A non-heme iron(IV)-oxo complex, [(TMC)FeIV(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was formed by oxidation of an iron(II) complex ([(TMC)FeII]2+) with dioxygen (O2) 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 O2 activation pathway. The autocatalytic radical chain reaction is initiated by scandium ion-promoted electron transfer from BPh4- to [(TMC)FeIV(O)]2+ to produce phenyl radical (Ph •). The chain propagation step is composed of the addition of O2 to Ph• and the reduction of the resulting phenylperoxyl radical (PhOO•) by scandium ion-promoted electron transfer from BPh4- to PhOO• to produce phenyl hydroperoxide (PhOOH), accompanied by regeneration of phenyl radical. PhOOH reacts with [(TMC)FeII]2+ to yield phenol (PhOH) and [(TMC)FeIV(O)]2+. Biphenyl (Ph-Ph) was formed via the radical chain autoxidation of BPh3 by O2. The induction period of the autocatalytic radical chain reactions was shortened by addition of a catalytic amount of [(TMC)FeIV(O)]2+, whereas addition of a catalytic amount of ferrocene that can reduce [(TMC)FeIV(O)] 2+ resulted in elongation of the induction period. Radical chain autoxidation of BPh4- by O2 also occurred in the presence of Sc(OTf)3 without [(TMC)FeIV(O)] 2+, initiating the autocatalytic oxidation of [(TMC)Fe II]2+ with O2 and BPh4- to yield [(TMC)FeIV(O)]2+. Thus, the general view for formation of non-heme iron(IV)-oxo complexes via O2-binding iron species (e.g., FeIII(O2•-)) without contribution of autocatalytic radical chain reactions should be viewed with caution.