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

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Abstract
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-dimethylbenzy)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° without rotation about the bridgehead-to-methylene bond (“isolated rotation (IR)”) and rotation about the methylene-to-aryl bond by 180° synchronous with rotation about the bridgehead-to-methylene bond by 120° (“gear motion (GM)”). The GM process predominates in peri-unsubstituted derivatives, while the IR process predominates in triply peri-substituted ones.

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