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Chemistry of Magnesium and Zinc Complexes Supported by Bulky Ancillary Ligands and Their Applications in the Ring-Opening Polymerization Studies of Cyclic Esters

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Chemistry of Magnesium and Zinc Complexes Supported by Bulky Ancillary Ligands and Their Applications in the Ring-Opening Polymerization Studies of Cyclic Esters Chemistry of Magnesium and Zinc Complexes Supported by Bulky Ancillary Ligands and their Applications in the Ring-Opening Polymerization Studies of Cyclic Esters DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Pasco M. Wambua, B.Sc., M.Sc. Graduate Program in Chemistry The Ohio State University 2014 Dissertation Committee: Professor Malcolm H. Chisholm, Advisor Professor James A. Cowan Professor T.V. RajanBabu Professor Yann Guezennec Copyright by PASCO WAMBUA 2014 Abstract A series of magnesium and zinc complexes supported by bulky ancillary ligands of the form (TMP)MgnBu(L), (BDI*)MgnBu(L) and (TMP)Zn(X) were synthesized where TMP = 1,5,9-trimesityldipyrromethene, BDI* = (3-[2,6-diisopropylphenyl)amino]- 5-[(2.6-diisopropylphenyl)imino]-2,2,6,6-tetramethyl-hept-3-ene), L = THF, 2-MeTHF, i pyridine and DMAP, X = N(SiMe3)2, OPr and OCMe2CO(O)Et. These were all characterized and applied in the study of ring-opening polymerization (ROP) of cyclic esters. All complexes ring-open and sustain the polymerization of lactide (LA) and ε- caprolactone (ε-CL) in solution. The magnesium catalyst series was shown to initiate ROP via a β-H atom transfer generating an alkoxide and sequential elimination of 1- butene. The TMP supported catalyst systems show a good stereoselectivity in the ring- opening polymerization of rac-Lactide (rac-LA) with Pr values ranging between 0.80 - 0.96 with higher values observed in THF solvent. Copolymerization of rac-LA with ε- CL, when both monomers are present at the same time in a reaction mixture, only led to the ROP of rac-LA. Initial ROP of ε-CL with subsequent addition of the rac-LA resulted in stereoblock PLA-b-PCL but the vice-versa could not be attained. This was shown to be as a result of chelation of the ketonic group to the metal center that upon ROP of LA inhibits the enchainment of ε-CL. This was further proved by the synthesis of the model compound (TMP)Zn(OCMe2COOEt) where its carbonyl group, in solution and solid state, was shown to be chelated. ii Role of solvent was investigated and we indeed observed that THF accelerated the ROP of ε-CL while suppressing that of LA. A detailed explanation of this phenomenon is given in chapter 4. iii Dedication This document is dedicated to my entire family and in particular my parents, Joseph and Beata Wambua. iv Acknowledgments I wish to convey my deepest gratitude to my mentor and advisor, Professor Malcolm. H. Chisholm. His timely guidance, continuous and persistent mentoring to bring out the best in me, his patience and understanding are priceless and I cannot thank him enough. I would also like to express my gratitude to my committee members Professor T.V. RajanBabu and Professor James A. Cowan for their contributions towards my graduate school growth and success. I wish to also acknowledge Dr. Judith Gallucci and Christopher Durr for their contributions to my dissertation in solving the crystal structures, Dr. Tanya Young for all the cumulative hours she spend assisting me in the NMR spectroscopy facility and Professor Sherwin Singer for his words of encouragement over the years. I wish to sincerely thank Chisholm group members for their support, past and present, in particular Dr. Kittisak Choojun, Dr. Ruaraidh McIntosh, Dr. Chandrani Chatterjee, Dr. Vesal Naseri, Dr. Alexandre Bernard and Vagulejan Balasanthiran. I want to also express my gratitude to the Department of Chemistry and Biochemistry, The Ohio State University for granting me the opportunity to pursue my graduate studies. Lastly, I wish to thank my family members who have stood by me through my ups and downs during my graduate school journey. My Parents Joseph and Beata Wambua, my siblings John, Martin, Thomas and Paul Wambua, my Aunty Victoria v Ndonye, my cousin Celina Ndonye and her family Dr. Patrick Wachira, Foe and Naike. I wish to thank my wife Jane Mburu and sons Brown and Sonic Muindi. vi Vita December 2005 .............................................B.S. Chemistry, Moi University, Kenya 2006 – 2008....................................................Senior Chemist, ARM Cement Ltd, Mombasa, Kenya August 2011 ..................................................M.Sc. Chemistry, The Ohio State University January 2009 to present ................................Graduate Research Associate and Graduate Teaching Assistant, Department of Chemistry and Biochemistry, The Ohio State University Publication 1. Chisholm, M.H.; Choojun, K.; Gallucci, J. C.; Wambua, P.M. Chem. Sci. 2012, 3, 3445-3457 Fields of Study Major Field: Chemistry vii Table of Contents Abstract ............................................................................................................................... ii Dedication .......................................................................................................................... iv Acknowledgments............................................................................................................... v Vita .................................................................................................................................... vii Table of Contents ............................................................................................................. viii List of Tables ................................................................................................................... xiii List of Figures ................................................................................................................... xv CHAPTER 1: INTRODUCTION ................................................................................. 1 1.1 Review on Grignard reagents ............................................................................... 1 1.1.1 Preparation and solvent effects of Grignard reagents ................................... 1 1.1.2 Coordination of Grignard reagents in solution and solid state ..................... 4 1.1.3 Reaction mechanism for Grignard reagents .................................................. 5 1.1.4 Highly reactive functionalized Grignard reagents ........................................ 8 1.2 Chemistry of organozinc reagents ...................................................................... 12 1.2.1 History of organozinc chemistry ................................................................. 12 1.2.2 Transmetallation reactions of organozinc reagents..................................... 15 viii 1.2.3 Nature and structure of organozinc reagents .............................................. 17 1.3 Green biodegradable polymers........................................................................... 18 1.3.1 Mechanism for ring-opening polymerization of lactide ............................. 20 1.3.2 Lactide stereocontrol ................................................................................... 23 1.3.3 Complexes for lactide ring-opening polymerization (ROP) ....................... 25 1.4 Summary ............................................................................................................ 29 CHAPTER 2: METHODS FOR EXPERIMENTS AND GENERAL CONSIDERATIONS ........................................................................................................ 31 2.1 General considerations ....................................................................................... 31 2.2 Physical measurements ...................................................................................... 32 2.3 General procedure for lactide ring opening polymerization .............................. 32 2.4 General procedure for studying the kinetics of rac-lactide polymerization....... 33 2.5 General procedure for studying the kinetics of ε-caprolactone polymerization. 33 13 2.6 CO2 insertion experiments ............................................................................... 34 2.7 General reactions with alcohols, secondary amines, and ketones ...................... 34 2.8 Crystallographic studies ..................................................................................... 34 CHAPTER 3: CONCERNING THE CHEMISTRY OF MAGNESIUM COMPLEXES SUPPORTED BY TMPH = 1,5,9-TRIMESITYLDIPYRROMETHENE AND BDI*H = (3-[(2,6 DIISOPROPYLPHENYL) AMINO]-5-[(2,6- DIISOPROPYLPHENYL) IMINO]-2,2,6,6-TETRAMETHYL-HEPT-3-ENE). ............ 36 ix 3.1 Introduction ........................................................................................................ 36 3.2 Chemistry of (TMP)MgnBuTHF)....................................................................... 38 3.2.1 Synthesis of (TMP)MgnBu(THF) ............................................................... 38 3.2.2 Solution NMR studies of (TMP)MgnBu(THF) ........................................... 38 3.2.3 Reaction of (TMP)MgnBu(THF) with benzophenone and diphenyl methanol .................................................................................................................... 41 3.2.4 Reactions of (TMP)MgnBu(THF) with lactide ........................................... 44 3.2.5 Reactions of (TMP)MgnBu(THF) with carbon dioxide .............................. 45 3.3 Single X-ray crystallographic studies ................................................................ 47 3.3.1 (TMP)MgnBu(THF) .................................................................................... 47 3.3.2 (TMP)MgOtBu(THF) .................................................................................. 51 3.4 Ring-opening polymerization
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