Open Access

Puneet Gupta

• The high selectivity of biological transformations taking place in Nature have long inspired synthetic chemists to develop analogous chemical processes. Similarly, transient intermediates identified in chemical transformations often provide a basis to understand biological processes. Therefore, new insights gained in biological studies are often useful for chemistry and vice versa. Proteins, and catalytically active enzymes, are among the most essential units of living cells. Metalloproteins or -enzymes, i.e., proteins or enzymes that contain transition metal ions such as copper, nickel, iron or zinc are often involved in processes like (1) metal-ion storage and transport, (2) exchange of electrons with the environment in catalysis and electron transfer reactions, and (3) dioxygen storage, transport, and metabolization. For decades, copper-mediated biological oxidations have spurred a great deal of interest among synthetic and catalytic chemists. Copper enzymes such as dopamine β-monooxygenase (DβM), peptidylglycine α-hydroxylating monooxygenase (PHM),particulate methane monooxygenase (pMMO) and tyrosinase activate molecular oxygen (O2) and incorporate one of the oxygen atoms selectively into C−H bonds yielding hydroxylated organic substrates. Remarkable progress in bioinorganic research has led to the development of a large number of copper-based model systems supported by various nitrogen donor ligands that bind O2, cleave the O−O bond, and/or afford hydroxylation reactions similar to copper enzymes. These synthetic model systems have helped to understand the structureactivity relationships of their biological role models and supporting theoretical studies have contributed substantially to the development of the field. Specifically, several density functional theory (DFT) studies have provided detailed mechanistic insights into coppermediated aliphatic and aromatic hydroxylation reactions. Until to date, however, pertinent quantum chemical research still suffers from severe problems as to identify sufficiently accurate and efficient methods for mechanistic studies, and conflicting literature reports have created confusions within the scientific community. Therefore, the first aim of this thesis is to identify a DFT method well suited to describe copper-mediated hydroxylation reactions. With this method at hand a number of interesting hydroxylation reactions is investigated aiming at a detailed understanding of the underlying reaction mechanisms. The thesis is divided into four chapters of which the first, the introductory chapter, is further divided into three sections (1) copper proteins and enzymes, (2) copper-O2 reactivity in enzymes and (3) biomimetic Cu/O2 chemistry. The first section gives a brief overview of a number of copper enzymes. The second section provides a concise introduction to the biochemical transformations brought about by those copper enzymes that perform aliphatic and aromatic hydroxylation reactions. It is shown that such copper enzymes carry different types of active sites which are responsible for their specific biological functions. These copper enzymes with their biological function are the role models for synthetic chemistry. In the third section, biomimetic Cu/O2 chemistry, the insights gathered in the past 35 years of extensive research on copper-based synthetic model systems that mimic various aspects of copper-enzyme reactivity are reviewed. Various types of active copper sites have been realized in these synthetic model systems and a brief introduction to the respective reactivities towards C−H bonds is presented. We will specifically focus on isomerization processes of dinuclear active Cu2O2 sites and the specific reactivity aspects of these isomers, as these phenomena have been the subject of enormous research efforts aiming at the understanding of the function of the enzyme tyrosinase. Theory has been integral part of this research and density functional theory (DFT) has effectively taken over the role as a working horse in most studies. Therefore, the second chapter is devoted to an exposition of earlier DFT applications in mechanistic studies of Cu/O2 chemistry. We specifically highlight the problems related to the use of DFT in this field and illustrate the present state of knowledge. The third chapter of this thesis provides results and discussion of (1) DFT benchmark studies and (2) mechanistic studies. In the first section, the results of a careful benchmark study on the performance of various DFT methods to study the μ-η2:η2-peroxodicopper(II)/bis(μ-oxo)dicopper(III) core isomerization and the C–H hydroxylation processes are compared with available experimental reference data. We provide an assessment of the effects of relativity, counteranions, and dispersion on the reference reactions. The most suitable DFT method evolving from this study, BLYP-D/def2-TZVP including solvent and relativistic corrections, is applied in the next sections to investigate the mechanistic scenario underlying three copper-dioxygen mediated hydroxylation reactions of aliphatic and aromatic C–H bonds. Our mechanistic studies show that bis(μ-oxo)dicopper(III) complexes are capable of achieving selective aliphatic and aromatic C–H hydroxylations. The study of substituent effects in these reactions has further shown that the bis(μ-oxo)dicopper complex acts as an electrophile in hydroxylation. The fourth chapter presents the conclusions of our investigations. Part of the work presented in this thesis has been published in a peer reviewed journal and enclosed in appendix 1. Further research work, not presented in chapters 1-4, was conducted during my PhD time. This has led to two publications which are added in the appendix.