Mechanistic variations in the formation of benzocyclooctatetraene from benzobicyclo[4.2.0]octa-2,4,7-triene

Article abstract of DOI:10.1139/v96-004

Benzocyclooctatetraene (i.e., COT, 2) is the major product from the thermolysis and direct photolysis of 2,3-benzobicyclo[4.2.0]octa-2,4,7-triene (i.e., triene 1). It is also a minor product from the sensitized irradiation of 1. The mechanism of the reaction 1→2 was investigated using deuterium-labelled triene (i.e., 1a/b), which was prepared in a two-step procedure from a known triene mixture (1a) containing deuterium at both C-4 (43%) and C-5 (97%): the thermolysis of 1a led to labelled COT 2a, which upon direct irradiation (λ ≥ 310 nm) gave the deuterated triene mixture 1a/b in good chemical yield (61%) but in low quantum efficiency (Φ ≈ 0.0001). The COT produced from the direct irradiation (Corex filter) of 1a/b possessed different deuterium distributions than when generated thermally or from triplet sensitization (p-dimethylaminobenzophenone), as determined by ¹H NMR integration. The observed deuterium-labelling patterns are in accord with a Zimmerman di-π-methane rearrangement being involved in formation of the COT from S₁ of triene 1, while a mechanism proceeding by initial cleavage of the cyclobutene C₁-C₆ bond appears to operate during the thermal or sensitized (T₁) generations of 2.