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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 have been designed to study the effect of fluorination of the "face" ring on the edge-to-face (EtF) interactions with the C(Ar)-H bond of a phenyl ring or the nitrogen lone-pair of pyridine. Calculations established that in their more stable conformation the model systems adopt a tilted EtF disposition with the rim of the meta-substituted ring pointing towards the face of the para-substituted ring. Topomerization occurs by flipping of the meta-substituted ring, a process that involves the formation of an intermediate featuring an orthogonal EtF disposition of the arenes, which is less stable than the tilted arrangement. The energy barriers to the isomerization were determined by variable-temperature NMR spectroscopy and were well reproduced by DFT calculations. The variation in the energy barrier as a function of the substitution of the para-substituted ring could be rationalized well by a polar interpretation of the EtF interaction in the cases of model systems presenting the PyN...pi interaction but not in the cases of models featuring the C(Ar)-H...pi interaction.