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The Synthesis and Reactivity of Oxazoline Based Transition Metal and Alkaline Earth Metal Complexes Steven Ryan Neal Iowa State University

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The Synthesis and Reactivity of Oxazoline Based Transition Metal and Alkaline Earth Metal Complexes Steven Ryan Neal Iowa State University Iowa State University Capstones, Theses and Graduate Theses and Dissertations Dissertations 2012 From pincers to scorpionates: The synthesis and reactivity of oxazoline based transition metal and alkaline earth metal complexes Steven Ryan Neal Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/etd Part of the Chemistry Commons Recommended Citation Neal, Steven Ryan, "From pincers to scorpionates: The synthesis and reactivity of oxazoline based transition metal and alkaline earth metal complexes" (2012). Graduate Theses and Dissertations. 12417. https://lib.dr.iastate.edu/etd/12417 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Graduate Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. From pincers to scorpionates: The synthesis and reactivity of oxazoline based transition metal and alkaline earth metal complexes by Steven Ryan Neal A dissertation submitted to the graduate faculty in partial fulfillment for the degree of DOCTOR OF PHILOSOPHY Major: Organic Chemistry Program of Study Committee: Aaron D. Sadow, Major Professor Malika Jeffries-EL William Jenks Gordon Miller Nicola Pohl Iowa State University Ames, Iowa 2012 Copyright © Steven Ryan Neal, 2012. All Right Reserved ii Table of Contents Acknowledgments iv Abstract vi Chapter 1: Ligand effects on organomagnesium compounds 1 General Introduction 1 Thesis outline 6 References 7 Chapter 2: Palladium and rhodium complexes containing 1,3- bis(oxazolinyl)propyl (Probox) ligands: Macrocycles and pincer compounds 10 Abstract 10 Introduction 11 Results and Discussion 18 Conclusion 36 Experimental 37 References 46 Chapter 3: Optically active, bulky tris(oxazolinyl)borato magnesium and calcium compounds for asymmetric hydroamination/cyclization 50 Abstract 50 Introduction 50 Results and Discussion 52 Conclusion 68 Experimental 68 References 75 iii Chapter 4: Synthesis of tris(oxazolinyl)boratomagnesium complexes of nitrogen, phosphorus, and sulfur: A reactivity study of silicon–heteroatom bond formation and a mechanistic study of Si–N bond formation via cross– dehydrocoupling 78 Abstract 78 Introduction 79 Results and discussion 84 Conclusion 99 Experimental 100 References 114 Chapter 5: Tris(oxazolinyl)boratomagnesium mediated Si–C bond formation: A reactivity and mechanistic study 117 Abstract 117 Introduction 117 Results and discussion 120 Conclusion 135 Experimental 135 References 141 Chapter 6: Conclusion 144 General conclusions 144 Future directions 145 Appendix: NMR spectra used to determine % ee of pyrrolidines 147 iv Acknowledgments My years in graduate school can be summed up as a crazy rollercoaster ride. During my orientation, I remember Erin Rockafellow drawing this graph of happiness versus year in grad school. It started out very high since all of us were excited about this new adventure we were about to embark on. It remained fairly steady with a few bumps along the way as research progressed…until the third year. The drastic drop coincided with our prelim-exam; after that it was a gradual (bumpy) increase until this point where we dropped precipitously while writing our dissertation. As silly as I thought this was at the time, I realize looking back how true, and fun, the ride has been. At the helm of this rollercoaster is Dr. Aaron Sadow; how could I possibly have gotten anywhere without his advice, kind words, and “do more reactions?” There were days that I didn’t want to see him because I might yell and scream at him, and I am sure he would say the same about me, and days that I couldn’t wait for him to come by the lab to show him the latest spectra or kinetics plot. What I have learned from him can only be summarized by one word, everything; you have taught me everything I know about being a scientist. Thank you for being hard on me. To my committee, Dr. Malika Jeffries-EL, Dr. Nicola Pohl, Dr. William Jenks, and Dr. Gordon Miller, thank you for your exceptionally valuable time for discussions, POS meetings, and last minute signatures. The Sadow group, what do I say. I owe every one of you a huge thank you. To all past and present members – Dr. James Dunne, Ben Baird, Jiachun Su, KaKing Yan, Hung- An Ho, Ryang Kim, Dr. Andrew Pawlikowski, Stephanie Smith, Kuntal Manna, Debabrata Mukherjee, Songchen Xu, Dr. Barun Jana, Jing Zhu, Nicole Lampland, Naresh Eedugurala, Aradhana Pindwal, Zak Weinstein, Jacob Fleckenstein, Regina Reinig – you have had to put v up with my messy desk and bench for all these years. We have had way too many sandwiches from West Street Deli, enjoyed a few good beers at DG’s, and many many late nights that I will never forget. I hope we cross paths again someday, until then…sleep less. We have been blessed to have many undergraduates in the lab over the years – Shawna, Marlie, Tristan, Jared, Kate, Jooyoung, Brianna, Rick, Josh, Marissa, Megan, Yitzhak, Yixin, Jessica. While I hope I helped many of you become better scientists, you will never know just how much you taught me. Thank you for helping to keep me sane over the years, there is always time for a good laugh. Good luck to all of you in your adventures in life. To all the students, past and present, in the chemistry department, your insightful discussions, chemistry and non-chemistry related, have played a very important role in all parts of my graduate career; thank you. A huge thank you goes out to the staff of the chemistry department; we would all be completely lost without your constant reminders about seminars, deadlines, and the occasional open house participation request. You keep us all going. My family – Mom, Dad, Erin, Barb, Alan, Tisha, Jennifer – you have been such a strong influence over the years; how can I ever thank you for your love, support, an ear to complain to, and frequent e-mail jokes. And lastly, Ally…I don’t know how you did it. From the day we visited Ames and I had to explain to you that what you were looking at is a snow blower to now where we just laugh at snow. You have put up with my late nights, early mornings, and constant ‘yes I have to go back to work.’ You are an amazing woman; I love you very much. Benton, your infectious smile and persistent “hi” has made the writing process so difficult and fun all at the same time. You have been a blessing to my life. vi Abstract Multidentate oxazoline based ligands have played a critical role in transition metal chemistry. They have several key advantages over many other ligands: (i) strong chelating ability, (ii) chiral versions readily synthesized from commercially available amino acids, (iii) the chirality is in close proximity to the metal center. With this in mind, we set out to design new bis(oxazoline) ligands that would cyclometalate with late-metal centers and tris(oxazoline) ligands for d0 metal centers. We synthesized new achiral [bis(4,4-dimethyl-2-oxazolinyl)propane], ProboxMe2, and chiral [bis(4R-phenyl-2-oxazolinyl)propane], ProboxPh, ligands for use as pincer complex proligands. While the isopropyl derivative, ProboxiPr, was known, no transition metal complexes containing these ligands have been reported. We attempted to synthesize palladium pincer complexes with ProboxMe2 and ProboxiPr but isolated thermally robust dipalladium(II) macrocycles of the form [(Probox)PdCl2]2. Attempts with rhodium provided a similar rhodium(I) macrocycle with ProboxPh. Finally, a rhodium(III) pincer complex is obtained with ProboxMe2. The spectroscopic and structural characteristics along with their reactivity are described. Since cyclometalation of Probox based ligands proved to be much more difficult than anticipated, we changed gears to tris(oxazolinyl)phenylborate ligands. Our group had reported the synthesis of achiral tris(4,4-dimethyl-2-oxazolinyl)phenylborate, ToM, and chiral tris(4S-isopropyl-2-oxazolinyl)phenylborate, ToP, along with the synthesis of several metal complexes containing these ligands. Unfortunately, we found that the isopropyl group in ToP, when coordinated to a metal, rotates away from the metal center, presumably to avoid steric interactions with the metal center, thus lessening the stereochemical control during asymmetric transformations. We decided to design a new chiral tris(oxazolinyl)phenylborate vii ligand with tert-butyl groups at the 4-position on the oxazoline ring with the anticipation that the tert-butyl group will remain in close proximity to the metal center. Magnesium and calcium complexes bearing this ligand have been prepared and used as catalysts for the asymmetric hydroamination/cyclization of aminoalkenes with moderate success. We observed high conversion to the corresponding pyrrolidines with % ee’s that were significantly higher than any other group 2-catalyst system to date. ToMMgMe was found to be an efficient precatalyst for the cross-dehydrocoupling of amines and silanes. With evidence gathered through kinetic investigations, including a Hammett plot, a new mechanism was proposed that involves a nucleophilic attack of the magnesium amide on the silane followed by hydrogen transfer to magnesium and displacement of the newly formed silazanes. Rapid protonolysis of the magnesium hydride with amine completed the catalytic cycle. Additionally, kinetic studies on the action of M To MgMe with PhSiH3 were also conducted. These included Eyring analysis, isotope effect, and a Hammett plot; these data provided evidence that this process is very similar to the Si– N bond forming reactions catalyzed by ToMMgMe. Additionally, preliminary studies on M M catalytic hydrosilylation using To MgMe and To MgMe with B(C6F5)3 as precatalysts are reported. 1 Chapter 1: Ligand effects on organomagnesium compounds General Introduction The design of new ligands for catalytic reactions is a continuous process because of the need to improve existing transformations while developing new reactions for organic synthesis.
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