ing SO2 derivatives. What is more, this very powerful oxygen
transfer agent does not require prolonged reaction times
(usually a few seconds to a few minutes are sufficient) or
elevated temperatures. These characteristics seemed very
encouraging in our quest for a general method of transform-
ing episulfides to their corresponding episulfones.
(oil) in 88 and 95% yield, respectively. As with 12, the
electron-donating alkyl groups encouraged a slow elimination
of SO2 already at room temperature, as evident also from
the NMR spectra which indicated a continuous olefin
formation. It should be noted, however, that the episulfone
9-thiabicyclo[6.1.0]nonane-S,S-dioxide (17),8 mp 93-95 °C,
produced in 90% yield from the episulfide of cyclooctene
18, is relatively stable and does not lose the element of SO2
as easily as its noncyclic counterparts 15 and 16.
Naturally, the first set of experiments was conducted on
the parent episulfidesthe thiirane (1) itself. The synthesis
of thiirane S,S-dioxide (2) with TFDO had been completed
by Taylor in his original work8 in 41% yield after a few
hours reaction. Only a few seconds were needed for the HOF‚
CH3CN to perform the same reaction and form 2 in 90%
yield. The reactions of two other monoalkyl-substituted
thiiranes, methyl- and decylthiiranes (3 and 4), with TFDO
were also described in the above paper with the conclusion
that the larger the alkyl group, the more difficult the reaction
was going to be. Even the small methyl group in 3 was
already responsible for the formation of 17% of the respective
episulfoxide 5 contaminating the desired episulfone 6, which
was formed in 65% yield. With the much larger decyl group
attached to the thiirane ring 4, the major component, even
after prolonged reaction time with TFDO, was the epi-
sulfoxide 7 (52%) along with only 32% of the decylthiirane
S,S-dioxide (8). The picture was much simpler with HOF‚
CH3CN. No episulfoxides were formed, and the respective
episulfones 6 and 8 were obtained in 87 and 80% yield,
respectively, in reaction times of a few seconds to a few
minutes.
The episulfide of cis-stilbene 19 presents an interesting
case. While treatment with TFDO, or any other oxygen
transfer agent for that matter, produces only the stable
episulfoxide 20, treatment with HOF‚CH3CN leads to 1,2-
diphenylthiirane S,S-dioxide (21, 80% yield) with no traces
of the episulfoxide 20 (Scheme 1). Although known,13 21 is
thermally not very stable and starts to release SO2 slowly,
forming cis-stilbene.
Scheme 1. Oxidation of Episulfides to Episulfones
The reaction with HOF‚CH3CN could effectively deal also
with thiiranes possessing substituents other than pure alkanes.
Reacting epithiochlorohydrin (9) with 2.5 molar equiv of
HOF‚CH3CN (each molar equivalent provides one oxygen
atom only) for 4 min at 0 °C resulted in 80% of the hitherto
unknown chloromethylthiirane S,S-dioxide (10), mp 59-62
°C dec, stable enough at room temperature to be fully
analyzed (see the Supporting Information). Replacing the
electron-withdrawing chloromethyl group with the electron-
donating phenoxymethylene one (11) did not change the
outcome much, and it took 2 min for the new 2-phenoxy-
methylthiirane S,S-dioxide (12), mp 80-83 °C dec, to be
formed in 85% yield. The only difference observed between
10 and 12 was the fact that the latter tends to decompose
faster then the chloromethylene derivative and it loses slowly
the elements of SO2 even at room temperature.
Remembering that the production of episulfones using
TFDO is more difficult with larger alkyl chains (see above),
it was of interest to find whether increasing the steric
hindrance will have any effect on the route of the reaction
with HOF‚CH3CN. It seems that placing two alkyl groups
at positions 2 and 3 of the thiirane ring does not diminish
the ability of the reagent to transfer two oxygen atoms to
the sulfur atom. 1-Isopropyl-2-methylthiirane (13) and
2-methyl-1-pentylthiirane (14) produced, after a short contact
(several seconds) with a cold (0 °C) solution of the
acetonitrile complex of the hypofluorous acid, the corre-
sponding new episulfones 15 (mp 29-31 °C dec) and 16
The above results raise two interesting points. The first is
the sharp contrast between HOF‚CH3CN and the rest of the
oxygen transfer agents, the latter producing the stable
episulfoxides only, while the first delivers two oxygens to
the sulfur atom resulting exclusively in episulfones.10 The
second point is rather more of a challenge, and it concerns
the possibility of forcing the HOF‚CH3CN complex in
producing episulfoxides in a clean reaction. To relate to these
issues, we reacted first the episulfide 9 with m-CPBA
forming the episulfoxide 2214 in 75% yield. An equimolar
mixture of 9 and 22 was then prepared and reacted with only
one molar equiv of HOF‚CH3CN. The resulting reaction
mixture consisted of 45% episulfoxide 22 and a similar
amount of the episulfone 10, the balance being the starting
sulfide (Scheme 2). This outcome indicates that the reagent
reacted almost exclusively with the episulfide 9 leaving the
(13) (a) Tokura, N.; Nagai, T.; Matsumura, S. J. Org. Chem. 1966, 31,
349. (b) King, J. F.; Durst, T. Can. J. Chem. 1966, 44, 819.
(14) Kondo, K.; Negishi, A. Tetrahedron 1971, 27, 4821.
(11) Rozen, S.; Bareket, Y. J. Chem. Soc., Chem. Commun. 1994, 1959.
(12) Amir, E.; Rozen, S. Angew. Chem., Int. Ed. 2005, 44, 7374.
1214
Org. Lett., Vol. 8, No. 6, 2006