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 (ΔG≠ ≈ 25 kcal mol–1) compared to that in 3‐indenyltriptycene (ΔG≠ ≈ 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 (ΔG≠ ≈ 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 (ΔG≠ ≈ 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.

Recent Developments in Synthesis and Applications of Triptycene and Pentiptycene Derivatives

“…They further determined by 1 H NMR and 13 C NMR that the rotational barrier about the 2-indenyl-triptycenyl single bond in 7 is Ͻ9 kcal mol -1 and that this bond undergoes essentially free rotation at ambient temperature. [14] When the M(CO) 3 unit was attached to the indenyl moiety in an η 6 fashion, the simplicity of the 1 H and 13 C NMR spectra of 8a showed the time-averaged equivalence of the three blades of the paddlewheel, and the crystal structure of 8a also revealed that there were no steric interactions between the triptycene and the indenyl moieties, which all indicated that the paddlewheel 8a was free to rotate. [14] When the M(CO) 3 unit was attached to the indenyl moiety in an η 6 fashion, the simplicity of the 1 H and 13 C NMR spectra of 8a showed the time-averaged equivalence of the three blades of the paddlewheel, and the crystal structure of 8a also revealed that there were no steric interactions between the triptycene and the indenyl moieties, which all indicated that the paddlewheel 8a was free to rotate.…”

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

“…Fast rotation is observed when af lat rotor is attachedt oa n apical position of at rigonal stator( 20 and 21), however, even minor alterations of the structure may result in much higher rotational barriers as in 22-24.S uch striking differences (often > 10 kcal mol À1 ), can clearly be understood in terms of the closest approachd istance (CAD) between two interacting fragments and their multiple steric repulsions as showni n Scheme 7. [57] Similarly,arather high rotational barrier, 13.4 kcal mol À1 ,i na na-(2-adamantyl)naphthalene derivative 25 was quantified in ar ecent report by 1 H-1 HE XSY [58] -this barrier arises perhaps from cooperative mutual steric repulsion.…”

From allenes to tetracenes: Syntheses, structures, and reactivity of the intermediates

“…However, deprotonation engenders negative charge on the indenyl fragment and causes the organometallic tripod to migrate onto the five-membered ring, as in 59, thus blocking rotation of the paddlewheel and rendering one of the blades inequivalent to the other two. The system has been characterized in both its η 6 "OFF" and η 5 "ON" forms by NMR spectroscopy and by X-ray crystallography [44]. …”