Polarization Effects, Restricted Rotation, and Isomerization Phenomena

Vinylogous thioamides have been shown to exist in two rotational forms, (521) and (522) the barrier to rotation about the CS—C bond was found to be 2.5—3.1kcalmor higher than that found for corresponding enamino-ketones. 

The syn-anti isomerism of the sulphines (523) has been studied by means of n.m.r. spectroscopy by Bonini and his co-workers, who interpreted their results in terms of the occurrence of an isomerization process proceeding by rotation around the C=S bond rather than by inversion about the sulphur atom. The same conclusion was reached by Snyder and Harpp on the basis of an SCF—MO—CNDO investigation of the geometry, the electron distribution, and the syn-anti isomerism of sulphine itself and some of its simple derivatives. Tangermann and Zwanenburg have demonstrated that the ortho-methyl groups in the sulphoxide sulphines (524) and (525) are 

Walter and his co-workers have determined AP = 19.4 kcal mol for the rotation about the C—N bond in protonated thiobenzamide this value is 1.5 kcal mor higher than that determined for the corresponding rotation in thiobenzamide itself, a fact that lends strong support to the idea that protonation of thioamides takes place preferentiaily at the S atom. In accordance with expectations, the free energy of activation for rotation about the C—N bond in NN-dimethylselenoformamide 

Several recent papers deal with the restricted C—rotations in unsubstituted/ monosubstituted, NlsT-disubstituted, NN-disubsti-tuted, N,N,N -trisubstituted, and tetrasubstituted thioureas, together with the conformational aspects involved. The barriers to C—N rotation of thio- and seleno-urea were equal (AF = 12.8 kcal mol ) and above that of urea (AF = 11,0 kcal mol ). Restricted-rotation phenomena have been observed for substituted N-(tosylmethyl)thioureas, thiourea S-trioxides, and 3-aryl-2-thiohydantoins.  

Filleux-Blanchard and A. Durand-Couturier, Bull. Soc. chim. France, 1972, 4710. 

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