Hindered Rotation in Diphenylmethane Derivatives. Electrostatic vs Charge-Transfer and Homoconjugative Aryl−Aryl Interactions

Abstract

A series of p,p‘-disubstituted 7-phenyl-7-(2-fluorophenyl)norbornanes 5xy has been prepared, and the barrier (ΔG#) to 160° libration around the 2-fluoroaryl-norbornane bond has been measured by DNMR. There is spectroscopic evidence of strong homoconjugative and charge-transfer (CT) interactions between both aryl groups of 5xy. However, the relationship between ΔG# and the nature of the substituents X and Y is accounted for only by electrostatic interactions between both aryl groups in the ground state as well as in the transition state of the libration. Therefore, the notion of CT and aromatic homoconjugation as strong attractive forces between aryl groups should be definitively rejected.