-Cycloaddition of 2 H-Azirines with Nitrosoarenes: Visible-Light-Promoted Synthesis of 2,5-Dihydro-1,2,4-oxadiazoles

Article abstract of DOI:10.1021/acs.orglett.9b01416

A formal [3 + 2]-cycloaddition reaction of 2H-azirines with nitrosoarenes has been achieved under irradiation by visible light with the assistance of organic dye photoredox catalyst. This method utilizes nitrosoarenes as efficient radical acceptors and pr

Full text of DOI:10.1021/acs.orglett.9b01416

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
[3 + 2]-Cycloaddition of 2H‑Azirines with Nitrosoarenes: Visible-
Light-Promoted Synthesis of 2,5-Dihydro-1,2,4-oxadiazoles
Bao-Gui Cai,† Ze-Le Chen,† Guo-Yong Xu,§ Jun Xuan,*,†,§ and Wen-Jing Xiao*,‡
†Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, College of Chemistry &
Chemical Engineering, Anhui University, Hefei, Anhui 230601, People’s Republic of China
‡Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University,
152 Luoyu Road, Wuhan, Hubei 430079, People’s Republic of China
§Institute of Physical Science and Information Technology, Anhui University, Hefei, Anhui 230601, People’s Republic of China
S
* Supporting Information
ABSTRACT: A formal [3 + 2]-cycloaddition reaction of 2H-
azirines with nitrosoarenes has been achieved under
irradiation by visible light with the assistance of organic dye
photoredox catalyst. This method utilizes nitrosoarenes as
efficient radical acceptors and provides a green and powerful
method for a series of biologically important 1,2,4-oxadiazole
derivatives in moderate to good yields.
he oxadiazole skeleton is one of the most important five-
cyclization of in situ formed amidoxime derivatives.5 Despite
these elegant methods, further exploration of new approaches
for highly efficient preparation of structurally diverse 1,2,4-
oxadiazoles under green and benign reaction conditions is still
highly desirable.
T
membered nitrogen-containing heterocyclic compounds.1
In the oxadiazole family, molecules containing 1,2,4-oxadiazole
moieties not only are frequently found in many natural and
artificial products showing interesting physiological and
biological activities but also apply as the functionalized organic
materials (Figure 1).2 As representative examples shown in
The cycloaddition reaction is a powerful strategy to build
molecular complexity into heterocycles.6 In this regard,
nitrosoarenes have been extensively investigated due to their
high reactivity profile in the preparation of highly function-
alized heterocyclic compounds.7 The pioneering work toward
the [3 + 2]-cycloaddition of nitrosoarene as dienophiles was
demonstrated by the Studer group in 2011, in which
allylstannanes were used as the formal 1,3-dipoles.8 Shortly
after this discovery, [3 + 2]-cycloaddition of nitrosoarene with
donor−acceptor cyclopropanes (DAC) was also realized by
the same group.9 Compared with those well-established ionic
cycloaddition processes where nitrosoarenes are employed as
electrophiles, reactions of nitrosoarenes as radical acceptors
have been considerably overlooked.10 In another arena,
chemical reactions initiated by visible-light-induced photo-
redox catalysis have attracted much attention in the past
several years due to its environmental compatibility and
versatility in character.11 However, to the best of our
knowledge, visible-light-promoted transformation of nitro-
soarenes, especially in [3 + 2]-cycloadditions toward the
construction of heterocycles, has not yet been disclosed.
In 2017, Sheikh, Leonori, and co-workers reported a visible-
light-promoted decarboxylative radical addition/protonation of
carboxylic acids and nitrosoarenes, where nitrosoarenes serve
as suitable radical acceptors to form persistent nitroxyl radical
intermediates (Scheme 1a).12,13 Motivated by these findings
Figure 1. Some biologically important structures containing 1,2,4-
oxadiazole motifs.
Figure 1, phidianidines A and B were isolated from marine
opisthobranch mollusk Phidiana militaris, which exhibits
cytotoxicity against tumor and nontumor mammalian cell
lines in in vitro assays.3 Drugs containing a 1,2,4-oxadiazole
core, such as libexin, exhibit antitussive activity, while
oxolamine has anti-inflammatory activity.4 Driven by their
rich biological activities, the development of efficient and
practical methods for the synthesis 1,2,4-oxadiazole is of
widespread interest in synthetic organic chemistry. Tradition-
ally, 1,2,4-oxadiazole derivatives are accessed by 1,3-dipolar
cycloaddition of nitriles to nitrile oxides or thermally promoted
Received: April 23, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01416
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Organic Letters
Letter
a,b
Scheme 1. Nitrosoarenes as Radical Acceptors in Visible-
Light-Induced Photoredox Catalysis
Scheme 2. Substrate Scope of 2H-Azirines
and our ongoing research interest in the development of
heterocycle-oriented methodologies,14 we became interested in
further exploring the potential reactivity of nitrosoarenes with
other types of radical species, such as radical cation A,
generated from the ring opening of 2H-azirines under visible-
light irradiation.15 We assumed that the formed radical cation
B might undergo a SET reduction/intramolecular cyclization
sequence, thus providing a new avenue to access biologically
important 2,5-dihydro-1,2,4-oxadiazoles (Scheme 1b). Herein,
we describe the preliminary results of this study.
At the outset, 2,3-diphenyl-2H-azirine 1a and nitro-
sobenzene 2a were selected as model substrates to test the
feasibility of the designed process. Fortunately, condition
optimization revealed that the proposed formal [3 + 2]-
cycloaddition reaction did indeed occur with 52% isolated
yield in the presence of 5 mol % of 9-mesityl-10-
methylacridinium perchlorate16 as photoreodx catalyst in
degassed CH3CN, under irradiation with blue LED for 5 h.
Control experiments indicated that the photoredox catalyst,
blue LED irradiation, and degassing procedure are all crucial to
the process (for details of the optimization and control
experiments, see the Supporting Information). To our delight,
an almost similar yield was obtained when the reaction was
enlarged to 1.0 mmol scale (10 h, 51%).
a
1 (0.1 mmol), 2a (0.2 mmol), and PC (0.05 mmol) in degassed
b
CH3CN (1.0 mL) under irradiation with 24 W blue LEDs. Isolated
yield. The reaction was carried out on a 1.0 mmol scale. PC = 9-
mesityl-10-methylacridinium perchlorate.
c
We next turned our attention to further investigating the
substrate scope with respect to the nitrosoarene component
(Scheme 3). Further underscoring the efficiency of this [3 +
a,b
Scheme 3. Substrate Scope: Variation of Nitrosoarenes
After identifying the optimal reaction conditions, we set out
to explore the scope of this formal [3 + 2]-cycloaddition
process by reacting a series of substituted 2H-azirines 1 with
nitrosobenzene 2a. As the results in Scheme 2 revealed, the
electronic property variation on the R1-phenyl ring of 2H-
azirines 1 has no obvious effect on the reaction efficiency. Both
electron-rich (−Me, −Et, −OMe, −OPh) and electron-
deficient substituents (−F, −Cl) at the para-position were
suitable for this photocatalytic transformation, providing the
corresponding 1,2,4-oxadiazole derivatives 3ba−ga in good
yields (46−63%). Note that reactions with 2H-azirines bearing
R1-naphthyl (1h) and R1-heterocycles such as furan (1i) and
thiophene (1j) proceeded smoothly. The R1-aryl substituent
could be replaced by an alkyl substituent as documented for
the butyl congener, albeit with relative low yield (3ka, 21%).
Moreover, substituent modification on the R2-phenyl ring also
proved to be successful, giving the [3 + 2]-cycloaddition
adducts 3la−pa in average good yield. It is worth noting that
the structure of 3oa was unambiguously confirmed by X-ray
diffraction analysis. The 3-phenyl-2-vinyl-2H-azirine 3q is also
a suitable substrate for this photocatalytic [3 + 2]-cyclo-
addition process to give 2,5-dihydro-1,2,4-oxadiazole 3qa in
25% yield. The aryl groups in 2H-azirines 1 are crucial for the
high yields of 2,5-dihydro-1,2,4-oxadiazole products, which
might be because it can stabilize the formed radical-cation
intermediate B during the reaction (Scheme 6).15
a
1a, 1j, or 1l (0.1 mmol), 2 (0.2 mmol), and PC (0.05 mmol) in
degassed CH3CN (1.0 mL) under irradiation with 24 W blue LEDs.
b
Isolated yield. PC = 9-mesityl-10-methylacridinium perchlorate.
2]-cycloaddition process, 2,5-dihydro-1,2,4-oxadiazole prod-
ucts were obtained in reasonably good yields when the para-
and ortho-substituted nitrosobenzenes and 2-nitrosopyridine
(2h) were treated with 2H-azirines under the standard reaction
conditions (3ab−ae, 3jf, 3lg, and 3ah). With respect to
reaction efficiency, better results were observed for para-
substituted nitrosoarenes compared to ortho-substituted
substrates, which might be due to the steric effect. Note that
a nitroso compound bearing electron-rich amine motifs, such
as N-nitrosodiphenylamine 2i, did not react at all.
The preparative utility of this method can be demonstrated
by exploring the follow-up chemistry as depicted in Scheme 4.
B
DOI: 10.1021/acs.orglett.9b01416
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Organic Letters
Letter
conditions, and this ring-opening reaction included a radical
step. Stern−Volmer fluorescence quenching experiments
indicated that 3aa could efficiently quench the photoexcited
9-mesityl-10-methylacridinium perchlorate, where it presum-
ably engaged in a SET process in the reaction (Scheme 5c).
Moreover, the oxidative potential of 3aa in CH3CN was
determined to be +1.21 V vs SCE (see the SI), indicating that
direct oxidation of 3aa by the photoexcited 9-mesityl-10-
methylacridinium perchlorate (E1/2red* = +2.06 V vs SCE)16 is
thermodynamically favored.
Scheme 4. Follow-up Chemistry
On the basis of the above observations and the literature
reports, a plausible reaction mechanism is proposed in Scheme
6. The formation of radical cation intermediate B or B′ from
Scheme 6. Plausible Reaction Mechanism
Direct sunlight irradiation of the reaction of 1g with 2a
provided the desired 2,5-dihydro-1,2,4-oxadiazole 3ga with
moderate yield. It was found that reductive N−O bond
cleavage in 3aa with Mo(CO)6/NaBH4 afforded N-phenyl-
benzimidamide in 65% yield (see the SI). This provided us a
good option to transfer 1,2,4-oxadiazole derivatives to other
biologically important heterocycles, such as imidazole 4,18a
triazole 5,18b and thiadiazole 6,18c after a two-step simple
operation.
Regarding the mechanism, we found that this [3 + 2]-
cycloaddition process required continuous visible-light irradi-
ation based on the results of light “on−off” experiments
(Scheme 5a). In the reaction system, we can isolate a ring-
Scheme 5. Mechanistic Experiments
ring-opening of 2H-azirines under photocatalytic conditions
was the same as in previous reports.15 Radical addition of B or
B′ to nitrosobenzene affords radical cation species C, which
subsequently undergoes a single-electron reduction/intra-
molecular cyclization cascade to give the formal [3 + 2]-
cycloaddition adduct 3aa and regenerates the photoredox
catalyst to complete the first photocatalytic cycle. As a
competing process, the formed 1,2,4-oxadiazole can serve as
reductive quencher to reduce the photoexcited 9-mesityl-10-
methylacridinium perchlorate and generate a resonance-
stabilized radical cation E. Deprotonation of E afforded the
α-amino radical species F.17 The N−O cleavage of F provides
N-centered radical intermediate G, which is reduced by a low-
valent photoredox catalyst to complete the second photo-
catalytic cycle and provides the nitrogen anion F. Finally,
protonation of F gives the ring-opening byproduct.
In conclusion, we have developed a visible-light-promoted
[3 + 2]-cycloaddition reaction of 2H-azirines with nitrsoarenes.
The method utilizes nitrosoarenes as suitable radical acceptors,
which provides a green and efficient method to a series of
biologically important 1,2,4-oxadiazole derivatives in average
good yields. More importantly, the direct sunlight-irradiation
process and the easy synthetic transformation of the final 2,5-
dihydro-1,2,4-oxadiazoles to other biologically important
opening byproduct, and this is also the major reason for the
relatively low yields of the [3 + 2]-cycloaddition products in
some cases. To gain insight into the formation of this
byproducts, some control experiments were conducted
(Scheme 5b). Treatment of 3aa under the standard reaction
conditions provided byproduct 7 in 77% isolated yield after 2 h
irradiation. Control experiments revealed that both of
photoredox catalyst and light irradiation were essential for
the formation of 7 from 3aa. In addition, only a trace amount
of 7 was observed when 2.0 equiv of TEMPO was added to the
reaction system. These results clearly revealed that 7 was
generated from 3aa under photoredox catalytic reaction
C
DOI: 10.1021/acs.orglett.9b01416
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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valuable. The further discovery of new cycloaddition reactions
by using the nitrosoarenes as radical acceptors is currently
underway in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01416.
1
Experimental procedures, characterization data, and H,
and 13C NMR spectra (PDF)
Accession Codes
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CCDC 1904024 and 1904026 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: xuanjun@ahu.edu.cn.
*E-mail: wxiao@mail.ccnu.edu.cn.
ORCID
̈
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2011, 50, 11257−11260.
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Jun Xuan: 0000-0003-0578-9330
Wen-Jing Xiao: 0000-0002-9318-6021
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (No. 21702001), the Natural Science Foundation of
Anhui Province (No. 1808085MB47), the Open Fund for
Discipline Construction, Institute of Physical Science and
Information Technology, Anhui University, and a Start-up
Grant from Anhui University for financial support.
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