Photo-induced oxidative cleavage of C-C double bonds for the synthesis of biaryl methanoneviaCeCl3catalysis

Article abstract of DOI:10.1039/d1ob01002f

A Ce-catalyzed strategy is developed to produce biaryl methanonesviaphotooxidative cleavage of C-C double bonds at room temperature. This reaction is performed under air and demonstrates high activity as well as functional group tolerance. A synergistic Ce/ROH catalytic mechanism is also proposed based on the experimental observations. This protocol should be the first successful Ce-catalyzed photooxidation reaction of olefins with air as the oxidant, which would provide inspiration for the development of novel Ce-catalyzed photochemical synthesis processes.

Full text of DOI:10.1039/d1ob01002f

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Biomolecular Chemistry
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Photo-induced oxidative cleavage of C–C double
bonds for the synthesis of biaryl methanone via
CeCl3 catalysis†
Cite this: Org. Biomol. Chem., 2021,
19, 6781
Received 24th May 2021,
Accepted 14th July 2021
Pan Xie, * Cheng Xue, Dongdong Du and SanShan Shi
DOI: 10.1039/d1ob01002f
rsc.li/obc
A Ce-catalyzed strategy is developed to produce biaryl metha- double bonds to biaryl methanones.9 By just changing the
nones via photooxidative cleavage of C–C double bonds at room solvent from DMA to HFIP, Luo and co-workers successfully
temperature. This reaction is performed under air and demon- performed this reaction at room temperature and attained
strates high activity as well as functional group tolerance. A syner- similar results.10 Besides, some heterogeneous catalytic pro-
gistic Ce/ROH catalytic mechanism is also proposed based on the cesses were also developed. In 2019, Nam, Cho and their co-
experimental observations. This protocol should be the first suc- workers successfully used Sono-BiVO4 to realise the oxidative
cessful Ce-catalyzed photooxidation reaction of olefins with air as cleavage of C–C double bonds under light illumination.11
the oxidant, which would provide inspiration for the development Then, Das and co-workers developed an efficient aerobic oxi-
of novel Ce-catalyzed photochemical synthesis processes.
dative strategy by using polymeric carbon nitrides (PCNs) as a
heterogeneous photocatalyst.12 In the last few years, hetero-
geneous photocatalysis has gained fast development in the
field of oxidative cleavage of C–C double bonds, but in this
area, the development of homogeneous catalysis was still rela-
tively slow. In most cases, the utilization of O2 is necessary for
getting good results, but it would induce many operational
difficulties and increase experimental risks in most of the
cases. Meanwhile, the cost and restricted availability of
common photocatalysts also impeded their industrial appli-
cations. Therefore, further searching for cheap and readily
available photocatalysts to develop more efficient oxidative
strategies for the cleavage of C–C double bonds would be still
highly desirable (Scheme 1).
Biaryl methanones are important chemicals that exist exten-
sively in numerous natural products and biologically active
compounds (Fig. 1).1 Therefore, intense efforts have been
devoted to the synthesis of these carbonyl-containing
systems.2 Among many types of synthesis methods, oxidative
cleavage of olefins is a very attractive choice due to its atom
economy.3
In the beginning, O3 and toxic OsO4 have been used in ozo-
nolysis and Lemieux–Johnson oxidation reactions.4 After that,
hypervalent metals and peroxides were also used to cleave the
C–C double bonds of olefins.5 In these processes, stoichio-
metric amounts of oxidants are always desirable, so large
amounts of waste could be produced as the reaction
progresses.
Recently, rare earth complexes have started to become new
choices because they are cheap and readily available.13 As the
Undoubtedly, O2 is the “greenest” oxidant because H2O is
the only by-product of the oxidation reaction.6 Thus, increased
attention has been gradually paid to the in-depth study of sus-
tainable development. Mn, Fe, Cu and other metal catalysts
are firstly used in the aerobic oxidation of olefins to biaryl
methanone compounds.7 Sometimes, organic radicals are
suggested to behave like high-valence metals.8 Therefore, Jiao
and co-workers reported NHPI-catalysed cleavage of C–C
College of Chemistry and Chemistry Engineering, Shaanxi Key Laboratory of
Chemistry Additives for Industry, Shaanxi University of Science & Technology, Xi’an
710021, China. E-mail: pan.xie@sust.edu.cn
†Electronic supplementary information (ESI) available: Experimental procedures
and details, mechanistic and optimization results and NMR spectra. See DOI:
10.1039/d1ob01002f
Fig. 1 Drug molecules containing a diaryl ketone motif.
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ation. Previous work displayed that the addition of alcohols
could improve the catalytic efficiency for Ce-promoted photo-
chemical reactions.16a Thus, some commercially available alco-
hols were screened, and the results proved that TCE (trichlor-
oethanol) was the best choice. After that, the effects of solvents
were also examined. No matter what other solvents were used,
inferior results were obtained.
With the optimized conditions in hand, the substrate scope
of biaryl olefins was first investigated. As shown in Table 2, the
reaction tolerated electron-donating groups such as methyl
and alkoxyl groups at the ortho-, meta-, and para-positions of
the phenyl ring, providing the corresponding substituted
ketones in good yields. The difference of reactivity between
ortho- and para-methyl-substituted stilbene demonstrated a
slight steric hindrance effect on this transformation.
Furthermore, we found that substrates with electron-withdraw-
ing groups such as F, Cl, and nitro groups at any position of
the phenyl ring were effective under the standard conditions.
In some cases, olefin polymerizations were observed, which
Scheme 1 Present methodologies to produce biaryl methanones from
olefins.
most abundant rare earth element, cerium complexes demon-
strated unique luminescence properties because both the +3
and +4 oxidation states are accessible in certain Ce(III) com-
pounds.14 Thus, the applications of cerium compounds in
organic transformations have attracted much attention in
light-induced organic synthesis.15 By using a Ce-promoted
LMCT (ligand-to-metal charge transfer) process, a series of
novel photo-mediated organic transformations have been
developed in the past few years.16 Lately, we realized the
aqueous C–H aerobic oxidation protocol via a CeCl3-mediated
photocatalytic process.17 Inspired by this result, we hope to
extend the CeCl3-mediated photocatalysis to the oxidative clea-
vage of C–C double bonds.
Table 2 Substrate scope of biaryl olefinsa
Initially, the photo-promoted aerobic oxidation of 1,1-
diphenylethylene 1a was selected as a model reaction and air
was used as the oxidant (Table 1). The most commonly used
cerium salts were first examined, and CeCl3 demonstrated
better catalytic activity to give benzophenone in 52% yield.
Decreasing the catalyst loading made the result a little worse,
so 10 mol% CeCl3 was used as the catalyst for further examin-
Table 1 Optimization of the reaction parametersa
Entry
[Ce]
ROH
Solvent
Yieldb (%)
1
CeCl3
CeBr3
CeCl3
CeCl3
CeCl3
CeCl3
CeCl3
CeCl3
CeCl3
CeCl3
CeCl3
—
—
—
MeOH
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CCl3CH2OH
DMF
52
42
45
60
68
92
55
65
60
64
73
2
3c
4
5
6
7
8
9
10
11
EtOH
CCl3CH2OH
CCl3CH2OH
CCl3CH2OH
CCl3CH2OH
CCl3CH2OH
CCl3CH2OH
DMSO
THF
DCM
a 1a (0.2 mmol), catalyst (10 mol%), solvent (1.0 mL), room tempera- a 1a (0.2 mmol), CeCl3 (10 mol%), TCE (20 mol%), CH3CN (1.0 mL),
ture (25 °C), blue LED (4 W) and 40 h. b Isolated yield. c CeCl3 room temperature (25 °C), blue LED (4 W) and 40 h; isolated yield.
(5 mol%).
b Blue LED (2 W). c Blue LED (10 W).
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may be the reason for lower yields. To expand the π-conjugated was also obtained. Besides, the reaction is amenable for
system, naphthyl- and biphenyl-substituted olefins (Table 2, 1o scaling up: using 1 mol% Ce catalyst, the model oxidation
and 1p) were employed as substrates, and the corresponding could be run with 5 mmol 1a to provide the pure isolated
products were generated in 40% and 88% yields, respectively. product 2a in 81% yield (Scheme 2).
It is worth noting that the tolerance of some functional groups
To shed light on the mechanism of our recommended pro-
(methoxy, fluoride, chloride, dimethylamino, and nitro tocol, some control and quenching experiments were carried
groups) in this protocol offered potential possibilities for out (Table 4). The obtained results revealed that light, Ce cata-
further derivation. To our delight, some unusual substrates in lyst and air are necessary and exclusion of each one can lead to
the previous oxidative process of olefins, heterocycle derived complete inhibition of the oxidation reaction. In addition, the
biaryl olefins, were also found to be tolerated in this trans- target reaction was also inhibited by the traditional radical sca-
formation, furnishing the corresponding products in good vengers such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)
yields (Table 2, 2s–2u, 70%–76%).
and BHT (2,6-di-tertbutyl-4-methyl phenol), demonstrating a
To further explore the synthesis potential of this process, free-radical mechanism. Besides, the addition of CuCl2 led to
some α-methyl styrenes were subjected to this reaction under a trace amount of the product, which proved the contribution
the optimized reaction conditions. To our delight, our photo- of single-electron processes.18 Finally, further addition of
catalysis strategy exhibited medium to good reactivity towards benzoquinone to the reaction mixture reduced the product
different styrenes. With either electron-donating or -withdraw- yield. This observation confirmed the presence of superoxide
ing groups at any position of the phenyl ring, oxidative reac- radicals in the oxidation process.19
tions ran efficiently and gave the corresponding ketones in
Next, H218O was used instead of TCE to detect the actual
good yields (Table 3, 4a–4g). 2-(Prop-1-en-2-yl) naphthalene source of oxygen in the ketone product. After the reaction was
was also a suitable substrate, and good yield was obtained. completed, no 18O-labelled ketone was observed. Finally, the
However, inferior reactivity was observed when a pyridine sub- reaction was carried out in the presence of 18O2 (1 atm) and
stituted olefin was used, and only moderate yield was obtained the 18O-labelled product was generated in 70% yield. Both
(4i–4j), which may be due to the strong electron-withdrawing results demonstrated that the oxygen of the product was from
effect of the pyridine group. Next, we turned our interest to the
reactions of terminal styrene. The results demonstrated that
various para-substituted styrenes could successfully participate
in this process, furnishing substituted benzaldehydes in good
yields (Table 3, 4l–4q).
Additionally, we found that the reaction could proceed
smoothly, even if the catalyst loading was decreased to
0.5 mol%. By prolonging the reaction time, a satisfactory yield
Table 3 Substrate scope of α-methyl and terminal styrenea
Scheme 2 The reaction practicability.
Table 4 Control and quenching experimentsa
Entry
Additive
Yield (%)
1b
2c
3d
4
5
6
—
—
—
No reaction
No reaction
No reaction
No reaction
No reaction
Trace
TEMPO (2.0 equiv.)
BHT (2.0 equiv.)
CuCl2 (1.0 equiv.)
Benzoquinone (1.0 equiv.)
7
8%
a 1a (0.2 mmol), CeCl3 (10 mol%), TCE (20 mol%), CH3CN (1.0 mL),
a 1a (0.2 mmol), CeCl3 (10 mol%), TCE (20 mol%), CH3CN (1.0 mL), room temperature (25 °C), blue LED (4 W) and 40 h; isolated yield. b N2
room temperature (25 °C), blue LED (4 W) and 40 h; isolated yield.
atmosphere instead of air. c No light. d No catalyst.
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tent supplement to the existing reported catalytic systems. The
application of this new protocol and further investigations on
the mechanism are still ongoing in our laboratory.
Conflicts of interest
There are no conflicts to declare.
Scheme 3 18O-labeling experiment.
Acknowledgements
This work was financially supported by the Education
Foundation of Shaanxi Province (No. 18JK0105).
Notes and references
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