8914
J. Am. Chem. Soc. 1998, 120, 8914-8919
Diastereoselective Episulfidation of Strained Cyclic Alkenes by a
Thiophene Endoperoxide versus Epoxidation by Dimethyldioxirane
Waldemar Adam, Bettina Fr o1 hling,†,‡ Karl Peters, and Stephan Weink o1 tz*
†
§
,†
Contribution from the Institut f u¨ r Organische Chemie der UniVersit a¨ t, Am Hubland,
D-97074 W u¨ rzburg, Germany, and Max-Planck-Institut f u¨ r Festk o¨ rperforschung, Heisenbergstrasse 1,
D-70506 Stuttgart, Germany
ReceiVed February 6, 1998
Abstract: Thiophene endoperoxide 2, which was prepared by photooxygenation of thiophene 1, transfers a
sulfur atom (up to 92%) to strained cycloalkenes to form thiiranes when thermolyzed in their presence. The
diastereomeric pair cis/trans-cyclooctene (5b) reacted stereoselectively, which speaks for a concerted process
rather than open dipolar and/or diradical intermediates. The set of chiral cyclooctenols 5c-e was also
investigated, and the relative configurations of the respective thiiranes were assigned by chemical correlation
and NMR spectral and X-ray analysis. The first-order kinetics of the process clearly shows that the endoperoxide
2
itself is not the sulfur-transferring species, but it is thermally transformed to the intermediates I and II.
Whereas intermediate II is responsible for the competitive formation of elemental sulfur, intermediate I,
presumably an oxathiirane, is the active sulfur-transferring species. The episulfidation was compared with
the epoxidation by the related dimethyldioxirane, and both show the same qualitative trends in the
diastereoselectivity and the reactivity toward the alkenes 5a-e.
Introduction
cycloalkenes to form the corresponding thiiranes in moderate
to good yields (Scheme 1).
1
1
The epoxidation of alkenes is beyond doubt one of the most
important and best investigated synthetic transformations. A
variety of reagents are known, which may be used to transfer
Presently we report the full experimental details on this novel
sulfur transfer to a set of olefins for assessing the diastereose-
lectivity and to explore the mechanistic details of this potentially
1-8
an oxygen atom directly to an alkene.
Several enantiose-
1
2
lective epoxidations demonstrate the significance of this well-
explored methodology in organic chemistry.2 In contrast, the
direct episulfidation of alkenes is reported only for a few special
important reaction. Since oxathiiranes have been postulated
-7
11,13
as intermediates in the thermolysis of endoperoxides
and
since their sulfur-transfer potential has been already claimed,1
these transients are postulated as the active sulfur-transferring
agents in this episulfidation. Comparison with the related
epoxidation by the analogous dimethyldioxirane (DMD) sub-
stantiates this claim.
4
9
,10
examples and has yet not been synthetically applied.
To
transform alkenes to their episulfides, usually indirect methods
are used, of which the preparatively more useful ones are the
conversion of epoxides to their episulfides by thiocarbonyl-
containing reagents (e.g., thiourea) or the addition of a sulfenyl
chloride to an alkene and subsequent base-catalyzed ring
Results
1
0
closure.
Episulfidations. Thiophene 1 reacted quantitatively with
singlet oxygen at - 30 °C to form the endoperoxide 2, which
is persistent at - 20 °C in CDCl3 solution for several days; it
Recently, we have found that in the thermolysis of thiophene
endoperoxide 2 a sulfur atom is transferred directly to strained
†
Institut f u¨ r Organische Chemie der Universit a¨ t.
Undergraduate Research Participant (1996).
Max-Planck-Institut f u¨ r Festk o¨ rperforschung.
1
13
11
‡
§
was characterized by its H and C NMR data.
The
(
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S0002-7863(98)00425-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/25/1998