Visible-light-promoted aerobic oxidative cyclization to access 1,3,4-oxadiazoles from aldehydes and acylhydrazides

Article abstract of DOI:10.1016/j.tetlet.2014.02.022

A novel and practical access to symmetrical and unsymmetrical 2,5-disubstituted 1,3,4-oxadiazoles directly from aldehydes and acylhydrazides using visible light irradiation under an air atmosphere in the presence of eosin Y as an organophotoredox catalyst at rt is reported. This is the first example of oxidative cyclization of acylhydrazones employing air and visible light as inexpensive, readily available, non-toxic, and sustainable reagents.

Full text of DOI:10.1016/j.tetlet.2014.02.022

Tetrahedron Letters 55 (2014) 2065–2069
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Visible-light-promoted aerobic oxidative cyclization to access
1,3,4-oxadiazoles from aldehydes and acylhydrazides
⇑
Arvind K. Yadav, Lal Dhar S. Yadav
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel and practical access to symmetrical and unsymmetrical 2,5-disubstituted 1,3,4-oxadiazoles
directly from aldehydes and acylhydrazides using visible light irradiation under an air atmosphere in
the presence of eosin Y as an organophotoredox catalyst at rt is reported. This is the first example of oxi-
dative cyclization of acylhydrazones employing air and visible light as inexpensive, readily available,
non-toxic, and sustainable reagents.
Received 19 December 2013
Revised 8 February 2014
Accepted 10 February 2014
Available online 21 February 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Aerobic oxidation
Acylhydrazones
Eosin Y
Oxadiazoles
Photoredox catalysis
Visible light
The development of new modes for the catalytic activation of
organic molecules is among the fundamental aims in the field of
organic synthesis. In this sequence visible light photoredox cataly-
sis has recently received much attention in organic synthesis ow-
ing to ready availability, sustainability, non-toxicity, and ease of
handling of visible light.1 The pioneer work of the groups of McMil-
lan,2 Yoon,3 and Stephenson4 have reinvigorated the field of visible
light photoredox catalysis utilizing ruthenium and iridium com-
plexes as photoredox catalysts. Their ability to engage in single-
electron-transfer (SET) processes with organic substrates upon
photoexcitation with visible light is remarkable and has inspired
the development of new methodologies in organic synthesis be-
cause most of the organic compounds, do not absorb in the visible
region (400–800 nm).
However, these transition metal based photocatalysts disadvan-
tageously exhibit high cost, potential toxicity, and low sustainabil-
ity. Recently, metal-free organic dyes such as eosin Y, fluorescein,
Rose Bengal, nile red, perylene, and rhodamine B have been used
as economically and ecologically superior surrogates for Ru(II)
and Ir(II) complexes in visible-light-promoted organic transforma-
tions involving SET.5 The five-membered aromatic heterocycles
comprise important structural motifs in the field of pharmaceutical
chemistry possessing biological activities like antifungal, antiviral,
and antibacterial properties.6 Among them, the 2,5-disubstituted
1,3,4-oxadiazole motifs are unique due to their optoelectronic
properties.7 They also find applications in the field of material sci-
ence because of their good electron-transporting and hole-blocking
abilities.8
To date a plethora of methods have been developed for the syn-
thesis of 1,3,4-oxadiazoles. The most common of them, utilizes oxi-
dative cyclization of acylhydrazones with oxidizing agents such as
13
ceric ammonium nitrate,9 chloramines T,10 Br2,11 KMnO4,12 FeCl3,
and iodine.6 The major drawbacks associated with them include
the use of expensive, hazardous materials, multistep operations
and limited substrate scope. In the context of oxidative organic
transformations, aerobic oxygen has recently received an upsurge
as a hot topic of research.14 It is an ideal source of oxygen, cost
effective, oxidant, readily available, and efficiently complies with
the principles of green chemistry. To the best of our knowledge,
the literature records only one method for the synthesis of 1,3,4-
oxadiazoles utilizing atmospheric O2 as a terminal oxidant but it
employs transition metal catalysis.7 Inspired by organocatalytic
visible-light-mediated
aerobic
oxidative
transforma-
tions5b,d,g,j,15,16a and our successful efforts for one-pot heterocyliza-
tion reactions,15b,16 the present oxidative cyclization of
acylhydrazones to 1,3,4-oxadiazoles was devised as depicted in
Scheme 1. On the basis of earlier observations and properties of or-
ganic dyes used as photoredox catalysts,5 eosin Y was chosen as
the photocatalyst for the present study.
In general, organosulfur/nitrogen compounds have been fre-
quently used as precursors in radical reactions because they form
⇑
Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533.
E-mail address: ldsyadav@hotmail.com (Lal Dhar S. Yadav).
http://dx.doi.org/10.1016/j.tetlet.2014.02.022
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2066
A. K. Yadav, Lal Dhar S. Yadav / Tetrahedron Letters 55 (2014) 2065–2069
H
N
2,5-disubstituted 1,3,4-oxadiazoles via oxyradicals would extend
the substrate scope for visible-light-mediated organic syntheses.
In order to work out the envisaged protocol, a key reaction was
conducted with acylhydrazone A in DMF containing 2 mol % of
eosin Y under an air atmosphere (without air bubbling) by irradia-
tion with visible light (green light-emitting diodes (LEDs),
k = 535 nm) at rt. The reaction delivered the desired 2,5-disubsti-
tuted 1,3,4-oxadiazole (3a) in 15% isolated yield after 24 h
(Table 1, entry 1). Following this experiment, a series of control
experiments were performed, which demonstrated that an organic
base is essential to give the desired product in a high yield (93%)
(Table 1, entry 2) and iPr2NEt was found to be the best base
(Table 2, entry 2 vs entries 6, 7, and 9). There was no product for-
mation or it was formed in traces in the absence (À) of any one of
the reagents/catalyst (Table 1, entries 3–5). The reaction did not
proceed satisfactorily when a household 18 W fluorescent lamp
was used instead of green LEDs (Table 1, entry 6 vs 2). Notably,
the same result was obtained on using O2 (balloon) instead of an
air atmosphere (Table 1, entry 8 vs 2), while under the degassed
condition or under a nitrogen atmosphere no product formation
was detected (Table 1, entries 5 and 7). These results establish
that visible light, base, photocatalyst, and air are all essential (+)
for the reaction and support the photocatalytic model of the
reaction.
R2
N
N
eosinY, DMF, green LEDs
, rt
N
R2
R1
iPr2NEt, O2
(air), 12-20 h
O
O
R1
H
Scheme 1. Visible-light-promoted aerobic oxidative cyclization of acylhydrazones.
Table 1
Screening and control experimentsa
H
N
N
iPr NEt
2
N
H
Ph
eosin Y (2 mol%),
, air
N
Ph
Ph
O
3a
O
DMF, visible light, 12-24 h, rt
Ph
A
Entry
Visible light
Eosin Y
Air
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
+
+
À
+
+
+
+
+
+
+
+
+
+
+
À
+
+
+
+
+
+
+
+
+
+
+
24
12
24
24
24
12
24
12
12
12
12
15c
93
n.r.d
n.r.
n.r.
49e
trace
93
À
+
N2
O2
+
+
+
93f
10
11
62g
46h
a
i
Reaction conditions: A (1.0 mmol), eosin Y (2 mol %), Pr2NEt (2.0 equiv), DMF
Next, the reaction conditions were optimized with respect to
solvents and the catalyst loading. In all the tested solvents (DMF,
DMSO, MeOH, and EtOH) the yield of 3a was >50% (Table 2), which
indicates that the reaction is not very sensitive to reaction media.
DMF was the best solvent in terms of the reaction time and yield
(Table 2, entry 1), hence it was used throughout the present work.
When the amount of the catalyst was decreased from 2 to 1 mol %,
the yield of 3a considerably reduced (Table 2, entry 3), but the use
of 3 mol % of the catalyst did not affect the yield (Table 2, entry 1).
Under the established reaction conditions in hand, the reaction
was tried in a one-pot procedure starting directly from an aldehyde
1a and an acylhydrazide 2a to give the desired product 3a as de-
picted in Scheme 2. To our delight, it worked well and a number
of symmetrical and unsymmetrical 2,5-disubstituted 1,3,4-oxadi-
azoles were successfully synthesized starting directly from various
aldehydes 1 and acylhydrazides 2 (Tables 3 and 4). This clearly
shows that the reaction is very mild and applicable to aryl, alkyl,
heteroaryl, and alicyclic substrates, and tolerates considerable
functional group variations like MeO, Br, Cl, and NO2 in the sub-
strates 1 and 2. Regardless of differences in the electronic and ste-
ric properties of substrates 1 and 2, they afford the desired product
3 in good to excellent yields (70–96%). However, aldehydes 1 and
hydrazides 2 with an electron-donating group on the aromatic ring
appear to react faster and afford marginally higher yields in com-
parison to those bearing an electron-withdrawing group (Table 3,
entries 2 and 3 vs entries 4–7; and Table 4, entries 2 and 3 vs en-
tries 4 and 8).
(3 mL), green LEDs 2.6 W, 161 lm irradiation under an air atmosphere at rt.
b
Isolated yield of the product 3a, n.r. = no reaction.
The reaction was conducted without Pr2NEt base in DMF.
The reaction was carried out in the dark.
c
d
e
f
i
The reaction was carried out using 18 W CFL (compact fluorescent lamp).
i
The reaction was performed with 3.0 equiv of Pr2NEt.
g
h
i
Reaction was performed with 1.0 equiv of Pr2NEt.
Reaction was performed with 1.0 mol % of Eosin Y.
Table 2
Optimization of reaction conditionsa
H
N
N
N
H
Ph
eosin Y (mol%), base, air
N
Ph
Ph
O
O
solvent, green LEDs, 12-18 h, rt
Ph
A
3a
Entry
Eosin Y (mol %)
Base
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
3
2
1
2
2
2
2
2
2
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
DBU
DABCO
iPr2NEt
Et3N
DMF
DMF
DMF
MeOH
EtOH
DMF
DMF
DMSO
DMF
12
12
12
18
18
18
18
12
18
93
93
46
76
65
55
58
85
67
a
Reaction conditions: A (1.0 mmol), eosin Y (1–3 mol %), solvent (3 mL), base (2.0
equiv), green LEDs 2.6 W, 161 lm irradiation under an air atmosphere at rt.
b
Isolated yield of the product 3a.
On the basis of the above observations and the literature prece-
dents,5,15,16a,17–19 a plausible mechanism involving photoredox
catalysis for the oxidative cyclization of acylhydrazones is depicted
in Scheme 3. On absorption of visible light, the organophotoredox
catalyst eosin Y (EY) is excited to its singlet state 1EY⁄ which
through inter system crossing (ISC) comes to its more stable triplet
state 3EY⁄ and undergoes a single electron transfer (SET). 3EY⁄ may
radicals very readily.5f,i,j,15b–d,17 Surprisingly, visible-light-
promoted organic transformations involving oxyradicals as
intermediates have been far less extensively studied.4a,b,16b Thus,
the present intramolecular cyclization of acylhydrazones to
H
N
N
H
N
DMF, 60 o
2-4 h
C
eosin Y, green LEDs
iPr2NEt, O2 (air), 12-20 h, rt
N
Ph
Ph
N
H2N
+
PhCHO
Ph
Ph
O
O
O
Ph
H
1a
3a
2a
A
without isolation
Scheme 2. One-pot sequential reaction for synthesis of 2,5-disubstituted 1,3,4-oxadiazoles directly from aldehydes and acylhydrazides.

A. K. Yadav, Lal Dhar S. Yadav / Tetrahedron Letters 55 (2014) 2065–2069
2067
Table 3
Scope of aldehydesa
N
N
H
N
(i) DMF, 60 oC, 2-4 h
R1CHO
+
R1
H2N
O
3
(ii) eosin Y (2 mol%),green LEDs,
iPr2NEt, O2 (air), 12-20 h, rt
2a
1
O
Entry
1
Aldehydes 1
CHO
Product 3
Timeb (h)
12
Yieldc,d (%)
N
N
93
95
O
3a
1a
CHO
N
N
2
3
12
12
O
3b
1b
CHO
CHO
N
N
O
3c
96
84
81
O
1c
O
N
N
O
3d
4
5
6
15
15
20
Br
1d
CHO
Br
N
N
O
Cl
3e
1e
Cl
CHO
N
N
O
76
86
O2N
1f
CHO
3f
O2N
N
N
O
3g
7
20
1g
NO2
NO2
CHO
1h
N
N
8
9
12
12
12
83
85
89
O
3h
N
CHO
N
1i
O
3i
CHO
N N
10
O
1j
3j
CHO
N N
11
12
15
15
85
83
O
1k
3k
CHO
CHO
N
N
O
3l
1l
O
13
14
12
12
85
82
O
S
O
S
1m
3m
3n
N
N
O
CHO
1n
N
N
CHO
N
N
N
15
16
84
O
N
1o
3o
a
For the experimental procedure, see Ref. 20.
Time required for step (ii).
b
c
Isolated yield of the products 3.
d
All compounds are known and were characterized by comparison of their mp, IR, NMR, and MS data with those reported in the literature.6–8

2068
A. K. Yadav, Lal Dhar S. Yadav / Tetrahedron Letters 55 (2014) 2065–2069
Table 4
Scope of acylhydrazidesa
H
N
OHC
+
N
N
(i) DMF, 60 oC, 2-4 h
R2
NH2
R2
(ii) eosin Y (2 mol%), green LEDs,
iPr2NEt, O2 (air), 12-20 h, rt
O
O
2
1a
3
Entry
1
Acylhydrazides 2
Product 3
Timeb (h)
12
Yieldc,d (%)
O
N
N
NHNH2
O
3a
92
2a
O
N
N
NHNH2
O
3b
2
3
12
12
93
95
2b
O
N
N
NHNH2
O
O
3c
O
O
2c
N
N
NHNH2
O
3f
4
5
6
7
20
12
12
16
20
77
87
82
85
70
O2N
O2N
2d
O
N N
O
NHNH2
O
3j
2e
N
N
NHNH2
O
3k
2f
O
N N
NHNH2
O
N
N
3o
2g
O
N
O
N
NHNH2
NO2
8
O2N
3p
NO2
O2N
2h
a
For the experimental procedure, see Ref. 20.
Time required for step (ii).
b
c
Isolated yield of the products 3.
d
All compounds are known and were characterized by comparison of their mp, IR, NMR, and MS data with those reported in the literature.6–8
R2
Br
Br
R2
N
H
N
H
SET
N
O
O
HO
Br
N
O
R1
O
Br
R1
eosin Y
eosin Y
Photocatalytic
eosin Y*
COOH
B
C
(ai
r)
O2
cycle of eosin Y
SET
-H+
iPr2NEt
visible light
Br
O2
Br
eo
sin Y
N
O
O
HO
Br
O2
H
N
N
R2
N
N
eosin Y
vis. light, air
eosin Y
N
Br
R1
R2
R2
R1
O
COOH
O
O
H
R1
H
HO2
A
3
D
Photoredox catalytic cycle of eosin Y
hv
ISC
Eosin Y
(EY)
EY
1EY*
3EY*
oxidant
EY
reductant
SET
Scheme 3. Photoredox catalytic cycle of eosin Y for the aerobic oxidative cyclization of acylhydrazones to 1,3,4-oxadiazoles.
undergo both reductive15a–c and oxidative5a,b,21 quenching. A SET
from B to 3EY⁄ generates acyl radical C, which undergoes intramo-
lecular cyclization (5-endo-trig) to form D followed by attack of O2À
to give the product 3, successively. The formation of superoxide
radical anion (OÀ2 ) during the reaction was confirmed by the detec-
tion of the resulting H2O2 using KI/starch indicator.18

A. K. Yadav, Lal Dhar S. Yadav / Tetrahedron Letters 55 (2014) 2065–2069
2069
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In conclusion, we have developed a novel transition metal-free
organocatalytic method for the synthesis of 1,3,4-oxadiazoles
directly from aldehydes and acylhydrazides in a one-pot procedure
in the presence of inexpensive eosin Y as a powerful organophoto-
redox catalyst at rt. The reaction utilizes visible light, a base, and O2
(air) as reagents. This protocol is a superior alternative to the exist-
ing methods for the synthesis of 1,3,4-oxadiazoles with several
advantages in terms of green chemistry parameters such as sus-
tainability, eco-friendly reagents, high efficiency, and one-pot sim-
ple operation under mild conditions. This methodology widens the
scope of substrates for visible light photoredox reactions.
Acknowledgments
We sincerely thank the SAIF, Punjab University, Chandigarh, for
providing spectra. One of us (A.K.Y.) is grateful to the CSIR, New
Delhi, for the award of a Senior Research Fellowship.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.02.022.
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3
and 4) from aldehydes and acylhydrazides: A solution of an aldehyde 1
(1.0 mmol) and an acylhydrazide 2 (1.0 mmol) in DMF (3 mL) was heated at
60 °C for 2–4 h to form the corresponding acylhydrazone (as monitored by
TLC). Then, eosin Y (2.0 mol %) and iPr2NEt (2.0 equiv) were added and the
mixture was irradiated with green LEDs (2.6 W, 161 lm) with stirring under an
air atmosphere at rt for 12–20 h. After completion of the reaction (monitored
by TLC), water (5 mL) was added and the mixture was extracted with EtOAc
(3 Â 5 mL). The combined organic phase was dried over MgSO4, filtered, and
evaporated under reduced pressure. The resulting product was purified by
silica gel column chromatography using a gradient mixture of hexane/ethyl
acetate as eluent to afford an analytically pure sample of 3. All the products are
known compounds and were characterized by the comparison of their spectral
data with those reported in the literature.6–8
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C.; Valdes-Aguilera, O. M. Adv. Photochem. 1993, 18, 315.