Wang and Jiang
JOCArticle
SCHEME 1. Progression of Strategies for Dihydroxylation and
Oxidative Cleavage of Olefins
SCHEME 2. Palladium-Catalyzed Oxidative Cleavage of
1-Methoxy-1,2-diphenylethene in MeOH
TABLE 1. Optimization of Reaction Conditions for the Palladium-
Catalyzed Oxidative Cleavage of trans-Stilbene
a
b
entry
Pd cat.
additive
solvent
yield (%)
1
2
3
4
5
6
7
8
9
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
2
2
2
2
2
2
ZnCl
2
MeOH
MeOH
DMSO
DMF
toluene
t-BuOH
trace
36
trace
trace
35
38
43
79
1
2
oxidants are still used extensively. Therefore, the develop-
ment of alternative protocols for alkene dihydroxylation and
cleavage that would be cleaner and amenable to large-scale
synthesis would constitute a breakthrough in both green
chemistry and organic synthesis.
H
H
H
H
H
H
H
2
O
2
O
2
O
2
O
2
O
2
O
2
O
CH COOH
3
PTSA
CH SO
CF COOH
88 (85)
81
77
1
1
1
1
0
1
2
3
3
3
H
3
PdCl
Pd dba
2
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
14
On the other hand, the utilization of molecular oxygen as
oxidant is one of the most important goals in oxidation
chemistry. Palladium-catalyzed aerobic oxidation reactions
have been shown to be highly versatile in preparing fine
chemicals. In particular, the development of the Wacker
process in the late 1950s had a significant impact on the
chemical industry. Herein, we report an efficient oxidation
of a wide range of olefins to 1,2-diols, aldehydes, or ketones
catalyzed by palladium with O as the sole oxidant. These
2
3
trace
16
21
8
0
14
15
16
17
PdI2
2
H O
1
3
Pd(NO
Pd/C
3
)
2
H
H
H
2
O
2
O
2
O
1
4
a
Reaction conditions: All reactions were performed with 1a (1 mmol),
pressure of O (8 atm), Pd catalyst (2 mol %), and additive (20 mol %) in
1
5
2
b
3
mL of solvent for 24 h, 100 °C. Determined by GC analysis of crude
reaction mixture with naphthalene as an internal standard; the number in
parentheses is the yield of isolated product based on complete alkene
consumption.
2
reactions constitute an economically attractive and environ-
mentally friendly synthesis of 1,2-diols and carbonyl com-
pounds and may be suitable for large-scale operations.
in control experiments that the cleavage reaction of 1-methoxy-
1,2-diphenylethene in MeOH afforded methyl benzoate
Results and Discussion
16
Scheme 2). This has encouraged us to explore the oxidative
(
cleavage reactions of olefins. We examined the cleavage reaction
of trans-stilbene under different reaction conditions, and the
results are summarized in Table 1. As expected, a combination
Palladium-Catalyzed Oxidative Cleavage of Alkenes with
Oxygen. In 2008, we disclosed a Lewis acid-promoted and
palladium-catalyzed oxidative cleavage of a carbon-carbon
of Pd(OAc) with ZnCl in MeOH, the most effective system for
2
triple bond with O in which alkyne is split into carboxylic
2
2
16
the cleavage reaction of alkynes, led to the formation of methyl
benzoate with a trace of benzaldehyde (Table 1, entry 1). With
the goal of optimizing the reaction for the formation of ben-
zaldehyde, ZnCl was removed and the desired product was
2
obtained in 36% GC yield (Table 1, entry 2).
Subsequently, a series of other solvents were evaluated,
ester in alcohols. As a starting point for this work, we observed
(
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and H O provided the best result (Table 1, entries 2-7). It is
2
worth mentioning that the use of water as solvent is more
advantageous than the use of organic solvent for environ-
mental considerations. Further exploration revealed that the
yield could be improved when a Brønsted acid was utilized as
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1
7
the additive in the reaction. Among the Brønsted acids
investigated, PTSA (p-toluenesulfonic acid monohydrate)
was the best choice (Table 1, entries 8-11). Different palla-
dium species were also tested, such as PdCl , Pd dba , PdI ,
(
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322 J. Org. Chem. Vol. 75, No. 7, 2010