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New Stereoselective Reactions to Form Amido Alkyl C-N and Vinyl Triflate C-O

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New Stereoselective Reactions to Form Amido Alkyl C-N and Vinyl Triflate C-O NEW STEREOSELECTIVE REACTIONS TO FORM AMIDO ALKYL C-N AND VINYL TRIFLATE C-O BONDS VIA CARBOCATION INTERMEDIATES & ULTRAFAST SILICON FLUORINATION METHODOLOGIES FOR APPLICATIONS IN PET IMAGING by Mohammed Alhuniti A Dissertation Submitted to the Faculty of The Charles E. Schmidt College of Science in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Florida Atlantic University Boca Raton, Florida December 2014 ACKNOWLEDGEMENTS I would like to thank my advisor, Prof. Salvatore D. Lepore, for his excellent guidance throughout my doctoral studies. Prof. Lepore is truly an outstanding mentor who devotes his time to teach more than organic chemistry. I want to thank my committee members Professors William Roush, Stéphane Roche, and Lyndon West for their time and insightful advice on my research. I thank Susan Lepore for her efforts to facilitate my research in the Lepore group. I also thank all former and current members of the group. Finally, I wish to express my gratitude to my family who have continually supported me. I sincerely appreciate the endless encouragement of my mother, my brothers, and my lovely wife and daughters. iv ABSTRACT Author: Mohammed Alhuniti Title: New Stereoselective Reactions to Form Amido Alkyl C-N and Vinyl Triflate C-O Bonds via Carbocation Intermediates & Ultrafast Silicon Fluorination Methodologies for Applications in PET Imaging. Institution: Florida Atlantic University Thesis Advisor: Dr. Salvatore D. Lepore Degree: Doctor of Philosophy Year: 2014 We report here the development of a Lewis acid catalyzed method for the dehydrative coupling of cyclic alcohols and nitriles to form amides with retention of configuration. By contrast, the formation of amides by nitrile trapping of carbocations (Ritter reaction) usually affords racemic product. The present reaction was accomplished by first converting alcohol starting materials to their corresponding chlorosulfites in situ. Even after an extensive search, only copper (II) salts were able to produce the desired conversion of these chlorosulfites to amides though with low catalytic turnover. Improving the turnover without deteriorating the stereochemical outcome was eventually accomplished by a careful selection of the reagent addition sequence and through the removal of gaseous byproducts. This Ritter-like coupling reaction proceeds in good yields with secondary cyclic alcohols under mild conditions. The stereochemical outcome v is likely due to fast nucleophilic capture of a non-planar carbocations (hyperconjomers) stabilized by ring hyperconjugation. In a second project, we demonstrate that TMSOTf in the presence of several metal catalysts converts alkynes to vinyl triflates under mild conditions. Current methods for the formation of vinyl triflates directly from alkynes generally involve harsh conditions and are exclusively selective for the E-isomer. Further study and optimization revealed that internal alkynes are converted to the Z-vinyl triflate product though with modest selectivity. The reaction efficiently converts aliphatic and aromatic terminal alkynes as well as internal alkynes to their corresponding vinyl triflate products. A mechanism is put forward to explain the unique role of silicon in this system. In this mechanism, we propose a silyl vinyl triflate intermediate that undergoes protodesilylation to afford the vinyl triflate product. Importantly, we believe that silicon may play a similar role in other recently reported reactions In a final project pursued in collaboration with NIH researchers, we describe our development of ultrafast silicon fluorination techniques for eventual applications in PET. The demand for physiologically stable organosilicon 18F-fluorides required compounds with a high degree of steric hindrance at the silicon center. This in turn greatly slows the rate of fluorination using current methods which often entail high temperatures and very polar solvents. Our initial solution to this problem centered on the use of metal chelating units attached to silicon substrates to serve as leaving groups. We reasoned that such leaving groups would stabilize negative charge developed on the silicon center in the TS and thus lead to a faster fluorination. Using these leaving groups, we observed fast radiofluorination of bulky silicon substrates at room temperature in 15 minutes without vi the need for the commonly used phase transfer reagents. Similar rate enhancements were 18 also observed with cyclotron-produced F-fluoride (t1/2 = 109.7 min, β+ = 97%). Based on our proposed mechanism for fluorination rate enhancement with chelating leaving groups, we reasoned that similar results could be achieved even without first attaching a chelating leaving group to a silicon center. As a result, we developed the concept of a Crown Ether Nucleophilic Catalyst (CENC). The use of these new phase transfer agents allows for efficient sequestration and recovery of cyclotron-derived K18F. Using these CENC/K18F complexes, we observed rapid radiofluorination of silicon substrates (5 min) which is significantly faster than currently reported methods. vii NEW STEREOSELECTIVE REACTIONS TO FORM AMIDO ALKYL C-N AND VINYL TRIFLATE C-O BONDS VIA CARBOCATION INTERMEDIATES & ULTRAFAST SILICON FLUORINATION METHODOLOGIES FOR APPLICATIONS IN PET IMAGING List of Figures……....................................................................................................... viii List of Tables…............................................................................................................ ix List of Scheme……...................................................................................................... x CHAPTER ONE STEREORETENTIVE COPPER (II) CATALYZED RITTER REACTION OF SECONDARY CYCLOALKANOLS…..…. 1 1.1 Introduction …................................................................................................... 1 1.2 Ritter reaction…………………………............................................................. 3 1.3 Development of stereoretentive copper (II) catalyzed Ritter reactions of secondary cycloalkanols....................................................... 14 1.4 Proposed catalytic cycle ….…........................................................................... 25 1.5 Experimental section........................................................................................... 26 CHAPTER TWO ZINC (II) CATALYZED CONVERSION OF ALKYNES TO VINYL TRIFLATE IN THE PRESENCE OF SILYL TRIFLATE......................................................... 32 2.1 Introduction………………………………........................................................ 32 2.2 Synthesis of vinyl triflate …………………....................................................... 36 2.3 Development of zinc (II) catalyzed conversion of alkynes to vinyl triflates in the presence of silyl triflates...................................................................................... 40 viii 2.4 Mechanistic study ………………………..……............................................... 45 2.5 Experimental section………………….............................................................. 51 CHAPTER THREE SILICON-NUCLEOPHILIC ASSISTED LEAVING GROUP (S-NALG)………………….………..................... 57 3.1 Introduction………………………………….................................................... 57 3.2 18F-Organosilicon radiotracers…………........................................................... 63 3.3 Enhanced nucleophilic fluorination and radiofluorination of organosilanes appended with potassium-chelating leaving groups.......................................... 67 3.4 Mechanistic considerations................................................................................ 76 3.5 Enhanced radiofluorination of organosilanes using Nucleophilic crown ether phase transfer agents.................................................................................. 76 3.6 Experimental section.......................................................................................... 81 Appendix……………………………………………….…………………………… 86 References……………………………………………….………………………….. 134 ix LIST OF FIGURES Figure 1 Major NOE correlations observed Bach and coworkers in carbocation 22................ 6 Figure 2 Plot of LUMO orbital of three cyclohexyl carbocations......................................... 11 Figure 3 Stabilization modes for the hyperconjomers of cyclohexyl cation.......................... 12 Figure 4 Mechanistic rational for the formation of stereoretentive amide................................ 14 Figure 5 Selected correlations from 2D-NOESY spectrum for 58 and 59............................. 22 Figure 6 Proposed catalytic cycle for stereoretentive amidation…….……….............. 26 Figure 7 Catalytic cycle for Heck coupling………..……............................................. 34 Figure 8 Structure of (+)-vernolepin.............................................................................. 36 Figure 9 Comins reagent................................................................................................ 38 Figure 10 Triflic acid addition to 113 in the presence and the absence of Lewis acid..........…..……….…………..….... 49 Figure 11 Common electrophilic fluorinating reagents................................................. 60 Figure 12 Structure of kryptofix 2.2.2 (K 2.2.2)............................................................ 62 Figure 13 Structures of common SiFA building blocks................................................ 66 Figure 14 Radiofluorination of substrates 173−178.....................................................
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