Photoenolization of 2-(2-Methyl Benzoyl) Benzoic Acid, Methyl Ester: Effect of E Photoenol Lifetime on the Photochemistry

Photolysis of 3 in argon-saturated 2-propanol led to formation of 5 via intermolecular H-atom abstraction followed by lactonization. Irradiation of 4 in 2-propanol gave compounds 6 and 7 that also come from intermolecular H-atom abstraction. In contrast, photolysis of an oxygen-saturated solution of 3 in 2-propanol yields products 8, 9, and 10, which were all formed from intramolecular H-atom abstraction and trapping of the corresponding biradical with oxygen. Laser flash photolysis of 3 in methanol showed formation of biradical 3BR (λmax 330 nm, and τ = 50 ns) via intramolecular H-atom abstraction as the main photoreactivity of 3. Biradical 3BR decayed into photoenols 3Z and 3E (λmax 390 nm, τ = 6.5 μs and τ = 162 μs, respectively). In comparison, laser flash photolysis of 4 yielded photoenols 4Z and 4E (λmax 390 nm, τ = 15 μs and τ = 3.6 ms, respectively). Thus photoenol 3E is unusually short-lived, and therefore it does not undergo the intramolecular lactonization as we have observed for the analogous ...