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Chemistry - a European journal 26(34), 7556-7562 (2020). doi:10.1002/chem.202000664, The enzyme tyrosinase contains a reactive side‐on peroxo dicopper(II) center as catalytically active species in C−H oxygenation reactions. The tyrosinase activity of the isomeric bis($μ$‐oxo) dicopper(III) form has been discussed controversially. The synthesis of bis(μ‐oxo) dicopper(III) species [Cu$_2$($μ$‐O)$_2$(L1)$_2$](X)$_2$ ([O1](X)$_2$, X=PF$_6^−$, BF$_4^−$, OTf$^−$, ClO$_4^−$), stabilized by the new hybrid guanidine ligand 2‐{2‐((dimethylamino)methyl)phenyl}‐1,1,3,3‐tetramethylguanidine (L1), and its characterization by UV/Vis, Raman, and XAS spectroscopy, as well as cryo‐UHR‐ESI mass spectrometry, is described. We highlight selective oxygenation of a plethora of phenolic substrates mediated by [O1](PF$_6$)$_2$, which results in mono‐ and bicyclic quinones and provides an attractive strategy for designing new phenazines. The selectivity is predicted by using the Fukui function, which is hereby introduced into tyrosinase model chemistry. Our bioinspired catalysis harnesses molecular dioxygen for organic transformations and achieves a substrate diversity reaching far beyond the scope of the enzyme., Published by Wiley-VCH, Weinheim

CuI complexes of the form K[(R3P)Cu(pinF)], in which (pinF)2− is the bidentate, oxygen-donating ligand perfluoropinacolate, were synthesized and characterized. Low-temperature oxygenation of the K[(R3P)Cu(pinF)(PR3)] species resulted in a trisanionic bis(μ3-oxo) trinuclear copper(II,II,III) core characterized by UV/Vis spectroscopy (λmax [nm] = 330, 535, 630), cryospray-ionization mass spectrometry, and X-band electron paramagnetic resonance spectroscopy (derivative resonance at 3300 G, Δms=2 at 1500 G). The kinetic behavior of the trimeric {Cu3O2} species was quantified by stopped-flow spectroscopy and the associated electronic structures were investigated by DFT calculations. An asymmetric {Cu3O2} species, AsTpinF, which bears a structure similar to multicopper oxidases, forms prior to full formation of the symmetric trinuclear core, SyTpinF. The trimer catalytically oxidizes para-hydroquinone to benzoquinone (a form of oxidase chemistry).

Tyrosinase model systems pinpoint pathways to translating Nature's synthetic abilities for useful synthetic catalysts. Mostly, they use N-donor ligands which mimic the histidine residues coordinating the two copper centres. Copper complexes with bis(pyrazolyl) methanes with pyridinyl or imidazolyl moieties are already reported as excellent tyrosinase models. Substitution of the pyridinyl donor results in the new ligand HC(3-tBuPz)(2)(4-CO2MePy) which stabilises a room-temperature stable mu-eta(2):eta(2)-peroxide dicopper(II) species upon oxygenation. It reveals highly efficient catalytic activity as it hydroxylates 8-hydroxyquinoline in high yields (TONs of up to 20) and much faster than all other model systems (max. conversion within 7.5 min). Stoichiometric reactions with para-substituted sodium phenolates show saturation kinetics which are nearly linear for electron-rich substrates. The resulting Hammett correlation proves the electrophilic aromatic substitution mechanism. Furthermore, density functional theory (DFT) calculations elucidate the influence of the substituent at the pyridinyl donor: the carboxymethyl group adjusts the basicity and nucleophilicity without additional steric demand. This substitution opens up new pathways in reactivity tuning.