Unexpected Pyrolytic Behaviour of Substituted Benzo[c]thiopyran and Thieno[2,3-c]thiopyran S,S-dioxides

Flash vacuum pyrolysis (FVP) of benzo[c]thiopyran S,S-dioxide 1 results in formation of indene and 2-vinylbenzaldehyde as previously described. A range of eight analogues with various substitution patterns are found to behave differently. In general there is no extrusion of SO₂ to give products analogous to indene, but unsaturated carbonyl products analogous to 2-vinylbenzaldehyde are formed in most cases by way of ring expansion to a 7-membered ring sultine, extrusion of SO, and intramolecular hydrogen atom transfer. Other processes observed include formation of anthracene via an isomeric 7-membered sultine with loss of SO, CO and methane or butane, and formation of 4-ethylidene-4,5-dihydrocyclobuta[b]thiophenes by way of SO loss, a radical rearrangement and extrusion of acetone. The analogues with a halogen substituent at position 8 on the benzene ring require a higher temperature to react and give naphthalene resulting from net elimination of HX and SO₂. The X-ray crystal structure of 1 is also reported.