Thiocyanation of N,N‐Dialkylhydrazonoyl Bromides: An Entry to Sulfur‐Containing 1,2,4‐Triazole Derivatives

Abstract

A simple and practical one‐pot protocol involving sequential C–H bromination and thiocyanation of readily available aldehyde‐derived N,N‐dialkylhydrazones has been established for the synthesis of 5‐thioxo‐1,2,4‐triazolium inner salts. Application of the method toward the synthesis of value‐added 3‐sulfanyl‐1,2,4‐triazoles through N→S alkyl group transfer has also been demonstrated.

Direct Electrochemical Synthesis of Sulfur‐Containing Triazolium Inner Salts

“…[ 4 ] In 2017, Monteiro and co‐workers reported an entry to sulfur‐ containing 1,2,4‐triazole derivatives by using N , N ‐dialkylhydrazonoyl bromides and sodium thiocyanate (NaSCN) (Scheme 1a). [ 5 ] Starting from aldehyde hydrazone derivatives, this protocol required stoichiometric N ‐bromosuccinimide (NBS) as a brominating agent. 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).…”

Sustainable Synthetic Approaches Involving Thiocyanation and Sulfur- Cyanation: An Update

“…Pyridine and KNTf 2 were produced at the same time. Allenyl thiocyanates (35) were prepared by an efficient cascade reaction starting from propargyl amines (34), by treatment with AgSCF 3 and KBr (Scheme 13) [13]. In this transformation, the isothiocyanate intermediate is formed in situ, then a [3,3]-sigmatropic rearrangement occurs to give allene products in high yields.…”

“…In that case, after the bromination of the hydrazone starting material, a thiocyanate salt is added to the reaction mixture to form an intermediate that subsequently performs a thiocyanate-to-isothiocyanate isomerization. The newly generated compound is then cyclized to lead to valuable 1,2,4-triazolium species (80) [34].…”

Sustainable Synthetic Approaches Involving Thiocyanation and Sulfur- Cyanation: An Update

“…Allenyl thiocyanates (35) were prepared by an efficient cascade reaction starting from propargyl amines (34), by treatment with AgSCF 3 and KBr (Scheme 13) [13]. In this transformation, the isothiocyanate intermediate is formed in situ, then a [3,3]-sigmatropic rearrangement occurs to give allene products in high yields.…”