1750
J . Org. Chem. 1998, 63, 1750-1751
mixture of RuCl3‚3H2O (4.3 mmol),6 MgCl2‚6H2O (43.2
mmol), and AlCl3‚H2O (14.4 mmol) was dissolved in distilled
water (30 mL). To an aqueous solution of Na2CO3 (74.7
mmol) and NaOH (0.13 mol) was slowly added the above
solution, and then the resulting solution was heated at 65
°C for 18 h with stirring. The obtained slurry was cooled to
room temperature and filtered, followed by washing with
distilled water and drying at 110 °C for 12 h (5.25 g). The
structure of hydrotalcite was confirmed by its XRD pattern,
and the basal spacing was estimated to be 7.9 Å. [Anal.
Calcd for Mg6Al2Ru0.5(OH)16CO3‚nH2O (n ) 8): Mg, 20.1;
Al, 7.4; Ru, 7.0. Found: Mg, 19.5; Al, 7.4; Ru, 7.3. XPS:
Ru 3d5/2 ) 281.0 eV,7 FWHM ) 2.5 eV.] A typical example
for the oxidation of alcohols is as follows. Into a reaction
vessel with a reflux condensor were placed cinnamyl alcohol
(0.80 g, 6.0 mmol), Mg6Al2Ru0.5(OH)16CO3 (0.90 g), and
toluene (15 mL). The resulting mixture was stirred at 60
°C under an O2 atmosphere. After 24 h, hydrotalcite was
separated by filtration. GC analysis of the filtrate showed
a quantitative yield of cinnamaldehyde. Removal of the
solvent under reduced pressure followed by column chro-
matography on silica yielded the product of cinnamaldehyde
(0.734 g, 92%). Isolated hydrotalcites were washed with 10
wt % Na2CO3 (aq) solution (30 mL) and water, which could
be reused as a catalyst without an appreciable loss of activity
for the above oxidation; the first, second, and third runs of
the reuse experiments gave cinnamaldehyde over 95% GC
yields.
In the oxidation of cinnamyl alcohol, various hydrotalcites
were examined as a catalyst in the presence of molecular
oxygen, which is shown in Table 1. Generally, cinnamyl
alcohol was oxidized to give cinnamaldehyde as a main
product under the above reaction conditions. We found that
hydrotalcites containing ruthenium in the Brucite-like layer
showed the highest catalytic activity for the oxidation among
the hydrotalcites with many transition metals, e.g., Fe, Ni,
Mn, V, and Cr. The most effective anion in the interlayer
of Ru-hydrotalcites is carbonate ion (Table 1, run 1).8
With respect to solvents, toluene, chlorobenzene, and
benzene were good solvents, giving 95, 93, and 92% yields
of cinnamaldehyde for 8 h, respectively. Reactions in
n-hexane, acetonitrile, 1,2-dichloroethane, and cyclohexane
solvents led to 70-86% of cinnamaldehyde, while use of
methanol resulted in a 27% yield. The oxidation at 80 °C
gave the highest yield of cinnamaldehyde; the increase of
the reaction temperature over 100 °C resulted in low
selectivity, whereas cinnamyl alcohol was perfectly con-
sumed.
Heter ogen eou s Oxid a tion of Allylic a n d
Ben zylic Alcoh ols Ca ta lyzed by Ru -Al-Mg
Hyd r ota lcites in th e P r esen ce of Molecu la r
Oxygen
Kiyotomi Kaneda,* Toyokazu Yamashita,
Tsuyoshi Matsushita, and Kohki Ebitani
Department of Chemical Science and Engineering,
Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560, J apan
Received October 27, 1997
Selective oxidation of alcohols using various kinds of
reagents has been widely studied because the functional
transformation to aldehydes and ketones plays an important
role in many organic syntheses.1 But even at present,
stoichiometric reagents such as Cr and Mn have been used
for the above selective transformation. From the stand-
points of atom economy and environmental demand for
chemical reactions,2 much attention has been paid to
development of the metal catalyst systems using molecular
oxygen as an oxidant.3 Hydrotalcites of layered materials
consist of a cationic Brucite layer and anionic compounds
in the interlayer.4 Various kinds of metal elements, which
are expected to act as active sites of catalysts, can be
introduced in the Brucite layer. In this paper, we report
that the hydrotalcites having Ru in the Brucite layer showed
high catalytic activity for oxidation of allylic and benzylic
alcohols in the presence of molecular oxygen.5 This hetero-
geneous catalyst has the advantages of not only the use of
molecular oxygen but also of a simple workup procedure
than other homogeneous oxidizing reagents. Furthermore,
this catalyst is reusable without an appreciable loss of the
activity and selectivity for the oxidation.
Oxidation of various kinds of allylic and benzylic alcohols
using Mg6Al2Ru0.5(OH)16CO3 in a toluene solvent was carried
out at 80 °C under an oxygen atmosphere. The hydrotalcite
showed high catalytic activities for many allylic and benzylic
alcohols, while saturated and nonallylic alcohols such as
cyclohexylmethanol and 2-octanol had low reactivity for the
oxidation. Typical examples for the oxidation of allylic and
benzylic alcohols are summarized in Table 2. Cinnamyl
alcohol and its derivatives can be easily oxidized to give
corresponding R,â-unsaturated aldehydes in almost quan-
Various hydrotalcites were prepared according to a modi-
fied procedure in the literature.4 A representative example
is for the hydrotalcite having ruthenium in the Brucite layer
and CO3 anion in the interlayer, Mg6Al2Ru0.5(OH)16CO3. A
* To whom correspondence should be addressed. Tel.: +81-6-850-6260.
Fax: +81-6-850-6296. E-mail: kaneda@cheng.es.osaka-u.ac.jp.
(1) (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.; J ohn
Wiley & Sons, Inc.: New York, 1992. (b) Hill, C. L. Advances in Oxygenated
Processes; Baumstark, A. L., Ed.; J AI Press, Inc.: London, 1988; Vol. 1, p
1. (c) Hudlucky, M. Oxidations in Organic Chemistry; ACS Monograph
Series; American Chemical Society: Washington, DC, 1990. (d) Sheldon,
R. A.; Kochi, J . K. Metal-Catalyzed Oxidations of Organic Compounds;
Academic Press: London, 1981.
(6) Purchased from N. E. Chemcat. Co. Ltd.
(7) This Ru 3d2/5 XPS peak position is assigned to an oxidized ruthenium.
X-ray absorption measurements on the Ru K-edge also showed the state of
Ru in the hydrotalcite is a cationic form. Therefore, the Ru element in the
hydrotalcite is present in an oxidized state and a possibility of metallic Ru
in the hydrotalcite can be excluded.
(8) The order of the d-spacing of the Ru-hydrotalcite with various
interlayer anions is as follows: DS (26.0 Å) > TA (14.1 Å) > ND (8.6 Å) >
SA (8.1 Å) > Cl (7.95 Å) ≈ AcO (7.94 Å) ≈ CO3 (7.92 Å) ≈ SO4 (7.92 Å). The
d-spacing of the Ru-hydrotalcite is not related to the oxidation activity. Most
of the active basic sites are probably located on the outer surface of the
Ru-hydrotalcite.
(2) Trost, B. M. Science 1991, 254, 1471; Angew. Chem., Int. Ed. Engl.
1995, 34, 259.
(3) (a) Kaneda, K.; Fujii, M.; Morioka, K. J . Org. Chem. 1996, 61, 4503.
(b) Kaneda, K.; Fujie, Y.; Ebitani, K. Tetrahedron Lett. 1997, 38, 9023.
(4) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173.
(5) (a) Kaneda, K.; Ueno, S.; Imanaka, T. J . Chem. Soc., Chem. Commun.
1994, 797. (b) Kaneda, K.; Ueno, S.; Imanaka, T. J . Mol. Catal. A: Chem.
1995, 102, 135. (c) Kaneda, K.; Ueno, S. Heterogeneous Hydrocarbon
Oxidation; Warren, B. K., Oyama, S. T., Eds.; ACS Symposium Series No.
638; American Chemical Society: Washington, DC, 1996; Chapter 22, p 300.
S0022-3263(97)01965-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/27/1998