Nuclear magnetic resonance spectroscopy. Conformational equilibration of cycloheptane and cycloheptene derivatives

Abstract

A study of the temperature dependence of the ^(19)F nuclear magnetic resonance (nmr) spectra of 4,4-difluoro-1,1-dimethylcycloheptane has shown that, at low temperatures, this substance exists predominantly in the
\ntwist-chair conformation with the methyl groups on the axis carbon atom. 1 ,1-Difluoro-4,5-trans-dibromocycloheptane, at low temperatures, exists predominantly in two different Conformations with the fluorine atoms at the axis carbon atom. 5,5-Difluorocycloheptene, at low temperatures, exists in a single conformation best represented as the chair. Activation parameters have been obtained for the conformational equilibration of these derivatives.
\nFor 1,1- difluorocycloheptane, 1,1,3,3-tetrafluorocycloheptane, and 4,4-difluoro-1-tert-butylcycloheptane the conformational equilibria are still rapid on the nmr time scale at low temperatures and only qualitative conclusions can be drawn about their conformations.