Pyrenophane strained

The effect of curvature on the aromatic character of polyaromatic hydrocarbons (PAHs) can be studied by comparing planar PAHs and their curved analogues. In order to address this issue a variety of [n](2,7)pyrenophanes (n = 7-10) in which the pyrene (22) moiety is strongly distorted from planarity have been synthesized and studied45. The degree of distortion from planarity and therefore the strain in these systems is controlled by the length and type of the tether that connects the two remote ends (positions 2 and 7) of 22. 

Three successive diamagnetic species can be detected when 22 is reduced with lithium metal in THF-rf8 (Figure 8) a protonated pyrene monoanion (22-H-), a dianion (222-) and finally a monoanion that incorporates the atoms of a former solvent molecule (solvent cleavage) (23)46. It has been shown that the reduction of the strained pyrene systems [7](2,7)pyrenophane (24), [2]metacyclo[2](2,7)pyrenophane (25), [8](2,7)pyreno-phane (26), [9](2,7)pyrenophane (27) and [10](2,7)pyrenophane (28) with lithium metal can afford totally different results than those encountered in the reduction of the planar parent pyrene system47. The reduction of these systems shows that the strain in the pyrene moiety profoundly affects its reactivity. 

Other oxidants like thallium(III) oxide, vanadium(V) oxyfluoride, palladium ) acetate, and ruthenium(IV) tetrakis(trifluoracetate) have been developed as powerful tools for the intramolecular biaryl coupling reaction [7,93,113]. Nevertheless, DDQ is still one of the most versatile reagents in oxidative coupling reactions (see Scheme 14 and 29 [82,114]). The highly strained dioxa[8](2,7)pyrenophane (65), portraying an overall curvature of nearly 90° for the pyrene subunit, was finally obtained from the mefa-cyclophanediene (66) by dehydrogenation with DDQ in refluxing benzene in 67% yield [114]. 

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