Browsing by Subject "Reduction (Chemistry)"
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Item Hydrogen-mediated carbon-carbon bond formations: applied to reductive aldol and Mannich reactions(2007) Garner, Susan Amy, 1980-; Krische, Michael J.Hydrogen gas is the cleanest and most cost-effective reductant available to mankind, and the use of hydrogen gas in catalytic hydrogenation reactions is one of the oldest and most utilized organic reactions. Although catalytic hydrogenation has been practiced in industry on enormous scale, the use of hydrogen gas as a terminal reductant in C-C bond forming reactions has been limited to processes involving the migratory insertion of carbon monoxide such as: alkene hydroformylation and the Fischer-Tropsch reaction. A significant advance to the field of synthetic organic chemistry would be the expansion of C-C bond forming reactions beyond reductive coupling via carbon monoxide insertion. Herein, related metal catalyzed reductive couplings to [alpha],[beta]-unsaturated compounds in the presence of reducing agents such as: silane, borane, and hydrogen are reviewed. The following chapters discuss the development of hydrogen-mediated reductive aldol and Mannich reactions. The results from this body of work clearly demonstrate that hydrogen-mediated C-C bond forming reactions are emerging as a powerful tool for synthetic chemists.Item Rh-catalyzed reductive coupling under hydrogenation conditions and nucleophilic catalysis via phosphine conjugate addition(2007) Kong, Jongrock, 1972-; Krische, Michael J.At the threshold of the 21st centry, a new set of challenges is defined by the need to develop sustainable means of preparing chemical commodities demanded by society. Hence, such concepts as atom economy, step economy, and 'green chemistry' have become the requirements for the development of synthetic reactions. Hydrogenation is one of the most powerful catalytic methods which successfully satisfy the stated requirements of modern chemistry. Accordingly, catalytic hydrogenation has been tremendously utilized in industrial settings. The profound impact of hydrogenation portended a powerful approach to reductive carbon-carbon bond formation under hydrogenation conditions, resulting in the discovery of the Fischer-Tropsch process and hydroformylation. However, since this discovery, processes have restricted to the incorporation of a single carbon monoxide unit. Even though there are a few seminal contributions, systematic efforts toward the development of hydrogen-mediated carboncarbon bond forming processes beyond hydroformylation have been absent from the literature. In an exciting advance, the Krische group has shown that it is possible to reductively couple two or more organic molecules simply through their exposure to gaseous hydrogen in the presence of a metal catalyst. This finding has led to the development of a broad, new family of hydrogen-mediated C-C bond formation. Herein, related to hydrogen-mediated C-C bond formation, the overview of metal catalyzed intermolecular reductive coupling in the presence of reducing agents such as borane, silane, alane, metal, and hydrogen is presented. Chapter 2 describes systematic approaches to the development of hydrogen-mediated C-C bond formation and successful preliminary results achieved by our research group. Chapters 3 and 4 will describe the further extension of these hydrogen-mediated C-C bond formations including (1) hydrogen-mediated reductive couplings of conjugated alkynes with iminoacetates, (2) hydrogen-mediated reductive couplings of 1,3-enynes with [alpha]-ketoesters, and (3) hydrogen-mediated multicomponent reductive couplings. The development of catalytic systems for the nucleophilic activation of enones using phosphine catalysts has received attractive attention. Recently, an intramolecular variant of the Rauhut-Currier reaction was developed in our lab. To further extend nucleophilic phosphine catalysis, we have sought to develop new catalytic methodology via phosphine conjugate addition. Chapter 5 describes two new methodologies related to their area: (1) catalytic cycloallylation via nucleophilic phosphine catalysis and (2) allylic amination of Morita-Baylis-Hillman acetates.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 Selective reduction of nitrogen dioxide by ammonia(Texas Tech University, 1972-12) Cheng, Yang-lehNot availableItem The role of proton donors, solvation, and additives in SmI2 mediated reduction of ketones(Texas Tech University, 2004-12) Chopade, Pramod RameshSamarium diiodide is a versatile reducing reagent commonly utilized in organic synthesis. A variety of reduction, reductive coupling, and sequential reactions are mediated by SmI2 with optimum reaction outcomes. However, little is known about its mode of action. A systematic investigation was carried out to unravel the mechanism of this extremely useful reagent. The first portion of this study deals with the role of commonly utilized proton donors such as alcohols and water in SmI2 mediated reduction reactions. A linear relationship between the acidity of the proton donor and the rate of acetophenone reduction was observed in the absence of complexation between a proton donor and SmI2. The proton source must be sufficiently acidic to protonate the carbanion intermediate formed upon initial reduction by SmI2. Water has a much higher affinity for SmI2 than the alcohols examined in this study. As a result, complexation between water and SmI2 was observed. Methanol was also found to complex to SmI2 at concentrations approaching 1 M. The unique reductants formed as a result of complexation between SmI2 and proton donor react with acetophenone through a mechanistically distinct pathway. The second portion is related to the role of HMPA in reduction of β-alkoxyketones utilizing SmI2. Addition of HMPA was found to increase reaction rates and affect stereochemical outcome of these reactions. Syn-1,3-Diols were predominantly the major products, However, addition of HMPA and presence of a bulky protecting functionality such as SiMcg-, resulted in reversal of stereoselectivity and provided the anti-\,3-diols as the major product due to inhibition of coordination. Chelation was found to play an important role in the stereoselectivity of these reduction reactions. The third portion deals with the role of solvation in SmI2 mediated reduction reactions. The preparation and reactivity of SmI2 in different solvents such as THF, acetonitrile, and DME was studied. SmI2 in these solvents are distinct species and reduce βhydroxyketones via different mechanistic pathways. SmI2-THF and SmI2-DME solvates gave predominandy the syn-l,3-âiols in excellent yields and diastereoselectivity. Their reactions follow similar pathways. SmI2-acetonitrile solvates however, reduced βhydroxyketones via a different mechanism and provided anti-l,3-diols in modest stereoselectivity. The concentration of methanol utilÍ2ed as proton donors was shown to have significant impact on reaction outcome in acetonitrile. Finally, the effect of triethylamine-water mixture on SmI2 mediated reduction of βhydroxyketones was studied. These reactions are usually completed in less than five minutes and quantitative yields of reduced products were obtained in high stereochemical control. The use of SmI2 in reduction of 1,4-hydroxyketones was also explored. Thus, mechanistic and synthetic aspects of SmI2 mediated reduction of ketones were studied.