Chalcogen chiral ylides for the catalytic asymmetric epoxidation of aldehydes: From sulfur to selenium and tellurium

Article abstract of DOI:10.1080/10426500590905824

The reaction of novel chiral selenonium and telluronium ylides was investigated with aldehydes and compared to the sulfur analogues. (2R,5R)-2,5-Dimethylselenolane was prepared and reacted as a catalyst for the benzylidenation of aldehydes. Disubstituted epoxides were readily prepared with a (surprising) absence of diastereoselectivity, and with enantiomeric excesses higher than 90%. The reaction of a tellurium analogue, (2S,5S)-2,5- diethyltellurolane, afforded the oxirane in very moderate yield and e.e.'s in the range of 62-82%. Though this was less productive, it is the first report of a chiral telluronium ylide leading to an asymmetric epoxidation of aldehydes. Copyright Taylor & Francis Inc.

Full text of DOI:10.1080/10426500590905824

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Phosphorus, Sulfur, and Silicon
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Chalcogen Chiral Ylides for
the Catalytic Asymmetric
Epoxidation of Aldehydes:
From Sulfur to Selenium and
Tellurium
Patrick Metzner a , Jean-François Brière a & Hiroya
Takada a
a ENSICAEN-Université , Caen, France
Published online: 01 Feb 2007.
To cite this article: Patrick Metzner , Jean-François Brière & Hiroya Takada (2005)
Chalcogen Chiral Ylides for the Catalytic Asymmetric Epoxidation of Aldehydes: From
Sulfur to Selenium and Tellurium, Phosphorus, Sulfur, and Silicon and the Related
Elements, 180:3-4, 965-968, DOI: 10.1080/10426500590905824
To link to this article: http://dx.doi.org/10.1080/10426500590905824
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Phosphorus, Sulfur, and Silicon, 180:965–968, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500590905824
Chalcogen Chiral Ylides for the Catalytic Asymmetric
Epoxidation of Aldehydes: From Sulfur to Selenium
and Tellurium
`
Jean-Franc¸ois Briere
Hiroya Takada
Patrick Metzner
ENSICAEN-Universite´, Caen, France
The reaction of novel chiral selenonium and telluronium ylides was inves-
tigated with aldehydes and compared to the sulfur analogues. (2R,5R)-2,5-
Dimethylselenolane was prepared and reacted as a catalyst for the benzylidena-
tion of aldehydes. Disubstituted epoxides were readily prepared with a (surprising)
absence of diastereoselectivity, and with enantiomeric excesses higher than 90%.
The reaction of a tellurium analogue, (2S,5S)-2,5-diethyltellurolane, afforded the
oxirane in very moderate yield and e.e.’s in the range of 62–82%. Though this was
less productive, it is the first report of a chiral telluronium ylide leading to an
asymmetric epoxidation of aldehydes.
Keywords Asymmetric synthesis; epoxides; selenonium ylide; telluronium ylide
INTRODUCTION
Epoxides are versatile intermediates in organic synthesis,1–3 as they are
easily opened up by a variety of nucleophiles. We have been interested
in a straightforward connective route for their access in an enantioen-
riched fashion: the reaction of chiral sulfur ylides with aldehydes,4–6
complementary route to the asymmetric oxidation of alkenes.
a
We have previously designed and used C2 symmetric enan-
tioenriched sulfides,7,8 (2R,5R)-2,5-dimethylthiolane and (2R,5R)-2,5-
diethylthiolane, as catalysts9 for the ylide-mediated conversion of
aldehydes to oxiranes. A simple one-pot procedure was achieved with
excellent yields, good enantio- and diastereo-selectivities, but some
limitations were met: modest kinetic rates, scope . . . This has led
us to explore the selenium and tellurium10–12 chiral congeners with
Received January 28, 2004; accepted October 13, 2004.
Address correspondence to Patrick Metzner, Laboratoire de Chimie Mole´culaire et
Thio-organique (UMR CNRS 6507), ENSICAEN, 6, boulevard du Mare´chal Juin, 14050
Caen, France. E-mail: metzner@ismra.fr
965

966
J.-F. Brie`re et al.
potential advantages: enhanced nucleophilic character for both the ini-
tial chalcogenide13 and the intermediate ylide. Their reaction with alde-
hydes is much less known than in the sulfur series, but some groups14–19
have reported that epoxidation is feasible. However, previous to our
work, no asymmetric epoxidation was known with these ylides.
(2R,5R)-2,5-Dimethylselenolane was synthesized in two steps
from (2S,5S)-hexanediol and its hypervalent dibromide adduct was
characterized.20 The selenolane proved to be an efficient catalyst
(0.2 equiv) for the benzylidenation of aromatic aldehydes (Scheme 1).
The reaction is indeed faster than in the sulfur series. The enantiomer
excess is usually higher than 90%. On the other hand, equal amounts
of trans and cis diastereomers were observed. In the sulfur series, the
trans diastereoisomer is largely predominant for the formation of stil-
bene oxides. It has been explained6,21 by a reversible formation of the
betaine leading to the cis epoxide. We do not have evidence for the origin
of the stereochemistry in the selenium series.
SCHEME 1
A similar study was launched with tellurium ylides. Reaction
of the bis(mesylate) of (2S,5S)-hexanediol with lithium telluride22
led to (2R,5R)-2,5-dimethyltellurolane, but some difficulties were
met, including volatility and epimerization. Less volatile (2S,5S)-
2,5-diethyltellurolane (Scheme 2) was then preferred. Though this

Catalytic Asymmetric Epoxidation of Aldehydes
967
SCHEME 2
preparation was more successful than the one with the two methyl
groups, it was sometimes not reproducible, and epimerization could also
be observed. We suspect a radical cleavage of a C–Te bond by residual
Et3B, and recombination.
A stoichiometric amount of (2S,5S)-2,5-diethyltellurolane was re-
acted with benzyl bromide, 4-chlorobenzaldehyde, and sodium hydrox-
ide in a 9:1 t-butanol/H2O mixture (Scheme 3). We were glad to observe
that the reaction proceeds but after 48 h the yield was only 35%. The
enantiomeric excess was rather moderate: 62%. Under similar condi-
tions, a catalytic amount (20%) of tellurolane was tested. For this sol-
vent system, we were limited to a yield of 24%, with an e.e. of 68%. A
higher asymmetric induction was achieved under biphasic conditions,
with dichloromethane and concentrated sodium hydroxide. Our highest
e.e. was 82%.
SCHEME 3
Thus, tellurium ylides were formed and epoxidation was observed.
A competitive olefination pathway18,23 was also observed, but to a lim-
ited extent (<10%). Even if the expected process is not very produc-
tive, this is the first report of a chiral telluronium ylide for asymmetric
epoxidation.
The diastereoselectivity is in favor of the trans isomer. This result is
similar to the sulfur series, but surprisingly very different from the sele-
nium series. At this stage, without any further experimental evidence,
it appears difficult to rationalize this behavior.
During the course of this study, Tang and co-workers disclosed24 the
use of telluronium ylides, from (2R,5R)-2,5-dimethyltellurolane, for the
asymmetric synthesis of vinyl cyclopropanes.

968
J.-F. Brie`re et al.
TABLE I Synthesis of 2-(4-Chlorophenyl),3-Phenyloxirane by
Reaction of (2S,5S)2,5-Diethyltellurolane with Benzyl Bromide and
4-Chlorobenzaldehyde
e.e.
Equivalent
of auxiliary
d.r.
(R,R) Yield
Solvent
Base
Time (h) (trans/cis)
(%)
(%)
1
0,2
0,2
tBuOH/H2O (9/1) 2 NaOH
tBuOH/H2O (9/1) 2 NaOH
48
72
72
85:15
90:10
97:3
62
68
82
35
24
28
CH2Cl2
aq. NaOH (50%)
1 nBu4NBr
In conclusion, these are the first reports on the use of enantiopure
selenium and telluronium ylides in asymmetric synthesis of epoxides.
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