ABSTRACT

The Balz-Schiemann reaction and related reactions are among the most important and simplest reactions used for the laboratory-scale synthesis of fluoroaromatics, and are particularly attractive since no special equipment or precautions often associated with traditional fluorination methodology are required. The reaction was developed in 1927 by Balz and Schiemann, although it is often simply referred to as the Schiemann reaction.' The process is a two-step route to fluoroaromatics starting from anilines, although the reaction is often preceded by aromatic nitration and reduction stages to generate the aniline precursors. The first step of the reaction involves the diazotization of an aniline (Figure 3.1). The usual Sandmeyer-type chemistry (preparation and decomposition of the aryldiazonium fluoride in aqueous solution) is unsuitable for the synthesis of fluoroaromatics, owing to the instablilty of the diazonium fluoride. Instead, the diazonium is prepared as the insoluble tetrafluroborate salt, which, unusually for a diazonium salt, is stable enough to be isolated. This step is actually the key stage in the reaction and stems from an original observation by Bart.2 The second stage involves the thermal decomposition of the aryldiazonium fluoride to give the fluoroaromatic, typically in high yields of around 70% (Figure 3.2). Yields are highly dependent on the nature of the substrate, in particular nitro-, hydroxy-, and carboxyl-substituted aromatics generally give poor yields.