The Chemistry of Aryllead(IV) Tricarboxylates. Reaction with Aromatics to Give Biaryls

Abstract

Aryllead(IV) tricarboxylates have been shown to react in trifluoroacetic acid with a range of aromatics to give biaryls. With aromatics more reactive than toluene the yields are generally good and the method provides a useful synthesis of unsymmetrical biaryls. A study of the mechanism of the reaction indicates an electrophilic substitution, although at least with the more reactive substrates this does not involve a free aryl cation. It is suggested that, for these compounds, the formation of a π-complex in which the aryl-lead bond is still intact is rate determining.

For benzene and toluene, which give low yields in the above reaction, a marked improvement in the yield of the biaryl can be achieved by using AlCl₃ in place of trifluoroacetic acid. Al(OCOCF₃)₃, Al(OCOCF₂CF₃)₃ and Al(OCOCF₂CF₂CF₃)₃ have been shown to be equally effective in promoting the reaction, and, unlike AlCl₃, do not suffer the disadvantage of causing alkyl and/or aryl group migrations.