Polarization Effects and Restricted Rotation

Several recent papers deal with the restricted C—N rotations in monosubstituted, NN -disubstituted, and NNN -trisubstituted thioureas, together with the conformational aspects involved. N-Alkyl-thioureas were found to exist both in the cis- and the trans-form, but N-arylthioureas appeared to exist solely in the intramolecularly hydrogen-bonded form (363). A similar form (364) was found to be predominant in l-alkyl-3-aryl- and 1,3-diaryl-thioureas, whereas both of the isomers (365) and (366), but not the sterically unfavourable form (367), were detectable in the case of l,3-dialkylthioureas. Reeves and his coworkers, by means of a total n.m.r, line-shape fitting, have evaluated the 

Recent studies on the restricted C—N rotation in NN-dialkylthio-carbamoyl chlorides, - tetra-alkylthiuram disulphides, - and tetra-alkylthiuram monosulphides revealed that the barrier to rotation in the first-mentioned compounds (AF = 18—19 kcalmol- ) is ca. 2 kcal mol higher than for the corresponding carbamoyl chlorides and ca, 3 kcal mol higher than for similar N-substituted thiuram disulphides. With identical N-substituents, the barrier to rotation in thiuram monosulphides is slightly higher than that in thiuram disulphides, an effect which was attributed both to the lower double-bond character of the C—N bond and the lower steric hindrance to rotation about this bond in the latter type of compound.  

Several groups of workers have studied the C—N rotational effects in various NN-disubstituted thiono- and dithio-carbamates. Apparently, 

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