Thermal decomposition of s‐2‐benzothiazole thiocarbonates

Abstract

Thermal decompositions of S‐2‐benzothiazole thiocarbonates have been investigated. The results are best rationalized by an Sni ion‐pair return mechanism from which S‐substituted benzothiazoles are the major products obtained. The ready ionization of the carbon‐oxygen bond of thiocarbonates at elevated temperature renders a concerted Sni mechanism (internal substitution with allylic rearrangement) inoperative. Of particular interest is the mixed benzo‐thiazole‐2‐thiol and 2‐thione compound isolated from the decomposition of 2 (or 3)‐halogen‐substituted ethyl (or propyl) thiocarbonates. Evidence is presented that the thione 14 results from the nucleophilic opening of the cyclic intermediate, 2,3‐dihydrothiazolo[2,3‐b]benzothi‐azolium chloride (20). The latter is readily formed under the experimental condition by intramolecular cyclization of the corresponding 2‐(2‐chloroethyl)benzothiazole (23), a primary product from the decomposition of S‐2‐benzothiazole 2‐chloroethylthiocarbonate (7).