Rotational isomerism involving an acetylenic carbon: Synthesis of and barrier to rotation around C(sp)-C(sp(3)) bonds in bis(1,4-disubstituted 9-triptycyl)ethynes

The substituent effect on the rotational barriers of acetylenic to tetrahed ral C-C bonds was studied for the title compounds carrying Me or OMe substi tuents at the I,il-positions in the 9-triptycyl groups. These compounds wer e prepared by use of the Diels-Alder and the Sonogashira reactions from a 9 -ethynylanthracene derivative. The kinetic parameters were determined by th e total lineshape analysis of the signals observed by the variable temperat ure H-1 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 ste ric interactions between the peri substituents. The acetylenic carbons an s ignificantly deformed from the linear geometry in the X-ray structure of th e tetramethyl compound. The substituent effect on the rotamer population is also discussed with the aid of the MM calculation.