[3.2] Metacyclophanes. Conformational Studies

The energy barriers to ring inversion of the 11-membered aromatic-aliphatic rings in a number of 2-substituted [3.2]metacyclophanes were determined from the temperature dependence of their proton magnetic resonance spectra. The energy barriers to ring inversion were found to be dependent upon the steric nature of the 2-substituent and lie between 15.8 and 19.1 kcal/mole in the temperature range 60-120°. Bulky 2 substituents were found to produce conformational changes in 2-monosubstituted derivatives with respect to 2,2-symmetrically disubstituted derivatives. A half-life at 0° of 2.4 sec for the stable conformers of [3.2]metacyclophane-2-carboxylic acid is predicted from the activation energy parameters for ring inversion obtained from the nmr data. In comparison, [2.2]metacyclophane is estimated to have a barrier to ring inversion of greater than 26-28 kcal/mole.