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The Catalytic Decarbonylation of Unstrained Ketones Mediated by Platinum(II) Complexes

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The Catalytic Decarbonylation of Unstrained Ketones Mediated by Platinum(II) Complexes The Catalytic Decarbonylation of Unstrained Ketones mediated by Platinum(II) Complexes Julia Paula Sarju Doctor of Philosophy University of York Chemistry September 2016 Abstract Abstract Cyclohexanone, an unstrained ketone, was found to undergo decarbonylation in the presence of soluble platinum(II) complexes of the general formula [Pt(tolpy)Cl(L)] (where tolpy is 2-(4-tolyl)pyridine and L is a neutral ligand) to afford the platinum carbonyl complex [Pt(tolpy)Cl(CO)] as well as carbon monoxide, methane and butane. As such, the activation of the cyclohexanone must proceed via the cleavage of three carbon–carbon bonds. However, the stoichiometric balance of the reaction required additional hydrogen, which implied a coupled transfer hydrogenation step. As part of a mechanistic investigation, a number of novel cycloplatinated complexes were prepared and characterised and their ability to catalyse the decarbonylation reactions was investigated. Many of them were identified as active catalyst precursors and, in particular, this was found to be true for [Pt(tolpy)Cl(CO)], suggesting that the reaction is catalytic. It was commonplace for reactions to be accompanied by decomposition to what was assumed to be colloidal platinum. In addition to cyclohexanone, a range of other carbonyl-containing substrates were investigated and examples of cyclic and acyclic ketones as well as aldehydes were found undergo decarbonylation under the conditions employed. A mechanistic investigation was undertaken involving in situ spectroscopic studies, dynamic light scattering, deuterium labelling and mercury poisoning experiments. A mechanism for the decarbonylation of cyclohexanone is proposed whereby fragmentation and transfer hydrogenation take place to afford acetaldehyde, which then undergoes decarbonylation to afford methane. For the family of complexes of formula [Pt(tolpy)Cl(L)] prepared to study the mechanism, a combination of spectroscopic and computational techniques were employed to study the structural and bonding properties of the complexes and the relative trans-influence of the ligands. An ordered series for the trans- influence of the ligands was identified using bond lengths obtained from analyses of single crystal X-ray data and this trend was also consistent with quantum chemical calculations. The trend was also analysed for possible correlations with chemical shifts and coupling constants obtained from 1H, 13C{1H}, 15N and 195Pt{1H} NMR spectroscopy. 2 List of Contents List of Contents Abstract ………………………………………………………………………………..2 List of Contents ……………………………………………………………………... 3 List of Tables ….…………………………………………………………………….11 List of Figures .……………………………………………………………………...16 Acknowledgements ……………………………………….…………………….…29 Author’s Declaration ……………………………………………………………….30 Chapter One: Introduction to Decarbonylation Reactions .......................... 31 1.1.0 Decarbonylation ................................................................................... 31 1.2.0 Cleavage of C–C Single Bonds ............................................................ 31 1.3.0 Strategies for Promoting Decarbonylation Reactions ........................... 33 1.3.1 Ring Strain ........................................................................................ 33 1.3.2 Chelation/Coordination ..................................................................... 35 1.4.0 Active Substrates for Decarbonylation Reactions ................................. 36 1.4.1 Ketones ............................................................................................. 36 1.4.2 Aldehydes ......................................................................................... 39 1.4.3 Esters ................................................................................................ 40 1.5.0 Applications of Decarbonylation Reactions .......................................... 41 1.5.1 Synthesis of Natural Products ........................................................... 41 1.5.2 Sustainable Chemistry Applications of Decarbonylation ................... 44 1.6 Reaction Mechanisms Reported for Decarbonylation Reactions ............. 46 1.7 Conclusion ............................................................................................... 50 Chapter Two: Transition Metal Mediated Decarbonylation ......................... 51 3 List of Contents 2.1 Previous Work ..................................................................................... 51 2.1.1 Initial Study into the Decarbonylation Reactions ........................... 53 2.2 Aims ........................................................................................................ 55 2.3 The Decarbonylation of Cyclohexanone .............................................. 56 2.3.1 Head-space Analysis Methodology ............................................... 56 2.3.2 Identification of the Hydrocarbon Products .................................... 58 2.3.3 Quantification of Methane and Butane Produced by the Decarbonylation of Cyclohexanone ........................................................... 59 2.4 Identification of Solvent System for the Decarbonylation of Cyclohexanone Mediated by [Pt(tolpy)Cl(S-dmso)] (3) ................................. 60 2.4.1 Results of the Control Reactions ................................................... 61 2.4.2 Decarbonylation of Cyclohexanone in Different Solvents .............. 63 2.5 Substrate Scope ...................................................................................... 67 2.5.1 Cyclic Ketones .................................................................................. 67 2.5.2 Linear Ketones .................................................................................. 70 2.5.3 Ketones Substituted in the -Position ............................................... 71 2.5.4 Non-ketone Containing Substrates: Ethers, Ester, Aldehydes, Carboxylic Acids and Alcohols ................................................................... 74 2.5.5 Summary of the Results from the Substrate Scope Experiments ..... 76 2.6 The Activity of Different Metal Catalysts .............................................. 79 2.6.1 Variation of Ligand, L, for the Family of Platinum Complexes, [Pt(tolpy)Cl(L)] ............................................................................................ 79 2.6.2 PtII Complexes with other Chelating Ligands .................................... 82 2.6.3 Rhodium Complexes......................................................................... 83 2.6.4 Heterogeneous Catalysis .............................................................. 84 4 List of Contents 2.6.5 Comparison of Active Systems ......................................................... 84 2.7 Summary and Outlook ............................................................................. 86 Chapter Three: Mechanistic Investigation .................................................... 87 3.1 Introduction .............................................................................................. 87 3.1.1 Main Aims ......................................................................................... 87 3.1.2 Photochemical Decarbonylation Reactions ....................................... 88 3.1.3 Transfer Hydrogenation .................................................................... 90 3.1.4 Transition Metal Nanoparticle Catalysis ............................................ 93 3.2 Determination of the Source of Energy Required to Drive the Decarbonylation Reactions............................................................................ 98 3.2.1 Is the Decarbonylation of Cyclohexanone a Thermal or Photochemical Process? .................................................................................................... 98 3.2.2 Determination of the Onset Temperature .......................................... 98 3.3 Radical Process ...................................................................................... 99 3.4 Transfer Hydrogenation ......................................................................... 100 3.4.1 Addition of Transfer Hydrogenation Donors .................................... 100 3.4.2 UV-Vis Spectroscopic Analysis ....................................................... 102 3.4.3 Ketone Substrates Revisited ........................................................... 103 3.5 Deuterium Labelling Studies .................................................................. 106 3.5.1 Cyclohexanone ............................................................................... 106 3.5.2 Cycloheptanone .............................................................................. 111 3.5.3 Cyclooctanone ................................................................................ 112 3.6 Observation of New Platinum Species Formed During the Decarbonylation Reactions .................................................................................................... 113 5 List of Contents 3.6.1 Formation and Decarbonylation Activity of [Pt(tolpy)(-Cl)] (1) ....... 113 3.6.2 Formation and Decarbonylation Activity of [Pt(tolpy)Cl(H-tolpy)] (2) ................................................................................................................. 114 IV 3.6.3 Formation of [Pt (tolpy)2Cl2] (11) .................................................... 114 3.7 In Situ Infra-red (IR) Spectroscopy .................................................... 116 3.7.1 In Situ IR Spectroscopy of the Decarbonylation of
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