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.