The Structure and Reactivity of 5-0xo- and 5-Hydroxy-substituted 5, 6-dihydro-4-phenyl-4 H-1, 3, 4-thiadiazines

Abstract

The structures of 5-oxo- and 5-hydroxy-substituted 5, 6-dihydro-4-phenyl-4 H-1, 3, 4thiadiazine, [1] and [2], were discussed in terms of resonance stabilization based on the spectral data and the extended Hckel molecular orbital calculations. The structures of [1]is best interpreted by weak combination of two adjacent, local resonance structures of 3π-2π-2πr(-S-C=N-) and 2π-2π2π(-N-C=O). On the other hand, [2] takes single butadiene-type resonance structure of 3π-2 π-2 π-2π(-S-C=N-N).
As to the chemical properties, while [1] was ine r t to electrophiles, nucleophiles and radical reagents, [2] was unstable to weak acids and other electrophiles giving hardly separable complex mixtures. Treatment of [2] with alcohols, thiols and amines readily gave, however, the corresponding 5-alkoxy-, 5-alkylthio- and 5-alkylamino-4-phenyl-5, 6dihydro-4 H-1, 3, 4-thiadiazines in almost quantitative yield.
The high reactivity of [2] towards electrophiles an d nucleophiles may be ascribed to the resonance stabilization of the carbocation intermediate, 5, 6-dihydro-4 H-1, 3, 4-thiadiazinium cation, formed by heterolysis of the carbon-oxygen bond.