Ionic intermediates in the first stages of the reaction of 3,4-dihydro-6-methyl-3-thioxo-1,2,4-triazin-5(2H)-one with propargyl bromide

Tautomerism of 3,4-dihydro-6-methyl-3-thioxo-1,2,4-triazin-5(2H)-one and it s anion were studied by 6-31G* nonempirical calculations. Of the 13 possibl e tautomers of the former in the gas phase the only stable is the 2H,4H-thi one tautomer, and of the 6 possible tautomers of the anion the least energi es are characteristic of the 2H- and 4H-thione ones. According to IR and UV spectral data, in methanol the 2H-thione tautomer of the anion is only sta bilized. This tautomer enters substitution reaction with propargyl bromide to give an S-propargyl derivative whose anion is an intermediate of heteroc ylization reaction. The alkylation and heterocyclization reactions are prob ably orbital-controlled.