Restricted Rotation Involving the Tetrahedral Carbon. LIX. Stereodynamics of Singly peri-Substituted 9-(3,5-Dimethylphenoxy)triptycene Derivatives

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Abstract
Five singly peri-substituted derivatives of 9-(3,5-dimethylphenoxy)triptycene with F, OCH3, CH3, Br, and C(CH3)3 as the peri-substituent were synthesized and their dynamic NMR behavior was studied. In any of the compounds, two m-methyl groups in the phenoxyl moiety were anisochronous at low temperatures and the signals coalesced into a single peak on raising the temperature. Lineshape analysis afforded the energy barriers to the rate process ranging from 10.1 kcal mol−1 (1 calth=4.184 J) for the F compound to 17.6 kcal mol−1 for the C(CH3)3 one. Molecular mechanics calculations (MM2) were performed on closely related model compounds to obtain the relative steric energies of the ground states and the possible transition states. The results from the dynamic NMR and the molecular mechanics studies suggest the following: When the peri-substituent is small (F and OCH3), the complete gear rotational circuit with the passing of the aryl group over the peri-substituent as the rate-determining step is the lowest-barrier process, while when the peri-substituent is bulky (CH3, Br, and C(CH3)3), the “isolated” rotation of the aryl group at the ap site has a lower barrier and is responsible for the observed dynamic NMR behavior. In the 1,3-di-t-butyl derivative, restricted rotation of the 1-t-butyl group was observed below −75 °C, the free energy of activation being 9.5 kcal mol−1.

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