Proton acid catalyzed hydride transfer from alkanes to methylated benzyl cations. 3. Solvated alkanes as hydrogen-donating intermediates

The hydride-transfer reaction between isobutane (and other tertiary alkanes) and methylated benzyl cations 2-X (X = OCH3, CH3, F, Br, H, NO2) is shown to be proton acid catalyzed. Scrambling of transferred hydride (or deuteride, tritide) occurs with solvent protons during the actual transfer step per se. The reaction between isobutane or isobutane specifically labeled with deuterium or tritium at the tertiary position and cations 2-X leads to differently labeled products 3-x. The amount of scrambling between solvent protons and transferred label depends on the acidity of the solvent, the temperature, and the isotopic mass of transferred hydrogen. Tritiation of the solvent leads to tritiated products 3-X. A termolecular transition state is postulated for the reaction. In this transition state, a presumably concerted transfer of a proton-hydride pair from a proton acid-alkane complex (solvated alkane) occurs towards the benzyl cation. The distinction between solvated alkanes (RH.HZ)+ and free, protonated alkanes (RH2)+ is discussed. CNDO/2 calculations show that a bonding interaction occurs between a C, symmetrical protonated isobutane molecule and a benzyl cation at R = 2.8 A. These calculations support the postulated transition state, Le., the transfer of an activated hydrogen molecule in which strong charge polarization is present or, in other words, an unsymmetrical proton-hydride pair