NMR parameters for 1,3-dioxanes: evidence for a homoanomeric interaction

Abstract

¹ H and ¹³C NMR spectra have been recorded for a series of 1,3-dioxanes, 2-oxo-1,3,2-dioxathianes, 1,3,2-dioxaphosphorinanes and 2-thioxo-1,3,2-dioxaphosphorinanes. which are held in a chair conformation. In all of them, the spectral parameters are compatible with the equatorial C(5)–H bond being weaker and longer than the axial C(5)–H bond [e.g.¹J_C(5)H(5eq) < ¹J_c(5)H(5ax)(a reversed Perlin Effect), and ³J_H(4)H(6eq) < ³J_H(4eq)H(5ax)]. This is the reverse of what is usually observed in cyclohexanes, or at the 2-position in tetrahydropyrans, where the sequence is ascribed to an n→σ*(anomeric) interaction between the α-oxygen and the axial C(5)–H bond. It is suggested that this reversal may be due to an n→σ*(homoanomeric) interaction between the β-oxygen and the equatorial C(5)–H bond, through a W-arrangement of orbitals.

This interpretation is supported by ab initio 6-31 G* calculations on the 1,3-dioxane molecule, which show that the equatorial C(5)–H bond is weaker and longer than the axial C(5)–H bond.