Selectivity as a function of anionic base properties in the gas-phase reactions of dimethyl sulfite

Article abstract of DOI:10.1021/ja00158a018

The thermally equilibrated gas-phase reactions of dimethyl sulfite (CH₃OSO₂CH₃), in a helium bath gas at 300 K, with a variety of anions ranging from amide, phenide, and allyl anions to nitromethide and hydrosulfide, have been quantitatively examined using the flowing afterglow technique. A master reaction scheme, composed of four competitive reaction pathways, is presented to account for the experimental observations. Of primary importance in selectivity is the structure of the anionic nucleophile and then its basicity. Strongly basic, localized, heteroatomic nucleophiles yield larger amounts of elimination products compared to localized carbon bases of comparable basicity. While most anions yield methyl substitution products, localized bases are much more nucleophilic at carbon than delocalized carbon bases. For nucleophilic attack at sulfur the opposite trend is found, delocalized carbon bases being most nucleophilic at this site. Only the weakly basic cyclopentadienide and nitromethide anions were found to react with dimethyl sulfite solely by termolecular adduct formation. Anomalies in the observation for acetaldehyde enolate, cyanomethide, and anilide anions are suggestive of reaction via the heteroatomic sites of these nucleophiles. The new level of chemical understanding developed from the overall reaction scheme defined will help design future gas-phase experiments to probe specific reaction mechanisms.