Tandem pseudopericyclic processes in the cyclization of α-diazonitriles to 5-halo-1,2,3-triazoles. Scope and limitations

A series of α-carbonyl diazoacetonitriles has been synthesized and subjected to reaction with hydrogen halides. The scope and limitations of this reaction for preparing 5-halo-1,2,3-triazoles have been determined. The experimental energy of this cyclization is 15–20 Kcal mol. A mechanism of this reaction consists of two steps—addition of HHal to α-carbonyl diazonitriles via a pseudopericyclic group transfer reaction leads to formation of intermediate α-diazohaloimines which in subsequent hetero-electrocyclic reactions afford 5-halo-1,2,3-triazoles. Indeed, the cyclization of α-carbonyl diazonitriles is a series of tandem pseudopericyclic processes.