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Autoxidation of Carbanions: Occurrence of Electron-Transfer Reactions Edward George Janzen Iowa State University

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Autoxidation of Carbanions: Occurrence of Electron-Transfer Reactions Edward George Janzen Iowa State University Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1963 Autoxidation of carbanions: occurrence of electron-transfer reactions Edward George Janzen Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Organic Chemistry Commons Recommended Citation Janzen, Edward George, "Autoxidation of carbanions: occurrence of electron-transfer reactions " (1963). Retrospective Theses and Dissertations. 2509. https://lib.dr.iastate.edu/rtd/2509 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 Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. This dissertation has been 64-3792 microfilmed exactly as received JANZEN, Edward George, 1932- AUTOXIDATION OF CARBANIONS. OCCURRENCE OF ELECTRON-TRANSFER REACTIONS. Iowa State University of Science and Technology Ph.D., 1963 Chemistry, organic University Microfilms, Inc., Ann Arbor, Michigan AUTOXIDATIQN OF CARBANIONS. OCCURRENCE OF ELECTRON.TRANSFER REACTIONS ty Edward George Janzen A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Major Subject: Organic Chemistry Approved: Signature was redacted for privacy. In Charge of Major Work Signature was redacted for privacy. Head of Major Departmen Signature was redacted for privacy. Dean of/lraduate Iowa State University Of Science and Technology Ames, Iowa 1963 ii TABLE OF CONTENTS Page I. INTRODUCTION 1 II. LITERATURE 3 A. Carbanion Reactions 3 B. Stable Free Radicals 15 C. Electron Transfer 26 HI. RESULTS AND DISCUSSION 37 A. Introduction 37 B. Solvent Effects 41 C. Relative Rates and Products 5^ 1. Products as a function of solvent 58 2. Rates and products as a function of structure 65 a. Nitro compounds 65 b. Fluorene derivatives 73 c. Toluene derivatives 84 d. Diphenylmethane derivatives 104 e. Base-addition-oxidation 110 3* Rates as a function of added catalyst 118 4. Rates as a function of base concentration 135 D. Spontaneous Dimerization 153 E. Electron Transfer 166 1. Free radicals from carbanion oxidation 166 2. Electron exchange 187 3. Spontaneous free radical formation 192 a. Alkyl nitroaromatics 192 b. g-Nitrotoluene kinetics 228 c. 1,4-Dianions 254 4. Electron transfer involving solvent 258 5. Electron transfer from carbanions 286 6. Electron acceptors 301 ill Page IV. CONCLUSIONS 306 V. EXPERIMENTAL 313 A. Methods 313 B. Solvents 323 C. Bases 324 D. Chemicals 325 VI. APPENDIX 332 A. Ketyls 332 B. New Free Radicals 3^5 C. Dimethyl Sulfoxide Adduets 353 D. pKa's of Weak Acids 357 VII. REFERENCES 362 VIII. ACKNOWLEDGMENTS 370 iv "How is bread made?" "I know that!" Alice cried eagerly. "You take some flour" "Where do you pick the flower?" the White Queen asked. "In a garden, or in the hedges?" "Well, it isn't picked at all," Alice explained; "it's ground -" "How many acres of ground?" said the White Queen. "You mustn't leave out so many things." Lewis Carroll 1 I. INTRODUCTION Chemistry of today is becoming the practice of electron mechanics. Whereas it sufficed for the Fathers of Chemistry to be sure of the atomic and molecular constitution of the products of a small number of reactions the emphasis has swung to puzzling out electronic mechanisms of as many transformations as possible sometimes merely to allow classification. The largest part of organic chemistry has been mechanistically classified into electrophilic or nucleophilic reactions. Electrophilic attack; (1) Nucleophilic attack: The net result is the very popular curved arrow-directing two-electron transfer. It would appear that one-electron transfer mechanisms have not been given equal consideration. Feasible mechanisms for electrophilic and nucleophilic reactions should include K solvent cage 2 (4) R® >; X » a* _ r R - C-.. + X / V solvent cage Evidence suggesting that free radical intermediates are produced via the above formulation is the discovery of free radical species in solutions of aluminum trichloride under Friedal Crafts conditions (1) and the isolation of 2,3-dimethyl-2,3-diphenylbutane from certain Wurtz reactions run in cumene (2). This study is concerned with the reaction of oxygen with carbanions sometimes considered a two-electron transfer process. + (5) R ® + Ô - 0: » R - 0. - 0:" The mechanism most consistent with the results in this thesis involves one-electron transfer steps: solvent cage R00* -5—D © R00 The occurrence of one-electron transfer from carbanions to common charge- transfer acceptors has been discovered and the wide generality of the reaction investigated. (7). R®+ A —• R' + A' A = e.£., nitrobenzene (8) R + A -R"+A" 3 II. LITERATURE A. Carbanion Reactions A detailed literature review of the reactions of oxygen with carb­ anions was made by A. J. Moye in 1961 (3). Of necessity the literature discussed in this section will contain only selected topics concerning recent oxidation studies in various solvents, stable free radicals and electron-transfer processes. In part the choice in subject matter will reflect the author's past and present interests during the course of this study. The classic solvents for carbanion reactions of organometallies have been ethers which suitably solvate the reagents made in situ from the free metal and an organohalide. A more highly ionizing medium is required to produce carbanions from their conjugate acids. Although sodium etboxide in absolute ethanol is a suitable solvent system for the production of many enolate anions in alkylation reactions (4), sodsmide in liquid ammonia ( 5), t-butyl alcohol containing potassium t-butoxlde (6), or benzyltrimethylaanonium methoxi.de (Triton B) in pyridine (7) have been used to ionize more weakly acidic compounds. The most useful solvents for ionic organic reactions including carbanion formation are dipolar aprotic solvents such as dimethyl sulfoxide, dimethylforeamide and sulfolane (8). Zaugg and co-workers studied the effect of the addition of small quantities of polar substances on the rate of alkylation of the sodium enolates of malonic esters in benzene (9)• They found rate enhancement to be large when dipolar aprotic solvents were used, e.g., rates for 4 hexamethylphosphoramide, pyridine-N-oxide, dimethyl sulfoxide and dimethylformamide were 21, 13, 6.3 and 5*8 times faster than the same reaction in benzene. Cram et al. (10) reported rate enhancement in potassium methoxide 9 catalyzed racemization reactions to be of the order of 10 in dimethyl sulfoxide as compared to the same reaction in methanol. Since the appearance of Cram's early observations numerous reactions involving anions have been found to proceed at faster rates and produce better yields in dipolar aprotic solvents than in previously used solvents. A review on the topic of solvation in dipolar aprotic solvents by Parker (8) contains a list of reactions which can be run to advantage in these solvents. It is thought that the higher reactivity of anions in dipolar aprotic solvents is due to a lower degree of solvation in these solvents. As a result of this effect anions are oxidized at a mercury electrode at lower potentials in aprotic dipolar solvents than in water. Only one series of examples is mentioned by Parker (8) studied by Kolthoff and Coetzee (11) who made a comparison of the potentials observed in water or aceto- nitrile for hydroxide, halides and thiocyanide. The base-catalyzed autoxidation of mercaptans has been studied in alcohol, ethers and dimethylformamide (IMF) ty Wallace and Schriesheim (12). The rate was found to be first order in mercaptan and increased in solvent mixtures containing MF. Stronger bases also increased the rate (sodium t-butoxide^> ethoxide^> methoxide and sodium^ potassium^ rubidiumcesium t-butoxide). An electron transfer mechanism was suggested involving dimerization of thiyl radicals (12, 13), 5 (9) RSH+B" > RS " + BH (10) RS " + 02 * RS* + -02" = (11) RS " + *02" > RS* + 02 (12) 2RS* > R-SSR = (13) 02 + BH * OH " + B " + i02 (HO2"+ B") T which agreed with the overall stoichiometry observed. (14) 2RSH + i02 + B " » R-S-S-R + OH ~ + BH The assumption was made that the mercaptan was completely ionized and that -02" and 0^ existed only at steady state concentrations. From these assumptions first order kinetics could be derived. (15) in (RSH) / (RSH)0 = kt The rates of oxidation of various nercaptans in methanol were approxi­ mately in the order of the expected stabilities of the anion, although benzylmercaptan oxidized almost twice as fast as n-butyl mercaptan in spite of the fact that the former is more acidic than the latter by 2 pK& units; in methanol, benzyl mercaptan n-butyl mercaptan )> g-aminothio- phenol y cyclohexyl mercaptan thiophenol £-ni tro thiophenol. itrothiophenol was stable to OîQrgen but its attempted oxidation in MF X was not reported. 1 mechanism involving electron-transfer from mereaptide ion to oxygen 6 seems reasonable (Equation 9)• However, there seems to be no justification for the assumption that a rapid electron transfer occurs between mercaptide anion and superoxide anion (both negatively charged species) or that base cleavage of peroxide is fast to yield hydroxide and oxygen, except that these steps produce the correct stoichiometry. In a further study of autoxidation in IMF the picolines were oxidized in the presence of potassium t-butoxide under 4 atm. oxygen pressure and at ambient temperatures to produce 59$» 70$ and 80$ yields of ^jS- and _£-picolinic acids after 10 min. (14). No dimeric products were found. The oxidation of pyrrole was observed in t-butyl alcohol containing potassium t-butoxide but products could not be identified. In discussing the effect of solvent on the rate of autoxidation of mercaptide ions the following solvated anion was drawn (13). f2. R—S —— C—0——Na H I The possibility exists in the oxidation of carbanions in IMF that a direct nucleophilic displacement occurs, which yields a new carbanion much more vulnerable to oxidation.
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