Visible-Light-Promoted Efficient Aerobic Dehydrogenation of N-Heterocycles by a Tiny Organic Semiconductor Under Ambient Conditions

Article abstract of DOI:10.1002/ejoc.202000170

An efficient reusable catalytic system has been developed based on perylene diimide (PDI) organic semiconductor for the aerobic dehydrogenation of N-heterocycles with visible light. This practical catalytic system without any additives proceeds under ambient conditions. The minute aggregates of PDI molecules on the surface of SiO₂ nanospheres form tiny organic semiconductors, resulting in high-efficiency photo-oxidative activity. Notably, the robustness of this method is demonstrated by the synthesis of a wide range of N-heteroarenes, gram-scale experiments as well as reusability tests.

Full text of DOI:10.1002/ejoc.202000170

A Journal of
Accepted Article
Title: Visible-Light-Promoted Efficient Aerobic Dehydrogenation of N-
Heterocycles by Tiny Organic Semiconductor under Ambient
Conditions
Authors: Kunyi Yu, Hanjie Zhang, Chenliang Su, and Yongfa Zhu
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000170
Link to VoR: http://dx.doi.org/10.1002/ejoc.202000170
Supported by

European Journal of Organic Chemistry
10.1002/ejoc.202000170
COMMUNICATION
Visible-Light-Promoted Efficient Aerobic Dehydrogenation of N-
Heterocycles by Tiny Organic Semiconductor under Ambient
Conditions
[
a]
[a]
[b]
[a]
Kunyi Yu, Hanjie Zhang, Chenliang Su,* Yongfa Zhu*
[
a]
K. Y. Yu, H. J. Zhang, Pro Y. F. Zhu
Department of Chemistry
Tsinghua University
Beijing 100084, PR China
E-mail: zhuyf@tsinghua.edu.cn
URL: http://www.zhuyfgroup.com
Pro C. L. Su
[
b]
International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, Engineering Technology
Research Center for 2D Materials Information Functional Devices and Systems of Guangdong Province, Institute of Microscale Optoelectronics,
Shenzhen University
Shenzhen 518060, PR China
Email: chmsuc@szu.edu.cn
URL: http://2dchem.szu.edu.cn/
Supporting information for this article is given via a link at the end of the document.
Abstract: An efficient reusable catalytic system has been developed
based on perylene diimide (PDI) organic semiconductor for the
aerobic dehydrogenation of N-heterocycles with visible light.
This practical catalytic system without any additives proceeds under
ambient conditions. The minute aggregates of PDI molecules on the
reusability.[1k] In recent years, semiconductor photocatalysis with
heterogeneous advantages has been widely used in
2 2
photocatalytic splitting water into H and O , the degradation of
organic pollutants and organic synthesis.[ To the best of our
knowledge, only two cases of aerobic dehydrogenation of N-
heterocycles by semiconductors have been reported. Operation
7]
2
surface of SiO nanospheres form tiny organic semiconductors,
resulting in high-efficiency photo-oxidative activity. Notably, the
robustness of this method is demonstrated by the synthesis of a wide
range of N-heteroarenes, gram-scale experiments as well as
reusability tests.
temperatures up to 80 ℃ and pure oxygen was required in the
catalytic system;[ TEMPO as a co-catalyst was
8]
mpg-C
required in the TiO
3
N
4
catalytic system.[9] Therefore, an efficient
2
semiconductor photocatalytic protocol that functions under
ambient conditions is highly desired.
N-heteroarenes are considered privileged structure in nature
products, medicinal chemistry and material science.[1] Therefore,
the development of mild methods for synthesis of N-heteroarenes
has attracted the interest of organic synthetic chemists.[2] The site-
selective introduction of substituents onto N-heteroarenes is a
formidable challenge, while this is relatively straightforward to
achieve for the corresponding N-heterocycles. For example,
tetrahydroquinoline can easily undergo 6-site electrophilic
substitution functionalization, and the corresponding substituted
quinoline can be obtained by dehydrogenation.[3] Therefore, the
dehydrogenation of N-heterocycles to N-heteroarenes is
considered a viable method.
Perylene diimide (PDI) has been widely used as a basic
structural unit for fabricating organic photo-functional materials in
recent years due to its unique optical and electronic properties.
[10]
Compared to some semiconductor photocatalysts (TiO
), easy chemically modified PDIs can be used as functional
photocatalysts in various photo-promoted chemical
2
and
3 4
C N
transformation. In 2014, König et al. reported a monomolecular
PDI-AN, which can reductively cleave inert C‒X bond due to
[11]
accumulating the energy of two photons. In 2016, Duan et al.
reported a metal-organic framework Zn-PDI, which can achieve
the photo-oxidation of benzyl alcohol and benzyl amine.[12] In the
same year, our group reported a self-assembled supramolecule
PDINH, capable of photo-oxidative degradation of organic
When compared to thermally promoted organic reaction, the
utilisation of visible-light energy as a driving force for organic
transformation is appealing, as it is a mild, safe, and easy-to-
[13]
pollutants. In 2017, Chen et al. reported a covalent immobilized
PDI-SN, which could be used for photo-reductive degradation of
halogenated organic pollutants.[14] PDIs exhibit a variety of
attractive photocatalytic activities in various phase states,
prompting us to develop a new photocatalytic system based on
PDI for the photocatalytic aerobic dehydrogenation of N-
heterocycles.
[
4]
handle energy resource . In general, there are two subclasses of
visible-light-promoted dehydrogenation of N-heterocycles:
acceptorless[
1e, 5]
and acceptor assisted,
[3, 6]
depending on
whether hydrogen acceptors are involved. Acceptor assisted
dehydrogenation, especially aerobic oxidative dehydrogenation
using oxygen as a cheap and readily available oxidant, has the
advantages of scalability and operational simplicity. From the
point of view of sustainable development, heterogeneous
catalysts are preferable due to facile product separation, catalyst
Firstly, we examined three heterogeneous photocatalysts
based on PDI (Figure 1): PDINH (self-assembly supramolecular),
PDI-SN (covalent immobilized to SiO
2
nanospheres) and PDI-U[15]
1
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European Journal of Organic Chemistry
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COMMUNICATION
Information). In addition, the fluorescence emission spectra of the
supramolecular PDINH and PDI-SN supported the hypothesis
mentioned above (see Figure S8 in the Supporting Information).
Unlike the thick layer of the polymer PDI-U semiconductors, PDI-
SN consists of minute aggregates of PDI molecules on the
2
surface of SiO nanospheres, which form thin layers of PDI
semiconductors, resulting in a higher photogenic carrier transport
efficiency, and superior photo-oxidative activity. In other words,
the particular aggregation of PDI molecules in PDI-SN ensured
both photo-oxidative activity and the high catalytic efficiency.
Next, we carried out solvent optimization experiments. The
current catalyst has catalytic activity in many polar solvents such
as DMA, DMF, DMSO, MeCN, and water. However, no product
was obtained when using methanol, which probably acted as a
role of electron donor and quenched the photo-generated holes
Figure 1. PDIs in different phase states.
(
see Table S3 in the Supporting Information). In order to
determine the necessity of each component, namely the catalyst,
LED irradiation and air, a number of control experiments were
conducted and the results indicated that they were all
indispensable components (Table 1, entries 5-7). In addition,
wavelengths of LED light sources were studied, with 400 nm
leading to the highest catalytic activity (see Table S2 in the
(
polymerisation with urea) in the aerobic dehydrogenation of N-
heterocyclic amines under ambient conditions (see Figure S1, b
in the Supporting Information), utilising tetrahydroquinoline (THQ)
as the model substrate. To our delight, good catalytic activity was
exhibited by all three catalysts (Table 1, entries 1-3). The
heterogeneous catalysts remained stable following the catalytic
process, apart from supramolecular PDINH, which was
structurally destroyed. In particular, the highest photo-oxidative
activity was exhibited by PDI-SN, which had a larger specific
Supporting
semiconductors TiO
exhibited poor activity under standard reaction conditions (Table
, entries 8-9).
Information).
Comparatively,
the
inorganic
2
and organic semiconductor bulky g-C
3 4
N
1
2
2
surface area than PDI-U (27.7 m /g for PDI-SN and 12.1 m /g for
PDI-U). Interestingly, the free monomer PDI-AN (condensation of
PDI with 2, 6-diisopropyl aniline as showed in Figure 1) did not
display any photo-oxidative activity (Table 1, entry 4); the
significant differences in catalytic activity between the free
monomer PDI-AN and PDI-SN prompted us to examine the phase
state of the PDI molecules in PDI-SN. Compared with the free
monomer PDI-AN, the wide peak in the UV-Vis absorption
spectrum of PDI-SN implied that the PDI molecules exist in
aggregation state in PDI-SN (see Figure S8 in the Supporting
Table 2. Aerobic dehydrogenation of cyclic amines.[a][b]
Table 1. Optimization of the reaction conditions.[a][b]
[
a]. Reaction conditions: cyclic amines (1, 0.4 mmol), PDI-SN (10 mg), DMA (2
mL), ambient air atmosphere under 18 W 400 nm LED irradiation overnight. [b].
Isolated yield.
[
a]. Reaction conditions: THQ (1a, 0.2 mmol), catalyst (5 mg), DMA (1 mL),
ambient air atmosphere under 18 W 400 nm LED irradiation overnight. [b]. Yield
determined by GC analysis with an internal standard (dodecane). [c]. Isolated
yield. [d]. In the absence of light. [e]. under Ar atmosphere.
With
the
optimal
reaction
conditions
in
hand,
this practical catalytic protocol was then applied to the aerobic
dehydrogenation of diverse cyclic amines. As shown in Table 2,
2
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European Journal of Organic Chemistry
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COMMUNICATION
various substituted tetrahydroquinolines and indolines were
compatible and the expected dehydrogenated products were
obtained in good to excellent yields. Tetrahydroquinolines bearing
a methyl, substituted at 2-, 3-, 4- and 6- positions were well
tolerated; the 2-position substitution afforded slightly lower yields.
It is probably due to the higher degree of steric hindrance at the
As agglomeration of heterogeneous nanocatalysts is
ubiquitous deficient,[ we examined the reusability of the present
catalyst by performing six consecutive experimental runs (see the
Supporting Informataion for details of the recycling procedure). To
our delight, no significant reduction (Figure 2) were observed in
yield (83 - 91%). In addition, no significant changes were seen in
the UV and IR spectra of the catalyst isolated by centrifugal
filtration, compared with the fresh one (see Figure S3 in the
Supporting Information).
2k]
2-position compared to that at the other positions (Table 2, 2b-
2
e). variety of 6-substituted tetrahydroquinolines were
A
successfully subjected to this practical catalytic system; electron-
deficient substitutions gave superior results (Table 2, 2f-2h). The
readily oxidised phenol, as well as aniline fragments were likewise
compatible, thus the 8-substituted hydroxyquinoline and
aminoquinoline being produced in 93% and 88% yield,
respectively (Table 2, 2i-2j). Similarly, various substituted
indolines gave the corresponding indoles in good yields (Table 2,
2
k-2q), except for 2-indoleic acid (Table 2, 2r). The ternary
heterocyclic dihydroacridine likewise performed successful (Table
, 2s). Finally, a heterocyclic substrate containing two nitrogen
2
atoms were investigated and the corresponding quinoxaline was
obtained in good yield (Table 2, 2t).
Table 3. Aerobic dehydrogenation of cyclic amines generated in situ.[a][b]
Figure 2. Recycling test of the PDI-SN catalyst. Reaction conditions: THQ (1a,
0
4
.4 mmol), PDI-SN (20 mg), DMA (2 mL), ambient air atmosphere under 18 W
00 nm LED irradiation overnight.
To evaluate the scalability of the aerobic dehydrogenation
protocol, a gram-scale experiment was performed (see the
Supporting Information for details of the gram-scale reaction
procedure). We found that the reactivity was no significant altered
in this case, and the gram-scale reaction proceeded smoothly,
giving the product in an isolated yield of 82% by a prolonged
reaction time of up to 72 h, and with a combination light source
(
Scheme 1).
[
a]. Reaction conditions: o-diaminobenzene (3, 0.4 mmol), Benzylamine or
aldehyde (4, 0.44 mmol), PDI-SN (10 mg), and DMA (2 mL), ambient air
atmosphere under 18 W 400 nm LED irradiation overnight. [b]. Isolated yield.
Scheme 1. Gram-scale preparation for the aerobic dehydrogenation of THQ.
Reaction conditions: THQ (1a, 10 mmol), PDI-SN (500 mg), DMA (50 mL),
ambient air atmosphere under 4x30 W 400 nm LED irradiation 72 h.
Encouraged by these results, we attempted to apply
this practical catalytic protocol to cyclic amines generated in situ.
As shown in Table 3, we investigated a strategy whereby
benzylamine was the precursor for the imine; it reacted with o-
phenylenediamine to form a cyclic amine intermediate, after
which dehydrogenative aromatization afforded 2-phenyl-
benzoimidazole (Table 3, 5a) in an isolated yield of 91%.
Benzylamines with varying electronic properties were compatible,
including those substituted with electron-donating group, such as
a methoxy (Table 3, 5b), and electron-withdrawing group, such as
nitro group (Table 3, 5c); the corresponding products were
obtained in 87% and 93% yield, respectively. In addition, the
combinations of o-hydroxyaniline and o-mercaptoaniline with
benzylamines were studied, and the corresponding 2-phenyl-
benzoxazole (Table 3, 5d) and 2-thiophene benzoxazole (Table
In order to determine the possible intermediates and reaction
mechanism, a series of quenching and trapping experiments were
performed (Figure 3).[16] The detection of reactive oxygen species
was conducted firstly. The reaction proceeded unaffectedly in the
presence of TBA,[17]
a well-known hydroxyl radical (HO•)
quenching agent, which indicated that no HO• was involved.
Similarly, the reaction is unaffected by the presence of singlet
1
) agent DBACO, [18] which indicated that
oxygen quenching ( O
2
1O
was not the reactive oxygen species. In the presence of
superoxide radical (O2 ) quenching agent BHT, a significantly
2
•
–
[19]
reduction in yield suggested that the reaction mechanism entail a
•
–
O2 , and analogous results were obtained in nitrotetrazolium blue
chloride (NBT) control experiments. [20] The reduction in reactivity
in the presence of the photo-generated electron quenching agent
AgOTf, [21] likewise supported the existence of O2 intermediates.
The reaction was severely hampered in the presence of photo-
generated holes quenching agent EG.[22] Reduction quenching
3, 5e) were generated in 69–73% yield under standard reaction
condition. Finally, vanillic aldehyde was not oxidized to vanillic
acid in this photo-oxidative system, and the desired product
benzimidazole derivative (Table 3, 5f) was obtained in a yield of
•–
78%.
3
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European Journal of Organic Chemistry
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COMMUNICATION
generally required participation by the photo-generated holes and
fluorescence quenching experiments indicated electrons transfer
from tetrahydroquinoline to organic semiconductor (see Figure S9
in the Supporting Information). The presence of TEMPO reduced
the reaction yield, which supported a free radical pathway.[23]
In summary, we have reported an efficient semiconductor
photocatalytic system for the aerobic dehydrogenation of N-
heterocycle under ambient conditions. This reusable
photocatalytic system proceeds under very mild conditions: in the
ambient air atmosphere, without additional heat sources and
additives. The minute aggregates of PDI molecules on the surface
Finally, H
2 2
O as a possible by-product was also detected in a
[
24]
typical horseradish peroxidase experiment.
2
of SiO nanospheres form tiny PDI semiconductors, resulting in
high-efficiency photo-oxidative activity. This practical catalytic
protocol is attractive, and offers a wide scope related to the
synthesis of quinoline, indole, benzimidazole, benzothiazole and
benzoxazole derivatives. The mechanistic studies show that
•
–
photo-generated holes and O2 are pivotal components in the
reaction pathway.
Acknowledgements
This work was partly supported by Chinese National Science
Foundation (21437003, 21673126, 21761142017, 21621003, and
2
(
1972094), Beijing Municipal Science and Technology Project
Z181100005118007), Collaborative Innovation Center for
Regional Environmental Quality, Shenzhen Peacock Plan
KQJSCX20170727100802505 and KQTD2016053112042971),
Figure 3. Experimental results of mechanism research. Reaction conditions:
THQ (1a, 0.2 mmol), quenching agent (0.4 mmol), PDI-SN (5 mg), DMA (1 mL),
ambient air atmosphere under 18 W 400 nm LED irradiation overnight. TBA =
tert-butyl alcohol; BHT = butylated hydroxytoluene; DABCO = bicyclo[2.2.2]-1,4-
diazaoctane; AgOTf = silver triflate; EG = ethylene glycol; TEMPO = 2,2,6,6-
tetramethylpiperidinyl-N-oxide radical.
(
Guangdong Special Support Program, and Pengcheng Scholar
program. The authors thank Dr. Chuntian Qiu, Shenzhen
University for providing important advice in preparing manuscripts.
Based on the above experimental results and known
literatures,[3,
5a, 5b, 5d, 8]
we proposed a possible reaction
Conflict of interest
mechanism (Scheme 2). Firstly, oxidative photo-generated holes
and reductive photo-generated electrons are produced by
excitation of the organic semiconductor PDI-SN, respectively.
Then, the substrate 1a is oxidized by photo-generated holes
through single electron transfer (HAT),[25] thereby producing the
The authors declare no conflict of interest.
Keywords: aerobic dehydrogenation • N-heterocyclic amines •
perylene diimide • organic semiconductor • visible light
corresponding cation radical A; O
photo-generated electrons, thereby producing a O2 , respectively.
Coherently, the radical cation A is oxidized by O2 to form an
imine cation intermediate B, via the so-called hydrogen-atom
transfer (HAT) mechanism.[3] Consequently, Imine intermediate C
2
from the air is reduced by
•
–
•
–
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4
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European Journal of Organic Chemistry
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COMMUNICATION
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support the single electron transfer (SET) from tetrahydroquinoline to
photo-induced holes (see supporting information).
[26] H. Li, J. Jiang, G. Lu, F. Huang, Z. Wang, Organometallics 2011, 30,
3131-3141.
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