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

Abstract

[reaction: see text] 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 (lambda(max) 330 nm, and tau = 50 ns) via intramolecular H-atom abstraction as the main photoreactivity of 3. Biradical 3BR decayed into photoenols 3Z and 3E (lambda(max) 390 nm, tau = 6.5 micros and tau = 162 micros, respectively). In comparison, laser flash photolysis of 4 yielded photoenols 4Z and 4E (lambda(max) 390 nm, tau = 15 micros and tau = 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.

Comparison of photoenolization and alcohol release from alkyl-substituted benzoyl benzoic esters

“…We assign this absorption to Z-3B and E-3B, based on comparison with the transient spectra of 3A and 3C. 8,10 The TD-DFT calculations also support this assignment as they revealed that the major absorption bands of Z-3B and E-3B are located at 426 nm (f = 0.1975) and 437 nm (f = 0.0699) in methanol, respec- tively. The lifetimes of Z-3B and E-3B in methanol are 15 ms and >3 ms, respectively.…”

“…6) We assign this absorption to triplet biradical 2B on the basis of its similarity to the transient spectra of the analogous 1,4-biradical 2A. 9,10 The TD-DFT calculations support this assignment because the major calculated electronic transition for 2B is located at 336 nm (f = 0.0871) in the gas phase and at 334 nm (f = 0.1104) in methanol. In oxygen-saturated solutions, the absorption owing to 2B was quenched.…”

“…In comparison, 1A, which has an ortho-methyl group, fails to release its alcohol moiety upon irradiation (Scheme 1), even though transient spectroscopy shows that the major reactivity of 1A involves intramolecular H-atom abstraction to form Z-3A and E-3A. 10 It was concluded that the short lifetime of E-3A prevented it from undergoing intramolecular lactonization and releasing its alcohol.…”

Ruthenium‐Catalyzed Enantioselective Hydrogenation/Lactonization of 2‐Acylarylcarboxylates: Direct Access to Chiral 3‐Substituted Phthalides

“…Methyl 2‐(2‐methylbenzoyl)benzoate ( 1 o ): R f =0.30 (PE/EA=15:1); colorless oil: 5.1 g, 50 % yield; 1 H NMR (400 MHz, CDCl 3 , 25 °C): δ =7.93 (dd, J= 7.6, 1.2 Hz, 1 H), 7.62–7.53 (m, 2 H), 7.45 (dd, J= 7.6, 1.2 Hz, 1 H), 7.39–7.35 (m, 1 H), 7.30 (d, J= 7.6 Hz, 1 H), 7.20 (dd, J= 7.6, 1.2 Hz, 1 H), 7.12 (t, J= 6.8 Hz, 1 H), 3.61 (s, 3 H), 2.66 ppm (s, 3 H); 13 C { 1 H} NMR (100 MHz, CDCl 3 , 25 °C): δ =198.0, 166.5, 142.0, 139.2, 136.3, 131.5, 131.4, 131.3, 130.7, 129.6, 129.3, 128.1, 124.9, 51.7, 20.9 ppm.…”

Photoenolization of o‐Methylvalerophenone Ester Derivative

“…In addition, alcohols can also be released through photoenolization of ester derivatives, as long as the photoenolization is followed by spontaneous intramolecular lactonization between the ester and the enol alcohol (Scheme 3) ( [14][15][16][17][18].…”