Restricted Rotation Involving the Tetrahedral Carbon. XLV. Appearance of a Maximum in the Rotational Barriers of 9-(1,1-Dimethyl-2-phenylethyl)triptycenes at a Medium-sized peri -Substituent

Abstract

Rotational isomers of seven peri-substituted 9-(1,1-dimethyl-2-phenylethyl)triptycene derivatives were stereoselectively synthesized and classical kinetics on rotational isomerization were studied. While compounds with relatively small peri-substituents (F and OCH3) showed higher barriers than the peri-unsubstituted one, the peri-chloro compound had almost the same barrier with and those carrying bulkier peri-substituents (Br, CH3, and CF3) showed lower barriers than the peri-unsubstituted one. These apparently anomalous phenomena may be explained in terms of large molecular deformation of these highly congested compounds.