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Electron Transfer and Hydride Transfer Reactions of Copper Hydrides

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Electron Transfer and Hydride Transfer Reactions of Copper Hydrides Electron Transfer and Hydride Transfer Reactions of Copper Hydrides Michael Scott Eberhart Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2016 © 2016 Michael Eberhart All Rights Reserved Abstract Electron Transfer and Hydride Transfer Reactions of Copper Hydrides Michael Scott Eberhart Copper hydrides such as [Ph3PCuH]6 (Stryker’s Reagent) are textbook reagents in organic chemistry for the selective hydrogenation of α,β-unsaturated carbonyl compounds. Despite their widespread use both stoichiometrically and catalytically, there are many important questions about polynuclear copper hydrides that have not been answered. I have investigated the electron transfer chemistry of [Ph3PCuH]6 and related copper + hydrides. Copper hydrides (E1/2 = –1.0 to –1.2 V vs FcH/FcH ) are good one-electron reducing agents. Stopped-flow techniques have allowed the detection of electron transfer intermediates in copper hydride reactions. The fate of the copper containing products after electron transfer or hydride transfer + reactions has been investigated. An unusual cationic copper hydride, [(Ph3P)7Cu7H6] was found to be the major product of these reactions. Methods of converting this species back to [Ph3PCuH]6 have been investigated. The chemistry of this cationic species plays an important role in catalytic use of copper hydrides. Table of Contents List of Schemes, Figures and Tables iv 1 Chapter 1- Structure of Copper Hydrides 1 1.1 Introduction 1 1.2 Synthesis of Copper Hydrides 2 1.3 Solid State Structures of Copper Hydrides Ligated by Monodentate Phosphines 4 1.4 Solution Structure of Copper Hydrides 8 1.5 Low Nuclearity Copper Hydrides 15 1.6 Electronic Structure of Copper Hydrides - DFT Molecular Orbital Calculations 19 General Procedures 20 DFT Calculations 21 Synthesis of [(p-anisyl)3PCuH]6 21 i Synthesis of [ PrPh2PCuH]6 22 Synthesis of [BnPh2PCuH]6 22 Preparation of [(dppbz)CuH]3 Crystals for X-ray Diffraction 23 Improved Procedure for Preparing [(dppbz)CuH]3 24 Chapter 1 References 27 Chapter 2 - Electron Transfer from Copper Hydrides1 31 i 2.1 Motivation to study the electrochemistry of copper hydrides 31 2.2 Electron Transfer in Molecular Transformations 31 2.3 Electron Transfer for Energy Conversion 33 2.4 Determining Redox Potentials 35 2.5 Non-aqueous Redox Potentials 37 2.6 Voltammetry of Hexameric Copper Hydrides 38 2.7 Spectroelectrochemistry of [Ph3PCuH]6 45 2.8 Voltammetry of [(dppbz)CuH]3 46 2.9 Determining the Electron Transfer Self-Exchange Rate for [Ph3PCuH]6. 49 Cyclic Voltammetry 56 Chapter 3 Copper Hydride Reagents for Small Molecule Transformations 64 3.1 Introduction 64 3.2 Electron Transfer to Small Molecules 66 3.3 Electron Transfer to Pyridinium Cations 67 3.4 Attempts to Open Epoxides to Primary Alcohols 71 3.2 Is Electron Transfer Relevant in the Hydrogenation of α,β-Unsaturated Carbonyl Compounds? 77 3.3 Reactions of [Ph3PCuH]6 with Trityl Cations and Trityl Radicals. 83 Chapter 3 References 87 Chapter 4 – Regeneration of Copper Hydrides in Catalysis 93 ii 4.1 Introduction 93 4.2 Regeneration of Copper Hydrides following Electron Transfer 95 4.3 Catalytic Reduction of decamethylferrocenium 96 4.3 Structural Characterization of An Unexpected Cationic Hepta-copper Hydride 104 + 4.4 Chemical Reactions of the [(Ph3P)7Cu7H6] 106 Cyclic Voltammetry 110 Appendix – X-ray Structures 114 Figure A-1. Thermal ellipsoid (50% probability) plot for [(dppbz)CuH]3. 114 Table A-1. Crystal data and structure refinement for [(dppbz)CuH]3. 114 Figure A-2. Thermal ellipsoid (50% probability) plot for [(p-anisyl)3PCuH]6. 115 Table A-2. Crystal data and structure refinement for [(p-anisyl)3PCuH]6. 115 i Figure A-3. Thermal ellipsoid (50% probability) plot for [ PrPh2PCuH]6•THF. 116 i Table A-3. Crystal data and structure refinement for [ PrPh2PCuH]6•THF. 116 Figure A4. Thermal ellipsoid (50% probability) plot for [BnPh2PCuH]6•C6H6. 118 Table A-4. Crystal data and structure refinement for [BnPh2PCuH]6•C6H6. 118 Figure A-5. Thermal ellipsoid (50% probability) plot for [(Ph3P)7Cu7H6][B(C6F5)4]. 119 Table A-5. Crystal data and structure refinement for [(Ph3P)7Cu7H6][B(C6F5)4]. 119 iii List of Schemes, Figures and Tables Scheme 1.1. Synthesis of hexameric copper hydrides can be accomplished with inexpensive reagents and 1 atm H2. .................................................................................................................... 4 Scheme 1.2. Possible mechanism for the formation of copper hydride hexamers from lower nuclearity copper hydrides. ............................................................................................................. 6 Figure 1.1. Core Ellipsoid Plots for Hexameric Copper Hydrides and Selected Crystallographic Data. ................................................................................................................................................ 8 Figure 1.2. Extinction Coefficient Determination for [Ph3PCuH]6. .............................................. 9 Figure 1.3. 1H NMR showing the resonance of the hydride ligand in [Ph3PCuH]6 at 263 K (top) and 303 K (bottom). ...................................................................................................................... 11 1 Figure 1.4. H NMR Spectrum showing the hydride ligand resonance of [(p-anisyl)3PCuH]6 at 200 K. ............................................................................................................................................ 13 2 Figure 1.5. Caulton’s Cu2(μ-H)2[κ -CH3C(CH2PPh2)3]2 structure. ............................................. 16 Figure 1.6. Structures of the dimeric Bertrand36 and Sadighi10 Copper Hydrides ....................... 17 Scheme 1.3. The dimeric structure on the right is favored; there is no evidence for the structure on the left in solution or in the solid state. .................................................................................... 18 38 Figure 1.7. Frontier Molecular Orbitals for [H3PRhH2]6 (left) and [H3PCuH]6 (right). ........... 20 Figure 1.8. HOMO (left) and LUMO (right) Calculated for [H3PCuH]6 .................................... 20 3+ Scheme 2.1. Steps in the racemization of optically pure Co(en)3 . ............................................ 32 Figure 2.1. Potential as a function of time in a cyclic voltammetry experiment. ........................ 36 Figure 2.2. – Cyclic voltammogram of [Ph3PCuH]6. ................................................................... 39 iv Figure 2.3. Cyclic voltammogram of [tolyl3PCuH]6.................................................................... 40 Figure 2.4. Cyclic voltammogram of [(p-anisyl)3PCuH]6. .......................................................... 40 Figure 2.5. Increasing Scan rate causes an increase in ΔE for [Ph3PCuH]6. ............................... 41 Figure 2.6. Cyclic Voltammogram of [Ph3PCuH]6 showing second (irreversible) oxidation. .... 43 Figure 2.7. Cyclic Voltammogram of [tolyl3PCuH]6 showing second (irreversible) oxidation. 43 Figure 2.8. Cyclic Voltammogram of [(p-anisyl)3PCuH]6 showing multiple oxidations. ........... 44 +● Figure 2.9. [Ph3PCuH]6 with λmax = 655 nm. ......................................................................... 45 Figure 2.10. Spectroelectrochemical oxidation of [Ph3PCuH6]. .................................................. 46 Figure 2.11. Cyclic voltammogram of [(dppbz)CuH]3. ............................................................... 47 Figure 2.12. Cyclic voltammogram of [(dppbz)CuH]3 in 1,2-difluorobenzene. .......................... 49 Figure 2.13. Effects of temperature on the NMR line width of the hydride ligands in +● [Ph3PCuH]6/[Ph3PCuH]6 in THF-d8. ......................................................................................... 52 Figure 2.14. NMR line width of the hydride ligands in [Ph3PCuH]6 in THF-d8 with four different +● concentrations of [Ph3PCuH]6 . .................................................................................................. 53 +● Figure 2.15. Measured NMR line width as a function of concentration of [Ph3PCuH]6 in solution at 200 K. .......................................................................................................................... 54 +● Table 2.1. Rate constants for electron transfer self-exchange in [Ph3PCuH]6/[Ph3PCuH]6 at low temperature. .................................................................................................................................. 54 Figure 3.1. A possible structure for a mononuclear copper hydride [LCuH] (L = CH3C(CH2PPh2)3) ......................................................................................................................... 65 Scheme 3.1. Mechanistic pathways leading to different products from net hydride transfer to a N-acyl pyridinium cation. ............................................................................................................. 68 v Table 3.1. Dihydropyridine product ratios obtained when an N-acyl pyridinium cation is treated with various transition metal hydrides. ......................................................................................... 69 Figure 3.2. UV-vis spectra obtained from reaction of [Ph3PCuH]6 and N- carbophenoxypyridinium in a stopped-flow. ................................................................................ 71 Scheme 3.2. Electron transfer to a terminal epoxide should result in a primary alcohol while hydride transfer
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