Restricted Rotation Involving the Tetrahedral Carbon. XXXV. Stereodynamics of 9-(3,5-Dimethylbenzyl)styrene Derivatives

Article abstract of DOI:10.1246/bcsj.54.473

Stereodynamics of a variety of 9-(3,5-dimethylbenzyl)triptycene derivatives was studied.Atropisomerism about the bridgehead-to-methylene bond was realized in a triply peri-substituted derivative, 8,13-dichloro-1,4-dimethyl-9-(3,5-dimethylbenzyl)triptycene: the activation enthalpy for the ap--> +/-sc conversion was 26.4 kcal/mol.Barriers to rotation about the bridgehead-to-methylene bond in a variety of triply, doubly, and singly peri-substituted derivatives were obtained by DNMR method.DNMR behavior of the diastereotopic m-methyl groups in the benzyl moiety revealed the dual mechanisms of the methyl exchange process; rotation about the methylene-to-aryl bond by 180 deg without rotation about the bridgehead-to-methylene bond ("isolated rotation (IR)") and rotation about the methylene-to-aryl bond by 180 deg synchronous with rotation about the bridgehead-to-methylene bond by 120 deg ("gear motion (GM)").The GM process predominates in peri-unsubstituted derivatives, while the IR process predominates in triply peri-substituted ones.