71. “The Photoreversible Addition of Sulfur Dioxide to Benzobenzvalene: A new Approach to the Benzoprefulvene Biradical”
    Burger, U.; Gmünder, C.; Schmidlin, S.; Bernardinelli, G.
    Helv. Chim. Acta 1990, 73, 1724-1729.

Benzobenzvalene (naphthvalene; 1) is shown to add SO₂ to a lateral bicyclobutane bond with formation of a sulfone 2 and a ‘γ-sultine’ 3. The structure of the latter is unambiguously established by X-ray diffraction. Both adducts extrude SO₂ upon direct photolysis at 254 nm and regenerate 1 accompanied by naphthalene in a 1:3 ratio. This result is interpreted in terms of a reversible homolytic cleavage leading, for both, 2 and 3, to the same sulfinyloxy biradical 5, which by loss of SO₂ gives the benzoprefulvene biradical 6. The latter in its singlet state undergoes ring closure to 1, or it opens to give naphthalene.

DOI : 10.1002/hlca.19900730617 

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