Double C--H Activation of Ethane by Metal-Free SO sub(2) super(.+) Radical Cations

The room-temperature C--H activation of ethane by metal-free SO sub(2) super(.+) radical cations has been investigated under different pressure regimes by mass spectrometric techniques. The major reaction channel is the conversion of ethane to ethylene accompanied by the formation of H sub(2)SO sub(2) super(.+), the radical cation of sulfoxylic acid. The mechanism of the double C--H activation, in the absence of the single activation product HSO sub(2) super(+), is elucidated by kinetic studies and quantum chemical calculations. Under near single-collision conditions the reaction occurs with rate constant k=1.010 super(-9) ( plus or minus 30%)cm super(3)s super(-1)molecule super(-1), efficiency=90%, kinetic isotope effect k sub(H)/k sub(D)=1.1, and partial H/D scrambling. The theoretical analysis shows that the interaction of SO sub(2) super(.+) with ethane through an oxygen atom directly leads to the C--H activation intermediate. The interaction through sulfur leads to an encounter complex that rapidly converts to the same intermediate. The double C--H activation occurs by a reaction path that lies below the reactants and involves intermediates separated by very low energy barriers, which include a complex of the ethyl cation suitable to undergo H/D scrambling. Key issues in the observed reactivity are electron-transfer processes, in which a crucial role is played by geometrical constraints. The work shows how mechanistic details disclosed by the reactions of metal-free electrophiles may contribute to the current understanding of the C--H activation of ethane. Two at once: The SO sub(2) super(.+) radical cation assists an exothermic and effective conversion of ethane to ethylene by double C--H activation at room temperature. Charge and spin effects are strictly related to geometrical constraints of short-lived intermediates, like the ion-molecule complexes containing the ethyl radical and the ethyl cation, respectively (see figure).