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Development of Green and of Polymer-Supported DEVELOPMENT OF GREEN AND OF POLYMER-SUPPORTED OXIDIZING AGENTS FOR OXIDATION OF ALCOHOLS by SYED JAVED ALI, M.Tech., B.Tech. A THESIS IN CHEMISTRY Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved David Birney Chairperson of the Committee Satomi Niwayama Accepted John Borrelli Dean of the Graduate School May, 2006 ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my mentor, Dr. David Birney, for his ever-inspiring scientific guidance, for his constant encouragement and support and for his unfathomable patience. I hold him in very high esteem for being such an excellent teacher and a wonderful human being. Words would only depreciate my admiration and my gratitude for all that he has done. I would like to thank Dr. Satomi Niwayama for agreeing to be my thesis committee member and for her valuable comments on my work. My thanks are also due to Dr. Pramod Chopade for helping me understand some of the chemistry, Ms. Paramakalyani Martinelango for being a great friend and for help with this manuscript. I thank my very best friend, Pradip, and Anwesa for their moral support and always being there for me. I would like to thank my parents for their love, sacrifice and continued prayers and blessings, and my friends back home who I know genuinely care for my well-being. ii TABLE OF CONTENTS ACKNOWLEDGEMENTS……………………………………………………………….ii LIST OF SCHEMES……………………………………………………………………...v LIST OF TABLES………………………………………………………………………..vi LIST OF FIGURES……………………………………………………………………...vii CHAPTER 1. INTRODUCTION…………………………………………………………………...…1 1.1 An Overview of Oxidizing Agents for Alcohols……………………………...1 1.1.1 With Strong Oxidizing Agents……………………………………....1 1.1.1.1 Chromium Compounds…………………………………….1 1.1.1.2 Manganese Compounds……………………………………3 1.1.1.3 Ruthenium Compounds……………………………………4 1.1.1.4 Other Metal-Based Oxidants……………………………….5 1.1.2 By Catalytic Dehydrogenation………………………………………5 1.1.3 The Oppenauer Oxidation…………………………………………...7 1.1.4 With Dimethyl Sulfoxide Reagents…………………………………8 1.1.5 With Hypervalent Iodine Reagents………………………………….9 1.2. Use of Nitroxyl Radicals…………………………………………………….11 1.2.1 Structure and Stability……………………………………………...11 1.2.2 Synthesis of Nitroxyl Radicals……………………………………..12 1.2.3 Redox Reactions…………………………………………………...13 1.2.4 Mechanistic considerations………………………………………...14 1.2.5 Significance of this research……………………………………….18 2. EXPERIMENTAL…………………………………………………………………….19 2.1 General Methods……………………………………………………………..19 2.2 Synthesis of PEG-supported TEMPO………………………………………..19 2.3 General procedure for oxidation of alcohols with PEG-TEMPO……………24 2.4 General procedure for TEMPO-TCCA oxidation of alcohols……………….25 iii 3. RESULTS AND DISCUSSION………………………………………………………26 3.1 Factors Affecting Frequency of Absorption of the C=O Group……………..26 3.1.1 Inductive and Resonance Effects…………………………………..26 3.1.2 Effects of Conjugation……………………………………………..27 3.1.3 Steric Effects……………………………………………………….27 3.1.4 Ring strain effects………………………………………………….28 3.2 Discussion of experimental data……………………………………………..28 3.2.1 Oxidation of benzylic alcohols to aromatic ketones……………….28 3.2.2 Oxidation of alcohols to aliphatic and unsaturated ketones……….32 3.2.3 Oxidation of alcohols to cyclic aliphatic ketones………………….33 3.2.4 Oxidation of alcohols to form aromatic aldehydes………………...35 3.2.5 Oxidation of alcohols to form aliphatic/ unsaturated aldehydes…..39 4. CONCLUSION………………………………………………………………………..41 REFERENCES…………………………………………………………………………..42 iv LIST OF SCHEMES 1.1 Oxidation of alcohols to carbonyl compounds……………………………............. ..1 1.2 Preparation of pyridinium chlorochromate (PCC)………………………………......2 1.3 Oppenauer oxidation of an alcohol to a carbonyl compound……………………... ..7 1.4 Alcohol oxidation with DMSO…………………………………………………… ..8 1.5 Synthesis of the Dess-Martin periodinane……………………………………….... 10 1.6 Disproportionation of a nitroxyl radical with an α- hydrogen………………….…..12 1.7 Redox reactions of TEMPO………………………………………………………. 13 1.8 Disproportionation of the nitroxyl radical……………………………………….....14 1.9 Possible mechanistic pathways for oxidation with TEMPO……………………… 15 1.10 Mechanism of the TEMPO-TCCA oxidation system…………………………….17 2.1 Synthesis of PEG-supported TEMPO…………………………………………….. 20 2.2 Synthesis of mesylate of modified PEG……………………………………….…...23 v LIST OF TABLES 3.1 Oxidation of Alcohols to form Aromatic Ketones...................................................30 3.2 Oxidation of Alcohols to form Aliphatic and Unsaturated Ketones........................32 3.3 Oxidation of Alcohols to form Cyclic Aliphatic Ketones........................................33 3.4 Oxidation of Alcohols to form Aromatic Aldehydes...............................................35 3.5 Oxidation of Alcohols to form Aliphatic/Unsaturated Aldehydes...........................39 vi LIST OF FIGURES 1.1 Structures of some chromium based oxidizing agent.................................................2 1.2 Structure of IBX........................................................................................................10 1.3 Resonance structure of the nitroxyl radical..............................................................11 1.4 Conjugated and non-conjugated nitroxyl radicals....................................................11 1.5 Complex formation in the acyclic intermediate........................................................16 3.1 Inductive and resonance effects influencing C=O shift…........................................27 3.2 Effect of conjugation.................................................................................................27 3.3 A resonance structure for 4-methoxybenzaldehyde showing decreased double bond character...........................................................................................................38 3.4 s-cis conformation for (Z)-hex-2-enal......................................................................40 vii CHAPTER 1 INTRODUCTION The selective oxidation of primary alcohols and secondary alcohols into their corresponding aldehydes (or carboxylic acids) and ketones is one of the most important transformations in modern organic synthesis. A myriad of oxidizing agents have been developed to affect this transformation shown in Scheme 1. Tertiary alcohols resist oxidation by conventional oxidizing agents unless they are dehydrated in acidic media to alkenes, which subsequently undergo oxidation. In modern synthetic chemistry there is still a demand for mild and selective reagents for the oxidation of alcohols in presence of other oxidizable groups. R1 O R1 CHOH C O R2 R2 R1= H, Alkyl or Aryl R2= Alkyl or Aryl Scheme 1.1 Oxidation of alcohols to carbonyl compounds 1.1 An overview of oxidizing agents for alcohols Primary and secondary alcohols can be oxidized to the corresponding carbonyl compounds in five main ways1 1.1.1 With strong oxidizing agents 1.1.1.1 Chromium compounds Traditionally, the reagents most commonly used are based on high oxidation state transition metals, particularly Cr(VI). Chromic acid (H2CrO4), a strong oxidizing agent, 2- can be prepared by acidification of sodium or potassium salts of chromate (CrO4 ) or 2- dichromate (Cr2O7 ). Oxidation of primary alcohols by this method leads to the 1 formation of carboxylic acids in most instances, due to the rapid hydration and subsequent oxidation of aldehydes with chromic acid. Pyridinium chlorochromate (PCC), prepared by dissolving chromium trioxide in aqueous HCl and adding pyridine2 (Scheme 2), was developed for the oxidation of primary alcohols to aldehydes. Secondary alcohols 3 are oxidized to ketones by chromic acid as well as by PCC. CrO3 + HCl + CrO3Cl N N H Scheme 1.2 Preparation of pyridinium chlorochromate (PCC) Pyridinium chlorochromate, tetra-n-butylammonium hydrogen chromate (Fig 1a) and pyridinium dichromate (Fig 1b) are typical representatives of an important series of chromium (VI) oxidants that may be formally regarded as lipophilic derivatives of the monomer form of chromic acid H2CrO4 or of the dimeric form H2Cr2O7. They find increasing use as mild reliable oxidants for alcohols.4 O n-Bu N-O O N 4 2- Cr2O7 Cr Cr O N HO O N O H 2 a b c Figure 1.1 Structures of some chromium based oxidizing agents 2 Jones’ reagent is a solution of chromium trioxide in dilute H2SO4 which also affects the oxidation of primary allylic alcohols to aldehydes and secondary alcohols to ketones. Other chromium trioxide reagents have been prepared by adsorption of chromium trioxide on Celite, silica gel, or on anion exchangers. Among non-acidic reagents which are suitable for oxidations of acid-sensitive substrates, is Collin’s reagent (Fig 1c), a complex of chromium trioxide with two molecules of pyridine. It is prepared by the addition of chromium trioxide in small portions to pyridine, with continuous stirring and cooling the mixture in an ice bath.2 The disadvantages of this reagent are that a) it is very hygroscopic and is easily converted into the insoluble dipyridinium dichromate b) a six fold quantity of the reagent might be required to obtain best yields in some instances and c) saturated aliphatic alcohols often give low yields. Though chromium based oxidations are effective, they are relatively expensive and have serious drawbacks in terms of green chemistry and environmental impact. They generate stoichiometric amounts of heavy-metal waste and Cr(VI) is a proven carcinogen.5,6 1.1.1.2 Manganese compounds The best known manganese based oxidants are manganese
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