Copper-catalyzed ortho-oxidation of phenols by dioxygen (tyrosinase mimics) do yields catechols as primary products

Single Chain Polymeric Nanoparticles to Promote Selective Hydroxylation Reactions of Phenol Catalyzed by Copper

Metal-containing single chain polymeric nanoparticles (SCPNs) can be used as synthetic mimics of metalloenzymes. Currently, the role of the folded polymer backbones on the activity and selectivity of metal sites is not clear. Herein, we report our findings on how polymeric frameworks modulate the coordination of Cu sites and the catalytic activity/selectivity of Cu-containing SCPNs mimicking monophenol hydroxylation reactions. Imidazole-functionalized copolymers of poly(methyl methacrylate-co-3-imidazolyl-2-hydroxy propyl methacrylate) were used for intramolecular Cu-imidazole binding that triggered the self-folding of polymers. Polymer chains imposed steric hindrance which yielded unsaturated Cu sites with an average coordination number of 3.3. Cu-containing SCPNs showed a high selectivity for the hydroxylation reaction of phenol to catechol, >80%, with a turnover frequency of >870 h^-1 at 60 °C. The selectivity was largely influenced by the flexibility of the folded polymer backbone where a more flexible polymer backbone allows the cooperative catalysis of two Cu sites. The second coordination sphere provided by the folded polymer that has been less studied is therefore critical in the design of active mimics of metalloenzymes.

A bio-inspired synthesis of oxindoles by catalytic aerobic dual C-H functionalization of phenols

We report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of oxygen.

Nitrogen-containing heterocycles are fundamentally important to the function of pharmaceuticals, agrochemicals and materials. Herein, we report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of molecular oxygen (O₂). Our method is inspired by the biosynthesis of melanin pigments (melanogenesis), but diverges from the biosynthetic polymerization. Mechanistic analysis reveals the involvement of Cu^II-semiquinone radical intermediates, which enable dehydrogenative carbon–heteroatom bond formation that avoids a catechol/quinone redox couple. This mitagates the deleterious polarity reversal that results from phenolic dearomatization, and enables a high-yielding phenolic C–H functionalization under catalytic aerobic conditions. Our work highlights the broad synthetic utility and efficiency of forming C–N bonds via a catalytic aerobic dearomatization of phenols, which is currently an underdeveloped transformation.

Photoelectrocatalytic Oxidation of o-Phenols on Copper-Plated Screen-Printed Electrodes

A novel and sensitive detection method based on photoelectrocatalytic oxidation of o-diphenols was demonstrated on a copper-plated screen-printed carbon electrode (designated CuSPE) in pH 8 phosphate buffer solution. The o-diphenols can be detected amperometrically through electrochemical oxidation at a low applied potential of -0.1 V versus Ag/AgCl, where the CuSPE is much less subject to interfering reactions. The mechanism that induces good selectivity of the CuSPE is explained in terms of the formation of a cyclic five-member complex intermediate (Cu(II)-o-quinolate). A prototype homemade flow through cell design is described for incorporating the photoelectrode and light source. Electrode irradiation results in a large increase in anodic current. The oxidative photocurrents produced by irradiation increase with light intensity presumably because of the formation of semiconductor Cu(2)O. The principle used in this study has an opportunity to extend into various research applications.

Selective detection of o-diphenols on copper-plated screen-printed electrodes

Selective detection of o-diphenols (e.g., catechol, dopamine, and pyrogallol) in the presence of simple phenols, m- and p- derivative diphenols, and ascorbic acid has been demonstrated on copper-plated screen-printed electrodes (CuSPEs) in pH 7.4 phosphate buffer solution. The CuSPE showed an unusual catalytic response at -0.05 V versus Ag/AgCl selectively to o-diphenolic compounds. The o-diphenols can thus be determined amperometrically through direct electrochemical oxidation in low potentials (approximately 0 V), where the CuSPE is much less subject to interfering reactions. Such a catalytic phenomenon cannot be observed on conventional Pt and glassy carbon electrodes. The selective mechanism is explained in terms of the formation of cyclic five-member complex intermediate (Cu(II)-o-quinolate). Most important of all, the common drawbacks of electrode fouling through polymerization were completely overcome in this system.

Novel tert-butyl migration in copper-mediated phenol ortho-oxygenation implicates a mechanism involving conversion of a 6-hydroperoxy-2,4-cyclohexadienone directly to an o-quinone

Copper mediated ortho-oxygenation of phenolates may proceed through the generation of a 6-peroxy-2,4-cyclohexadienone intermediate. To test this theory, we studied the fate of sodium 4-carbethoxy-2, 6-di-tert-butylphenolate, where the ortho-oxygenation sites are blocked by tert-butyl groups. Using the Cu(I) complex of N, N-bis(2-(N-methylbenzimidazol-2-yl)ethyl)benzylamine, isolation of the major oxygenated product and characterization by single-crystal X-ray crystallography and NMR spectroscopy revealed it to be 4-carbethoxy-3,6-di-tert-butyl-1,2-benzoquinone, resulting from a 1, 2-migration of a tert-butyl group. The independently prepared 6-hydroperoxide is transformed by the Cu(I)- (or Cu(II)-) ligand complex to the same o-quinone. The observed 1,2-migration of the tert-butyl group appears to reflect an electron demand created by rearrangement of the postulated peroxy intermediate. A mechanism proceeding alternatively through a catechol and subsequent oxidation to the o-quinone seems ruled out by a control study demonstrating that the requisite intermediate to catechol formation would instead eliminate the 2-tert-butyl group.