Reactions of lead tetra-acetate. Part XXI. Acyloxylation, plumbylation, and oxidative dimerisation of some benzenoid compounds in the presence of trifluoro- or trichloro-acetic acid

Abstract

The oxidation of benzene, chlorobenzene, toluene, and anisole by lead tetra-acetate in trifluoroacetic acid or, in some cases, trichloroacetic acid has been studied. The products include biaryls, aryl trifluoroacetates (usually analysed as phenols), aryl acetates, and aryl-lead compounds, in proportions which vary markedly with the aromatic compound and the conditions; for example, in trifluoroacetic acid, benzene gives phenol in up to ca. 80% yield whereas anisole gives mainly the dimethoxybiphenyls, and in trichloroacetic acid anisole gives about 50% of bis-p-methoxyphenyl-lead bistrichloroacetate, the reactions of which are discussed. Evidence is adduced for the occurrence of two oxidation pathways. One involves electrophilic plumbylation of the aromatic compound, which can be followed by heterolysis to give an aryl cation; the other is a process involving, formally, electron-transfer from the aromatic compound to lead(IV). The relative importance of the two not only depends on the electron-releasing ability of the benzenoid substituents but is also sensitive to the acid strength of the medium.