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The First General Electron Transfer Reductions of Carboxylic Acid Derivatives Using Samarium Diiodide

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The First General Electron Transfer Reductions of Carboxylic Acid Derivatives Using Samarium Diiodide The First General Electron Transfer Reductions of Carboxylic Acid Derivatives Using Samarium Diiodide A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy In the faculty of Engineering and Physical Sciences 2014 Malcolm Peter Spain School of Chemistry Malcolm Spain PhD Thesis Contents Abstract .................................................................................................................... 5 Declaration................................................................................................................ 6 Copyright statement .................................................................................................. 7 Acknowledgements ................................................................................................... 9 Abbreviations .......................................................................................................... 10 Chapter 1. Introduction ........................................................................................... 13 1.1 Introduction to samarium diiodide .................................................................. 13 1.2 Reduction of ketones and aldehydes ............................................................. 15 1.3 Reduction of carboxylic acid derivatives ....................................................... 23 Chapter 2. Investigations of the SmI2–H2O system ................................................. 27 2.0 Preliminary studies of the SmI2–H2O system ................................................. 27 2.1 Directing group effects ................................................................................... 31 2.1.1 Optimisation ............................................................................................ 32 2.1.2 Rates of directed reductions.................................................................... 34 2.1.3 Substrate scope ...................................................................................... 36 2.1.4 Mechanistic studies ................................................................................. 38 2.2 Mechanistic studies of lactone reduction ....................................................... 42 2.2.1 Dependence of lactone reduction on water concentration ....................... 43 2.2.2 Dependence of radical stability upon water concentration ....................... 48 2.3 Aromatic reductions ....................................................................................... 58 2.3.1 Determination of the effective redox potential of SmI2–H2O systems ...... 59 2.3.2 Optimisation of hydrocarbon reductions .................................................. 61 1 Malcolm Spain PhD Thesis 2.3.3 Mechanism of hydrocarbon reductions .................................................... 63 2.3.2 Substrate scope ...................................................................................... 68 2.4 - Reductions of barbituric acids ..................................................................... 70 2.4.1 Optimisation ............................................................................................ 71 2.4.3 Mechanistic studies ................................................................................. 73 Chapter 3. Investigations of the SmI2–amine–H2O system ..................................... 78 3.1 Ester Reduction ............................................................................................. 78 3.1.1 Ester reduction optimisation studies ........................................................ 79 3.1.2 Substrate scope of the ester reduction .................................................... 82 3.1.3 Mechanism of ester reduction ................................................................. 86 3.2 Acid Reduction .............................................................................................. 96 3.2.1 Acid reduction optimisation studies ......................................................... 96 3.2.2 Substrate scope of the acid reduction ..................................................... 97 3.2.3 Mechanism of acid reduction................................................................. 100 Chapter 4. New divalent lanthanide complexes ..................................................... 107 4.1 Effective redox potential of TmI2–MeOH ...................................................... 107 4.2 The role of proton donors ............................................................................ 108 4.3 TmI2-mediated reduction of carboxylic acid derivatives ............................... 110 Chapter 5. Synthesis of samarium diiodide ........................................................... 116 5.1 Investigation of commercially available samarium(II) iodide solutions .......... 117 5.2 Preparation of samarium diiodide solutions ................................................. 120 5.2.1 Methods of preparation ......................................................................... 120 5.2.2 The influence of water ........................................................................... 121 2 Malcolm Spain PhD Thesis 5.2.3 The influence of oxygen ........................................................................ 122 5.2.4 The influence of peroxides .................................................................... 123 5.2.5 Preparation procedure .......................................................................... 124 5.2.6 Imamoto’s procedure ............................................................................ 125 5.2.7 Effect of the quality of samarium metal on the preparation of samarium(II) iodide ............................................................................................................. 126 5.2.8 Activation of samarium metal ................................................................ 127 5.2.9 Stability of solutions of samarium(II) iodide ........................................... 130 5.3 Reduction of Meldrum’s acids (practical protocol for the use of SmI2–H2O Solutions) .......................................................................................................... 132 Chapter 6. Conclusions and Future Work ............................................................. 134 Chapter 7. Experimental ....................................................................................... 135 General experimental ........................................................................................ 135 Chapter 2. SmI2–H2O ........................................................................................ 138 Chapter 2.0 Preliminary studies of the SmI2–H2O system .............................. 138 Chapter 2.1 Directing group effects ................................................................ 139 Chapter 2.3 Mechanistic studies of lactone reduction using SmI2–H2O .......... 159 Chapter 2.4 Aromatic Reductions .................................................................. 169 Chapter 3. SmI2–amine–H2O ............................................................................. 175 Chapter 3.1 Ester Reduction .......................................................................... 175 Chapter 3.2 Acid Reduction ........................................................................... 195 Chapter 4. New divalent lanthanide complexes ................................................. 211 Chapter 5. Synthesis of samarium diiodide........................................................ 214 3 Malcolm Spain PhD Thesis References ........................................................................................................... 217 4 Malcolm Spain PhD Thesis Abstract The University of Manchester School of Chemistry Malcolm Peter Spain Doctor of Philosophy The First General Electron Transfer Reductions of Carboxylic Acid Derivatives Using Samarium Diiodide 2014 The development of new methods for the reduction of carboxylic acid derivatives is described. The ability to reduce these carbonyl derivatives through radical intermediates provides an orthogonal approach as compared with hydride based reductions. Initial experiments focused on the development of the SmI2–H2O system, where we have shown that chelation effects can be utilised to facilitate reduction of cyclic esters. Furthermore, a revised mechanism for the SmI2–H2O mediated reduction of lactones is discussed, and the effective reduction potential of the system determined. Also described is the optimisation of barbituric acids using SmI2–H2O to give the corresponding hemiaminal product. Next, experiments towards the development of a more reactive SmI2-based system are presented; where we have demonstrated that the SmI2–amine–H2O system is capable of the reduction unactivated carboxylic acid derivatives. The reductions of carboxylic esters and acids are described with mechanistic discussions. In addition, the design of a new divalent lanthanide system based on thulium diiodide is described. The addition of proton sources to TmI2 increases the effective reduction potential and facilitates unprecedented reactivity with amides. An investigation into the preparation of the reagent is also described, which has been one of the key factors developing all of the chemistry presented. 5 Malcolm Spain PhD Thesis Declaration No portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification at this or any other university or other institute of learning. Part of this work
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