Stereoelectronic Effects in the Gas Phase. 2. Negative Ion Reactions of 1,3-Dithianes and 1,3-Dithiane 1-Oxides

Article abstract of DOI:10.1021/ja00201a015

Reactions of gaseous anions (methoxide, hydroxide, and thermal electrons) with cis-4,6-dimethyl-1,3-dithiane and the corresponding axial and equatorial 1-oxides have been investigated using the techniques of ion cyclotron resonance (ICR) spectroscopy and pulsed positive-negative ion chemical ionization (PPNICI) spectroscopy.Deprotonation to (M-H)(1-) ions and extensive fragmentation to ions m/z 99 and 101 were observed for all three compounds with all three reactant anions.When compounds labeled with deuterium specifically at the C2 position were used, it was foundthat deprotonation occurred at C2 and elsewhere in the molecule.The axial hydrogen at C2 was removed as readily or more so than the equatorial hydrogen, depending on the reactants and conditions of ion generation. (These results differ from the corresponding condensed-phase reactions, which show strong selectivity for C2 equatorial deprotonation).Deuterium isotope effects were estimated to be 1.2 and 1.3 for ions generated by MeO(1-) and e, respectively.Exchange (H/D) between hydroxide and cis-4,6-dimethyl-1,3-dithiane-2-d2 was insignificant, although exchange was observed in comparable reactions of hydroxide with 1,3-dithiane-d2 and bis(methylthio)methane-d2.Stereoelectronic effects that may contribute to selectivity in solution do not account for the gas-phase results.Ab initio calculations at the 3-21G(^*) level applied to methanedithiol and the anion (HS)2CH(1-) (as models for the 1,3-dithiane system) provide insight into the nature of the gas-phase reactions.Possible reaction pathways are discussed.