2
Tetrahedron
as-prepared Pd/ZrO2 was characterized by XRD, TEM, and
UV–vis.
furnished satisfactory yields. Unfortunately, aliphatic sulfonyl
hydrazides were not adaptable substrates for this protocol,
presumably because of the relative instability of the sulfonyl
radicals generated in situ.
The prepared 3 wt % Pd/ZrO2 was characterized by XRD,
TEM, and UV–vis respectively. First, it is obvious that the
diffraction peaks of the Pd/ZrO2 could correspond to the pure
ZrO2 entirely from Figure 1a, indicating that the structure of ZrO2
remained unchanged after the metal NPs were loaded, which may
be account of the the metal content is inferior to the detection
limit and/or the crystallinity of the surface metal NPs is poor .
Moreover,as can be seen from the TEM image (Figure 1b), the
Pd nanoparticles disperse equally on the surface of the ZrO2.
Finally, UV–vis spectra (Figure 1c) shows that the light
absorption of Pd/ZrO2 is stronger than ZrO2 both in the UV and
visible range, suggesting that the enhanced light absorption arises
from the dispersion of Pd nanoparticles on the ZrO2 surface.
Table 1. Optimization of the reaction conditionsa
Entry
1
Catalyst
Solvent
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
i-PrOH
Dioxane
DMSO
H2O
Yieldb (%)
3%wt Au/ZrO2
3%wt Pd/CeO2
3%wt Pd/ZrO2
4%wt Pd/ZrO2
5%wt Pd/ZrO2
ZrO2
13
70
2
3
92
4
68
5
25
6
10
7
3%wt Pd/ZrO2
3%wt Pd/ZrO2
3%wt Pd/ZrO2
3%wt Pd/ZrO2
3%wt Pd/ZrO2
3%wt Pd/ZrO2
3%wt Pd/ZrO2
3%wt Pd/ZrO2
3%wt Pd/ZrO2
70
8
75
9
65
10
11
12
13
14
15
63
n.r
MeCN
THF
Trace
68
DMF
10
Figure 1. (a) The X-ray diffraction patterns for pure 3 wt %
Pd/ZrO2 (blue) and ZrO2 (black). (b) TEM image of 3 wt %
Pd/ZrO2 (c) UV–vis spectra of pure ZrO2 (black), 3 wt %
Pd/ZrO2 (red).
EtOH
n.rc, n.rd
a
Reaction conditions: 1a (0.6 mmol), catalyst (50 mg), solvent (2 mL), O2,
irradiation under incandescent light (0.4 Wcm-2) at 45oC for 24 h.
First, we explored the photocatalytic activity of Pd NPs
toward sulfonyl hydrazides. To optimize the reaction conditions,
we chose 4-methyl benzenesulfonyl hydrazide (1a) as the
substrate. The reaction was performed in the presence of 3 % wt
Pd/ZrO2 as a photoredox catalyst, O2 as the oxidant and EtOH as
the solvent at ambient temperature under irradiation of
incandescent light (0.4 Wcm-2) for 24 h. To our delight, we
obtained a 92% HPLC yield (Table 1, entry 3) of the desired
thiosulfonate (2a). As shown in Table 1, the application of 4
wt % Pd provided a lower yield of 2a, and 5 wt % Pd gave only a
small amount (Table 1, entries 4 and 5). Pd NPs is nanoparticles
of nonplasmonic transition metals, so, we then used Au/ZrO2,
because it has been well demonstrated to have localized surface
plasmon resonance (LSPR),13 but the result was a poor yield of
2a (Table 1, entry 1). When 3 wt % Pd/CeO2 was used as the
photoredox catalyst, no improvement in the transformation was
observed (Table 1, entry 6). The use of ZrO2 as a catalyst led to a
10% yield of product (Table 1, entry 2). It is important to note
that visible light and a catalyst are essential in the reaction (Table
1, entries 15).
b HPLC yield
c No Light
d No Catalyst
We then screened for the sulfenylation of TsNHNH2 (1a)
under Pd NPs catalysis. Investigations of protic and aprotic
solvents under identical catalysis conditions showed that EtOH
was the best medium for thiosulfonate formation (Table 1, entry
3). A product yield of 92% was eventually obtained in HPLC
with an 80% isolated yield after 24 h.
The advantages of heterogeneous catalysts exist in their good
stability, easy separation and fantastic recyclability. As expected,
the recycled photocatalyst showed good reusability in five cycles
only with a slight decrease in its activity, which was reveled on
Figure 2a.
The time course of the catalytic activities was then tested. As
shown in Figure 2b, the yield of 2a increased with increasing
visible light irradiation time. In the first 3 hours of the reaction,
photogenerated electrons entered the conduction band of Pd and
electron generation on the surface charge transfer complex
improved.14 The yield of 2a increased slowly from the fourth
hour. As shown in Figure 2b, the yield increased over time and
the optimal visible light irradiation time was 12 h.
With these results in hand, we then explored the scope of our
method by changing the aryl sulfonyl hydrazide substrate. It was
possible to convert sulfonyl hydrazides bearing either electron-
donating or electron-withdrawing groups to their corresponding
thiosulfonates with excellent yields. It is noteworthy that the
presence of neutral substituents on the aromatic groups had a
significant effect on the efficiency of the conversion and
Table 2. Scope of Arylsulfonyl Hydrazides Using Pd/ZrO2 as
Photocatalysts