Double CH activation of ethane by metal-free SO2.+ radical cations

The room-temperature CH activation of ethane by metal-free SO2.+ 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 H2SO2.+, the radical cation of sulfoxylic acid. The mechanism of the double CH activation, in the absence of the single activation product HSO2+, is elucidated by kinetic studies and quantum chemical calculations. Under near single-collision conditions the reaction occurs with rate constant k=1.0×10−9 (±30 %) cm3 s−1 molecule−1, efficiency=90 %, kinetic isotope effect kH/kD=1.1, and partial H/D scrambling. The theoretical analysis shows that the interaction of SO2.+ with ethane through an oxygen atom directly leads to the CH activation intermediate. The interaction through sulfur leads to an encounter complex that rapidly converts to the same intermediate. The double CH 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 CH activation of ethane.