5-Bromo- and 5-chloro-3-thiolene-2-one : Allylic rearrangement in 5-bromo-3-thiolene-2-one

5-Bromo- and 5-chloro-3-thiolene-2-one have been prepared by acid-catalysed dealkylation of 5-bromo- and 5-chloro-2-t-butoxythiophene, respectively. On storage in the liquid state at −20°, 5-bromo-3-thiolene-2-one undergoes allylic rearrangement to 3-bromo-3-thiolene-2-one. At room temperature 5-bromo-3-thiolene-2-one rapidly decomposes to a black tar with formation of a mixture of products among which 3-bromo-3-thiolene-2-one, 3-thiolene-2-one, hydrogen bromide, and hydrogen sulphide have been identified. The NMR spectra of 5-bromo- and 5-chloro-3-thiolene-2-one in CCl₄ show that they exist almost exclusively as such, and have been analysed according to the ABX case. From the theoretical spectra it is deduced that J_AX(J₃₄) and J_BX(J₄₅) are of the same sign.