AN INVESTIGATION OF THE RETRO-DIELS-ALDER REACTION AS A METHOD FOR THE GENERATION OF DIATOMIC SULFUR

The cyclic disulfides 2,3-dithiabicyclo[2.2.2]oct-5-ene 5, 1,4-dihydro-2,3-benzodithiin 6 and hexahydro-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.