Gas-phase Base-induced Elimination-reactions In Onium Intermediates .1. Competitive Substitution and Elimination Mechanisms In the Attack of Amines On Diethylmethyloxonium Ions

The elimination/substitution branching ratio assocd. with the attack of amines on diethylmethyloxonium ions has been studied in the gas phase by using the method of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Increasing the base strength causes enhancement of the efficiency of the elimination process relative to substitution. The same trend is obsd. upon increasing the excitation energy of the encounter between the oxonium ion and the base. At low encounter excitation energies, nucleophilic displacement is flanked by an entropically favored E2 elimination mechanism. At high encounter excitation energies, these reactions are superseded by alternative entropically favored processes, characterized by an initial one-electron transfer from the amine to the oxonium ion leading to a long-lived electrostatically bound adduct involving the amine radical ion and an alkyl radical loosely bound to an ether mol. Collapse of this adduct may involve an alkyl radical-amine ion-radical recombination (an SRN1-like mechanism) or, alternatively, a hydrogen transfer from the present neutrals to the mol. ion of the base (an ER1 mechanism). These proposed mechanisms are unprecedented in the field of gas-phase pos. ion-mol. chem.