terminal alkenes, producing the corresponding methylketones
in good to excellent yield. Gratifyingly, higher alkenes were
oxidized to the corresponding methylketones without detectable
olefin isomerization, which is considered to be a common side
reaction in Wacker oxidation. It is worth mentioning that the
aryl alkenes with an electron-donating substituent at the phenyl
ring gave higher reactivity towards the Wacker product (entries
8, 10, 12, 14 vs. 6), whereas an electron-withdrawing group led
to decreasing in the yield of the methyl ketone (entry 16). As a
consequence, an electronic effect of substituents on the phenyl
ring had great influence on the reactivity of the double bond.
It is important to note that the aryl chloride was not activated
under these conditions (entry 16).
In conclusion, we showed that EC as an alternative green
solvent is of considerable potential in Wacker oxidations using
oxygen as the sole oxidant. Besides its environmentally benign
characters, the advantages of using EC as the reaction medium
in the present study appear particularly interesting in widened
substrate scope, facile separation of the product, and stabilizing
effect on the colloidal Pd nanoparticle. Further investigation of
organic carbonates as ecofriendly reaction media for catalysis
and organic transformation is currently under investigation in
our laboratory.
Acknowledgements
Financial support of this work from the National Natural
Science of Foundation of China (No. 20421202, 20672054,
20872073) and the 111 project (B06005), and the Committee of
Science and Technology of Tianjin is gratefully acknowledged.
Furthermore, good to excellent yields and selectivity were
◦
attained by just increasing the reaction temperature to 100 C
and elongating the reaction time to 24 h (entries 7, 9, 11, 13, 15,
17, Table 3). Accordingly, to the best of our knowledge, this work
presented herein is the first example of the aryl alkenes subjected
to the cocatalyst-free and/or ligand-free Wacker oxidation
system.
Another benefit of employing EC as the alternative solvent is
the facile separation of the product. After reaction, the product
is very easy to extract with n-hexane (for higher alkenes) or ethyl
ether (for aryl alkenes), and the catalyst is allocated exclusively
in the solid EC phase under the cooling conditions.
Notes and references
‡ General experimental procedure for the palladium-catalyzed direct
O2-coupled Wacker oxiation of 1-dodecene in EC: CAUTION: The
procedure described below is conducted in closed reaction apparatus that
is under pressure. It must only be carried out by using the appropriate
equipment and under rigorous safety precautions. The reaction was
carried out in a 50 mL stainless-steel autoclave with an inner glass and
a magnetic stirrer. A mixture of PdCl2 (1.77 mg, 0.01 mmol), NaOAc
(4.1 mg, 0.05 mmol), and 3 mL EC was placed in the inner glass and
stirred at 50 ◦C for 30 min. Then, the substrate (0.5 mmol) and H2O
(0.01 mL), 1 MPa of O2 were successively introduced. The mixture was
stirred for 12 h at 60 ◦C. After the reaction, the reactor was quickly
cooled to 0 ◦C in ice water. The excess of O2 was depressurized slowly.
The resultant mixture was then extracted with an appropriate solvent
(n-hexane was used for higher alkenes, and ethyl ether for aryl alkenes)
(5 mL ¥ 3), dried over MgSO4. The solvent was removed, and the residue
was subjected to column chromatography with ethyl acetate/petroleum
(1 : 20 to 1 : 10) as eluent to obtain the desired product.
Among the reported Wacker oxidations using oxygen as the
sole oxidant, Fujimoto and Kunugi19 reported that the addition
of a material with high surface area, such as active charcoal,
could increase the reaction rate substantially by dispersing the
Pd(0) and facilitating its reoxidation. It is also worth mentioning
that PC could be used to stabilize the colloidal Pd nanoparticle
for the ligand-free Heck type reaction.13e To further gain insight
into the effect of EC on the present Wacker process, we set
out to verify the involvement of palladium nanoparticles in
our catalyst system. Indeed, transmission electron microscopy
(TEM) examination of a mixture of PdCl2/NaOAc in EC stirred
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◦
for 30 min at 50 C, revealed the presence of highly dispersed
colloidal Pd nanoparticles (Fig. 1, left). After reaction, the
nanoparticles began to cluster, resulting in formed agglomerates
with less surface area, which presumably accounts for the
lower catalytic activity (Fig. 1, right), being consistent with the
experimental result of the second run of the catalyst system using
1-dodecene as a model substrate under the identical reaction
conditions giving 28% yield of dodeca-2-one (entry 5, Table 3).
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Fig. 1 TEM images of colloidal Pd nanoparticle: (a) before reaction,
(b) after reaction.
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