Organic Sulfur Mechanisms. 13. Sulfene Formation by a Reverse Diels–Alder Reaction

Dimethyl 2,2-dioxido-4-phenyl-2-thiabicyclo[2.2.2]octa-5,7-diene-5,6-dicarboxylate (5), a molecule purposely designed to undergo a thermal reverse Diels–Alder reaction to form sulfene and an aromatic co-fragment, has been synthesized. Thermolysis of 5 gave dimethyl biphenyl-2,3-dicarboxylate (7) and, in addition, (a) in the presence of an enamine (6) gave the appropriate sulfene–enamine adduct (8), and (b) with an excess of p-toluidine gave methanesulfon-p-toluidide (10), in agreement with the above expectation. In the absence of sulfene traps the only product identified, in addition to 7, was sulphur dioxide; further experiments failed to determine the fate of the methylene group but serve to exclude certain possibilities, including fragmentation of sulfene to sulphur dioxide and a trappable carbenoid species.