Proton Shifts in Gaseous Arenium Ions and Their Role in the Gas-Phase Aromatic Substitution by Free Me3C+ and Me3Si+ Cations

The gas-phase reaction of Me3C+ with benzene and toluene has been studied at 720 Torr, in the temperature range from 47 to 120-degrees-C with the radiolytic technique, using suitably deuterated substrates. The results underline the kinetic role of the 1,2-H(D) shifts in the primary charged intermediate, the sigma-complex I. When the latter originates from Me3C+ attack at the para position of p-MeC6H4D, the D shift rate is relatively slow, ca. 5 x 10(7) s-1 at 47-degrees-C, with an activation energy E(a) = 7.6 +/- 0.2 kcal mol-1 and a preexponential factor A = 10(12.9+/-0.4). Ions I from the Me3C+ attack on benzene or at the meta positions of toluene have lifetimes shorter than a few nanoseconds, and the 1,2-H shift, while still kinetically significant in the overall benzene reaction, is unaffected by addition of strong bases to the gas at pressures up to 8 Torr, which ensures deprotonation of I within 1 ns. Since 1,2-H(D) shifts compete with back dissociation of I, their rate influences the net alkylation rate, which accounts for the kinetic isotope effect in excess of 2 observed in the competitive tert-butylation of C6H6 and C6D6, the first one reported for a thermal ionic reaction in the gas phase. Analogous experiments involving the reaction of Me3Si+ with benzene and toluene show that H(D) shifts are remarkably slow from the ipso-silylated position, consistent with its high site basicity demonstrated by previous mass spectrometric and radiolytic studies.