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 (g(1) = 2.069, g(2) = 2.007, g3 = 1.963), which directly reacts with benzene at -80 degrees C with the rate constant k(2) = 0.6 M-1 s(-1) , and with toluene with k(2) > 1 M(-1)s(-1), thus giving evidence for its key role in the selective oxygenation of aromatic substrates. (C) 2018 Elsevier B.V. All rights reserved.