Complete σ* intramolecular aromatic hydroxylation mechanism through O2 activation by a Schiff base macrocyclic dicopper(I) complex

Albert Poater,Miquel Solà

doi:10.3762/bjoc.9.63

Abstract

In this work we analyze the whole molecular mechanism for intramolecular aromatic hydroxylation through O2 activation by a Schiff hexaazamacrocyclic dicopper(I) complex, [CuI2(bsH2m)]2+. Assisted by DFT calculations, we unravel the reaction pathway for the overall intramolecular aromatic hydroxylation, i.e., from the initial O2 reaction with the dicopper(I) species to first form a CuICuII-superoxo species, the subsequent reaction with the second CuI center to form a μ-η2:η2-peroxo-CuII2 intermediate, the concerted peroxide O–O bond cleavage and C–O bond formation, followed finally by a proton transfer to an alpha aromatic carbon that immediately yields the product [CuII2(bsH2m-O)(μ-OH)]2+.

Highlights

Bearing in mind the key role of dioxygen in biology, in particular toward Cu and Fe metal centers, being involved in the catalytic cycle of proteins, including dinuclear copper-active sites, such as hemocyanin, tyrosinase and catechol oxidases [1,2,3,4,5,6,7], either transporting or activating O2, its comprehension is still underway
A hot topic is still to unravel, either experimentally or by calculations, which of the side-on μ-η2:η2peroxo and bis(μ-oxo) isomeric Cu2O22+ cores are present, and in the case that they exist, to study the feasibility of their interconversion [15,16,17,18,19], tuning either the metallic complex or the reaction conditions [20,21,22,23]. Both Cu2O22+ cores have been proposed to be the active species in the aromatic hydroxylation process
Apart from the high instability with respect to the peroxo intermediate, from d it is necessary to overcome a barrier of 22.3 kcal·mol−1, which rules out the role of h in the reaction pathway a→g

Summary

Introduction

Bearing in mind the key role of dioxygen in biology, in particular toward Cu and Fe metal centers, being involved in the catalytic cycle of proteins, including dinuclear copper-active sites, such as hemocyanin, tyrosinase and catechol oxidases [1,2,3,4,5,6,7], either transporting or activating O2, its comprehension is still underway. This step leads to the cleavage of the O–O bond and consists of a direct and concerted attack on the closest carbon atom of the aromatic ring to form species e through a barrier of 20.8 kcal·mol−1.

Results

Conclusion