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Theoretical Study of the Oxidation of Phenolates by the [Cu 2 O 2 ( N , N ′‐di‐ tert ‐butylethylenediamine) 2 ] 2+ Complex
Author(s) -
Liu Yan Fang,
Yu Jian Guo,
Siegbahn Per E. M.,
Blomberg Margareta R. A.
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201203052
Subject(s) - chemistry , bond cleavage , electron transfer , copper , proton , density functional theory , redox , reaction mechanism , concerted reaction , yield (engineering) , catechol , crystallography , ion , quinone , cleavage (geology) , photochemistry , stereochemistry , computational chemistry , inorganic chemistry , catalysis , materials science , organic chemistry , physics , quantum mechanics , fracture (geology) , composite material , metallurgy
Experiments have shown that the μ‐η 2 :η 2 ‐peroxodicopper(II) complex [Cu 2 O 2 ( N , N ′‐di‐ tert ‐butylethylenediamine) 2 ] 2+ rapidly oxidizes 2,4‐di‐ tert ‐butylphenolate into a mixture of catechol and quinone and that, at the extreme temperature of −120 °C, a bis‐μ‐oxodicopper(III)phenolate intermediate, labeled complex A , can be observed. These experimental results suggest a new mechanism of action for the dinuclear copper‐containing enzyme tyrosinase, involving an early OO bond‐cleavage step. However, whether phenolate binding occurs before or after the cleavage of the OO bond has not been possible to answer. In this study, hybrid density functional theory is used to study the synthetic reaction and, based on the calculated free‐energy profile, a mechanism is suggested for the entire phenolate‐oxidation reaction that agrees with the experimental observations. Most importantly, the calculations show that the very first step in the reaction is the cleavage of the OO bond in the peroxo complex and that, subsequently, the phenolate substrate coordinates to one of the copper ions in the bis‐μ‐oxodicopper(III) complex to yield the experimentally characterized phenolate intermediate ( A ). The oxidation of the phenolate substrate into a quinone then occurs in three steps: 1) CO bond formation, 2) coupled internal proton and electron transfer, and 3) electron transfer coupled to proton transfer from an external donor (acidic workup, experimentally). The first of these steps is rate limiting for the decay of complex A , with a calculated free‐energy barrier of 10.7 kcal mol −1 and a deuterium kinetic isotope effect of 0.90, which are in good agreement with the experimental values of 11.2 kcal mol −1 and 0.83(±0.09). The tert ‐butyl substituents on both the phenol substrate and the copper ligands need to be included in the calculations to give a correct description of the reaction mechanism.