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.