PHOTOLYSIS AND THERMAL DECAY OF THE SULFUR-DIOXIDE ADDUCTS OF BENZOBENZVALENE

Benzobenzvalene 1 is shown to add SO2 with formation of a strained sul fone 2 and an isomeric gamma-sultine 3. Photochemically (254 nm) the t wo adducts interconvert. Moreover, SO2 is extruded during the irradiat ion of 2 or 3 with regeneration of 1 and formation of naphthalene. Thi s result is interpreted in terms of a reversible homolytic cleavage le ading to an intermediate sulfinyloxy biradical 5 and subsequent format ion 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 co nsist of a cycloreversion giving an intermediate sulfene 9. This can b e trapped with N-phenyl maleimide (NPMI). Key structures are ascertain ed by X-ray analysis.