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 photoenol 1E. Photoenol 3Z decays back to 3
via an intramolecular 1,5-H shift, whereas photoenol 3E reforms 3 efficiently via the solvent with
the aid of the ortho ester group. The intramolecular lactonization of photoenols 1E and 3E must
be a slow process, presumably because the photoenols are rigid and the hydroxyl group is inhibited,
by intramolecular hydrogen bonding, from acquiring the correct geometry for lactonization. Thus
only photoenols that are resistant to reformation of their ketone via the solvent are long-lived enough
to undergo lactonization and release the alcohol moiety.