Trifluoromethanesulfonic acid: a novel solvent for the electrophilic fluorination of fluoroaromatics

Abstract

Trifluoromethanesulfonic acid has been discovered to be an excellent new solvent for promoting the direct elemental fluorination of aromatics. Fluorobenzene has been successfully fluorinated to a mixture of 1,4-difluorobenzene (31%) and 1,2-difluorobenzene (7%) in CFCl₃–CF₃SO₃H (5%), but no further improvement is observed by the addition of boron trifluoride. When the reaction is carried out in only CFCl₃, the main reaction pathway is the 1,2- and 1,4-addition of fluorine to fluorobenzene forming cyclohexenes of molecular formula C₆H₅F₅, and only a small amount of 1,4-difluorobenzene is produced. Although both 1,2- and 1,3-difluorobenzene are fluorinated to 1,2,4-trifluorobenzene in CFCl₃–CF₃SO₃H (10%), 1,4-difluorobenzene does not undergo the same electrophilic substitution reaction.