LEWIS ACID CATALYSIS OF PHOTOCHEMICAL REACTIONS. 10. SPECTROSCOPY AND PHOTOCHEMISTRY OF 2-QUINOLONES AND THEIR LEWIS ACID COMPLEXES

The spectroscopic properties, photodimerization, and cross-cycloaddition reactions of 2-quinolone and three of its methylated derivatives have been investigated in the absence and presence of the strong Lewis acid BF3. Comparison of quinolone, which forms a hydrogen-bonded dimer, and its N-methylated derivatives establishes that hydrogen bonding has little effect on these properties. Methylation at C-4 also has little effect on spectroscopic properties, but does retard photodimerization and result in the formation of photoene products in competition with cycloadduct formation with alkenes. All of the quinolones form strong complexes with BF3. Complex formation results in changes in the NMR, absorption, and fluorescence spectra of the quinolones and in their photochemical behavior. Complexation is proposed to occur on oxygen for all of the quinolones resulting in changes in electron populations that have been probed using GAUSSIAN 86 calculations. A decrease in the energy of the oxygen nonbonding orbitals upon complexation results in a change in the configuration of the lowest singlet state from n,pi* to pi,pi* upon complexation. This change results in an increase in singlet lifetime and a change in cycloaddition mechanism from triplet (stepwise) to singlet (concerted) upon complexation.