Deprotonation of Tertiary Amine Cation Radicals. A Direct Experimental Approach

Article abstract of DOI:10.1021/ja00205a014

The tertiary amine cation salt p-An2NCH3^.+AsF6(1-) was prepared by oxidation of the corresponding amine with dioxygenyl hexafluoroarsenate.The cation radical salt was characterized by EPR spectroscopy, by magnetic susceptibility, by UV-vis spectroscopy, and by single-crystal X-ray diffraction.The reaction of the salt with quinuclidine produced at 1:1 ratio of p-An2NCH3 and the adduct 1.Four possible mechanisms were considered for this reaction: a proton-transfer mechanism, an electron-transfer mechanism, and two mechanisms involving a cation radical/base complex.Stopped-flow kinetics were used to determine that the reaction rate was first-order in p-An2NCH3^.+AsF6(1-) and first-order in quinuclidine.A comparison of the reaction rate for p-An2NCH3^.+AsF6(1-) vs p-An2NCD3^.+AsF6(1-) provided an isotope effect of 7.68(7) at 15.1 deg C.The combined kinetic data ruled out all but a rate-limiting proton-transfer mechanism.The temperature dependence of the reaction rate provided activation parameters for the deprotonation: ΔH(excit.) = 3.7(1) kcal/mol and ΔS(excit.) = -22.3(4) cal/molxdeg.Reaction of p-An2NCH3^.+AsF6(1-) with four substituted quinuclidine bases provided a Broensted β of 0.63.The isotope effects for reaction with the substituted quinuclidine bases were measured, and a plot of k_H/k_D vs pK_a was found to have a maximum ca. 8 pK_a units lower than the pK_a of the cation radical.This was explained in terms of the differing widths of the potential energy wells for the X-H bonds being broken and being made.