Photolysis dynamics of cyclopropyl and oxiranyl aryl ketones: Drastic ring-activation effect of oxygen

Although 1-(o-tolyl)-1-benzoylcyclopropane (I) and 2-(o-tolyl)-2-benzoyloxirane (II) are structurally analogous, they go through dramatically different reaction pathways upon absorption of photons. I yields a single photoproduct, while ²H₂-substituted II gives birth to three different photoproducts. The structural and kinetic features of the entire reaction intermediates have been measured with laser flash techniques. I at T₁ undergoes intramolecular hydrogen transfer at 2.3 × 10⁸ s^-1 from the methyl group to the carbonyl group to form a biradical intermediate, which transforms into a cyclized photoproduct (φ 0.14; 5 × 10⁷ s^-1). However, the carbonyl group of II at T₁ can abstract a hydrogen atom from the oxiranyl ring (6 × 10⁷ s^-1) as well as from the methyl group (2.3 × 10⁸ s^-1). The biradical intermediate from the former process immediately rearranges into a photoproduct (φ 0.025), while the one from the latter transforms into a new oxiranyl ring-opened intermediate (1.1 × 10⁸ s^-1) or a cyclized photoproduct (φ 0.075; 3.3 × 10⁸ s^-1). The new intermediate rearranges into another photoproduct (φ 0.025; 3.7 × 10⁷ s^-1). The dramatic changes of II are theoretically attributed to the drastic increase in the acidity and instability of the three-membered ring with oxygen substitution.