Oligo(cyclohexylidene) oximes and derivatives as probe molecules for long-range substituent effects on 13C NMR chemical shifts

Abstract

For a series of rodlike aliphatic oximes (1–4) the influence of the oxime substituent on the ¹³C NMR chemical shifts has been studied. Various 2D NMR techniques were applied for the unequivocal assignment of their ¹H and ¹³C resonances. For bicyclohexylidene oximes 1–2 long-range substituent effects on the ¹³C NMR chemical shifts of aliphatic carbon atoms of the six-membered rings due to the presence of the oxime group are discernible up to positions six carbon–carbon bonds distant from the iminyl carbon! The ¹³C NMR data obtained for bicyclohexyl oximes 3–4 reveal that in this series the effect is limited to carbon atoms which are five bonds distant from the iminyl carbon. The observed differences between the two series is attributed to the presence of an olefinic double bond in 1–2, which becomes polarized by the electric field of the oxime substituent.