The Ru-Cu-Al-hydrotalcite-catalysed oxidation of alcohols to aldehydes or ketones

Article abstract of DOI:10.1055/s-2001-14595

A new heterogeneous ruthenium-copper-hydrotalcite catalyst for the efficient and selective conversion of a variety of aliphatic, allylic and aromatic alcohols to either aldehydes or ketones is described. Suitable co-oxidants include iodosylbenzene, tetrabutyl ammonium periodate and, depending on substrate, oxygen.

Full text of DOI:10.1055/s-2001-14595

LETTER
869
The Ru-Cu-Al-Hydrotalcite-Catalysed Oxidation of Alcohols to Aldehydes or
Ketones
a
a
a
b
Holger B. Friedrich, * Farida Khan, Nirad Singh, Martin van Staden
a)
School of Pure and Applied Chemistry, University of Natal, Durban, 4041, South Africa
b)
National Metrology Laboratory, Council for Scientific and Industrial Research, Pretoria, 0001, South Africa
Fax +27-31-2603091; E-mail: friedric@nu.ac.za
Received 21 March 2001
Abstract: A new heterogeneous ruthenium-copper-hydrotalcite
catalyst for the efficient and selective conversion of a variety of al-
iphatic, allylic and aromatic alcohols to either aldehydes or ketones
is described. Suitable co-oxidants include iodosylbenzene, tetrabu-
tyl ammonium periodate and, depending on substrate, oxygen.
Key words: alcohol, oxidation, ruthenium, hydrotalcite, XPS
Heteronuclear catalysts have often been found to be more
1
active than their individual mononuclear components. A
classical example is the Pd/Cu/O -based Wacker process
for the industrial preparation of acetaldehyde. To our
2
2
Scheme 1 Proposed redox behaviour of the cations in the Brucite
sheet
knowledge there has been a single prior report of a bime-
tallic Ru/Cu system for the partial oxidation of alcohols.
The report describes a mixture of mononuclear ruthenium
and copper containing compounds which shows fairly low
rates of conversion and selectivity - possibly because the
Typical oxidation procedures involved adding activated 4
Å molecular sieves and toluene (6 mL) to a Schlenk tube,
after which the substrate (0.478 mmol) and isobutyl-meth-
acrylate (80 L) as internal standard were added. For the
3
Ru and Cu are not in close proximity. This prompted us
to investigate a new Ru/Cu-containing hydrotalcite sys-
tem as a catalyst for the oxidation of alcohols - a process
of key importance in synthetic organic chemistry. The hy-
drotalcite permits the bringing together of different metals
within the same compound, whilst it also contains inter-
layer spaces which are reactive environments. Some pre-
vious applications of hydrotalcite-type anionic clays
include uses as catalysts for the polymerisation of alkene
oxides, aldol condensation, reforming, alcohol synthesis,
methanation, Baeyer–Villiger oxidations and the photox-
reactions using O as co-oxidant, the reactions were car-
2
ried out at 60 C in O saturated toluene under an atmo-
2
sphere of oxygen. Alternatively, either iodosylbenzene
(
PhIO) or tetrabutylammonium periodate (TBAP) (0.717
mmol) was added as co-oxidant and the reactions carried
out in N saturated toluene at 60 °C under nitrogen. The
2
hydrotalcite (250 mg) was added last in all cases. The re-
actions were followed by gas chromatography. Product
isolation involved filtering off the hydrotalcite and molec-
ular sieves, which could then be reused, followed by chro-
matography and/or fractional distillation, as required.
4
,5
idation of isopropyl alcohol to acetone. An advantage of
the hydrotalcite catalysts is that they are heterogeneous
catalytic systems, thus permitting the easy removal of the
catalysts from the reaction medium. Whilst a number of
efficient homogeneous ruthenium catalysts for the mild
The results of the oxidation reactions are shown in Table
1
. As can be seen, the Ru-Cu-HT was found to be a very
efficient and selective oxidant for the variety of alcohols
investigated when either PhIO or TBAP were used as co-
oxidants. The efficiency of oxygen as co-oxidant depends
on the alcohol substrate, with conversions ranging from
6
,7
oxidation of alcohols are known, difficulties with prod-
uct separation remain and often the expensive catalyst
cannot be re-used after workup.
The Ru-Cu-Al-hydrotalcite (Ru-Cu-HT) was prepared by 20% to 100%. Low activity with oxygen has been ob-
8
adding a solution of RuCl .3H O (0.40 g, 1.53 mmol), served for some other hydrotalcite catalysts. No over-ox-
3
2
CuCl ·6H O (2.61 g, 15.3 mmol) and AlCl ·6H O idation products, namely carboxylic acids, were detected
2
2
3
2
(
1.23 g, 5.1 mmol) in water (10 mL) to a solution of in any of the reactions. Similarly, no attack of sensitive
Na CO (1.10 g) in 46.4 mL 1 M NaOH. The mixture was functional groups, e.g. double bonds, nitro groups or het-
2
3
stirred at 65 C for 18 h, after which the green product was eroatoms was detected, with the inertness of cinnamyl
filtered off, washed with water and dried at 110 C for chloride indicating that organohalides are not attacked.
1
2 h. The product was ground into a powder before use. Also no evidence of allylic rearrangement of double
EDS confirmed the presence of Ru, Cu and Al and flame bonds, as would e.g. be seen by the formation of citronel-
9
emission spectroscopy showed the ruthenium content to lal from geraniol, was observed. The mildness of these
be 6.4%.
oxidants is also emphasised by the selective conversion
Synlett 2001 No. 6, 869–871 ISSN 0936-5214 © Thieme Stuttgart · New York

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70
H. B. Friedrich et al.
Table 1 The oxidation of alcohols to aldehydes or ketones with various co-oxidants
LETTER
of furfuryl alcohol to furfuraldehyde, since furfuryl al- The Ru-Cu-HT catalyst with the above co-oxidants is less
cohol reacts explosively with more vigorous oxidants.10
efficient for large electron rich molecules. Thus cholester-
ol is oxidised to 5-cholesten-3-one by the Ru-Cu-HT-O2
system in low yield. Use of TBAP or PhIO as co-oxidant
results in a number of products. A similar effect was ob-
served for the RuCl (PPh ) -PhIO catalysed oxidation of
cholesterol where the substrate was reported to have de-
composed during the attempted oxidation.12 No reaction
with cholesterol occurs with 4-methylmorpholine N-ox-
ide as co-oxidant.
In the absence of co-oxidants, the catalyst shows low ac-
tivity in air. The recyclability of the catalyst was investi-
gated in detail on the oxidation of cinnamyl alcohol
2
3 3
(
Table 2). Storing the catalyst under nitrogen between re-
cycles results in a steady decrease in the activity of the cat-
alyst. However, storing the catalyst in air greatly increases
the catalysts lifetime, with quantitative yields of cinnam-
aldehyde still being obtained after each recycle, albeit af-
ter slightly longer reaction times. A similar effect has been XRD analysis of the Ru-Cu-HT catalyst shows features
observed for a supported osmium cis-hydroxylation cata- that are typical of all hydrotalcites, namely sharp and in-
1
1
lyst.
tense lines at low values of the 2 angle and less intense
and generally asymmetric lines at the higher angular
Synlett 2001, No. 6, 869–871 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
The Ru-Cu-Al-Hydrotalcite-Catalysed Oxidation of Alcohols to Aldehydes or Ketones
871
ed supported oxidants15,16 and Ru-Co-HT catalysts, with
8
Table 2 Oxidation of cinnamayl alcohol to cinnamaldehyde with
the recycled Ru-Cu hydrotalcite stored under different environments
the advantage over the latter that aliphatic primary alco-
hols are also oxidised in very high yield. Advantages over
clayfen include a more general applicability, easier syn-
thesis and indefinite stability.
17
Acknowledgement
We thank the NRF, URF and NMT Electrodes for support and Mar-
tin Onani for helping to prepare this manuscript.
References and Notes
(
(
1) Adams, R. D. Polyhedron 1988, 7, 2251-2252.
2) Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, Prentice-
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(
(
(
3) Minmoun, H.; Malmaison, R.; de-Seine, H. German patent
1
979, 29 20 678.
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1
5) Kaneda,K.; Ueno, S.; Imanaka, T. J. Chem. Soc., Chem.
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values. The presence of both sharp and diffuse non-basal
reflections indicates a partially disordered structure. The
basal spacing was determined to be 7.56 Å vs. 7.92 Å re-
(
(
6) Friedrich, H. B. Platinum Metals Rev. 1999, 43, 94-102.
7) Dijksman, A.; Arends, I.W.C.E.; Shelden, R.A. Chem.
Commun. 1999, 1591-1592.
8
ported for a Ru-Mg hydrotalcite.
(
(
8) Matsushita, T.; Ebitani, K.; Kaneda, K. Chem. Commun.
1999, 265-266.
XPS measurements on a newly prepared catalyst showed
binding energies of 282.1 eV and 932.8 eV due to Ru(VI)
and Cu(II), respectively. Measurements on a recycled cat-
alyst showed a binding energy of 282.1 eV due to Ru(VI),
whilst XPS measurements on a spent catalyst (after a sto-
ichiometric reaction with cinnamyl alcohol to give cinna- (11) Herrmann, W. A.; Kratzer, R. M.; Blümel, J.; Friedrich, H. B.;
maldehyde in 100% yield) showed binding energies of
9) Markó, I.E.; Gautier, A.; Tsukazaki, M.; Llobet, A.; Mir-
Plantalech, E.; Urch, C.; Brown, S.M. Angew. Chem., Int. Ed.
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th
(
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Fischer, R. W.; Apperley, D. C.; Mink, J.; Berkesi, O. J. Mol.
Catal. A 1997, 120, 197-205.
2
82.2 eV, 933.0 eV and 935.3 eV, corresponding to
(
(
12) Müller, P.; Godoy, J. Tetrahedron Lett. 1981, 22, 2361-2364.
13) Moulder, J.; Stickle, W.S.; Sobol, P.E.; Bomben K.D. in
Handbook of X-Ray Photoelectron Spectroscopy, Perkin
Elmer Corp., 1992.
1
3
Ru(VI), Cu(II) and Cu(0). This suggests that copper is
involved in the oxidation of Ru(III) to Ru(VI) in the syn-
thesis of the hydrotalcite and in the regeneration of the ac-
tive catalytic species, Ru(VI), as shown in the scheme. (14) Lee, D.G.; Congson, L.N.; Spitzer, U.A.; Olson, M.E. Can. J.
The scheme is further supported by the observation that
the catalyst acts as a two electron oxidant, since it con-
verts cyclobutanol to cyclobutanone.1
Chem. 1984, 62, 1835-1839.
15) Friedrich, H. B.; Singh, N. Tetrahedron Lett. 2000, 41, 3971-
(
4
3974.
(
16) Hinzen, B.; Ley, S. V. J. Chem. Soc., Perkin Trans.1 1997,
In conclusion, the Ru-Cu-HT catalyst was found to be
1907-1908.
easy to synthesise, stable, recyclable and to oxidise effi- (17) Cornelis, A; Laszlo, R. Synthesis 1985, 909-919.
ciently and selectively aliphatic, allylic and aromatic alco-
hols to their corresponding oxidation products under mild
conditions. The catalyst compares well to recently report-
Article Identifier:
1
437-2096,E;2001,0,06,0869,0871,ftx,en;D07501ST.pdf
Synlett 2001, No. 6, 869–871 ISSN 0936-5214 © Thieme Stuttgart · New York