Structural equilibria determined by attractive steric interactions. 1,6-Dialkylcyclooctatetraenes and their bond-shift and ring inversion investigated by dynamic NMR spectroscopy and molecular mechanics calculations

Abstract

1,6-Dialkylcyclooctatetraenes equilibrate with their 1,4-dialkylcyclooctatetraene isomers by a bond-shift process which is slow on the NMR timescale at ambient temperature. The relative stability of these isomers is measured from NMR signal intensities, and attractive alkyl–alkyl steric interactions in the 1,6-isomer are invoked to explain its predominance. Molecular mechanics calculations bear out this explanation. Various features of attractive steric interactions appear from these results and are discussed. Barriers to the bond-shift and to some ring inversion processes are reported.