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High-Turnover Ch Borylation of Arenes with (Pocop)

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High-Turnover Ch Borylation of Arenes with (Pocop) HIGH-TURNOVER C-H BORYLATION OF ARENES WITH (POCOP) IRIDIUM COMPLEXES, THE SYNTHESIS OF GROUP 9/10 (POCS) COMPLEXES AND THE STUDY OF TRIFLYLOXY-SUBSTITUTED CARBORANES A Dissertation by LOREN PAUL PRESS Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Chair of Committee, Oleg V. Ozerov Committee Members, Donald J. Darensbourg Michael B. Hall Jodie L. Lutkenhaus Head of Department, Simon W. North August 2016 Major Subject: Chemistry Copyright 2016 Loren P. Press ABSTRACT Over the last century, transition metal-catalyzed C-H functionalization has emerged as one of the most important topics in synthetic chemistry. One type of C-H functionalization, known as C-H borylation, generates organoboron reagents directly from hydrocarbon substrates. The synthetic utility of organoboron compounds is well established and numerous systems have been developed for the catalytic C-H borylation of alkyl and aromatic substrates since the reaction was first conceived over 20 years ago, yet catalysts that support high TONs remain limited to a handful of examples. Here we present highly active POCOP iridium complexes for the catalytic C-H borylation of arenes. In favorable cases, TONs exceeding 10,000 have been observed. The synthesis and isolation of multiple complexes potentially relevant to catalysis permitted examination of several key elementary reactions. We found C-H activation at Ir(I) here is in contrast to the olefin-free catalysis with state-of-the-art Ir complexes supported by neutral bidentate ligands, where the C-H activating step is understood to involve trivalent Ir-boryl intermediates. Next, we investigated the stoichiometric reactivity of a (POCOP)Ir(boryl)2 complex with various small molecules under thermolytic conditions. Transition metal- boryl complexes are ubiquitous in the literature and have been identified as key intermediates in several critical chemical transformations including C-H borylation and hydroborylation chemistries. The (POCOP)Ir(boryl)2 complex was found to undergo ii several stoichiometric transformations including reduction of CO2 to CO, 1,2- diborylation of ethylene and the selective protonation of one boryl ligand. The synthesis and characterization of novel POCS pincer complexes of nickel, palladium and iridium are described. The modular design of the POCS pincer ligand allowed the exploration of monomeric and bridging ligand designs akin to PNN and PCN complexes. – The selective B-H functionalization of the mono-anionic carborane [HCB11H11] with one or three triflyloxy (OTf) groups is described. The mono-triflyloxy substituted carborane can be halogenated to give pentabromo and decachloro derivatives with – preservation of the B–OTf linkage. The use of [HCB11Cl10OTf] as a weakly coordinating anion is demonstrated. iii DEDICATION To my Mom, Dad and Grammy Rose. iv ACKNOWLEDGEMENTS I would like to thank my committee chair and research advisor Prof. Oleg Ozerov for his guidance and patience during my time at Texas A&M University. He has taught me an approach to synthetic chemistry that is world-class. I would also like to thank my committee members, Prof. Michael Hall, Prof. Don Darensbourg and Prof. Jodie Lutkenhaus. I want to give a special thanks to Dr. Rita Silbernagel for her friendship and support. I want to thank Billy McCulloch for his companionship and for helping me think outside of the box. I would also like to acknowledge: Dr. Jessica DeMott, Dr. Morgan MacInnis, Dr. Rodrigo Ramirez, Dr. Dan Smith and Chris Pell. Thank you for the good times. To all of the members of the Ozerov group that I had the privilege to work with, it has been a pleasure. I want to recognize Dr. Weixing Gu and Dr. David Herbert, among others, for helping me throughout my formative years as a synthetic chemist. I also want to extend my gratitude to Dr. Yohannes Rezenom at the Laboratory for Biological Mass Spectrometry as well as the Texas A&M Chemistry Department NMR staff and instrumentation. I would like to thank the lovely Miss Martha Todd for all that she has done for me. Above all, I want to thank to my Mom, Dad and family for their encouragement and support through thick and thin. v NOMENCLATURE FG functional group DG directing group EAS electrophilic aromatic substitution py pyridine bpy 2,2’-bipyridine dtbpy 4,4’-di-tert-butyl-2,2’-dipyridyl DMAP 4-dimethylaminopyridine TMS trimethylsilyl OEt2 diethyl ether iPr isopropyl tBu tert-butyl Et ethyl Me methyl Cp* pentamethylcyclopentadienyl Ind indenyl COD cyclooctadiene COE cyclooctene COA cyclooctane TBE 3,3-dimethyl-1-butene TBA 2,2-dimethylbutane vi Ar aryl Ph phenyl OAc acetate THF tetrahydrofuran L ligand Bpin 4,4,5,5-tetramethyl-1,3,2-dioxaboryl ligand HBpin 4,4,5,5-tetramethyl-1,3,2-dioxaborolane B2pin2 4,4,4',4',5,5,5',5'-octamethyl-2,2'-bi-1,3,2-dioxaborolane HOTf trifluoromethanesulfonic acid OTf trifluoromethanesulfonate TON turnover number TOF turnover frequency DHBTA dehydrogenative borylation of terminal alkynes HDF hydrodefluorination rt room temperature dmpe 1,2-bis(dimethylphosphino)ethane dppe 1,2-bis(diphenylphosphino)ethane NMR nuclear magnetic resonance vii TABLE OF CONTENTS Page ABSTRACT .......................................................................................................................ii DEDICATION .................................................................................................................. iv ACKNOWLEDGEMENTS ............................................................................................... v NOMENCLATURE .......................................................................................................... vi TABLE OF CONTENTS ............................................................................................... viii LIST OF SCHEMES ......................................................................................................... xi LIST OF FIGURES .......................................................................................................... xv LIST OF TABLES .......................................................................................................... xix CHAPTER I INTRODUCTION AND LITERATURE REVIEW .................................... 1 1.1 Introduction .............................................................................................................. 1 1.2 Types of C-H bond functionalization ....................................................................... 3 1.3 Early examples of C-H activation and functionalization ......................................... 7 1.4 Directed C-H functionalizations............................................................................... 9 1.5 Transition metal catalyzed arene C-H bond borylation ......................................... 12 1.5.1 Introduction to transition metal catalyzed arene C-H bond borylation ........... 12 1.5.2 Mechanism of L2Ir(Bpin)3 catalyzed C-H borylation ..................................... 14 1.5.3 Selectivity in Ir catalyzed arene C-H borylation ............................................. 16 1.6 Homogeneous transition metal-catalyzed alkane dehydrogenation ....................... 20 1.6.1 Alkane dehydrogenation background .............................................................. 20 1.6.2 Ir pincer complexes for alkane dehydrogenation ............................................ 21 1.6.3 Recent advances in alkane dehydrogenation catalyzed by Ir pincer complexes ................................................................................................................. 24 1.6.4 Synthesis of alkanes and aromatics using Ir pincer catalysts .......................... 29 1.7 Introduction to carboranes ...................................................................................... 31 1.7.1 Carborane functionalization ............................................................................ 34 CHAPTER II HIGH-TURNOVER AROMATIC C-H BORYLATION CATALYZED BY POCOP-TYPE PINCER COMPLEXES OF IRIDIUM ................... 36 viii 2.1 Introduction ............................................................................................................ 36 2.2 Results and discussion ............................................................................................ 39 2.2.1 Synthesis of (POCOP)Ir(H)(Cl) and (POCOP)Ir(olefin) precatalysts ............ 39 2.2.2 Catalytic arene borylation studies with (POCOP)Ir pre-catalysts ................... 42 2.2.3 Preparative scale catalytic arene borylation using (POCOP)Ir precatalysts ... 45 2.2.4 Comparison with the ITHM arene borylation system ..................................... 48 2.2.5 Synthesis of relevant (POCOP)Ir compounds ................................................. 49 2.2.6 XRD and NMR characterization of Ir hydrido-boryl complexes .................... 55 2.2.7 C-H borylation mechanistic analysis ............................................................... 59 2.3 Conclusion .............................................................................................................. 66 2.4 Experimental .......................................................................................................... 68 2.4.1 General considerations .................................................................................... 68 2.4.2 Synthesis of compounds .................................................................................. 69 CHAPTER III SMALL MOLECULE ACTIVATION
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