Pseudorotation and ring inversion in 1-substituted cyclooctenes

Abstract

The stereodynamics of a series of 1-substituted cyclooctenes have been studied by dynamic NMR spectroscopy and molecular mechanics calculations and compared with previous results. Two separate processes are identified in the NMR spectra, with substituent dependent barriers between 7.6 and 8.2 kcal mol^–1 for the first process and between 5.3 and 8.2 kcal mol^–1 for the second process.

A consideration of models and calculations suggests that the ground state conformation is a twist boat–chair, that the former process is a pseudorotation through a chair–chair conformation, and that the latter is a pseudorotation through boat–boat conformations. Both pseudorotations are necessary to achieve ring inversion, i.e. the interconversion of enantiomeric conformations, so inevitably ring inversion will always have the higher of the two barriers measured. However, the rate determining pseudorotation may change as the size or rigidity of the 1 -substituent increases. A substituent conjugating with the double bond, or which is sterically large, affects the boat–boat pseudorotation more than the chair–chair one.