Wacker-type oxidation and dehydrogenation of terminal olefins using molecular oxygen as the sole oxidant without adding ligand

Article abstract of DOI:10.1021/ol500218p

An efficient and economical palladium-catalyzed oxidation system has been identified. The oxidation system, characterized by not adding ligand and using molecular oxygen as the sole oxidant, can realize the Tsuji-Wacker oxidation of terminal olefins and especially styrenes to methyl ketones; in addition, this system can achieve tandem Wacker oxidation-dehydrogenation of terminal olefins to α,β-unsaturated ketones.

Full text of DOI:10.1021/ol500218p

Letter
pubs.acs.org/OrgLett
Wacker-Type Oxidation and Dehydrogenation of Terminal Olefins
Using Molecular Oxygen as the Sole Oxidant without Adding Ligand
Yu-Fei Wang, Ya-Ru Gao, Shuai Mao, Yan-Lei Zhang, Dong-Dong Guo, Zhao-Lei Yan, Shi-Huan Guo,
and Yong-Qiang Wang*
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of Chemistry &
Materials Science, Northwest University, Xi’an 710069, P. R. China
S
* Supporting Information
ABSTRACT: An efficient and economical palladium-catalyzed oxidation system
has been identified. The oxidation system, characterized by not adding ligand and
using molecular oxygen as the sole oxidant, can realize the Tsuji−Wacker
oxidation of terminal olefins and especially styrenes to methyl ketones; in
addition, this system can achieve tandem Wacker oxidation−dehydrogenation of
terminal olefins to α,β-unsaturated ketones.
Since styrenes are valuable but challenging substrates for
Tsuji−Wacker oxidation, we first chose them to study the
reaction. In preliminary experiments, styrene was treated with
10 mol % of Pd(OAc)2 and trifluoroacetic acid (TFA, 8 equiv)
in DMSO/H2O (10:1) under an oxygen atmosphere (O2
balloon) at room temperature for 24 h. We found the
conversion of the reaction was low (<30%)(Table 1, entry
1). Increasing the temperature could greatly improve the
reaction rate, but higher temperature led to a significant drop in
the yield due to the decomposition of starting material and
product (entries 2−4). Changing the solvent did not enhance
reactivity (entry 5). By investigating the amount of TFA, we
found that 1 equiv of TFA was the optimal amount to afford
the desired product in 86% yield, and in the absence of
trifluoroacetic acid, the reaction would proceed with much low
conversion (entries 6−9). Using other palladium sources and
acids, the yield of acetophenone could not be improved (entries
10−14). When air was used as oxidant instead of oxygen, the
reaction rate and the yield obviously dropped (entry 15). When
the loading of Pd(OAc)2 was lowered, e.g., to 5 mol %, the
reaction was slow and would proceed incompletely (entry 16).
Accordingly, the reaction conditions were optimized as follows:
Pd(OAc)2 (10 mol %), TFA (1 equiv) under oxygen
atmosphere in DMSO/H2O (10:1) at 70 °C.
After identifying the optimized conditions, we examined the
scope of stryenes (Table 2). We were pleased with the
generality of this method. Styrenes possessing either electron-
withdrawing groups or electron-donating groups were found to
smoothly react and provided acetophenones in good to
excellent yields (76−95%, entries 1−8). The position of the
substituted group on the phenyl ring almost did not affect the
reactivity (entries 4−6). Sterically hindered 2,4,6-trimethylstyr-
ene also readily converted into 2,4,6-trimethylacetophenone in
76% yield (entry 9). In all cases, the reaction was of complete
he palladium(II)-catalyzed oxidation of terminal olefins to
Tmethyl ketones, known as the Tsuji−Wacker oxidation, is
one of the most important reactions that has extensive synthetic
applications in laboratories and industries.1−4 Classically, the
reaction is conducted with catalytic PdCl2 and stioichiometric
CuCl under aerobic atmosphere. Palladium decomposition,1b
olefin isomerization,2c,5 chlorinated byproducts,1b and copper
waste have been recognized as considerable limitations for
oxidation.6 To address these limitations, many ligands of
palladium have been developed,2d,7 and many oxidants, such as
nitrous oxide,8 H2O2,9 1,4-benzoquinone (BQ),10 pressurized
oxygen,11 and tert-butyl hydroperoxide (TBHP),7c,12 have been
used. These modified methods undoubtedly widen the range of
application of the Tsuji−Wacker oxidation. However, they also
increase the cost of the oxidation and demand higher practical
operation skills for the oxidation. Therefore, it is still highly
desirable to develop a facile and efficient Wacker-type oxidation
that does not rely on the ligand and uses ambient-pressure
oxygen as the sole oxidant.
Acetophenones, as valuable intermediates for the manufac-
ture of a variety of pharmaceuticals, perfumes, resins, pesticides
and other chemical products, can be reasonably obtained from
styrenes by Tsuji−Wacker oxidation.13 Nevertheless, styrenes
are problematic substrates for the reaction due to facile
polymerization and oxidative cleavage to benzaldehydes and/or
benzoic acids.14 In general, molecular oxygen can be applied for
the Tsuji−Wacker oxidation of alkyl-substituted terminal
olefins,7d while a stronger oxidant, such as TBHP, is required
for such oxidation of stryenes.12b,15 A breakthrough was
obtained by Reiser et al. in 2010, who employed chiral
pseudo-C2-symmetrical bis(isonitrile) as ligand to realize
palladium-catalyzed Tsuji−Wacker oxidation of styrenes using
molecular oxygen as oxidant. Herein, we report a palladium(II)-
catalyzed Tsuji−Wacker oxidation of olefins and especially
styrenes to methyl ketones using ambient-pressure oxygen as
the sole oxidant without introducing any extra ligand.
Received: January 21, 2014
Published: March 7, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions
Table 2. Substrate Scope of the Palladium-Catalyzed Wacker
Oxidant Reaction
a
f
entry
cat.
acid (equiv) temp (°C) time (h) yield (%)
1
2
3
4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(TFA)2
PdCl2
TFA (8)
TFA (8)
TFA (8)
TFA (8)
TFA (8)
TFA (5)
TFA (1)
TFA (0.5)
rt
50
70
90
70
70
70
70
70
70
70
70
70
70
70
70
24
12
12
12
12
12
12
12
12
12
12
12
12
12
24
24
25
66
74
54
b
5
<56
81
6
7
8
86
77
c
9
<10
79
10
11
12
13
TFA (1)
TFA (1)
TFA (1)
TFA (1)
Acids (1)
TFA (1)
TFA (1)
nd
30
Pd(OH)2
Pd(PPh3)2Cl2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
nd
<45
57
d
14
e
15
g
h
16
48
a
Reaction conditions: 1a (1 mmol), catalyst (0.1 mmol), O2 (1 atm),
and acid in DMSO/H2O (10:1, 5 mL) at the specified temperature.
b
Solvents used instead of DMSO: EtOAc, DMF, THF, n-hexane,
toluene, acetone, CH3CN, dioxane, Et2O, CH2Cl2, CHCl3, CH3NO2,
c
d
EtOH. No acid. HCO2H, AcOH, PhCO2H, tartaric acid, p-
toluenesulfonic, salicylic acid, boric acid, oxalic acid, maleic acid,
e
f
g
fumaric acid. Using air as oxidant. Isolated yield. Pd(OAc)2 (5 mol
h
%) was used. The conversion was 52%.
Markonikov selectivity, and no benzaldehydes or polymeric
products could be detected by analyzing the reaction mixtures.
Aliphatic terminal olefins were also compatible with the
reaction to provide the products in good yields and complete
Markonikov selectivity (entries 10−14). In addition, under the
standard reaction conditions, the reaction of aliphatic olefins
was obviously faster than that of styrenes.
To our surprise, when 4-phenyl-1-butene was used as
substrate, we attained a sole product, trans-4-phenyl-3-buten-
2-one in 87% yield (Scheme 1). This means that the
Pd(OAc)2−TFA oxidative system has more potential to realize
deeper oxidation of the products to generate α,β-unsaturated
ketones. It is well-known that α,β-unsaturated ketones are
versatile synthetic intermediates that are frequently prepared via
multistep routes,16 for instance, α-bromination−dehydrobromi-
nation,17 sulfoxide or selenoxide elimination,18 Saegisa
oxidation,19 etc. In 2012, Zhao et al. and Stahl et al.,
respectively, reported Pd(II)-catalyzed aerobic dehydrogen-
ations of carbonyl compounds to form corresponding α,β-
unsaturated carbonyl compounds.20 In the two studies, the
same ligand of palladium, 4,5-diazafluorenone, was exploited.
More recently, White et. al. reported an elegant tandem Wacker
oxidation−dehydrogenation to generate α,β-unsaturated ke-
tones directly from terminal olefins.21 In the approach, they
used Pd(CH3CN)4(BF4)2 (10 mol %) and PhI(OAc)2 (25 mol
%) as cocatalyst and 1,4-benzoquinone (2 equiv) as oxidant.
Inspired by this research and our above findings, we intended
to develop a palladium-catalyzed tandem Wacker oxidation−
dehydrogenation of terminal olefins using molecular oxygen as
the sole oxidant and without the need of extra ligand. To our
delight, the Pd(OAc)2−TFA oxidative system was indeed able
to accomplish the conversion (Table 3). 4-Aryl-1-butenes,
a
Reaction conditions: alkene (1 mmol), Pd(OAc)2 (0.1 mmol), O2 (1
atm) and TFA (1 mmol) in DMSO/H2O (10:1, 5 mL) at 70 °C.
b
c
d
Isolated yield. 10 mmol of styrene was used. At 100 °C.
bearing an electron-withdrawing or electron-donating sub-
stituent on aromatic ring, all smoothly underwent tandem
Wacker oxidation−dehydrogenation to deliver the desired α,β-
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Organic Letters
Letter
Scheme 1. Palladium-Catalyzed Tandem Wacker
Scheme 2. Plausible Mechanism of the Reaction
Oxidation−Dehydrogenation of 4-Phenyl-1-butene
Table 3. Substrate Scope of the Reaction of Terminal Olefins
a
to α,β-Unsaturated Ketones
the formation of palladium enolate followed by β-hydride
elimination to afford the α,β-unsaturated ketone,20,24 and the
Pd0 can be reoxidized to Pd(O2CCF3)+ by O2 in the presence
of TFA to complete the catalytic cycle.25
In summary, we have identified a simple, efficient, and
economical Pd(II)-catalyzed ligandless oxidation system using
molecular oxygen as the sole oxidant at ambient pressure. The
catalytic system can oxidize terminal olefins and especially
styrenes to methyl ketones. The reaction is of complete
Markonikov selectivity, and no aldehydes or polymeric
products can be detected. The catalytic system can also
facilitate tandem Wacker oxidation−dehydrogenation to
generate α,β-unsaturated ketones directly from terminal olefins.
The method should have many applications in synthetic
chemistry. Studies of the detailed mechanism and applications
of this catalytic system are in progress.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures including spectroscopic and
analytical data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: wangyq@nwu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (NSFC-20972126, 21272185),
the Program for New Century Excellent Talents in University
of the Ministry of Education China (NCET-10-0937), and the
Education Department of Shaanxi Provincial Government
(09JK776).
a
Reaction conditions: alkene (1 mmol), Pd(OAc)2 (0.1 mmol), O2 (1
atm), and TFA (1 mmol) in DMSO/H2O (10:1, 5 mL) at 70 °C.
b
c
Isolated yield. Yield in parentheses was obtained with 10 mmol of 4-
d
e
phenyl-1-butene. At 100 °C. TFA (0.5 mmol) was used; piperidine
(0.1 mmol) was added.
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