An Experimental Study of the Effect of Substituents on the Aromatic-Aromatic Interactions in Some Dithia-[3,3]-metaparacyclophanes.

Simple model systems based on the 2,11-dithia-[3,3]-metaparacyclo-phane skeleton were designed and synthesized to study the effect of substituents on the intramolecular aromatic-aromatic interactions between benzene rings. X-Rays structures established that in their more stable conformation these metaparacyclophanes feature partially overlapped aromatic rings (interplanar distance of about 3.5 Å) with the planes of the aromatic systems arranged in a slightly tilted disposition (interplanar angles ranging from 5 to 19°). Calculations showed that these derivatives underwent topomerization by flipping of the meta-substituted ring over the para-substituted one, a process in which the two rings adopted a continuum of edge-to-face dispositions, including also an orthogonal one, which were less stable than the starting face-to-face arrangement. The energy barriers to the isomerization process were experimentally determined by variable temperature NMR spectroscopy, using an internal temperature standard to safely assess even minor differences in energy (estimated experimental error ± 0.1 kJ mol-1). The variation of the barriers as a function of the different substitution on the interacting ring was small and apparently not related to the effect of the substituents on the polarity of the -systems. An explanation based on the charge penetration effect seemed more suitable to rationalize the observed trends in the barriers.