Aromatic C[sbnd]H oxidation by non-heme iron(V)-oxo intermediates bearing aminopyridine ligands

Direct data on the reactivity of the nonheme iron(V)-oxo intermediates toward substituted benzenes at −70 °C have been obtained. The intermediates are generated in the catalyst systems 1/CH₃CO₃H/RCOOH (1 = [(PDP*)₂Fe^III ₂(μ-OH)₂](OTf)₄, PDP* = N,N′-bis(3,5-dimethyl-4-methoxypyridyl-2-methyl)-(S,S)-2,2′-bipyrrolidine) and 2/CH₃CO₃H/RCOOH (2 = [(TPA*)₂Fe^III ₂(μ-OH)₂](OTf)₄, TPA* = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine), RCOOH are various linear and branched carboxylic acids). It has been found that only one type of iron(V)-oxo intermediates 1a^RCOOH with the proposed structure [(PDP*)Fe^V=O(OC(O)R)]²⁺ (g₁ = 2.071–2.072, g₂ = 2.007, g₃ = 1.959–1.960) can be observed in the systems 1/CH₃CO₃H/RCOOH, whereas in the systems 2/CH₃CO₃H/RCOOH, two types of iron(V)-oxo intermediates 2a^RCOOH (g₁ = 2.070–2.071, g₂ = 2.004, g₃ = 1.960–1.962) and 2a (g₁ = 2.075, g₂ = 2.011, g₃ = 1.964) are formed, with the proposed structures [(TPA*)Fe^V=O(OC(O)R)]²⁺ and [(TPA*)Fe^V=O(OH)]²⁺, respectively. For linear carboxylic acids, intermediate 2a strongly predominates in the reaction solution just after the reaction onset. When RCOOH is branched carboxylic acid, the concentration of 2a^RCOOH can be higher than the concentration of 2a just after the reaction onset at −70 °C; then, 2a^RCOOH converts into 2a within 5-10 min at this temperature. The reactivity of the iron-oxo species toward substituted benzenes is higher for more electron-rich substrates, varying in the following order acetophenone < chlorobenzene < benzene < toluene, which is consistent with the electrophilic aromatic substitution mechanism. In agreement with this, the highest turnover numbers in aromatic oxidation (up to 25) were obtained for toluene. The second-order rate constants for the reaction of intermediates 1a^RCOOH and 2a with acetophenone and chlorobenzene have been evaluated. The aromatic hydroxylation reactivities of the observed intermediates vary in the following order 2a^RCOOH < 2a < 1a^RCOOH.