Browsing by Subject "Addition reactions"
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Item Role of heteroatom chelation in addition and reduction reactions(Texas Tech University, 2004-08) Davis, ToddThe ability to control stereochemistry through transition metal chelation has been under investigation since Cram's seminal work in the 1950's. Chelation-control can bring about organization in the transition state or intermediate of a reaction. Due to the high degree of order in a chelated transition state or intermediate, high diastereoselectivities and enantioselectivities are observed in the product distribution. The research presented herein focuses in two areas: (1) The ability of lanthanides, in particular samarium diiodide (Sml2), to promote chelation-control in the reduction of â-hydroxyketones, and (2) The ability of fluorine to act as a template for chelation-control synthesis. Since 1980, the use of lanthanides, especially samarium diiodide, has found a unique role in synthetic organic chemistry. Although Sml2 has been shown to mediate a variety of organic transformations, the mechanistic details on how these reactions occur is still under investigation. The reduction of â-hydroxyketones using Sml2 has recently been discovered as an efficient method to synthesize anft-l,3-diols in high yields and diastereoselectivities. This research investigates the mechanistic details of this efficient reduction of â-hydroxyketones. The role of substrate, solvent, and proton source has been investigated to determine the optimal conditions for reduction of â-hydroxyketones with high diastereoselectivity. The second part of this work focuses on the ability of fluorine to act as a template for chelation-control synthesis. Fluorine containing organics have become of importance to medicinal chemists and the pharmaceutical industry. The introduction of fluorine into a pharmaceutical agent has been shown to have dramatic increases in the biological activity in comparison to the non-fluorinated analogue. Although these molecules display increased biological activity, the synthesis of fluorine containing substrates is extremely difficult and costly. The focus of this project was to determine the ability of electronegative fluorine to interact with a Lewis acid and promote a chelated transition state or intermediate. Our investigations have shown that fluorine can chelate to lithium and titanium containing Lewis acids to yield reduction and addition to pendant a-carbonyls in high yields and diastereoselectivities. Excellent diastereoselectivity was observed in the reduction and addition reactions of a - and â-fluoroketones providing products through a chelated intermediate or transition state. This study provides evidence that fluorine is an excellent facilitator for chelation-controlled synthesis..Item Theoretical and experimental studies on the reactivities of conjugated ketenes(Texas Tech University, 1998-08) Ham, SihyunThe reactivity of imidoylketene was examined using ab initio molecular orbital theory. MP4(SDQ)/6-31G*//MP2/6-31G* calculations on the conformations of imidoylketene as well as transition states for several of its reactions show parallels between the reactivity of imidoylketene and its oxygen analog formylketene. All reactions proceed via concerted, planar (or nearly so) transition structures regardless of the number of electrons involved. Calculated activation energies are remarkably lower than those for a pericyclic process, as expected from the case of formylketene. The reactions are interpreted in light of their pseudopericyclic orbital topology. N-Propylacetacetimidoylketene was produced by the solution pyrolysis of t-butyl N-propyl 3-amino-2-butenoate. Selectivities of acetimdoylketene toward various polar reagents were measured for the first time in a series of competitive trapping reactions. Significant steric and electronic discriminations of this ketone were observed, suggesting further synthetically useful applications. These experimental reactivity trends indirectly provide support for the planar, pseudopericyclic transition structures predicted by ab initio calculations. The mechanism of the reactions of nitrosoketene to form cyclic nitrones (which leads stereoselective synthesis of a-amino acids) was investigated using ab initio molecular orbital theory (MP4(SDQ)/6-31G*//MP2/6-31G* + ZPE). The direct [3+2] cycloadditions of nitrosoketene with ketones are calculated to be favored over the alternative [4+2] pathway via concerted, asynchronous, pseudopericyclic transition states. The detailed conformations and the reactivity of nitrosoketene toward sterically and electronically different ketones render useful information of the synthetic route for the biologically important reactions. Transition structures for a series of eight cheletropic decarbonylations were optimized at the MP4(SDQ)/D95**//MP2/6-3lG* + ZPE level. Dramatic differences in activation energies and in exothermicities are discussed in terms of the molecular orbital topology. A fundamental question regarding pseudopericyclic orbital overlap is addressed, specifically, how many and what type of orbital orthogonahties in the reaction sites are needed for a reaction to be pseudopericyclic. Generalizations regarding the characteristics of the pseudopericyclic reactions are made to provide a better understanding of the "allowedness" and "favoredness" of the orbital topologies.Item Titanium (IV) halide mediated Baylis-Hillman and related reactions(Texas Tech University, 2001-05) Gao, JuNot available