Construction of Bicyclic 1,2,3-Triazine N-Oxides from Aminocyanides

Article abstract of DOI:10.1021/acs.orglett.0c03952

Using a facile and cost-effective method, nine bicyclic 1,2,3-triazine 2-oxides were synthesized from o-aminocyanide substrates through an unusual nitration cyclization. The reaction mechanism was studied experimentally and theoretically. Moreover, nine 1

Full text of DOI:10.1021/acs.orglett.0c03952

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Construction of Bicyclic 1,2,3-Triazine N‑Oxides from Aminocyanides
Yuji Liu, Xiujuan Qi, Wenquan Zhang,* Ping Yin,* Ziwu Cai, and Qinghua Zhang*
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ABSTRACT: Using a facile and cost-effective method, nine bicyclic 1,2,3-
triazine 2-oxides were synthesized from o-aminocyanide substrates through
an unusual nitration cyclization. The reaction mechanism was studied
experimentally and theoretically. Moreover, nine 1,2,3-triazine 3-oxides
were also obtained in good yields.
riazines are a special class of heterocyclic compounds
which are extensively studied for potential applications in
Although several monocyclic 1,2,3-triazine compounds have
been reported over the past decades, 1,2,3-triazine N-oxides
have attracted little attention. It was reported that the N-oxide
functionality on aromatic heterocycles can enhance their
pharmacological and energetic properties.17−19 Few reports
on preparation of 1,2,3-triazine N-oxides include a direct
oxidation to 1,2,3-triazine 1-, 2-, and 3-oxides with m-
chloroperoxybenzoic acid (m-CPBA)20−23 and amino diazoti-
zation to a diazonium salt followed by reactions with a
neighboring oxime to afford 1,2,3-triazine 1- or 3-oxide.24,25
However, the majority of these reactions suffer from low
tolerance of functional groups, multiple byproducts, and low
selectivity of N-oxide positions. Thus, universal methods for
the synthesis of 1,2,3-triazine N-oxide do not exist. Addition-
ally, very little is known about the construction of bicyclic
1,2,3-triazine N-oxides. In recent efforts to develop new
energetic compounds, two bicyclic compounds based on 1,2,3-
triazine 2-oxide and 1,2,3-triazine 3-oxide skeletons were
synthesized by us and Shreeve.26,27 They showed promising
detonation performances as primary explosives and fluorescent
energetic materials. However, the reaction scope and
mechanism of these methods for constructing other bicyclic
1,2,3-triazine N-oxides remain unclear.
T
agrochemicals,1,2 pharmaceuticals,3−5 energetic materials,6,7
and synthetic intermediates.8,9 Among the three possible
isomers of triazine, 1,2,3-triazine is more favored than the
1,2,4- and 1,3,5-triazine isomers, largely due to their potent
efficacy and less side effects as pharmaceuticals,10 as well as
higher energy content as energetic materials (Figure 1a).11
Figure 1. Structures and synthetic methods of 1,2,3-triazine.
In this work, we are interested in preparing a variety of
bicyclic 1,2,3-triazine N-oxides from different o-aminocyanide
substrates based on five- or six-membered aromatic rings. In
method (I), o-aminocyanide is reacted with NaN3 to give o-
tetrazolylamine derivatives, followed by a one-step ring-closure
reaction to yield the target products of bicyclic 1,2,3-triazine 2-
oxides. In the first step reaction, 12 six- or five-membered
However, 1,2,3-triazine is also the least studied due to
synthetic difficulties in constructing such a catenated nitrogen
chain in an aromatic ring. It was first prepared by Pinnow et al.
in 1896 via reaction of 2-amino-N′-hydroxybenzimidamide
with nitrous acid.12 In 1910, Chandross et al. reported another
approach via a pyrolytic rearrangement of 1,2,3-triphenylcy-
clopropyl azide.13 Over the next 60 years, there were no
significant advances in its synthesis until Igeta et al. reported
the preparation of neat 1,2,3-triazine with an oxidative
rearrangement from 1-aminopyrazole (Figure 1b).14−16 Even
now, it is still a great challenge to develop new facile and
universal methods to prepare 1,2,3-triazine derivatives.
Received: November 29, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03952
© XXXX American Chemical Society
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aromatic heterocycle precursors were prepared, including
pyridine, pyrazine, pyrimidine, pyridazine, and pyrazole
(Scheme 1). Most substrates gave high yields from 73% to
Table 1. Optimization for Nitrating Conditions for 2b
Scheme 1. Reaction of o-Aminocyanides 1a−1l with NaN3
a
entry
nitrating agent
temp (°C)
yield (%)
1
2
3
4
5
6
7
8
HNO3/H2SO4
HNO3/Ac2O
HNO3/TFAA
NO2BF4/MeCN
HNO3/TFAA
HNO3/TFAA
HNO3/TFAA
HNO3/TFAA
−5
−5
−5
−5
−15
5
65
72
63 (5)
70
83
43 (12)
15 (42)
0 (64)
rt
50
a
Yields in parentheses are separated yields for 3b-1.
Scheme 2. Nitration of o-tetrazolylamines, 2a−2l to 1,2,3-
a
Triazine 2-Oxide
82%. The reaction of pyridines 1b−1e to form 2b−2e gave
slightly higher yields than the diazine-based analogues 1f−1h
possibly because the higher nitrogen content of a six-
membered ring depressed the efficiency of this transformation.
Five membered azoles 2i−2j all were formed in high yields of
80% to 82%. The reaction between 2-aminobenzonitrile (1a)
and NaN3 is noteworthy. When catalyzed by NHMe2·HCl, an
unexpected tricyclic compound 5,6-dihydrotetrazolo[1,5-c]-
quinazoline (2a-1) was separated. With NH4Cl, another
tricyclic compound tetrazolo[1,5-c]quinazolin-5-one (2a-2)
was formed. It is likely that the methylene or carbonyl moiety
came from the solvent DMF. When the solvent was replaced
with DMA, the desired product 2a was finally obtained in a
yield of 64%.
The second step in method I is a tandem nitration−
cyclization which forms 1,2,3-triazine 2-oxides. Here, 2b is
used as the substrate, and several commonly used nitrating
agents are selected for this reaction. The results are shown in
Table 1. It should be noted that the HNO3/TFAA nitrating
system gives a modest yield of 63% at −5 °C because some
byproducts, detected on TLC, were detrimental to the
recovery of the product. However, when the reaction was
repeated at −15 °C, a pure product with an isolated yield up to
83% was obtained. Therefore, a mixture of HNO3/TFAA was
chosen to be the optimal nitrating system. From entries 5−8, it
was found that temperature has a major impact on the reaction
yield. The highest isolated yield was found at −15 °C. As the
temperature was increased, more byproducts began to form
and the isolated yield dropped significantly. At 50 °C, only a C-
nitrated 1,2,3-triazine compound, 6-nitro-4-oxo-4,8-
dihydropyrido[2,3-d][1,2,3]triazine 2-oxide (3b-1), was sepa-
rated as the only product. Also, a trace amount of 5-nitro-3-
tetrazolylpyridin-2-amine (3b-2) was separated from the
nitration mixture with column chromatography. Its structure
was confirmed with single-crystal XRD.
a
Thermal ellipsoid plots were drawn at 50% probability level.
new o-tetrazolylamines 2a−2l prepared in step I, were readily
transformed to 1,2,3-triazine 2-oxides 3a−3l, with yields from
60% to 89%. It is interesting that some of the 1,2,3-triazine 2-
oxides (3b, 3e, 3i, 3j) bear an azide on the 4-position, while
others hold a carbonyl at the same position. The mechanism
for the formation of carbonyl products was proposed by Zhou
et al.,28,29 while the mechanism for azide products will be
discussed later. For phenyl compounds 2a and 2l, nitro
moieties were inevitably introduced to the ring even at −15
°C, giving dinitro products 3a-1 and 3l-1, with a yield of 60%
and 64%, respectively. For heterocycles, nitro-free products
After the condition optimization of the second step reaction
in method I, the substrate scope was expanded to other six- or
five-membered aromatic heterocycles (Scheme 2). Most of the
B
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could be prepared by carefully controlling the temperature to
no more than −10 °C. It was also found that the reactions of
the diazines 3f−3h did not fully proceed to 1,2,3-triazine 2-
oxides and only nitramine products were separated. The
structures of 3b, 3d-1, 3e, 3f-1, and 3h-1 were confirmed by
single-crystal XRD.
Scheme 3. Reactions of o-Aminocyanides 1a−1m with
Hydroxylamine
To gain more insight into the nitration mechanism, quantum
calculations were carried out to study the reaction
intermediates and transition states for the nitration of 2b. A
plausible mechanism is proposed in Figure 2. Its energy profile
reaction conditions. The results are listed in Table 2. From
entries 1−3, temperature has a crucial influence on the yield.
Figure 2. Calculated reaction mechanism. Numbers in parentheses
are calculated energy barriers (in kcal/mol).
Table 2. Optimization of Diazotization Reactions for 4a
is also shown (more calculation details could be found in the
Supporting Information). The reaction from IM-1 to TS-1 has
a relatively high energy barrier of 39.7 kcal/mol and is
considered as the key step of the whole reaction process. The
intermediate IM-2 already bears a 1,2,3-triazine skeleton and is
dehydrated to IM-3, with a huge energy release of 37.1 kcal/
mol. A final tetrazole−azide transformation yields the product
3b with another energy drop of 8.7 kcal/mol. Calculated
results reveal that initially a nitramine intermediate IM-1 was
formed. It was further dehydrated via a complicated process to
generate the product 3b. According to this mechanism, the
nitrogen atom of the N-oxide comes from nitric acid. To verify
this mechanism experimentally, a 15N-labeled KNO3 with 10%
abundance was used in place of nitric acid in the nitrating
reaction. The 15N NMR spectrum of 15N-labeled 3b shows a
single peak with chemical shift at 334.14 ppm (liquid NH3 as
external standard), which is assigned to the nitrogen atom of
the N-oxide. This is in good agreement with the calculated
mechanism.
After the successful syntheses of the bicyclic 1,2,3-triazine 2-
oxides, the syntheses of bicyclic 1,2,3-triazine 3-oxide were
explored. In this synthetic method, o-aminocyanide reacts
readily with hydroxylamine to give o-aminoamidoxime. This is
followed by diazotization and cyclization to the product 4-
amino-1,2,3-triazine 3-oxide. Twelve different six- and five-
membered precursors were prepared (Scheme 3). Reactions
for most substrates achieved medium-to-high yields up to 92%.
The reaction for compound 4l showed the highest yield, which
could be attributed to the electron-donating methoxy group on
the ring. Imidazole, 1k, has a low reactivity with aqueous
NH2OH, giving rise to a low conversion yield even after a
couple of days. Pyrimidine derivative 1h, upon reacting with
NH2OH, gave a pair of E/Z isomers of 4h-E/Z.30,31
entry
diazotization agent
temp (°C)
yield (%)
1
2
3
4
5
6
7
8
NaNO2/HCl
NaNO2/HCl
NaNO2/HCl
NaNO2/H2SO4
NaNO2/AcOH
iAmONO/HBF4
tBuONO/AcOH
NOBF4/MeCN
−5
10
rt
−5
−5
−5
−5
−5
72
46
0
23
64
23
45
0
The highest yield of 72% was achieved at −5 °C. Increasing
the temperature to 10 °C resulted in a lower yield of 46%. At
room temperature, no 1,2,3-triazine 3-oxide was detected on
TLC from the reaction. Different diazotization agents had
varied reactivities in the diazotization reaction. A mixture of
NaNO2/HCl led to the highest yield of 72%. Replacing the
acid with H2SO4 significantly reduced the yield to 23%. Using
AcOH as the acid resulted in a modest yield of 64%. Other
diazotization agents such as isoamyl nitrite or tert-butyl nitrite
gave relatively lower yields up to 45%.
After the determination of the best optimized conditions, the
reaction scope was expanded to other oxime compounds
prepared in Scheme 3. Among these new compounds, phenyl
and pyridine derivatives 4a−4e, 4k gave smooth reactions with
medium-to-high yields from 65% to 79% (Scheme 4). The
structure of 1,2,3-triazine 3-oxide was further confirmed with
single-crystal XRD for 5a. However, the diazine compounds
4f−4h showed very different reactivity. They all gave
chloroxime products 5f-1, 5g-1, and 5h-1 instead of the
expected 1,2,3-triazine 3-oxides. It could be inferred that the
formation of a 1,2,3-triazine 3-oxide or a chloroxime are
competing reactions. The reaction pathway depends on the
The second step is a diazotization reaction followed by a
cyclization to 1,2,3-triazine 3-oxide. 2-Amino-N′-hydroxyben-
zimidamide (4a) was selected as the substrate to optimize the
C
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Science and Technology, Mianyang 621900, China;
Email: zhangwq-cn@caep.cn
Scheme 4. Diazotization of o-Aminoamidoxime 4a−4m to
1,2,3-Triazine 3-Oxide
Ping Yin − School of Material Science & Engineering, Beijing
Institute of Technology, Beijing 100081, China; Institute of
Chemical Materials, China Academy of Engineering Physics,
Mianyang 621900, China; School of Material Science and
Engineering, Southwest University of Science and Technology,
Mianyang 621900, China; orcid.org/0000-0002-2870-
8225; Email: pingyin@bit.edu.cn
Qinghua Zhang − School of Material Science & Engineering,
Beijing Institute of Technology, Beijing 100081, China;
Institute of Chemical Materials, China Academy of
Engineering Physics, Mianyang 621900, China; School of
Material Science and Engineering, Southwest University of
Science and Technology, Mianyang 621900, China;
orcid.org/0000-0002-4162-7155;
Email: qinghuazhang@caep.cn
Authors
Yuji Liu − School of Material Science & Engineering, Beijing
Institute of Technology, Beijing 100081, China; Institute of
Chemical Materials, China Academy of Engineering Physics,
Mianyang 621900, China; School of Material Science and
Engineering, Southwest University of Science and Technology,
Mianyang 621900, China
relative reactivities of the two amino groups. The amino group
with a higher reactivity forms diazonium salt and proceeds
through the final product.
Xiujuan Qi − School of Material Science & Engineering,
Beijing Institute of Technology, Beijing 100081, China;
Institute of Chemical Materials, China Academy of
Engineering Physics, Mianyang 621900, China; School of
Material Science and Engineering, Southwest University of
Science and Technology, Mianyang 621900, China
Ziwu Cai − School of Material Science & Engineering, Beijing
Institute of Technology, Beijing 100081, China; Institute of
Chemical Materials, China Academy of Engineering Physics,
Mianyang 621900, China; School of Material Science and
Engineering, Southwest University of Science and Technology,
Mianyang 621900, China
In conclusion, a series of bicyclic 1,2,3-triazine 2-oxides and
1,2,3-triazine 3-oxides were synthesized from o-aminocyanides
in good yields using two simple methods. The ring-closing
reaction mechanism of bicyclic 1,2,3-triazine 2-oxide was
investigated with the aid of quantum calculations and isotope-
labeled experiments, respectively. These reactions provide
efficient approaches to the construction of bicyclic 1,2,3-
triazine N-oxides from a variety of o-aminocyanide substrates,
which are also applicable to the synthesis of 1,2,3-triazine
heterocycles with high nitrogen content under mild conditions.
This work provides useful insights for developing new fused-
ring 1,2,3-triazine derivatives that usually require lengthy or
multistep synthetic routes.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03952
ASSOCIATED CONTENT
* Supporting Information
■
Notes
sı
The authors declare no competing financial interest.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03952.
ACKNOWLEDGMENTS
■
Experimental procedures, characterization data, calcu-
lation details, and crystallographic data (PDF)
This work was supported from the National Natural Science
Foundation of China (No. 21875228 and No. 21975231) and
the Presidential Foundation of CAEP (No. YZJJLX2017003).
Accession Codes
CCDC 2035798−2035800, 2035802−2035807, and 2047071
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.ca-
m.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.
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AUTHOR INFORMATION
Corresponding Authors
■
Wenquan Zhang − School of Material Science & Engineering,
Beijing Institute of Technology, Beijing 100081, China;
Institute of Chemical Materials, China Academy of
Engineering Physics, Mianyang 621900, China; School of
Material Science and Engineering, Southwest University of
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