Oxidation of Thiols to Disulfides with Molecular Oxygen
309
an increase in substrate solubility suggest that subcritical water may be useful as a
replacement for environmentally unacceptable solvents for a number of organic
reactions [21]. The oxidation of thiols to disulfides in subcritical water were carried
ꢀ
out as isothermal experiments at 100 C using different amounts of oxygen
(
Scheme 1). As shown in Table 1, all thiols converted to the corresponding di-
sulfides, which were isolated in excellent yields. Because aliphatic and aromatic
thiols were oxidized to disulfides in the absence of an organic solvent and metal
salts=complexes, our method is indeed very green chemistry.
The amount of dissolved oxygen in water at atmospheric pressure was deter-
ꢀ
mined according to Henry’s law [28] (at 25 C the solubulity of O is of
2
À3
1
.296 Á 10 mol=kg H O). This value was used initially as shown in the Table 1
2
as entry A. The amount of oxygen was regulated by its pressure. All oxidations were
3
performed by adding 280 cm water, one equivalent of substrate, and two equivalents
oxygen. Since longer reaction times did not improve the yields, the time was opti-
mized at 2 h for all oxidations. On the other hand, when oxygen pressure was
increased, the yields were not increased beyond 20 bar; an increase in the
amount of oxygen led to decomposition of the starting materials converting them
into tars. All products were characterized by their spectral data and comparison
with reported data. The high-pressure and high-temperature system was used in
all reactions.
In conclusion, we developed a practical procedure for the oxidation of thiols to
their corresponding disulfides in subcritical water with molecular oxygen in the
absence of metal catalysts.
Experimental
+
Mps were determined on an Electrothermal 9100 apparatus. IR spectra were recorded on a Win First
+
+
1
Satellite model spectrophotometer. H NMR spectra were obtained using a 400 MHz Bruker DPX
instrument.
General Procedure
ꢀ
Oxidations were caried out at 100 C in a stainless steel pressure reactor equipped with N and O inlet,
2
2
pressure gauge, safety valve, digital temperature reader, heater, and magnetic stirrer. The total pressure
was kept at 40 bar by N . A glass vessel was inserted into the reactor to avoid the catalytic effect of
2
3
steel and corossion. The reactor was charged with thiol and 280 cm H O. All the valves of the reactor
2
were tightly closed during preheating. N was supplied through a tube into the liquid phase directly.
2
Then the desired oxygen pressure was applied to the vessel through a stainless steel tube into the liquid
phase directly, and the total reaction time was 2 h. After the reaction was completed, the reactor was
3
cooled to room temperature and the reaction mixture was extracted with ether (3Â15 cm ). The
combined organic layer was dried (MgSO ) and evaporated on a rotary evaporator under reduced
4
pressure. Then the product was chromatographed over silica gel using ethyl acetate=n-hexane (1=4) as
eluent. Evaporation of the solvent gave fairly pure solids, which were crystallized; the melting point of
1
solid compounds was checked and the solids and oils were identified by IR and H NMR.
Acknowledgement
We are greatful to Mersin University Research Council and TUBITAK (The Scientific and Technical
Council of Turkey) for supporting this work (Grant No: TBAG-2235).