Aluminium(III) Chloride-Catalyzed Three-Component Condensation of Aromatic Aldehydes, Nitroalkanes and Sodium Azide for the Synthesis of 4-Aryl-NH-1,2,3-triazoles

Article abstract of DOI:10.1002/adsc.201600098

An aluminium(III) chloride-catalyzed three-component reaction of aromatic aldehydes, nitroalkanes, and sodium azide has been developed; this reaction sequence can be applied to a broad substrate scope and affords the corresponding 4-aryl-NH-1,2,3-triazoles in good to excellent yields. The milder reaction conditions and easier operation make this AlCl₃-catalyzed protocol more advantageous for the synthesis of 4-aryl-NH-1,2,3-triazoles. (Figure presented.).

Full text of DOI:10.1002/adsc.201600098

UPDATES
DOI: 10.1002/adsc.201600098
Aluminium(III) Chloride-Catalyzed Three-Component Condensa-
tion of Aromatic Aldehydes, Nitroalkanes and Sodium Azide for
the Synthesis of 4-Aryl-NH-1,2,3-triazoles
Qinquan Hu,a Yi Liu,a Xiaocong Deng,a Yanjun Li,b,* and Yunfeng Chena,*
a
School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, Peopleꢀs Republic
of China
E-mail: yfchen@wit.edu.cn
b
Department of Chemistry, Wuhan University of Science and Technology, Wuhan 430081, Peopleꢀs Republic of China
E-mail: yanwatercn@wust.edu.cn
Received: January 22, 2016; Revised: March 21, 2016; Published online: May 3, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600098.
Abstract: An aluminium(III) chloride-catalyzed
three-component reaction of aromatic aldehydes,
nitroalkanes, and sodium azide has been developed;
this reaction sequence can be applied to a broad
substrate scope and affords the corresponding 4-
aryl-NH-1,2,3-triazoles in good to excellent yields.
The milder reaction conditions and easier operation
make this AlCl3-catalyzed protocol more advanta-
geous for the synthesis of 4-aryl-NH-1,2,3-triazoles.
the synthesis of diverse 1,2,3-triazoles by post-N-func-
tionalizations, such as post-alkylation or arylation of
NH-1,2,3-triazole for synthesis of N2-substituted 1,2,3-
triazoles.[11] However, to a certain extent, the synthe-
sis of NH-1,2,3-triazole is still a challenge in contrast
to the direct synthesis of substituted 1,2,3-triazoles.
Methods for the synthesis of 4-aryl-NH-1,2,3-triazoles
are usually confined to the cycloaddition of alkynes
and TMSN3 followed by deprotection of the TMS
group.[12] A direct Pd-catalyzed cyclization of vinyl
bromides and NaN3 was also developed, which did
not need the deprotection step.[13] The condensation
of nitroalkenes and NaN3 is a popular method for the
synthesis of NH-1,2,3-triazoles. Zefirov initially re-
ported the reaction of sodium azide with nitrostyrenes
to synthesize NH-triazoles.[14] Then the method was
revised by Zard with improved yields, but was still
Keywords: aluminium(III) chloride catalysis; NH-
1,2,3-triazoles; sodium azide; three-component re-
action
1,2,3-Triazoles are one of the most significant classes limited by potential substrate polymerization.[15] Re-
of heterocyclic compounds, and they have found cently, Guan has made an adventurous breakthrough
widespread applications in diverse areas, such as me- by introducing PTSA (p-methylbenzenesulfonic acid)
dicinal chemistry,[1] materials chemistry[2] and synthet- to this cycloaddition reaction, which inhibited the cy-
ic organic chemistry.[3] In the past decades, many ap- clotrimerization of the nitroolefins and promoted the
proaches for the synthesis of 1,2,3-triazoles have been cycloaddition effectively.[16]
developed. The most popular method is the Cu-cata-
Multicomponent reactions are a kind of powerful
lyzed azide-alkyne cycloaddition (Cu-AAC),[4] which tool for the assembly of different molecules in
gives the 1,4-disubstituted 1,2,3-triazoles with high ef- a single synthetic operation, which not only is high
ficiency. Complementarily, other transition metal-cat- step-economical, but also avoids the separation of the
alyzed click protocols such as Ru-AAC,[5] Ir-[6] and reaction intermediates.[17] In 2008, the Shi group re-
Ag-AAC[7] have allowed the synthesis of 1, 5-disubsti- ported a proline-catalyzed three-component reaction
tuted or 1,4,5-trisubstituted 1,2,3-triazoles. Recently, of aldehyde, b-alkylnitroalkene, and NaN3 to synthe-
several other methods for the synthesis of different size 4,5-disubstituted-1,2,3-NH-triazoles.[18] Then,
1,2,3-triazole derivatives were reported, including or- a three-component reaction based upon the Julia re-
ganocatalyzed azide-ketone cycloaddtions,[8] three- agent, aldehyde, and NaN3 was conducted by the Chu
component reactions,[9] metal-free and azide-free group.[19] Lately, Lin developed another three-compo-
methods.[10] Within the 1,2,3-triazole categories, the nent reaction based upon aldehyde, nitroalkane and
NH-1,2,3-triazole is an important class which has NaN3.[20] However, the reaction efficiency was hin-
shown wide applications in many fields, especially in dered by the requirement of slow addition of nitroal-
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UPDATES
Qinquan Hu et al.
kane and excess amounts of NaHSO3/Na2SO3 yield (entry 1). Then other sets of conditions were
(2 equivalents) and NaN3 (4 equivalents) under an screened based on the initial test, as shown in Table 1.
argon atmosphere. At the same time, the overall In the absence of catalyst, 3a could be also isolated in
yields were not high, especially those with aromatic 58% yield (entry 2). It seemed that the NaN3 served
aldehydes bearing either strongly electron-donating as both base and reaction component to promote for-
or electron-withdrawing groups gave lower yields.
mation of the nitroalkene, and further react with ni-
We are interested in the synthesis and properties of troalkene to give the target product. When PTSA was
new 1,2,3-triazoles and related heterocycles.[21] The used as catalyst, a slight improvement was observed.
development of a more simple and efficient method Furthermore, on lowering the reaction temperature to
for the synthesis of NH-1,2,3-triazoles is one of the 708C, the yield of 3a could be increased to 78%
major goals for us. In this paper, we report an AlCl3- (entry 5). We proposed that the low reaction tempera-
catalyzed three-component condensation of aromatic ture could prevent NaN3 from decomposition under
aldehydes, nitroalkanes and NaN3 for the synthesis of acidic conditions, however, an increase in the amount
4-aryl-NH-1,2,3-triazoles. This reaction proceeds of NaN3 did not improve the product yield obviously
under mild conditions and affords the products with (entry 6), at the same time, the reaction only gave
high yields and reaction efficiency. Benzaldehyde and 72% yield without any catalyst at 708C (entry 7), indi-
nitromethane were selected for the initial conditions cating that the acid catalyst was still playing the key
screening. The results are summarized in Table 1.
role in the reaction. So the catalyst was changed to
Initially we proposed that the condensation of ben- a Lewis acid. The reaction gave 3a in 72% yield when
zaldehyde with nitromethane would occur smoothly FeCl3 was used as catalyst (entry 8), while ZnBr2 and
under acid conditions, such as AcOH to afford the ni- CuCl2 as catalysts did not give better results. It was
troalkene intermediate, then the nitroalkene could gratifying that a 95% isolated yield was obtained
[22]
subsequently react with NaN3 to give the NH-1,2,3- (entry 11) when AlCl3 served as catalyst. The subse-
triazole. Firstly, the reaction was carried out using quent screening of different solvents (entries 11–13)
AcOH as catalyst and DMSO as solvent at 1108C, revealed DMSO as the optimal solvent. Further low-
the desired 1,2,3-triazole 3a was obtained in 63% ering of the temperature (508C, entry 15) significantly
decreased the reaction rate, while a higher tempera-
ture (1108C, entry 16) was not advantageous for the
yield either.
Table 1. Optimization of the reaction conditions.[a]
With the optimized reaction conditions established,
we then explored the substrate scope. The results are
summarized in Scheme 1. The AlCl3-catalyzed three-
component reaction showed good functional group
tolerance and has proved to be a general method for
the synthesis of 4-aryl-NH-1,2,3-triazoles. Irrespective
of whether the functional groups on aryl rings are
electron-withdrawing, or electron-donating, they all
gave excellent yields. By contrast, hydroxy, methoxy
and nitro substituted aromatic aldehydes gave rela-
tively lower yields than other groups due to their po-
tential for coordinating with AlCl3, which were differ-
ent with the PTSA-catalyzed results.[16] Aliphatic alde-
hydes, such as phenylacetaldehyde and butyraldehyde,
were also tested for this three-component reaction,
however, these reactions could not give desired prod-
ucts.
Generally, the condensation of NaN3 with nitroal-
kenes was a popular method for synthesis of 4-aryl-
NH-1,2,3-triazoles. However, some nitroalkenes were,
to some extent, hard to be obtained, for example,
ester group-substituted nitroalkenes were obtained
usually with lower yields or under harsh reaction con-
ditions.[9b,23] Based on these considerations, the 2-ethyl
nitroacetate has been chosen for this three-compo-
nent reaction (Scheme 2). It was exciting that the re-
actions resulted in high efficiency, which gave the
ester group-substituted NH-1,2,3-triazoles with high
Entry
Cat.
Solvent
Temp. [oC]
Yield [%][b]
1
2
3
4
AcOH
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
110
110
110
90
70
70
70
70
70
70
70
70
70
r.t.
50
63
58
67
72
78
80
72
72
68
66
95
70
NR
NR
41
78
TsOH
TsOH
TsOH
TsOH
5
6[c]
7
8
9
FeCl3
ZnBr2
CuCl2
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
10
11
12
13
14
15[d]
16
CH3CN
DMSO
DMSO
DMSO
110
[a]
Reaction conditions: benzaldehyde (212 mg, 2 mmol), ni-
tromethane (183 mg, 3 mmol), NaN3 (156 mg, 2.4 mmol),
and catalyst (0.2 mmol), solvent (8 mL); reaction time,
overnight, in air.
Yields of isolated products.
3 mmol (1.5 equiv.) NaN3 were used.
[b]
[c]
[d]
32% unreacted benzaldehyde was recovered.
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Aluminium(III) Chloride-Catalyzed Three-Component Condensation
Scheme 1. Substrate scope of the reaction. Reaction conditions: aromatic aldehydes (2 mmol), nitroalkanes (3 mmol), NaN3
(2.4 mmol), and AlCl3 (0.2 mmol), DMSO (8 mL); overnight, in air. Yields of isolated products are given.
yields. So far, this method is the optimal way to syn- 1,2,3-triazoles with excellent yields under operational-
thesize ester group-substituted NH-1,2,3-triazoles.[19,24] ly convenient conditions and showed a toleration of
To further identify the reaction process and under- a wide range of functional groups. Besides the easily
stand the efficiency of this three-component reaction, available reaction materials, this method avoided the
certain control experiments were performed separation of the nitroalkene intermediates, especially
(Scheme 3). The condensation of benzaldehyde and those with sensitive functional groups like nitrovinyl-
nitromethane barely occurred when AlCl3 was used as pyridine, thus improved the overall yields. Further ap-
catalyst at 708C (Scheme 3a). At the same time, the plication of the simple protocol for synthesis of func-
condensation of nitroalkene with NaN3 by using AlCl3 tional 4-aryl-NH-1,2,3-triazoles with unique properties
as catalyst at room temperature could also give 3a are under investigation in our group.
with 96% yield in 2 h (Scheme 3b), which revealed
that the formation of NH-1,2,3-triazole was a fast pro-
cess. These results indicated that the fast formation of
triazole could drive the initial slow Henry reaction,
which potential inhibited the polymerization tendency
Experimental Section
of nitroalkene intermediate. To further identify the
role of AlCl3, 2-pyridine-carboxaldehyde was chosen
as the substrate, when the AlCl3 was changed to
Typical Procedure for the Synthesis of 3a
The mixture of benzaldehyde (1a, 212 mg, 2 mmol), nitro-
methane (2a, 183 mg, 3 mmol), NaN3 (156 mg, 2.4 mmol),
PTSA, the triazole 3n was obtained in only 47% yield
and AlCl3 (27 mg, 0.2 mmol), were stirred in 8 mL DMSO
(Scheme 3c), indicating that the AlCl3 should be
at 708C under air. After 5–8 h (as monitored by TLC), the
a Lewis acid to activate the nitro group to benefit the
solution was extracted with EtOAc (3”40 mL). The com-
bined organic layers were dried over anhydrous Na2SO4,
and the solvent was evaporated in vacuo. The resulting mix-
ture was chromatographed on silica gel by eluting with ethyl
azide nucleophilic addition.[25]
In conclusion, we have developed an efficient and
convenient AlCl3-catalyzed three-component conden-
sation reaction of aromatic aldehydes, nitroalkanes
acetate/petroleum ether to afford 3a as a white solid; yield:
and sodium azide. The reactions gave 4-aryl-NH- 276 mg (95%).
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Acknowledgements
We are grateful to Key Projects of Hubei Provincial Depart-
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