Systematic Evaluation of the anomeric Effect in 2-(Arylseleno)-1,3-dithianes. Evidence for stabilizing Orbital Interactions

Article abstract of DOI:10.1021/jo00251a019

The effects of substitution on the magnitude of the anomeric effect in 2-<(4-substituted-phenyl)seleno>-1,3-dithianes are described.The conformational equilibria were determined by direct measurement of the individual conformer resonances in the 77Se and 1H NMR spectra at the slow-exchange limit.The magnitude of the anomeric effect varies with substituent in the order NO2 > CF3 > Cl > F = Me > OMe > NMe2 , providing systematic evidence for n_S -> ?^*_C-Se stabilizing orbital interactions and the existence of the anomeric effect in S-C-Se fragments.Evidence for the mechanism of transmission of the electronic effects is also presented.Thus, analysis of the results by means of a dual substituent parameter approach indicated best correlation with ?_I and ?^o_R substituent constants and a slightly greater contribution of the resonance effect than the polar effect.The positive ρ values are interpreted in terms of the accommodation of effective negative charge on the selenium atom in the axial conformer.Further evidence for such increased negative charge is provided by the differences in 13C chemical shifts for the aromatic carbons between the two conformers for a particular substituent and also by the trends in 77Se chemical shifts as a function of substituent for the two conformers.The solvent dependence of the conformational equilibria for selected compounds shows no apparent correlation with dielectric constant although significant anomeric effects and substituent effects are stil observed in a polar medium.The results are interpreted in terms of the dominance of the orbital interaction component over the electrostatic component.