Studies in restricted rotation. Part I. Barrier to rotation in some 3-arylcyclohexenone derivatives

Temperature dependence of the n.m.r. spectra of three 2-aryl-6-oxocyclohex-1-enylacetic acid derivatives (I)-(III) has been studied and the free energy of activation of internal rotation about the aryl-cyclohexenone bond in each determined from the coalescence temperature. The 1-naphthyl derivative (I) possesses an appreciable energy barrier (ca. 22 kcal mol-1) which is lowered considerably by the introduction of a methoxy-group adjacent to the pivot bond [as in (II)]. Some plausible explanations are suggested