A Ceric Ammonium Nitrate N-Dearylation of N-p-Anisylazoles Applied to Pyrazole, Triazole, Tetrazole, and Pentazole Rings:  Release of Parent Azoles. Generation of Unstable Pentazole, HN₅/N₅^-, in Solution

Abstract

The reaction of cerium(IV) ammonium nitrate (CAN) with a range of N-(p-anisyl)azoles in acetonitrile or methanol solvents leads to N-dearylation releasing the parent NH-azole and p-benzoquinone in comparable yields. The scope and limitations of the reaction are explored. It was successful with 1-(p-anisyl)pyrazoles, 2-(p-anisyl)-1,2,3-triazoles, 2-(p-anisyl)-2H-tetrazoles, and 1-(p-anisyl)pentazole. The dearylation renders the p-anisyl group as a potentially useful N-protecting group in azole chemistry. The azole released in solution from 1-(p-anisyl)pentazole is unstable HN5, the long-sought parent pentazolic acid. p-Anisylpentazole samples were synthesized with combinations of one, two, and three 15N atoms at all positions of the pentazole ring. The unstable HN5/N5- produced at -40 degrees C did not build up in the solution but degraded to azide ion and nitrogen gas with a short lifetime. The 15N-labeling of the N3- ion obtained from all samples proved unequivocally that it came from the degradation of HN5 (tautomeric forms) and/or its anion N5- in the solution.

Azide Chemistry – An Inorganic Perspective, Part II[‡][3+2]‐Cycloaddition Reactions of Metal Azides and Related Systems

“…[545] A detailed reexamination of the reaction of (NH 4 ) 2 Ce(NO 3 ) 6 with p-anisylpentazoles carrying 15 N-labels in different positions led to the conclusion that the azide ion observed results from the degradation of N 5 -(N 5 -Ǟ N 3 -+ N 2 ) which, however, is too short-lived to be observed in NMR spectroscopy. [546] Theoretical calculations support the view that the pentazolate anion (or HN 5 ) has been created in these experiments in solution.…”

Kinetic Stability and Propellant Performance of Green Energetic Materials

“…[21] The decomposition barrier for the cyclic N 5 À anion has been estimated to be 26 kcal mol À1 by using high-level ab initio calculations. [21] As the detection of the pentazole anion in the condensed phase by NMR spectroscopy has proven difficult [22][23][24] it is still a topic of ongoing research. [25][26][27] The stability of many aryl pentazoles is well established; their decomposition barriers are typically around 20 kcal mol À1 .…”

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

“…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]. Thus, the quest for its synthesis and isolation goes on.…”