Restricted Rotation Involving the Tetrahedral Carbon. LVII. Stereodynamics of 9-(2-Alkylphenoxy)-1,4-dimethyltriptycenes

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Abstract
1,4-Dimethyl-9-phenoxytriptycene and its derivatives carrying a methyl, isopropyl, or t-butyl group in the o-position of the phenoxyl moiety were synthesized and their dynamic NMR behavior was studied. These molecules are considered to constitute a bevel gear system with a two-toothed and a three-toothed wheels and the dynamic NMR behavior is best explained in terms of gear rotation. The o-isopropyl derivative, for example, shows the energy barriers of 17.2 and 10.8 kcal mol−1 for the ap\ightleftharpoons±sc and +sc\ightleftharpoons−sc gearing processes, respectively. The population of the ±sc rotamer increases with the bulkiness of the o-alkyl group: The steric congestion among the o-alkyl group, the oxygen atom, and the peri-methyl group is severer in ap than in ±sc. Results of molecular mechanics calculations on these molecules are discussed.

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