IPSO AROMATIC SUBSTITUTION A thesis presented for the degree of Doctor of Philosophy in Chemistry in the University of Canterbury by R.J. MARTYN Christchurch New Zealand 1985 PHYSICAl,; SCIENCES _LIBRARY THESIS TABLE OF CONTENTS CHAPTER Page ABSTRACT i 1 GENERAL INTRODUCTION 1 1.1 Electrophilic Aromatic Halogenation. .. 1 1.2 Consequences of Ipso-Attack . " . 2 1.3 Reactions of Wheland Intermediates 4 1.4 The Chlorination of Polysubstituted Phenols with Chlorine . 10 2 CHLORINATION OF PHENOLS AND ANI LINES IN ACETIC ACID AND HYDROCHLORIC ACID: A STRUCTURAL STUDY . 19 2.1 Chlorination of 3-Chloro-6-methylaniline and 2,5-dimethylaniline . .. 19 2.2 Chlorination of 2,4-Dichloro-6-methyl­ phenol, 2,3-Dimethylaniline and 3-Chloro- 2-methylphenol . .. 23 2.3 Chlorination of 2,3,6-Trimethylphenol, 2,6-Dimethylaniline and 2,6- Dimethylphenol . 26 2.4 Chlorination of 2,5-Dimethylphenol, 3,5-Dimethylphenol, 2,3,4,6-Tetrachloro- 5-methylphenol, 2,3-Dimethylphenol and 2,3,4,5-Tetrachloro-6-methylphenol . .. 30 2.5 Chlorination of 3-Methylphenol 42 3 FORMATION OF 6-CHLORO-6-METHYLCYCLOHEXA-2,4- DIENONES FROM POLYSUBSTITUTED PHENOLS . 43 3.1 Introduction 43 3.2 Reaction of Polysubstituted Phenols with Chlorine in Carbon Tetrachloride and Pyridine . 44 3.3 Mechanism of 2,4-Dienone Formation . 58 CHAPTER Page 4 REACTIONS OF 6-CHLORO-6-METHYLCYCLOHEXA-2,4- DIENONES WITH CHLORINE IN ACETIC ACID 62 4.1 Introduction . 62 4.2 Chlorine addition to 6-Chloro-6-methyl- cyclohexa-2,4-dienones in Acetic Acid 63 4.3 Discussion: Chlorine addition to 6-Chloro-6-methylcyclohexa-2,4-dienones 76 5 EXPERIMENTAL METHODS · · · · · · · · 84 5.1 Apparatus, Materials and Instrumentation 84 5.2 Experimental section Relating to Chapter 2 · · · · · . · · · · · · · · 86 5.3 Experimental Section Relating to Chapter 3 · · · · · . · · · · · · · · 100 5.4 Experimental section Relating to Chapter 4 · · · · · · · · · · · · 112 5.5 Selected Preparations of Chlorophenols · 127 APPENDIX 1 Single-Crystal X-ray Structure Analyses . .. 131 APPENDIX 2 Chlorination of 2-Nitrophenols; Formation of (E)-(2RS-5RS)-2,4,5,6,6-Pentachloro~2-methyl- 6-nitrohex-3-enoic Acid. • . .. 174 REFERENCES 181 ACKNOWLEDGEMENTS 215 LIST OF FIGURES, TABLES AND DIAGRAMS Page Page Scheme 1 17 Block A 16 Scheme 2 196 Block B 18 Scheme 3 197 Block C 186 Scheme 4 198 Block D 187 Scheme 5a 200 Block E 188 Scheme 5b 200 Block F 189 Scheme 6 208 Block G 190 Scheme 7 212 Block H 191 Scheme 8 214 Block I 192 Block J 193 Block K 194 Block L 195 Block M 199 Block N 201 Block 0 202 Block P 203 Block Q 204 Block R 205 Block S 206 Block T 207 Block U 209 Block V 210 Block W 211 Block X 213 LIST OF FIGURES, TABLES AND DIAGRAMS Page Page Table 2.1 36 Figure 1 166 Table 3.1 61 Figure 2 166 Table 4.1 82 Figure 3 167 Table 1 150 Figure 4 167 Table 2 151 Figure 5 168 Table 3 152 Figure 6 169 Table 4 153 Figure 7 169 Table 5 154 Figure 8 170 Table 6 155 Figure 9a 171 Table 7 156 Figure 9b 171 Table 8 157 Figure 10 172 Table 9 158 Figure 11 172 Table 10 159 Figure 12 173 Table 11 160 Figure 13 180 Table 12 161 Table 13 162 Table 14 163 Table 15 164 Table 16 165 i ABSTRACT In the first part of this thesis the conformation and stereochemistry of a number of polychlorocyclohex-3-enones, formed by the reaction of chlorine in acetic acid and hydrochloric acid with polysubstituted phenols (or anilines), are discussed. Those polychlorocyclohex-3-enones with the H(Cl)CS or the Me(Cl)CS structural features were shown to exist in a twist-boat conformation with the CS-Cl bond in the flagpole orientation. In contrast, two polychlorocyclohex- 3-enones with gem-dichloro substituents at CS, were shown to be conformationally mobile in solution. The alicyclic ring of two 4,4,S-trichlorocyclohex-2-enones were also shown to exist in twist-boat conformations, but with the CS-Cl bond in the equatorial orientation. A satisfactory correlation between the lH n.m.r. and infrared spectroscopic data and the known structures in the solid state indicates that these polychlorocyclohex-3-enones and polychlorocyclohex- 2-enones adopt conformations in solution close to those observed in the solid state. Extensive use of single-crystal X-ray structure analysis was made in the above structural studies; some thirteen structure analyses are reported in this thesis. In the second part of this thesis are discussed the reactions of polysubstituted 2-methylphenols with chlorine in carbon tetrachloride in the presence of pyridine to give 6-chloro-6-methylcyclohexa-2,4-dienones. These 6-chloro-6- methylcyclohexa-2,4-dienones arise from ipso chlorine attack on the phenol ortho to the hydroxy function. It was shown that attack ipso to a methyl group occurred in preference to ii attack ipso to a chlorine atom. Where both ortho positions of the phenolic substrate are methyl substituted, the site of ipso chlorine attack is affected by the meta substituents. In the third part of this thesis the additions of chlorine to 6-chloro-6-methylcyclohexa-2,4-dienones to give polychlorocyclohex-3-enones and polychlorocyclohex-2-enones are discussed. These addition reactions proceed by three distinct reaction mechanisms, 2,3-, 4,5- and 2,5-chlorine addition. The 2,3-chlorine addition reaction was shown to be powerfully acid-catalysed. In contrast, the 4,5- and 2,5-chlorine additions were shown to be only mildly acid­ catalysed. Reaction mechanisms which accommodate these observations are discussed. Finally, the formation of an acyclic pentachloro hex-3-enoic acid by the chlorination of 4-chloro-2-methyl- 6-nitrophenol (130) in acetic acid and hydrochloric acid is described and a probable mode of formation suggested. 1. CHAPTER 1 GENERAL INTRODUCTION 1.1 Electrophilic Aromatic Halogenation Conventional electrophilic substitution reactions of aromatic compounds involving the halogens F, Cl, Br, I are now generally well understood and a ntimber of good reviews have been written on this topic. l ,2,3,4 Less well understood are reactions, often observed alongside electrophilic substitution reactions, in which the product is unexpected in terms of conventional substitution. Many such reactions, previously termed anomalous or non-conventional, have been reported. For instance, the bromination of t-butylbenzene yields bromobenzene in addition to the expected productsr ortho-, meta- and para- bromo-t-butylbenzene. 5 tBu Br tBu tBu tBu Br Br2/H2°., dioxane 38% 7% Bf It is probable that the bromobenzene is formed by electrophilic attack of bromine on the carbon bearing the t-butyl group, followed by loss of the comparatively stable t-butyl carbocation. Other non-conventional reactions, such as side-chain modification have been reported. For instance, the chlorination of hexamethylbenzene has been shown to give the side-chain chlorinated chloromethylpentamethyl benzene as the product. 6 2. Cl (l2 .. A(OH The initial step in this reaction is attributable to electrophilic attack of chlorine at a carbon bearing a methyl group. The term ipso was introduced by Perrin and Skinner7 to denote attack by a reagent at a substituted nuclear position. Subsequently many of the previously observed, anomalous or non-conventional reactions could be described in terms of the consequences of ipso aromatic attack. 1.2 Consequences of Ipso-Attack The Wheland intermediates (w's) from a mono-substituted benzene are of two kinds, those (Wo ' wP ' Wm) in which the electrophileis attached to an unsubstituted position, and that rlf in which it is attached to the. position at which the substituent X is attached. x x x x x E E H E 3. Observation of the intermediates WOI wml wp is rare since generally the chemistry associated with these forms is simple, rapid proton loss. However, direct observation of ~ has proved possible under exceptional circumstances. For instance, the reaction between hexamethylbenzene and N02+, under conditions in which bases and nucleophiles are absent, has been shown to give the ~e below, by l3C n.m.r. 8 x Other stable wi 's are obtained when a stabilising group is situated para to the position of ipso attack. For instance the reaction of N,N-dimethyltoluidine with N02+ gives the stabilised wiMe below. 9 Me + NO Z ) N(MelZ The wf can undergo a variety of reactions, for example, return to reactants, migration of the ipso group, side-chain modification, capture of a nucleophile or ipso substitution. These reactions are considered in the next section. 4. 1.3 Reactions of the Wheland Intermediates x (al Nucleophilic Capture of ~ The Wheland intermediate, with its cationic character, is susceptible to nucleophilic attack. Many examples have been reported in the field of electrophilic nitration of aromatic compounds. One in particular, the nitration of o-xylene in acetic anhydride, was important in attracting attention to ipso attack in nitration. 10 Me Me Me Me Me Me 16% OAe 51% The major product was found to be the acetoxy-o-xylene (51%) along with the expected nitro-o-xylenes. Further studies carried out on a number of methyl benzenes showed that the ratio of nitration to acetoxylation was dependent upon the nature of the substrate but independent of its concentration.lO,ll It was suggested that the mechanism involved capture of the wfe by acetate followed by elimination of nitrous acid to give the aryl acetate product. Me Me N02 Me Me Me Me Me Me NO· AeO-) ) -4 - HN0 2 H OAe OAe 5 • This mechanism was confirmed by the isolation of the stereoisomeric adducts, Me Me H OAc formed during the nitration ofo-xylene,lO,ll Similar products were obtained for a large number of aromatic substrates, mainly by Fischer and co-workers.