Gas-phase acid-induced nucleophilic displacement reactions. 4. A stereochemical probe for the existence and the relative stability of cyclic halonium ions in the gas phase

A comprehensive examn. of the existence and the relative stability of gaseous 3-membered cyclic butene halonium ions (I) was carried out by establishing the stereochem. of the acid-induced displacement by nucleophiles (e.g., H2O, H2S, etc.). I were obtained, in the gas phase, from the reaction of radiolytically formed Broensted (CH5+ and Et+) and Lewis (Et+ and Me2F+) acids with 2,3-dihalobutanes. The stereoisomeric distribution of the neutral 3-halobutan-2-ols (or thiols) formed in these reactions strongly implies the intermediacy of I, whose stability in the gas phase depends on the nature of the halogen involved, increasing in passing from Cl to Br. No evidence for the occurrence of stable cyclic 2,3-butene fluoronium ions was obtained by the same procedure. Other factors, namely the strength of the radiolytic gaseous acid catalysts and the configuration of the starting dihalobutane, were involved in detg. the stereochem. of the substitution reaction examd. A close correspondence exists between these results and those from strictly related mass spectrometric and condensed phase investigation.