The sulfoxide bond as stereochemical probe in methyl‐1,4‐thiazane‐1‐oxide. Conformational preference in 1,4‐thiazane‐l‐oxide

Abstract

AbstractOxidation of 3‐methyl‐l,4‐thiazane provides two sulfoxide isomers. The isomer with the axial sulfoxide bond is formed in threefold excess. Comparison of 1H NMR spectral characteristics of cis‐ and trans‐3‐methyl‐l,4‐thiazane‐l‐oxide with these of 1,4‐thiazane‐1‐oxide points out that in the conformational equilibrium of the latter the sulfoxide bond in axial, position is somewhat favoured. From the “slow exchange” 13C NMR spectrum we estimate the axial sulfoxide isomer to be present for 57% (ΔG° = 0.46 kJ/mol ± 0.35 kJ/mol) at 203 K. Estimation from proton shifts and interproton couplings result in an axial population ranging from 53 to 69%. The presence of the N‐lone pair orbital influences substantially the conformational behaviour, as follows from the increased axial sulfoxide character in the protonated and N‐acetyl‐derivatives.