Superactive Mg-Al-O-t-Bu hydrotalcite for epoxidation of olefins

Article abstract of DOI:10.1055/s-1998-1919

Epoxidation of unfunctionalised olefins and α,β-unsaturated ketones has been accomplished at an amazingly and unprecedented rate by superactive Mg-Al-O-t-Bu-hydrotalcite solid base catalyst prepared in our laboratory.

Full text of DOI:10.1055/s-1998-1919

November 1998
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Superactive Mg-Al-O-t-Bu Hydrotalcite for Epoxidation of Olefins
B.M. Choudary,* M. Lakshmi Kantam, B. Bharathi and Ch. Venkat Reddy
Indian Institute of Chemical Technology, Hyderabad 500 007, India
* Fax: 0091 40 7173378; E-mail: iict@ap.nic.in; IICT Communication No.: 4054
Received 24 July 1998
Abstract: Epoxidation of unfunctionalised olefins and α,β-unsaturated
ketones has been accomplished at an amazingly and unprecedented rate
by superactive Mg-Al-O-t-Bu-hydrotalcite solid base catalyst prepared
in our laboratory.
In the present endeavour, we describe a magnificent display of
superactivity in the epoxidation of unfunctionalised as well as electron
deficient olefins employing I as a base catalyst and hydrogen peroxide
as an oxidant under a set of different conditions as described in schemes
1 and 2.
Hydrotalcites consist of Brucite-like layers having positive charge
countered by anionic species intercalated to form neutral materials. The
basicity and activity can be easily tuned up choosing a set of hetero
elements, changing the element ratio, and anion species intercalated
from a pool of wide range of multioptions.1-3 Recently, the exquisite
solid bases are of increased attention in view of their ubiquitous
characteristics and are being considered for possible replacement of
soluble bases in the base induced chemical processes to offer
environmentally benign technologies.4-7
Epoxides are versatile building blocks for organic synthesis.8
Hydrotalcite carbonates as synthesised and the calcined samples have
been probed for epoxidation of unfunctionalised and electron deficient
olefins respectively and are found to be active.6, 9-12 However, the rates
of the reactions are slow. We effected very recently modifications on
hydrotalcites to induce aldol reactions successfully. Incorporation of t-
butoxide in the interlayers of hydrotalcite by anion-exchange provided a
simple methodology for the preparation of Mg-Al-O-t-Bu-hydrotalcite
[I] for the selective synthesis of aldols.13
Scheme 1
Scheme 2
The results of epoxidation of α,β-unsaturated ketones catalysed by Mg-
Al-O-t-Bu-hydrotalcite and calcined hydrotalcite6 are compiled in Table

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reaction even after prolonging the reaction time. This certainly indicates
that t-butoxide ion is intercalated in the catalyst I. The epoxidation of
terminal olefins to racemic epoxides in excellent yields is noteworthy
and assumes further significance since value added optically pure
epoxides and 1,2-diols could be easily obtained from the racemic
epoxides through kinetic resolution.8
Therefore, the amazing activity of Mg-Al-O-t-Bu-Hydrotalcite catalyst
in the epoxidation of various olefins is ascribed to the anion O-t-Bu
paired with Mg and Al of the main frame hydrotalcite since the reaction
was practically very slow with potassium t-butoxide and a mixture of
potassium t-butoxide and Mg-Al-NO3 hydrotalcite in separate
experiments conducted under identical conditions. In other words, the
intercalated anionic tert. butoxide species in conjunction with Mg and
Al projected superactivity in the epoxidation reactions. In view of the
superactivity expressed both in epoxidation of unfunctionalised and
electron-deficient olefins, the present catalyst can be a potential
candidate to offer environmentally benign chemical technologies.
Acknowledgement: We gratefully acknowledge the financial support of
this work by Commission of the European Communities [CII*-CT94-
0050 (DG12 HSMU)].
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Interestingly, with potassium t-butoxide in place of I, the reaction [Table
1, entry 1] took 10 h to afford a reasonable yield of 80%. In a separate
experiment [Table 1, entry 1] conducted by adding Mg-Al-NO3
hydrotalcite and potassium t-butoxide to the reaction medium, the
reaction afforded 85% conversion in 12h and there was no further
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