Epoxidation of Allylic Alcohols by Sandwich-Type Polyoxometalates
the criterion of sufficiently high catalyst activity. A good
example of remarkably high catalytic activity in hydrogen-
peroxide-mediated oxidations is provided by the disub-
stituted manganese(II) POM, namely [WZnMn2(II)(Zn-
W9O34)2]12-, which displays high selectivity for the
epoxidation of alkenes and the oxidation of alcohols to
their carbonyl products by 30% H2O2 in a biphasic
system.4a,b Other isostructural analogues with disubsti-
tuted noble metals [Rh(III), Pd(II), Pt(II), and Ru(III)]
have also been shown to exhibit activity in alkene and
alkane oxidations by H2O2 and t-BuOOH.5 In fact, the
ruthenium derivative was effective in the oxidation of
adamantane even by molecular oxygen.6 Very recently,
F IGURE 1. Chiral hydroperoxides used as inductors as well
as oxygen donors for asymmetric oxidation.
we have demonstrated the efficacy of the oxidatively and
solvolytically resistant sandwich-type polyoxometalates
for the epoxidation of the chiral allylic alcohols 1 by
hydrogen peroxide (30%).7 High (>95%) chemo-, regio-,
and diastereoselectivity have been achieved in the ep-
oxidation of 1,3- or 1,2-allylically strained alcohols. These
epoxidations were also efficiently catalyzed by a non-
10 mol %) and the pronounced “ligand deceleration”,
which both undoubtedly reduce the catalytic efficiency.10-12
In recent years, alternatively, catalytic enantioselective
oxidations have been conducted by combining optically
active hydroperoxides 4 (Figure 1; these organic hydro-
peroxides are potentially dangerous and should be handled
with care) and achiral metal complexes.13 An early
example on the asymmetric epoxidation of allylic alcohols
under titanium catalysis employed optically active sugar-
derived (4e-g)13a,b or simple secondary alkyl (4a -c)
hydroperoxides,13c which gave er values of 75:25 at best.
Very recently, more successful was the VO(OiPr)3-
catalyzed asymmetric epoxidation with the sterically
demanding TADDOL-derived hydroperoxide TADOOH
(4d , ) [(4R,5R)-5-[(hydroperoxydiphenyl)methyl]-2,2-
dimethyl-1,3-dioxolan-4-yl]diphenylmethanol) in the pres-
ence of an achiral hydroxamic acid ligand, which dis-
played higher (up to 86:14) er values for the epoxy
alcohols.14 In addition, we have utilized such alkyl
hydroperoxides in the asymmetric Weitz-Scheffer ep-
oxidation of R,â enones and obtained er values of up to
95:5.13e Especially high enantioselectivities (er values up
to 99:1) have been achieved with TADOOH as chiral
oxygen source in the asymmetric Weitz-Scheffer epoxi-
dation,theBaeyer-Villiger oxidation,andin sulfoxidations.13f
The good performance of these POM catalysts with
aqueous H2O2 prompted us to use sandwich POMs as
epoxidation catalysts in combination with organic hy-
droperoxides, which are often superior to those by H2O2
in regard to selectivity.15 Furthermore, the successful
application of chiral hydroperoxides in asymmetric oxida-
tions encouraged us to employ them as chiral oxygen
sources to develop POM-catalyzed asymmetric oxidations.
Indeed, in a very recent report, high enantioselectivities
(er values up to 95:5) have been accomplished with
TADOOH in the catalytic epoxidation of allylic alcohols
by the oxovanadium(IV)-disubstituted, sandwich-type
transition-metal-substitutedPOM,e.g.,[WZn3(ZnW9O34)2]12- 7
.
Despite the wide set of POM-catalyzed transforma-
tions, there is to date no report on POM-catalyzed
asymmetric oxidations. Conventionally, catalytic asym-
metric oxidations are achieved through the combination
of chiral metal complexes with achiral oxygen sources.
Prominent examples are the Sharpless-Katsuki epoxi-
dation of allylic alcohols8 and the J acobsen-Katsuki
epoxidation of unfunctionalized alkenes.9 Chirally modi-
fied vanadium complexes have also received considerable
scrutiny in asymmetric catalysis.10 A highly promising
asymmetric epoxidation of allylic alcohols was reported
by Sharpless as early as 1977, in which enantiomeric
ratio (er) values of up to 90:10 have been obtained by
the combination of VO(acac)2 and chiral hydroxamic acid,
with tert-butyl hydroperoxide as oxygen source.11 Re-
cently, Yamomoto and co-workers achieved a remarkable
improvement in the existing vanadium-based asymmetric
protocol that allows optically active epoxy alcohols to be
obtained with er values of up to 98:2.12 The disadvantages
in all of these vanadium-mediated asymmetric epoxida-
tions are the requirement of a multifold excess of the
chiral ligand with respect to the vanadium catalyst (1-
(5) Neumann, R.; Khenkin, A. M. Inorg. Chem. 1995, 34, 5753-
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