NMR study of stereoelectronic anomeric and homoanomeric effects on the axial and equatorial CH bonds in 1,3-diazacyclohexanes and 1,5-diazabicyclo[3.2.1]octanes

Abstract

The ¹H and ¹³C NMR spectra of a series of 1,3-di-tert-butyl-1,3-diazacyclohexanes 6, and of 1,5-diazabicyclo[3.2.1]octanes 7, have been investigated in an attempt to find evidence regarding a stereoelectronic interaction of the non-bonding electron pairs on nitrogen or of the C–N bonding electron pairs in the ring, with the axial and equatorial CH bonds of the α- or β-methylene groups. In 6, the orbital carrying the non-bonding electron pair lies antiperiplanar to the axial α-CH bond; a significant n_N→σ*_CH 3-orbital interaction (the anomeric effect) is sterically possible, and this shows up in the values of δ_H and of ¹J_CH. In 7, the doubly-occupied orbital is orientated gauche to both the axial and equatorial α-CH bonds, and no anomeric effect is apparent. In 7 however, in contrast to 6, the stereochemistry is appropriate for a W-plan n_N→σ*_CH 4-orbital interaction with the equatorial β-CH bond, and comparison of the NMR data of 6 and 7 suggests that such a homoanomeric effect may operate, though it is much weaker than in the analogous 1,3-dioxanes. No evidence could be found for a σ_CN→σ*_CH 3-orbital interaction.