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Exploring the reactivity of nickel in the formation and scission of C-X (X = C, N, O, S) bonds Chiu, Wei Ling

Abstract

In this Thesis, we explore the reactivity of organometallic complexes of nickel towards activating bonds in esters, thioesters, and oxaziridines, as well as study the mechanism by which nickel mediates the trifluoromethylthiolation of aryl chlorides. Chapter 1 delves into modern developments in activating and forming C–F bonds using group 10 transition metal complexes. Emphasis is placed on the challenges and strategies employed, such as ligand design and additive choice, to achieve selective C-F bond activation and formation. In Chapter 2, we explore the reactivity of a low valent nickel complex with esters and thioesters. Stoichiometric cross-coupling of thioesters proceeds via initial C(acyl)-S bond insertion by nickel, followed by transmetallation with phenylboronic acid to form thioethers. Esters react via C(aryl)-O bond insertion and transmetallation to afford heterobiaryls. We discuss how our attempts at rendering the above transformations catalytic led to the identification of nickel complexes that prevent catalytic turnover. We also collaborate with the Kennepohl group at UBC to study the degree of Ni(0) versus Ni(II) character in a series of bisphosphine nickel complexes, obtaining evidence that the bonding in the Ni π-complexes exhibits major backbonding from the bisphosphine ligand with minor contributions from σ-bonding. In Chapter 3, we show that nickel can cleave the weak N–O bond in oxaziridines to generate a variety of organonickel complexes. We study the fragmentation of an oxazanickelacyclobutane and test the potential catalytic applications of the complexes toward the synthesis of amides and oxazoles. However, we observe the formation of nickel complexes that are thermodynamic sinks which impede catalysis. In Chapter 4, we seek to uncover the mechanism of the Ni-catalyzed trifluoromethylthiolation of aryl chlorides using AgSCF₃. using a combination of stoichiometric experiments, cyclic voltammetry, and NMR spectroscopic studies. We identify a major side reaction that forms a nickel fluoride complex and obtain evidence that suggests a pathway involving inner sphere electron transfer from silver to nickel. By studying the reactivity of nickel complexes and their roles in catalytic reactions, we hope to inform scientists on improved ligand design, substrate choice, and reaction conditions that would lead to more efficient catalysis.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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