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Synthesis, Structure, and Reactivity of Early Transition Metal SYNTHESIS, STRUCTURE, AND REACTIVITY OF EARLY TRANSITION METAL PRECATALYSTS BEARING (N,O)-CHELATING LIGANDS by Philippa Robyn Payne B.Sc., University of Ottawa, 2008 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemistry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) December 2013 © Philippa Robyn Payne, 2013 ABSTRACT The synthesis, structure, and reactivity of early transition metal complexes containing (N,O)-chelating ancillary ligands are described. The ligands investigated include ureates, pyridonates, amidates, and sulfonamidates. These related ligands generate four-membered metallacycles when bound to the metal center in a κ2-(N,O) fashion. The zirconium and tantalum complexes have been examined in terms of their activity and selectivity as precatalyst systems for hydroamination or hydroaminoalkylation. A chiral cyclic ureate ligand has been synthesized from enantiopure L-valine for application in zirconium-catalyzed asymmetric hydroamination of aminoalkenes. Chiral zirconium complexes, prepared in situ from two equivalents of the urea proligand and tetrakis(dimethylamido) zirconium, promote the formation of pyrrolidines and piperidines in up to 12% ee. Isolation of an asymmetric bimetallic zirconium complex containing three bridging ureate ligands confirms that ligand redistribution occurs in solution and is most likely responsible for the low enantioselectivities. Mechanistic investigations focusing on the hydroaminoalkylation reactivity promoted by a bis(pyridonate) bis(dimethylamido) zirconium precatalyst expose a complex catalytic system in solution. Stoichiometric investigations reveal the formation of polymetallic complexes upon addition of primary amines. The kinetic and stoichiometric investigations are most consistent with a bimetallic catalytically active species. A series of mono(amidate) tantalum amido complexes with varying steric and electronic properties have been synthesized via protonolysis. Solid-state and solution-phase characterization indicate that the amidate substituents influence the observed binding mode of ii the ligand. Salt metathesis and protonolysis routes to the synthesis of mixed tantalum chloro amidate complexes are investigated. Sulfonamide proligands react with pentakis(dimethylamido) tantalum to generate well-defined monomeric complexes containing a κ2-(N,O) bound sulfonamidate. The hemilabile (N,O)-chelating amidate ligands, which generate four-membered metallacycles, are the most active of the precatalysts examined for the intermolecular hydroaminoalkylation of terminal olefins with secondary amines. The substrate scope of a mono(amidate) tetrakis(dimethylamido) tantalum complex has been examined for the α-alkylation of unprotected piperidine, piperazine, and azepane N- heterocyclic amines. The lack of reactivity with pyrrolidine substrates is examined by quantum chemical calculations and isotopic labeling studies. Two (N,O)-chelating ureate ligands are also successful ancillary ligands for this transformation and, with a C1-symmetric chiral ureate complex, enantioselective α-alkylation of piperidine is observed. iii PREFACE Parts of the research conducted for this thesis were carried out collaboratively with other members of the Schafer research group. I, in consultation with my supervisor Dr. Laurel Schafer, designed and performed all of the experiments described herein except in the following instances. The initial concept and synthetic approach to the synthesis of the cyclic urea proligand was designed by Dr. David C. Leitch (Chapter 2). Compound 16 was synthesized and characterized through X-ray diffraction by Dr. Patrick Eisenberger (Chapter 3). The amidate ligands 22 and 23 (Chapter 4) and the related tantalum complexes were synthesized and characterized by Benedict J. Barron, an undergraduate researcher, under my supervision. The X- ray diffraction data for the crystalline mixture of compounds 29 and 30 were refined by Dr. Nicholas C. Payne. The DFT calculations were performed by Dr. J.M. Lauzon (Chapter 5). The N-heterocyclic substrate screening was performed in collaboration with Dr. Patrick Eisenberger (Table 5.1, Chapter 5). Jacky Yim performed the synthesis and characterization for compound 47 (Table 5.1, entry 3). The solid-state molecular data presented herein was collected by Neal Yonson, Jacky Yim, or Scott Ryken while I performed the final refinements. The following publications have been reported based on this work. Any work from these papers that I did not directly carry out, with the exceptions listed previously, does not appear in this thesis or is appropriately referenced. I wrote these manuscripts with editorial assistance from the co-authors listed, except for the Tetrahedron where I was involved with the editing but not the initial draft. Payne, P. R.; Bexrud, J. A.; Leitch, D. C.; Schafer, L. L. Can. J. Chem. 2011, 89, 1222. (Chapter 2) iv . Garcia, P.; Payne, P. R.; Chong, E.; Webster, R. L.; Barron, B. J.; Behrle, A. C.; Schmidt, J. A. R.; Schafer, L. L. Tetrahedron 2013, 69, 5737. (Chapter 4) . Payne, P. R.; Garcia, P.; Eisenberger, P.; Yim, J. C.-H.; Schafer, L. L. Org. Lett. 2013, 15, 2182. (Chapter 5) v TABLE OF CONTENTS ABSTRACT ................................................................................................................................... ii PREFACE ..................................................................................................................................... iv TABLE OF CONTENTS ............................................................................................................ vi LIST OF TABLES ....................................................................................................................... xi LIST OF FIGURES ................................................................................................................... xiii LIST OF SCHEMES ................................................................................................................. xix LIST OF ABBREVIATIONS AND ACRONYMS .................................................................xxv ACKNOWLEDGEMENTS ......................................................................................................xxx DEDICATION.......................................................................................................................... xxxi CHAPTER 1: Synthesis, structure, and reactivity of early transition metal precatalysts bearing (N,O)-chelating ligands. ...................................................................................................1 1.1 Introduction ..................................................................................................................... 1 1.2 Mono anionic, monodentate ligands based on N- and O-donors. ................................... 5 1.2.1 Multidentate, monoanionic ligands of (O,O)-, (N,N)-, and (N,O)-compounds .......... 6 1.2.2 Amidate complexes for hydroamination and hydroaminoalkylation........................ 11 1.2.3 Ureate complexes for hydroamination ...................................................................... 18 1.2.4 Pyridonate complexes for hydroamination and hydroaminoalkylation .................... 22 1.2.5 Sulfonamidate complexes for hydroamination ......................................................... 25 1.3 Scope of thesis .............................................................................................................. 28 CHAPTER 2: C1-symmetric ureate complexes of zirconium for the asymmetric hydroamination of unactivated aminoalkenes ..........................................................................31 vi 2.1 Introduction ................................................................................................................... 31 2.1.1 Asymmetric hydroamination of unactivated olefins ................................................. 31 2.1.2 Rare-earth metal systems .......................................................................................... 33 2.1.3 Group 4 metal systems .............................................................................................. 34 2.1.4 Expansion of substrate scope using a tethered bis(ureate) zirconium catalyst ......... 36 2.1.5 Scope of chapter ........................................................................................................ 37 2.2 Results and discussion .................................................................................................. 38 2.2.1 Ligand and complex .................................................................................................. 38 2.2.2 Intramolecular hydroamination ................................................................................. 41 2.2.3 Isolated bimetallic complex ...................................................................................... 43 2.3 Conclusions ................................................................................................................... 47 2.4 Experimental ................................................................................................................. 48 2.4.1 General methods ....................................................................................................... 48 2.4.2 Materials ................................................................................................................... 49 2.4.3 General experimental procedure ..............................................................................
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