Conformations and Internal Mobility of Side Chains in Heterosubstituted Hexaalkylbenzenes. Isopropyl/Ethyl and Isopropyl/Cyclopropyl Systems

Abstract

Variable-temperature NMR spectra of hexaalkylbenzenes containing both isopropyl and ethyl or cyclopropyl groups, in conjunction with empirical force-field calculations, have shown that the alkyl groups in these heterosubstituted benzenes retain the conformation they individually adopt in the homosubstituted analogues: isopropyl groups maintain a bisected conformation, whereas ethyl or cyclopropyl groups adopt a perpendicular one. Two site-exchange processes have been observed in heterosubstituted benzenes containing vicinal isopropyl groups: a low-energy process, which leads to fast rotation of all alkyl groups except for the isopropyl groups, and a high-energy process that corresponds to the unfreezing of the gear-locked isopropyl groups. The magnitude of the barriers for both processes has been determined for 1,2-diethyl- and 1,2-dicyclopropyl-3,4,5,6-tetraisopropylbenzene and for 1,2-diisopropyl-3,4,5,6-tetracyclopropylbenzene. © 1985, American Chemical Society. All rights reserved.