Ambident reactivity of anisole and p-iodoanisole toward phenylium cations and evidence for ipso-attack in cationic phenylation

Abstract

The phenylation of p-iodoanisole from the thermolysis of benzenediazonium tetrafluoroborate affords, among other products, 4-methoxybiphenyl and 4-iododiphenyl ether, providing evidence for the occurrence of ipso-attack and attack on oxygen by C₆H₅⁺. In the case of anisole, the oxygen atom of the methoxy-substituent is also attacked by C₆H₅⁺ giving rise to diphenyl ether via the initial formation of diphenylmethyloxonium ion followed by intermolecular demethylation. We have demonstrated that no intramolecular rearrangement occurs with the oxonium ion. The results are consistent with a mechanism involving kinetically predominant C-phenylation of both substrates. Boron trifluoride (a Schiemann reaction product) promotes deiodination of p-iodoanisole and its phenylated isomers. This deiodination invalidates the conclusions drawn from isomeric distributions; therefore the presence of BF₃ must be carefully controlled in a mechanistic study.