[22391-97-5] · CoF4K · Potassium Tetrafluorocobaltate(III) · (MW 174.03)
(moderately strong reagent used for the fluorination of a wide range of organic substrates in the solid/vapor phase)
Physical Data: mp unknown; d ca 3.9 g cm-3.
Solubility: reacts violently with water and hydroxylic solvents. Will fluorinate most organic compounds at relatively low temperatures.
Form Supplied in: pink-purple solid when pure, darkens on exposure to moisture; not commercially available.
Preparative Methods: usually prepared by the reaction of potassium cobaltate(II) with elemental fluorine at temperatures in the range 250-350 °C, usually in a stirred bed or static bed reactor preferably constructed of nickel or less satisfactorily of copper. The reagent is reacted with organic substrates in the same reactor in the manner described for reactions of Cobalt(III) Fluoride.2 Its crystal structure has been reported by Hoppe.3
Handling, Storage, and Precautions: normally prepared and used in situ. Can be stored under dry conditions under nitrogen for a considerable period. Releases HF on contact with moist air and must be handled with suitable protection. This reagent should be handled in a fume hood.
Until recently, the whole of the published work on the reactions of this reagent have been by the Birmingham group over the period 1962-1993.
Whereas cobalt(III) fluoride is a very indiscriminate fluorinating agent which generally produces saturated fluorocarbons and fluorohydrocarbons with the loss of functional groups, KCoF4 is a much milder reagent and affords more lightly fluorinated materials, often without affecting functionality and unsaturation present in the substrate. This makes it especially useful for the fluorination of heterocyclic materials.
The fluorination of hydrocarbons to give perfluorocarbons by CoF3 has been described,2 but there is relatively little known concerning the preparation of the potentially useful fluorohydrocarbons (HFCs) with this reagent. With its milder reactivity, probably due to its lower oxidation potential relative to CoF3,4 KCoF4 was an obvious reagent to use for this kind of fluorination. The fluorination of ethane (the only full study so far reported5) was shown to yield a mixture of fluorinated ethanes (eq 1). It is notable that even at 420 °C there is still only a low degree of fluorination; for comparison, CoF3 at 165 °C affords almost exclusively penta- and hexafluoroethane in equal proportions. This work has recently been repeated with essentially the same results in a Japanese patent.6
The fluorination of alkenes with KCoF4 has again only been sparsely studied. The original work (reported only in the patent literature) describes the fluorination of vinyl halides (eqs 2-4). These fluorinations provide a very simple one-step route to a number of halofluorocarbons which are quite difficult to prepare by conventional fluorine technology. The fluorination of ethylene has been described and yields essentially the same products as found for the fluorination of ethane.5,6
The fluorination of arenes by KCoF4 might be expected, if the proposed mechanism for high valency metal fluoride fluorinations is tenable, to be significantly different from that of CoF3.7 This proved to be so, as shown for the case of benzene (eq 5). Fluorobenzene and p-difluorobenzene gave essentially the same products in the same proportions, suggesting that they are intermediates in the fluorination. The fluorination of polycyclic arenes has been investigated for naphthalene (eq 6)8 and tetralin,9 anthracene (eq 7), phenanthrene (eq 8), and pyrene (eq 9).10 The compounds from these reactions have been used as sources of the corresponding perfluoroarenes.10,11
A wide range of heterocycles have been fluorinated, ranging from THF to 4-methylpyridine and some polyfluoropyridines. At temperatures between 200 and 400 °C, fluorination of THF yields a mixture of six products (eq 10). In this reaction there are two major products, again illustrating the relative mildness of the reagent.12 This is further illustrated by the fluorination of 2-methyltetrahydrofuran,13 where essentially only one product is isolated (eq 11).
The fluorination of thiophene (eq 12), tetrachlorothiophene (eq 13),14 and 2-methylthiophene (eq 14)15 have also been carried out, with results essentially similar to those obtained for THF.
Fluorination of 1-methylpyrrole at 200-240 °C gave a mixture of products slightly different from the pattern of the other heterocycles in that they are all saturated (eq 15).16
Fluorination of pyridine (eq 16) is a notable exception in the fluorination of heterocycles in that the major products are all ring-opened, although small amounts of fluoropyridines are detected.17 Fluorination of polyfluoropyridines (eqs 17 and 18), however, lead to ring-retained compounds.18
Limited studies of the fluorination of functional group-containing compounds have been made. In strong contrast to CoF3 fluorinations, the functional group is largely retained. Thus, fluorination of diethyl ketone (eq 19) yields a mixture of fluorinated ethanes and a nonvolatile fraction of acyl fluorides which was isolated by reaction withe ethanol to give a mixture of the ethyl esters of 2-fluoro- and 2,2-difluoropropionic acids.19 This clearly comes from some kind of Lewis acid-induced cleavage of the ketone followed by fluorination, since at low temperatures the unfluorinated acyl fluoride is the sole product. The fluorination of halo ketones leads mostly to cleavage products (e.g. fluorotrichloromethane from hexachloroacetone) but some fluorodichloro esters (ca. 25%) are isolated. The fluorination of esters (eqs 20 and 21) and acyl halides yields mixtures of fluorinated esters.20
Clearly there is scope for optimization of these fluorinations since the compounds formed are extremely difficult to prepare by other routes; even though the yields here are relatively modest, they nonetheless do yield working quantities of the desired products in a simple single-step reaction. There is but one example of the fluorination of nitriles and the main result of value which comes from the work is that MeCN is a good solvent for fluorinations at or below its boiling point.21 The fluorination of ethers has also been reported without significant differences from the results obtained from cobalt trifluoride fluorination.22
It is clear that potassium tetrafluorocobaltate is a useful fluorinating agent but has the constraints of needing special apparatus and a good reliable source of elemental fluorine.
Paul L. Coe
University of Birmingham, UK