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APPLICATION of Pd-PEPPSI COMPLEXES in REGIOSELECTIVE CROSS-COUPLING and NATURAL PRODUCT TOTAL SYNTHESIS

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APPLICATION of Pd-PEPPSI COMPLEXES in REGIOSELECTIVE CROSS-COUPLING and NATURAL PRODUCT TOTAL SYNTHESIS PART I: APPLICATION OF Pd-PEPPSI COMPLEXES IN REGIOSELECTIVE CROSS-COUPLING AND NATURAL PRODUCT TOTAL SYNTHESIS PART II: ROOM TEMPERATURE, ADDITIVE-FREE CARBON-SULFUR COUPLING USING NOVEL NHC-Pd COMPLEXES JENNIFER LYN FARMER A DISSERTATION SUBMITTED TO THE FACULTY OF GRADUATE STUDIES IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY GRADUATE PROGRAM IN CHEMISTRY YORK UNIVERSITY TORONTO, CANADA DECEMBER, 2015 © JENNIFER LYN FARMER, 2015 ABSTRACT The first part of this research is focused on the development of an efficient method for the regioselective Suzuki-Miyaura cross-coupling of allylboronic acid pinacol ester derivatives with aryl and heteroaryl halides using Pd-PEPPSI-IPent (Pyridine Enhanced Pre-catalyst Preparation Stabilization and Initiation) that would be selective for the linear product. Pd-PEPPSI-IPent showed high selectivity (>97%) at the α−carbon of the allylboron reagent to generate the desired linear isomer under mild reaction conditions (i.e., THF, 70 °C, 5M KOH). In the case of trisubstituted allylboronates with different substituents on the olefin, minor olefin geometry isomerization was observed (E/Z ≈ 80/20). Using this method, isoprenylated natural products methylated cathafuran A and lakoochin A were synthesized. Although the prenyl and geranyl side chains could be installed onto advanced 2-arylbenzofuran building blocks with high α−selectivity, significant amounts of an unwanted side product were observed. A dramatic solvent effect on the selectivity for the Directed ortho Metalation (DoM) of 1,5-dichloro-2,4-dimethoxybenzene, a key building block in our synthetic strategy to cathafuran A and lakoochin A, was observed. A series of time-course and labelling studies has revealed that deprotonation occurs exclusively at C3 followed by isomerization of the anion to C6 in THF. In contrast, when DoM was performed in Et2O, deprotonation again occurred selectively at C3, but now no isomerization occurs. Labeling studies also revealed that deuterium has an enormous kinetic isotope effect ii (KIE) that suppresses not only the original DoM reaction at C3 when deuterium is present there, but also suppresses isomerization to C6 when the label is at that site. The second part of this research focused on the development and application of new, easily activated NHC–PdII pre-catalysts featuring a trans-oriented morpholine ligand in carbon-sulfur cross-coupling chemistry. [(IPent)PdCl2(morpholine)] was identified as the most active pre-catalyst, effectively coupling a variety of deactivated aryl halides with aryl, alkyl, and silyl thiols at ambient temperature. Mechanistic studies revealed that, in contrast to other common NHC–PdII pre-catalysts, these complexes are rapidly reduced to the active NHC–Pd0 species in the presence of KOtBu, thus avoiding the need for additives, external activators, or cumbersome pre-activation steps. iii DEDICATION This dissertation is dedicated to my late grandparents: Norma Farmer, Margaret and Henry Russell. Thank you for teaching me the value of hard work and that if you beilieve in yourself you can accomplish great things. iv ACKNOWLEDGEMENTS This thesis would not have been possible without the help and support of many people. First, I would like to express my deepest gratitude to my supervisor, Professor Michael G. Organ, for his continuous encouragement, advice, patience, and support throughout my graduate studies. I will never forget the lessons learned under his guidance, both professional and personal. I would also like to extend thanks to my research committee advisors: Prof. Arturo Orellana for teaching me the importance of designing experiments smartly and for his constructive criticism; and Prof. William Pietro for always encouraging me to think outside the box. In addition, I would like to thank my internal and external examiners, Prof. Barry Loughton and Prof. Kalman Szabó, respectively, for agreeing to examine this thesis. A special thanks to Dr. Howard Hunter, not only for his assistance with the NMR spectroscopy studies, but also for his willingness to share his immense knowledge of NMR spectroscopy. Next, I would like to thank all past and present members of the Organ group. Each and every one of you made my experience as a graduate student an enjoyable one. Thank you for all your help, friendship and unforgettable memories both in and outside of the laboratory. Finally, I would like to thank my friends and family for all their love and support throughout the years. Thank you mom, dad, sister, step parents, and the MacMillan’s for all your encouragement and free meals! A special thanks to my partner, David, for enduring my unpredictable working hours, endless conversations about my research, and always believing in me even at times when I did not believe in myself. v TABLE OF CONTENTS 1 ABSTRACT ................................................................................................................. ii 2 DEDICATION ............................................................................................................. iv 3 ACKNOWLEDGEMENTS .......................................................................................... v 5 LIST OF TABLES ........................................................................................................ x 6 LIST OF FIGURES .................................................................................................... xii 7 LIST OF SCHEMES .................................................................................................. xv LIST OF ABBREVIATIONS... .............................................................................. xviii PUBLICATIONS .................................................................................................... xxiii Part I: Application of Pd-PEPPSI Complexes in Regioselective Cross- Coupling and Natural Product Total Synthesis………………………….1 CHAPTER 1 – Introduction ............................................................................................ 2 1.1 The Importance of Isoprenylated Arenes .............................................................. 2 1.2 Traditional Prenylation Methods ........................................................................... 5 1.3 Palladium-Catalyzed Allylation Reactions .......................................................... 12 1.3.1 Prenyl Electrophiles ...................................................................................... 12 1.3.2 Prenyl Nucleophiles ...................................................................................... 19 1.4 Ancillary Ligands ................................................................................................ 25 1.4.1 Phosphine Ligands ........................................................................................ 25 1.4.2 N-Heterocyclic Carbene (NHC) Ligands ...................................................... 30 1.5 Pd-PEPPSI Pre-catalysts ...................................................................................... 33 1.6 Plan of Study ........................................................................................................ 35 vi CHAPTER 2 – Regioselective Coupling of 3,3-Disubstituted Allylboronic Acid Pinacol Ester Derivatives onto Aromatic Rings using Pd-PEPPSI IPent (53) .......... 37 2.1 Background .......................................................................................................... 37 2.2 Preparation of Regioisomers 61a and 61b. .......................................................... 38 2.3 Optimization Study for the Regioselective Suzuki-Miyaura Cross-coupling of Prenylboronic Acid Pinacol Ester 60a .......................................................................... 40 2.4 Substrate Study .................................................................................................... 44 2.5 Regioselective Suzuki-Miyaura Cross-coupling Reaction – Mechanism of Transmetallation ........................................................................................................... 46 2.6 Conclusion ........................................................................................................... 52 3 CHAPTER 3 – Application of Regioselective Suzuki-Miyaura Coupling using Pd- PEPPSI-IPent in Total Synthesis. .................................................................................. 53 3.1 Background .......................................................................................................... 53 3.2 Overview of Synthetic Approach ........................................................................ 55 3.3 Construction of 2-arylbenzofuran core ................................................................ 56 3.4 Introduction of Allylbornic Acid Pinacol Ester Derivatives onto 2- arylbenzofurans 109 and 111. ....................................................................................... 67 3.4.1 Optimization Study ....................................................................................... 67 3.4.2 Synthesis of methylated lakoochin A and cathafuran A ............................... 70 3.5 Attempted O-dealkylation of Aryl Methyl Ethers ............................................... 74 3.6 Conclusion ........................................................................................................... 76 CHAPTER 4 – Investigating the Effects of Solvent and Kinetic Isotope Effects (KIEs) in the Directed Ortho Metalation (DoM) of 1,5-Dichloro-2,4- dimethoxybenzene ..........................................................................................................
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