The electrophilic amination of carbanions allows the preparation of a wide range of amines through an unconventional C-N bond-forming reaction. The concepts behind the varied synthetic approaches, classified by the nature of the aminating agent and of the organometallic species, are discussed. The mild operational conditions, the high selectivity, and the availability of the starting materials are good assets of these processes which nicely complement each other. New appealing and flexible routes can be devised, leading in several cases to the synthesis of otherwise not easily accessible N-containing compounds.
The -C and 1 7 0 (~tural abundance) chemical shifts of several mono-and di-methyl ring-substituted thiane 1-oxides and thiane 1,l-dioxides are reported. The cis and trans isomers of methyl-substituted thiane l-oxide are readily identified by -C and 170 NMR. In particular, the "0 NMR signals of axial SO groups are found several ppm upfield of those of the equatorial counterparts. The proportion of axial and equatorial conformers of S a n e 1-oxide in Merent solvents has been measured by low-temperature -C NMR. In THF the proportion of the axial conformer is nigher than in CDzClz, whereas in CDcl, or CHF&l the conformational preference is reversed and the equatorial conformer is slightly favoured. INTRODUCI'IONWe report a 13C and 170NMR study of thiane 1-oxide, thiane 1,l-dioxide and several of their methyl ring-substituted derivatives.The conformational energy, AG,a%AG,,a for an SO group of a six-membered ring was first estimated to be approximately 5.4 kJ mol-' (1.3 kcal mol-I), on the basis of thermal and hydrogen chloride catalysed equilibration of cis-and trans-4-tea-butylthiane 1-oxide.' Later studies showed that the conformational energy for the parent compound, thiane 1-oxide (l), was much lower, i.e. 0.8 kJ mol-' (0.2 kcal mol-l) as measured by 'H NMR.' Force-field calculations have been found to be in agreement with the NMR At present, the reasons governing the conformational preference of thiane 1-oxide are not well understood. According to an early interpretation, 1,3 the axial preference of the sulphinyl oxygen in 1 is accounted for in terms of an unusually attractive van der Waals interaction between the oxygen atom and the syn-axial protons in the axial conformer. In agreement with this interpretation, when the axial hydrogen at C-3 is replaced with a methyl group, as in 3,3-dimethylthiane 1-oxide, only the equatorial conformer is observed at low ternperat~re.~ However, according to a more recent interpretation the axial preference of thiane 1-oxide is best ascribed to the destabilizing interactions existing between the S-0 bond and the adjacent C-€3 bonds: whereas there are only two such interactions in the axial conformer, four are present in the equatorial conformer.4a Comparison of the experimental results with the theoretical predictions cannot be made correctly, however, until the influence of the * Author to whom correspondence should be addressed. solvent in the conformational equilibrium of 1 is known. The position of the conformational equilibrium is dependent on the solvent in several saturated heterocycles. This is the case, for instance, for some derivatives of 1,3-dioxane7 for which a strong dependence on the dielectric constant of the solvent was found for the conformational properties.6A better knowledge of the conformational preference of a sulphinyl group is also of relevance in the highly debated question of the stereochemistry of aHtmethylation of sulphoxides. It has been shown, for example, that in six-membered ring sulphoxides the stereochemistry of this reaction depends on the ...
Abstract-The13C 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. / / ' / /'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, \
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