Atropisomerism in Doubly peri-Substituted 9-(2-Methylbenzyl)triptycene Derivatives, 8-Chloro- and 8-Bromo-1,4-dimethyl-9-(2-methylbenzyl)triptycenes

Abstract

Abstract The 13C dynamic NMR study revealed that 9-(2-methylbenzyl)triptycene has an energy barrier to gear rotation of 11.8 kcal mol−1 (1 cal = 4.184 J). Introduction of substituents at two of the peri-positions was expected to raise the barrier enough for atropisomerism to be realized in this system. Actually, two stable rotational isomers, ap and sc*(9S*), were separately isolated for 8-chloro- (7) and 8-bromo-1,4-dimethyl-9-(2-methylbenzyl)triptycene (8). Classical kinetic studies in toluene-d8 solutions revealed that the isomerization barriers (ΔG≠ at 350 K) are 26.4 and 27.2 kcal mol−1 for 7 and 8, respectively. Isomerization pathways were investigated by molecular mechanics calculations, which suggested that gear rotational pathway was favored. In either compound, the sc* (9S*) rotamer was found to thermally isomerize to the ap rotamer in the solid state. X-Ray crystallographic analysis of the rotameric crystals showed that the rotamers had different crystal structures and thus the crystal would be destroyed during the isomerization.