Introduction
Energetic materials are widely used in the military and aerospace fields
as explosives or propellants [1]. The future research and
development direction of energetic materials is mainly to synthesize
more suitable high-energy, low-sensitivity new energetic materials. In
1998, 1,1-diamino-2,2-dinitroethylene (FOX-7) was first synthesized by
Latypov et al. [2]. Its energy is close to the typical high-energy
materials RDX and HMX [3] that have been put into production now.
Its sensitivity is low, and close to the typical low-sensitivity
energetic materials TATB [4]. Therefore, studying the
characteristics of FOX-7 is of great significance for the search for
high-energy and low-sensitivity energetic materials. There is a highly
polarized carbon-carbon double bond in the middle of the FOX-7 molecule,
and the carbon-carbon double bond is flanked by two amino groups and two
nitro groups. This nitroenamine structure has a unique electronic
“push-pull” property. Due to the electron-pushing effect of the amino
group and the electron-pulling effect of the nitro group, a stable
carbon-carbon double bond structure is formed, and various isomers and
resonance structures can also be formed through electron rearrangement
[5-7]. The positively charged end of the carbon-carbon double bond
can be attacked by the nucleophile, resulting an addition reaction is
carried out. Since the amino group is an electron-pushing group and is
positively charged, the substitution reaction of the amino group can be
reacted with nucleophile. In addition, the nitrogen atom of the amino
group is sp3 hybridized and has a lone pair of
electrons, which can also react with electrophilic reagents. [8,9].
Based on this “push-pull” electronic structure, there are many
reactive sites on the molecular structure of FOX-7, so a variety of
potential new energetic materials can be synthesized.
The nucleophilic substitution reaction of single amino group is an
important reaction type based on FOX-7. In the reaction of FOX-7 with
methylamine, an amino group on FOX-7 is substituted by methylamine to
generate 1-amino-1-methylamino-2,2-dinitroethylene (AMFOX-7)
[10],In the above reaction, the ring formation reaction based on
both amino groups are substituted is difficult to proceed, and it may be
caused by the steric hindrance effect according previous reports
[11,12]. The reaction that two amino groups are substituted into a
ring is also an important reaction type based on the FOX-7 reaction. In
the reaction of FOX-7 with ethylenediamine, both amino groups on FOX-7
is substituted, resulting in a cyclic
2-dinitromethylene-1,3-diazacyclopentane (DNDZ) product at the original
position of the two amino groups. There is no single amino-substituted
product in the above reaction, which may be due to the strong stability
and lower energy of the cyclic compound. In addition, due to the
existence of lone pair electrons on both amino groups of FOX-7, such a
structure itself can also react as a nucleophile, and the cyclic
compound 4,5-dihydroxy-2-(dinitromethylene)-imidazolidine (DDNI)
[13] can be generated from the reaction of FOX-7 with glyoxal based
on this. In addition, other types of reactions can occur with FOX-7
[14-17].
The polyazole ring has great potential as the molecular framework of
energetic materials [18], and Click reaction is still the main
synthetic method for synthesizing 1,2,3-triazole rings [19]. New
energetic material molecules based on molecular framework of polyazole
ring can be constructed by substitution reaction of amino group on
FOX-7. If the mechanism of substitution of amino groups into ring can be
clarified, a new method for the synthesis of polyazole rings may be
developed based FOX-7. In scheme 1, an important synthetic work on amino
substitution is reported by Xu et al. [20], including: FOX-7 and
methylhydrazine (CH3NHNH2) is cyclized
to form 2-methyl-5-nitro-1 ,2,3-triazol-4-amine (MNTzA), and
1,5-bis(1-amino-2,2-dinitrovinyl) (BADCh) is formed by mono-amino
substitution of two FOX-7 and carbohydrazide (CHZ). The possible
synthesis mechanism of MNTzA is also proposed by Xu, but through our
verification, a more suitable reaction mechanism is found. Moreover, the
mechanism of mono-amino substitution and ring formation in the reaction
between FOX-7 and CHZ is also compared.