Journal of Molecular Catalysis A: Chemical 195 (2003) 253–262
Oxidation of monoterpenes with hydrogen peroxide
catalysed by Keggin-type tungstoborates
I.C.M.S. Santos, M.M.Q. Simões , M.M.M.S. Pereira, R.R.L. Martins,
M.G.P.M.S. Neves, J.A.S. Cavaleiro, A.M.V. Cavaleiro
∗
Department of Chemistry, Campus Universitario de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal
Received 19 July 2002; accepted 18 October 2002
Abstract
The Keggin-type anions [Mn
III
(H
2
O)BW
11
O
39
]
6-
and [BW
12
O
40
]
5-
were used for the first time as catalysts in studies of
oxidation of monoterpenes by hydrogen peroxide. The oxidation of geraniol 1, nerol 2,(+)-3-carene 3, thymol 4 and carvacrol
5 was examined. The reactions were carried out in acetonitrile, at room temperature (for 1 and 2) or at reflux (for 3, 4 and 5).
Compounds 1 and 2 were preferentially epoxidised at the C
2
–C
3
double bond, whereas 3, under controlled reaction conditions,
could afford only the -epoxide. The oxidation of 4 and 5 yielded a mixture of benzoquinones. For tetrabutylammonium salts,
the Mn
III
substituted anion was found to be a more efficient catalyst than [BW
12
O
40
]
5-
. The infrared spectra of the recovered
solids at the end of reaction showed that the Keggin anions were still present, but the electronic spectra of the manganese
residues indicate that oxidation of Mn
III
had taken place.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: Keggin polyoxotungstates; Tungstoborates; Manganese; Hydrogen peroxide; Catalysis; Oxidation; Terpenes
1. Introduction
The development of efficient catalysts for the
selective oxidation of organic compounds in mild
and environmental friendly conditions is an active
field of research. An interesting line of study is the
search for effective environmentally clean catalytic
reactions to transform cheap natural compounds into
valuable intermediates for organic synthesis both in
the laboratory and in industry.
The use of Keggin-type polyoxometalates as cata-
lysts in oxidative transformations, like epoxidation of
olefins and hydroxylation of saturated hydrocarbons,
∗
Corresponding author. Tel.: +351-234-370-734;
fax: +351-234-370-084.
E-mail address: ana@dq.ua.pt (A.M.V. Cavaleiro).
has been well documented during the last decade
[1–8]. In particular, transition metal substituted
Keggin-type heteropolytungstates [M(H
2
O)XW
11
-
O
39
]
(n -m)-
(X = P, Si, etc.), in which a transition
metal cation, M
m+
, is co-ordinated to the binding
sites of a lacunary heteropolyanion [XW
11
O
39
]
n -
,
have been found to catalyse several types of oxidative
reactions with a variety of oxidants and substrates
[6–8]. The interest of these species, presenting a metal
centre surrounded by a stable inorganic environment,
for oxidative catalysis was noticed about 16 years ago
[9,10] and was followed by a large surge of work.
Most of this work is concerned with polyoxoanions
having phosphorous or silicon as the heteroatom X
[6–8].
We report here the use of the heteropolytungstate
[Mn
III
(H
2
O)BW
11
O
39
]
6-
as catalyst in the oxidation
1381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII:S1381-1169(02)00582-4