Browsing by Subject "Hydrogenation"
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Item Carbon-carbon bond formation via catalytic hydrogenation and transfer hydrogenation : application in the total synthesis of bryostatin 7(2012-08) Lu, Yu, active 2012; Krische, Michael J.Under the conditions of transfer hydrogenation employing ortho-cyclometallated iridium C,O-benzoate catalysts, two protocols of iterative chain elongation of 1,3-diols to furnish 1,3-polyols were developed. First, one-directional chain elongation employing mono-protected 1,3-diols as starting materials was achieved. In all cases, high levels of catalyst-directed enantioselectivity and diastereoselectivity were observed. Then, double asymmetric allylation of 1,n-glycols to deliver C₂-symmetric adducts with exceptional level of enantioselectivity was devised. Iterative two-directional elongation of 1,3-diols to furnish 1,3-polyols with high level of catalyst-directed diastereoselectivity was then achieved. Implementation of this methodology and other hydrogenative C-C bond formations proved to be effective means for the preparation of a known bryostatin A-ring fragment and the total synthesis of bryostatin 7.Item Controlling selectivity in novel transition metal catalyzed carbon-carbon bond forming hydrogenations(2012-05) Zbieg, Jason Robert; Krische, Michael J.; Anslyn, Eric V.; Siegel, Dionicio R.; Brodbelt, Jennifer S.; Liu, Hung-WenThe focus of my graduate research in the Krische group has been the development of catalytic carbon-carbon bond forming reactions with an emphasis on controlling diastereo- and enatio-selectivity in transfer hydrogenative couplings. The broad goal of our research program has been the development and implementation of efficient green methods for carbonyl addition employing [pi]-unsaturates as surrogates to preformed organometallic reagents, thus enabling byproduct free variants of traditional carbanion chemistry. This dissertation shows the new reactions that I have developed toward this goal. These reactions includes new metal catalyzed approaches for carbonyl crotylation, aminoallylation, and vinylogous reformatsky aldol reactions.Item Density functional theory calculations in materials science and catalysis(2019-06-13) Chai, Wenrui; Henkelman, Graeme; humphrey, simon; Mullins, Charles B; hwang, gyeongAs scientists are pushing the limit of technology, experimental trial and error explorations are becoming increasingly unaffordable, especially when accurate manipulation of materials at the atomic level is essential. The need for theoretical oversight is ever increasing while traditional empirical theories are becoming obsolete. Density Functional Theory (DFT) is therefore developed as a portable theoretical tool that can shed light on many different fields based on first principle quantum mechanics. It is enjoying increasing popularity for its ability to provide information complementary to experimental characterization methods, as well as its astounding prediction power that can potentially vastly increases experimental output. In this dissertation, I showcase how DFT can be used to support experimental endeavors by providing insights and validations otherwise unobtainable and deepen our understanding of a variety of subjects. DFT calculations are also used to validate and explain experimental characterization results. For isolated Pt atom and Pt clusters with few atoms, the Pt dband center and hydrogen binding energy were calculated, used in conjunction with cyclicvoltammetry data to characterize the Pt atom/clusters and explain the observed activity towards hydrogen evolution reaction. For methodology development, DFT calculations are used to provide a plausible mechanism for the technique of hydrogen elimination monitoring in ultraviolet photodissociation of proteins for mass-spectroscopy to solve protein structures. It shows that depending on the degree of hydrogen bonding engagement, backbone cleavage can take place first and prevent succeeding hydrogen transfer. The results explained why the technique is a reliable method for finding protein structure information. Towards the development of materials, DFT calculations are used to find reaction mechanisms for hydrogenation using carbon-nitrogen-phosphorous pincer Fe catalysts and to find causes for geometry change for post-synthetically modified metal-organic frameworksItem Gold-surface-mediated hydrogenation chemistry(2013-05) Pan, Ming, active 2013; Mullins, C. B.High surface area catalysts have been studied and applied in a wide range of chemical reactions and processes. The related microscopic details of surface chemistry are important and can be effectively explored employing surface science techniques. My dissertation focuses on investigations of catalytic properties of gold, primarily using vacuum molecular beam techniques, temperature programmed desorption (TPD) measurements, reflection-absorption infrared spectroscopy (RAIRS), and density functional theory (DFT) calculations. I conducted fundamental studies of hydrogenation reactions on a H atoms pre-covered Au(111) single crystal surface with co-adsorption of various chemical compounds, including acetaldehyde (CH₃CHO), acetone (CH₃COCH₃), propionaldehyde (CH₃CH₂CHO), water (H₂O), and nitrogen dioxide (NO₂). These studies allow better understanding of hydrogenative conversions facilitated by gold catalysts, which show great promise in hydrogenation applications but for which relevant fundamental studies are lacking. The experimental results unravel the unique and remarkable catalytic activity of gold in hydrogenation reactions: i) H atoms weakly absorb on the Au(111) surface and have a low desorption activation energy of ~ 28 kJ/mol; ii) acetaldehyde can be hydrogenated to ethanol at a low temperature of < 200 K; iii) propionaldehyde can be hydrogenated to 1-proponal (CH₃CH₂CH₂OH) on H pre-covered Au(111) whereas 2-propanol (CH₃CH(OH)CH₃) cannot be formed in the reaction of acetone with hydrogen atoms; iv) a coupling reaction of aldehyde-aldehyde or aldehyde-alcohol is observed on the H pre-covered Au(111) surface at temperatures lower than 200 K and this reaction can produce various ethers (symmetrical or unsymmetrical) from aldehydes and alcohols with the corresponding chain length; v) co-adsorbed H atoms have a strong interaction with water on the gold model surface and induce the dissociation of the O-H bond in water, which cannot be dissociated on the clean surface; vi) we observed a facile reaction of NO₂ reduction on H covered Au(111) and NO is produced at 77 K, yielding high NO₂ (100 %) conversion and selectivity towards NO (100 %) upon heating the surface to ~ 120 K. These studies indicate the exceptional catalytic activity of gold and enhance the understanding of surface chemistry of classical supported Au-based catalysts at the molecular scale.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 Metal oxide support effects on the hydrogenation of cyclohexene and crotonaldehyde using microwave synthesized rhodium and iridium nanoparticles(2017-09-15) Polanco, Luis Ruben; Humphrey, Simon M.Nanoparticles (NPs) are an exciting new class of materials with unique physical and chemical properties, which have been studied for applications in semiconductors, drug delivery, heavy metal sequestration, and heterogeneous catalysis. The last decade has seen an exponential growth in noble metal NP catalysis research. The scarcity and price of these metals has created a need for more highly efficient catalysts and the surface-area-to-volume ratio of NPs can alleviate that demand. Highly selective catalysis is still dominated by homogeneous catalysts, but their lack of recyclability makes them unusable for industrial settings where durability is the top priority. The Humphrey group has pioneered the synthesis of monometallic, core-shell, and alloyed noble metal NPs of different sizes and morphologies, facilitated by microwave heating. However, support media effects have not been studied in the group, as strong metal-support interactions (SMSI) and hydrogen spillover have been shown to alter the observed catalytic activities. v Herein, mono metallic Rh NPs (~5nm) have been immobilized on amorphous metal oxides (SiO₂, Al₂O₃, TiO₂, Nb₂O₅, and Ta₂O₅) to study the effects these supports play in the hydrogenation of alkenes and the chemoselectivity hydrogenation of α,β-unsaturated aldehydes. Cyclohexene was utilized as a model alkene to assess the reactivities of said catalysts. In addition, controlled growth of Ir NPs in aqueous media is in development. Computational and experimental data has shown higher selectivity towards unsaturated alcohol products from the hydrogenation of α,β-unsaturated aldehydes and ketones. Unfortunately, not much research has been done Ir NPs due to their small particles sizes. Viscous solvents are typically used in NP synthesis to avoid particle agglomeration, but Ir since NPs don’t typically grow past 2 nm, other solvents can be used during synthesis. This allows the use of less viscous, greener solvents, such as water. Herein, the synthesis of Ir NPs in water is explored and the largest free-standing Ir NPs (2.98 nm) are presented. Also, 2.71 nm Ir NPs can be achieved after 1 minute, making them desirable for large sacel synthesis of these materials.Item Optimized synthesis of PdAu bimetallic nanoparticles using microwave irradiation as heterogeneous catalysts for compositional dependent hydrogenation reaction(2016-05-19) Kunal, Pranaw; Humphrey, Simon M.; Jones, Richard A.Synthesis of atom-efficient, cost-effective, and recyclable catalysts is quintessential for large scale production of chemicals of strategic importance. Expertise of Humphrey research group in microwave assisted synthesis of noble metal nanoparticles (NPs) has been implemented for synthesis of PdAu NPs, and a thorough investigation of their catalytic performance for cyclohexene hydrogenation was completed. Different reaction times were evaluated for their effects on extent of alloying and size distributions of NPs. Varying concentration of metal precursors used during the synthesis of Pd₅₀Au₅₀ alloy NPs led to appreciable size tunability. Microwave heating (μWI) was also effective in rapidly exchanging organic functionalization of the synthesized Pd₅₀Au₅₀ alloy NPs from a polymeric to monomeric capping agent. A comparison of μWI and conventional heating (CvH) methods showed that μWI led to superior PdAu alloy NPs with better monodispersity and size control. Using optimized reaction conditions, relative compositions of the two metals were controlled over a wide range. Synthesized PdAu alloy NPs of different compositions were thoroughly characterized using a variety of techniques and then tested for catalytic hydrogenation of cyclohexene using a continuous flow gas phase reactor. Pd59Au41 alloy NPs showed the best catalytic performance among all the catalysts studied. Continuing efforts towards further improving Pd-containing bimetallic NPs systems are currently made with the future goal of comparing catalytic performances of PdAu NPs with PdAg NPs in order to develop more economical and versatile catalysts.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 Synthesis of palladium-gold alloy nanoparticle catalysts for the reduction of nitrite in water(2016-05-06) Seraj, Sarah; Werth, Charles J.; Humphrey, Simon M.Hydrogenation using palladium-based (Pd-based) catalysts has emerged as a promising treatment method for nitrate in drinking water. However, low catalytic activity and longevity can be a barrier to widespread adoption over conventional treatment methods. Controlling catalyst structure at the molecular scale is one approach to improving catalytic activity and longevity. Intermetallic palladium-gold nanoparticle (PdAu NP) alloy catalysts of varying composition were synthesized for nitrite reduction using a polyol reduction method and microwave-assisted heating. The average size of PdAu NPs was 4.1 ± 2.2 nm. Enhanced nitrite reduction has been previously observed for Pd combined with Au in a core-shell NP structure, but has not been studied for intermetallic PdAu alloy NPs. Moreover, the mechanism by which Au enhances Pd-catalyzed nitrite reduction is not well understood. The PdAu NPs were loaded into an amorphous silica support and evaluated for nitrite reduction in a batch reactor. Reaction followed pseudo first-order kinetics for greater than 80% of conversion. Catalyst activity showed volcano-like behavior with varying composition .... All PdAu alloys were significantly more active for nitrite reduction compared to pure Pd NPs, despite Au being catalytically inactive for hydrogenation. Sulfide fouling and catalyst longevity studies were conducted. The presence of Au in the catalyst structure did not appear to enhance resistance to sulfide fouling. Moreover, catalyst activity was reduced upon repeated cycles of nitrite reduction. Further investigation is required to understand the mechanism for catalyst deactivation.Item Transition metal catalyzed C-C bond formation under transfer hydrogenation conditions(2013-05) Leung, Joyce Chi Ching; Krische, Michael J.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.Item Transition metal catalyzed carbonyl additions under the conditions of transfer hydrogenation(2011-05) Patman, Ryan Lloyd; Krische, Michael J.; Anslyn, Eric V.; Siegel, Dionicio R.; Brodbelt, Jennifer S.; Kerwin, Sean M.The efficient construction of complex organic molecules mandates that an assortment of methods for forming C-C bonds be available to the practicing synthetic chemist. The addition of carbon based nucleophiles to carbonyl compounds represents a broad class of reactions used to achieve this goal. Traditional methodology requires the use of stoichiometrically preformed organometallic reagents as nucleophiles in this type of reaction. However, due to the moisture sensitivity, excessive preactivation and inevitable generation of stoichiometric waste required for the use of these reagents, alternative methods have become a focus of the synthetic organic community. The research presented in this dissertation describes a new class of C-C bond forming reactions enabled through catalytic transfer hydrogenation. Here, the development and implementation of efficient green methods for carbonyl addition employing π-unsaturates as surrogates to preformed organometallic reagents is described. Additionally, this research describes the first systematic studies toward using alcohols as electrophiles in carbonyl allylation, propargylation and vinylation reactions.Item Transition metal catalyzed reductive couplings under hydrogenative and transfer hydrogenative conditions(2010-08) Williams, Vanessa Monet; Krische, Michael J.; Martin, Stephen F.; Magnus, Philip D.; Laude, David A.; Liu, Hung-WenEnvironmental 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.Item Transition metal-catalyzed reductive C-C bond formation under hydrogenation and transfer hydrogenation conditions(2008-12) Ngai, Ming-yu, 1981-; Krische, Michael J.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.Item Transition metal-catalyzed reductive C-C bond forming hydrogenation/transfer hydrogenation and applications in the total synthesis of (+)-roxaticin(2010-12) Han, Soo Bong, 1975-; Krische, Michael J.; Magnus, Philip D.; Cowley, Alan H.; Kerwin, Sean M.; Siegel, Dionicio R.By simply hydrogenating enones in the presence of aldehydes at ambient temperature and pressure, aldol adducts are generated under neutral conditions in the absence of any stoichiometric byproducts. Using cationic rhodium complexes modified by tri(2-furyl)phosphine, highly syn-diastereoselective reductive aldol additions of vinyl ketones are achieved. Finally, using novel monodentate TADDOL-like phosphonite ligands, the first highly diastereo- and enantioselective reductive aldol couplings of vinyl ketones were devised. These studies, along with other works from our laboratory, demonstrate that organometallics arising transiently in the course of catalytic hydrogenation offer byproduct-free alternatives to preformed organometallic reagents employed in classical carbonyl addition processes. Existing methods for enantioselective carbonyl allylation, crotylation and tert-prenylation require stoichiometric generation of pre-metallated nucleophiles, and often employ stoichiometric chiral modifiers. Under the conditions of transfer hydrogenation employing an ortho-cyclometallated iridium C,O-benzoate catalyst, enantioselective carbonyl allylations, crotylations and tert-prenylations are achieved in the absence of stoichiometric metallic reagents or stoichiometric chiral modifiers. Moreover, under transfer hydrogenation conditions, primary alcohols function dually as hydrogen donors and aldehyde precursors, enabling enantioselective carbonyl addition directly from the alcohol oxidation level.