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.