On the Mechanism of Base-Induced Gas-Phase Elimination Reactions of Thioethers

Article abstract of DOI:10.1021/ja00259a003

The base-induced gas-phase elimination reactions of diethyl sulfide have been studied by using the method of Fourier transform ion cyclotron resonance mass spectrometry.Reaction with the nitrogen bases NH2^-, EtNH^-, and Me2N^- preferentially proceeds via an α',β-elimination mechanism, where α' proton abstraction of the sulfide results in the formation of an α-thio carbanion, which undergoes an intramolecular syn elimination to generate the ethyl thiolate anion.Reaction with OH^-, MeO^-, and F^- exclusively proceeds via an E2 mechanism.However, for the reaction with OH^-, rapid exchange is observed within the reaction complex between the α-hydrogens of the sulfide and the hydroxide hydrogen prior to E2 elimination.For the E2 eliminations the α- and β-deuterium isotope and leaving group effects have been determined as a function of the base strength.As in our previous study of the mechanism of base-induced gas-phase elimination reactions of ethers, the isotope and leaving group effect data are interpreted in terms of a variable E2 transition-state structure.Combining the results of our previous study with those of the present study has led to the conclusion that the perturbation of the transition-state character effected by changing the leaving group or the base follows the rules established for condensed phase β-elimination reactions.For the nearly thermoneutral elimination reaction of diethyl sulfide induced by F^- it is concluded that two reaction mechanisms are operative, both characterized by a bent proton transfer: an E2C mechanism involving a two-side attack of F^- on the β-hydrogen and α-carbon of the sulfide, yielding free ethyl thiolate anions and a syn elimination involving a two-side attack of F^- on the β-hydrogen and the leaving group, generating HF solvated ethyl thiolate anions.