The Intramolecular Interaction of Thiophene and Furan with Aromatic and Fluoroaromatic Systems in Some [3.3]Meta(heterocyclo)paracyclophanes: A Combined Computational and NMR Spectroscopic Study

Four thiophene- and furancontaining [3.3]meta(heterocyclo)paracyclophanes were designed and synthesized to study the intramolecular interaction between standard heteroaromatic rings and tetra-H- or tetra-F-substituted benzenes. A complete conformational analysis, carried out by DFT calculations and variable-temperature NMR techniques, showed that, despite their structural similarity, these adducts have different conformational preferences and undergo different types of isomerizations depending on the nature of the heterocycle. The thiophene-derived adducts adopted a parallel stacked arrangement of the aromatic systems in the ground-state conformations. Their isomerization pathways involved a thiophene ring-flip process passing through an edge-to-face arranged transition state in which the heterocycle is perpendicular to the benzene platform and its sulfur atom points toward the center of that ring. The threshold energy barrier to the ring-flip process was higher by 10 kJmol-1 in the case of the adduct featuring the perfluorinated benzene. This difference was rationalized by assuming that the ground-state conformations of the H- and F-substituted compounds have different stability. On the contrary, the furan-derived adducts were shown by calculations and NMR spectroscopy to adopt, in their groundstate conformations, a perpendicular edge-to-face disposition of the rings with the oxygen atom pointing toward the benzene platform. The adoption of this arrangement was confirmed by Xray crystallography. In the case of these compounds, the isomerization process involved distortion of the CH2SCH2 bridges connecting the aromatic systems and the adoption of transitionstate geometries for which the rings were arranged in a parallel-stacked orientation. Once again a very nice agreement was observed between the predicted and the experimentally determined geometries and pathways. In the case of the furan-containing compounds, the threshold barriers were found to be much lower in energy that those observed for the thiophene derivatives. Remarkably, they were virtually independent of the presence of fluorine atoms on the platform benzene ring.