Mixed [2.2]Cyclophanes of Pyrene and Benzene

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An examination of the literature on [2.2]cyclophanes reveals a loose relationship between the relative sizes of the two ‘half-cyclophanes’ (as measured by the parameter Δd) and the limitations of the dominant general synthetic approaches. Direct coupling methods tend to be successful only for systems with Δd values below 1.0 Å, whereas ring-contraction-based approaches are usually viable for systems with Δd values up to 2.0 Å. For the very few known systems with Δd values greater than 2.0 Å, aromatization-based approaches are the only ones that have been successful. The syntheses of two [2.2]cyclophanes with very large Δd values, [2]paracyclo[2](2,7)pyrenophane (17) (Δd = 4.25 Å) and [2]metacyclo[2](2,7)pyrenophane (18) (Δd = 5.04 Å) are presented here. The syntheses hinge on a valence isomerization/dehydrogenation reaction. The crystallographically determined bend angle, θ, for 18 is 96.1°. Cyclophane 18 undergoes a degenerate conformational flip, the energy barrier for which was determined to be 18.9 kcal mol–1 by DNMR.