Gas-Phase Acid-Induced Nucleophilic Displacement Reactions. Stereochemistry of Inter- and Intramolecular Substitutions at Saturated Carbon

Article abstract of DOI:10.1021/ja00529a038

The stereochemistry of gas-phase nucleophilic displacement by water on a number of positively charged intermediates was investigated under different experimental conditions.The ionic intermediates were generated in the gas phase at atmospheric pressure by attack of radiolytically formed Broensted (CH5⁺, C2H5⁺) and Lewis (C2H5⁺, CH3FCH3⁺) acids on selected mono- and bifunctional substrates.Isolation and identification of their neutral substituted products allowed us to demonstrate that, under the used experimental conditions, gas-phase acid-induced inter- and intramolecular nucleophilic displacement reactions occur via predominant (64-98percent) inversion of configuration at the reaction center.The yield and the stereoisomeric distribution of the substituted products were found to depend on either the nature of the gaseous acid used to generate the charged intermediates or the concentration of the added base (NH3 or H2O).Product distribution from bifunctional substrates is characterized by the presence of minor amounts of substituted derivatives retaining the original configuration of their neutral precursors.Their formation is ascribed to the occurence of an extensive neighboring group participation effect (an HO-3 process) on the displacement reaction, resulting in a double inversion of the reaction centers.A mechanistic model is proposed for gas-phase nucleophilic substitutions at atmospheric pressures, and compared with those from the related low-pressure ICR (ion cyclotrone resonance mass spectrometry) and solution-chemistry studies.