Restricted Rotation Involving the Tetrahedral Carbon. LXII. Rotational Isomerism, Structure, and Isolation of Triptycene Rotamers that Carry Secondary Alkyl Group at 9-Position

Abstract As an example of a set of rotational isomers of triptycenes that carry a secondary alkyl group at the 9-position, 9-alkyl-1,8-difluorotriptycenes (alkyl = isopropyl or 3-hydroxy-1-methylpropyl) were synthesized and their rotational isomerism was examined by means of NMR spectroscopy. For the isopropyl compound, the ap and sc isomers were observed in a 6.2 : 1 ratio in chloroform-d, but the two diastereomeric rotamers were inseparable. X-Ray analysis of the compound reveals that all molecules take ap conformation about the C9–Calkyl bond in crystal. In contrast three rotamers of 3-hydroxy-1-methylpropyl compound could be isolated by HPLC as stable entities at room temperature. Barriers to rotation in chloroform-d were found to be 26.6, 26.3, 25.0, 24.6, 24.8, and 24.9 kcal mol−1 for sc*(S*) → ap(R*), sc*(S*) → sc*(R*), ap(R*) → sc*(S*), ap(R*) → sc*(R*), sc*(R*) → ap(R*), and sc*(R*) → sc*(S*) processes, respectively, at 39.5 °C.