Synthesis and Electrophilic Heterocyclization of 3-Alkenylsulfanyl-5-phenyl-1,2,4-triazines under the Action of Iodine and Bromine

Derivatives of 1,2,4-triazine-3-thione are interesting objects for study due to their wide range of application as medicinal drugs, substances with optoelectronic properties, and precursors for the synthesis of new pyridine systems by the Diels–Alder reaction. In this study, alkylation of 5-phenyl-2,3-dihydro-1,2,4-triazine-3-thione (1) with 3-chloro-2-methylprop-1-ene, 2,3-dibromoprop-1-ene, 1-bromo-3-methylbut-2-ene and 4-bromobut-1-ene yielded previously unknown 3-(2-methylprop-1-enyl)-, 3-(2-bromoprop-1-enyl)-, 3- prenylsulfanyl-, and 3-butenylsulfanyl-5-phenyl-1,2,4-triazines (2a–2d). The 1H NMR spectrum of compound 2b showed a downfield shift of the SCH2-group and the vinyl group proton signal by 0.48 and 0.60 ppm compared with similar signals in the spectrum of 3‑allylsulfanyl-5-phenyl-1,2,4-triazine. This may be due to the content of the bromine atom in the allyl fragment. The weakest signal in the 13C NMR spectra of compounds 2а–2d at 171.86–173.68 ppm belongs to the aromatic carbon atom of the triazine ring in the third position (C‑3) bonded to a sulfur atom and two nitrogen atoms. Using electrophilic heterocyclization of metallyl-, bromolyl-, prenyl-, and butenyl sulfides 2a–2d, the synthesis of new fused heterocyclic systems of the ionic type with a bridging nitrogen atom was carried out. At the same time, heterocyclization of the 2a, 2b compounds yielded [1,3]thiazolo[3,2-b][1,2,4]triazinium halides; heterocyclization of compounds 2c, 2d gave [1,3]thiazino[3,2-b][1,2,4]triazinium halides. The 1H NMR spectra of triazinium halides show a characteristic downfield shift of the H-6 aromatic proton signal compared to the similar signal in the spectrum of the initial sulfides 2а–2d. The 13C NMR spectra of triazinium halides feature a shift of the signal of the aromatic carbon atom bonded to the sulfur atom and two nitrogen atoms (to the range of 162.56–172.42 ppm), which can be explained by the occurrence of a positively charged nitrogen atom in their structure.