Electronic Structure,Molecular Conformation and Reactivity of Benzonorbornadiene Systems

From the photoelectron spectra of benzonorbornadiene ( 1 ) and its 9-isopropylidene derivative ( 2 ) the energy and an approximate wave function for the highest occupied orbitals is obtained. With these results the differential reactivity of the systems in Diels-Alder additions with ‘inverse electron demand’ is discussed on the basis of frontier orbitals. For the ground state of 2 similar non-bonded interactions between the isopropylidene unit and the double bond or the benzene ring are indicated. Conformational equilibria in 9-aza derivatives of 1 are discussed in view of these findings.