Photochemistry of cyclic vicinal tricarbonyl compounds. Laser flash photolysis study of the reaction of indane-1,2,3-trione and its 5-methoxy derivative with olefins

Abstract

The rate constants for the quenching of indane-1,2,3-trione (1) and 5-methoxyindane-1,2,3-trione (2) triplets by olefins, in degassed benzene solution, have been measured by laser flash photolysis. The alkenes studied included acyclic, cyclic, isolated and conjugated dienes, and enol ethers. No quenching was observed when irradiation was performed in the presence of olefins substituted with electron-accepting groups such as maleic anhydride, dimethyl fumarate, dimethyl maleate or chalcone. The plots of log k_qversus the ionization potential for cyclohexene, 2-methylbut-1-ene, 2-methylbut-2-ene, 2,3-dimethylbut-2-ene, trans-penta-1,3-diene, ethyl vinyl ether and ethyl prop-1-en-1-yl ether are linear with a slope of −2.7/eV (r = 0.98) for 1 and −2.6/eV (r = 0.95) for 2. The magnitude of the slope is in agreement with a mechanism involving a partial charge transfer complex, which then leads to product formation. A comparison of the reactivity of 1 and 2 toward olefins shows that a similar mechanism operates for the quenching processes of these two triketones.