The conformational analysis of saturated heterocycles. Part XXXIV. The influence of lone pair orientation on the differential proton chemical shift of adjacent methylene groups and on Jgem values

Abstract

The differential chemical shifts (Δ_ae) and geminal coupling constants (J_gem) between axial and equatorial protons of the 2-position CH₂ groups in hexahydropyrimidines, hexahydro-1,3,5-triazines, and tetrahydro-1,3-oxazines have been compared with the proportions of lone-pairs axial in these compounds. It is concluded that whereas Δ_ae can provide a qualitative indication of the orientation of lone pairs, no simple quantitative correlation exists. The variations found in J_gem showed no systematic correlation with the conformational equilibrium of adjacent N-alkyl groups, in the compounds now studied.