Rotational Isomerism Involving an Acetylenic Carbon: Synthesis of and Barrier to Rotation around C(sp)-C(sp3) Bonds in Bis(1,4-disubstituted 9-triptycyl)ethynes

The substituent effect on the rotational barriers of acetylenic to tetrahedral C-C bonds was studied for the title compounds carrying Me or OMe substituents at the 1,4-positions in the 9-triptycyl groups. These compounds were prepared by use of the Diels-Alder and the Sonogashira reactions from a 9-ethynylanthracene derivative. The kinetic parameters were determined by the total lineshape analysis of the signals observed by the variable temperature 1H NMR for five compounds. The rotational barriers are in the range of 9—15 kcal mol-1, and are enhanced as the steric size of the substituents is increased (H < OMe < Me). This substituent effect is attributed to the destabilization of the transition state of the rotation process, where the two triptycyl groups are almost eclipsed about the central axis, by the steric interactions between the peri substituents. The acetylenic carbons are significantly deformed from the linear geometry in the X-ray structure of the tetramethyl compound. The substituent effect on the rotamer population is also discussed with the aid of the MM calculation.