First Generation of Pentazole (HN₅, Pentazolic Acid), the Final Azole, and a Zinc Pentazolate Salt in Solution: A New N‐Dearylation of 1‐(p‐Methoxyphenyl) Pyrazoles, a 2‐(p‐Methoxyphenyl) Tetrazole and Application of the Methodology to 1‐(p‐Methoxyphenyl) Pentazole.

Abstract

Ceric ammonium nitrate (CAN) in methanol-water gave a new N-dearylation of a series of substituted 1-(p-methoxyphenyl) pyrazoles and a 2-(p-methoxyphenyl)tetrazole producing p-benzoquinone and the parent azole in a mole for mole ratio. Application of this reaction to 1-(p-methoxyphenyl) pentazole at 240 °C produced p-benzoquinone. 15 N NMR spectra suggest that pentazole, HN 5 , was also produced and held in solution as N 5 2 with Zn 2+ ion. The 15 N signal from N 5 2 was 210.0 ± 2.0 ppm in agreement with calculated values.The existence of an all-nitrogen aromatic azole ring, R-N 5 , the pentazoles, was proved by Clusius and Hurzeler 1 in Zurich and Huisgen and Ugi 2 in Munich. 3 Since then a significant goal of heterocyclic chemistry has been to prepare the parent pentazole, HN 5 . Ozonolytic degradation of the aryl ring was attempted 4 and recently revisited with controlled limited amounts of ozone, 5 but HN 5 or its anion N 5 2 was not found. Following the recent discovery of N 5 + as a reasonably stable cation in inorganic salts 6,7 there has been renewed experimental interest in allotropes of nitrogen and in N 5 2 in particular. There has also been much theoretical interest in these types of species. [8][9][10][11][12] Metallocene pentazole derivatives have been predicted as viable forms of N 5 2 , but bidentate forms MN 5 (M = Na, K, MgCl + , CaCl + ) and M(N 5 ) 2 (M = Na, K, Mg, Ca) and unidentate Zn(N 5 )Cl and Zn(N 5 ) 2 are calculated to have lower energy. 13 Recently it has been suggested the N 5 + .N 5 2 is a viable entity. 14 Following our recent establishment of the mechanism of formation of aryl pentazoles from aryldiazonium ions and azide ion, 15 we have been endeavouring to generate HN 5 and its anion N 5 2 . Recently N 5 2 has been detected for the first time from high voltage collisions of 4-pentazolylphenolate ion, N 5 -C 6 H 4 O 2 , in the mass spectrometer. 16 We are therefore prompted to make a preliminary report of our work.To remove an aryl group from an arylpentazole without destruction of the pentazole ring a mild oxidizing agent and the appropriate aryl pentazole must be chosen. Ceric (IV) ammonium nitrate (CAN) has been used to remove p-MeOC 6 H 5 -groups from p-methoxyphenyl ethers 17,18 and N-p-methoxyphenyl azetidines. [19][20] The methoxyphenyl ring is removed as p-benzoquinone and the use of H 2 18 O has proved that the two quinone oxygens arise from water in the solvent. 18 Successive one-electron oxidations are considered to produce a quinoneimine dication which is hydrolysed. 20 We tested this reaction for the first time with an azole substrate using the 1-N-pmethoxyphenyl pyrazole series shown in Fig. 1. It proved successful and was optimised at low temperatures. The pbenzoquinone is easily detected and estimated. Its presence indicates a mole for mole presence of the azole. High temperatures are not appropriate for this reaction because the azole generated may undergo addition reactions with pbenzoquinone. 21 We also attempted the reaction with the nonmetallic one-electron ...

Theoretical Design of Green Energetic Materials: Predicting Stability, Detection, Synthesis and Performance

“…Finally, in 2002 and 2003, after extensive theoretical studies, the PZ was detected in two independent mass spectrometry experiments [56,57]. The detection in solution was reported in 2003 [58], but the NMR-assignment was subsequently questioned [59]. Although a recent theoretical study has provided support for the original assignment [60], it is clear that PZ at best was generated as a transient species [61].…”

“…Considering the difficulties in detecting PZ, even with modern spectroscopical methods, it is tempting to speculate that some of their experiments actually resulted in the desired product. Butler and coworkers used Ceric ammonium nitrate (CAN) in methanolwater at −40°C for N-dearylation of 1-p-methoxyphenylpentazole to produce benzoquinone [58,61]. They suggest that the reaction afforded pentazole (HN 5 ), which was held in solution as PZ complexed to Zn 2+ .…”

“…The barrier for the phenylpentazole is lower than that for fluorobenzene, but still too high to make the reaction feasible. Inspired by the work of Butler and coworkers [58], we investigated the effect of complexation of Zn 2+ to the N 5 ring. This reduces the barrier for the dissociation by 6 kcal/mol.…”

Recent Advances in the Chemistry of N5+, N5− and High‐Oxygen Compounds

“…Attempts to prepare cyclo-N 5 À salts in bulk by either the oxidative cleavage of the C À N bond using ozone [37,45] or the reductive one using alkali metals in liquid ammonia [37], had been unsuccessful. There was only one claim for the preparation of the cyclo-N 5 À anion in solution and its identification by 15 N NMR spectroscopy [54]. Using (NH 4 ) 2 Ce(IV)(NO 3 ) 6 (CAN) in aqueous methanol at À 40 8C, the para-methoxyphenyl group in para-methoxyphenylpentazole was oxidized to para-benzoquinone, supposedly yielding the zinc salt of the pentazolate anion as coproduct.…”

“…Using (NH 4 ) 2 Ce(IV)(NO 3 ) 6 (CAN) in aqueous methanol at À 40 8C, the para-methoxyphenyl group in para-methoxyphenylpentazole was oxidized to para-benzoquinone, supposedly yielding the zinc salt of the pentazolate anion as coproduct. The following 15 N NMR evidence was presented [54] for the formation of N 5 À : (i) the observation of a signal at À 10 AE 2 ppm which is close to the value predicted [54,55] for N 5 À ; and (ii) the observation of two signals at À 283 and À 147 ppm which were attributed to the terminal and central nitrogen atoms, respectively, of N 3 À , the expected decomposition products of N 5 À [15]. Because the synthesis of the arylpentazole starting material was carried out with azide which was singly 15 N-labeled only in the terminal position, the appearance of the 15 N-label in all positions of the azide decomposition product would be strong evidence for the formation of an intermediate N 5 À anion in which all nitrogens have become equivalent (Eq.…”