Internal Rotation in peri-Phenylnaphthalenes

A 180º rotation about a phenyl-naphthyl bond is expected to be effectively blocked for derivatives of the peridiphenylnaphthalenes, in which steric requirements force the phenyl rings to assume a face-to-face conformation. However, surprisingly low rotational energy barriers have been found. The preparation and the measurement of the barrier to phenyl ring rotation of a derivative of the highly crowded 1,4,5,8-tetraphenylnaphthalene system is described. The barrier for this substance is 14.9 kcal/mol compared with 16.4 kcal/mol determined for 1,8-diphenylnaphthalene; both of these barriers are much lower compared with the 33.5 kcal/mol reported for the stereotopically similar [3,4]paracyclophane. The differences are discussed in terms of a rotational transition state having large deformations of the naphthalene ring.