Visible-light-initiated catalyst-free oxidative cleavage of (Z)-triaryl-substituted alkenes containing pyridyl motif under ambient conditions

Article abstract of DOI:10.1039/d1gc00716e

The catalyst-free oxidative cleavage of (Z)-triaryl-substituted alkenes bearing a pyridyl motif in ambient air under the irradiation of blue LEDs at room temperature has been developed. The reaction was facile and scalable, and proceeded with good functional group tolerance, affording pharmaceutically useful 2-acyl pyridines. The electron paramagnetic resonance (EPR) studies together with control experiments showed that the singlet oxygen (¹O₂) and superoxide anion (O₂˙^?) are the reactive oxidants. The¹O₂generation mechanism correlated well with the photophysical properties of the substrate (Z)-triaryl-substituted alkenes, the excited state of which was proved to serve as the triplet sensitizer for the generation of¹O₂

Full text of DOI:10.1039/d1gc00716e

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Visible-light-initiated catalyst-free oxidative
cleavage of (Z)-triaryl-substituted alkenes
containing pyridyl motif under ambient
conditions†‡
Cite this: DOI: 10.1039/d1gc00716e
a
Wenjing Li,a Shun Li,a Lihua Luo,a Yicen Ge,b Jiaqi Xu, a Xueli Zheng,
a
Maolin Yuan,a Ruixiang Li, a Hua Chena and Haiyan Fu
*
The catalyst-free oxidative cleavage of (Z)-triaryl-substituted alkenes bearing a pyridyl motif in ambient air
under the irradiation of blue LEDs at room temperature has been developed. The reaction was facile and
scalable, and proceeded with good functional group tolerance, affording pharmaceutically useful 2-acyl
pyridines. The electron paramagnetic resonance (EPR) studies together with control experiments showed
that the singlet oxygen (1O2) and superoxide anion (O2•−) are the reactive oxidants. The 1O2 generation
mechanism correlated well with the photophysical properties of the substrate (Z)-triaryl-substituted
alkenes, the excited state of which was proved to serve as the triplet sensitizer for the generation of 1O2.
Received 24th February 2021,
Accepted 6th April 2021
DOI: 10.1039/d1gc00716e
rsc.li/greenchem
safe oxidant has become a good choice for CvC bond oxi-
dation, especially activated by clean and renewable light
Introduction
Compounds bearing the carbonyl moiety, such as aldehydes irradiation. The existing protocols of alkene photooxidation
and ketones, are of great synthetic interest in pharmaceutical typically require external photocatalysts, such as Fukuzumi
development, chemical biotechnology, industrial processes catalyst 10-methylacridinium perchlorate (AcrH+ClO4−),12
and materials science.1 Among various synthetic approaches, 5,10,15-triphenyl-20-(4-hydroxy-phenyl)-21H,23H-porphyrin
direct oxidative cleavage of unsaturated bonds of alkenes is a (TPP-OH),13 9,10-dicyanoanthracene (DCA),14a,b dimethoxy-
classic and facile way to acquire carbonyl compounds.2 benzene,14c Eosin Y,14d polymeric carbon nitrides (PCN),14e
Traditionally, CvC bonds can be cleaved by ozonolysis,3
Lemieux–Johnson oxidation,4 transition metal (Ru,5 Au,6 Pd,7
Pt,8 Fe,9 Cu10) catalyzed oxidation, or thermally initiated
radical processes11 (Scheme 1, eqn (a)). However, these
methods often employ super-stoichiometric oxidants, a series
of specific additives, or heavy metals as catalysts under harsh
reaction conditions. As a result, large amounts of hazardous
and toxic waste are generated afterwards, which does not meet
the requirements of green and sustainable chemistry.
Due to the increasing demand for environmentally friendly
synthesis, molecular O2 as an abundant, clean, cheap, and
aKey Laboratory of Green Chemistry & Technology, Ministry of Education College of
Chemistry, Sichuan University, Chengdu, 610064, P. R. China.
E-mail: scufhy@scu.edu.cn
bCollege of Materials, Chemistry and Chemical Engineering, Chengdu University of
Technology, No.1 3rd Erxian Road East, Chengdu, 610059, P. R. China.
E-mail: geyicen@cdut.edu.cn
†Dedicated to Prof. Christian Bruneau for his outstanding contribution to
catalysis.
‡Electronic supplementary information (ESI) available. CCDC 2055782. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1gc00716e
Scheme 1 Methodologies for the oxidative CvC bond cleavage of the
alkene.
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Table 1 Optimization of the reaction conditionsa
disulfide,15a,c and I2
(Scheme 1, eqn (b)). Under the
15b
irradiation of light, the photocatalysts are excited to higher
energy states from their ground states, thereby exhibiting stron-
ger oxidizing ability. The excited photocatalysts oxidize the
alkene to give the corresponding radical cation, which then
12
reacts with either molecular O2 or reactive oxygen species
such as singlet oxygen (1O2)13 and superoxide anion (O2•−),14
thus facilitating the oxidative cleavage of CvC bonds.
Alternatively, the alkene oxidation reaction could also occur via
photoinitiated radical addition processes, for example, visible-
light-mediated I2 and disulfide catalytic systems.15 However, the
requirement of an external photosensitizer in these systems
makes them less attractive owing to economic and environ-
mental concerns. In this regard, the development of reactions
without external photocatalysts or photomediators is attracting
growing interest. Recently, several elegant organic transform-
ations have been successfully achieved in the absence of a
photocatalyst with photoactive substrates.16 For example, in
2018, Wang17 reported a selective remote C–H trifluoro-methyl-
ation of aminoquinolines with CF3SO2Na under visible-light
irradiation in air without an external photocatalyst. Later, Wei18
described the visible-light-driven reaction of aryldiazo sulfones
with thiols to synthesize unsymmetrical sulfoxides by employ-
ing O2 as an oxidant. Most recently, He19 developed the visible-
Entry
Light source
Solvent
Yieldb (%)
Benzaldehyde
1
2
3
4
5
6
7
8
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
Blue LEDs
UV lamps
Dark
DMSO
DMAc
DMF
DCE
THF
CH3CN
1,4-Dioxane
Toluene
EtOH
Isopropanol
MeOH
MeOH
MeOH
MeOH
60
67
52
Trace
Trace
Trace
Trace
Trace
81
70
83
39
NR
42
45
40
NR
NR
NR
NR
NR
58
50
63
Trace
NR
NR
9
10
11
12
13
14c
Blue LEDs
NR
a Reaction conditions: (Z)-S1 (0.10 mmol), solvent (1 mL), ambient air,
30 W blue LEDs, room temperature, time. b Isolated yields. c N2
atmosphere.
light-induced decarboxylative acylation of quinoxalin-2(1H)-ones contrast, alcohol solvents such as ethanol (EtOH), isopropanol
with α-oxo carboxylic acids in ambient air. The reactivity of this (IPA), and methanol (MeOH) could increase the yields over
transformation was attributed to the singlet 1O2 formed from 70% (entries 9–11). The highest yield obtained was 83% in
the ground state triplet O2 (3O2) via energy transfer with the MeOH. It should be noted that benzaldehyde was always
photoexcited substrate. Nevertheless, further explanations about detected along with compound 1 in around 40–60% yields.
the reason why the substrate could act as a photosensitizer of When the reaction was irradiated under UV light, however,
3O2 to produce 1O2 are still lacking.
only 39% yield of the product was formed (entry 12). The
Herein, we describe an oxidative cleavage of (Z)-triaryl-substi- control experiments showed that both light and O2 in air play
tuted alkenes containing a pyridyl motif in ambient air as a a crucial role in the process, as no reaction occurred either
green oxidant. The reaction gave rise to a series of 2-acyl pyri- without light or in a nitrogen atmosphere (entries 13 and 14).
dines under the irradiation of blue LEDs without an external It was noteworthy that besides the oxidation product there is
photocatalyst. To the best of our knowledge, it represents the also a small amount of (Z)-S1 which was isomerized to its
first catalyst-free alkene photooxidation system. Mechanistic E-isomer under photooxidation conditions (Table S2‡). In the
studies revealed that the reaction was enabled by photoinduced case of pure (E)-S1 as the substrate, while the E → Z isomeriza-
1O2 and O2 which were produced via the energy transfer and tion of alkenes also occurred to a different extent, no CvC oxi-
•−
single electron transfer (SET) process, respectively, between the dation product was detected (Table S3‡). In addition,
triplet excited state of the substrate and molecular O2.
triphenylethylene and other Z-configurational heteroaryl sub-
2-Pyridyl containing triaryl-substituted alkenes were syn- stituted alkenes failed to be photooxidized under the optimal
thesized by using our previously developed pyridinium salts conditions (Table S4‡). These results revealed that both the
alkenylation reaction.20 We initiated our investigations by irra- Z-configuration and pyridyl motif were responsible for the
diating the solution of (Z)-2-(1,2-diphenyl-vinyl)-6-phenylpyri- realization of the present external catalyst-free photooxidation
dine (Z)-S1 with a 30 W blue LEDs (380–410 nm) in ambient reaction.
air at room temperature. As shown in Table 1, solvents had a
With the optimal reaction conditions in hand, we next
great influence on the reaction. Polar solvents such as examined the substrate scope (Scheme 2). In general, this pro-
dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), tocol was highly efficient with a broad substrate scope. The use
and N,N-dimethylformamide (DMF) showed moderate activi- of various electron-rich substituents in the 3-, 4-, or 5-position
ties, affording the desired product phenyl(6-phenylpyridin-2- of the pyridine ring had little influence on the oxidation
yl)methanone 1 in 52–67% yields (Table 1, entries 1–3). Non- process and the desired 2-acyl pyridine 2–6 were formed in
polar solvents, such as dichloroethane (DCE), tetrahydrofuran 70–90% yields. Notably, the substrate bearing a methyl group
(THF), acetonitrile (CH3CN), 1,4-dioxane, and toluene, were in the 5-position of the pyridine ring required a longer time to
found to be not suitable for this reaction and only led to the afford ketone 6 in moderate yield, probably due to the
formation of a trace amount of the product (entries 4–8). In increased steric hindrance around the reaction centre. A series
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Scheme 2 The substrate scope of (Z)-triaryl-substituted alkenes. Reaction conditions: S2–S42 (0.10 mmol), MeOH (1–3 mL), ambient air, 30 W
blue LEDs, room temperature, time. Isolated yields.
of electron-rich and electron-poor functional groups on the –CN, –OMe, –CF3, alkyls, and heterocycles were well tolerated
adjacent phenyl rings were well tolerated, which delivered the on the 1-pyridyl motif.
desired 2-acyl pyridine 7–13 in 63–79% yields. In addition,
To exemplify the potential applications of the developed
substrates with heterocyclic, cyclohexyl, methyl, or branched process, (Z)-2-(1,2-diphenylvinyl)-6-phenylpyridine (Z)-S1 was
alkyl groups at the 2-position of the pyridine moiety were reacted in MeOH on a 30-times scale under the standard con-
reacted smoothly, providing the corresponding products 14–20 ditions with a prolonged reaction time, and the desired
in moderate to excellent yields (69–88%). Meanwhile, the elec- product phenyl(6-phenylpyridin-2-yl)methanone 1 was isolated
tronic properties of the 1-aryl group had a negligible effect on in a moderate yield of 75%, demonstrating the good scalability
the reaction, delivering moderate to high yields of the corres- of this method. Inconsistent with its versatile derivatization in
ponding 2-acyl pyridine 21–24.
pharmaceutical synthesis,21 the resulting 2-acyl pyridine was
Even for the steric demanding substrates with the bulkier efficiently converted into the corresponding alfa-pyridyl-substi-
3,5-dimethylphenyl group, the CvC bonds were still cleaved tuted phenylmethane 43,22a benzyl alcohol 44,22b benzylic
efficiently to give the corresponding 2-acyl pyridine 25–42 in amine 45 22c by reduction, hydrogenation, and reductive amin-
good yields. Again, various functional groups, such as –F, –Cl, ation in good yields (Scheme 3a–c). In addition, pyridyl hydra-
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piperidinyloxyl (TEMPO) or 2,6-ditertbutyl-4-methylphenol
(BHT) was added into the reaction system, the yield was dra-
matically decreased, revealing that the reaction was likely to
proceed involving the radical species (Table 2, entries 1 and 2).
Furthermore, when performing the model reaction in the pres-
ence of 2 equiv. of 1O2 scavengers, such as 1,4-diazabicyclo
[2,2,2]octane (DABCO) and NaN3,23 only a trace amount of oxi-
dation product 1 was obtained (entries 3 and 4). In addition,
endoperoxide 49 as the [4 + 2] cycloaddition product was
detected in a reaction system containing the 1O2 trapping
reagent anthracene (entry 5, see the ESI‡ for details).24
Moreover, the yield of the photooxidation product of (Z)-S1
decreased significantly to 36%, when an 1O2 consuming
reagent 1,3-diphenylisobenzofuran (DPBF) was present
(Fig. 1).25 The above experimental results strongly suggested
that 1O2 was generated during the transformation. In addition,
a superoxide radical anion (O2•−) might also be involved in
this oxidation process, since none of the desired product was
Scheme 3 Large-scale synthesis of 1 and its further derivatization.
Reaction conditions: (a) NaI (0.5 mmol), HBr (0.625 mmol), CH3COOH detected in the presence of O2•− scavengers, such as 2,2-diphe-
(0.5 mL), H3PO2 (0.75 mmol), 115 °C, 12 h; (b) AgOTf (6 mol%), KHMDS
(20 mol%), H2 (20 bar), Toluene (1 mL), 80 °C, 72 h; (c) NH4OAc
(0.5 mmol), NH3·H2O (0.75 mL), Zn (0.25 mmol), EtOH (1 mL)/H2O
(0.5 mL), 80 °C, 3 h; (d) NH2NH2·H2O (0.5 mmol), CH3COOH (5 μL),
EtOH (0.5 mL), 80 °C, 12 h; (e) NH2OH·HCl (0.5 mmol), NaOAc
(0.5 mmol), H2O (0.5 mL)/MeOH (50 μL), 60 °C, 12 h; (f) 1) NH2NH2·H2O
(0.375 mmol), CH3COOH (0.025 mmol), EtOH (1 mL), 80 °C, 6 h; (2) Cu
(OAc)2 (0.0125 mmol), EtOAc (2 mL), 80 °C, 12 h.
nyl-1-picrylhydrazyl (DPPH) and benzoquinone (entries 6 and
7).26 Finally, light on–off experiments showed that the product
formation was completely suppressed in the absence of light,
ruling out a radical chain pathway (Fig. S3‡).
Electron paramagnetic resonance (EPR) studies were per-
formed to further probe the nature of the radical species
formed during the reaction (Fig. 2). With the addition of the
zone 46 and oxime 47 were also readily prepared by the con-
densation of 1 with hydrazine and hydroxylamine in moderate
yields of 58% and 51%, respectively (Scheme 3d and e).22d,e
Interestingly, the treatment of 1 with NH2NH2.H2O followed by
Cu(II)-promoted cyclization gave rise to a triazolo-fused pyri-
dine derivative 48 in 73% overall yield (Scheme 3f).22f
To understand this transformation, some control experi-
ments were conducted and the results are shown in Table 2.
First, when the typical radical scavenger 2,2,6,6-tetramethyl-1-
Table 2 Quenching experiments for the oxidative CvC bond cleavage
of the alkenea
Fig. 1 UV-vis absorption spectra of (Z)-S1 (5 × 10−5 M−1) and DPBF (5 ×
10−5 M−1) in MeOH under blue LEDs irradiation.
Entry
Quenchers
Yieldb
Verified reactive species
1
2
3
4
5c
6
7
TEMPO
BHT
DABCO
NaN3
Anthracene
DPPH
Benzoquinone
Trace
Trace
NR
Trace
6
NR
Trace
Radical
Radical
1O2
1O2
1O2
•−
O2•−
O2
a Reaction conditions: (Z)-S1 (0.10 mmol), quenchers (0.20 mmol),
MeOH (1 mL), ambient air, 30 W blue LEDs, room temperature, 12 h.
b Yields were determined by GC-MS. c Endoperoxide 49 was detected.
Fig. 2 EPR spectra of the 1O2 adduct with TEMP: magnetic induction
intensity B (mT) (Left); g-factor (Right).
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Fig. 3 (a) UV-vis absorption of (Z)-S1 (1 × 10−5 M−1) in DCM (black line),
CH3CN (red line) and DMF (blue line); (b) phosphorescence spectra of
(Z)-S1 and (E)-S1 (1 × 10−5 M−1) in 2-MeTHF, 77 K.
Fig. 4 Cyclic voltammogram.
radical spin trapping reagent 2,2,6,6-tetra-methylpiperidine
(TEMP) to the methanol solution of (Z)-S1 (pre-treated by
irradiation with blue-LEDs), a triplet EPR signal with a g-value
of 2.0063 and an aN value of 16.2 G was observed, and it was
assigned to the TEMP-trapped 1O2 species.17,27 The same
signal was also detected in the solution of (E)-S1 under identi-
cal conditions (Fig. S11‡). However, no EPR signal was
detected in the case of triphenylethylene (Fig. S12‡).
To elucidate the mechanism of 1O2 generation, the UV-vis
absorption spectra of (Z)-S1 in dilute DCM (dichloromethane),
Scheme 4 The proposed mechanism.
CH3CN, and DMF solution (1 × 10−5
M
−1) were obtained as
shown in Fig. 3a. Negative solvatochromism (namely, the
absorption wavelength blue-shifted with increasing solvent
polarity) was observed in the weak absorption band (355 nm-
In addition, the cyclic voltammetry (CV) measurement
(Fig. 4) demonstrated the oxidation peaks (vs. Ag/Ag+) at 1.00
V, 1.38 V for (Z)-S1, 1.56 V for (E)-S1, and 1.52 V for triphenyl-
ethylene, respectively. Evidently, (Z)-S1 could be easily oxi-
dized, while the oxidation of (E)-S1 and triphenylethylene was
difficult.
1
400 nm), which was tentatively assigned to nπ*. In addition,
the hyperfine structure of the phosphorescence spectrum was
assigned to a triplet state with 3ππ* character. The energy levels
1
3
of the nπ* (3.12 eV) and ππ* (2.97 eV) states for (Z)-S1 were
determined from the highest energy peak of its phosphor-
escence emission band at 77 K in 2-MeTHF (Fig. 3b), and the
Based on the above mechanistic studies and the related lit-
erature results,13,14,29 a plausible mechanism is proposed in
Scheme 4. First, the substrate (Z)-S1 (S0) was excited with light
to produce a singlet-excited state (S1) with 1nπ* character,
which readily underwent intersystem crossing to the triplet
state (T1) (denoted as (Z)-S1*) due to their small singlet–triplet
energy gap. Then the long-lived 3ππ*-featured triplet (Z)-S1*
state sensitized the ground-state triplet oxygen 3O2 to highly
reactive singlet 1O2 via the energy transfer (ET) process.30
Subsequently, the resultant 1O2 underwent [2 + 2] cyclo-
addition with (Z)-S1 to give dioxetane A (path a). Alternatively,
a SET process occurred between (Z)-S1* and molecular O2 to
generate a superoxide radical anion (O2•−) and the (Z)-S1•+ B,14
combination of them could also afford dioxetane A (path b).
Finally, the cleavage of dioxetane A generated the desired
product 1 and benzaldehyde.
1
onset of the nπ* band in more polar DMF, respectively. Thus,
from the absorption and emission spectra, the energy differ-
ence between the singlet 1nπ* and triplet 3ππ* excited states
(ΔEST) was calculated as 0.15 eV. According to the El-Sayed’s
rules,28 such a low ΔEST value enabled the facile intersystem
crossing (ISC) process from the 1nπ* to 3ππ* excited state.
Nevertheless, the transition of 3ππ* to the ground state was
spin-forbidden, and thus the lifetime of the triplet 3ππ* excited
3
1
states was long enough for the sensitization of O2 to O2 via
the energy transfer process. In contrast, the phosphorescence
of (E)-S1 was very weak, which indicated a lower triplet yield in
the case of (E)-S1. The different phosphorescence between the
two isomers might be ascribed to their structural difference
(Fig. S8 and S9‡). The maximum wavelength of the UV-vis
absorption spectra of (E)-S1 exhibited a pronounced red-shift
in comparison with that of (Z)-S1 (326 nm vs. 290 nm)
(Fig. S5‡), suggesting that (E)-S1 has a more conjugated π
Conclusions
system, which might be caused by the more co-planarity of (E)-
1
S1 (Fig. S7‡). The relatively localized nπ* state should have a In conclusion, the first external catalyst-free oxidative cleavage
similar energy level for both Z and E isomers, but the 3ππ* of CvC bonds with (Z)-triaryl-substituted alkenes has been
energy level of (E)-S1 is even lower due to its more conjuga- developed in ambient air under the irradiation of blue LEDs.
tional feature, thus resulting in higher ΔEST.
This oxidation reaction was enabled by employing photoactive
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(Z)-triaryl-substituted alkenes bearing a 2-pyridyl group, thus
affording a variety of pharmaceutically important 2-acyl pyri-
dines with high efficiency. Mechanistic studies revealed that
the photoactivated substrate served as a triplet sensitizer to
react with 3O2 to produce highly reactive 1O2 or O2•−, which
afterwards oxidatively cleaved the CvC bonds of the substrate.
The main advantages of this process include using aerobatic
air as the oxidant, catalyst-free and mild conditions, and sus-
tainable light as the energy resource.
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Acknowledgements
We thank the National Natural Science Foundation of China
(No. 22072099 and 21871187) and the Collaborative Fund of
Luzhou Municipal Government and Sichuan University (No.
2018CDLZ-13) for financial support. We also thank Professor
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