The Existence of Second-Row Anomeric Interactions. Conformational Analysis of 2-Substituted 5-Methyl-5-aza-1,3-dithiacyclohexanes

Article abstract of DOI:10.1021/ja00199a038

The conformational analysis of several 2-substituted 5-methyl-5-aza-1,3-dithiacyclohexanes made the evaluation of S-C-Y anomeric interactions possible, where Y = SCH3, SC6H5, COC6H5, CO2CH2CH3, and CO2C6H5.The relative order of the effects as a function of substituent is similar to that previously encountered in the corresponding 2-substituted 1,3-dithianes; however, the effects are smaller in magnitude in the title compounds, and we attribute this to the decreased ring dipole, which leads to a diminished influence of dipole-dipole interactions on these equilibria.Nevertheless, the observed anomeric effects are significant and can be explained in terms of endo and exo hyperconjugative interactions, which indicate that second-row elements are able to perticipate in anomeric interactions.Plots of ln K versus 1/T are linear, permiting evaluation of the enthalpic and entropic contributions to the S-C-Y anomeric effect.For the S-C-S segment, the effect is of enthalpic origin and overcomes unfavorable steric and entropic components in the axial conformer.For the compounds incorporating S-C-C(O) segments, the entropy term is mainly responsible for the axial predominance; the measured ΔH⁰ ca. 0 for these derivatives still reflects a significant enthalpic anomeric effect owing to the countervailing steric effects in the axial conformers.