Browsing by Subject "Transition metal catalysts"
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Item The Use of Soluble Polyolefins as Supports for Transition Metal Catalysts(2012-10-19) Hobbs, Christopher EugeneThe use of polymer supports for transition metal catalysts are very important and useful in synthetic organic chemistry as they make possible the separation and isolation of catalysts and products quite easy. These polymer-bound ligands/catalysts/reagents can, often, be recovered and recycled numerous times and typically yield products in high purity, negating the need for further purification steps (i.e. column chromatography). Because of this, interest in these systems has garnered international attention in the scientific community as being ?Green?. Historically, insoluble, polymer-supports (i.e. Merrifield resin) were used to develop recoverable catalysts. This has the advantage of easy separation and isolation from products after a reaction; because of their insolubility, such supported catalysts can be easily removed by gravity filtration. However, these catalysts often have relatively poor reactivity and selectivity when compared to homogeneous catalysts. Because of this disadvantage, our lab has had interest in the development of soluble polymer-supports for transition metal catalysts. We have developed several separation methods for these soluble polymer-bound catalysts. These include thermomorphic liquid/liquid and solid/liquid as well as latent biphasic liquid/liquid separation techniques. This dissertation describes the use of both, latent biphasic liquid/liquid separation systems and thermomorphic solid/liquid separation systems. In order to perform a latent biphasic iii liquid/liquid separation, a polymer-bound catalyst must have a very high selectivity for one liquid phase over the other. Our lab has pioneered the use of polyisobutylene (PIB) oligomers as supports for transition metal catalysts. Previous work has shown that these oligomers are > 99.96 % phase selectively soluble in nonpolar solvents. This has allowed us to prepare PIB-supported salen Cr(III) complexes that can be used in a latent biphasic liquid/liquid solvent system. The synthesis of these complexes is quite straightforward and such species can be characterized using solution state 1H and 13C NMR spectroscopy. Also, these complexes can be used to catalyze the ring opening of meso epoxides with azidotrimethylsilane (TMS-N3) and can be recovered and recycled up to 6 times, with no loss in catalytic activity. To perform a thermomorphic solid/liquid separation, a polymer-bound catalyst that is completely insoluble at room temperature but soluble upon heating must be used. Our lab has pioneered the use of polyethylene oligomers (PEOlig) as supports for transition metal catalysts. Such PEOlig-supported catalysts are able perform homogeneous catalytic reactions at elevated temperatures (ca. 65 ?C), but, upon cooling, precipitate out of solution as solids while the products stay in solution. This process allows for the easy separation of a solid catalyst from the product solution. Described herein, is the development of PEOlig-supported salen-Cr(III) complexes and PEOlig-supported NHC-Ru complexes. The preparation of these complexes is also straightforward and such species can be characterized using solution state variable temperature (VT) 1H and 13C NMR spectroscopy. In the case of the PEOlig-supported salen-Cr(III) complex, it was found to be a recoverable/recyclable catalyst for the ring opening of epoxides with TMS-N3 and could be reused 6 times with no loss in activity. The PE-supported NHC-Ru complex was able to be used as a recyclable ring closing metathesis (RCM) catalyst and could be used up to 10 times.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.