The 13C NMR spectra have been determined of: (i) aliphatic compounds having at one end a functionalized sulphur atom (-SH, -S-, -SMe, -S(O)Me, -SO,Me and -S-'Me,) and (ii) saturated sulphur heterocycles variously substituted at the S-atom (\S, \SO '\SO, and 'xS Me). The results are discussed in terms of the familiar deshielding effects for CL-and B-carbons and shielding effects for y-carbons, exerted by the sulphur atom itself and/or by the atoms or groups of which the sulphur function is made up. The 11-effect of the S-atom appears to be nearly independent of the nature of the S-function and of comparable magnitude to that of an aliphatic carbon (-2.5 t--3.0 ppm). Surprisingly, however, a S-CH, group shields the carbon in y position with respect to CH, by an amount (-5.4 ppni) which is more than twice that (-2.5 ppm) exerted by the aliphatic y-carbon on the S-CH, carbon itself. As to the cyclic compounds, the shieldings of the G(-and 8-carbons can be rationalized in terms of the conformational orientation of the substituent at sulphur, and the equilibrium distribution of the conformers. The results confirm the great value of 13C NMR for configurational and conformational assignment of S-heterocycles.
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WITH the exception of a recent paper by Buchanan and collaborators, dealing with aryl methyl sulphides, sulphoxides and sulphones,l only scattered information is available in the literature on the 13C chemical shifts of sulphur-containing organic compounds. The present report contributes some systematic observations on the effect of such sulphur functions as: thiol, -SH, the sulphone, >SO, and the sulphonium cation, >S 1-R, on the 13C chemical shifts of typical aliphatic and S-heterocyclic structures.
The main purpose of this work was to derive empirical criteria (additivity rules) to be used as groundwork for the conformational analysis of saturated sulphur heterocycles. The more numerous and systematic observations concern sulphoxides and sulphonium cations. We particularly concentrated on these functions since they are known to be pyramidal and configurationally stable. Therefore, when an integral part of a ring, they were expected to produce characteristic 13C chemical shift effects that, while permitting facile configurational assignment, would also greatly aid the thiolate anion, -S-, sulphide, -S-, sulphoxide, ,S-0, \ * Author to whom correspondence should be addressed.
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