Ph3P
Ph3P
O
observed, and no PPh3 detected, on addition of 15 mM PPh3.
Me
Re
Me
Re
Addition of a further 5 mM increment of PPh3 showed very
small amounts of PPh3. Beyond that, more phosphine did not
lead to more Ph3PNS. These findings indicate the presence of a
rhenium(vii) sulfide that easily transfers a sulfur atom to a
phosphine to generate ReV and phosphine sulfide. Most of
product of the MTO/H2S reaction exists as ReVII on reaction
with H2S. On the basis of direct information, however, we
cannot claim that another valence state, such as ReVI, is not
involved.
This is a novel catalytic system that effects the desulfuriza-
tion of episulfides at ambient temperature and pressure. The
reaction is stereospecific and tolerant to functional groups.
This research was supported by the U.S. Department of
Energy, Office of Basic Energy Sciences, Division of Chemical
Sciences, under contract W-7405-Eng-82.
S
+
slow
O
O
O
O
O
Ph3P
S
Me
Re
O
O
Ph3P
S
Scheme 1
to react with H2S. Reaction with 2-mercaptomethyl-
thiophenol23,24 gave a new dimeric yellow rhenium(v) complex
D [eqn. (3)], as reported earlier.25
O
Me
S
SH
MeReO3
Re
(3)
S
Notes and references
S
SH
Re
S
† MTO, propylene sulfide and cyclohexene sulfide were purchased. The
other episulfides were synthesized. NMR spectra were referenced to solvent
peaks: d 7.15 for C6D6 and d 1.93 for CD3CN.
‡ In a typical experiment, hydrogen sulfide was bubbled into 2 mM MTO
in CD3CN for about 5 min, as a black solid separated. The excess of H2S
was removed by sparging with argon, whereupon PPh3 and the episulfide
were added. The reaction was monitored by NMR, and yields obtained
based on proton integration relative to the solvent or to an internal standard
of Ph3CH. The chemical shifts of the olefin products agreed well with
reference data.
§ In a typical experiment, carried out at room temperature, H2S was bubbled
through a 2 mM MTO in CD3CN for 5 min; excess H2S was then removed
by purging with argon. To this solution PPh3 (125 mM) was added, followed
by the thiirane (100 mM). The reaction was followed by 1H NMR at 25 °C.
The reaction also worked very well on a 2 g scale.
Me
O
D
With D in hand it was possible to test the mechanism in
Scheme 1, according to which 2 equiv. of episulfide should be
desulfurized without phosphine. Fig. 2 shows the kinetics of an
experiment with D and a four-fold excess of propylene sulfide.
According to the amounts of material formed and remaining, D
reacts with 2 equiv. of the thiirane, establishing rhenium(v) as
the active catalyst. Furthermore, the reaction can be made
catalytic in rhenium by the addition of PPh3 and the use of much
less D at the outset, although a longer time is required than with
the use of the aforementioned MTO/H2S procedure. The same
rate of desulfurization as given by MTO/H2S was achieved by
bubbling H2S through the dimer solution before the addition of
the episulfide and phosphine (Fig. 2). Addition of H2S changes
the color of the solution from yellow to pink, consistent with
conversion of the ReNO group of D to ReNS; the species
responsible could not be isolated.
The question still remains, regarding the original MTO/H2S
system, of whether the reactions utilize a sulfur analogue of
MTO or a rhenium(v) species that might have been formed in
the system. The black solid obtained on reaction of MTO with
H2S is an oligomeric alkylrhenium sulfide. Transition metal
sulfides, including high oxidation state rhenium sulfides, are
known to oligomerize and some of them have been charac-
terized. 26 When the black solid obtained on reaction with H2S
was treated with propylene sulfide, 4% desulfurization was
observed in 2 h (starting with 5 mM MTO), suggesting the
formation of at least some rhenium(v). The constitution of the
major part was deduced by these experiments. First H2S was
bubbled through 20 mM MTO in CD3CN until all the MTO had
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1
reacted (by H NMR), the excess then being removed by an
argon purge. Its reaction with successive 5 mM increments of
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PPh3 was then monitored by 31P NMR. Only Ph3PNS was
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Fig. 2 Decrease in the concentration of propylene sulfide as followed 1H
NMR: (8) stoichiometric experiment, with [propylene sulfide] = 30 mM,
[D]
= 7.5 mM; (2) catalytic experiment, with [propylene sulfide] =
100 mM, [PPh3] = 125 mM and [D] = 2 mM, at room temperature.
Communication 9/01708I
1004
Chem. Commun., 1999, 1003–1004