COMMUNICATION
Unique gold chemoselectivity for the aerobic oxidation of allylic
alcohols{
Alberto Abad, Carles Almela, Avelino Corma* and Hermenegildo Garc´ıa*
Received (in Cambridge, UK) 3rd May 2006, Accepted 30th May 2006
First published as an Advance Article on the web 16th June 2006
DOI: 10.1039/b606257a
HAuCl4 on colloidal ceria (5 nm),18 the resulting material was
Gold nanoparticles supported on nanocrystalline ceria has been
reduced by the solvent-free oxidation of 1-phenylethanol giving
gold nanoparticles with a crystallite size of 2–5 nm. Pd–
hydroxyapatite and Au–Pd/TiO2 were prepared following ref. 16
and 17 respectively (see supplementary information{).
found to be more active and chemoselective than palladium
and gold(core)–palladium(shell) nanoparticles for the aerobic
oxidation of allylic alcohols.
Alcohol oxidation to aldehydes or ketones is one of the pivotal
functional group transformations in organic chemistry.1 Classically
this oxidation is performed in high yields using stoichiometric
amounts of transition metal oxidants or halooxoacids. Although
these reagents are quite general in scope, they are no longer tole-
rable from a green chemistry point of view.2 Development of envi-
ronmentally-friendly catalytic oxidations that can be used for any
alcohol, including multifunctional ones as allylic alcohols, would
constitute a significant breakthrough in organic chemistry.3–5 The
ideal oxidant should be molecular oxygen at atmospheric pressure
and the reaction should be performed under solventless conditions
or (for solid alcohols) in green organic solvents. In addition, the
catalyst should be moisture-stable and reusable.
Gold nanoparticles supported on cerium oxide,6 other inorganic
oxides,7–9 active carbon10–13 or even unsupported gold nanopar-
ticles14 have recently attracted considerable attention since these
catalysts are able to promote the selective oxidation of alcohols.
Very recently we have presented the benefit of using nanoparti-
culated CeO2 instead of the conventional ceria as a support for
gold nanoparticles resulting in a highly active, selective and
recyclable catalyst for the oxidation of alcohols into aldehydes and
ketones using oxygen at atmospheric pressure as oxidant in the
absence of solvent and base.15 High turnover frequencies for the
oxidation of selected alcohols with oxygen under mild condition
have been achieved using Pd supported on hydroxyapatite,16 and
titania-supported gold(core)–palladium (shell) nanoparticles.17 An
air and moisture stable oxidation catalyst has, in order to be useful
for organic chemists, to give not only high conversion with some
selected alcohols, but has to be chemoselective when the molecule
contains other oxidizable functional groups. In this case, one
important family of alcohols that has industrial relevance are
allylic alcohols. In this common type of alcohol chemoselective
oxidation of hydroxyl groups avoiding oxidation, isomerization or
polymerization of the olefinic group is required.
Air oxidation of allylic alcohols was carried out at 393 K,
PO2 = 105 Pa and without solvent (unless specified). The metal
substrate molar ratio was varied between 1–125 6 1024
,
depending on the alcohol. The results obtained with the different
catalysts are given in Table 1. In this Table the percentage of
isomerization represents products where double bond migration
has occurred, while the selectivity column refers to the formation
of the a,b-unsaturated carbonyl compound that is the desired
product. The rest up to 100% corresponds to unidentified
polymerized by-products. For comparison and in addition to
Au/CeO2, Pd–hydroxyapatite, Au–Pd/TiO2 as well as two related
solids have also been included as catalysts. One of the
unprecedented catalysts consists of a CeO2 support that contains
simultaneously independent gold and palladium nanoparticles
(Au–Pd/CeO2), and the other consists of gold nanoparticles
supported on TiO2 (Au/TiO2) (this catalyst was supplied by the
International Gold Council).19
The most salient feature of Table 1 is the high yields and the
unique chemoselectivity attained using supported gold nanoparti-
cles as catalysts for the production of a,b-unsaturated ketones.
Minimum yields over 90% are obtained for the solventless
oxidation except in the case of 2-octen-1-ol, for which the presence
of an organic solvent is necessary in order to obtain the desired
a,b-unsaturated aldehyde with high yield (see entry 16 and
footnote b). In this context, it is worth noticing that in addition
to solvent-less conditions Au–CeO2 is also very active and selective
for the oxidation of primary allylic alcohols in organic solvents
such as toluene.
Particularly, in the oxidation of cinnamyl alcohol to cinnamal-
dehyde, Au–CeO2 combines excellent selectivity (see entry 20 in
table 1) and the highest turnover frequency (538 h21) among those
reported so far: Ru–Al2O3(27 h21),20,21 Ru–Co(38 h21),21,22 Pd–
hydroxyapatite (73 h21),16,21 Au–Pd–TiO2 (97 h21),17 Pt–Bi–Al2O3
(144 h21).21
We will show that for the relevant oxidation of allylic alcohols,
gold presents unique selectivity when compared with Pd or Au–Pd
catalysts. Au/CeO2 was prepared by deposition–precipitation of
We checked the reusability of the Au–CeO2 catalyst in the case
of trans-carveol oxidation. The solid was recovered by filtration,
washed with a 1 M aqueous solution of NaOH and dried in
vacuum. This recovered Au–CeO2 sample was used in a
consecutive run without observing any decay in its catalytic
activity.
Instituto de Tecnolog´ıa Qu´ımica(CSIC-UPV), Universidad Polite´cnica
de Valencia, Avda. de los Naranjos s/n, 46022, Valencia, Spain.
E-mail: acorma@itq.upv.es; Fax: +34 96 3877809; Tel: +34 96 3877800
{ Electronic supplementary information (ESI) available: Catalysts pre-
paration and characterization. See DOI: 10.1039/b606257a
Table 1 also reveals that the presence of palladium in the
catalyst is highly detrimental for chemoselectivity. This low
3178 | Chem. Commun., 2006, 3178–3180
This journal is ß The Royal Society of Chemistry 2006
Abad, Alberto
