RESTRICTED ROTATION INVOLVING THE TETRAHEDRAL CARBON .62. ROTATIONAL-ISOMERISM, STRUCTURE, AND ISOLATION OF TRIPTYCENE ROTAMERS THAT CARRY SECONDARY ALKYL GROUP AT 9-POSITION

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-C(alkyl) 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-degrees-C.