1,2-Diethoxyethane catalyzed oxidative cleavage of gem-disubstituted aromatic alkenes to ketones under minimal solvent conditions

Article abstract of DOI:10.1016/j.cclet.2020.01.036

Aerobic oxidation using pure dioxygen gas as the oxidant has attracted much attention, but its application in synthetic chemistry has been significantly hampered by the complexity of catalytic system and potential risk of high-energy dioxygen gas. By employing 1,2-diethoxyethane as a catalyst and ambient air as an oxidant, an efficient protocol for the construction of various aryl-alkyl and diaryl ketones through oxidative cleavage of gem-disubstituted aromatic alkenes under minimal solvent conditions has been achieved.

Full text of DOI:10.1016/j.cclet.2020.01.036

Journal Pre-proof
1,2-Diethoxyethane catalyzed oxidative cleavage of gem-disubstituted
aromatic alkenes to ketones under minimal solvent conditions
Kai-Jian Liu, Ji-Hui Deng, Tang-Yu Zeng, Xin-Jie Chen, Ying Huang,
Zhong Cao, Ying-Wu Lin, Wei-Min He
PII:
S1001-8417(20)30053-X
https://doi.org/10.1016/j.cclet.2020.01.036
CCLET 5434
DOI:
Reference:
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Chinese Chemical Letters
Received Date:
Revised Date:
Accepted Date:
21 November 2019
1 January 2020
17 January 2020
Please cite this article as: Liu K-Jian, Deng J-Hui, Zeng T-Yu, Chen X-Jie, Huang Y, Cao Z,
Lin Y-Wu, He W-Min, 1,2-Diethoxyethane catalyzed oxidative cleavage of gem-disubstituted
aromatic alkenes to ketones under minimal solvent conditions, Chinese Chemical Letters
(2020), doi: https://doi.org/10.1016/j.cclet.2020.01.036
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Chinese Chemical Letters
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Communication
1,2-Diethoxyethane catalyzed oxidative cleavage of gem-disubstituted aromatic
alkenes to ketones under minimal solvent conditions
Kai-Jian Liua, Ji-Hui Denga, Tang-Yu Zenga, Xin-Jie Chenc, Ying Huangb, Zhong Caob, Ying-Wu Lind,
Wei-Min Hea
a Department of Chemistry, Hunan University of Science and Engineering, Yongzhou 425100, China
b Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, Changsha University of Science and Technology, Changsha
410114, China
c School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
d School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001, China
 Corresponding author.
E-mail address: weiminhe2016@yeah.net
Graphical abstract
By employing 1,2-diethoxyethane as a catalyst and ambient air as an oxidant, an efficient protocol for the
construction of various aryl-alkyl and diaryl ketones through oxidative cleavage of gem-disubstituted aromatic
alkenes under minimal solvent conditions has been achieved.
ARTICLE INFO
ABSTRACT
Aerobic oxidation using pure dioxygen gas as the oxidant has attracted much attention, but its
application in synthetic chemistry has been significantly hampered by the complexity of
catalytic system and potential risk of high-energy dioxygen gas. By employing 1,2-
diethoxyethane as a catalyst and ambient air as an oxidant, an efficient protocol for the
construction of various aryl-alkyl and diaryl ketones through oxidative cleavage of gem-
disubstituted aromatic alkenes under minimal solvent conditions has been achieved.
Article history:
Received 21 November 2019
Received in revised form 2 January 2020
Accepted 10 January 2020
Available online
Keywords:
Ketone
Oxidation
Ambient air
Oxidative cleavage
Radical chemistry has become indispensable tools in modern
organic chemistry [1]. Most of the free-radical reactions rely on
transition-metal salts/ligands or environmentally-unfriendly
organocatalysts, resulting in the formation of plenty of
unexpected side-products and residual catalysts [2]. The
disadvantages restrict the utility of these radical reactions,
especially in case of preparation of artificial drugs. As a
consequence, the development of simple and efficient radical
reaction for clean organic synthesis is strongly desirable.
Molecular oxygen is a clean, abundant and sustainable natural
resource [3]. The radical process using molecular oxygen as a
sole oxidant under transition metal- and organocatalysts-free
conditions is undoubtedly the ideal and promising route to
addressing the aforementioned challenges.

Table 1
The oxidative cleavage of alkenes to corresponding carbonyl
compounds is one of the most valuable transformations in
organic synthesis, given the significance of these products for the
fine and bulk chemical industry [4]. During the past decades, a
series of protocols for the cleavage of alkenes to carbonyl
compounds have been established, including Harries ozonolysis
[5], (super)stoichiometric oxidation [6], transition-metal catalysis
[7], and other methods [8]. Among those strategies, the
transition-metal-catalyzed oxidative scission of gem-disubstituted
aromatic alkenes to aryl-alkyl and diaryl ketones using eco-
friendly dioxygen (1 atm) as the sole oxidant has attracted
extensive attention due to its environmental benign, high
efficiency, and good functional-group-tolerance. From 2015 to
2016, Xiao’s group [7c] and Wang’s group [7e] successively
reported Fe(III) - and Cu(II)-ligand complex catalyzed oxidation
of alkenes to carbonyl compounds with O2 as an oxidant (Scheme
1a-b). Very recently, Feng and He reported a bismuththiol
promoted bis(diphenylphosphino)ferrocene-catalyzed oxidative
scission of alkenes under ligand-free conditions (Scheme 1c) [7g].
However, these oxidation reactions have one or more of the
following shortcomings, such as the non-commercially available
ligands, expensive additives, and the inevitable residual
transition-metal, which limits their applicability in pharmacy
industry. In the other hand, the homogeneous transition-metal-
catalyzed oxidations generally requires a large excess amount of
volatile organic solvent as a reaction medium, which leads to a
potentially flammable and possibly explosive mixture in the
presence of strongly oxidative and highly energetic pure
dioxygen gas [9]. In comparison with pure dioxygen gas, ambient
air is a more abundant natural resource and a safer oxidant which
ideally fulfils the requirements of green and sustainable
chemistry [10]. To the best of our knowledge, there is no
precedence of a radical oxidative scission of gem-disubstituted
aromatic alkenes initiated solely by air under transition-metal-
salts and additive-free conditions. Therefore, the development of
efficient, safe and practical protocol for the oxidative cleavage of
gem-disubstituted aromatic alkenes to carbonyl compounds is
highly desirable.
Optimization of reaction conditions.a
Yield (%)b
Temp.
(oC)
Conv.
(%)
Entry
Solvent (equiv.)
2a
3a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1,2-diethoxyethane (6)
1,2-dimethoxyethane (6)
2-ethoxyethyl ether (6)
2-methoxyethyl ether (6)
bis(methoyxethyl)ether (6)
1,4-dioxane (6)
2-methoxyethanol (6)
1,2-diethoxyethane (7)
1,2-diethoxyethane (5)
---
90
90
90
90
90
90
90
90
90
90
100
80
70
60
100
87
85
81
91
83
63
100
49
0
99
84
83
81
89
82
63
99
48
0
1
3
2
0
2
1
0
1
1
0
1,2-diethoxyethane (6)
1,2-diethoxyethane (6)
1,2-diethoxyethane (6)
1,2-diethoxyethane (6)
100
81
63
6
99
81
62
6
<1
0
1
0
a Conditions: All reactions were carried out in a 5 mL round-bottom
flask in the presence of 1a (0.6 mmol), solvent, air.
b Yield was determined by GC-MS.
At the outset of our study, we performed the metal-free
oxidation of α-menthylstyrene (1a) with ambient air in 1,2-
diethoxyethane (6 equiv.) at 90 C, a 99% GC-MS yield of the
o
desired acetophenone (2a) and 1% GC-MS yield of 2-methyl-2-
phenyloxirane (3a) based on 100% conversion of the starting
material 1a was obtained after 12 h (Table 1, entry 1). As
predicted, the nature of reaction medium exerted a significant
influence on the oxidation efficiency. Compared with the slightly
lower yields of 2a provided by dialkyl-end-capped ether solvents
(entries 2-6), inferior reaction outcomes were obtained when
monoalkyl-end-capped ethers were used as the reaction media
(entry 7). The exploration on the loading of 1,2-diethoxyathane
(entries 8,9 and 1) suggested that 6 equiv. of 1,2-diethoxyathane
was a suitable amount. No oxidation product was detected by
GC-MS analysis in the absence of 1,2-diethoxyethane (entry 10).
The investigation result of the screening of reaction temperature
indicated that the current oxidation could be
a
thermodynamically controlled process (entries 11-14). No benefit
o
was observed by elevating the temperature to 100 C (entry 11).
Decreasing the reaction temperature resulted in a lower yield of
2a (entries 12 and 13) and only 6% yield of 2a was observed at
60 oC (entry 14).
Scheme 1. Catalytic oxidative cleavage of gem-disubstituted
aromatic alkenes.
In continuation of our efforts to develop environment benign
synthetic reactions [11], herein we present a clean, safe and
sustainable protocol for the oxidative cleavage of gem-
disubstituted aromatic alkenes to aryl-alkyl and diaryl ketones by
employing in situ generated peroxide in 6 equiv. of 1,2-
diethoxyethane under ambient air as the radical initiator (Scheme
1d).

excellent yield under the optimal conditions (2y–2aa). Finally,
substrate bearing fluorene motif also provided the desired ketone
2ab in 83% yield.
All the previous reported oxidative cleavage of aromatic
alkenes requires the tedious post-processing procedures to
remove the organic solvent, catalyst, additives and organic by-
products, which will prevent subsequent direct chemical
transformations. The present oxidation reaction is highly efficient
and avoids using a large amount of volatile organic solvent,
external catalyst as well as additives. In order to prove the
efficiency and cleanness of the developed transformation, a series
of one-pot transformations were carried out. As shown in Scheme
3, with cheap and abundant α-methylstyreen as the starting
material, various valuable ketone downstream products (4a–7a)
were easily obtained in good yields through one-pot
oxidation/derivatization reactions [12].
Scheme 2. Substrate scope. All reactions were carried out in a 5 mL
Scheme 3. One-pot transformation from 1a.
round-bottom flask in the presence of
1 (0.6 mmol), 1,2-
diethoxyethane (3.6 mmol), air, 90 oC. Isolated yields are reported.
The generality of this eco-friendly oxidation reaction was then
investigated under the above optimal conditions (Scheme 2). α-
Methylstyreens bearing electron-neutral, -rich, or -deficient
substituents at the para-, meta- or ortho-positions of the benzene
ring underwent the transformation to generate the desired
acetophenone derivatives with good to excellent yields (2a−2k).
These experimental results suggested that neither electronic
effects nor steric hindrance of α-methylstyreens has appreciable
influence on the oxidation reaction outcome. Notably, these
halogen substituents (F, Cl and Br) are useful entities amenable
to further transformations in organic synthesis. When 4-(prop-1-
en-2-yl)phenol or 4-(prop-1-en-2-yl)aniline was used as substrate,
no reaction was observed. α-Methyl fused-aromatic and aromatic
heterocycles can also be used as the reaction substrates and
delivered the expected oxidative products (2l–2r) in good yields,
thus further enhancing the scope of the developed eco-friendly
oxidation reaction. It is notable that an N-heterocyclic group (1r)
is well tolerated in this present oxidation reaction, but was
incompatible in the previous reported transition metal-catalyzed
oxidations. α-Alkylsubstituted styreens with different carbon
chain lengths and isomeric structures did not significantly affect
the yields of this present transformation (2s–2x), even in the
presence of halogen group (2w and 2x). Other than these α-
alkylsubstituted styreens, various 1,1-diarylethylenes proved to
be effective reaction substrates, producing the desired products in
Fig. 1. The time course experiment for the oxidation. Conditions: 1a
(0.6 mmol), 1,2-diethoxyathane (3.6 mmol), air, 90 oC; GC-MS
yields were reported.
The effect of oxidation time on the conversion of α-
menthylstyrene (1a) was investigated under the standard
conditions and the results were shown in Fig. 1. In the initial
reaction stage of 4 h, both acetophenone (2a, 16%) and 2-methyl-
2-phenyloxirane (3a, 24%) were generated along with the
consumption of the 1a (40% conversion), suggesting that the
conversion of 1a to 7a is the dominant reaction. After reaction
for 6 h, 29% yield of 2a and 34% yield of 3a based on 63%
conversion of raw material 1a were detected by GC-MS. As the
oxidation reaction exceeded 12 h, 1a and 3a was almost
completely converted into 2a. These results suggest that the

oxidation of 1a is more liable to proceed through phenyloxirane
pathway, indicating a different mechanism compared with
previous reports [7g].
detected by GC-MS analysis in the absence of 1,2-
diethoxyethane. Taken together, these experimental results
manifested that the 1,2-diethoxyethane not only served as a
reaction medium but also acted as a catalyst in the developed
oxidation (Scheme 4d). Considering that this type of oxidation
reactions with ether as the solvent may involve the in situ
generated ether peroxide, we attempted to detect this compound
by GC-MS, but all efforts failed. It is probably due to the
unstable of ether peroxide in high temperature. In addition,
approximately 5% of 1,2-diethoxyethylane was lost during the
reaction (detected by GC-MS), which might be caused by system
error.
Scheme 5. Plausible reaction mechanism.
Scheme 4. Control experiment.
A reasonable reaction mechanism was proposed based on the
above-mentioned observations and previous related reports
(Scheme 5) [7g, 14]. Firstly, 1,2-diethoxyethane reacted with
dioxygen under heating conditions to produce a peroxyl radical A
(detected by EPR), which oxidized α-menthylstyrene 1a to form
2-methyl-2-phenyloxirane 3a (detected by GC-MS) and
regenerate the 1,2-diethoxyethane. Finally, decomposition of 3a
afforded the desired product 2a along with the regeneration of
HCHO by-product. The cleavage of alkene 1 processed through
dioxetane (3b) as a minor pathway cannot be completely
excluded.
Fig. 2. Electron paramagnetic resonance (EPR) experiments.
In summary, we have established an efficient and sustainable
protocol for the 1,2-diethoxyethane catalyzed oxidative cleavage
of alkenes to corresponding ketones (aryl-alkyl ketones and
diaryl ketones) by using ambient air as the sole oxidant. The
developed methodology is compatible with reaction substrates
bearing various valuable functional-groups, including
heterocycles and fused ring groups. The commercially available
inexpensive 1,2-diethoxyethane plays a dual role as both a
catalyst and reaction medium. Other favorable features of the
present oxidation reaction are as follows: (a) ambient air can be
employed as the sole oxidant which is abundant and safe to
handle; (b) The present protocol is ideally suited for one-pot
sequential synthesis, owing to its clean reaction conditions,
minimal amount of solvent and high to quantitative yields.
To further gain insight into the reaction mechanism, several
control experiments were carried out (Scheme 4). The 2-methyl-
2-phenyloxirane 3a was transformed into the acetophenone 2a in
94% GC-MS yield (Scheme 4a), suggesting that the
phenyloxirane might be a key intermediate during the present
oxidation reaction. Only a trace amount of oxidation product 2a
was detected by GC-MS analysis in the presence of a free-radical
inhibitor (BHT or TEMPO) under the optimal reaction conditions
(Scheme 4c). Both the results of radical trapping experiment and
radical-clock experiment [(1-cyclopropylvinyl)benzene, 1v]
indicated that the present reaction might proceed through a free-
radical pathway. To clarify that a free-radical species is involved
in the oxidation process, the electron paramagnetic resonance
(EPR) experiments were performed. As shown in Fig. 2a, when
2-diethoxyethane was exposed under oxygen atmosphere at
90 °C for 5 min, by using a trapping reagent of 5,5-dimethyl-1-
pyrroline N-oxide (DMPO), we successfully observed a weak
signal with four characteristic hyperfines of the DMPO adduct of
the peroxyl radical (DMPO-OOH) (g = 2.002, A N = 1.46 mT,
AH = 1.37 mT). Prolonged the heating time to 20 min, strong
signals were observed, which thereby confirmed the formation of
peroxyl radicals in the reaction (Fig. 2b) [13]. The investigation
on the oxidants under the nitrogen atmosphere revealed that air
was pivotal in the reaction system to ensure an efficient
proceeding of the metal-free oxidation reaction (Scheme 4c). A
moderate yield of 2a was observed in the presence of 0.1 equiv.
of 1,2-diethoxyethane, whereas no oxidation product was
Declaration of interests
The authors declare that they have no known
competing financial interests or personal
relationships that could have appeared to influence
the work reported in this paper.
Acknowledgement
We are grateful for financial support from the Hunan Provincial
Natural Science Foundation of China (Nos. 2019JJ20008 and
2019JJ40090).

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