Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 110-113). Also available on the Internet.

Carbon-carbon bond forming reactions are vital to the synthesis of natural products and pharmaceuticals. In 2003, the 200 best selling prescription drugs reported in Med Ad News are all organic compounds. Synthesizing these compounds involves many carbon-carbon bond forming processes, which are not trivial and typically generate large amounts of waste byproducts. Thus, development of an atom economical and environmentally benign carbon-carbon bond forming methodology is highly desirable. Hydrogenation is one of the most powerful catalytic reactions and has been utilized extensively in industry. Although carbon-carbon bond forming reactions under hydrogenation conditions, such as, alkene hydroformylation and the Fischer-Tropsch reaction are known, they are limited to the coupling of unsaturated hydrocarbons to carbon monoxide. Recently, a breakthrough was made by the Krische group, who demonstrated that catalytic hydrogenative C-C bond forming reactions can be extended to the coupling partners other than carbon monoxide. This discovery has led to the development of a new class of carbon-carbon bond forming reactions. Herein, an overview of transition metal-catalyzed reductive couplings of [pi]-unsaturated systems employing various external reductants is summarized in Chapter 1. Chapters 2-4 describe a series of rhodium- and iridium-catalyzed asymmetric hydrogenative couplings of various alkynes to a wide range of imines and carbonyl compounds. These byproduct-free transformations provide a variety of optically enriched allylic amines and allylic alcohols, which are found in numerous natural products, and are used as versatile precursors for the synthesis of many biologically active compounds. Transfer hydrogenation represents another important class of reactions in organic chemistry. This process employs hydrogen sources other than gaseous dihydrogen, such as isopropanol. The Krische group succeeded in developing a new family of transfer hydrogenative carbon-carbon bond formation reactions. Chapter 5 presents two novel ruthenium- and iridium-catalyzed transfer hydrogenative carbonyl allylation reactions. The catalytic system employing iridium complexes enables highly enantioselective carbonyl allylation from both the alcohol and aldehyde oxidation level. These systems define a departure from the use of preformed organometallic reagents in carbonyl additions that transcends the boundaries of oxidation level. / text

Carbon-carbon bond forming reactions are fundamental transformations for constructing structurally complex organic building blocks, especially in the realm of natural products synthesis. Classical protocols for forming a C-C bond typically require the use of stoichiometrically preformed organometallic reagents, constituting a major drawback for organic synthesis on process scale. Since the emergence of transition metal catalysis in hydrogenation and hydrogenative C-C coupling reactions, atom and step economy have become important considerations in the development of sustainable methods. In the Krische laboratory, our goal is to utilize abundant, renewable feedstocks, so that the reactions can proceed in an efficient and atom-economical manner. Our research focuses on developing new C-C bond forming protocols that transcend the use of stoichiometric, preformed organometallic reagents, in which [pi]-unsaturates can be employed as surrogates to discrete premetallated reagents. Under transition metal catalyzed transfer hydrogenation conditions, alcohols can engage in C-C coupling, avoiding unnecessary redox manipulations prior to carbonyl addition. Stereoselective variants of these reactions are also under extensive investigation to effect stereo-induction by way of chiral motifs found in ligands and counterions. The research presented in this dissertation represents the development of a new class of C-C bond forming transformations useful for constructing synthetic challenging molecules. Development of transfer hydrogenative C-C bond forming reactions in the form of carbonyl additions such as carbonyl allylation, carbonyl propargylation, carbonyl vinylation etc. are discussed in detail. Additionally, these methods avoid the use of stoichiometric chiral allenylmetal, propargylmetal or vinylmetal reagents, respectively, accessing diastereo- and enantioenriched products of carbonyl additions in the absence of stoichiometric organometallic byproducts. By exploiting the atom-economical transfer hydrogenative carbonyl addition protocols using ruthenium and iridium, preparations of important structural motifs that are abundant in natural products, such as allylic alcohols, homoallylic alcohols and homopropargylic alcohols, become more feasible and accessible. / text

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Atomically thin group-VI transition metal dichalcogenides (TMDC) has been emerging as a family of intrinsic 2-dimensional (2D) crystals with a sizeable bandgap in the visible and near infrared range, satisfying numerous requirements for ultimate electronics and optoelectronics. This intrinsic 2D crystal also provides a perfect platform for physics study in 2D semiconductors. The characteristic inversion symmetry breaking presented in monolayer TMDCs leads to non-zero but contrasting Berry curvatures and orbital magnetic moments at K/K’ valleys located at the corners of the first Brillouin zone. These features provide an opportunity to manipulate electrons’ additional internal degrees of freedom, namely the valley degree of freedom, making monolayer TMDC a promising candidate for the conceptual valleytronics. Besides, the strong spin-orbit interactions and the subsequent spin-valley coupling demonstrated in 2D TMDCs open potential new routes towards quantum manipulation. In this thesis, I give a brief review on the background and our progress of the physics study in 2D TMDCs (MoS2, WS2) via optical spectroscopy. Particularly, our experimental approach on the excitonic effect, valley dependent circular dichroism, and the spin-valley coupling in monolayer and bilayer TMDCs are elaborated in individual chapters. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Environmental concerns have birthed an awareness of how we conduct ourselves as citizens of this planet. To reduce environmental impact, we have learned that we must be responsible stewards in all ranges of life: from buying locally grown food to how scientific research and industrial processes are executed. In the realm of chemical research, "green chemistry" has initiated the development of new, sustainable methods that make use of atom economy, step economy, and utilize renewable materials to minimize waste and production of toxic by-products. The formation of carbon-carbon bonds lies at the very heart of organic synthesis, and traditional methods for forming such bonds generally require the use of at least one stoichiometrically preformed organometallic reagent. This corresponds to at least one equivalent of metallic waste byproduct. The in situ formation of alkyl metal nucleophiles for carbonyl additions via hydrogenation of [pi]-unsaturates represents an alternative to use of preformed organometallic reagents. Comprising nearly 90% of the atoms in the universe, hydrogen is vastly abundant and very cheap. The Krische group seeks to contribute new technologies which make use of catalytic hydrogenation and transfer hydrogenation in the reductive coupling of basic chemical feedstocks. / text

published_or_final_version / Chemistry / Master / Master of Philosophy

In this dissertation, I present the results of my research regarding hybrid semiconductor interfaces between organic and inorganic semiconductors. Using photoemission spectroscopy, I elucidate the important role of defect-induced electronic states within the inorganic semiconductor phase. These states significantly affect both the energy level alignment and the charge carrier dynamics at the hybrid interface. I demonstrate that the behavior of these hybrid semiconductor interfaces is complex and not well characterized by current models for organic semiconductor interfaces. Specifically, I show that hybrid interfaces host unique electronic phenomena that depend sensitively on the surface structure of the inorganic semiconductor. I also demonstrate new applications of photoemission spectroscopies that enable the direct analysis of important properties of inorganic semiconductors, including charge carrier behavior near hybrid interfaces and the electronic character of defect-induced energy levels. The research presented here focuses on two different n-type inorganic semiconductors, tin disulfide (SnS₂) and zinc oxide (ZnO). SnS₂ is a layered transition metal dichalcogenide that presents an atomically flat and inert surface, ideal for sensitively probing electronic interactions at the hybrid interface. To probe the electronic structure of the SnS₂ surface, I used a variety of organic molecules, including copper phthalocyanine, vanadyl naphthalocyanine, chloro-boron subphthalocyanine, and C₆₀. ZnO has a complex surface structure that can be modified by simple experimental procedures; it was therefore used as a tunable semiconductor substrate where the effects of altered electronic structure can be observed. By carefully studying the origin of hybrid interfacial interactions, these research projects provide a first step in explicitly elucidating the fundamental mechanisms that determine the electronic properties of hybrid interfaces.

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