TRANSANNULAR ORBITAL INTERACTION BETWEEN KETONE AND OLEFIN CHROMOPHORES DETECTED BY CIRCULAR-DICHROISM AND C-13-NMR SPECTROSCOPY - DIMETHANONAPHTHALENONES AND TRIMETHANOANTHRACENONES

A series of delta,epsilon-unsaturated ketones and their saturated ketone analogs were synthesized on the dimethanonaphthalene skeleton with endo-endo (1), endo-exo (2), and exo-exo (3) configurations. The corresponding norbornylogs (trimethano-anthracenes) with zeta,eta-unsaturated ketones (4, 5, and 6) and their saturated analogs were also synthesized. Their C-13-NMR and circular dichroism (CD) spectra were measured in order to examine transannular orbital interactions. Transannular orbital interaction was detected by a relatively more shielded C-13=O resonance in the unsaturated ketones. The shielding was largest when the C=O and C=C chromophores could interact through sigma-pp orbital overlap, as in 1. That is, orbital interaction through space is an important component of the ground state of 1. In the CD spectra, the ketone n-pi* DELTAepsilon(max) values were typically larger for the unsaturated ketones, as compared with their saturated ketone analogs. The n-pi* DELTAepsilon(max) enhancements fall off with increasing interchromophoric distance, as seen in the trimethanoanthracenes. The presence of ''charge-transfer'' CD bands near 230 nm in the unsaturated ketones suggests a component of ''through-space'' orbital interaction. The intensity of the CD charge-transfer bands is greatest when the chromophores are oriented for sigma-pp orbital interaction. Orbital interaction through space appears to be an important component of the excited state of 1 and 2, and even 4, but it appears to play a less important role in 3, 5, and 6.