Synthesis of fluorinated benzaldehydes and benzoyl fluorides using halex methodology

Preparation of Highly Fluorinated Compounds

“…These fluorinations are not simply solution-phase processes, because some reaction undoubtedly occurs on the surface; indeed, the surface area of the metal fluoride is extremely important to reactivity and in some cases it has been demonstrated that the amount of solid metal fluoride is important [31]. Methods of activating metal fluorides (usually potassium fluoride) fall into two broad classes: (a) increasing the surface area of the metal fluoride by spray drying [38,39], freeze drying [40], recrystallising from methanol [41] or absorbing onto a solid inert support such as calcium fluoride [42], alumina [43], graphite [44] or a polymer [45]; or, (b) increasing the solubility of the metal fluoride in aprotic solvents by the addition of coordinating crown ethers [46,47] such as 18-crown-6 or a phase-transfer catalyst such as tetraphenylphosphonium bromide [48,49] or a tetra-alkylammonium salt [50]. Fluoride ion is a relatively strong base which has been used to effect a large number of base-catalysed reactions in general organic synthesis [34,35] and so, if forcing conditions are required for a particular halogen exchange reaction, the limiting feature can be proton abstraction by fluoride ion from the solvent or the substrate.…”