14
15
16
17
0
0
0
0
g
15
0
g
RuCl2(PPh3)3g,h
RuCl2(PPh3)3
35i
a
b
2.0 wt%. Determined by GLC (gas–liquid chromatography) based
Department of Energy and Hydrocarbon Chemistry, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
E-mail: wadaken@scl.kyoto-u.ac.jp; Fax: +81 75 383 2479;
Tel: +81 75 383 2482
w Electronic supplementary information (ESI) available: Full experi-
mental procedures for the reactions, results of filtration and
reusability tests, and characterization of the catalysts. See DOI:
10.1039/b901830a
c
d
e
on 2a. Reaction time, 6 h. At 150 1C. PPh3 (0.10 mmol) was
added. 1,1,3-Trimethyl-1H-indene was obtained, quantitatively.
f
g
h
Homogeneous catalyst. From ref. 5, 1a 4.0 mmol, 2a 4.0 mmol,
Ru cat. 0.20 mmol, allyl acetate 30 mmol, THF 8.0 cm3 under CO
i
(10 atm) at 200 1C for 40 h. The product was 3-methyl-1-phenylbut-
2-en-1-one.
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(entry 9). These results strongly suggest that the present
transfer-allylation was not a carbonyl–ene reaction8 promoted
by acidic sites of the catalysts.
Under the present reaction conditions, the homogeneous
Ru3(CO)12 catalyst gave 4a in a lower yield than with the
heterogeneous Ru/CeO2 catalyst (entry 15), and
5
RuCl2(PPh3)3 was not effective at all (entry 16). Note that
the RuCl2(PPh3)3-catalysed reaction required much more
severe conditions to afford an a,b-unsaturated ketone,
3-methyl-1-phenylbut-2-en-1-one, in low yield.5
The results obtained by the Ru/CeO2-catalysed transfer-
allylation–isomerization from a series of homoallyl alcohols to
aldehydes under the optimized reaction conditions are shown
in Table 2.y The reaction of 1a with aromatic aldehydes 2
proceeded smoothly to give desired ketones 4 in good to high
yields. The substituents on the aromatic ring of 2 did not affect
the reaction (entries 2–6), and 2-naphthaldehyde (2g) was also
applicable (entry 7). Unexpectedly, the reaction of 1b with
benzaldehyde 2a gave 4h in a poor yield (entry 8). Aliphatic
aldehydes, such as 1-octanal, gave a complex mixture due to
the side reactions represented by an aldol condensation.
The time-course of the reaction of 1a with 2a clearly reflects
the sequential transfer-allylation and isomerization of an
intermediate, 5a (Fig. 1).
Fig. 1 Time-course of the reaction of 1a with 2a during the initial
10 h; (a) yield of 4a (’) and (b) yield of 5a (n, see inset). The reaction
conditions were identical with those in Table 2.
the solid surface, and we believe that CeO2 acts as a
macro-ligand, which stabilizes the catalytically active ruthenium
species.
The structure of the Ru/CeO2 catalyst was investigated by
spectroscopic analyses (see the ESI for detailsw). A nitrogen
adsorption–desorption measurement of the Ru/CeO2 catalyst
showed that its pore size was distributed in the mesopore
region (pore diameter: 7 nm; BET surface area: 92 m2 gꢁ1).
Other supports examined also possessed pores in the range
of mesopores. XRD analysis of the freshly prepared
Ru/CeO2 catalyst showed a broad pattern due to micro-
crystalline CeO2. The absence of patterns corresponding to
crystalline RuO2 indicated the formation of well-dispersed
ruthenium species. The diffuse reflectance FTIR measurement
of the Ru/CeO2 catalyst showed a distinct band at 975 cmꢁ1
that could be assigned to a ruthenium oxo species.13 Note that
there were no signs of the RuQO species for ruthenium
catalysts supported on other carriers such as TiO2. A previous
XANES study revealed the presence of a five-coordinate
ruthenium species in the Ru/CeO2 catalyst prepared by a
The Ru/CeO2 catalyst was used repeatedly at least three
times without a loss in the catalytic activity after calcination at
400 1C for 30 min. (see Table S3 in the ESIw).
It is important to investigate whether the reaction actually
proceeds on the surface of the solid catalyst.11,12 After the
reaction of 1a and 2a at 170 1C for 24 h, the solid catalyst
was removed by cold filtration. Subsequent addition of 1a
(1.0 mmol) and 2a (0.50 mmol) to the clear filtrate, and heating
of the mixture at 170 1C for an additional 24 h resulted in no
increase of 4a. ICP analysis revealed that the filtered solution
contained only 0.0014 mmol of ruthenium species. Note that
hot filtration of the solid catalyst after 3 h suppressed the
further formation of product 4a (see the ESIw). These results
strongly suggest that the present catalytic reaction occurred on
similar method.13
A temperature-programmed reduction
(TPR) measurement in a 1.9% H2–Ar stream revealed that
ruthenium species on CeO2 were reduced at around 72 1C,
which was lower than that in the reduction of bulk RuO2
(ca. 105 1C). These results supported our idea of the formation
of a well-dispersed five-coordinate ruthenium(IV) oxo species
(Scheme 1) on the surface of Ru/CeO2,13 not on other
supported Ru catalysts. Consequently, these oxo species
would be reduced to form catalytically active low-valent
ruthenium species on CeO2.
Table 2 Transfer-allylation from homoallyl alcohols 1 to aromatic
aldehydes 2 catalysed by Ru/CeO2
Although the reaction mechanism is not clear yet, we
believe that the initial step might consist of the oxidative
addition of an OH group of 1 to an active ruthenium species.
The following b-allyl elimination5 would give 3 and an
(allyl)ruthenium intermediate, which would react with an
aldehyde to give the homoallyl alcohol (5). The formation of
Entry
Alcohol
Ar
Yield of 4 (%)a
1
2
3
4
5
6
7
8
1a (R = Me)
Ph (2a)
69 (4a)
78 (4b)
93 (4c)
(52) (4d)
64 (4e)
(78) (4f)
(89) (4g)
19 (4h)
4-CH3C6H4 (2b)
4-CH3OC6H4 (2c)
4-Me2NC6H4 (2d)
4-FC6H4 (2e)
4-ClC6H4 (2f)
2-Np (2g)
1b (R = H)
Ph (2a)
a
Determined by GLC based on 2. Figures in the parentheses show
isolated yields.
Scheme 1 The proposed structure of the surface ruthenium species in
the freshly prepared Ru/CeO2 catalyst.
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the stable allyl species on the surface of the Ru/CeO2 catalyst
has already been proposed on the basis of spectroscopic
studies,13 when propene (C3H6) was treated by the Ru/CeO2
catalyst. A further mechanistic study including observation of
the catalytically active species is now in progress.
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K. Kaneda, Tetrahedron Lett., 2005, 46, 4283; (c) K. Mori,
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Chem., 2006, 71, 7455; (e) T. Mitsudome, K. Nose, K. Mori,
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In conclusion, the first heterogeneous Ru/CeO2-catalysed
sequential transfer-allylation–isomerization from homoallyl
alcohols to aldehydes to give saturated ketones has been
developed. This simple solid oxide catalyst showed a comparable
or superior performance to exquisite homogeneous transition
metal complex catalysts. No additives such as phosphines,
bases or CO were required, which facilitated reuse of the
Ru/CeO2 catalyst, and this feature is obviously attractive
and advantageous from synthetic, industrial, and environmental
perspectives.
5 T. Kondo, K. Kodoi, E. Nishinaga, T. Okada, Y. Morisaki,
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T.K. acknowledges financial support of a Grant-in-Aid for
Scientific Research on Priority Areas (No. 19020029,
Advanced Molecular Transformations of Carbon Resources)
from Ministry of Education, Culture, Sports, Science and
Technology, Japan.
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Notes and references
z Preparation of
a Ru/CeO2 catalyst. Supported catalysts were
prepared by the impregnation method. 1.0 g of CeO2, prepared by
the treatment of cerium(III) nitrate with aqueous ammonia, was added
to a solution of 42 mg of Ru3(CO)12 in 10 cm3 of THF in air at room
temperature. After impregnation, the resulting light yellow powder
was calcined in air at 400 1C for 30 min to afford the Ru(2.0 wt%)/CeO2
catalyst as a dark brown powder.
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y Catalytic transfer-allylation–isomerization reaction. Homoallyl
alcohol 1 (1.0 mmol), aldehyde 2 (0.50 mmol), and mesitylene
(2.0 cm3) were placed in a glass Schlenk tube with a balloon under
an argon atmosphere together with 125 mg of the Ru(2.0 wt%)/CeO2
catalyst (0.025 mmol as Ru). The reaction mixture was stirred on a hot
stirrer at 170 1C with a cooling block for 24 h. The products were
identified by GC-MS and NMR measurements, and quantified by GC
using naphthalene as an internal standard.
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4114 | Chem. Commun., 2009, 4112–4114