Barriers to rotation about the amide (N–CO) and sulphenamide (N–S) bonds in methyl N-arylsulphenyl-N-benzylurethanes. A simple molecular orbital model to explain substituent effects on sulphenamide rotational barriers

Abstract

Barriers to rotation about both the N–S and N–CO bonds have been measured in a series of substituted methyl N-arylsulphenyl-N-benzylurethanes, XC₆H₄SN(CH₂C₆H₅)CO₂CH₃, by analysis of the temperature dependent n.m.r. spectra of the benzyl methylene protons. N–S Torsional barriers increase linearly with increasing electron demand of the substituents (σ^– substituent constant), while N–CO barriers are substituent independent. This result is inconsistent with a mechanism for S–N torsion involving (p–d)π conjugation between the nitrogen lone pair and sulphur. An alternative rationale is proposed, based on a simple perturbational molecular orbital model. Application of the model to various experiments is discussed, with the conclusion that it adequately rationalizes all the currently available (and sometimes inexplicable) results on sulphenamides.