One-Electron Reduction of Carbonium Ions. II. Reduction of the Tropylium Ion and the Triphenylmethyl Cation with Metallic Powders and Metallic Ions

Abstract

Abstract The reaction of tropylium fluoroborate in an aqueous solution has been carried out at room temperature with powders of various metals (Mg, Al, Mn, Zn, Cr, Fe, Cd, Co, Ni, Sn, Pb, Sb, Cu, Hg, and Ag) under heterogeneous conditions and with several metallic ions (Cr(II), Ti(III), Sn(II), Cu(I), Fe(II), Fe(III), Hg(I), Hg(II), and Ag(I)) in a homogeneous solution. All these reagents, except Ag, Fe(III), Hg(I), Hg(II), and Ag(I), reduce with ease the tropylium ion to give the tropyl radical, which then rapidly couples to bitropyl; their reducing powers, estimated by comparing the yields of bitropyl under the same reaction conditions, correlate with their oxidation electrode potentials and oxidation-reduction potentials. The ferric ion, the mercurous ion, the mercuric ion, and the silver ion afforded no bitropyl, but only an oxidation product of the tropylium ion, i.e., benzaldehyde. Silver powder gives neither a reduction nor an oxidation product under these reaction conditions. The reduction has also been carried out in an acetonitrile solution, and the reducibility of tropylium fluoroborate with powders of Zn, Fe, Ni, Sb, Cu, and Ag has been compared with that of triphenylmethyl fluoroborate under heterogeneous conditions. Both the tropylium ion and the triphenylmethyl cation are smoothly reduced by Zn, Fe, Ni, Sb, and Cu in this solvent. The tropylium ion, however, is not reduced by silver powder, while the triphenylmethyl cation is reduced to the triphenylmethyl radical. This greater reducibility of the triphenylmethyl cation is consistent with what could be anticipated from the data of half-wave potentials, ionization potentials, and H.M.O. calculation reported with respect to these two carbonium ions.