The Photochemistry of 5,6-Dimethylidene-2-norborananone. Synthesis and Diels-Alder Reactivity of 2,3-Dimethylidenebicyclo-[2.1.1]hexane

2,3-Dimethylidenebicyclo 2.1.1]hexane (4) was isolated from direct irradiation (253.7 nm) of 5,6-dimethylidene-2-norbornanone (3) . Quenching experiments at 253.7 nm, as well as direct and sensitized irradiations at >300 nm suggested that a high vibrationally excited S₁- or a S₂-state is required for the photodecarbonylation of 3 in contrast with other β, γ-unsaturated ketones for which α-cleavage occurs with lower excitation-energy. The new diene 4 reacted toward tetracyano-ethylene (k (1mol^?1 s^?1))=(3.1±0.34) · 10^?3) in toluene and (6.2±0.11) · 10^?3 in benzene only 60 times more slowly than 2,3-dimethylidenenorbornane (5) and ca. 850 times as fast as 2,3-dimethylidene-syn1,4,5,6-tetramethylbicyclo2.1.1]-hexane (9) .