Catalyst-Free Selective Oxidation of Diverse Olefins to Carbonyls in High Yield Enabled by Light under Mild Conditions

Article abstract of DOI:10.1021/acs.orglett.9b02569

The selective oxidation of olefins, in particular, aromatic olefins to carbonyls, is of significance in organic synthesis. In general, stoichiometric toxic oxidants or a high-cost catalyst is required. Herein we report a novel and practical light-enabled protocol for the synthesis of carbonlys in high yield through a catalyst-free oxidation of olefins using H₂O₂ as a clean oxidant. A broad scope of carbonyls can be synthesized in high yield, and no catalyst or toxic oxidant is required.

Full text of DOI:10.1021/acs.orglett.9b02569

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Catalyst-Free Selective Oxidation of Diverse Olefins to Carbonyls in
High Yield Enabled by Light under Mild Conditions
Weiwei Yu and Zhongkui Zhao*
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P. R.
China
S
* Supporting Information
ABSTRACT: The selective oxidation of olefins, in particular,
aromatic olefins to carbonyls, is of significance in organic
synthesis. In general, stoichiometric toxic oxidants or a high-
cost catalyst is required. Herein we report a novel and
practical light-enabled protocol for the synthesis of carbonlys in high yield through a catalyst-free oxidation of olefins using
H2O2 as a clean oxidant. A broad scope of carbonyls can be synthesized in high yield, and no catalyst or toxic oxidant is
required.
these oxidants not only are expensive and toxic in themselves
he selective oxidative scission of olefins into correspond-
Ting carbonyls is a widely practiced transformation in but also a large amount of waste is yielded, with some being
highly toxic.
synthetic chemistry, and the corresponding products (e.g.,
benzaldehyde, benzophenone, etc.) are important intermedi-
ates for pharmaceuticals, fragrances, agrochemical chemicals,
food additives, and other chemical industries.1 It is surprising
and regrettable that to date, this transformation is still
commonly practiced by using an ozonolysis method, despite
the serious safety issues with the use of ozone (O3) as an
oxidant, the high requirements for reaction equipment, and the
large amount of waste generated during the two-step
production process (Scheme 1a).2 To overcome the problems
from the ozonolysis process, a chemical oxidation with
stoichiometric oxidants, such as meta-chloroperoxybenzoic
acid, KMnO4, CrO2Cl2, PhIO/HBF4, and OsO4, has been
considered as an effective alternative (Scheme 1b).3 However,
In recent years, great attention has been devoted to the
development of various catalyst systems (homogeneous and
heterogeneous catalysts) to selectively cleave CC double
bonds of olefins with clean oxidants (O2 and H2O2) driven by
thermal energy4 or economic and clear light energy5 (Scheme
1c). Compared with the conventional chemical oxidation
method, catalytic oxidation features a promising green protocol
because both O2 and H2O2 are atom-efficient, easy to handle,
and safe and produce H2O as the solo byproduct.6 Never-
theless, the use of catalysts (especially homogeneous catalysts)
typically suffers from the drawbacks regarding the toxic metal
contaminants left in products, the separation and recyclability
of precious catalysts, the oxygen sensitivity, the nontrivial
preparation, and so on.7 In particular, the trace toxic metal
residue is totally unacceptable in the pharmaceuticals and food
additives. Compared with homogeneous catalysts, although
heterogeneous catalysts show their advantages in solving the
problem concerning catalyst separation and reuse, they are
generally inferior to homogeneous catalysts in activity and
selectivity. Moreover, the heterogeneous catalysts applied to
these reactions generally require the participation of metals or
metal oxides and even some precious metals, which
undoubtedly increases the cost.1e,8 In addition, the preparation
process of catalysts also leads to resource waste and
environmental pollution. It was initially discovered that the
oxidative cleavage of styrene to benzaldehyde could take place
under UV-light irradiation. Unfortunately, only a very low yield
(only 29%, with 91% selectivity for styrene for 12 h) was
obtained.9 Therefore, developing a new, efficient, and practical
catalyst-free strategy for the selective oxidation of olefins is of
great significance but still remains a challenge.
Scheme 1. Background of the Research
Received: July 23, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02569
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Organic Letters
Letter
a
Herein we conceived a novel and efficient catalyst-free light-
driven strategy in which the produced active oxygen species
from H2O2 selectively oxidatively cleaves olefins to their
corresponding carbonyls in high yield at room temperature
(Scheme 1d). This protocol completely overcomes all
problems that appear in the presence of catalysts and lays a
solid foundation for the large-scale industrial production of
carbonyls, one kind of widely practiced compound in many
chemical industries. Moreover, this catalyst-free light-enabled
protocol has great potential to be extended to the practiced
synthesis of other types of useful compounds through the
oxidative activation process.
Table 2. Substrate Scope of Olefins
At the outset, we began our studies with styrene 1a as a
model substrate listed in Table 1. To our delight, styrene can
Table 1. UV-Light-Initiated Catalyst-Free Selective
Oxidation of Styrene under Different Reaction Conditions
a
entry
solvent
CH3CN
THF
EtOH
acetone
CH3CN + H2O
CH3CN
CH3CN
CH3CN
time (h)
conv. %
sel. %
1
2
3
4
5
6
7
4
4
4
4
4
6
8
8
55
8
5
26
30
79
96
93
95
84
74
94
93
95
b
8
a
Reaction condition unless otherwise noted: 0.1 mmol 1a, 1.2 mmol
H2O2, 1 mL of solvent, air, 20 °C, UV light determined by gas
b
chromatography (GC). Without light irradiation. Note: 0.6 mL of
CH3CN + 0.4 mL of H2O.
be successfully converted into benzaldehyde 1b with 55%
conversion and 93% selectivity under light irradiation if
acetonitrile (CH3CN) is chosen as an efficient reaction
medium (entry 1). However, the conversion of styrene is
very low in tetrahydrofuran (THF) and ethanol (EtOH)
solutions (entries 2 and 3), only 8 and 5%, respectively.
Compared with CH3CN, acetone as the solvent leads to not
only a much lower conversion but also a dramatically lower
selectivity from 93 to 74% (entry 4). Introducing H2O into
CH3CN reduces the conversion of styrene but maintains the
selectivity (entry 5). Thus choosing an appropriate solvent is
essential for the catalyst-free light-enabled oxidative cleavage of
CC double bonds of olefins, and CH3CN can be a
promising solvent. By optimizing the reaction time (Figures
S1 and S2 and entries 6 and 7 of Table 1), 96% conversion of
1a with 95% selectivity of 1b has been achieved. Note that
without light irradiation, no product is obtained (entry 8),
suggesting that the oxidation ability of H2O2 itself is not
sufficient to cleave CC double bonds of olefins and UV light
is indispensable.
a
Reaction condition unless otherwise noted: 1 mmol substrate, 12
mmol H2O2, 10 mL of CH3CN, air, 20 °C, UV light, determined by
GC.
toward carbonyls has been achieved. Regardless of the type
(−F, −Cl, −Br) and position (ortho, meta, para) of halogen
groups with electron-withdrawing capacity, the halogenated
styrenes 2a−6a can be oxidized to corresponding halogenated
aldehydes 2b−6b in good GC yield (81−97%). Notably, aryl
olefins bearing an electron-donating group (−CH3, 7a) can
also proceed with the catalyst-free oxidation reaction to form
4-methylbenzaldehyde 7b. 3-Phenylpropylene 8a is also well
From above, a novel, effective, and clean light-enabled
strategy concerning the selective oxidative cleavage of CC
double bonds of olefins to their corresponding carbonyls under
simple, catalyst-free, and mild conditions has been established.
Next, we explored the substrate scope with respect to diverse
olefins (Table 2). The developed catalyst-free light-driven
strategy works well on diverse substrates, and a high selectivity
B
DOI: 10.1021/acs.orglett.9b02569
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Organic Letters
Letter
tolerated and can be translated into phenylacetaldehyde 8b in
36% GC yield. Moreover, delightfully, two typical α-
substituted styrenes including 2-phenyl-1-propene 9a and
1,1-diphenylethylene 10a are successfully converted to
acetophenpone 9b and benzophenone 10b in 86 and 95%
GC yield, respectively. In a word, we have successfully
developed a novel, effective, green, and practical strategy to
produce carbonlys through the selective oxidative cleavage of
CC double bonds of olefins with a broad substrate scope
under mild conditions, and no any catalyst or toxic oxidant is
required. In addition, the isolated yields (isol. yield) of various
products are shown in Table 2, and the experimental details are
shown in the Supporting Information. The satisfactory isol.
yield concerning the transformation of various substrates can
be obtained by the developed synthetic method.
without the characteristic peaks of DMPO−·O2− adducts.
Furthermore, 1a cannot be oxidized without H2O2 under an
oxygen atmosphere (entry 6). The two results are contrary to
the results of a controlled experiment with BQ as the
scavenger. The reason that the introduction of BQ reduces
the conversion of 1a is that the brown-yellow CH3CN solution
of BQ (Figure S5a) has a strong absorption of UV light (Figure
S5b). Therefore, the conversion of 1a significantly decreased in
the controlled experiments with BQ not because ·O2− in the
reaction system was captured by BQ but because of the
absorption of UV light by BQ, which leads to the significant
reduction of the available light energy of H2O2. So the reactive
species in the catalyst-free oxidation of styrene with H2O2
under UV light is ·OH rather than ·O2− . The above results
also explain why the conversion of olefin decreases if THF,
EtOH, or acetone is used as a solvent to replace CH3CN
(entries 2−4). This can be ascribed to the quenching of ·OH
by these solvents. Moreover, the introduction of water to
partially replace CH3CN leads to a decrease in 1a conversion
(entry 5) for two reasons: The first reason is that the presence
of H2O inhibits the decomposition of H2O2 into ·OH and
H2O, and the second reason is that compared with CH3CN as
the solvent, the dispersion of olefins becomes worse in the
mixed solvent of water and CH3CN because the olefins are
insoluble in water.
To get further insights into the reaction mechanism of the
developed catalyst-free selective oxidative cleavage of CC
double bonds of olefins to their corresponding carbonlys, a
series of controlled experiments were executed by using 1a as
the model molecule. The results are summarized in Table 3. In
Table 3. Controlled Experiments To Explore the
Mechanism of UV-Light-Initiated Free Catalyst Oxidation
a
of Styrene
On the basis of the above results and analysis, a plausible
mechanism for the catalyst-free selective oxidation of olefins
with H2O2 under UV-light irradiation is proposed and is
illustrated in Figure 1. H2O2 can decomposed into ·OH and
entry
light
addition
H2O2
atm.
conv. %
79
sel. %
93
1
2
3
UV
UV
vis
UV
UV
UV
×
×
×
√
×
√
√
√
×
air
air
air
air
air
O2
b
4
TBA 0.1 mL
BQ 50 mg
×
61
18
95
97
5
6
a
Reaction condition unless otherwise noted: 0.1 mmol styrene, 1.2
mmol H2O2, 1 mL CH3CN, 6 h, 20 °C, UV light determined by GC.
b
0.9 mL CH3CN.
the absence of H2O2, the oxidation reaction of 1a does not
occur under UV light irradiation (entry 2), suggesting that the
reactive species in reaction systems are all derived from H2O2
decomposition. Under the visible light (>420 nm), no product
is formed, even if H2O2 is also provided in the reaction system
(entry 3). This is mainly because H2O2 is not able to absorb
light with a wavelength of >320 nm (Figure S3). As is known
to all, to explore the plausible reaction mechanism, it is
important to determine the types of reactive species in the
reaction system. It is a powerful tool to verify the types of
reactive species by carrying out the controlled experiments
with different radical scavengers.10 In this study, 4-
benzoquinone (BQ) and tert-butyl alcohol (TBA) were used
as the scavengers for superoxide radical (·O2−) and hydroxyl
radicals (·OH), respectively. When TBA was added to the
reaction system, a certain decrease in the conversion of 1a
appeared (79−61%) with proximity selectivity (entries 1 and
4), indicating that ·OH plays a role in the oxidation of 1a. The
introduction of BQ significantly reduces the conversion of 1a
to 18% (entry 5), suggesting that ·O2− also is a reactive species
in the oxidation of 1a. However, the electron spin resonance
(ESR, Figure S4) patterns using 5,5-dimethyl-1-pyrroline N-
oxide (DMPO) as a superoxide radical trapping agent show
only four typical characteristic peaks for DMPO−·OH adducts
Figure 1. Proposed reaction mechanism.
react with styrene to form radical A. Then, the radical A further
reacts with ·OH and generates the intermediate B. Finally, the
corresponding carbonyl is obtained by the rearrangement of
intermediate B.
In summary, we have developed an efficient catalyst-free
light-initiated protocol for the selective oxidation of diverse
olefins to their corresponding carbonyls in high yield and with
high selectivity using H2O2 as a clean oxidant under green and
mild conditions. UV light, H2O2, and the special CH3CN are
indispensible to promote the smooth reaction. The protocol
effectively avoids the use of stoichiometric toxic oxidants.
Moreover, the common problems caused by the use of
catalysts like the separation and regeneration of the catalyst,
the metallic residue in products, the easy deactivation, and the
high cost can also be overcome. This work not only presents an
efficient, practical, and green method for synthesizing diverse
carbonyls in high yield but also opens the door to synthesizing
a variety of useful compounds though a light-driven selective
oxidation process under catalyst-free conditions.
C
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Organic Letters
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02569.
Detailed experimental procedure, characterization meth-
ods, and additional data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zkzhao@dlut.edu.cn.
ORCID
Zhongkui Zhao: 0000-0001-6529-5020
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was financially supported by the National Natural
Science Foundation of China (U1610104 and 21676046).
■
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