Hui Xu et al. / Chinese Journal of Catalysis 41 (2020) 1468–1473
1471
tron-withdrawing groups displayed greater activity than those
possessing electron-donating groups. Finally, anion C combined
with H+ to form H2O2. The synthesis of asymmetric disulfides
proceeded smoothly, wherein mercaptan 3 rapidly performed a
nucleophilic attack on the newly formed symmetric disulfide
with the assistance of TEA.
Table 4
Visible-light photocatalytic oxidative synthesis of asymmetric disulfides
with O2 on anatase TiO2.
In summary, we have developed a visible-light-induced
strategy for the aerobic oxidation of thiols to disulfides. In this
context, anatase TiO2 served as a multifunctional platform, not
only for complexation with thiols, but also for electron transfer
to form O2•‒. When various thiols were examined, the method
exhibited excellent reaction selectivity, functional group toler-
ance, and substrate scope with high isolated yields (> 90%).
Notably, the substrates with electron-withdrawing groups
could fulfil the transformation in less than 10 min. The concise
design contributed to the atom-economic and energy-saving
concepts of sustainable chemistry. Simultaneously, it was also
an efficient strategy for the synthesis of some asymmetric di-
sulfides with TEA as an additive.
N
S
S
4m,94%*
Reaction conditions: benzenethiol (0.6 mmol), t-butylthiol (1.8 mmol),
TEA (2.0 mmol), anatase TiO2 (ST-01, 30 mg), 520 nm green LEDs (3 W
× 4), CH3CN (1 mL), O2 (0.1 MPa), 5 min. * Reaction time, 2 h.
electronic or steric effects exhibited minimal loss to the present
system, as seen by their corresponding yields. In the case of 4k,
we also examined 2k as a substrate under dark conditions
lacking TiO2 wherein 4k was found at 98% yield. This finding
suggested that the symmetric disulfide was not thermodynam-
ically stable and therefore could transform into the asymmetric
disulfide in the presence of mercaptan 3 and base. As for prod-
uct 4m, the electrical properties of the N atom had a negative
effect on the nucleophilic substitution between 2m and 3. Fi-
nally, this asymmetric synthesis was also compatible with thiol
1n.
A plausible mechanism for the visible-light photocatalytic
selective oxidation of thiols to disulfides on TiO2 is depicted in
Scheme 2. It was assumed that prior to photocatalysis, the thiol,
a heteroatom substrate, was easily adsorbed onto the surface of
TiO2 via weak coordination at the initial stage. Then, visi-
ble-light irradiation motivated an electron from the thiol to be
transferred to the conduction band (cb) of TiO2 while circum-
venting the valence band (vb). At the same time, sulfur radical
A and H+ were released. The unpaired electron was easily
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Scheme 2. Visible-light photocatalytic selective oxidation of thiol into
disulfide with O2 on anatase TiO2.