Molar paramagnetic solvent NMR shifts of galvinoxyl in toluene. Kinetics of the reaction between galvinoxyl and tertiary amines at high radical concentrations

Abstract

AbstractThe molar paramagnetic solvent NMR shifts of galvinoxyl (1, G′) in toluene were measured at 34°C and found to be 2.21 and 2.39 ppm mol−1 for the aliphatic and the aromatic proton signals of toluene, respectively. The implications of the inequality of the two molar shifts are discussed. The paramagnetic solvent shift of G. in toluene in the presence of tertiary amines decays according to a first‐order rate law. The apparent first‐order rate constants were found to be linearly related to the initial concentration of the amine. The kobs vs. [R3N]0 regression lines in some cases exhibit a positive intercept, indicating the involvement of the solvent in the reaction. The logarithms of the second‐order rate constants are shown to be linear with respect to the ionization potentials of the amines. The reaction products of galvinoxyl with n‐Bu3N are hydrogalvinoxyl, GH and the enamine n‐Bu2NCH=CHEt whereas, with N/N‐dimethylaniline. the products are hydrogalvinoxyl and the hemiaminal ether of formaldehyde, derived from the combination of the radical PhN(Me)CH2. with G. through the oxygen of the latter. A mechanistic scheme is proposed involving a rate‐determining electron‐transfer step.