Epoxidation of alkenes using alkyl hydroperoxides generated in situ by
catalytic autoxidation of hydrocarbons with dioxygen
Takahiro Iwahama, Gou Hatta, Satoshi Sakaguchi and Yasutaka Ishii*
Department of Applied Chemistry, Faculty of Engineering & High Technology Research Center, Kansai University,
Suita, Osaka 564-8680, Japan. E-mail: ishii@ipcku.kansai-u.ac.jp
Received (in Cambridge, UK) 2nd November 1999, Accepted 10th December 1999
Olefins were smoothly epoxidized under O
presence of a hydrocarbon such as ethylbenzene or tetralin,
using N-hydroxyphthalimide (NHPI) and Mo(CO) as
2
(1 atm) in the
of the present epoxidation by the addition of molecular sieves
4A (MS-4A) was observed, and thus the conversion of 1 and
selectivity of epoxide 3 reached 81% and 70%, respectively (run
2). Using tetralin (2b) instead of 2a, it was found that the
selectivity of epoxide 3 was considerably improved (run 3).
However, when toluene (2c) was employed as a hydrocarbon
source, the selectivity of 3 was lowered to 44% (run 4). This is
believed to be due to the occurrence of allylic hydrogen atom
abstraction from 1 in competition with the benzylic hydrogen
6
catalyst; the present reaction involves autoxidation of the
hydrocarbon assisted by NHPI and epoxidation of alkenes
6
with the resulting hydroperoxide catalyzed by Mo(CO) ; cis-
alkene was epoxidized in a stereospecific manner to form the
corresponding cis-epoxide in high yield.
8
The epoxidation of alkenes using molecular oxygen via a
atom abstraction of 2c by PINO. Although AIBN was used as
catalytic process is a challenging subject in the field of
oxidation chemistry. Since the direct epoxidation of alkenes
with molecular oxygen, which lies in triplet ground state, is
a radical source instead of NHPI, 1 was difficult to epoxidize to
3 (run 5). It is very interesting to note that the present
epoxidation was induced even at room temperature to afford
1
9
inhibited, epoxidation using O
2
is carried out in the presence of
epoxide 3 in high selectivity (97%) at 31% conversion (run 7).
a compound like an aldehyde which serves as an active oxygen
carrier. Although there have been many reports on the metal-
On the other hand, metal complexes such as MoO
2 2
(acac) ,
2
VO(acac) and TiO(acac) were found to be inadequate for the
2
2
catalyzed epoxidation of alkenes by O
2
in the presence of
as
present reaction (runs 8–10). Although molybdenum(VI) com-
aldehydes, only a limited number of methods using O
2
plexes have high catalytic activity for the epoxidation of alkenes
3
10
terminal oxidant have been developed. There has been long-
standing interest in the epoxidation of alkenes with alkyl
hydroperoxides, as an active oxygen carrier, generated in situ
2 2
with tert-butyl hydroperoxide, MoO (acac) was found to
depress the formation of the hydroperoxide from 2b under these
conditions (run 8).
On the basis of these results, the epoxidation of various
from hydrocarbons and O
however, only one report has appeared on the epoxidation of
alkenes by the use of cumene and O with heteropolyox-
2
. To the best of our knowledge,
2
alkenes with O using 2a or 2b was examined in the presence of
2
2 6
catalytic amounts of NHPI, Co(OAc) and Mo(CO) under
4
ometalates as catalyst. A major difficulty in the epoxidation is
attributed to the following: (i) autoxidation of aldehydes takes
place very fast, at least two orders of magnitude faster than that
of hydrocarbons; (ii) as a result, the epoxidation using
hydrocarbons must be carried out under severe reaction
conditions; and (iii) the epoxidizing ability of alkyl hydro-
selected reaction conditions (Table 2).
trans-Oct-2-ene (trans-1) was epoxidized with excellent
stereoselectivity to give trans-2,3-epoxyoctane (trans-3) (trans+
cis = > 99+1) with 88% selectivity together with a small
amount of octane-2,3-diol (4%) in 78% conversion. Similarly,
the epoxidation of cis-1 gave cis-2,3-epoxyoctane (cis-3)
(cis+trans = 99+1) in good selectivity. It is noteworthy that the
present epoxidation of cis-olefin proceeds with nearly complete
stereoselectivity to give cis-epoxide, although the metal-
peroxides is considerably lower than that of peracids or
acylperoxy radicals derived from aldehydes and O
stepwise procedures are commonly utilized in epoxidations
using alkyl hydroperoxides. For example, the Halcon process
consists of the aerobic oxidation of ethylbenzene to a-
hydroperoxyethylbenzene, and the Mo-catalyzed epoxidation
5
2
. Therefore,
catalyzed epoxidation of cis-olefins using an aldehyde and O
2
leads to a mixture of cis- and trans-epoxides.11 This is because
the resulting alkylperoxyl radical can abstract the hydrogen
6
of propylene with the a-hydroperoxyethylbenzene. Conse-
quently, development of an epoxidation system using a
hydroperoxide generated in situ from ethylbenzene and molec-
ular oxygen is very attractive from the synthetic and industrial
points of view.
Recently, we have shown that hydrocarbons are efficiently
2
oxidized with O by N-hydroxyphthalimide (NHPI), which
7
serves as a radical catalyst under mild conditions. In this
oxidation, hydrocarbons are converted into oxygen-containing
compounds such as alcohols or ketones through alkyl hydro-
peroxides (B) as transient intermediates. If the alkyl hydro-
peroxides formed can be utilized as oxidants, it is possible to
epoxidize alkenes using hydrocarbons and molecular oxygen.
Here we report the Mo-catalyzed epoxidation of alkenes with
hydroperoxides generated in situ by the NHPI-catalyzed aerobic
oxidation of hydrocarbons such as ethylbenzene (Scheme 1).
Table 1 shows representative results for the epoxidation of
oct-2-ene (1) with O
source.† The epoxidation of 1 using ethylbenzene (2a) in the
presence of NHPI (10 mol%), Co(OAc) (0.1 mol%) and
Mo(CO) (5 mol%) at 60 °C under O (1 atm) gave epoxide 3
in 61% selectivity at 67% conversion (run 1). An improvement
2
using hydrocarbons as a hydroperoxide
2
6
2
Scheme 1
Chem. Commun., 2000, 163–164
This journal is © The Royal Society of Chemistry 2000
163