Effect of different carboxylic acids on the aromatic hydroxylation with H₂O₂ in the presence of an iron aminopyridine complex

In this contribution, the effect of the structure of the catalytic additive – carboxylic acid – on the catalytic performance of the iron based catalyst [(PDP)Fe(OTf)₂], 2 (PDP = N,N′-bis(2-pyridylmethyl)-(S,S)-2,2′-bipyrrolidine) in the selective aromatic oxidation of alkylbenzenes with H₂O₂ is presented. Eight (linear and branched) carboxylic acids have been tested; in the presence of 2-ethylhexanoic acid, the system [(PDP)Fe(OTf)₂]/RCOOH/H₂O₂ has demonstrated the highest substrate conversion and the highest selectivity for oxygen incorporation into the aromatic ring (up to > 99%) at the same time. Low-temperature EPR spectroscopic study of the system [(PDP)Fe(OTf)₂]/2-ethylhexanoic acid/H₂O₂ witness the presence of the low-spin perferryl intermediate 2a^EHA with small g-factor anisotropy (g₁ = 2.069, g₂ = 2.007, g₃ = 1.963), which directly reacts with benzene at −80 °C with the rate constant k₂ = 0.6 M⁻¹s⁻¹, and with toluene with k₂ > 1 M⁻¹s⁻¹, thus giving evidence for its key role in the selective oxygenation of aromatic substrates.