Gas-phase acid-induced nucleophilic displacement reactions. Stereochemistry of inter- and intramolecular substitutions at saturated carbon

The stereochem. of gas-phase nucleophilic displacement by H2O on a no. of pos. charged intermediates was studied. The ionic intermediates were generated in the gas phase at atm. pressure by attack of radiolytically formed Broensted (CH5+, Et+) and Lewis (Et+, Me2F+) acids on selected mono- and bifunctional substrates. Isolation and indentification of their neutral substituted products showed that, under the exptl. conditions, gas-phase, acid-induced, inter- and intramol. nucleophilic displacement reactions occur via predominant (64-98%) inversion of configuration at the reaction center. The yield and stereoisomeric distribution of the substituted products depend on either the nature of the gaseous acid used to generate the charged intermediates or the concn. of the added base (NH3 or H2O). Product distribution from bifunctional substrates is characterized by the presence of minor amts. of substituted derivs. retaining the original configuration of their neutral precursors. Their formation is ascribed to the occurrence of an extensive neighboring-group-participation effect (an HO-3 process) on the displacement reaction, resulting in double inversion of the reaction centers. A mechanistic model is proposed for gas-phase nucleophilic substitutions at atm. pressures, and compared with those from related low-pressure ICR (ion-cyclotron resonance) mass spectrometry and soln.-chem. studies.