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C-H Borylation of Terminal Alkynes, the Reductive Cyclization

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C-H BORYLATION OF TERMINAL ALKYNES, THE REDUCTIVE CYCLIZATION OF ALKYNYLBORONATES, AND THE SYNTHESIS AND ANALYSIS OF PINCER-LIGATED RHODIUM COMPLEXES A Dissertation by CHRISTOPHER JAMES PELL 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, Michael B. Hall Franҫois P. Gabbaї Perla B. Balbuena Head of Department, Simon W. North December 2017 Major Subject: Chemistry Copyright 2017 Christopher J. Pell ABSTRACT Due to the utility of organoboron reagents in the synthesis of pharmaceuticals and other industrially relevant compounds, the transition metal-catalyzed C-H borylation of organic molecules has become a very popular area of research. Since the Ozerov group’s initial discovery of an iridium complex that can perform catalytic dehydrogenative borylation of terminal alkynes (DHBTA), we have made further investigations into the C-H borylation of terminal alkynes and the synthetic applications of the alkynylboronate products. Here we describe a pincer-ligated palladium complex as the first group 10 catalyst for DHBTA. Unlike the iridium systems, the palladium catalyst also catalyzed a competing alkyne hydrogenation reaction, which could be suppressed using elemental mercury or phosphines as additives. We then turned our attention to (PNP)Ir complexes as catalysts for the DHBTA of 1,6-enynes and 1,6-diynes. These substrates could be borylated and isolated in moderate to excellent yields, and they could serve as substrates for rhodium-catalyzed reductive cyclization to form five-membered cyclic compounds containing pendant C-B bonds. It was found that borylated 1,6-diynes tethered by amines or an ether linkage would form 3,4-bis(methylpinacolboryl)-substituted pyrroles or furans during reductive cyclization. Investigations into C-C coupling catalyzed by pincer-ligated rhodium complexes will also be discussed. First, we describe the synthesis of a series of PCP-based pincer- ligated rhodium complexes and how they performed as catalysts for stereoselective ii alkyne dimerization. While stereoselectivity was not achieved with any of the PCP- ligands on rhodium, these systems were faster and longer-lived than the (PNP)Rh alkyne dimerization catalysts previously reported by our group. Later, the fate of (PNP)Rh as a potential catalyst for Negishi coupling will be discussed. It was observed that although (PNP)Rh could perform all three fundamental steps in Negishi coupling (oxidative addition of an aryl halide, transmetallation, and reductive elimination), it was not a suitable catalyst. This was due to rhodium’s ability to insert into the carbon-zinc bond of the organozinc reagent to form a catalytically inactive species. Finally, the synthesis, characterization, and electronic structure of complexes of (PNP)Rh bearing small perfluorocarbon ligands will be described. iii DEDICATION To Mom and Dad for showing me that hard work pays off. iv ACKNOWLEDGEMENTS I would like to thank my research advisor Dr. Oleg Ozerov for his guidance over the past five years as well as during my 2011 summer semester as an REU student. I appreciate his time and all the opportunities that he has provided me. His mentorship has allowed me to develop into a successful chemist and an independent thinker. I would also like to thank my committee members Dr. Michael Hall, Dr. François Gabbaï, and Dr. Perla Balbuena. I would also like to thank the Ozerov research group, especially my elders who mentored me in navigating graduate school and becoming a better chemist: Sam Timpa, Morgan MacInnis, Chun-I Lee, Loren Press, Billy McCulloch, Jessica DeMott, Yanjun Zhu, Rafael Huacuja, Rodrigo Ramirez, Jill Davidson, and Chandra Mouli Palit. I also appreciated having Wei-Chun Shih as a classmate, and I am proud to see Bryan Foley continue the project of alkyne C-H borylation. To all of those still working in the Ozerov group, I wish you the best of luck. Above all, I would like to thank my family who stood by me and made all of my accomplishments possible. Thanks to my mom, dad, and Tom for instilling in me all the values that have helped me succeed in graduate school. Finally, I would like to thank the love of my life, Yanyan Wang, who inspires me to be a better person. I am proud to have spent time at Texas A&M University. The spirit and traditions of A&M are contagious, and I am glad that I have become a part of this community. v CONTRIBUTORS AND FUNDING SOURCES This work was supervised by a thesis committee consisting of Professor Oleg V. Ozerov, Professor Franҫois P. Gabbaї, and Professor Michael B. Hall of the Department of Chemistry and Professor Perla B. Balbuena of the Department of Chemical Engineering. Assistance with automated flash column chromatography for the work in Chapter 3 was provided by Yanyan Wang and Professor Donald J. Darensbourgh (Department of Chemistry, TAMU). The XRD structures in Chapter 5 were solved by Wei-Chun Shih during his time as a graduate student in the Department of Chemsitry at Texas A&M Univsersity. The XRD structures in Chapter 6 were solved by David Herbert during his time as a postdoctoral research associate in the Department of Chemistry at Texas A&M University. The synthesis and characterization of some compounds in Chapter 6 were completed by Yanjun Zhu (Department of Chemsitry, TAMU), and several experiments were conducted by Rafael Huacuja (Department of Chemistry, TAMU). All computational analysis in Chapter 6 was performed by Professor Russel P. Hughes of the Department of Chemistry at Dartmouth College. All other work conducted for this dissertation was completed by the student independently. This work was made possible in part by the U.S. National Science Foundation (grants CHE-0944634, CHE-1300299, and CHE-1565923 to O. V. O.) and the Welch vi Foundation (grant A-1717 to O. V. O.). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the U.S. National Science Foundation or the Welch Foundation. vii NOMENCLATURE Ar Aryl BArF20 Tetrakis(pentafluorophenyl)borate B2pin2 Bis(pinacolato)diboron BINAP 2,2’-Bis(diphenylphosphino)-1,1’-binaphthyl BIPHEP 2,2’-Bis(diphenylphosphino)-1,1’-biphenyl Boc tert-Butoxycarbonyl CAAC Cyclic (alkyl)(amino) carbene COD Cyclooctadiene COE Cyclooctene Cp Cyclopentadienyl Cp* 1,2,3,4,5-Pentamethylcyclopentadienyl DBA Dibenzylideneacetone DFT Density Functional Theory DHBTA Dehydrogenative Borylation of Terminal Alkynes Dipp 2,6-diisopropylphenyl Et Ethyl FLP Frustrated Lewis Pair HBpin Pinacolborane, 4,4,5,5-tetramethyl-1,3,2-dioxaborolane HBdan 1,8-naphthalenediaminatoborane iPr iso-propyl MIDA N-methylimidodiacetic acid viii NBO Natural Bond Order NBS N-bromosuccinimide n-Bu n-butyl NHC N-heterocyclic carbene NLMO Natural Localized Molecular Orbitals NMR Nuclear Magnetic Resonance OA Oxidative Addition OAc Acetate OTf Triflate Ph Phenyl PMP 1,2,2,6,6-pentamethylpiperidine py pyridine RE Reductive Elimination TBE tert-butyl ethylene, 2,2-dimethyl-3-butene tBu tert-butyl THF Tetrahydrofuran TM Transmetallation Ts Toluenesulfonamide WBI Wiberg Bond Indices XRD X-Ray Diffraction ix TABLE OF CONTENTS Page ABSTRACT .......................................................................................................................ii DEDICATION .................................................................................................................. iv ACKNOWLEDGEMENTS ............................................................................................... v CONTRIBUTORS AND FUNDING SOURCES ............................................................. vi NOMENCLATURE ....................................................................................................... viii TABLE OF CONTENTS ................................................................................................... x LIST OF SCHEMES ....................................................................................................... xiv LIST OF FIGURES ......................................................................................................... xix LIST OF TABLES .........................................................................................................xxii CHAPTER I INTRODUCTION AND LITERATURE REVIEW .................................... 1 1.1 Introduction .............................................................................................................. 1 1.1.1 Introduction to Organoboron Chemistry ........................................................... 1 1.1.2 Synthesis of Organoboron Reagents and the Beginning of C-H Borylation ..... 3 1.1.3 Summary – Alkynylboronates in Context ......................................................... 7 1.2 Synthesis of Alkynylboron Compounds ................................................................ 10 1.2.1 Traditional Synthesis of Alkynylboron Reagents ........................................... 10 1.2.1.1 Synthesis of Alkynylboronates ................................................................. 10 1.2.1.2 Synthesis of Ethynyl N-Methylimidodiacetic Acid Boronates ................ 12 1.2.1.3 Synthesis of Alkynyl Trifluoroborate Salts .............................................. 13 1.2.2 Dehydrogenative Borylation of Terminal Alknyes ......................................... 13 1.2.3 Formation of Alkynyl Triarylborates with Frustrated Lewis Pairs ................. 19 1.2.4 Formation
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