Synthesis of Triazolium Inner Salts by Thiocyanation of Aldehyde‐Derived Hydrazones

Abstract

An ammonium persulfate mediated thiocyanation reaction of aldehyde‐derived hydrazones has been carried out under metal‐free conditions at ambient temperature to afford the corresponding 5‐thioxo‐1,2,4‐triazolium inner salts. A library of triazolium inner salts that contain a wide range of functional groups were synthesized in high yields. The reaction proceeds through a thiocyanation followed by isomerization to give the isothiocyanate and subsequent cyclization steps.

Direct Electrochemical Synthesis of Sulfur‐Containing Triazolium Inner Salts

“…Based on related literature reports [ 6,14 ] and our studies, a plausible mechanistic pathway was proposed for the electrochemical oxidative thiocyanation/isomerization/cyclization of aldehyde hydrazone, using 1a as a model substrate, based on the direct electrochemical initiation model (Scheme 5). [ 7c ] Due to the much lower oxidative potential of NaSCN than aldehyde hydrazone 1a , SCN – is first initiated by the anodic oxidation, leading to the generation of pseudo‐halogen (SCN) 2 A , which subsequently undergoes electrophilic addition with 1a to produce cationic intermediate B .…”

“…In 2018, Hajra and co‐workers explored the direct C(sp 2 )—H thiocyanation of aldehyde‐derived hydrazones with ammonium thiocyanate (NH 4 SCN) for the preparation of triazolium inner salts by using stoichiometric K 2 S 2 O 8 as an external oxidant inevitably (Scheme 1a). [ 6 ] However, despite undisputable advances, it is difficult to obviate the use of stoichiometric halogen or chemical redox reagents.…”

I2/DMSO-Promoted Synthesis of Diaryl Sulfide- and Selenide-Embedded Arylhydrazones

“…In this regard, the ambient nucleophilic character of the −NHNCH– moiety of arylhydrazones plays a vital role to activate the azomethine (−NCH−) carbon toward electrophilic substitution. In the past decades, substantial synthetic outcomes of azomethine C­(sp 2 )–H functionalization of arylhydrazones in the presence of various catalysts, oxidants, or visible light are already reported. , Recently, Hajra and co-workers have reported an ammonium persulfate-mediated thiocyanation of arylhydrazones using NH 4 SCN in 1,2-dichloroethane (DCE) medium . However, a thorough investigation for aromatic C­(sp 2 )–H bond functionalization of arylhydrazones using a variety of chalcogenyl-substituents is still necessary from a biological point of view.…”

Organocatalytic Oxidative C–H Amination of Aldehyde Hydrazones with Azoles at Ambient Temperature

“…To the best of our knowledge, there has been no report on the direct oxidative C–H amination of aldehyde-derived hydrazone derivatives with azoles. With our continuous efforts to functionalize the C–H bond of hydrazone derivatives (Scheme B, ii), herein we disclose a report on the method of organocatalytic oxidative C–H amination for the synthesis of azolylmethylidenehydrazine derivatives at room temperature (Scheme B, iii).…”