Direct hydroxylation of benzene and aromatics with H2O2 catalyzed by a self-assembled iron complex: evidence for a metal-based mechanism

Abstract

An iminopyridine Fe(II) complex, easily prepared in situ by self-assembly of cheap and commercially available starting materials (2-picolylaldehyde, 2-picolylamine, and Fe(OTf)₂ in a 2 : 2 : 1 ratio), is shown to be an effective catalyst for the direct hydroxylation of aromatic rings with H₂O₂ under mild conditions. This catalyst shows a marked preference for aromatic ring hydroxylation over lateral chain oxidation, both in intramolecular and intermolecular competitions, as long as the arene is not too electron poor. The selectivity pattern of the reaction closely matches that of electrophilic aromatic substitutions, with phenol yields and positions dictated by the nature of the ring substituent (electron-donating or electron-withdrawing, ortho-para or meta-orienting). The oxidation mechanism has been investigated in detail, and the sum of the accumulated pieces of evidence, ranging from KIE to the use of radical scavengers, from substituent effects on intermolecular and intramolecular selectivity to rearrangement experiments, points to the predominance of a metal-based S_EAr pathway, without a significant involvement of free diffusing radical pathways.