Browsing by Subject "Dehalogenation"
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Item Degradatiom of environmental pollutants using rhodium hydrides.(2009-08) Peterson, Alicia AnnThe objective of this work was to study the dehalogenation of environmental pollutants mediated by rhodium hydrides. The product distribution and mechanism of dehalogenation was explored and the information obtained can possibly be applied to improve future remediation strategies. In Chapter 2, the dehalogenation of chlorinated and fluorinated ethylenes was explored using (PPh 3 ) 3 RhCl and Et 3 SiH, and counter-intuitively, vinyl fluoride was dehalogenated 6 times faster than vinyl chloride. This study established substrate scope and preferences for the Et 3 SiH and (PPh 3 ) 3 RhCl catalytic system. In Chapter 3, the mechanism for dehalogenation of chlorinated and fluorinated ethylenes was elucidated using H 2 as the reducing agent with the pre-catalyst (PR 3 ) 3 RhCl. These results were compared to those from using Et3SiH as the reducing agent. Dehalogenation using (PPh 3 ) 3 RhCl and either H 2 or Et 3 SiHsupport an insertion/β-chloride elimination mechanism; however the two systems display distinct differences. Based on these differences, the dominant pathway for Et 3 SiH is proposed to involve rhodium(I), while the H 2 system is proposed to primarily involve rhodium(III). In Chapter 4, a heterogeneous catalytic system using Rh/Al 2 O 3 as the catalyst and H 2 as the reducing agent was investigated. Consistent with the homogenous system of (PPh 3 ) 3 RhCl and H 2 , the data from this system also supports an insertion/β-Cl-elimination mechanism as the dominant degradation pathway. Ultimately, the goal of this work was to facilitate the preparation of engineered pump-and-treat strategies that will function to effectively degrade environmental pollutants to benign products with no halogen substituents.Item Radical involvement in cobalt- and nickel-mediated dehalogenation reactions.(2009-11) Kliegman, Sarah Isabella Morningstar .Halogenated chemicals represent a large and toxic class of environmental pollutants. Although regulation of certain halogenated organics resulted in decreasing production of these chemicals in the United States since the 1970s, others were yet unknown when the regulations were written and production is increasing. In many cases, halogenated organics are persistent in the environment, bioaccumulative, and bioactive, causing toxicological concerns. As such, environmental scientists have studied the processes by which these chemicals can be broken down, and the products that form in these breakdown reactions. In some cases, the toxic effects associated with halogenated organic pollutants can be ameliorated by complete dehalogenation, while incomplete dehalogenation or other transformations can result in the production of harmful compounds. The mechanisms of these transformations are in most cases not yet well understood, but a fundamental understanding of these reactions helps in the development of effective remediation strategies, and informs the fundamental chemistry inherent to these reactions. Concern about halogenated environmental pollutants has led to investigations of a number of means of dehalogenation including biological attenuation. Microbially mediated dehalogenation represents a major transformation pathway for halogenated pollutants in the environment. Metal-containing cofactors have been implicated in these processes including cobalamin (vitamin B12), factor F430, and hemitin. These cofactors are responsible for the reductive dehalogenation of environmental pollutants. These reactions can proceed by a various intermediates, but one of particular interest is the formation of radicals. Radicals have at least one unpaired electron, and as such are highly reactive and transient intermediates. These features can make them difficult to study but their powerful reactivity underscores their importance in environmental transformations. Radical intermediates are often proposed but rarely fully understood in a range of environmental systems. In this thesis, the role of radicals in dehalogenation reactions is explored with particular attention to cobalamin-mediated and nickel-mediated reactions. The mechanism of cobalamin-mediated dechlorination has been studied extensively and evidence for both outer-sphere (radical based) and inner sphere (nonradical based) mechanisms has been presented. In this thesis the literature concerning cobalamin-mediated dehalogenation is reviewed in detail (Chapter 1) and a mechanistic study on the role of radicals in cobalamin-mediated dechlorination of chloroethylenes reconciles previously seemingly contradictory data (Chapter 2). Similarly, both radical and nonradical pathways have been invoked in nickel-mediated dehalogenation of a variety of substrates. Nickel-mediated dehalogenation has not been studied as extensively as cobalt-mediated reactions and the understanding is complicated by the fundamental chemistry of nickel complexes. In order to better understand the chemistry of reduced nickel complexes, particularly their reaction with halogenated organics, a series of nickel complexes was synthesized and characterized (Chapter 3). The relationship between reduced transition metal complexes and their ligands is inextricably linked to whether and how radical intermediates are formed in these systems. The reactivity of two reduced nickel complexes precursors show that these complexes are highly sensitive to slight changes in ligand structure (Chapter 4).