Letter
reaction is not as good as that of DMF (entries 7 and 8). In
addition, changing the temperature and the amount of K2S
cannot improve the yield (Table 1, entries 9−12). Intriguingly,
when K CO was removed from the reaction mixture, the yield
2
3
increased to 99% (Table 1, entry 13). Finally, we want to find
the optimal reaction time (Table 1, entry 14). When the
reaction time was decreased to 12 h, the material remained,
and the target product was obtained with an isolated yield of
8
6%. After a series of optimizations of the conditions, the
optimized reaction conditions were obtained: ethene-1,1-
Figure 1. Fe−S cluster and Fe−S compelx catalyst.
diyldibenzene 1a (0.3 mmol), K S (0.6 mmol), and FeCl
2
3
(
20 mol %) in DMF (2 mL) at 80 °C for 14 h.
With the optimal reaction conditions in hand, the substrate
S3• was discovered more than 40 years ago and can be
−
scope for the synthesis of 2 was investigated. First, we
investigated the reaction activity of 1,1-diarylethylene sub-
strates. The results summarized in Scheme 2 show that 1 with
easily formed by the reaction of elemental sulfur with KOH in
19
•−
DMF at room temperature. Recently, applications of S3
species for the synthesis of organic compounds have gradually
increased, but almost all of them are used in the synthesis of
organic sulfur compounds (Scheme 1c).
2
0
Scheme 2. Substrate Scope of 1,1-Diaryl-Substituted
The new
a−c
Alkenes
applications in the conversion of other functional groups
involving S3• have not yet been reported. It is well-known that
iron−sulfur proteins can be used as beneficial redox catalysts
−
21
involving electron transfer. Encouraged by research about the
21a
“iron−sulfur world” hypothesis,
we hypothesize that a
system similar to iron−sulfur protein can accelerate the
selective aerobic oxidation of terminal alkenes. Herein, we
demonstrate an iron−sulfur complex, formed by the selective
aerobic oxidation of terminal alkenes to carbonyl compounds
•
−
(
Scheme 1d) with a simple mixture of FeCl with S
3 3
generated in situ from K S, promoted the reaction.
2
Initially, ethene-1,1-diyldibenzene 1a was treated with K2S
(
2.0 equiv), K CO (1.0 equiv), and FeCl (20 mol %) in
2
3
3
DMF at 80 °C for 8 h. From this reaction, benzophenone 2a
was formed in 91% yield (Table 1, entry 1). To improve the
efficiency, we tried other diffirent catalysts, changed the
amount of catalyst, and found that FeCl (20 mol %) has the
3
best catalytic effect (Table 1, entries 2−6). Next, DMSO and
DMA were tried as the reaction medium, but the effect on the
a
a
Table 1. Optimization of the Reaction Conditions
Raction conditions: 1 (0.30 mmol), K S (0.60 mmol), FeCl (20
2 3
b c
mol %), DMF (2 mL), O (1 atm), 80 °C, 14 h. Isolated yields. 1
2
(
(
0.30 mmol), K S (1.20 mmol), FeCl (40 mol %), DMF (2 mL), O
1 atm), 80 °C, 14 h.
2
3
2
b
just one electron-donating group (EDG) such as methyl,
phenyl, or methoxy or an electron-withdrawing group (EWD)
such as chloro, bromo, or trifluoromethyl at the para or meta
positions took part in this reaction, smoothly affording 2d−2k
in good yields. However, when there is a substituent in the
ortho position, the yield is significantly reduced, and target
products 2b and 2c are obtained in moderate yields. In
addition, when there are two substituents on the substrate,
whether the substituents are on the same benzene ring or on
two benzene rings, the target product (2l−2n) can be obtained
with yields of 88−96%. To our delight, 1,4-bis(1-phenylvinyl)-
benzene can be converted into target product 2o in 86% yield
under the action of K S and FeCl . Notably, when there are
two kinds of alkenes in the reactants, they can selectively
oxidize aryl-substituted alkenes. For example, 1-(allyloxy)-4-(1-
phenylvinyl)benzene can selectively afford target product 2p in
a good yield of 84%.
Next, other substituted alkenes have also been studied, and
the results are listed in Scheme 3. First, when 1,1-arylalkyl-
substituted alkenes were used as substrates, the effect on the
entry
catalyst (mol %)
temp (°C)
K S (equiv)
yield (%)
2
1
2
3
4
5
6
FeCl (20)
80
80
80
80
80
80
80
80
60
100
80
80
80
80
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
−
91
84
trace
84
37
trace
83
51
70
88
38
3
Fe(OAc) (20)
2
Cu(OAc) (20)
2
FeCl (10)
3
FeCl (5)
3
−
c
7
d
FeCl (20)
3
8
FeCl (20)
3
9
FeCl (20)
3
1
1
1
1
1
0
1
2
3
4
FeCl (20)
3
FeCl (20)
3
2
3
FeCl (20)
NR
99
86
3
e
f
FeCl (20)
2.0
2.0
3
FeCl (20)
3
a
Raction conditions: 1a (0.30 mmol), K S (0.60 mmol), catalyst (20
mol %), K CO (0.3 mmol), solvent (2 mL), O (1 atm) 14 h.
Isolation yields. The solvent is DMA. The solvent is DMSO.
2
2
3
2
,
b
c
d
e
f
Without K CO . Time of 12 h.
2
3
4
706
Org. Lett. 2021, 23, 4705−4709