Mono-N-oxidation of heterocycle-fused pyrimidines

Article abstract of DOI:10.1039/d0dt03260c

Three nitrogen-rich heterocyclic compounds containing the diamino-pyrimidine mono-N-oxide moiety were synthesizedviamild oxidation reactions. Oxidation of the furazano-pyrimidine compound (1) with a mixture of trifluoroacetic anhydride (TFAA) and hydrogen peroxide (50%) gave the nitrate salt (3). All of the compounds were characterized by NMR spectra, elemental analysis, and single-crystal X-ray diffraction. They show high thermal stability and good detonation performance as well as low sensitivity.

Full text of DOI:10.1039/d0dt03260c

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Mono-N-oxidation of heterocycle-fused
pyrimidines†
Yongxing Tang, *‡a,b Kejia Li,‡a Ajay Kumar Chinnam, b Richard J. Staples
Cite this: Dalton Trans., 2021, 50,
2143
c
b
and Jean’ne M. Shreeve
*
Three nitrogen-rich heterocyclic compounds containing the diamino-pyrimidine mono-N-oxide moiety
were synthesized via mild oxidation reactions. Oxidation of the furazano-pyrimidine compound (1) with a
mixture of trifluoroacetic anhydride (TFAA) and hydrogen peroxide (50%) gave the nitrate salt (3). All of the
compounds were characterized by NMR spectra, elemental analysis, and single-crystal X-ray diffraction.
They show high thermal stability and good detonation performance as well as low sensitivity.
Received 5th November 2020,
Accepted 20th January 2021
DOI: 10.1039/d0dt03260c
rsc.li/dalton
the N-oxide moiety into fused heterocyclic compounds is still
limited.
Introduction
Heterocyclic N-oxides have been widely used as structural
As a result of the absence of such materials and our conti-
motifs in various fields such as pharmaceutical agents, func- nuing interest in the synthesis of fused compounds with the
tional organic compounds, ligands and energetic materials.1–6 N-oxide moiety, a strategy was developed for the mild oxidation
In the field of energetic materials, the N-oxide moiety which is reactions of several nitrogen-rich fused pyrimidine rings (1, 4,
another source of oxygen and possible replacement for the 6) which give energetic fused mono-N-oxides (2, 5, 7). All of
nitro group has attracted more and more attention.7 them have high decomposition temperatures, low sensitivities
Introducing this functionality can improve density, heat of for- (>40 J, >360 N) and good detonation performances.
mation and oxygen balance while enhancing detonation per- Additionally, the site selectivity for N-oxidation of fused pyri-
formance. Additionally, compared with nitro organic explo- midine was investigated theoretically.
sives, the N-oxide moiety can delocalize the molecule and
reduce mechanical sensitivity.8–11
In recent years, many N-oxide formation reactions have
been reported, such as the formation of the furoxan ring,12–14
azoxy link,15,16 and N-oxidation on nitrogen-rich heterocyclic
compounds17 (Fig. 1). Some oxidants such as hydrogen per-
oxide, trichloroisocyanuric acid (TCAA), m-chloroperoxybenzoic
acid (mCPBA), and Oxone have been proved to be effective for
N-oxide formation. However, these N-oxidation reactions have
focused mostly on single ring or bicyclic rings,18–20 which only
require mild oxidants. For fused heterocyclic compounds,
strong oxidants such as HOF/CH3CN are necessary because of
the low reactivity of the nitrogen atom in conjugated fused
compounds.21–23 Likely because of this, the introduction of
Results and discussion
Synthesis
The initial fused compound (1) was prepared based on the lit-
erature.24 Compounds 4 and 6 are commercially available. The
initial investigation for the introduction of the N-oxide moiety
began with 1, which was treated with a mixture of 50% hydro-
gen peroxide and acetic anhydride at room temperature for
24 h. However, only the starting material (1) was recovered.
Surprisingly, the mono-N-oxide product (2) was isolated in
high yield (63%) when the reaction temperature was raised to
50 °C for 1.5 h (Scheme 1). The structure of 2 is supported by
single-crystal X-ray diffraction analysis. However, the introduc-
tion of more than one N-oxide group failed when the reaction
aNanjing University of Science and Technology, Nanjing, 210094, China.
E-mail: yongxing@njust.edu.cn
bDepartment of Chemistry, University of Idaho, Moscow, Idaho, 83844-2343, USA.
E-mail: jshreeve@uidaho.edu; Fax: (+1) 208-885-5173
cDepartment of Chemistry, Michigan State University, East Lansing, Michigan 48824,
USA
†Electronic supplementary information (ESI) available: Crystal structure analysis
and NMR spectra. CCDC 2040720–2040723. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/d0dt03260c
‡These authors contributed equally to this work.
Fig. 1 Different N-oxide moieties on heterocyclic rings.
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Fig. 2 (a) Molecular structure of 2·H2O. (b) Molecular structure of 3. (c)
Molecular structure of 5·HCl·2H2O. (d) Molecular structure of 7·2HCl.
Scheme 1 Synthesis of compounds 2, 3, 5 and 7.
time was extended. Other oxidants such as m-chloroperoxyben-
zoic acid (mCPBA) and 12% sodium hypochlorite (NaClO) were
also examined for the oxidation of 1. mCPBA was found to be
an effective oxidant when reacted with 1 in acetonitrile at
reflux for 2 h to give 2 in good yield (70%).
An attempt to oxidize 1 using a mixture of 50% hydrogen
peroxide and trifluoracetic anhydride at room temperature for
1 h led to the formation of a nitrate salt (3) in a low yield
(16%). However, no product was obtained if the reaction time
was extended to 2 h. The nitrate salt (3) was also prepared in a
high yield (85%) by reacting 1 with 15% nitric acid.
Attempted reactions of 5,7-diamino-2H-[1,2,3]triazolo[4,5-d]
pyrimidine (4) and 2,6-diamino-9H-purine (6) with a mixture
of 50% hydrogen peroxide and acetic anhydride gave only
unreacted starting materials. However, when a mixture of
H2O2 (50%) and trifluoroacetic anhydride (TFAA) was used, the
mono N-oxide derivatives 5 and 7 were obtained. Compound 5
was characterized with 1H and 13C NMR spectra as well as
elemental analysis. Compound 7 was characterized with 1H
NMR and elemental analysis. The 13C NMR spectrum was not
obtained due to its poor solubility even in d6-DMSO. To
confirm the structures of 5 and 7, the crystal structures of their
hydrochloride salts were obtained.
well as water molecules in the crystal of 2·H2O, six kinds
of hydrogen bonds between oxygen atoms and nitrogen
atoms were observed. The detailed information is given in
the ESI.†
Compound 3 crystallizes in the monoclinic space group
P21/n with a high calculated crystal density of 1.859 g cm−3 at
100 K. At 273 K, the density is 1.812 g cm−3 according to the
volume expansion equation (ρ273 K = ρT/(1 + αv(273 − T)); αv =
1.5 × 10−4 −1).25 The crystal structure is shown in Fig. 2b. The
K
bond lengths of C4–N5 and N6–C3 are 1.306(2) and 1.312(2) Å,
respectively, which are close to the C-amino bond lengths in
2·H2O. From the packing diagram in Fig. S2,† it is seen that
intermolecular hydrogen bonds are formed, such as N1–
H1⋯O2 and N6–H6A⋯O3.
Compound 5·HCl·2H2O crystallizes in the monoclinic space
group P21/n with four molecules per unit cell (Z = 4). The mole-
cular structure is shown in Fig. 2c. As can be seen, the proto-
nation of 5 has occurred at the oxygen atom (O1). The bond
length of N1–O1 is 1.377(2) Å, which is longer than that in
2·H2O, while the bond length (1.382(3) Å) of C3–C4 is a little
shorter. As expected, the fused cation is a nearly planar struc-
ture with the torsion angles of N2–C3–C4–N5 = −179.75(18)°
and N3–C3–C4–C1 = −179.16(17)°. In the packing diagram
(Fig. S3†), the fused cations form a layer-by-layer stacking
structure with a distance of 3.21 Å. In addition, extensive
Crystal structure
Compound 2·H2O crystallizes in the tetragonal space group hydrogen bonds are also found among the fused cation, chlor-
P42/n with eight molecules per unit cell (Z = 8). The mole- ide and water molecules.
cular structure is shown in Fig. 2a. Due to the presence of
Compound 7·2HCl crystallizes in the triclinic space group
ˉ
water molecules, the calculated crystal density is 1.448 g P1 with two molecules per unit cell (Z = 2). As depicted in
cm−3 at 100 K. The fused-ring shows a planar structure that Fig. 2d, the molecule is protonated at both oxygen atom (O1)
is supported by the torsion angles of N2–C2–C3–N4 = 179.7 and nitrogen atom (N4). The bond length of N1–O1 is 1.385(3)
(3)° and N3–C2–C3–C4 = 179.8(3)°. The bond length of N1– Å, which is close to that in 5·HCl·2H2O. The bond length of
O2 is 1.342(2) Å, which is comparable to those in C2–C4 is 1.384(4) Å. Because of the planar structure of the
ICM-102.19 The bond length of C2–C3 is 1.415(3) Å. Due to fused cation, they are stacked with π–π interactions with a sep-
the presence of the N-oxide moiety and amino groups as aration of 3.01 Å (Fig. S4†).
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Table 1 Physiochemical and energetic properties of the compounds
and comparison with TNT, TATB and LLM-105
Physicochemical and energetic properties
A noteworthy feature of the oxidation reaction of the nitrogen-
rich fused pyramidine ring is the site-selectivity in favor of the
introduction of the N-oxide moiety at the position between the
two C-amino groups. To better understand the selectivity,
density functional theory (DFT) calculations and orbital-
weighted dual descriptor for the oxidation were employed.26 In
Fig. 3, the green region and the blue region represent positive
and negative isosurfaces, respectively. In addition, positive/
negative values represent electrophilic/nucleophilic attack sus-
ceptibility. It is seen that the positive isosurface distributes
ρb
ΔfHc
νD
Pe
[GPa] [J]
ISf
FS g
[N]
a
d
Td
Compounds [°C] [g cm−3
]
[kJ g−1
]
[m s−1
]
2
3
280 1.80
247 1.82
303 1.72
309 1.66
295 1.654
350 1.94
342 1.913
220 1.845
1.27
1.61
0.58
0.47
−0.26
−0.60
−0.06
−0.91
7696
8563
7854
6609
6824
8201
8498
8613
21.7
30.1
21.5
13.9
19.4
28.0
30.6
31.6
>40 >360
>40 >360
>40 >360
>40 >360
15
50
20
5·H2O
7
TNTh
TATBh
LLM-105h
FOX-7
>353
>360
360
24.7 >360
around the nitrogen atom between the two C-amino groups, a Thermal decomposition temperature (onset) under nitrogen gas
(DSC, 5 °C min−1). b Measured densities, gas pycnometer at room
indicating that this position is easily attacked by an electrophi-
lic reagent. Therefore, the N → O bond was formed since the
active oxygen seeks electrons as an electrophilic reagent. These g Friction sensitivity. h Ref. 28.
results account for the experimentally observed selectivity that
favors the position between the two C-amino groups.
temperature. c Calculated heat of formation. d Calculated detonation
velocity. e Calculated detonation pressure. f Impact sensitivity.
The thermal behavior of the four compounds was deter-
mined by differential scanning calorimetry (DSC) at a heating
rate of 5 °C min−1. The neutral fused-ring compounds show
excellent thermal stabilities (>280 °C). Among them, 7 exhibits
the highest decomposition temperature at 309 °C, while the
decomposition temperatures (onset) of 2 and 5 are 280 °C and
303 °C, respectively. The nitrate salt 3 also has a high
decomposition temperature (onset) of 247 °C. Considering the
high decomposition temperatures, they may have potential
applications in heat-resistant explosives (Td > 250 °C).
The gas-phase heats of formation were calculated based on
G2 method. The solid-state heats of formation were obtained
Fig. 4 Electrostatic potential (ESP) of (a) 1, (b) 2, (c) 4, (d) 5, (e) 6, (f) 7.
by subtracting the enthalpy sublimation from the gas phase
heats of formation.27 The results are given in Table 1. All of
the compounds have positive heats of formation. The densities
were measured by using a gas pycnometer at room tempera-
ture. Compound 2 has a density of 1.80 g cm−3, while its
Molecular electrostatic potential (ESP) is a very useful
descriptor in understanding reactive sites for electrophilic/
nucleophilic reactions and sensitivities since it is related to
the electron density. The negative (blue) regions are related to
electrophilic reactivity and the positive (red) ones to nucleophi-
lic reactivity. As is shown in Fig. 4a, c and e, the negative (blue)
regions were mainly localized on the nitrogen and oxygen
atoms. Considering the actual nitrogen oxidation reaction, it is
focused on the two nitrogen atoms on the pyrimidine. In com-
paring 1 (Fig. 4a) with 2 (Fig. 4b), the introduction of N-oxide
makes the blue region more negative, indicating that the less
negative nitrogen atoms (between the two C-amino groups)
were easily attacked by active oxygen. In addition, for 2
(Fig. 4b), 5 (Fig. 4d) and 7 (Fig. 4f), all negative regions are
much larger than the positive regions, suggesting that they are
insensitive.
nitrate salt (3) has a slightly higher density of 1.82 g cm−3
.
Using the measured densities and the calculated heats of
formation, the detonation velocity (ʋD) and the detonation
pressure (P) was evaluated using EXPLO5 v6.05.02 program.29
Among them, the nitrate salt (3) shows the highest detonation
properties with a detonation velocity of 8563 m s−1 and a deto-
nation pressure of 30.1 GPa, which are a slightly better than
those of traditional insensitive energetic materials (TATB and
LLM-105). In addition, according to the values of the impact
sensitivities and friction sensitivities (IS: >40 J; FS: >360 N),
they are impact and friction insensitive, indicating they have
potential application as insensitive energetic materials.
Conclusions
In conclusion, an efficient synthesis of the nitrogen-rich fused
diamino pyrimidine mono-N-oxide moiety has been achieved.
The reactive site for N-oxidation was studied by theoretical cal-
culation and supported by experimental results. The oxidation
Fig. 3 The orbital-weighted dual descriptor isosurfaces (value
+0.001 and −0.002), using a width of the Gaussian function (Δ) of 0.01
a.u. for 1, 4 and 6 at the b3lyp/6-31+g(d,p) level.
=
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of furazan fused diamino pyrimidine with a mixture of TFAA Dualflex, HyPix diffractometer equipped with an Oxford
and 50% H2O2 gives a nitrate salt. The structures were con- Cryosystems low-temperature device, operating at T = 100.00
firmed by single-crystal X-ray diffraction analysis. All of the (10) K. The structures were solved with the ShelXT32 solution
compounds have high decomposition temperatures and they program using dual methods and by using Olex2 1.3-alpha33
are insensitive to impact and friction (IS > 40 J, FS > 360 N). as the graphical interface. The model was refined with ShelXL
The nitrate salt (3) has properties comparable with TATB. 2018/334 using full matrix least squares minimisation on F2.
Furthermore, the N-oxidation reaction of the fused compound
will be very useful in the design of new energetic compounds
and other heterocyclic compounds.
Synthesis
Synthesis of 2. Method a: Hydrogen peroxide (50%, 2 mL)
was added dropwise to acetic anhydride (5 mL) at 0 °C.
Compound 1 (0.34 g, 2.0 mmol) was added to the solution and
Experimental section
the reaction mixture heated and stirred at 50 °C for 90 min.
Caution! Those compounds with mono-oxides are potential
The precipitate (2) (0.21 g, 63%) was collected by filtration,
energetic materials that might explode under certain con-
ditions such as impact, friction or electric discharge.
Appropriate safety precautions including safety goggles, face
shields and gloves should always be used all the time.
washed with water (10 mL) and dried in air.
Method b: m-Chloroperoxybenzoic acid (mCPBA, 1.5 g) was
added to a suspension of compound 1 (0.40 g, 2.4 mmol) in
acetonitrile (20 mL), then the reaction mixture was refluxed for
3 h. The light-yellow solid (2) (0.28 g, yield: 70%) was collected
by filtration, washed with acetonitrile (10 mL) and diethyl
General
All reagents were purchased in analytical grade and were used
ether (10 mL), dried in air.
as supplied. 1H and 13C NMR spectra were recorded on a
1
2: Light-yellow solid. Td (onset): 280 °C. H NMR (d6-DMSO):
δ 9.31 (s, 2H), 8.65 (s, 1H), 8.08 (s, 1H) ppm. 13C NMR (d6-
Bruker AVANCE 300 nuclear magnetic resonance spectrometer.
Chemical shifts for 1H and 13C NMR spectra are given with
˜
DMSO): δ 154.5, 153.7, 141.1, 135.0 ppm. IR (KBr): v = 3353,
respect to external (CH3)4Si (1H and 13C). [D6]DMSO was used
as a locking solvent unless otherwise stated. The thermal beha-
viors were determined by using a dry nitrogen gas purge and a
heating rate of 5 °C min−1 on TA Instruments Q2000. IR
spectra were recorded using KBr pellets with Thermo Nicolet
AVATAR 370. Density was determined at room temperature by
employing a Micromeritics AccuPyc II 1340 gas pycnometer.
Elemental analyses (C, H, N) were performed with a Vario
Micro cube Elementar Analyser. Impact sensitivity were
measured by using a standard BAM Fallhammer and friction
sensitivity were determined by a BAM friction tester.
1650, 1617, 1500, 1433, 1373, 1294, 1228, 1159, 1080, 1016,
907, 874, 858, 798, 759, 707, 628 cm−1. Elemental analysis for
C4H4N6O2 (168.11): calcd C 28.58, H 2.40, N 49.99%. Found: C
28.59, H 2.54, N 50.13%.
Synthesis of 3. Method a: Hydrogen peroxide (50%, 1.5 mL)
was added dropwise to a solution of trifluoroacetic anhydride
(5 mL) in dichloromethane (20 mL) at 0 °C. Then compound 1
(0.50 g, 3.0 mmol) was added to the solution and the reaction
mixture was stirred at room temperature for 1 h. After remov-
ing the solvent by air, ethanol (10 mL) was added to the
residue and the precipitate was collected by filtration. It was
washed with ethanol (3 mL) and dried in air. 0.10 g, yield:
16%.
Computational methods
The gas-state enthalpies of formation were calculated based on
G2 method. The solid-state heats of formation (for 2, 5 and 7)
were calculated based on Trouton’s rule according to eqn (1)
(T represents either the melting point or the decomposition
Method b: Compound 1 (0.50 g, 3.0 mmol) was added to
nitric acid (15%, 5 mL), the reaction mixture was heated to
50 °C and stirred for 0.5 h. After removing the dilute nitric
acid, the residue was recrystallized with EtOH/H2O. 0.55 g,
yield: 85%.
temperature
when
no
melting
occurs
prior
to
decomposition).27
1
3: White solid. Td (onset): 247 °C. H NMR (d6-DMSO): 9.93
(s, 1H), 9.59 (s, 1H), 8.58 (s, 1H), 8.24 (s, 1H) ppm. 13C NMR
ΔHsub ¼ 188=J molꢀ1 Kꢀ1 ꢁ Tð1Þ
ð1Þ
˜
(d6-DMSO): δ 157.7, 154.8, 1531, 136.0 ppm. IR (KBr): v = 3383,
For energetic salt (3), the solid-phase enthalpy of formation
is obtained using a Born−Haber energy cycle.30,31
3336, 3122, 1687, 1585, 1483, 1384, 1289, 1178, 1046, 968, 898,
834, 780, 717, 648, 598 cm−1. Elemental analysis for C4H5N7O4
(215.13): calcd C 22.33, H 2.34, N 45.58%. Found: C 22.55, H
2.51, N 45.70%.
Crystal structure analysis
A yellow needle-shaped crystal (2·H2O) with dimensions 0.20 ×
Synthesis of 5·H2O. Hydrogen peroxide (50%, 1.5 mL) was
0.03 × 0.03 mm3, a colourless plate-shaped crystal (3) with added dropwise to a solution of trifluoroacetic anhydride
dimensions 0.16 × 0.10 × 0.04 mm3, a yellow plate-shaped (5 mL) in dichloromethane (20 mL) at 0 °C. Then 4 (0.50 g,
crystal (5·HCl·2H2O) with dimensions 0.21 × 0.15 × 0.05 mm3, 3.3 mmol) was added to the solution and the reaction mixture
and a colourless needle-shaped crystal (7·2HCl) with dimen- was stirred at room temperature for another 3 h. After remov-
sions 0.17 × 0.05 × 0.02 mm3 were mounted on a nylon loop ing the solvent by air, water (20 mL) was added to the residue
with paratone oil. Data were collected using a XtaLAB Synergy, and the solution was neutralized with solid sodium bicarbon-
2146 | Dalton Trans., 2021, 50, 2143–2148
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ate. The precipitate was collected by filtration, washed with
water (20 mL) and dried in air.
5·H2O: (0.37 g, yield: 60%). White solid. Tm: 223 °C. Td
(onset): 303 °C. 1H NMR (d6-DMSO): δ 7.59 (s, 1H) ppm. 13C
4 E. A. Chugunova, V. A. Samsonov, A. S. Gazizov,
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˜
NMR (d6-DMSO): δ 153.2, 145.6, 143.7, 118.6 ppm. IR (KBr): v =
3354, 3151, 3017, 1703, 1638, 1618, 1563, 1418, 1392, 1308,
1244, 1214, 1158, 997, 904, 800, 765, 738, 626 cm−1. Elemental
analysis for C4H7N7O2 (185.14): calcd C 25.95, H 3.81, N
52.96%. Found: C 25.85, H 3.51, N 52.40%.
7 P. Politzer and J. S. Murray, Cent. Eur. J. Energ. Mater., 2017,
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Synthesis of 7. Hydrogen peroxide (50%, 1.5 mL) was drop-
wise added to a solution of trifluoroacetic anhydride (5 mL) in
dichloromethane (20 mL) at 0 °C. Then 6 (0.50 g, 3.3 mmol)
was added to the solution and the reaction mixture was stirred
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water (20 mL) was added to the residue and the solution was Eur. J. Org. Chem., 2018, 2018, 1004–1010.
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NMR (d6-DMSO): δ 12.60 (br), 8.75 (s, 2H), 7.95 (s, 1H), 7.55 (s,
2H) ppm. 13C NMR (d6-DMSO): δ 151.2, 148.4, 147.7, 139.5,
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˜
111.0 ppm. IR (KBr): v = 3460, 3130, 2811, 1676, 1625, 1524,
1473, 1415, 1346, 1256, 1202, 1176, 947, 902, 834, 721, 696, 13 D. Fischer, T. M. Klapötke and J. Stierstorfer, Eur. J. Inorg.
626 cm−1. Elemental analysis for C5H6N6O (166.14): Calcd C
36.15, H 3.64, N 50.58%. Found: C 35.84, H 3.69, N 50.00%.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
Financial support of the Office of Naval Research (N00014-16-
1-2089), and the Defense Threat Reduction Agency (HDTRA 1-
15-1-0028) is gratefully acknowledged. The Rigaku Synergy S
Diffractometer was purchased with support from the National
Science Foundation MRI program (1919565). This work was
supported by the National Natural Science Foundation of
China (21905135), the Natural Science Foundation of Jiangsu
Province (BK20190458), the Fundamental Research Funds for
the Central Universities (30919011270) and the Large
Equipment Open Funding of Nanjing University of Science
and Technology.
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