Lewis Acid Catalysis of Photochemical Reactions. 8. Photodimerization and Cross-Cycloaddition of Coumarin

The effect of Lewis acid complexation upon the electronic structure and photochemical behavior of coumarin has been investigated. Changes in IR and ¹H NMR spectra upon complexation of coumarin with BF₃or EtAlCl₂are indicative of 1:1 complexation of the Lewis acid with the carbonyl oxygen. Changes in the UV spectrum and the observation of room-temperature fluorescence from the coumarin-BF₃complex are attributed to increased energy of the n, π* singlet state relative to the fluorescent π, π* singlet state. Dimerization is found to occur via the reaction of either the singlet or triplet complex with ground-state coumarin. The singlet-state reaction selectively yields the syn head-to-tail dimer while the triplet-state reaction yields the anti head-to-head dimer. The singlet- and triplet-state complexes also react with simple alkenes. Addition of the singlet complex with the isomeric 2-butenes occurs with retention of configuration. The enhanced reactivity of complexed vs uncomplexed coumarin is attributed to its increased singlet-state lifetime and electrophilicity.