Mechanism of arylation of nucleophiles by aryllead triacetates. Part 3. Concerning their reaction with phenols and X-ray molecular structure of p-methoxyphenyllead triacetate

Abstract

(6-Methoxy-2,3,4-trimethylphenyl)phenyllead diacetate 17, the methyl ether of one of the two possible intermediates in the ortho phenylation of 3,4,5-trimethylphenol by phenyllead triacetate, is synthesised and shown to be unchanged in CHCl₃–pyridine at 60 °C even after 6 days, thus excluding diaryllead diacetates as intermediates in this supposed ligand coupling process. A study of the arylation of a number of methyl substituted phenols by p-methoxyphenyllead triacetate shows that, in general, the rate of ortho arylation increases with methyl substitution; however, ortho methyl groups have a much greater effect than those in the meta and para positions. These results support the view that the arylation of phenols proceeds by an initial ligand exchange (or ligand exchange and pseudorotation) to produce an intermediate in which π-donation by the aryloxy ligand and electron release to the ortho position(s) are important factors. Phenols which are poor π-donors do not undergo the reaction.