Revisiting the Balz-Schiemann Reaction of Aryldiazonium Tetrafluoroborate in Different Solvents under Catalyst- and Additive-Free Conditions

Abstract

The thermal and photochemical Balz-Schiemann reaction in commonly used solvents was revisited under catalyst- and additive-free conditions. The study showed that using low- or non-polar solvents could improve the pyrolysis and photolysis of aryldiazonium tetrafluoroborates, enabling effective fluorination at a low temperature or under visible-light irradiation. PhCl and hexane were exemplified as cheap and reliable solvents for both reactions, providing good to excellent yields of aryl fluorides from the corresponding diazonium tetrafluoroborates. The combination of slight heating with visible-light irradiation was beneficial for the transformation of stable aryldiazonium tetrafluoroborates. Nevertheless, the electronic and steric nature of aryldiazonium tetrafluoroborates still had a pivotal effect on both fluorinations even in these solvents.

Scheme 1. Catalyst- and Additive-Free Balz–Schiemann Reactions of Aryldiazonium Tetrafluoroborates in Chlorobenzene or Hexane at 60–90 °C

The reactions were carried out with 0.5 mmol of aryldiazonium tetrafluoroborates (1) in PhCl or hexane (2 or 4 mL) at 60–90 °C under an air atmosphere in a sealed tube. Isolated yields. Solvent-free conditions. Yields were determined by ¹⁹F NMR analysis of the reaction mixtures using 1-fluoronaphthalene or PhF as an internal standard.

Scheme 2. Blue- or Purple-LED-Irradiated Fluorination of Aryldiazonium Tetrafluoroborates in Chlorobenzene or Hexane under Catalyst- and Additive-free Conditions

The reactions were carried out with 0.5 mmol of aryldiazonium tetrafluoroborates (1) in PhCl or hexane (2 or 4 mL) under blue (435–455 nm) or purple (385–415 nm) LED irradiation and an air atmosphere in a sealed tube. Isolated yields. Solvent-free conditions.