Browsing by Subject "Reductive Elimination"
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Item Carbon-Heteroatom Reductive Elimination and Catalysis Utilizing (POCOP)Rh and (POCOP)Co Complexes(2014-07-29) Timpa, Samuel DTransition metal catalyzed cross-coupling reactions of aryl halides have revolutionized the synthesis of organic molecules. These reactions, which are commonly catalyzed by group 10 metals, have found applications including natural product synthesis, pharmaceuticals, and agrochemicals. Pd catalyzed cross-coupling reactions have undergone the greatest development due to their wide applicability, high efficiency, and selectivity. The success of Pd is attributed to its ability to readily traverse between Pd(0) and Pd(II) oxidation states, which is essential to the mechanistic steps oxidative addition and reductive elimination. The utility of transition metals outside of group 10 has largely been limited to Cu, but more recently several examples of Rh catalyzed cross-coupling reactions have been described. These examples propose a Rh(I)/Rh(III) cycle analogous to the Pd(0)/Pd(II) catalytic cycle involving aryl halide oxidative addition, transmetallation, and product forming reductive elimination; however, there has been little experimental evidence to support these claims. Examples of aryl halide oxidative addition to Rh(I) have been reported, but examples of reductive elimination from Rh(III) are less prevalent. Pincer ligands, tridentate ligands that typically coordinate in a meridional fashion, provide an excellent scaffold for the examination of both oxidative addition and reductive elimination at Rh due to their ability to access to three-coordinate Rh(I) and stable five-coordinate Rh(III) complexes. The ability of the (PNP)Rh center to undergo each of the stoichiometric reactions of catalytic C-C coupling reactions, including aryl halide oxidative addition and C-C reductive elimination, has been established. This dissertation describes the ability of the (POCOP)Rh system to catalytically form C-C as well as C-N and C-S bonds. Several proposed catalytic intermediates have been isolated and their reactivity examined to gain insight into the mechanism of these catalytic transformations. C-N and C-S reductive elimination from Rh(III) have been closely examined, with results providing insight to their respective steric and electronic properties. In addition, the potential for (POCOP)Rh systems to undergo C-F reductive elimination were also examined both theoretically and experimentally. Finally, early investigations into the synthesis of (POCOP)Co complexes will be described, with an emphasis on demonstrating the aptitude for this system to experience concerted reductive elimination. Numerous (POCOP)Co complexes were isolated and characterized, including Co(II) and stable Co(III) compounds.Item Organometallic Chemistry Supported by the PNP Pincer Framework for Both Early and Late Transition Metals(2012-08-20) Brammell, Christina 1987-Tridentate "pincer" ligands provide a unique balance of stability and reactivity in organometallic chemistry. The development of diarylamido-based PNP pincer ligands has led to many applications in catalysis, including the potential to facilitate unique chemical transformations at transition metal centers. The main objective of this thesis was to explore transition metal chemistry supported by the PNP pincer framework for both early and late transition metals. In Chapter I, the history behind the design and synthesis of pincer complexes is described. The advantages and disadvantages of various pincer ligands are reviewed to show the reasoning behind the synthesis of the PNP pincer framework. Chapter II discusses the synthesis of novel Hf and Ta complexes involving the PNP ligand. Reactions of (PNP)HfCl3 with large alkyl Grignards led to double alkylation and triple alkylation was achieved with methyl Grignard. (PNP)HfMe3 and (PNP)Hf(CH2SiMe3)2Cl displayed remarkably irregular coordination environments about hafnium, in contrast to the approximately octahedral structure of (PNP)HfCl3. (PNP)HfMe3 was found to be thermally stable at 75 degrees C, whereas thermolysis of (PNP)Hf(CH2SiMe3)2Cl under similar conditions led to a mixture of products. The major decomposition product is believed to be a Hf alkylidene complex on the basis of in situ NMR spectroscopic observations (e.g., delta 248.2 ppm in the 13C{1H} NMR spectrum). The reaction of (PNP)TaF4 with an excess of ethyl Grignard led primarily to the double alkylation product, (PNP)Ta(CH2CH3)2F2. Repeating this reaction in the presence of excess ethyl Grignard and dioxane resulted in the formation of an ethylene complex, (PNP)Ta(=CHCH3)(C2H4). In Chapter III, a C-C reductive elimination study is described comparing two pincer ligand scaffolds: Me(PNP) ligand and TH(PNP) ligand. The tied ligand has previously been found to be more sterically demanding than the untied ligand, which has allowed for faster N-C cleavage, faster oxidative addition and a more selective alkyne dimerization catalyst. This study reveals that the tied ligand complex, TH(PNP)Rh(C6H4CF3)(Ph), undergoes slower reductive elimination of p-Ph-C6H4CF3 (< 4% after 7 h at 38 degrees C; t1/2 = 7.7 h at 64 degrees C; t1/2 = 2.13 h at 75 degrees C) than Me(PNP)Rh(C6H4CF3)(Ph) (t1/2 = 15.6 min at 38 degrees C).