Organocatalytic Oxidative Amidation of Aldehydes with Tetrazoles to Construct 2,5-Diaryl 1,3,4-Oxadiazoles

Article abstract of DOI:10.1002/cjoc.201500598

A practical and metal-free oxidative amidation of aldehydes with tetrazoles into 1,3,4-oxadiazoles has been developed by employing tetrabutylammonium iodide (TBAI) as catalyst and tert-butyl hydroperoxide (TBHP) as oxidant. A wide range of 2,5-disubstituted 1,3,4-oxadiazoles can be conveniently generated in moderate to good yields. Gram-scale reaction was also realized in this catalytic system..

Full text of DOI:10.1002/cjoc.201500598

COMMUNICATION
DOI: 10.1002/cjoc.201500598
Organocatalytic Oxidative Amidation of Aldehydes with
Tetrazoles to Construct 2,5-Diaryl 1,3,4-Oxadiazoles
Jing Cao and Liang Wang*
School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China
A practical and metal-free oxidative amidation of aldehydes with tetrazoles into 1,3,4-oxadiazoles has been de-
veloped by employing tetrabutylammonium iodide (TBAI) as catalyst and tert-butyl hydroperoxide (TBHP) as oxi-
dant. A wide range of 2,5-disubstituted 1,3,4-oxadiazoles can be conveniently generated in moderate to good yields.
Gram-scale reaction was also realized in this catalytic system.
Keywords 1,3,4-oxadiazoles, oxidative amidation, tetrazoles, aldehydes, organocatalysis
Introduction
free protocol for one-pot synthesis of 2,5-diaryl
1,3,4-oxadiazoles in the presence of di-tert-butyl perox-
ide.[21] However, long reaction time and large amount of
radical promotor were still the major limitations.
1,3,4-Oxadiazoles are important five-membered het-
erocycles and have shown wide applications in pharma-
ceutical chemistry as well as material science.[1-4] A se-
ries of methods have been developed to construct
1,3,4-oxadiazoles during the past decades. Among them,
dehydrative cyclization of 1,2-diacylhydrazines and ox-
idative cyclization of N-acylhydrazones are the most
common strategies (Scheme 1). However, the former
reaction is conducted in the presence of SOCl2, PPA,
POCl3 or H2SO4.[5-9] For the latter one, various oxidants
including PbO2, Br2, chloramine T, HgO/I2, KMnO4,
CAN, hypervalent iodines, FeCl3, and I2/K2CO3 were
employed.[10-16] Harsh reaction conditions, hazardous
materials, as well as limited substrate scope are the ma-
jor limitations associated with these procedures. Huis-
gen reaction is also an old but attractive approach to
afford the 1,3,4-oxadiazoles, however, the formation of
acylated tetrazole intermediates requires a large excess
of carboxylic acid anhydride or acid chloride at elevated
temperature.[17-20] We also reported a metal- and base-
Recently, the direct oxidative amidation of aldehydes
with amines in the presence of various catalysts and ox-
idants have attracted much attention.[22-36] This elegant
approach makes use of cheap and readily available
starting materials. Through this protocol, some preacti-
vation steps or the use of unfriendly reagents can be
avoided, thereby improving the reaction atom-efficiency
and decreasing waste. Inspired by these methods, we
envisioned that the acylated tetrazoles could be synthe-
sized via direct oxidative coupling reaction between
5-aryl-2H-tetrazoles and aldehydes. Subsequently,
1,3,4-oxadiazoles were generated via simple thermal
rearrangement (Huisgen 1,3,4-oxadiazoles synthesis).
Herein, we report an organocatalyzed one-pot synthesis
of 1,3,4-oxadiazoles by employing tetrabutylammonium
iodide (TBAI) as catalyst and tert-butyl hydroperoxide
(TBHP) as oxidant.
Scheme 1 Strategies for synthesis of 1,3,4-oxadiazoles
Classical str ategies
O
O
H
O
N
N
dehydrative
cyclization
oxidative
R1
R2
R1
R2
R1
R2
HN NH
NH
N
O
cyclization
This work
N
O
N
N
N
O
N
N
TBAI, TBHP
N
N
H
+
R2
N
R1
R1
R2
R1
H
N
R2
-N2
-2H
O
*
E-mail: lwcczu@126.com. Tel.: 0086-0519-86330263; Fax: 0086-0519-86330251
Received August 19, 2015; accepted September 17, 2015; published online October 28, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500598 or from the author.
Chin. J. Chem. 2015, 33, 1239—1243
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Cao & Wang
Table 1 Optimization of reaction conditionsa
COMMUNICATION
Experimental
O
N
N
N N
Chemicals were used as received without special pu-
rification unless stated otherwise. Aryl tetrazoles were
prepared according to the literature.[37] 1H NMR and 13C
NMR spectra were recorded on a Bruker Advance 400
analyzer in chloroform-d (CDCl3) using TMS as an in-
ternal standard. The coupling constants J are given in
Hz. Melting points (m.p.) are determined with an Op-
timelt MPA 100 apparatus and are not corrected. Mass
spectrometry was performed on TSQ Quantum LC-MS.
Catalyst, Oxidant
solvent
N
+
H
H
Ph
2a
Ph
Ph
N
Ph
O
1a
3a
Entry Catalyst
Oxidant
TBHP
t/h
3
Solvent Yieldb/%
EtOAc 93(69c, 80d, 85e)
1
n-Bu4NI
2
n-Bu4NBr TBHP
n-Bu4NCl TBHP
3
EtOAc 86
3
12
3
EtOAc 20
4
NH4I
TBHP
TBHP
TBHP
TBHP
TBHP
－
EtOAc 31
General procedure for the synthesis of 2,5-disubsti-
tuted 1,3,4-oxadiazoles
5
NH4Br
KI
3
EtOAc trace
EtOAc 27
6
3
A sealed tube equipped with a magnetic stirrer bar
was charged with aryl tetrazole (0.5 mmol), aldehyde
(1.0 mmol), TBAI (0.05 mmol, 10 mol%), and TBHP
(3.0 equiv., 70% aqueous) in EtOAc (2 mL). The reac-
tion mixture was stirred at 120 ℃ for 3－12 h. After
reaction, the mixture was allowed to cool to room tem-
perature. The solvent was then removed under vacuum,
and the residue was purified by silica gel chromatogra-
phy using a mixture of petroleum ether/EtOAc to afford
the desired product 3.
7
I2
3
EtOAc 33
8
－
3
EtOAc trace
EtOAc NRf
EtOAc NRf
EtOAc NRf
EtOAc NRf
EtOAc NRf
EtOAc NRf
9
n-Bu4NI
－
3
10
11
12
13
14
15
16
17
18
19
H2O2
H2O2
O2
3
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
3
3
PIDA
Oxone
TBHP
TBHP
TBHP
TBHP
TBHP
3
3
3
DCE
65
3
Acetone trace
CH3CN 70
DMSO NRf
EtOAc 84g
Results and Discussion
3
Our initial studies focused on the model reaction of
5-phenyl-2H-tetrazole 1a with benzaldehyde 2a. En-
couragingly, product 3a was obtained in 93% yield with
TBAI as the catalyst and TBHP as oxidant at 120 ℃ in
ethyl acetate. When the temperature was decreased,
lower yields were obtained. This was because the de-
composition of acylated tetrazole was incomplete under
lower temperature, while by-product was generated at a
much higher temperature (Table 1, Entry 1). Among the
catalysts screened, TBAB also showed good catalytic
activity, while other catalysts including Bu4NCl, NH4I,
NH4Br, KI, and I2 gave unsatisfactory yields (Table 1,
Entries 2－7). This indicated that iodide was not essen-
tial for this reaction. No desired products were obtained
in the absence of either a catalyst or an oxidant. Switch-
ing oxidants to H2O2, O2, PIDA and Oxone halted the
reaction (Table 1, Entries 10－14). Further studies on
the solvents and oxidant loadings showed that ethyl ac-
etate and 3 equiv. of TBHP were the best choices (Table
1, Entries 15－19).
With the optimized conditions in hand, a series of
substituted tetrazoles were employed to explore the
generality of this protocol (Table 2). Generally, nearly
all of the investigated tetrazoles coupled with aldehydes
smoothly to provide the 1,3,4-oxadiazoles in good to
excellent yields (75%－93%). Various functional groups
such as methyl, chloro, fluoro, trifluoromethyl, and
methoxyl survived well under the reaction conditions
except the para-hydroxyl substituted tetrazole, which
provided the desired product 3n in trace amount. Steric
hindrance also showed negligible influence on the reac-
tion (3c, 88%).
3
3
a Unless otherwise specified, all reactions were carried out using
1a (0.5 mmol) and 2a (1 mmol) in a solvent (2 mL) with 10 mol%
of catalyst and oxidant (3.0 equiv.) at 120 ℃. b Isolated yield.
c Reaction at 80 ℃. d Reaction at 100 ℃. e Reaction at 130 ℃.
f No reaction. g 2 equiv. of TBHP was used.
Table 2 Reaction scope for aryl tetrazolesa
O
N
N
N N
O
N
TBAI, TBHP
R1
Ph
H
+
H
Ph
R1
120 oC, EtOAc
N
2
3
1
Entry Product R1
Yieldb/% m.p./℃lit.
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
Ph
93
138－139 (136－138)[38]
122－124 (121－122)[38]
110－111 (96－98)[39]
112－113 (112－113)[21]
161－162 (161－162)[38]
148－149 (151－152)[38]
153－154 (153－155)[21]
147－149 (149－150)[38]
－
4-CH3C6H4 92
2-CH3C6H4 88
3-CH3C6H4 90
4-ClC6H4
4-FC6H4
84
82
3g
3h
3i
4-CF3C6H4 75
4-MeOC6H4 80
4-OHC6H4 trace
9
a
Reaction conditions: 1 (0.5 mmol), 2 (1 mmol), TBAI (10
mol%), TBHP (3 equiv.), EtOAc (2 mL), 120 ℃, 3 h. Isolated
yields.
A series of aldehydes were then employed to further
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Chin. J. Chem. 2015, 33, 1239—1243

Organocatalytic Oxidative Amidation of Aldehydes with Tetrazoles
explore the scope of this protocol. The results were
summarized in Table 3. To our delight, aryl aldehydes
with various substituents such as methyl, methoxyl,
chloro, fluoro, trifluoromethyl, nitro, cyano, and methyl-
thio group coupled with 5-phenyl-2H-tetrazole 1a
smoothly to afford the desired products in moderate to
good yields (51%－91%). Aldehydes bearing strong
electron-withdrawing groups such as －CF3 and
－NO2 gave relatively lower yields even by prolonging
the reaction time to 12 h (3g, 51%; 3j, 58%, respec-
tively), while 4-formylbenzonitrile showed good reac-
tivity to afford product 3l in 81% yield. It is worth not-
ing that 4-(methylthio)benzaldehyde showed good
compatibility to the reaction, providing product 3m in
55% yield without over-oxidation. Obvious steric hin-
drance influence was also observed for ortho-substituted
aldehydes, and relatively lower yields were obtained (3n,
71%; 3o, 67%; 3p, 52%, respectively). 1-Naphthal-
dehyde also showed good reactivity to deliver 3q in
82% yield. Pleasingly, both furfural and 2-thiophenal-
dehyde survived the reaction conditions, and 2-thiophe-
naldehyde was efficiently transformed to product 3s in
91% yield. Finally, aliphatic aldehydes turned out to be
inert towards the reaction, and only trace amount of
products were observed.
A scale-up reaction was also performed to demon-
strate the practicability of the developed protocol (10
mmol scale, see Supporting Information). The reaction
proceeded smoothly under the optimized conditions to
provide the product 3a in 81% yield (Scheme 2).
Scheme 2 Gram-scale reaction
O
N
N
N N
TBAI (10 mol%)
N
+
H
Ph
R2
H
Ph
2a
N
Ph
TBHP (3 equiv.)
EtOAc (40 mL)
120 oC
O
1a
10 mmol
3a
1.80 g, 81%
To gain more insight into the mechanism, additional
control experiments were performed (Scheme 3). Pre-
viously, Ishihara and co-workers proposed that hypoio-
dite [IO]－ or iodite [IO2]－ was the active species in the
TBAI/TBHP system.[41,42] Later Wan and coworkers
suggested that iodine was generated in this system, and
it captured the single electron from the reactant.[33,43] In
this system, when we added TEMPO into the reaction,
the acylated TEMPO was obtained in excellent yields
using either n-Bu4NI or n-Bu4NBr as catalyst, which
indicated that an acyl or acetal radical may be involved
in this reaction (Scheme 3, Eq. 1). Moreover, the reac-
tion did not occur in the presence of stoichiometric
amounts of n-Bu4NOH and I2 (which should generate
[n-Bu4N]＋[IO]⎯ in situ). In the presence of n-Bu4NOH
and Br2, the coupling product was obtained in good
yield (Scheme 3, Eq. 2). Moreover, when benzoic acid
or tert-butyl benzoperoxoate was used instead of ben-
zaldehyde, no product was detected. All these results
Table 3 Reaction scope for aldehydesa
O
N
N
N
N
TBAI, TBHP
N
+
R2
Ph
R2
H
H
120 oC, EtOAc
N
O
Ph
2
1a
3
Entry Product R2
Yieldb/%
m.p./℃lit.
1
3b
3c
3d
3e
3f
4-CH3C6H4 87
2-CH3C6H4 84
3-CH3C6H4 86
122－124 (121－122)[38]
110－111 (96－98)[39]
112－113 (112－113)[21]
161－162 (161－162)[38]
148－149 (151－152)[38]
153－154 (153－155)[21]
147－149 (149－150)[38]
－
222－224 (222－223)[40]
169－171 (169－170)[38]
179－180 (178－180)[21]
112－114 (113－115)[21]
96－98 (95－98)[40]
153－155 (155－156)[21]
117－119 (120－121)[21]
120－122 (120－122)[16]
100－102 (101－102)[16]
112－114 (114－115)[38]
－
2
Scheme 3 Control experiments
3
4
4-ClC6H4
4-FC6H4
83
82
O
N
TBAI or TBAB
TBHP
N
O
5
N
N
H
(1)
(2)
O
N
+
H
Ph
Ph
TEMPO (2 equiv.)
6
3g
3h
3i
4-CF3C6H4 51
4-MeOC6H4 73
4-OHC6H4 trace
4-NO2C6H4 58
2a
1a
93% and 85%
7
8
N
N
N
N
O
conditions
N
9
3j
H
+
Ph
Ph
N
EtOAC, 120 oC
Ph
H
O
Ph
10
11
12
13
14
15
16
17
18
19
3k
3l
4-BrC6H4
73
2a
3a
1a
4-CNC6H4 81
4-MeSC6H4 55
a: I2 (1 equiv.), n-Bu4NOH (2 equiv.)
b: Br2 (1 equiv.), n-Bu4NOH (2 equiv.)
trace
3m
3n
3o
3p
3q
3r
3s
67%
2-ClC6H4
2-BrC6H4
71
67
O
O
Standard
O
conditions
1a +
O
HO
O
or
No reaction ! (3)
2-MeOC6H4 52
1-naphthyl 82
Standard
conditions
O
2-furyl
53
91
+
MeOH
2-thienyl
Ph
O
Ph
H
2a
3t
cyclohexyl trace
PhCH2CH2 trace
No reaction!
20
3u
－
Standard
conditions
O
a
Reaction conditions: 1a (0.5 mmol), 2 (1 mmol), TBAI (10
mol%), TBHP (3 equiv.), EtOAc (2 mL), 120 ℃. Isolated yields.
b Reaction for 12 h.
(4)
Ph
O
TsOH (0.2 equiv.)
26%
Chin. J. Chem. 2015, 33, 1239—1243
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1241

Cao & Wang
COMMUNICATION
supported the hypothesis that the intermediates in these
reactions are different with those previously reported. To
verify whether acyl cation was formed in this reaction, a
control experiment was conducted by addition of
methanol under standard conditions. It was found that
no methyl ester was detected. However, when an acid
catalyst (p-TsOH) was added, methyl ester was obtained
in 26% yield (Scheme 3, Eq. 4). It was assumed that the
addition of an acid catalyst facilitated the acetal forma-
tion between benzaldehyde and methanol. The lack of
methyl ester formation in the absence of an acetalyza-
tion catalyst reduces the likelihood of acyl iodide or acyl
radical intermediates as the key reactive species.
A proposed mechanism was presented based on the
above experimental results as well as the literature re-
ports (Scheme 4).[29-31,44,45] Initially, tert-butoxyl,
tert-butylperoxyl radicals, iodine and a hydroxyl anion
are generated via oxidation of n-Bu4NI by TBHP. The
tert-butoxyl or tert-butylperoxyl radical subsequently
abstracts a hydrogen atom from the aminal specie A,
which is generated from the reaction of tetrazole 1 and
the aldehyde 2, and the resulting radical species B is
further oxidized to acylated tetrazole C. Finally, a
1,5-dipole (nitrileimine) D is generated by elimination
of nitrogen of acylated tetrazole C, followed by cycliza-
tion to provide the desired product 3.
reaction was also realized in this catalytic system.
Acknowledgement
This project was financially supported by the Na-
tional Natural Science Foundation of China (No.
21302014) and the Natural Science Foundation for Col-
leges and Universities of Jiangsu Province (No.
13KJB150002).
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Chin. J. Chem. 2015, 33, 1239—1243
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