Photolysis and Thermal Decay of the Sulfur Dioxide Adducts of Benzobenzvalene

Benzobenzvalene 1 is shown to add SO2 with formation of a strained sulfone 2 and an isomeric γ-sultine 3. Photochemically (254 nm) the two adducts interconvert. Moreover, SO2 is extruded during the irradiation of 2 or 3 with regeneration of 1 and formation of naphthalene. This result is interpreted in terms of a reversible homolytic cleavage leading to an intermediate sulfinyloxy biradical 5 and subsequent formation of the benzoprefulvene biradical 6. The pyrolysis of 2 and 3 gives, in the gas phase (FVP) and in solution, 1H-indene-1-carboxaldehyde 8 and naphthalene. The key step of this thermal reaction is shown to consist of a cycloreversion giving an intermediate sulfene 9. This can be trapped with N-phenyl maleimide (NPMI). Key structures are ascertained by X-ray analysis