Novel Thermal Rearrangement of Fused Diaryl- v -Triazolium Salts to Neutral Indazole Derivatives. Fused Azolium Salts. 16

Alkylation of 1,3-diaryl- v -triazolo[1,5- a ]benzimidazole ( 1 ) by trimethyloxonium salt afforded selectively the 4-methyl quaternary salt 2, whereas reaction with dimethyl sulfate at elevated temperature resulted in ring transformation, implying ring opening to a nitrenium cation and a subsequent electrophilic ring closure to give a benzimidazolylindazole product 5 . Independent of the methylation, a similar easy thermal rearrangement was observed with alkylated 1,3-diaryl- v -triazolo[1,5- a ]benzimidazolium ( 2 → 4 ) and the related benzothiazolium salt ( 7 → 8 ). Extension of the reaction to diaryl- v -triazolopyridinium salt ( 9 → 10 + 11 ) allowed observation of an additional novel reaction path that provided further support for the supposed reaction mechanism.