Collarene Molecular Belts

Structural formulae of collarene maerotyeles. The [injparacyclophane backbone of pillararenes - contained within the eollarene framework -is illustrated in black, while the additional bridging methylene units are illustrated in gray. 

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Figure 12.6 Graphical illustration of the DFT (B3LYP/6-31G ) frontier molecular orbitals of the [12]collarene molecular belt 12.18b.

The synthesis (Scheme 12.8) of the first [l2]collarene molecular belt (12.18b) was achieved in our laboratory through the repeated application of Diels-Alder (DA) cycloadditions between the dienophile 12.20 and the diene 12.21, which proceeds with extraordinarily high diastereoselectivity. While the DA reaction between 12.20 and 12.21 could, potentially, lead to four different diastereoisomeric products, only a single diastereoisomer was actually observed in this reaction. This especially high diastereoselectivity is dictated primarily by electronic effects, which kinetically favor attack on the diene 12.21 from the endo face, while attack on the dieneophile 12.20 is preferred from the exo side. Density functional theoiy (DFT) calculations (Figure 12.7) reveal that the observed diastereoselectivity has its cause in the selective stabilization of the endo-exo transition state (12.19a). 

In supramolecular chemistry [31] iterative synthesis is frequently used for the construction of ribbon and belt shaped molecules [32]. The repeated Diels-Alder reaction is suitable for the preparation of beltenes, collarenes and cycla-cenes containing partially or fully unsaturated annelated six-membered rings. They are potential molecular receptors for appropriate substrates. Stoddart et al. synthesized the cyclacene 18 by Diels-Alder cyclization of the bisdiene 15 and the bisdienophil 16 [33] (Fig. 10). 

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