Direct Evaluation of the Reactivity of Nonheme Iron(V)–Oxo Intermediates toward Arenes

Abstract

The reactivity of nonheme iron(V)–oxo intermediates toward aromatic C–H oxidation at −70 °C has been directly evaluated. The intermediates were generated upon the interaction of the ferric complex [(PDP*)FeIII(μ-OH)2FeIII(PDP*)](OTf)4 (4◊; PDP* = N,N′-bis(3,5-dimethyl-4-methoxypyridyl-2-methyl)-(S,S)-2,2′-bipyrrolidine) with peracetic acid in the presence of acetic or 2-ethylhexanoic acid. The second-order rate constants (k2) for the reaction of substituted benzenes with iron–oxo intermediates [(PDP*)FeV═O(OC(O)R)]2+ at −70 °C were determined (R = CH3, 3-heptyl). For more electron rich arenes, much higher k2 values were observed, increasing in the order nitrobenzene < acetophenone < chlorobenzene < benzene < toluene, in accordance with the electrophilic aromatic substitution mechanism. The catalytic oxidation of mono- and dialkylbenzenes with H2O2 proceeded with good efficiency (up to 36.5 TN per Fe atom) and high selectivity toward aromatic oxidation products (up to 91%).