Synthesis and Properties of Monocyclic 5H-1,2-Oxathioles (Cyclic (alpha),(beta)-Unsaturated Sulfenic Acid Esters)

2,5,5-Trimethyl-3-hexyn-2-ol was successively treated with NaH and SCl2 in Et2O to provide 3-t-butyl-4-chloro-5,5-dimethyl-5H-1,2-oxathiole (1b), a class of (alpha),(beta)-unsaturated sulfenic acid ester, in 74% yield. Several S-unoxidized 5H-1,2-oxathioles were synthesized in similar ways. Analysis by cyclic voltammetry uncovered that the oxidation potentials of 1b, 3-(1-adamantyl)-4-chloro-5,5-dimethyl-5H-1,2-oxathiole (1c), and 4-chloro-5,5-dimethyl-3-phenyl-5H-1,2-oxathiole (1d) are +0.69, +0.68, and +0.78 V, respectively, and are lower than those of sulfides (thioanisole, +0.97 and dimethyl sulfide, +1.09 V), and also lower than that of an acyclic sulfenate ester (ethyl benzenesulfenate, +0.90 V). Thermolysis of 1c in boiling toluene furnished 2-(1-adamantyl)-3-chloro-4-methylthiophene in 81% yield.