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CONTENTS ACKNOWLEDGEMENTS VII SUMMARY VIII CHAPTER 1 INTRODUCTION 1.1 GENERAL 1 1.2 USES OF COUPLED PHENOLICS 4 1.2.1 Antioxidants 4 1.2.2 Other Uses 6 METHODS OF PREPARATION OF COUPLED PHENOLICS 6 1.3.1 General Types of Coupling Reaction Mechanisms 8 1.3 1.3.2 Chemical and Electrochemical Methods for Oxidatively Coupling Phenolics Chemical oxidative coupling 15 Vanadium(IV) and vanadium(V) 16 A (nitrosonaphtholato)metal complex 18 Activated manganese dioxide 21 Cupric salts 23 1.4 15 Electrochemical oxidative coupling 25 Direct electroche mical oxidations 25 Indirect electrochemical oxidations 29 OBJECTIVES AND MOTIVATION FOR THIS STUDY 32 I CHAPTER 2 2.1 2.2 EXPERIMENTAL MATERIALS 34 2.1.1 Reagents for Synthesis and Analysis 34 SYNTHETIC PROCEDURES 36 2.2.1 Reagents for Analysis 36 Preparation of 3,3’-di-t-butyl-4,4’-dihydroxybiphenyl 36 Preparation of 3,3’,5,5’-tetra -t-butyldiphenoquinone 36 Preparation of 3,3’,5,5’-tetra -t-butyl-4,4’dihydroxybiphenyl Preparation of 3,3’,5,5’-tetra -t-butyl-2,2’dihydroxybiphenyl 37 Preparation of 3,3’,5,5’-tetramethyl-2,2’Dihydroxybiphenyl 38 2.2.2 Preparation of Coupling Agents 38 Preparation of silver carbonate/celite 38 Preparation of barium manganate 39 Preparation of a (nitrosonaphtholato)metal complex (MnII(1-nnap)2) 2.3 37 39 Electrochemical preparation of cerium(IV) from cerium(III) using a divided cell 40 Preparation of silver oxide 42 EXPERIMENTAL PROCEDURES 43 2.3.1 Oxidative Coupling Reactions 43 Oxidation of alkylphenols using silver II carbonate/celite 43 Oxidation of alkylphenols using copper complexes of dicarboxylic acids 45 Oxidation of alkylphenols using manganese(III) acetate 44 Oxidation of alkylphenols using barium manganate 44 Oxidation of alkylphenols using a (nitrosonaphtholato)metal complex 45 Oxidation of alkylphenols using FeCl3 in an organic solvent 45 Oxidation of alkylphenols using FeCl3 without solvent 45 Oxidation of alkylphenols using Ag2O 46 Oxidation of alkylphenols using lead tetra -acetate 46 Oxidation of alkylphenols using Ce4+ 46 Oxidation of alkylphenols using potassium ferricyanide 47 2.3.2 Determination of Ce(III) Remaining After the Electrochemical 2.4 2.5 Oxidation of Ce(III) to Ce(IV) 47 2.3.3 Dealkylation of Dihydroxybiphenyls 48 ANALYTICAL TECHNIQUES 48 2.4.1 High Performance Liquid Chromatography (HPLC) 48 2.4.2 Nuclear Magnetic Resonance (NMR) Spectroscopy 50 2.4.3 Fourier Transform Infra Red (FTIR) Spectroscopy 50 2.4.4 Gas Liquid Chromatography-Mass Spectrometry (GC -MS) 51 2.4.5 Molecular Orbital Calculations 51 TERMS AND DEFINITIONS 52 III CHAPTER 3 3.1 3.2 DISCUSSION MODES OF PHENOLIC COUPLING 53 3.1.1 Molecular Orbital Calculations for the Coupling of Phenol 56 THE OXIDATIVE COUPLING OF 2-t-BUTYLPHENOL 58 3.2.1 The Range of Possible Products During the Oxidative Coupling of 2 -t-Butylphenol 60 3.2.2 Oxidative Coupling Reactions of 2-t-Butylphenol using Various Oxidants Vanadium(V) oxytrichloride and vanadium(IV) tetrachloride as coupling agents 65 Silver carbonate supported on celite as coupling Agent 63 65 Copper acetate, in the presence of a dicarboxylic acid, as coupling agent 70 Manganese(III) acetate as coupling agent 71 Barium manganate as coupling agent 72 Ferric chloride as coupling agent 73 Silver oxide as coupling agent 74 Potassium ferric cyanide, lead tetra-acetate, a (nitrosonaphtholato)metal complex and cerium(IV) sulphate as oxidants 75 3.2.3 Concluding Remarks on the Oxidative Coupling of 2-tButylphenol 3.3 75 THE OXIDATIVE COUPLING OF 2,6-DI-t-BUTYLPHENOL IV 76 3.3.1 Molecular Orbital Calculations for the Oxidative Coupling of 2,6-Di-t-Butylphenol 77 3.3.2 Oxidative Coupling Reactions of 2,6-Di-t-Butylphenol Using Various Oxidants 81 Silver oxide as coupling agent 83 Copper(II) acetate/oxalic acid as coupling agent 87 3.3.3 Concluding Remarks on the Oxidative Coupling of 2,6-Di-tButylphenol 3.4 88 THE OXIDATIVE COUPLING OF 2,4-DI-t-BUTYLPHENOL 88 3.4.1 Molecular Orbital Calculations for the Oxidative Coupling of 2,4-Di-t-Butylphenol 90 3.4.2 Oxidative Coupling Reactions of 2,4-Di-t-Butylphenol Using Various Oxidants 94 Ferric chloride as coupling agent 95 Silver oxide as coupling agent 97 Potassium ferric cyanide as coupling agent 100 Cerium as coupling agent 104 Identification of Ce(IV) as the preferred oxidant Oxidation in MeSO3 H mediated by Ce(IV) ions 104 107 Reaction mechanism for the oxidative coupling of 2,4-di-t-butylphenol using Ce(IV) 119 3.4.3 Concluding Remarks on the Oxidative Coupling of 2,4-Di-tButylphenol 126 V 3.5 THE OXIDATIVE COUPLING OF 2,4-DIMETHYLPHENOL 127 3.5.1 Oxidative Coupling Reactions of 2,4-Dimethylphenol Using Various Oxidants 131 Ferric chloride as coupling agent 132 Potassium ferric cyanide as coupling agent 136 Cerium(IV) as coupling agent 138 Reaction mechanism for the oxidative coupling of 2,4-dimethylphenol using Ce(IV) 138 3.5.2 Concluding Remarks on the Oxidative Coupling of 2,4Dimethylphenol 3.6 148 BUTYLATED PHENOLIC COUPLINGS: COMPARISONS 149 3.6.1 Reactions of 2-t-Butylphenol and 2,6-Di-t-Butylphenol with Ag2O and Cu(OAc)2 /Oxalic Acid 149 3.6.2 Reactions of 2,4-Di-t-Butylphenol and 2,6 -Di-t-Butylphenol with Ce(IV) in MeSO3H CHAPTER 4 151 CONCLUSION AND FINAL COMMENTS REFERENCES 159 APPENDIX 169 VI ACKNOWLEDGEMENTS The author wishes to express his sincere appreciation to: • My promoters, Dr B. Barton and Prof B. Zeelie, for their assistance and enthusiasm for this work. • The NRF and Port Elizabeth Technikon for financial support. • My fellow students, Mteza, Nigel, Daniël, Melissa and Knowledge for their moral support. • Dr S. Gouws, Dr G. Rubidge and Prof P. Loyson for their assistance. • The staff and students of the Department of Chemistry at the Port Elizabeth Technikon for their assistance and moral support. VII SUMMARY The oxidative coupling of 2,6-di-t-butylphenol under mild reaction conditions is well documented and the subject of many patents. However, the coupling of other monoand di- substituted phenols is not as well documented and thus there is scope for further investigation for providing a convenient, environmentally friendly and economically viable method for the oxidative coupling of these phenols. In this study, the oxidative coupling of a variety of alkylated phenolic substrates, 2-tbutylphenol, 2,6-di-t-butylphenol, 2,4 -di-t-butylphenol and 2,4-dimethylphenol, using a range of different oxi dizing agents, were investigated by means of experimental and/or theoretical means. The dibutylated aromatics provided the highest selectivities to their respective coupled products, with results obtained with the dimethyl analogue being only satisfactory, and that for 2 -t-butylphenol being totally inefficient. PM3 Molecular orbital (MO) calculations were used to predict the possible modes of coupling for the substrates 2,6 -di-t-butylphenol and 2,4-di-t-butylphenol, and these results were then compared with those obtained experimentally in the laboratory. Preliminarily, the coupling of unsubstituted phenolics was also assessed by means of MO calculations. Much emphasis was placed on Ce(IV) as the oxidant, and the reaction conditions under which it was used and the results that were obtained have not been reported before and are therefore novel. The oxidation of 2,4-di-t-butylphenol using Ce(IV) in the presence of methanesulphonic acid was optimized to afford high yields and selectivities to the desired ortho C-ortho C coupled product under mild reaction conditions. Various reaction parameters were also investigated in this case, such as varying the MeSO3 H concentration, the solvent, the reaction temperature, the reaction time, the substrate loading, the rate of oxidant addition and the substrate to oxidant ratio. Ce(IV) also gave a high selectivity to the para C-para C coupled product when VIII using 2,6-di-t-butylphenol as the substrate. However, it was not as effective with 2,4dimethylphenol, and even less so with 2-t-butylphenol. The oxidation reactions of 2-t-butylphenol and 2,4-dimethylphenol with various coupling agents were also investigated with the intention of obtaining high selectivities to the respective desired coupled products. In these studies, 2-t-butylphenol afforded a large number of products, irrespective of the oxidant used. The dimethyl analogue was more selective, but results were not optimal. It was clear that the number of substituents on the phenol ring, their nature and their position with regards to the hydroxyl moiety was of great importance and made a significant impact on the preferred coupling mode of the substrate. It was observed that steric effects also played a major role in the outcome of these reactions: 2,6-di-t-butylphenol never afforded any C-O coupled products whereas 2-t-butylphenol, 2,4-di-t-butylphenol and 2,4-dimethylphenol all appeared to undergo some C-O coupling. Finally, reaction mechanisms were provided for both the K3Fe(CN)6 and Ce(IV) work, these reacting in basic and acidic media, respectively. It was proposed that both of these mechanisms operate through the initial formation of the phenoxyl radical. IX CHAPTER 1 INTRODUCTION 1.1 GENERAL The chemical industry today is faced with major economic and environmental challenges. We as scientists have a responsibility towards the efficiency and profitability of the industry. We thus have to look at developing sustainable processes that have long -term economic and environmental viability. The chemical industry has been continually driven by this need for better quality products and much more effective and efficient production procedures, resulting in an industry that is currently well established and one that continues to grow.1 From an initial slow start in the 1850’s, the chemical industry has made tremendous strides in the field of organic synthesis, this being primarily due to enhanced competition between the various chemical companies, leading to increased numbers of products becoming commercially available.2 During the twentieth century, the industry has experienced exponential growth and this has led to a major improvement in both our living standards and life expectancy. Phenol and other phenolics are currently some of the more versatile and important industrial organic chemicals. Phenol itself was first isolated from coal tar by Runge.3 In 1843, C.F. Gerhardt prepared phenol by heating salicylic acid with lime; the resulting product was given the name ‘phenol’.4 Until World War II, phenol was essentially a coal tar extraction product, but due to an increased demand, synthetic methods replaced extraction from natural resources. Currently, only small amounts of phenol are obtained from coal tar (SASOL); larger quantities are being formed in coking or by the low pressure carbonization of wood and brown coal, as well as from oil cracking. The earlier methods of phenol synthesis via benzenesulphonic acid using alkali fusion (Scheme 1) and via chlorobenzene (Scheme 2)5 have since been replaced by more economically and environmentally friendly processes such as the Hock process, which utilizes cumene as substrate. 1 SO3H OH 1. NaOH, 300°C 2. H 3O+ Scheme 1: Preparation of phenol from benzenesulphonic acid (alkali fusion) Cl OH 1. aq. NaOH, 340°C, 2500 psi 2. + H 3O + NaCl Scheme 2: Preparation of phenol from chlorobenzene The Mitsui group is the world’s second largest producer of phenol through the Hock process. Acetone is produced as a byproduct in this process, but this is not deemed a disadvantage of the Hock method since there is also a high demand for acetone worldwide. The Hock process involves the alkylation of benzene with propene to afford isopropylbenzene (cumene); cumene is oxidized to the corresponding terthydroperoxide, which is then ultimately cleaved to yield phenol and acetone (Scheme 3). 2 OOH OH [O] cumene O H3O+ + acetone cumene hydroperoxide Scheme 3: The Hock process for the production of phenol Plants operating the cumene process are found in the USA, Canada, France, Italy, Japan, Spain, Eastern Europe and Germany, with an overall capacity of 5 000 000 tons per annum.6 By noting the Japanese production output and usage of phenol and phenolic resins (in tons) through the years 1996 to 2000, merely as an example, as contained in Table 1.1, one can better comprehend the importance of these compounds in an industrial capacity (Table 1.1).7 Table 1.1 Japanese production of phenol and phenolic resins (in tons) Increase Chemical/Year 1996 1997 1998 1999 2000 1999/2000 Phenol 768 833 851 888 916 3.2% Phenol Resins 294 303 259 250 262 4.8% Alkylphenols, such as xylenols, cresols, octylphenols and tert-butylphenols are generally produced by the alkylation of phenol with methanol or the corresponding olefins. Alkyphenols can the n be reacted further by oxidative coupling to form the 3 dihydroxybiphenyls, the focus of this investigation. All of these products have considerable economic importance because they are used to manufacture thermosets, insulating foams, adhesives, laminates, impregnating resins, and serve as raw materials for varnishes, herbicides and insecticides. 1.2 USES OF COUPLED PHENOLICS 1.2.1 Antioxidants One of the more important uses of many phenolic materials is their ability to serve as antioxidants. Antioxidants are merely compounds that are added to, or occur in, various materials, both living organisms and synthetic organic materials – antioxidants then readily react with free radicals that would otherwise damage the materials prematurely. The free radicals are normally the result of autoxidation, a process that occurs spontaneously all around us all the time due to the oxygen in the air. In human blood plasma, α-tocopherol, well known as a component of vitamin E, has proved to be the most efficient phenol derivative to date to trap damaging phenoxyl radicals (ROO •),8,9 caused by autoxidation, and therefore acts as an efficient antioxidant. Uninhibited free radical peroxidation in vivo is implicated in a wide variety of degenerate diseases such as cancer, heart disease, inflammation and even aging. Thus, over the last two decades, there has been a tremendous increase in the research of phenols as antioxidants.10,11 Phenols owe their efficient antioxidant activity to their ability to scavenge radicals by hydrogen or electron transfer in processes that are much faster than radical attacks on the substrate. The antioxidant property can be related to the readily abstractable phenolic hydrogen as a consequence of the relatively low bond dissociation enthalpy of the phenolic O-H group. Thus phenols and dihydroxybiphenyls are an extremely important class of antioxidants.12,13 4 To understand the antioxidant strength of phenols and diols, we need to discuss the reaction of molecular oxygen with organic molecules under mild conditions (autoxidation). It may be represented by the following chemical reactions (1 – 5). Initiation: production of RO • Propagation: R• Termination: + → O2 (1) ROO• (2) ROO• + ROO• + RO• → products (4) ROO• + PhOH → ROOH + PhO• (5) → RH ROOH + R• (3) While reaction 1 is very fast, having a rate constant of approximately 109 M-1s-1, reaction 4 is much slower at 10 1 M-1s-1. Oxidative degradation can therefore occur readily, and the use of low concentrations of antioxidants is thus important for all living organisms and for many commercial products in order to reduce the rate of degradation. Both phenols and dihydroxybiphenyls behave as antioxidants because of their ability to undergo reactions such as that shown in reaction 5, thus trapping potentially damaging peroxyl radicals. This is a much faster reaction than the attack of the peroxyl radicals on the organic substrate (reaction 3) due to the low bond dissociation energies for the oxygen-hydrogen bond in the hydroxyaromatic. The substituents on the aromatic ring have a profound effect on the ability of the phenol/diol to donate a hydrogen atom. Only those phenols bearing electron- donating substituents are active as antioxidants, particularly if these are at the ortho and/or para positions relative to the hydroxyl moiety. This is not unexpected since electron-donating groups are expected to lower the phenolic O-H bond dissociation enthalpy and thus increase the reaction rates with peroxyl radicals, implying a more efficient antioxidant. 5 1.2.2 Other Uses Dihydroxybiphenyls are used in toner resins to increase surface additive adhesion and to optimize cohesion between the toner particles.14 It also acts as a binder resin, thus eliminating the need for gels to be present in the toner, and enabling the magnetic brush development system to achieve consistent, high quality copy images.15 They are also used as inexpensive and simple starting materials for producing polycarbonate resins,16 which are used to reinforce rubber vulcanizates.17 Dihydroxybiphenyls are extensively used in coating agents,18 glass moulding 19 and infrared-reflecting colourants,20 and they are reacted with acid catalysts to form polymers which are used as a polymer scale deposition preventative agent.21 1.3 METHODS OF PREPARATION OF COUPLED PHENOLICS The diversity of phenol oxidation products offers interesting synthetic possibilities for the preparation of simple and polymeric molecules containing phenolic and/or quinoid structural elements; these can be formed from both like and unlike radical species.13,22 The successful synthesis of various natural products from phenols has been well documented from the 1950’s to the present.23-28 Biogenetic oxidative coupling routes were first investigated in 1957,29,30 and the prevalence of the overall coupling process in the biosynthesis of natural products was authenticated. Thus the oxidative coupling step has been found to be extremely important in the natural formation of compounds as diverse as lignins,31 lignans,32 tannins,33 plant pigments,22 and an estimated 10% of all known alkaloids.23 (Lignin is a complex biopolymer that accounts for 20-30% of the dry weight of wood. It is formed by the free radical polymerization of substituted phenylpropane units to yield polymers which have a number of functional groups such as aryl ethers, phenols and benzyl alcohols.34) 6 The major difficulty with oxidative coupling reactions of phenols is that a large variety of potential products are possible from a single substrate when carried out in the presence of various chemical or biological oxidants. This is because the phenolic molecules are able to undergo both carbon-carbon (Scheme 4 shows para-para coupling, though ortho-para coupling may also occur) and carbon-oxygen (Scheme 5) coupling reactions. OH + OH R HO OH R R R Scheme 4: Carbon-carbon oxidative coupling (showing para-para coupling) OH R + OH R HO O R R Scheme 5: Carbon-oxygen oxidative coupling The type of coupled product (whether C-C or C-O coupled) is also dependent on whether the ortho or para positions bear substituents or not. In addition to these two potential reaction products, the oxidative coupling of phenols also often allows for the formation of polymeric materials which, in general, are undesirable (though there are a few industrial processes where these are of great importance35,36). It has been reported that when carbon-oxygen coupling occurs, there is a tendency for further coupling to occur on the resultant substrate, and this leads to the formation of polymeric products.37 7 To understand the effect that both the nature of the reactant and oxidant has on the type of products that are formed, one must have an understanding of the various reaction pathways that are possible, from a mechanistic point of view. A summary of literature reports dealing with the various mechanisms is now briefly discussed. 1.3.1 General Types of Coupling Reaction Mechanisms The reaction pathway for the oxidative coupling of phenols has been extensively investigated.38,39 There are two main modes of coupling that may be highlighted. These are an external and an internal oxidation process. In the former, electrons are transferred from the phenolic compound to an external oxidizing agent, whilst the internal oxidation process involves an internal oxidation-reduction reaction in which one substrate molecule is oxidized whilst another is simultaneously reduced. Since there is no change in the net overall oxidation state, this process may be termed a “non-oxidative coupling (NOC)” reaction. In our investigations, only the external oxidative coupling process was studied. For this reason, literature reports dealing only with this mode are summarized here. External oxidative coupling reactions may be grouped into two separate classes, those involving free radical intermediates, and those that are non-radical in nature. These may further be subdivided into several general mechanistic types. a) Mechanisms involving free radical intermediates i) Direct coupling of two phenoxyl radicals (FR1) ii) Homolytic aromatic substitution (FR2) iii) Heterolytic coupling preceded by two successive one-electron oxidation steps (FR3) b) Mechanisms which are non-radical in character i) Heterolytic coupling preceded by a single two-electron transfer (NR1) ii) Concerted coupling and electron transfer (NR2) 8 It has previously been widely accepted that, in the field of phenol oxidations, the FR1 mechanism is the most viable (without discounting the FR2 mechanism). Most reviewers have included the FR3 mechanism in their discussions but have attached little importance to it. Until recently, no one has considered the NR1 and NR2 mechanisms as significant enough to warrant a discussion of them in this context. The para-para (C-C) coupling of a simple 2,6-disubstituted phenol is used to illustrate the five general types of processes (FR1, FR2, FR3, NR1 and NR2) as listed above. In all cases, the oxidized phenolic species is written as the neutral phenol molecule, and only intermediates are shown as unprotonated. The following scheme (Scheme 6) highlights the FR1, FR2 and FR3 mechanisms. 9 OH O R R O R R R -e -, -H + pathway (a) R phenoxy radical (1) FR1 coupling of two phenoxy radicals + (1) O FR2 R FR3 -e- R O O R R R R H H H + R R + (1) H -e- -H+ H -H+ OH R R O disproportionation (2) tautomerization OH R R H OH R R R R H H -2e- , -2H+ R R OH (4) OH (3) Scheme 6: The FR1, FR2 and FR3 free radical mechanisms 10 The degree of protonation of the phenolic species in each of these mechanisms depends on various factors, such as the acidity of the species, the nature of the solvent and the pH of the solution. The free radical processes are initiated by means of pathway (a) shown in Scheme 6. The first one-electron transfer from the disubstituted phenol (1) to an oxidant results in the formation of the phenoxyl radical which is stabilized by resonance, as shown in the following scheme (Scheme 7). O R O R R O R R O R R R Scheme 7: Resonance stabilization of the phenoxyl radical The formation of the phenoxyl radical is well attested, for example by ESR.40,41,42 (It has been shown9 that the subsequent dimerization thereof fits a diffusion-controlled model.) The phenoxyl radical is able to react in one of three ways, each leading to the same product (Scheme 6). • Firstly, it may homolytically combine with another phenoxyl radical by mechanism FR1 to afford compound (2). This dicyclohexadienone rapidly tautomerizes in protic media to the more stable aromatic biphenol product (3). • Secondly, the phenoxyl radical may react with the initial substrate (1) via mechanism FR2 to generate a dimeric radical. Upon loss of an electron and a proton from this new radical, (2) is formed once again. However, the dimeric radical may also disproportionate, leading to a dihydro product (4) as well as to (2). As yet, compounds such as (4), although analogous to similar products produced 11
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