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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry