Restricted Rotation Involving the Tetrahedral Carbon. XI. Barriers to Rotation and Conformational Preferences of Substituted 9-Isopropyltriptycenes

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Abstract
Several 9-isopropyltriptycene derivatives were prepared by addition of benzynes to 9-isopropylanthracenes. Rotation about the C9-Cpr bond of these compounds is found to be frozen at room temperature on the NMR time scale. Those compounds which carry a substituent at 1-position exist as dl-isomers, at least overwhelmingly. No sign of existence of meso-isomer is found. On the other hand, 9-isopropyl-2,4-dimethyltriptycene exists as a mixture of d, l, and meso isomers, composition of which is almost 1 : 1 : 1. Repulsive nonbonding interaction is a decisive factor to determine the conformational preference. Approximate methods for line shape analysis of two methyl groups in an isopropyl group are discussed. Activation energies for rotation about the C9-Cpr bond increase with the increase in the size of 1-substituent except for the methyl group. The smaller effective size of the methyl group than that expected from the van der Waals radius is attributed to cogwheeling arrangement of methyl and isopropyl groups at the transition state of rotation and/or the higher energy level of the ground state due to the severe interaction between methyl and isopropyl groups.