Nitrile sulphides. Part 3. Thermal fragmentation of 1,3,4-oxathiazoles: formation of nitrile sulphides in a retro-1,3-dipolar cycloaddition reaction

On thermolysis at ca. 160 °C 1,3,4-oxathiazoles undergo retro-1,3-dipolar cycloaddition forming nitrile sulphides and carbonyl-containing fragments. The nitrile sulphides either decompose to sulphur and nitriles or are trapped as their 1,3-dipolar cycloadducts in the presence of dipolarophiles (dimethyl acetylenedicarboxylate, ethyl cyanoformate, benzonitrile, ethyl propiolate). Similar ratios (1.32, 1.34, 1.33, 1.31) of 4- and 5-ethoxycarbonyl-3-(p-methoxyphenyl)isothiazole obtained from four sources [(2a), (3a), (3b), (3c)] of p-methoxybenzonitrile sulphide with ethyl propiolate provide strong evidence for product formation from a discrete intermediate nitrile sulphide rather than via direct interaction of precursor with dipolarophile. 2-Dichloromethylene-1,3,4-oxathiazoles, prepared by dehydrochlorination of 2-trichloromethyloxathiazoles, likewise fragment to nitrile sulphides, but attempts to trap dichloroketene were unsuccessful.