Theses and Dissertations - Department of Chemistry & Biochemistry

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Browsing Theses and Dissertations - Department of Chemistry & Biochemistry by Author "Arduengo III, Anthony J."

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    Fluoroalkoxy-functionalized carbenes for main group and transition metal complexes
    (University of Alabama Libraries, 2011) Runyon, Jason W.; Arduengo III, Anthony J.; University of Alabama Tuscaloosa
    This dissertation recounts the synthesis and use of imidazol-2-ylidenes for their applications as ligands for main group elements and transition metals and as sterically hindered Lewis bases for hydrogen activation. Chapter 1 reviews the background of carbenes as ligands for main group compounds and transition metal complexes. The history and development of chelating Martin-type fluoroalkoxy ligands to form unique bonding architectures is covered. The synthetic strategies employed to functionalize carbene ligands with fluoroalkoxy substituents is reviewed leading to the development of stable fluoroalkoxy imidazolium zwitterions. A series of new ionic fluoroalkoxy imidazol-2ylidenes is described along with a comparison of the electronic properties to neutral imidazol-2-ylidenes. These fluoroalkoxy imidazol-2-ylidenes were used to prepare chelated hypervalent main group compounds. The structural characterization of the hypervalent adducts is described along with effects due to chelation. Additionally, the characterization of unique fused heterocyclic byproducts formed from unstable main group chelates is reviewed. The synthesis and characterization of transition metal complexes utilizing these ligands are described. Special emphasis is focused on catalytically active transition metals complexes and the influence of the tridentate ligand on catalyst activity. This ligand was also used to allow for the stabilization of iron benzylidene complexes which may have applications towards olefin metathesis. Chapter 2 describes work on hydrogen activation as a project with the U.S. Dept. of Energy's Alabama Chemical Hydrogen Storage Initiative. This project takes advantage of sterically hindered Lewis acid-base pairs to activate small molecules. A wide range of main group element Lewis acids were explored with a variety of mixed results. "Abnormal" coordination of Lewis acids to the carbene backbone results in structures with unique properties and reactivity. These systems were utilized with the goal of reversible heterolytic hydrogen cleavage followed by hydrogen elimination under another set of circumstances.
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    Structure, heats of formation, and bond dissociation energies of group IIIa-group IVA-group VA molecules for chemical hydrogen storage systems
    (University of Alabama Libraries, 2010) Grant, Daniel Justin; Dixon, David A.; University of Alabama Tuscaloosa
    The potential of Group IIIA—IVA—VA compounds for chemical hydrogen storage have been evaluated from thermodynamic properties, heats of formation and bond dissociation energies (BDEs), from CCSD(T) calculations in conjunction with correlation consistent basis sets extrapolated to the complete basis set limit, including additional core-valence, scalar-relativistic, and atomic spin-orbit corrections. Geometry optimizations and frequencies were computed at the CCSD(T)/MP2 levels. Diatomic distances, frequencies, and anharmonic constants were obtained from a potential energy curve fit at the CCSD(T) level. Calculations show that AlH_3NH_3(g), AlH_3PH_3(g), [AlH_4^-][NH_4^+](s), [AlH_4^-][PH_4^+](s), and [BH_4^-][PH_4^+](s) can potentially serve as hydrogen storage systems, in addition to BH_3NH_3 and [BH_4^-][NH_4^+](s). Dehydrogenation of methyl-substituted ammonia boranes is most favorable across B-N where methylation at N reduces the reaction exothermocity, becoming more thermoneutral. The adiabatic π-bond energy is defined as the rotational barrier between the ground state and C_s transition state structures, the intrinsic π-bond energy as the adiabatic rotational barrier corrected for inversion, and σ-bond energy, as the adiabatic dissociation energy minus the adiabatic π-bond energy. Within the substituted boranes H_(3-n)BX_n (X = F, Cl, Br, I, NH_2, OH, and SH), fluorines have the largest BDEs while the second and third largest are for hydroxyl and amino. Hydride and fluoride affinities have been predicted to judge the Lewis acidities with the highest affinities found for BI_3, lowest for B(NH_2)_3, and within the boron trihalides, the acidity increases down the periodic table. Although the sequential dehydrogenation of diammoniosilane is exothermic, further dehydrogenation is largely endothermic, requiring an effective coupling process to remove three hydrogen molecules thermoneutrally. Except for methyliodosilane, methyl and halide substitution increases the Si-X and Si-C BDEs compared to the halosilanes and methylsilane, respectively. The differences in the adiabatic and diabatic BDEs in the PF_xO and SF_xO compounds are employed to explain trends in their stepwise BDEs. The adiabatic BDE for removal of fluorine from stable closed-shell SF_6 to give the unstable SF_5 radical is 2.8 times the BDE for removal of fluorine from the unstable SF_5 radical to give stable closed-shell SF_4. Simlar principles govern the BDEs of the phosphorous fluorides and the phosphoro and sulfur oxofluorides.
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    Synthetic applications of (E)-α-trialkylsilyl-α,β-unsaturated esters
    (University of Alabama Libraries, 2017) Johnson, David Allen; Jennings, Michael P.; University of Alabama Tuscaloosa
    This dissertation details the use of (E)-α-trialkylsilyl-α,β-unsaturated esters for three novel methodologies. This document is divided into four chapters. The first chapter will relate pertinent background information about enolates and extended dienolates that will be revisited in subsequent chapters. The second chapter will recount a γ-deprotonation-α-protonation sequence resulting in (E)-α-trialkylsilyl-β,γ- unsaturated esters. It explores potential reasons for poor a-regioselective protonations present in the literature. The intermediate extended dienolate was also trapped which allowed for confirmation of its stereochemistry. A method to transform (Ε)-α-trialkylsilyl-α,β-unsaturated esters into chiral allyl silanes will be examined in the third chapter. This will involve a Cu(I) catalyzed conjugate addition involving Grignard reagents followed by a diastereoselective protonation. This is important because α-silyl-α,β-unsaturated esters have a scant record in the literature as Michael acceptors. Finally, the last chapter will relate the culmination of work performed in chapter two and direct application of ideas presented in chapter one, which consists of using the extended dienolate with known sterochemistry for a tandem diastereoselective aldol- Peterson olefination process. These methodologies will provide much needed light on the synthetic utility of (E)-α-trialkylsilyl-α,β-unsaturated esters.