Ipso aromatic substitution

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 C5, were shown to be conformationally mobile in solution. The alicyclic ring of two 4,4,5-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 ¹H 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 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.