The catalytic activity is quite remarkable, particularly for the
substrates that are less easily oxidized by other systems. A striking
activity was observed for a series of cyclohexanols (entries 6–13).
In the case of 4-tert-Bu-cyclohexanol, for example, the activity
(TOF 5 3.5 h21) is comparable with that observed under aerobic
conditions both over Pd/MgO (TOF 5 3.5 h21 2a
and Ru/Al2O3
)
(TOF 5 2.1 h21),9 whereas for cyclooctanol the activity
(TOF 5 12.5 h21) is higher than that observed for both systems
(TOF 5 3.2 h21 for Pd/MgO and TOF 5 2.7 h21 for Ru/Al2O3).
However, if we consider productivity expressed as gproduct/ gcatalyst
h, Cu/Al2O3 turns out to be one order of magnitude more active
than the other two systems for the dehydrogenation of 4-tert-Bu-
cyclohexanol and two orders of magnitude for the dehydrogena-
tion of cyclooctanol.
Fig. 1 Conversion and selectivity values obtained in recycling Cu/Al2O3
for 3-octanol oxidation at 0.5 h.
The catalyst was pretreated in H2 at 543 K before use, but
reduction at a much lower temperature resulted in a still very
active material (entry 2). It should also be mentioned that these
materials are not pyrophoric even in the reduced state, nor do they
require anhydrous conditions.
To the best of our knowledge this is the first report on a
supported copper catalyst used in the liquid phase for selective
alcohol dehydrogenation reactions under such mild conditions and
of general use. As already shown for hydrogenation reactions,6a,11
the catalyst is efficient for at least six catalytic runs without
relevant loss in activity nor in selectivity (Fig. 1). The Cu content
before use and after six runs was found to be unchanged by
AAS analysis, while Cu in the filtrates was found to be absent by
GF-AAS. Moreover, TPR profiles before and after six runs
appeared to be identical, showing the high stability of the metallic
phase.
The very high selectivity towards reduction of styrene with
respect not only to ketones but also to a,b-unsaturated carbonyls is
shown by the reaction of carveol (entry 13) which was oxidized
with 88% selectivity under standard conditions and with 95%
selectivity only by using 2 equiv. of hydrogen acceptor. On the
other hand, in the absence of styrene dihydrocarvone could be
obtained in moderate yield directly from carveol, acting as both
the hydrogen donor and the acceptor (Scheme 1).
Practical concerns claim for the use of these catalysts for
synthetic purposes. In fact, copper catalysts prepared with this
technique revealed remarkable performances in different kinds of
transformations, varying from hydrogenation to cyclization and
acid catalysed reactions,8 always showing excellent selectivity and
good productivity, basic features for the application of hetero-
geneous catalysts to fine chemicals synthesis.
The reaction of (2)-menthol sheds some light on the reaction
mechanism. The oxidation of this substrate is rarely reported over
heterogeneous catalysts due to its very low reactivity.10 Its slow
oxidation over Cu/Al2O3 shows the relevant influence of steric
hindrance on this system, apparent also in the substituted
cyclohexanols series (entries 6–10) and from the comparison
between linear and branched substrates (entry 1 vs. 5). This effect is
so strong that if the -OH group is in an axial conformation, as in
neomenthol, the reaction rate is much faster (entry 12).
In all these reactions the use of these materials allows the set up
of a simple, safe and clean protocol.
Work is in progress to find the experimental conditions allowing
us to oxidize also primary alcohols and to better elucidate the
reaction mechanism.{
In the oxidation of both (2)-menthol and neomenthol a
menthone/isomenthone mixture was obtained. Epimerization at
C2 strongly suggests that the reaction proceeds through an enolic
intermediate, thus also justifying the inactivity of primary alcohols
and the differences in reaction rate between 3- and 2-octanol. This
trend of reactivity is the opposite to those observed with all the
other oxidation heterogeneous systems reported so far.2
Federica Zaccheria,a Nicoletta Ravasio,*b Rinaldo Psarob and
Achille Fusia
aUniversita` degli Studi di Milano, Via Venezian 21, 20133, Milano,
Italy
bCNR-ISTM, Via Golgi 19, 20133, Milano, Italy.
E-mail: n.ravasio@istm.cnr.it; Fax: ++39 02 50314405;
Tel: ++39 02 50314382
Notes and references
{ Cu/SiO2 and Cu/Al2O3 catalysts were prepared as already reported12
by using SiO2 (BET 5 320 m2 g21; PV 5 1.75 ml g21) and Al2O3
(BET 5 280 m2 g21; PV 5 1.15 ml g21, DavicatH SMR 24-847) from
Grace Davison, Worms, DE and Cu(NO3)2?3H2O from Carlo Erba.
1 (a) I. E. Marko`, A. Gautier, R. Dumeunier, K. Doda, F. Philippart,
S. M. Brown and C. J. Urch, Angew. Chem., Int. Ed., 2004, 43,
1588–1591 and references therein; (b) P. Gamez, I. W. C. E. Arends,
J. Reedijk and R. A. Sheldon, Chem. Commun., 2003, 19, 2414–2415.
2 (a) U. R. Pillai and E. Sahle-Demessie, Green Chem., 2004, 6, 161–165;
(b) for a review see T. Mallat and A. Baiker, Chem. Rev., 2004, 104,
3037–3058.
3 D. Kramer, in Methoden der Organischen Chemie (Houben-Weyl), ed.
E. Muller, G. Thieme, Stuttgart, 1973, vol. 7/2a.
Scheme 1
254 | Chem. Commun., 2005, 253–255
This journal is ß The Royal Society of Chemistry 2005
Zaccheria, Federica
