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

Abstract

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*)2FeIII2(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*)2FeIII2(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 1aRCOOH with the proposed structure [(PDP*)FeV=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 2aRCOOH (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*)FeV=O(OC(O)R)]2+ and [(TPA*)FeV=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 2aRCOOH can be higher than the concentration of 2a just after the reaction onset at −70 °C; then, 2aRCOOH 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 1aRCOOH and 2a with acetophenone and chlorobenzene have been evaluated. The aromatic hydroxylation reactivities of the observed intermediates vary in the following order 2aRCOOH <2a <1aRCOOH.