A non-heme iron(IV)-oxo complex, [(TMC)-Fe-IV(O)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was formed by oxidation of an iron(II) complex ([(TMC)Fe-II](2+)) with dioxygen (O-2) and tetraphenylborate (BPh4-) in the presence of scandium triflate (Sc(OTf)(3)) in acetonitrile at 298 K via autocatalytic radical chain reactions rather than by a direct O-2 activation pathway. The autocatalytic radical chain reaction is initiated by scandium ion-promoted electron transfer from BPh4- to [(TMC)Fe-IV(O)](2+) to produce phenyl radical (Ph-center dot). The chain propagation step is composed of the addition of O-2 to Ph-center dot and the reduction of the resulting phenylperoxyl radical (PhOO center dot) by scandium ion-promoted electron transfer from BPh4- to PhOO center dot to produce phenyl hydroperoxide (PhOOH), accompanied by regeneration of phenyl radical. PhOOH reacts with [(TMC)Fe-II](2+) to yield phenol (PhOH) and [(TMC)F-IV(O)](2+)+. Biphenyl (Ph-Ph) was formed via the radical chain autoxidation of BPh3 by O-2. The induction period of the autocatalytic radical chain reactions was shortened by addition of a catalytic amount of [(TMC)Fe-IV(O)](2+), whereas addition of a catalytic amount of ferrocene that can reduce [(TMC)F-IV(O)](2+) resulted in elongation of the induction period. Radical chain autoxidation of BPh4- by O-2 also occurred in the presence of Sc(OTf)(3) without [(TMC)Fe-IV(O)](2+), initiating the autocatalytic oxidation of [(TMC)Fe-II](2+) with O-2 and BPh4- to yield [(TMC)F-IV(O)](2+). Thus, the general view for formation of non-heme iron(IV)-oxo complexes via O-2-binding iron species (e.g., Fe-III(O-2(center dot-))) without contribution of autocatalytic radical chain reactions should be viewed with caution.