The Structure and Reactivity of 5-Oxo- and 5-Hydroxy-substituted 5,6-dihydro-4-phenyl-4 H-l, 3,4-thiadiazines

Abstract

The structures of 5-oxo- and 5-hydroxy-substituted 5,6-dihydro-4-phenyl-4 H-l, 3,4-thiadiazine, [1] and [2], were discussed in terms of resonance stabilization based on the spectral data and the extended H∗#x00FC;ckel molecular orbital calculations. The structures of [1] is best interpreted by weak combination of two adjacent, local resonance structures of 3pπ-2pπ-2pπ(-S-C=N-) and 2pπ-2pπ(-N-C=0). On the other hand, [2] takes single butadiene-type resonance structure of 3 pπ-2 pπ-2 pπ-2 pπ(-S-C=N-N). As to the chemical properties, while [1] was inert 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-5,6-dihydro-4 H-l, 3, 4-thiadiazines in almost quantitative yield. The high reactivity of [2] towards electrophiles and nucleophiles may be ascribed to the resonance stabilization of the carbocation intermediate, 5, 6-dihydro-4 H-l, 3, 4-thiadiazinium cation, formed by heterolysis of the carbon-oxygen bond. © 1987, The Chemical Society of Japan. All rights reserved.