The nucleophilic (halex) substitution of the chloro substituent by F- can be achieved in good yield by high temperature reaction with KF in tetramethylene sulfone in the presence of Tetraphenyl-phosphonium bromide, as a thermally stable phase-transfer catalyst. This method is general for chloro substituents activated by an ortho- or para-aldehyde group: Chem. Lett., 1355 (1988).
