DISTORTION OF OLEFIN AND CARBONYL PI-ORBITALS IN DIBENZOBICYCLO[2.2.2]OCTATRIENES AND DIBENZOBICYCLO[2.2.2]OCTADIENONES - UNSYMMETRIZATION OF PI-LOBES ARISING FROM PI-PI-ORBITAL INTERACTIONS

We have detected the unsymmetrical pi faces of the olefin group in 2-substituted dibenzobicyclo-[2.2.2]octatrienes (2-substituted 9,10-dihydro-9,10-ethenoanthracenes) and the carbonyl groups of 2-substituted and 3-substituted dibenzobicycla[2.2.2]octadienones (2-substituted and 3-substituted 9,10-dihydro-9,10-(11-ketoethano)anthracenes), wherein alpha-type overlaps of the pi orbitals are involved, in a similar manner to longicyclic conjugation. An intrinsically nonequivalent substituent at distal positions modulates the epoxidation and dihydroxylation of the olefin group and the reduction of the carbonyl group. Both systems exhibit similar substituent effects: an ''electron-withdrawing'' substituent such as a nitro or fluoro group gave a large to moderate bias (preferred syn attack with respect to the substituent) whereas an ''electron-donating'' methoxy substituent exhibited a negligible bias. Herein we interpret these biases or nonbiases in terms of unsymmetrization of pi lobes of the olefin and carbonyl pi orbitals, arising from nonequivalent pi-pi interactions rather than from an electron-donating or -withdrawing effect.