A theoretical and experimental investigation of acid catalyzed rearrangements of small [n]cyclophanes

The difference in reactivity of small [n]cyclophanes towards CF3CO2H is discussed in terms of charge densities, strain energies and proton affinities. These data are calculated with MNDO and MINDO/3 for para‐, meta‐ and ortho‐cyclophanes and for their ipso‐protonation products; an attempt is made to transform gas phase ΔH values into liquid phase ΔH0f values. Experimental evidence is presented that the acid catalyzed rearrangement of [5]paracyclophane to its ortho‐isomer proceeds via two consecutive 1,2‐carbon shifts without deprotonation; intermediate adducts were identified by NMR‐spectroscopy. Thus, a gradual shift in reaction pattern in the series [4]‐, [5]‐ and [6]paracyclophane is observed experimentally, in line with the calculational results.