An investigation of the retro Diels–Alder reaction as a method for the generation of diatomic sulfur

Abstract

The cyclic disulfides 2,3-dithiabicyclo[2.2.2]oct-5-ene 5, 1,4-dihydro-2,3-benzodithiin 6 and hexa-hydro-5,8-epoxy-2,3-benzodithiin 7 have been prepared by oxidation of the corresponding dithiols. Each of these compounds has been subjected to vapour phase pyrolysis in order to determine whether a retro Diels–Alder reaction occurs, leading to the formation of diatomic sulfur (S₂) and a diene. Compounds 5 and 7 both appeared to decompose in this way; in each case the expected diene was detected in the pyrolysate together with sulfur (S₈). Attempts to intercept S₂ and thus to obtain direct evidence for its formation were not successful. The benzodithiin 6 underwent an analogous loss of sulfur on vapour phase pyrolysis, but only as a minor reaction pathway, and this decomposition pathway was not detectable in solution pyrolyses. Solution pyrolysis of 6 in the presence of N-phenylmaleimide gave N-phenyl-2,3-naphthalimide 21 in low yield. cis-Cyclopentene-3,5-dithiol 9 has also been prepared but no evidence could be obtained for the formation of the disulfide 4 on oxidation. Reaction of the dithiol 9 with aldehydes and ketones in the presence of acids led to the formation of adducts (such as 12 and 13 from acetone) which were rapidly interconverted in acidic media.