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An aromatic saddle was designed from the hypothetical three-dimensional graphene with the negative Gaussian curvature (Schwarzite P192). Two aromatic saddles, tetrabenzo8circulene (TB8C) and its octamethyl derivative OM-TB8C, were synthesized by the Scholl reaction of cyclic octaphenylene precursors. The structure of TB8C greatly deviates from planarity, and the deep saddle shape was confirmed by single-crystal X-ray crystallography. There are two conformers with the S4 symmetry, which are twisted compared to the DFT structure (D2d). The theoretical studies propose that the interconversion of TB8C via the planar transition state (125 kcal mol(-1)) is not possible. However, the pseudorotation leads to a low-energy tub-to-tub inversion via the nonplanar transition state (7.3 kcal mol(-1)). The ground-state structure of TB8C in solution is quite different from the X-ray structure because of the crystal-packing force and low-energy pseudorotation. OM-TB8C is a good electron donor and works as the p-type semiconductor.
Sakamoto et al. (Tue,) studied this question.