Severe Energy Costs of Double Steric Interactions: Towards a Molecular Clamp

Abstract The factors determining the ease of rotation about carbon–carbon single bonds connecting two internally rigid fragments such as phenyl, indenyl, anthracenyl and triptycyl are analysed. The internal rotation barriers in these molecules have been estimated on the basis of kinetic data or variable‐temperature NMR measurements, and the crystal structures have been analysed in terms of steric strain. Computer simulation of the internal rotation indicates that the estimated Closest Approach Distance, CAD, between sterically interacting atoms of the two interconnected fragments can be a helpful parameter for evaluating their rotational freedom, but must be used with caution. Thus, the barrier to rotation of a 3‐indenyl moiety linked to the 9‐position of anthracene is very high (Δ G ≠ ≈ 25 kcal mol –1 ) compared to that in 3‐indenyltriptycene (Δ G ≠ ≈ 12 kcal mol –1 ) despite the fact that the nominal CADs in both cases are very similar. Moreover, dimeric 2‐methylindenyl fragments linked by a single bond at the 3‐position undergo relatively slow rotation (Δ G ≠ ≈ 14–15 kcal mol –1 ) owing to the simultaneous close approach of two pairs of sterically interacting hydrogens. Although the rotational barrier for a 2‐indenyl or phenyl moiety attached to the bridgehead atom of triptycene, or to the related dibenzobicyclo[2.2.2]octane system, is relatively low (Δ G ≠ ≈ 8–9 kcal mol –1 ), further extension of the bridge to dibenzobicyclo[2.2.4]dioxadecane leads to an activation energy barrier in excess of 23 kcal mol –1 , attributable to an intramolecular simultaneous “clamping” of the phenyl rings by the edges of the aromatic rings of the dibenzobicyclo[2.2.4]dioxadecane moiety. The X‐ray crystal structures of 15 molecules, including mono‐ and di‐indenyl‐anthracenes, racemic ‐ and meso ‐2‐methylindenyl dimers, phenyl‐ and indenyl‐triptycenes and ‐barrelenes, are reported.