Conformation Analysis of phenyl substitutents on bicycle[3.2.2]nona-3,6-dien2-ones

Abstract

The conformations of a phenyl ring situated in an axial position on 9-phenylbicyclo[3.2.2]nona-3,6-dien-2-one (4) and 9-methyl-9-phenylbicyclo[3.2.2]nona-3,6-dien- 2-one (5) were determined using a series of quantum chemical techniques, that is, semi-empirical MO, ab initio RHF, MP2 and DFT. The theoretical investigations suggested in chorus that, in accord with the conclusion we have previously drawn from 1H-NMR chemical shifts, the aromatic rings of the two compounds were oriented quite differently each other. This results were in contrast to the case of the phenylcyclohexane derivatives early reported by Allinger's group, where the phenyl ring in an axial position had the same perpendicular conformation due to syn-axial hydrogens repulsions, reflecting delicate differences in the magnitude of the intramolecular steric repulsions between the two cases. There might exist other key factors to influence the conformation of the phenyl ring of 4 and 5, i.e., so-called intra CH/π interactions. To elucidate the effects, we accomplished further calculations of model compound 10 and 11 whose double bond in question are reduced, leading to the exclusion of CH/π interaction.