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New Reactions with N-Heterocyclic Carbene Boranes and Amidine Boranes, and the Study

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New Reactions with N-Heterocyclic Carbene Boranes and Amidine Boranes, and the Study New Reactions with N-Heterocyclic Carbene Boranes and Amidine Boranes, and the Study of Initiators in the Radical Hydrostannation of Propargyl Silyl Ethers by Timothy R. McFadden B. S. in Chemistry with Honors, The Pennsylvania State University, 2010 Submitted to the Graduate Faculty of the Kenneth P. Dietrich School of Arts and Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2018 UNIVERSITY OF PITTSBURGH KENNETH P. DIETRICH SCHOOL OF ARTS AND SCIENCES This dissertation was presented by Timothy R. McFadden It was defended on December 22, 2017 and approved by Jan Beumer, Associate Professor, School of Pharmacy Kazunori Koide, Associate Professor, Department of Chemistry Peter Wipf, Distinguished University Professor, Department of Chemistry Committee Chair: Dennis P. Curran, Distinguished Service Professor and Bayer Professor, Department of Chemistry ii Copyright © by Timothy R. McFadden 2018 iii New Reactions with N-Heterocyclic Carbene Boranes and Amidine Boranes, and the Study of Initiators in the Radical Hydrostannation of Propargyl Silyl Ethers Timothy R. McFadden, Ph. D. University of Pittsburgh, 2018 The development of new reactions of N-heterocyclic carbene (NHC) boranes and amidine- boranes is described. Additionally, the mechanism of the AIBN- and triethylborane-initiated radical hydrostannation of propargyl silyl ethers is studied. Chapter 1 describes the reaction of NHC-boranes with dimethyl acetylenedicarboxylate (DMAD) to form alkenylboranes and boriranes. These products arise through formal single and double trans-selective hydroborations. The reaction conditions were optimized and applied to produce a small library of NHC-borane derived products. Alkenylboranes were formed as the major product from the reaction of N,N-dialkyl NHC-boranes with DMAD, while boriranes were the major product formed from the reactions of N,N-diaryl NHC-boranes. Scope and limitation studies were performed to show that borirane-formation is specific to the reaction of NHC- boranes and borohydrides with DMAD. Boriranes were not formed from the reaction of NHC- boranes with any other alkynes or from the reaction of amine-borane complexes with DMAD. Control experiments showed that the products could not be interconverted. The mechanism was probed by deuterium-labeling experiments. Chapter 2 describes the synthesis and characterization of amidine-boranes. These ligated borane complexes can be prepared in good yield from the reaction of heterocyclic amidines, such as 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) or 1,2-dimethylimidazole, with borane. These complexes are air- and water-stable solids at room temperature and do not decomplex in solution iv at elevated temperature. The amidine-boranes are demonstrated to be reactive towards acids and halogens, in addition to being competent aldehyde, ketone, and imine reducing agents. Compared to other ligated boranes, DBU-borane is a more reactive hydride donor. In Chapter 3, the azobisisobutyronitrile (AIBN)- and triethylborane (Et3B)/oxygen (O2)- initiated hydrostannation of propargyl silyl ethers to form alkenylstannanes is studied extensively. When Et3B/O2 is used to initiate the reaction, the resulting alkenylstannane is formed with high Z-selectivity, while little selectivity is observed when the reaction is initiated by AIBN. Recent publications asserted that the difference in selectivity is derived from differing reaction mechanisms of the two initiators. The role of initiator, temperature, and reaction time are probed to demonstrate that both AIBN and Et3B/O2 initiated the reaction by the same mechanism, but at differing efficiencies, leading to different product ratios. v TABLE OF CONTENTS LIST OF TABLES ......................................................................................................................IX LIST OF FIGURES ....................................................................................................................XI LIST OF SCHEMES ...............................................................................................................XIII LIST OF ABBREVIATIONS .................................................................................................XVI PREFACE...................................................................................................................................XX 1.0 SYNTHESIS OF BORIRANES AND ALKENYLBORANES BY HYDROBORATION REACTIONS OF N-HETEROCYCLIC CARBENE BORANES AND ELECTRON- DEFICIENT ALKYNES.............................................................................................................. 1 1.1 INTRODUCTION ....................................................................................................... 1 1.1.1 Hydroboration.................................................................................................. 1 1.1.2 Trans-selective hydroboration reactions ....................................................... 3 1.1.3 N-Heterocyclic carbene boranes ..................................................................... 7 1.1.4 Hydroborations with NHC-boranes ............................................................. 11 1.1.5 Synthesis of boriranes.................................................................................... 12 1.2 RESULTS AND DISCUSSIONS.............................................................................. 17 1.2.1 Hydroboration reactions of electron-poor alkynes with N-heterocyclic carbene boranes.......................................................................................................... 17 1.2.2 Discovery of a new borirane-forming reaction............................................ 21 vi 1.2.3 Optimization of the borirane-forming reaction .......................................... 24 1.2.4 Preparative synthesis of NHC-ligated boriranes......................................... 28 1.2.5 Determining the scope and limitations of borirane formation with other ligated boranes............................................................................................................ 35 1.2.6 Investigating the mechanism of borirane formation .................................. 42 1.3 CONCLUSIONS........................................................................................................ 50 1.4 EXPERIMENTAL .................................................................................................... 50 2.0 SYNTHESIS AND STUDY OF AMIDINE-BORANE COMPLEXES ........................ 68 2.1 INTRODUCTION ..................................................................................................... 68 2.1.1 Borohydride.................................................................................................... 68 2.1.2 Ligated boranes .............................................................................................. 69 2.2 RESULTS AND DISCUSSIONS.............................................................................. 72 2.2.1 Preparation of amidine-borane complexes .................................................. 72 2.2.2 Substitution reactions of amidine-boranes .................................................. 77 2.2.3 Ligand exchange reactions with DBU and DBU-borane............................ 84 2.2.4 Reductions with amidine-boranes ................................................................ 87 2.3 CONCLUSIONS........................................................................................................ 98 2.4 EXPERIMENTAL .................................................................................................... 99 3.0 STUDY OF THE AIBN- AND TRIETHYLBORANE-INITIATED HYDROSTANNATION OF PROPARGYL SILYL ETHERS............................................ 114 3.1 INTRODUCTION ................................................................................................... 114 3.1.1 Radical initiators .......................................................................................... 114 3.1.2 Azo initiators ................................................................................................ 115 vii 3.1.3 Triethylborane and O2................................................................................. 116 3.1.4 Hydrostannation reactions .......................................................................... 118 3.1.5 Hydrostannations of propargylic alcohol derivatives............................... 119 3.2 RESULTS AND DISCUSSIONS............................................................................ 128 3.2.1 Mechanistic hypotheses ............................................................................... 128 3.2.2 Preparative hydrostannations of propargyl alcohol derivatives ............. 130 3.2.3 The effects of initiator, temperature, and time on the resulting Z/E ratio of alkenylstannanes ...................................................................................................... 133 3.2.4 The kinetic ratio of alkenylstannanes is afforded when tributyltin hydride is the limiting reagent .............................................................................................. 137 3.2.5 Triethylborane can initiate the isomerization under forcing conditions 139 3.2.6 Tin exchange reactions of alkenylstannanes to probe the mechanism of isomerization............................................................................................................. 140 3.2.7 TEMPO inhibits the Et3B/O2-initiated hydrostannation of alkynes ....... 143
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