Thermal decomposition of s‐2‐benzothiazole thiocarbonates

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 ).