Scandium ion-enhanced oxidative dimerization and N-demethylation of N,N-dimethylanilines by a non-heme iron(IV)-oxo complex.

Oxidative dimerization of N,N-dimethylaniline (DMA) occurs with a nonheme iron(IV)-oxo complex, Fe(IV)(O)(N4Py) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), to yield the corresponding dimer, tetramethylbenzidine (TMB), in acetonitrile. The rate of the oxidative dimerization of DMA by Fe(IV)(O)(N4Py) is markedly enhanced by the presence of scandium triflate, Sc(OTf)(3) (OTf = CF(3)SO(3)(-)), when TMB is further oxidized to the radical cation (TMB(•+)). In contrast, we have observed the oxidative N-demethylation with para-substituted DMA substrates, since the position of the C-C bond formation to yield the dimer is blocked. The rate of the oxidative N-demethylation of para-substituted DMA by Fe(IV)(O)(N4Py) is also markedly enhanced by the presence of Sc(OTf)(3). In the case of para-substituted DMA derivatives with electron-donating substituents, radical cations of DMA derivatives are initially formed by Sc(3+) ion-coupled electron transfer from DMA derivatives to Fe(IV)(O)(N4Py), giving demethylated products. Binding of Sc(3+) to Fe(IV)(O)(N4Py) enhances the Sc(3+) ion-coupled electron transfer from DMA derivatives to Fe(IV)(O)(N4Py), whereas binding of Sc(3+) to DMA derivatives retards the electron-transfer reaction. The complicated kinetics of the Sc(3+) ion-coupled electron transfer from DMA derivatives to Fe(IV)(O)(N4Py) are analyzed by competition between binding of Sc(3+) to DMA derivatives and to Fe(IV)(O)(N4Py). The binding constants of Sc(3+) to DMA derivatives increase with the increase of the electron-donating ability of the para-substituent. The rate constants of Sc(3+) ion-coupled electron transfer from DMA derivatives to Fe(IV)(O)(N4Py), which are estimated from the binding constants of Sc(3+) to DMA derivatives, agree well with those predicted from the driving force dependence of the rate constants of Sc(3+) ion-coupled electron transfer from one-electron reductants to Fe(IV)(O)(N4Py). Thus, oxidative dimerization of DMA and N-demethylation of para-substituted DMA derivatives proceed via Sc(3+) ion-coupled electron transfer from DMA derivatives to Fe(IV)(O)(N4Py).