Spectroscopic and mechanistic investigations of Tyrosinase model complexes [electronic resource]
Keywords: Chemistry

Tyrosinase (Tyr) is a ubiquitous binuclear copper enzyme that oxygenates to form a [Mu]--[Eta]2:[Eta]2--peroxodicopper(II) (P) species and is capable of hydroxylating phenols to catechols and oxidizing catechols to quinones. While synthetic systems can create spectroscopically and structurally faithful models, the number of these complexes capable of exogenous phenolate hydroxylation is extremely limited, and the active oxidant in most cases has been identified not as the side-on peroxo species P, but an isomeric bis([Mu]--oxo)dicopper(III) species with an equatorially ligated phenolate(s), designated as an A species. Chapter 2 concerns the characterization of a simple bis([Mu]--oxo)dicopper(III) species (O) formed in the oxygenation of a Cu(I) complex containing the simplest of the peralkylated diamines, namely N, N, N', N'-tetramethylethylenediamine (TMED). This O species has been conspicuously absent from the literature, and the factors that complicated its characterization were identified. Using appropriate conditions, nearly full formation of the O species is achieved, which allowed its reactivity to be explored. Its thermal stability rivals the best of the dicationic O species, which is attributed to the oxidative stability of the methyl substituents of TMED. The simplicity of the TMED ligand creates an O species with an exposed Cu2O2 core capable of oxidizing benzyl alcohol to benzaldehyde, a reaction not shown by most peralkylated diamine O species. Chapter 3 expands the scope of characterized A species using a flexible, secondary diamine ligand, DBPD, and a variety of phenolates. The A species are formed at extreme solution temperatures (153 K) through the reaction of phenolates with the P species, initially generated by oxygenation of the [(DBPD)Cu(I)]+ complex. The decomposition of the A species is accompanied by regiospecific phenolate hydroxylation or dehalogenation of ortho-halogenated phenolates. The latter is a novel reactivity for any Cu/O2 complexes. This study support the general trend that the bis([Mu]-oxo)dicopper(III) species serves as the active hydroxylating reagent of phenolates in synthetic complexes; O--O bond cleavage of the P species precedes C--H bond hydroxylation. Chapter 4 investigates the reaction conditions necessary to promote phenolate hydroxylation over phenolate coupling through a series of di-tert-butyl-ethylenediamine ligands that form equilibrium mixtures of P and O species. The equilibrium position is determined primarily by the diamine ligand with greater tertiary amine ligation biasing the equilibrium toward the O species. Addition of basic counter-anions readily push the P [Equivalent] O equilibrium to a "pure" P state. The initial P [Equivalent] O equilibrium position correlates to the product distribution; P biased equilibria yield greater amounts of phenolate hydroxylation and O biased equilibria yield predominant coupled phenol product. The product distribution is also correlated to the accessibility to Cu2O2 cores with more accessible cores exhibiting more hydroxylation reactivity. This study suggests that the Cu2O2 species with moderate reduction potentials and exposed Cu2O2 cores will promote phenolate hydroxylation over phenolate coupling.