Investigation Of Gold-vinyl Intermediates Opens New Avenues

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Investigation Of Gold-vinyl Intermediates Opens New Avenues

Transcript Of Investigation Of Gold-vinyl Intermediates Opens New Avenues

INVESTIGATION OF GOLD-VINYL INTERMEDIATES OPENS NEW AVENUES IN GOLD CATALYSIS
Dieter Weber
A dissertation submitted to the faculty of the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in the Department of Chemistry.
Chapel Hill 2012
Approved by Professor Michel R. Gagné Professor Cynthia K. Schauer Professor Maurice S. Brookhart Professor Jeffrey S. Johnson Professor Joseph L. Templeton

© 2012 Dieter Weber ALL RIGHTS RESERVED
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ABSTRACT
DIETER WEBER: Investigation of Gold-Vinyl Intermediates Opens New Avenues in Gold Catalysis
(Under the direction of Professor Michel R. Gagné)
The mechanism of gold(I)-catalyzed intramolecular hydroarylation of allenes was studied in detail. While monitoring the reaction by NMR, an unexpected dinuclear gold-vinyl intermediate was observed, that was proposed to contain a geminally diaurated carbon center stabilized by an aurophilic interaction. Isolation of this intermediate was achieved by inhibiting catalytic turnover with a base. Addition of suitable ligands abstracted one gold unit and yielded a monogold-vinyl complex, which was structurally confirmed by X-ray diffraction analysis. The reactivity of both intermediates was studied.
To better understand the formation of digold-vinyl intermediates, a variety of arylgold(I) model complexes were synthesized. A mixture of mono- and digold-aryl complexes revealed averaged proton signals by NMR. The chemical shift of these resonances could be used to determine the percentage of monogold bound as digold, which allowed for quantification of anion and ligand effects on digold formation. In addition, it was observed that Brønsted acids affected the coordinating ability of anions through homoconjugate acid/base pairs.
Trends observed in model systems were confirmed by mechanistic studies on the intramolecular hydroalkoxylation of allenes. Depending on the reaction conditions a digold or a monogold resting state was observed. Kinetic isotope effect studies and isotope labeling experiments were conducted to identify the catalytic role of digold. Data suggested that the
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digold intermediate operated off cycle and acted as an inactive catalyst reservoir, which seemed inhibitory for efficient catalysis. Further mechanistic studies revealed that digold formation altered kinetic data as well, which complicated the interpretation of Hammett studies and the development of rate laws.
It was also found that silver and palladium intercepted gold-vinyl intermediates as well. Silver affected the mechanism of the intramolecular hydroarylation of allenes, which could explain previously observed silver effects in gold catalysis. Organogold complexes could engage in cross-coupling chemistry by transmetallating their organic group to Pd. Further studies led to a palladium-catalyzed homo-coupling of gold-aryl model complexes was observed, which was driven by a bimetallic Au(I)/Pd(0) RedOx process. Other new avenues in gold catalysis include the Lewis acid mediated activation of silver-free catalyst precursors and a new mechanistic proposal for the origin of chirality in gold-catalyzed reactions.
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TO MY FAMILY AND FRIENDS
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ACKNOWLEDGEMENT “The things you do for yourself are gone when you are gone, but the things you do for others remain as your legacy” – Kalu Kalu With this quote I would like to express thanks to everyone that has supported me. I would not be at this point of my life without your constant love, service, and sacrifices. I will not mention specific names or institutions, but I will hold these blessings in my remembrance, and do my best to carry forward a legacy of genuine love and service for others.
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TABLE OF CONTENTS
LIST OF TABLES ................................................................................................................... xi LIST OF SCHEMES............................................................................................................... xii LIST OF FIGURES ............................................................................................................... xvi LIST OF ABBREVIATIONS AND SYMBOLS .................................................................. xxi
Chapter 1. The Element Gold and Gold Catalysis – an Introduction.................................1 1.1 General Properties of the Element Gold ..........................................................................1 1.2 Gold in Organometallic Chemistry ..................................................................................7 1.3 Gold in Homogenous Catalysis .....................................................................................19 1.4 Enantioselective Gold Catalysis ....................................................................................30 1.5 Observation of Intermediates in Gold Catalysis ............................................................35
Chapter 2. Previous Work in the Gagné Research Group.................................................41 2.1 The Development of New Gold-Catalyzed Reactions...................................................41 2.2 Proposed Mechanisms ...................................................................................................44 2.3 Research Objectives and Motivation .............................................................................46
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Chapter 3. Gold-Vinyl Intermediates in the Intramolecular Hydroarylation .................48 3.1 Introduction....................................................................................................................48 3.2 Monitoring Catalysis by 31P NMR.................................................................................50 3.3 Isolation and Characterization of Intermediates ............................................................52 3.4 Reactivity of Intermediates ............................................................................................62 3.5 Variation of Anions........................................................................................................65 3.6 Protodemetallation and Competition Experiments ........................................................66 3.7 Variation of Phosphine Ligands.....................................................................................68 3.8 X-ray Crystal Structure of Digold Intermediate ............................................................73 3.9 Probing Different Allenes for Digold Intermediates .....................................................77 3.10 Experimental Data .......................................................................................................80
Chapter 4. Gold-Vinyl Intermediates in the Hydroalkoxylation of Allenes.....................82 4.1 Introduction....................................................................................................................82 4.2 Results with JohnPhos (Widenhoefer Group) ...............................................................82 4.3 Results with Ph3P (Gagné Group)..................................................................................85 4.3 Experimental Data .........................................................................................................90
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Chapter 5. Model Complexes Lead to a Better Understanding of Digold Formation.....92 5.1 Introduction....................................................................................................................92 5.2 Studies with Gold-Vinyl Model Complexes..................................................................94 5.3 Studies with Gold-Aryl Model Complexes....................................................................97 5.4 Experimental Data .......................................................................................................109
Chapter 6. Mechanistic Role of Digold Intermediates in Gold Catalysis .......................110 6.1 Introduction..................................................................................................................110 6.2 Mechanistic Models .....................................................................................................111 6.3 Kinetic Isotope Effect Studies and Isotope Labeling Experiments .............................113 6.4 Theoretical Discussion.................................................................................................118 6.5 Kinetic Data .................................................................................................................124 6.6 Hammett Studies ..........................................................................................................136 6.7 Experimental Data .......................................................................................................144
Chapter 7. Silver Effect in the Intramolecular Hydroarylation of Allenes ....................146 7.1 Introduction..................................................................................................................146 7.2 The Observation and Isolation of a Gold-Silver Resting State....................................147 7.3 Structural Proposals .....................................................................................................150 7.4 Reactivity of Gold-Silver Complex .............................................................................151
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7.5 Relevance for Catalysis................................................................................................153 7.6 Experimental Data .......................................................................................................155 Chapter 8. Dual Gold/Palladium Catalysis .......................................................................156 8.1 Introduction..................................................................................................................156 8.2 Palladium-Catalyzed RedOx Driven Homocoupling...................................................158 8.3 Experimental Data .......................................................................................................165 Chapter 9. New Avenues in Gold Catalysis .......................................................................166 9.1 Introduction..................................................................................................................166 9.2 Origin of Chiral Induction in Gold Catalysis...............................................................166 9.3 Lewis Acid Mediated Catalyst Activation ...................................................................168 9.4 Enantioselective Cope-Rearrangement ........................................................................169 BIBLIOGRAPHY ................................................................................................................171
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DigoldAvenuesAllenesGold CatalysisStudies