Angewandte
Chemie
DOI: 10.1002/anie.200805838
Heterogeneous Catalysis
Selective Isomerization of Epoxides to Allylic Alcohols Catalyzed by
TiO2-Supported Gold Nanoparticles
Christos Raptis, Hermenegildo Garcia,* and Manolis Stratakis*
Epoxides play a dominant role as valuable intermediates in
organic processes. They provide an array of addition products
through either nucleophilic opening or electrophilic activa-
tion, or they isomerize to carbonyl compounds or allylic
alcohols depending on the reaction conditions (acidic or
basic). Regarding their isomerization to allylic alcohols under
homogeneous reaction conditions, relatively few procedures
are known. The reaction with alkylamide bases[1] or Schloss-
erꢀs Li/K mixed superbases[2] converts epoxides into allylic
alcohols. Sharpless and Lauer[3] presented a method for the
isomerization of epoxides to allylic alcohols by using organo-
selenium chemistry. Under acidic reaction conditions, epox-
ides primarily isomerize to carbonyl compounds[4] by the
Meinwald rearrangement, and allylic alcohols are often
formed as by-products. The isomerization of epoxides to
allylic alcohols, through a radical pathway ([Cp2TiCl], Cp =
C5H5) and in moderate yields, is also known.[5]
An alternative unprecedented catalytic mechanism to
convert an epoxide into an allylic alcohol would require the
development of a mild heterogeneous bifunctional acid/base
catalyst. Upon activation of the epoxide moiety by the acidic
sites, the basic sites would abstract, in a rather concerted way,
a hydrogen atom from the a-carbon atom (with respect to the
epoxide functionality) to produce an allylic alcohol
(Scheme 1). Our concept is reminiscent of the use of
amphoteric aluminum alkoxides or amides[6] under homoge-
neous reaction conditions. Unfortunately, these reagents act
either in stoichiometric or multimolar amounts relative to the
epoxide.
There a few examples in the literature concerning the
isomerization of epoxides under heterogeneous catalysis.
Amphoteric oxides such as Al2O3, ZrO2, TiO2,[7] or Li3PO4
(the last for the industrial production of allyl alcohol from
propylene oxide)[8] have been used as catalysts. However, the
reactions generally proceed at elevated temperatures to form,
apart from allylic alcohols, mixtures of products. Indeed, in
tests of the isomerization of a model substrate (6,7-epoxyger-
anyl acetate (1), Table 1) over TiO2 or SnO2 in refluxing 1,2-
dichloroethane as the solvent (100 mg of TiO2 or SnO2 per
0.1 mmol of substrate), the allylic alcohol 1a was formed;
however, several cyclization products,[9] which are typical for
an acid-catalyzed pathway, also appear in comparable
amounts relative to 1a. Moreover, the reaction is extremely
slow as it proceeds to 15–20% conversion after 24 hours, and
requires a week to reach completion. As a result, new by-
products start to form and the reaction mass balance falls to
less than 60%.
To improve the reactivity/selectivity of the isomerization
process, we turned our attention to TiO2 supported on gold
nanoparticles, a material that has received tremendous
attention in recent years as a catalyst for alcohol[10a–e] and
aldehyde[10a–c,d] oxidation wherein O2 is employed as the
oxidant. It has been proposed that the TiO2 surface stabilizes
cationic gold species,[11] such as AuI, which are detectable by
X-ray photoelectron spectroscopy (XPS). We envisioned such
species acting as Lewis acid sites to activate the epoxide,
whereas the surrounding oxygen atoms from the TiO2 support
could act as basic sites and catalyze the isomerization
according to the concept presented in Scheme 1. Gold
nanoparticles have demostrated unique activity for promot-
ing chemical transformations of organic compounds which do
not occur by using conventional catalysts.[12] We were pleased
to find that Au/TiO2[13] was a perfect catalyst for our purposes.
Upon treatment of 1 with Au/TiO2 suspended in 1,2-dichloro-
ethane the reaction proceeds smoothly and goes to comple-
tion within two to three hours at 808C. Allylic alcohol 1a was
the only product formed in a yield greater than 90% after
isolation.
Scheme 1. Schematic representation of the isomerization of an epoxide
into an allylic alcohol using a bifunctional acid/base catalyst. LA=
Lewis acid, B=Lewis base.
[*] C. Raptis, Prof. Dr. M. Stratakis
Department of Chemistry, University of Crete
Voutes 71003, Iraklion (Greece)
Prompted by this result, we examined a series of epoxides
and found that the reaction is quite general; the yields are
high, and in many cases the product selectivity is remarkable.
The results are summarized in Table 1. Apart from 1,
terpenoid epoxides 2–4, which are prone to undergo acid-
catalyzed cyclization,[9] exclusively provide their isomeric
allylic alcohols 2a–4a in high yields. a-Pinene oxide (7), a
highly sensitive substrate towards acids,[14] reacts to primarily
give trans-pinocarveol (7a) and a mixture of campholenic
E-mail: stratakis@chemistry.uoc.gr
Prof. Dr. H. Garcia
Instituto de Tecnologia Quimica
CSIC-UPV Universidad Politecnica de Valencia
46022 Valencia (Spain)
E-mail: hgarcia@qim.upv.es
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2009, 48, 3133 –3136
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3133