Photoenolization as a Means To Release Alcohols

Abstract

We have designed molecules which release alcohols upon exposure to UV light independent of the reaction media, making it possible to liberate alcohols in a controlled manner in applications. Photolysis of 2-(2-isopropylbenzoyl)benzoate ester derivatives 4 in various solvents and in thin films results in the liberation of the alcohol moiety from the ester. The reaction mechanism for the release of the alcohol has been elucidated by time-resolved laser flash photolysis. Upon irradiation the triplet excited state of ketone, 4 is formed, and its lifetime can be estimated to be between 0.08 and 0.8 ns. The triplet excited state decays by efficient intramolecular H-atom abstraction to form a 1,4-biradical, 8, that has a lifetime of less than 17 ns and is trapped by molecular oxygen. In the absence of oxygen, biradical 8 intersystem crosses to form photoenols (Z)-9 and (E)-10 in a ratio of 5:2, respectively. Photoenol (Z)-9 has a lifetime of approximately 3000 ns in protic solvents and returns to the starting material through 1,5 intramolecular hydrogen transfer. The other isomer, (E)-10, is much longer lived (>1 ms) and releases the alcohol moiety through an intramolecular lactonization.

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.…”

“…8,9 The release from 1C is initiated by intramolecular H-atom abstraction by the triplet ketone in 1 to form 1,4-triplet biradical 2C, which intersystem crosses to E and Z-photoenols 3C. Z-3C is short-lived because it decays by a 1,5-H shift to regenerate 1C.…”

Controlled Release of Volatiles under Mild Reaction Conditions: From Nature to Everyday Products

“…In contrast to anthraquinone derivatives, the photoreduction of 2-benzoyl benzoates releases primary or secondary alcohols via a hydroxy radical intermediate which then eliminates the alcohol upon intramolecular cyclization (Scheme 9). [72,73] The release of geraniol from 24 requires an external hydrogen donor, such as 2-propanol, or an electron donor, such as a primary amine. [72] The introduction of an isopropyl substituent in the proximity of the carbonyl function, as for example in 25 or 26 (Scheme 9), allows the release of the corresponding fragrance alcohol independently of the reaction medium, for example, from a thin film of the corresponding compound.…”

Fluorescent Caged Compounds of 2,4-Dichlorophenoxyacetic Acid (2,4-D): Photorelease Technology for Controlled Release of 2,4-D

“…On the basis of the above strategy, several bioactive molecules including nucleic acids (7), amino acids, enzyme substrates, and catalysts of biochemical reactions (8) were chemically caged using PRPGs and released on irradiation by using a UV-visible light source. Recently, Gudmundsdottir's group also demonstrated the controlled release of fragrances under UV light over an extended period of time by chemically caging volatile alcohols such as geraniol using a PRPG (9).…”