The transformation of 4-nitroimidazole (3) into 1,4-dinitroimidazole (4) could be brought about by treatment with acetyl nitrate (AcONO2), which could
be generated in the mixed solution of acetic anhydride and fuming nitric acid in acetic acid in situ at low temperature [6]. It took two days to get
excellent yield (95 %) of 4. There are about 3 % of unreacted 3 as the major impurity, which can be removed by the extraction with methylene chloride.
This compound 4 is so thermally unstable at the elevated temperature (over 50 °C) that nitro group of 1-position can be deleted to return to 3.
2,4-Dinitroimidazole (5) was prepared from 4 by thermal rearrangement with refluxing in chlorobenzene [6]. The yield was 93 %. In general, the
synthesized 5 were contaminated with 3 in a small amount. It is probably that very small portion of 4 might go back to 3 during the rearrangement.
Along with above thermal decomposition, 3 in 5 can come from the contaminated 4. To minimized 3 in 5, it is important to get rid of 3 when making 4.
The 3rd nitration to 2,4,5-trinitroimidazole (6) was performed in the mixed acid condition. As the intermediate of potassium 2,4,5-trinitroimidazolate
(7), 6 was not stable enough that we could not isolate that. From the extracted ether solution of 6, the solid of 7 was precipitated with the
saturated K2CO3/KCl solution in good yield (60 %) [15]. It was reported that 6 could be manufactured via 2,4,5-triiodoimidazole, but we chose the
route through 2,4-DNI (5).
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