The oxidative cyclization of formazans to tetrazolium salts

Approach to High-Nitrogen Materials with Dual-Use Properties via Tetraaza-Nazarov Cyclization

“…In this report, we execute oxidation of a formazan to generate a tetrazolium salt, in which the pentadienyl system has nitrogen atoms at positions 1,2 and 3,4 (Scheme 2D). 46 The idea that this transformation is pericyclic and thus related to the processes in Scheme 1A−C has not been considered previously. In this paper, we examine the energetic profile of the cationic cyclization using DFT to explore the tetraaza-Nazarov hypothesis shown in Scheme 2D.…”

“…Pentannulations in which nitrogen atoms are incorporated into the pentadienyl system (“aza-Nazarov” cyclizations) have also been reported, but these cases are much less common. − Examples with one nitrogen atom at positions 1, ,,, 2, − and 3 of the pentadienyl system (Scheme B) as well as “diaza-Nazarov” cyclizations , with two nitrogen atoms (Scheme C) have been reported. In this report, we execute oxidation of a formazan to generate a tetrazolium salt, in which the pentadienyl system has nitrogen atoms at positions 1,2 and 3,4 (Scheme D) . The idea that this transformation is pericyclic and thus related to the processes in Scheme A–C has not been considered previously.…”

“…Although 4π electrocyclization was not explicitly proposed for the reactions, Hegarty et al suggested that a cationic path was operative in their 1975 study on the oxidative cyclization of formazans using bromine. 46 These researchers found that neither irradiation nor performing the reaction in the dark affected rate constants, which led them to rule out the radical pathway. In 2012, Samanta and co-workers demonstrated that I(III)mediated cyclizations of aryl acetamides were consistent with a 4π electrocyclization process, based on Hammett plot data.…”

“…A radical path was also considered, but it is considered unlikely based on relevant literature. Although 4π electrocyclization was not explicitly proposed for the reactions, Hegarty et al suggested that a cationic path was operative in their 1975 study on the oxidative cyclization of formazans using bromine . These researchers found that neither irradiation nor performing the reaction in the dark affected rate constants, which led them to rule out the radical pathway.…”

The Effect of Oxidizing Agents in the Preparation of 2,3,5‐triaryl‐2H‐tetrazolium Salts from 1,3,5‐triarylformazans

“…In view of this, and given the variety of methods reported with limited success for preparing scalable TTz, we conducted comparative studies with the aim of preparing 2-(4-nitrophenyl)-3,5-diphenyl-2H-tetrazolium chloride (NTC) from 1-(4-nitrophenyl)-3,5diphenylformazan (NF). NTC was chosen as a model compound rather than the commonly studied 2,3,5triphenyltetrazolium chloride [16][17][18][19][20][21][22][23], because NTC is of higher practical interest in recent TTz studies [7] and the addition of an electron-withdrawing substituent at N (2) would hinder the oxidation reaction [24], therefore representing a wider group of tetrazolium compounds. For our studies, the direct chlorination of NF with either chlorine or tert-butyl hypochlorite was deemed impractical, because of the high reactivity and hazard properties of these reagents.…”