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Nickel Mediated Reactions in a High-Speed Ball Mill Nickel mediated reactions in a high-speed ball mill A dissertation submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY (Ph.D.) in the Department of Chemistry of the College of Arts and Sciences Rebecca Haley B.S., Wheeling Jesuit University, 2013 Committee Chairs: James Mack, Ph.D. and Hairong Guan, Ph.D. Nickel mediated reactions in a high-speed ball mill Abstract High-speed ball milling (HSBM), a type of mechanochemistry, has become a popular method for synthesis. This newfound interest in mechanochemistry is because, unlike traditional synthesis, solvent is not necessary to facilitate a chemical reaction. With solvents representing a large portion of the waste generated from a reaction step, HSBM can be viewed as an environmentally friendly solution to reducing solvent waste. This dissertation will delve more specifically into combining catalysis, another prominent technique in green chemistry, with HSBM. A nickel catalyzed [2 + 2 + 2 + 2] cycloaddition of terminal alkynes, sans ligand, was developed mechanochemically. Nickel pellets were found to be the most successful source of nickel to produce moderate to high yields of cyclooctatetraene derivatives. Addition of 1,3-bis- (2,4,6-trimethylphenyl)-2-imidazolidinylidene (SIMes) yielded primarily cyclotrimerization products. This finding suggests that the cycloaddition mechanism in HSBM is similar to the one proposed for the cycloaddition conducted in solution. Finally, heat was applied to the cyclotetramerization reaction. A higher temperature did not alter the selectivity for tetramers over trimers but did decrease the time required to give high conversions of the alkynes to the cyclooctatetraene products. To further explore how nickel can be used in a ball mill, reduction of various types of unsaturated bonds was investigated. Nickel nanoparticles have become popular catalysts to perform reactions of this type. High-speed ball mills have been previously used to synthesize nickel nanoparticles, so we hypothesized that nickel pellets could display similar reactivity. To create an even simpler and inexpensive method, water was used as a source of hydrogen. Without ii heat, the reduction of aldehydes with nickel and water did not result in adequate conversions to alcohol products. However, stainless steel and water produced moderate to high yields of the alcohols. This result suggested that the reduction proceeded via a single electron transfer mechanism. To further investigate this phenomenon, the roles of iron, chromium, and nickel were studied individually. Iron was found to be the most effective electron donor, but the combination of nickel and iron in the stainless steel provided the most sustainable source of electrons. Mechanistic information was obtained to support a radical pathway. Finally, the substrate scope including various aldehydes, alkynes, and acetophenone was investigated. While using metals in their metallic form for catalysis is useful, there will still be situations where synthesizing a metal complex is more beneficial. Therefore, synthesis of these complexes from metallic starting material under HSBM conditions was explored. A nickel pincer complex was successfully synthesized from elemental nickel, suggesting that it is feasible to make organometallic complexes from such material. Additionally, HSBM has been advertised as a method greener than the traditional methods, although it is has been rarely quantified and compared to other methods using green chemistry metrics. A dithiocarbamate ligand was synthesized under both HSBM and solvent conditions. The greenness of this synthesis was then quantified using atom economy, e-factor, and EcoScale. Comparing these metrics showed that, counterintuitively, for the synthesis of the dithiocarbamate ligand, it was greener to perform the reaction in solution. iii iv Acknowledgements There are many people that deserve to be recognized in this section, but the first and foremost are my parents. You instilled the confidence in me to pursue whatever I wanted in life, and that is invaluable. I am beyond lucky to have never had to go a day without your encouragement, support, and love. I would also like to thank my Grammy for encouraging me to apply for the P.E.O. Scholar Award. I never could have imagined that I would actually receive such an honor. Not only was that a highlight of my graduate career, but the application process gave me an amazing community of forward-thinking, supportive, and intelligent women. My brother also deserves recognition for always being open to having conversations about science, music, and the combination of both. I have learned a lot from you. It is with my deepest appreciation that I thank my two advisors, Professor James Mack and Professor Hairong Guan. This mentorship was not what I anticipated as I entered graduate school, but it has made me a far better scientist and, more importantly, person than I could have hoped. The autonomy that Dr. Mack gives his students results in an independence that is truly unique. His guidance has shaped the way I approach teaching and mentoring in the best way possible, and I am excited to take the lessons I learned from him to my future academic career. I hope I can be just as good of a mentor to my students as he was to me. Hairong provides his students with a sense of trust that is crucial as one is learning how to navigate academic life. I never doubted that he would tell me the truth about my science, any aspect of graduate school, or life after graduate school. In addition to this, he shows immense care for his students. Even though he is a very busy person, if one of us has a problem or need, he puts us first. I can only hope that a small amount of his professionalism, attention to detail, and ability to navigate scientific questions has transferred v to me over the past five years. I am very grateful for their support of my personal and professional endeavors. With two advisors come many labmates. I am so happy to have gotten to know both the Guan and Mack group members over my time at University of Cincinnati. You have all made my time here so enjoyable. I have created lifelong friendships with many of you, and I am truly excited to see where our lives take us. I have also been fortunate to have had many talented undergraduate students. I would like to thank them for their hard work and I wish them the best in their future careers. Last but not least, I would like to thank my fiancé Jay Nellis for his endless support and encouragement. These past three years have been my happiest, and it is no coincidence that this is the same amount of time that we have known each other. You embraced the journey of having a significant other in graduate school, and you allowed me to be the best I could be. I could not have faced the challenges of graduate school nearly as well without you. Thank you for your endless support and encouragement. vi Nickel mediated reactions in a high-speed ball mill Table of Contents Chapter 1: Introduction 1.1 Green Chemistry 2 1.2 Catalysis 7 1.3 Solvent Waste and Solventless Reactions 9 1.4 High-Speed Ball Milling (HSBM) 11 1.5 Merging Catalysis and High-Speed Ball Milling 13 1.6 Research Objectives 20 Chapter 2: Ni-catalyzed [2 + 2 + 2 + 2] and [2 + 2 + 2] Cycloaddition Reactions in a High- Speed Ball Mill 2.1 Introduction 23 2.2 Investigation of Cycloaddition Reactions Under HSBM Conditions 24 2.3 Tuning the Selectivity 34 2.4 Concentration of Nickel in Products 47 2.5 Conclusions 49 2.6 Experimental 50 vii Chapter 3: Water and Stainless Steel Mediated Reduction Reactions 3.1 Introduction 67 3.2 Ni-Promoted Reactions and Initial Results 70 3.3 Optimization with Stainless Steel Media 74 3.4 Substrate Scope 77 3.5 Mechanistic Studies and EcoScale Evaluation 84 3.6 Conclusions 90 3.7 Experimental 91 Chapter 4: Synthesis of Dithiocarbamate Ligands and a Nickel Pincer Complex 4.1 Introduction 99 4.2 Dithiocarbamate Ligand Synthesis 101 4.3 Synthesis of a Nickel Pincer Complex 107 4.4 Conclusions 109 4.5 Experimental 110 Appendix 1: Reductive Cycloaddition Reactions 113 Appendix 2: 1H NMR and 13C NMR Spectra (Chapter 2) 129 Appendix 3: 1H NMR and 13C NMR Spectra (Chapter 3) 152 viii Appendix 4: Green Chemistry Metric Calculations (Chapter 4) A4.1 EcoScale Calculations 162 A4.2 Atom Economy Calculations 164 A4.3 E-Factor Calculations 165 ix Nickel mediated reactions in a high-speed ball mill List of Figures Chapter 1: Introduction Figure 1 “Valley of the Drums” waste site 5 Figure 2 Various types of mills with the motion that imparts energy 12 Figure 3 Computational diagram of Spex8000M mixer mill ball path 12 Figure 4 Screw-cap copper vial and 3/16” copper ball 17 Figure 5 View of stainless steel vial fitted with Ag foil 20 Chapter 2: Ni-catalyzed [2 + 2 + 2 + 2] and [2 + 2 + 2] Cycloaddition Reactions in a High- Speed Ball Mill Figure 1 ORTEP drawing of 5a at the 50% probability level 26 Figure 2 Visualized process of catalyst and product separation 30 Figure 3 Product distribution of cycloaddition promoted by nickel and SIMes 44 1 Figure 4 Superimposed H NMR (CDCl3 as the solvent) spectra of the isolated but unidentified oligomers and crude reaction mixture 47 Figure 5 ESI-MS of isolated, unidentified oligomers resulting from [2 + 2 + 2] cycloaddition reaction for ethyl propiolate 65 Chapter 3: Water and Stainless Steel Mediated Reduction Reactions Figure 1 Optimization of temperature for stainless steel and water-assisted reduction
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