Visible light-induced cyclization reactions for the synthesis of 1,2,4-triazolines and 1,2,4-triazoles

Article abstract of DOI:10.1039/c7cc04911k

A novel method for concisely synthesizing 1,2,4-triazolines via [3+2] cyclization under visible light is reported. These compounds can be easily converted into 1,2,4-triazoles under basic or photoredox conditions. The application of the 1,2,4-triazoles was also investigated via mild operations.

Full text of DOI:10.1039/c7cc04911k

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Visible Light-Induced Cyclization Reactions for the Synthesis of
1,2,4-Triazolines and 1,2,4-Triazoles
Hongyu Wang,† Yanfei Ren,† Kaiye Wang, Yunquan Man, Yanan Xiang, Na Li* and Bo Tang*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel method for concisely synthesizing 1,2,4-triazolines via reactions induced by visible light for the synthesis of 1,2,4-triazoles
[3+2] cyclization under visible light is reported. These compounds would be an interesting and significant method. 2H-azirines, as
can be easily converted into 1,2,4-triazoles under basic or highly important valuable intermediates for preparing various
photoredox conditions. The application of the 1,2,4-triazoles was functional compounds, could be catalyzed by transition metal
also investigated via mild operations.
catalysts or induced by UV light irradiation.7 Recently, Xiao and co-
workers found that 2H- azirines could be excited by
a
Nitrogen-containing moieties are vital parts of a large number of
medicinal compounds and naturally biological products.1 Thus, the
development of novel strategies for the efficient synthesis of these
heterocyclic compounds is highly important in the field of synthetic
chemistry. Notably, 1,2,4-triazolines and 1,2,4-triazoles represent
important structural units that exhibit anticancer and
anticonvulsant properties and which exist in numerous biologically
active compounds (Figure 1).2 Due to the significance of these
molecules, some strategies for their efficient synthesis have been
developed based on the metal-catalysis and organocatalysis.3 In
addition, due to being readily available, azodicarboxylates have
been employed as important reagents for the synthesis of these
compounds. For example, Tepe and co-workers reported the
efficient synthesis of 1,2,4-triazoles utilizing oxazolones and
azodicarboxylate (Scheme 1A) via two steps.4 Heinrich’s group
realized the [3+2] cyclization between azomethines ylides and
phenylazocarboxylates yielding the 1,2,4-triazoles via three steps
(Scheme 1B).5 To the best of our knowledge, although large number
of methods for preparing these products has been reported in the
photosensitizer under visible light, in order to synthesize oxazoles
and pyrroles.8 Inspired by the works, we speculated that 1,2,4-
triazoles might be synthesized by photoredox catalysis using 2H-
azirines and azodicarboxylates. Herein, we report the efficient
synthesis of 1,2,4-triazolines and 1,2,4-triazoles under visible light
via one step.
Initially, we examined the reaction of 2H-azirine 1a with
azodicarboxylate 2a in the presence of I (9-mesityl-10-methyl-
acridinium perchlorate). DCE was used as the solvent under blue
LED illumination. To our delight, the reaction was completed in 10
CO2H
N
N
N
N
N
N
Et
N
N
H
N
O
OMe
OH
DL 111-IT
R3
R1
N
R2
Cl
Triadimenol
OH
N
N
Ligand
Penipanoid A
past decades, mild operating conditions with metal-free, room Figure 1. Represent biological active compounds and ligands.
temperature, and fewer experimental steps are still challenging
Previous work:
projects for chemists.
R1
O
1) CH3CN, RT
2) NaOH, EtOH, reflux
ref.
R2
H
N
Recently, photoredox catalysis has emerged as
a powerful
R1
R3O2
C
N
N
A:
B:
O
N
N
CO2R3
N
N
approach in organic synthesis, and various novel reactions have
begun to be fully realized under visible light.6 Therefore, cyclization
R2
1) DBU (1 equiv)
2) MnO2 (2.5 equiv)
Ar
N
3) CF3CO2
H
Ar
N
Ar
N
CO2R
+
CO2tBu
CO2R
ref.
N
N
Ar
This work:
R2
N
R2
C
N
N
Photosensitizer
N
N
CO2R3
+
N
R1
and
R3O2
C
N
R1
R1
R2
N
10 W Blue LED
N
R3O2
C
R3O2
CO2R3
mild conditions
high yield
gram-scale
up to 99% yield
up to 92% yield
one step, two products
Scheme 1. The synthesis of 1,2,4-triazoles.
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min to give 3a in 91% yield (Table 1, entry 1). However, when the
Under the optimized conditions, we next investigated the scope
.
reaction was catalyzed by Ru(bpy)3Cl2 6H2O, Ir(ppy)2(dtbbpy)PF6 and of the [3+2] cyclization reactions with various azodicarboxylate and
EY, no products could be obtained (Table 1, entries 4-6). After 2H-azirines. In general, all of the reactions proceeded smoothly to
screening the solvents, no better results were obtained, and DCE afford the desired products in good and excellent yields. Different
was determined to be the superior choice (Table 1, entries 7-9). protecting groups of the azodicarboxylate, including ethyl, isopropyl,
Control experiments were also performed, and no reaction and benzyl groups, were all tolerated in the reactions. The electron-
occurred in the absence of the photosensitizer or visible light (Table withdrawing groups and electron-donating groups on the benzene
1, entry 2,3). Taken together, these results indicated that the [3+2] ring of the 2H-azirines all furnished the corresponding products
cyclization was induced by photoredox catalysis.
with good to excellent yields within the desired time period. The
naphthyl moiety of the 2H-azirines was also tolerated, with 62%
yield and 65% yield in 1h (Table 2). In particular, 3a can be scaled up
to gram-level affording 1.4g with 78% yield in 2 h. To further
explore the applications of the products 3, we subsequently
performed reactions between 3 and 4 under basic conditions. We
found that 1,2,4-triazoles with alkyl groups on the “N” site can be
obtained with good to excellent yields, irrespective of the electronic
and steric nature of the substituents on the benzene ring of 3 and 4
(Table 3).
Table 1. Screening the reaction conditions.a
When we were exploring the [3+2] cyclization reactions under
visible light, the desired 1,2,4-triazoles can be obtained stirring for a
longer time. Encouraged by the result, we then continued to
explore the direct aromatization of 3 induced by visible light under
photoredox catalysis. After screening the conditions of the reaction,
the reaction was efficiently catalyzed by I in the air with 99% yield
(in Ar, 35% yield), affording the corresponding product with high
aUnless otherwise noted, in all reactions, 1a (0.05 mmol) and 2a (0.075 mmol)
were catalyzed by
ClCH2CH2Cl.
I
( 5 mol%) in DCE (1 mL) under blue LED at 25 oC. DCE:
Table 3. Substrates scope of the tri-substituted 1,2,4-triazoles.a
Table 2. The scope of the [3+2] reactions.a
R
C
N
N
N
I (5 mol%)
R'
R''O2
C
N
+
N
N
CO2R''
10 W Blue LED,
R
R'
R''O2
DCE, 25 o
C
CO2R''
1
2
3
N
N
N
N
N
N
N
N
N
N
N
N
BnO2C
EtO2
C
EtO2C
iPrO2C
CO2Bn
CO2Et
CO2Et
CO2iPr
3a, 81% yield, 1 h
(78% yield, 2 h)b
3c, 60% yield, 1 h
3b, 62% yield, 1 h
3d, 47% yield, 41 h
Me
Et
F
Cl
N
N
N
N
N
N
N
N
N
N
N
N
EtO2C
EtO2C
EtO2
C
EtO2C
CO2Et
CO2Et
CO2Et
CO2Et
3e, 58% yield, 1 h
3f, 92% yield, 1 h
3g, 75% yield, 15 h
3h, 87% yield, 0.5 h
F
Cl
Br
MeO
N
N
N
N
N
N
N
N
N
N
N
N
EtO2C
EtO2
C
EtO2
C
EtO2C
CO2Et
3i, 71% yield, 0.5 h
Me
CO2Et
3j, 36% yield, 0.5 h
OMe
CO2Et
CO2Et
3k, 60% yield, 1 h
3l, 65% yield, 1.5 h
F
Br
N
N
N
N
N
N
N
N
N
N
N
N
EtO2C
EtO2
C
EtO2
C
EtO2
C
CO2Et
CO2Et
CO2Et
CO2Et
3p, 56% yield, 1 h
3n, 37% yield, 0.5 h
3o, 92% yield, 1 h
3m, 45% yield, 1 h
Me
Me
Me
N
Me
N
N
N
N
N
N
N
N
N
N
N
EtO2C
EtO2C
EtO2
C
EtO2
C
CO2Et
CO2Et
CO2Et
CO2Et
aUnless otherwise noted, the reactions were carried out with
(0.10 mmol) using NaH (0.10 mmol) in THF. bthe regioselectivity ratio
determined by 1H-NMR. cthe reactions were carried out with 3b (0.05 mmol) and
(0.10 mmol) using NaH (0.10 mmol) in THF.
3
(0.05 mmol) and
3q, 75% yield, 1 h
3r, 62% yield, 1 h
3s, 65% yield, 1 h
3t, 78% yield, 1 h
4
aUnless otherwise noted, in all reactions,
catalyzed by
(5 mol%) in DCE under blue LED at 25 oC. b1a (5 mmol) and 2a (7.5
mmol) were catalyzed by
(5 mol%) in DCE under blue LED at 25 oC.
1 (0.5 mmol) and 2 (0.75 mmol) were
I
4
I
2 | Chem. Commun., 2017, 00, 1-3
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yield in 1 h (See SI). Next, we investigated the influences of different
substituents, including ethyl ester, isopropyl ester and benzyl ester,
on the “N” of 3. All of the reactions proceeded well and furnished
the corresponding products with high yields within a short time
period. After the investigation of the electronic and steric nature of
the substituents on the benzene ring of 3, higher yields were
obtained when electron-withdrawing groups were present on the
benzene ring. By contrast, electron-donating groups, such as a
methoxyl, were detrimental to the reactions, as only 34% yield and
35% yield could be obtained (Table 4, 6h, 6l). Meanwhile, the
naphthyl groups were also tolerated. Furthermore, we also
explored the direct synthesis of 1,2,4-triazoles induced by the blue
LED via one step without any operations, 80% yield of 6a was
yielded by stirring for a longer time (Scheme 2A).
To gain insights into the aromatization of 3, we added TEMPO to
the reaction. However, only 35% yield was obtained after stirring
for 24 h, which suggested that the reaction included a radical step.
Though other oxidants were also tested in order to illustrate the
importance of this method, only DDQ could oxidize substrate 3a,
resulting in 88% yield over a long reaction time (24 h). Thus,
photoredox catalysis under visible light is critical for the
aromatization of 3a. Notably, 6a could be easily transformed using
LiAlH4 into the biologically active molecule 7,10 which has contra
gestational activity. The subsequent methylation of 7 affords 8,11
which has antibacterial activity. We also attempted the asymmetric
reaction between 1a and 2a using the chiral photosensitizer 9 as
the catalyst based on the ion-pair strategy;12 however, only 20% ee
Scheme 2. The mechanistic study and the transformations of 6a.
N
9 (5 mol%)
N
N
EtO2C
N
+
Ph
N
CO2Et
DCE, 10 W Blue LED, RT
Ph
N
EtO2C
1a
2a
CO2Et
3a, 95% yield, 20% ee
cat.
CF3
1 h
Me
CF3
Me
Me
O
O
O
O
Table 4. The scope of aromatization reactions of 3.a
P
N
Me
CF3
R
N
R
C
I (5 mol%)
N
N
10 W Blue LED,
DCE, 25 oC, air
9
N
N
R'O2
C
N
F3C
R'O2
CO2R'
6
3
Me
Scheme 3. The asymmetric [3+2] cyclization reaction.
N
N
N
N
N
N
N
N
N
N
N
N
BnO2
Br
C
EtO2C
EtO2C
iPrO2
C
6d, 65% yield, 1 h, 1.3:1b
6a, 99% yield, 1 h
6b, 75% yield, 1 h
6c, 81% yield, 0.5 h
MeO
F
Cl
N
N
N
N
N
N
C
N
N
N
N
C
N
N
EtO2
EtO2C
EtO2
EtO2C
6h, 34% yield, 0.5 h, > 20:1b
6g, 88% yield, 1 h, 1.5:1b
6f, 91% yield, 3.5 h, 1.2:1b
6e, 86% yield, 2 h, 1.3:1b
F
Me
Cl
N
N
N
N
N
N
N
N
N
EtO2
C
EtO2C
EtO2C
6i, 60% yield, 2 h, 1.5:1b
6j, 93% yield, 2.5 h, 1.5:1b
6k, 88% yield, 2 h, 1.3:1b
6k CCDC: 1552400
OMe
Scheme 4. The plausible reaction pathway.
F
Br
Me
N
N
N
N
was obtained. A detailed exploration of the asymmetric reactions
catalyzed by chiral photosensitizers is now underway in our lab.
Based on the above results and Xiao’s work, a plausible pathway
was proposed as shown in Scheme 4. Firstly, 2H-azrine is oxidized
by the photoredox catalyst under the visible light, and then 2-
azaallenyl radical cation A is generated sequentially. After that, 2a
will react with the radical cation A to form the new generated B,
N
N
N
N
EtO2C
N
N
N
N
EtO2C
EtO2C
EtO2C
6o, 98% yield, 0.5 h, 4:1b
6n, 55% yield, 1 h, > 20:1b
6l, 35% yield, 0.5 h, 1.2:1b 6m, 95% yield, 3 h, > 20:1b
Me
Me
Et
Me
N
N
N
N
N
N
N
N
N
N
N
EtO2
C
N
EtO2C
EtO2C
EtO2
C
6q, 41% yield, 1 h, > 20:1b
6r
, 78% yield, 2 h, > 20:1
6s, 80% yield, 0.5 h, 2:1b
b
6p, 72% yield, 0.5 h, > 20:1b
which will be reduced by
a low-valent photocatalyst. The
aUnless otherwise noted, in all reactions, 3 (0.05 mmol) were catalyzed by I (5
mol%) in DCE under blue LED. Supplementary crystallographic data for 6k can be
found in CCDC 1552400.9 bthe regioselectivity ratio determined by 1H-NMR.
corresponding product 3a is obtained via the catalytic cycle, and
then 3a is continued to be oxidized by the photoredox catalyst to
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the aromatization proceeded, the desired compound 6a is yielded.
In summary, we have developed a novel method in which 2H-
azirines and azodicarboxylates are utilized to efficiently synthesize
1,2,4-triazolines, with good to excellent yields, under visible light.
The 1,2,4-triazoles could be efficiently furnished when the 1,2,4-
triazolines reacted with benzyl bromides under basic conditions or
were directly catalyzed by photosensitizers under visible light.
Notably, the 1,2,4-triazoles can be transformed into biologically
active molecules via additional mild reactions. The asymmetric
cyclization reaction was also preliminarily investigated using a chiral
organic photosensitizer. Novel methods for synthesizing nitrogen-
containing compounds based on azodicarboxylates under visible
light are being developed in our lab, and our findings will be
reported in due course.
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This work was supported by 973 Program (2013CB933800),
National Natural Science Foundation of China (21390411,
21535004, 21422505, 21375081 and 21602125), Natural
Science Foundation for Distinguished Young Scholars of
Shandong Province (JQ201503), Natural Science Foundation of
Shandong Province (ZR2016BQ38) and a Project of Shandong
Province Higher Educational Science and Technology Program
(J16LC14) is gratefully acknowledged.
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CCDC 1552400
(
6k)
contains the supplementary
Notes and references
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Center via www.ccdc.cam.ac.uk/data_request/cif.
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DOI: 10.1039/C7CC04911K
Visible Light-Induced Cyclization Reactions for the Synthesis of
1,2,4-Triazolines and 1,2,4-Triazoles
Hongyu Wang, YanfeiRen, Kaiye Wang, Yunquan Man, Yanan Xiang, Na Li*and Bo Tang*
1,2,4-triazolines and 1,2,4-triazoles can be synthesized under the visible light via one step
without additional operations, which can be also scaled up to gram-level.