Highly Efficient Aromatic C-H Oxidation with H₂O₂ in the Presence of Iron Complexes of the PDP Family

The catalytic activity of a series of iron complexes of the PDP family (PDP=N,N'-bis(2-pyridylmethyl)-2,2'-bipyrrolidine) in the oxidation of aromatic substrates with H₂O₂ has been studied. In the presence of acetic acid, these complexes efficiently catalyze the oxidation of benzene and alkylbenzenes with high selectivity for oxygen incorporation into the aromatic ring (up to 93%), performing up to 84 catalytic turnovers. The parent complex, [(PDP)(OTf)₂], has demonstrated the highest catalytic efficiency and aromatic oxidation selectivity. The yield of products of oxidation of different substrates increases in line with increasing number of electron-donating alkyl groups of the substrates: halogenbenzenes<benzene<monoalkylbenzenes<dialkylbenzenes, which, together with the results of competitive oxidation experiments and the observation of inverse primary k_H/k_D (0.90-0.92), agrees with the S_EAr oxidation mechanism. Low-temperature EPR studies have witnessed the presence of low-spin (g₁=2.071, g₂=2.008, g₃=1.960) perferryl intermediates, demonstrating direct reactivity toward benzene. The oxidation of m-xylene in the presence of H₂ ¹⁸O has shown the same probability of ¹⁸O incorporation into the aromatic ring and the aliphatic moieties, which is indicative that both the aromatic and aliphatic oxidations are conducted by the same active species.