Microwave-expedited olefin epoxidation over hydrotalcites using hydrogen peroxide and acetonitrile

Article abstract of DOI:10.1016/S0040-4039(02)00426-4

An efficient microwave-assisted epoxidation of olefins is described over hydrotalcite catalysts in the presence of hydrogen peroxide and acetonitrile. This general and selective protocol is extremely fast and is applicable to a wide variety of substrates.

Full text of DOI:10.1016/S0040-4039(02)00426-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2909–2911
Microwave-expedited olefin epoxidation over hydrotalcites using
hydrogen peroxide and acetonitrile
Unnikrishnan R. Pillai, Endalkachew Sahle-Demessie* and Rajender S. Varma
National Risk Management Research Laboratory, Sustainable Technology Division, MS 443, US Environmental Protection
Agency, 26 West M. L. King Drive, Cincinnati, OH 45268, USA
Received 8 February 2002; revised 28 February 2002; accepted 1 March 2002
Abstract—An efficient microwave-assisted epoxidation of olefins is described over hydrotalcite catalysts in the presence of
hydrogen peroxide and acetonitrile. This general and selective protocol is extremely fast and is applicable to a wide variety of
substrates. Published by Elsevier Science Ltd.
Epoxidation of carbon–carbon double bonds is of
immense importance in organic chemistry due to the
synthetic utility of the ensuing epoxides.1–3 Hydrogen
peroxide (H2O2) is an ideal oxidant of choice for these
oxidations since it is relatively less expensive, environ-
mentally safe and forms innocuous water as the only
by-product.4 Epoxidation of olefins is usually con-
ducted under strongly alkaline conditions using bases
such as NaOH, Na2CO3 or KOH and H2O2 as the
oxidant. However, the usage of these strong bases is
highly undesirable as they result in the production of
large amounts of industrial waste. Additional methods
have been reported that involve reagents namely,
sodium hypochlorite,5 dioxiranes,6 sodium perborate in
the presence of a phase-transfer reagent,7 alumina sup-
ported KF,8 t-butyl hydroperoxide in the presence of a
titania–silica catalyst,9 tertiary arsine oxides and
H2O2,10 Ti(IV) silsesquioxane/MCM-41,11 Ti(IV)/
Sn(Ge)(IV)/MCM-41,12 modified Ti-MCM-41,13 Cu
and Ag catalysts,14 porphyrin-based catalysts,15 moly-
bdenum silicate,16 modified natural phosphates17 etc.
One of the main drawbacks of most of these processes
is the long reaction time required to achieve the desired
product yield. In addition, with increased environmen-
tal awareness, close attention is being paid to the
development of environmentally friendly protocols that
use relatively benign oxidants such as molecular oxygen
and hydrogen peroxide in combination with reusable
and recyclable solid catalysts.4
Hydrotalcites or anionic clays are homogenous basic
mixed hydroxides with a brucite-like layer structure.18
They are shown to be highly active catalysts for epoxi-
dation of olefins using H2O2 in the presence of a
nitrile,19,20 wherein H2O2 reacts with a basic hydroxyl
function on the hydrotalcite to form a perhydroxyl
anion (OOH−), which in turn, nucleophilically attacks
the nitrile to generate a peroxycarboximidic acid inter-
mediate (I, Scheme 1). The subsequent oxygen transfer
from the intermediate to the olefin results in the forma-
tion of epoxide and an amide. However, the reaction
usually involves, in addition to the nitrile, the use of an
organic solvent, which is less desired these days.
The use of microwave (MW) in enhancing chemical
transformations has been gaining attention in recent
years due to several advantages such as rapid reaction
rates and higher yield of pure products.21–23 This is a
consequence of the selective absorption of microwave
energy by polar molecules or polar transition state
intermediates formed during the course of the
reaction.22a
As part of the ongoing program on environmentally
benign chemical synthesis employing alternative reac-
tion conditions and media in our laboratory,21 herein
we report an expeditious and environmentally benign
NH
_
H3C
C
OOH
H3C
C
N
+
OOH
Keywords: microwave-expedited synthesis; olefins; epoxidation;
hydrotalcites; hydrogen peroxide.
I
* Corresponding
sahle-demessie.endalkachew@epa.gov
author.
Fax:
513-569-7677;
e-mail:
Scheme 1.
0040-4039/02/$ - see front matter. Published by Elsevier Science Ltd.
PII: S0040-4039(02)00426-4

2910
U. R. Pillai et al. / Tetrahedron Letters 43 (2002) 2909–2911
epoxidation protocol for olefins using hydrotalcite cata-
lysts in the presence of acetonitrile (MeCN) and H2O2
under MW-irradiation conditions.
Baeyer–Villiger oxidation product. The temperature of
the reaction mixture immediately after MW-irradiations
was found to be <60°C at 40% power level (480 W) and
<70°C at 60% power level (720 W), thereby suggesting
that the rate enhancement may not entirely be due to
thermal effect. The intermittent heating of the samples
for a short time (15 s, see experimental procedure)
followed by thorough mixing for 1–2 min in a stop-
pered vessel ensured minimum sample loss by evapora-
tion.
The results of MW-assisted olefin epoxidation over
hydrotalcite catalyst are summarized in Table 1 along
with the comparative reactions under conventional
heating conditions using excess acetonitrile. Appar-
ently, the selective epoxidation of a variety of olefins
occurs rapidly upon MW-irradiation including sub-
strates that are difficult to oxidize. As an example,
the epoxidation of sterically hindered isophorone is
difficult to accomplish.7,8,20c However, isophorone
(entry 13) could be easily oxidized to epoxy isophorone
(87% yield) using the MW-protocol. Mesityl oxide
(entry 12) also formed the epoxidation product in good
yield with no appreciable formation of any 1,2-diox-
olane by-products. The selectivity to epoxide is
extremely good with little or no formation of any
In conclusion, the olefin epoxidation occurs readily
over hydrotalcite catalysts upon MW exposure of the
mixture in hydrogen peroxide and acetonitrile. This is
presumably due to the polar nature of the reaction
intermediates (I, Scheme 1) that couple efficiently with
microwaves and hence the dramatic rate acceleration.
This approach significantly minimizes the longer reac-
tion times required in conventional heating of olefins
Table 1. MW-assisted olefin epoxidation over hydrotalcite catalysts using H2O2 and acetonitrilea

U. R. Pillai et al. / Tetrahedron Letters 43 (2002) 2909–2911
2911
with H2O2 and avoids the use of large excess of volatile
organic solvent usually employed.
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Experimental
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NaOH were obtained from Aldrich Chemicals and used
as such without any further purification. Magnesium
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aluminum
hydroxy
carbonate
(hydrotalcite),
Mg6Al2(OH)16CO3·4H2O, was prepared by co-precipita-
tion technique at a constant pH of 8 from a 0.5 M
solution of Mg(NO3)2·6H2O and Al(NO3)3·9H2O in
de-ionized water using a mixture of NaOH and Na2CO3
as the precipitants according to the procedure described
elsewhere.18,24 The solid precipitate was filtered, washed
several times with de-ionized water, dried in an oven at
100°C overnight and calcined in air at 400°C for 5 h. The
structure of hydrotalcite was confirmed by X-ray diffrac-
tion analysis of the dried material, which showed char-
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MW-assisted epoxidation of various olefins was con-
ducted in liquid phase in a stoppered 125 mL conical flask
using an unmodified domestic household microwave
oven (Panasonic, 1200W) equipped with inverter tech-
nology, which provides a realistic control of the
microwave power to the desired level. The MW oven was
operated at reduced MW-power level of 40% (480 W) for
low boiling olefins (<100°C) and at 60% power level (720
W) for high boiling olefins (>100°C). In all cases, the
samples were subjected to MW-irradiation for a period
of 15 s with 2 min mixing intervals.25 After completion
of the reaction, the mixture was extracted into diethyl
ether, the organic layer was separated and analyzed by
a Hewlett–Packard 6890 Gas Chromatograph using a
HP-5 5% phenyl methyl siloxane capillary column (30
m×320 mm×0.25 mm) and a quadruple mass filter
equipped HP 5973 mass selective detector. The product
yield reported is the GC yield. The purity of the products
was established by the GC–MS analysis of the samples.
Acknowledgements
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Lidstrom, P.; Tierney, J.; Wathey, B.; Westman, J. Tetra-
hedron 2001, 57, 9225; (c) Bose, A. K.; Banik, B. K.;
Lavlinskaia, N.; Jayaraman, M.; Manhas, M. S. Chemtech
1997, 27, 18.
U.R.P. is a postgraduate research participant at the
National Risk Management Research Laboratory
administered by the Oak Ridge Institute for Science and
Education through an interagency agreement between
the US Department of Energy and the US Environmental
Protection Agency.
23. Sala, G. D.; Giordano, L.; Lattanzi, A.; Proto, A.; Scettri,
A. Tetrahedron 2000, 56, 3567.
24. Unnikrishnan, R.; Narayanan, S. J. Mol. Catal. A (Chem.)
1999, 144, 173.
25. In a typical reaction procedure, the catalyst (0.25 g) was
mixed with the substrate (12.5 mmol), acetonitrile (4 mL)
and hydrogen peroxide (50 mmol) in a 125 mL conical flask
closed with a rubber stopper and subjected to MW
irradiation for 15 s. The sample was then thoroughly mixed
outside for 2 min and again irradiated for another 15 s. This
intermittent heating–stirring cycle was repeated for the total
irradiation time (see Table 1). The temperature of the bulk
reaction mixture was also measured immediately after the
irradiation. Forcomparisonpurposes, theexperimentswere
also conducted in the liquid phase under conventional
heating conditions in an oil bath.
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