O2 Activation and Selective Phenolate ortho Hydroxylation by an Unsymmetric Dicopper μ-η1:η1-Peroxido Complex

Solvent-Switched Oxidation Selectivities with O2 : Controlled Synthesis of α-Difluoro(thio)methylated Alcohols and Ketones

The solvent-switched hydroxylation and oxygenation of α-difluoro(thio)methylated carbanions with molecular oxygen under mild conditions are reported. This strategy tames the redox reactions of the in situ generated hydroperoxy difluoromethylsulfides, in which solvent-bonding can alter their reactivity and switch the oxidation selectivities. These controllable three-component reactions of gem-difluoroalkenes, thiols and molecular oxygen afford various useful α-difluoro(thio)methylated alcohols and ketones in high yields. Significantly, this protocol has been applied in the synthesis different bioactive molecules. Mechanism studies enable the detection of the hydroperoxy difluoromethylsulfide intermediates and exclude the thiol-based radical pathway.

Fluorescent Bis(guanidine) Copper Complexes as Precursors for Hydroxylation Catalysis

Bis(guanidine) copper complexes are known for their ability to activate dioxygen. Unfortunately, until now, no bis(guanidine) copper-dioxygen adduct has been able to transfer oxygen to substrates. Using an aromatic backbone, fluorescence properties can be added to the copper(I) complex which renders them useful for later reaction monitoring. The novel bis(guanidine) ligand DMEG2tol stabilizes copper(I) and copper(II) complexes (characterized by single crystal X-ray diffraction, IR spectroscopy, and mass spectrometry) and, after oxygen activation, bis(µ-oxido) dicopper(III) complexes which have been characterized by low-temperature UV/Vis and Raman spectroscopy. These bis(guanidine) stabilized bis(µ-oxido) complexes are able to mediate tyrosinase-like hydroxylation activity as first examples of bis(guanidine) stabilized complexes. The experimental study is accompanied by density functional theory calculations which highlight the special role of the different guanidine donors.

Efficient Biomimetic Hydroxylation Catalysis with a Bis(pyrazolyl)imidazolylmethane Copper Peroxide Complex

Bis(pyrazolyl)methane ligands are excellent components of model complexes used to investigate the activity of the enzyme tyrosinase. Combining the N donors 3-tert-butylpyrazole and 1-methylimidazole results in a ligand that is capable of stabilising a (μ-η(2) :η(2) )-dicopper(II) core that resembles the active centre of tyrosinase. UV/Vis spectroscopy shows blueshifted UV bands in comparison to other known peroxo complexes, due to donor competition from different ligand substituents. This effect was investigated with the help of theoretical calculations, including DFT and natural transition orbital analysis. The peroxo complex acts as a catalyst capable of hydroxylating a variety of phenols by using oxygen. Catalytic conversion with the non-biological phenolic substrate 8-hydroxyquinoline resulted in remarkable turnover numbers. In stoichiometric reactions, substrate-binding kinetics was observed and the intrinsic hydroxylation constant, kox , was determined for five phenolates. It was found to be the fastest hydroxylation model system determined so far, reaching almost biological activity. Furthermore, Hammett analysis proved the electrophilic character of the reaction. This sheds light on the subtle role of donor strength and its influence on hydroxylation activity.

Bioinspired copper(I) complexes that exhibit monooxygenase and catechol dioxygenase activity

New tripodal ligand L2 featuring three different pyridyl/imidazolyl-based N-donor units at a bridgehead C atom, from which one of the imidazolyl units is separated by a phenylene linker, was synthesized and investigated with regards to copper(I) complexation. The resulting complex [(L2)Cu]OTf (2(OTf)), the known complex [(L1)Cu]OTf (1(OTf); L1 differs from L2 in that it lacks the phenylene spacer) and [(L3)Cu]OTf (3(OTf)), prepared from a known chiral, tripodal, N-donor ligand featuring pyridyl, pyrazolyl, and imidazolyl donors, were tested as catalysts for the oxidation of sodium 2,4-di-tert-butylphenolate (NaDTBP) with O2. Indeed, they mediated NaDTBP oxidation to give mainly the corresponding catecholate and quinone (Q). None of the complexes 1(OTf), 2(OTf), and 3(OTf) is superior to the others, as yields were comparable and, if the presence of protons is guaranteed by concomitant addition of the phenol DTBP, the oxidation can also be performed catalytically. For all complexes stoichiometric oxidations under certain conditions (concentrated solutions, high NaDTBP content) were found to also generate products typical for metal-mediated intradiol cleavage of the catecholate with O2. As shown representatively for 1(OTf) this dioxygenation sets in at a later stage of the reaction. Initially a copper species responsible for the monooxygenation must form from 1(OTf)/NaDTBP/O2, and only thereafter is the copper species responsible for dioxygenation formed and consumes Q as substrate. Hence, under these circumstances complexes 1(OTf)-3(OTf) show both monooxygenase and catechol dioxygenase activity.

Observation of a Cu(II)(2) (μ-1,2-peroxo)/Cu(III)(2) (μ-oxo)(2) equilibrium and its implications for copper-dioxygen reactivity

Synthesis of small-molecule Cu2 O2 adducts has provided insight into the related biological systems and their reactivity patterns including the interconversion of the Cu(II) 2 (μ-η(2) :η(2) -peroxo) and Cu(III) 2 (μ-oxo)2 isomers. In this study, absorption spectroscopy, kinetics, and resonance Raman data show that the oxygenated product of [(BQPA)Cu(I) ](+) initially yields an "end-on peroxo" species, that subsequently converts to the thermodynamically more stable "bis-μ-oxo" isomer (Keq =3.2 at -90 °C). Calibration of density functional theory calculations to these experimental data suggest that the electrophilic reactivity previously ascribed to end-on peroxo species is in fact a result of an accessible bis-μ-oxo isomer, an electrophilic Cu2 O2 isomer in contrast to the nucleophilic reactivity of binuclear Cu(II) end-on peroxo species. This study is the first report of the interconversion of an end-on peroxo to bis-μ-oxo species in transition metal-dioxygen chemistry.

Double C(arom)-H activation associated with etheral oxygen insertion to phenazine architecture in oxidisable ruthenium(III) complexes: a mechanistic insight

Three examples of unusual double aromatic CH bond activation associated with insertion of etheral oxygen atom to phenazine architecture in Ru(III) complexes are reported. The chemical transformations have led to the formation of new Ru(IV) complexes with angular pentacyclo heterocyclic ligands. A mechanistic investigation indicates that the overall process is a combination of successive steps involving air (O2 ) and H2 O.

Electrophilic arene hydroxylation and phenol O-H oxidations performed by an unsymmetric μ-η(1):η(1)-O2-peroxo dicopper(II) complex

Reactions of the unsymmetric dicopper(II) peroxide complex [Cu(II)(2)(μ-η(1):η(1)-O(2))(m-XYL(N3N4))](2+) (1 O(2), where m-XYL is a heptadentate N-based ligand), with phenolates and phenols are described. Complex 1 O(2) reacts with p-X-PhONa (X = MeO, Cl, H, or Me) at -90 °C performing tyrosinase-like ortho-hydroxylation of the aromatic ring to afford the corresponding catechol products. Mechanistic studies demonstrate that reactions occur through initial reversible formation of metastable association complexes [Cu(II)(2)(μ-η(1):η(1)-O(2))(p-X-PhO)(m-XYL(N3N4))](+) (1 O(2)⋅X-PhO) that then undergo ortho-hydroxylation of the aromatic ring by the peroxide moiety. Complex 1 O(2) also reacts with 4-X-substituted phenols p-X-PhOH (X = MeO, Me, F, H, or Cl) and with 2,4-di-tert-butylphenol at -90 °C causing rapid decay of 1 O(2) and affording biphenol coupling products, which is indicative that reactions occur through formation of phenoxyl radicals that then undergo radical C-C coupling. Spectroscopic UV/Vis monitoring and kinetic analysis show that reactions take place through reversible formation of ground-state association complexes [Cu(II)(2)(μ-η(1):η(1)-O(2))(X-PhOH)(m-XYL(N3N4))](2+) (1 O(2)⋅X-PhOH) that then evolve through an irreversible rate-determining step. Mechanistic studies indicate that 1 O(2) reacts with phenols through initial phenol binding to the Cu(2)O(2) core, followed by a proton-coupled electron transfer (PCET) at the rate-determining step. Results disclosed in this work provide experimental evidence that the unsymmetric 1 O(2) complex can mediate electrophilic arene hydroxylation and PCET reactions commonly associated with electrophilic Cu(2)O(2) cores, and strongly suggest that the ability to form substrate⋅Cu(2)O(2) association complexes may provide paths to overcome the inherent reactivity of the O(2)-binding mode. This work provides experimental evidence that the presence of a H(+) completely determines the fate of the association complex [Cu(II)(2)(μ-η(1):η(1)-O(2))(X-PhO(H))(m-XYL(N3N4))](n+) between a PCET and an arene hydroxylation reaction, and may provide clues to help understand enzymatic reactions at dicopper sites.