The Intramolecular Edge-to-Face Interactions of an Aryl C-H Bond and of a Pyridine Nitrogen Lone Pair with Aromatic and Fluoroaromatic Systems in Some [3,3]-Metaparacyclophanes: A Combined Computational and NMR Study

Simple model systems based on the [3,3]-metaparacyclophane skeleton were designed to study the effect of fluorination of the “face” ring in the EtF interactions with the CAr-H bond of a phenyl ring or the nitrogen lone pair of pyridine. Calculations established that in their more stable conformation the model systems adopted a tilted EtF disposition with the rim of the meta-substituted ring pointing toward the face of the para-substituted ring. Topomerization occurred by flipping of the meta-substituted ring, a process that involved the formation of an intermediate featuring an orthogonal EtF disposition of the arenes, which was less stable than the tilted arrangement. The energy barriers to the isomerization were determined by variable temperature NMR spectroscopy, and well reproduced by DFT calculations. The variation of the barrier as a function of the substitution of the para-substituted ring could be nicely rationalized using a polar interpretation of the EtF interaction in the case of the model systems presenting the PyN••/ but not in the case of the models featuring the CAr-H/ interaction.