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Novel Oxidation Reduction Reactions in Trifluoromethanesulphonic Acid

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Novel Oxidation Reduction Reactions in Trifluoromethanesulphonic Acid NOVEL OXIDATION REDUCTION REACTIONS IN TRIFLUOROMETHANESULPHONIC ACID. by Rupert Philip Austin, A thesis presented to the University of London in partial fulfilment of the requirements for the degree of Doctor of Philosophy. Department of Chemistry University College London September 1993 ProQuest Number: 10018645 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10018645 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Acknowledgements 2 Acknowledgements. My sincere thanks are given to Professor J. H. Ridd for his help, encouragement, and supervision throughout this work. Thanks are also given to the technical staff of the Chemistry Department, especially Richard Waymark whose knowledge of n.m.r. spectrometers has been a great help. Thanks are also given to James Steel for his help with e.s.r. spectroscopy. To all my friends in the Chemistry Department, past and present - Stuart, Akbar, Simon, Rob C., Joe, Rob J., Duncan, John, Vem, Gurpreet, and many others - thanks for some of the lighter moments over the last three years. Finally, special thanks are given to my parents, family, and of course my wife Talbir, for their love, encouragement, and support throughout my years at University College. Abstract 3 Abstract. This thesis describes three apparently disparate chemical processes. However, all are linked by the fact that they each formally involve unusual oxidation reduction reactions made possible not by the oxidative power of the trifluoromethanesulphonic acid solvent, but rather by its high acidity. l,3-Dimethyl-2-chlorobenzene has been found to undergo an unusual reaction with 1,4-dinitrobenzene in trifluoromethanesulphonic acid at 70 °C. The principle products of the reaction after quenching with water are 4,4',4"- trichloro-3,5,3',5',3",5"-hexamethyltriphenylmethanol and 1,4-diaminobenzene. The reaction was shown to occur also with many aromatic nitro compounds in place of the 1,4-dinitrobenzene, but not with nitromethane. The overall mechanism of the reaction is discussed in terms of a complex sequence of oxidation reduction reactions, along with a number of forward and reverse benzylation reactions. Kinetic and acidity dependence studies were used to show that it is the 0 -protonated form of the aromatic nitro compound that facilitates the oxidation reduction processes, and protonated aromatic nitio compounds are proposed to have previously unknown oxidative properties. In light of the work described above, the mechanism of cyclisation of 2-alky Ini tiobenzenes to give 3-alkylanthianils in trifluoromethanesulphonic acid has been reassessed. This reaction has also been shown to be initiated by protonation of the nitro group, and the mechanism is discussed in terms of a rate-determining intramoleculai* hydrogen shift from the benzylic carbon atom to an oxygen atom of the protonated nitro group, followed by several rapid steps. 2-Nitrobenzyl alcohol, 2- benzylc/ibOc!:f, and 2-nitrosobenzaldehyde have been found to react with tiifluoromethanesulphonic acid to give Abstract 4 4-amino-3-carboxyphenyltriflate in good yield. The mechanism is discussed, and a cyclic nitrenium ion intermediate is proposed. This work also led to a brief study of the Bamberger rearrangement of N-phenylhydroxylamine in trifluoromethanesulphonic acid. Contents 5 Contents. Title Acknowledgements Abstract Contents Abbreviations Chapter 1. Introduction. 1.1 Trifluoromethanesulphonic acid. 10 1.2 Protonation of nitro compounds in highly acidic media. 11 1.3 Acidity functions. 14 1.3.1 Definition and measurement. 14 1.3.2 Correlation of rate coefficients with Hq - The Zucker-Hammett Hypothesis. 18 1.4 Relevant chemistry in strongly acidic systems. 21 1.4.1 Alkyl group migrations. 21 1.4.2 Transalkylation reactions. 23 1.4.3 Nitro group rearrangements. 24 1.4.4 The formation of anthranils in trifluoromethanesulphonic acid. 26 1.4.5 Nitronic acids and the Meyer reaction. 28 1.4.6 The Bamberger rearrangement and nitrenium ions. 32 1.5 Molecular orbital calculations. 37 Contents 6 Chapter 2. Redox reactions of aromatic nitro compounds in trifluoromethanesulphonic acid. 2.1 Introduction. 40 2.2 The reaction of l,3-dimethyl-2-chlorobenzene with trifluoromethanesulphonic acid. 41 2.3 The reactions of 2-chlorotoluene and of 3-chlorotoluene with 1.4-dinitrobenzene in trifluoromethanesulphonic acid. 45 2.4 The reaction between 3-chloro- l-[^^C]methylbenzene and 1.4-dinitrobenzene in trifluoromethanesulphonic acid. 46 2.5 Isolation of 4,4'-dichloro-3,3',5,5'-tetramethyldiphenylmethanol. 51 2 . 6 Studies of the reaction between 1 ,3-dimethyl- 2 -chlorobenzene and 1,4-dinitrobenzene in trifluoromethanesulphonic acid using n.m.r. spectroscopy. 53 2.7 The reaction between 1 ,3-dimethyl- 2 -chlorobenzene and other nitro compounds in trifluoromethanesulphonic acid. 57 2.8 Preliminary discussion of mechanism. 59 2.9 Attempted simplification of oxidation reduction reactions involving aromatic nitro compounds in trifluoromethanesulphonic acid. 65 2 . 1 0 Studies using the method of initial reaction rates. 69 2.10.1 Kinetics of the reactions of 1,3 -dimethyl-2-chlorobenzene in trifluoromethanesulphonic acid, with and without 1,4-dinitrobenzene. 69 2.10.2 Kinetics of the reaction between 1,3-dimethyl-2-chlorobenzene and other aromatic nitro compounds in trifluoromethanesulnhonic acid. 75 Contents 1 2.10.3 Acidity dependence kinetics of the reaction between l,3-dimethyl-2-chlorobenzene and 1,4-dinitrobenzene in trifluoromethanesulphonic acid. 85 2.11 Mechanism of oxidation. 90 2.11.1 Introduction. 90 2.11.2 Studies using e.s.r. spectroscopy. 93 2.11.3 Studies using deuterotrifluoromethanesulphonic acid. 94 2.11.4 Charge-transfer complex mechanism. 101 2.12 Final discussion of mechanism. 103 2.13 Appendix. 106 Chapter 3. The mechanism of formation of anthranils in trifluoromethanesulphonic acid. 3.1 Introduction. 109 3.2 Kinetic studies. 1 1 2 3.2.1 The reaction between 2-ethylnitrobenzene and trifluoromethanesulphonic acid, with and without the presence of 1,4-dinitrobenzene. 112 3.2.2 Acidity dependence kinetics of the reaction between 2-ethylnitrobenzene and trifluoromethanesulphonic acid. 115 3.4 Discussion of mechanism. 121 3.5 Molecular orbital calculations. 124 Contents 8 Chapter 4. The formation of 4-aminophenyltriflates in trifluoromethanesulphonic acid. 4.1 Introduction. 133 4.2 The reaction of 2-nitrobenzyl alcohol with trifluoromethanesulphonic acid. 133 4.3 The reaction of 2-nitrobenzyl chloride with trifluoromethanesulphonic acid. 137 4.4 The reaction of 2-nitrosobenzaldehyde with trifluoromethanesulphonic acid. 141 4.5 Mechanism of formation of 4-amino-3-carboxyphenyltriflate from 2-nitrosobenzaldehyde. 144 Chapter 5. Experimental. 5.1 Apparatus and reagents. 148 5.2 Synthesis and isolation of compounds, and n.m.r. spectroscopic studies. 150 5.3 Kinetics. 178 5.4 Molecular orbital calculations. 184 References. 185 Published work. Abbreviations 9 Abbreviations. Triflic - T rifluoromethanesulphonic. Triflate - Trifluoromethanesulphonate. n.m.r. - Nuclear magnetic resonance. h.p.l.c. - High performance liquid chromatography. e.s.r. Electron spin resonance. M Mol dm"^. DMCB - 1,3 -Dimethy 1-2-chlorobenzene. DNB - 1,4-Dinitrobenzene. TMAT - Tétraméthylammonium trifluoromethanesulphonate. Introduction 10 Chapter 1. Introduction. 1.1 Trifluoromethanesulphonic acid. The synthesis of trifluoromethanesulphonic (triflic) acid (CF 3 SO 3 H) was first reported in 1954 by Haszeldine and Kidd^ who obtained the acid by oxidation of bis(trifluoromethylthio)mercury with aqueous hydrogen peroxide, followed by treatment with barium carbonate in order to form barium trifluoromethanesulphonate. The pure acid was then obtained by distillation from a mixture of the barium salt and 1 0 0 % sulphuric acid (Scheme 1.1). CF3SSCF3 s ► (CF3ShHg > CF3SO3H.H2O CF3SO3KH2O + BaC03 ^ ------► (CF3S03h B a — ► CF3SO3H Scheme 1.1 An alternative synthesis involving the electrochemical fluorination (ECF) of alkylsulphonyl fluorides was also reported in 1954^ (Scheme 1.2). ™ _ 3HF aq. KOH 100 % H,SO. CH3SO2F — ► CF3SO2F----------------► CF3SO3K------------ ^ .....-V CF3SO3H -3 H2 Scheme 1.2 Introduction 11 This ECF method is attractive since it results in a very high yield of trifluoromethanesulphonic acid. Trifluoromethanesulphonic acid is a very powerful acid, and is certainly one of the strongest known monoprotic acids.^ Trifluoromethanesulphonic acid and its conjugate base have great thermal stability and resistance to oxidative and reductive cleavage. The fact that the acid itself does not cause sulphonation, and is not a strong oxidising agent renders it a good solvent for minimising side reactions that can intervene when other strong acids such as perchloric acid and sulphuric acid are used. Pure trifluoromethanesulphonic acid is a clear, colourless liquid with a boiling point of 162 °C (760 mm Hg) and a melting point of -43.5 °C. The acid is very hygroscopic and fumes in moist air until it is converted to the stable monohydrate, which is a solid
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