Potassium tetrafluorocobaltate

Considerable differences are evident in the results of fluonnations of toluene With potassium tetrafluorocobaltate at 340 C [25] and with cesium tetra-flnorocobaitate at 320 C [29] (Table 2)... 

Table 1. Comparison of Fluorination of Benzene with Manganese Trifluoride [7], Potassium Hexafluoronickelate [24], Silver Difluoride [6], and Potassium Tetrafluorocobaltate [6]...

Table 2. Fluorination of Toluene with Potassium Tetrafluorocobaltate at 340 °C [25] and Cesium Tetrafluorocobaitate at 360 C [29]...

Fluorination of aliphatic ethers at gentle conditions with cobalt trifluoride or potassium tetrafluorocobaltate do not give perfluorinated products and cause only negligible cleavage of the ether bond. Complex mixtures are formed from ethyl methyl ether and from diethyl ether [9] (equations 16 and 17)... 

As a starting material, tetrahydrofuran has little to recommend it except in rare cases, one of which is the preparation of fluorofurans. With cobalt(III) fluoride, tetrahydrofuran yields a mixture of polyfluoro derivatives from which alkali fusion removes HF leaving various fluorofurans including tetrafluorofuran and 2,3,4-trifluorofuran (1). Potassium tetrafluorocobaltate acts on tetrahydrofuran giving 2 as the main product and alkali fusion converts this into 2,5-difluorofuran. The fluorofurans all polymerize readily and are rather unresponsive to electrophilic reagents.17... 

A few examples exist in which halogen-halogen exchange gives rise to fluorothiophenes. Both addition and halogen exchange occurred when potassium tetrafluorocobaltate reacted with tetrachloro- or 2-iodo-thiophene [71JCS(C)346]. 

Ethane30 over cobalt(III) fluoride at 165=C gives mainly pentafluoroethane (38%) and hexafluoroethane (40%), together with small amounts of all the other possible C2FnH6 n isomers. In a batch reactor at 100°C, 1,1-difluoroethane (26%). 1,2-difluoroethane (30%), and 1,1,2-trifluoroethane (30%) arc the major products.25 Potassium tetrafluorocobaltate(III) is much less reactive than cobalt(III) fluoride and even at 420 C mainly gives30 a lower degree of fluorination 1,2-difluoroethane (25%), 1,1-difluoroethane (11%), 1,1,2-trifluoroethane (29%), and 1,1,2,2-tetrafluoroethane (10%). 

When toluene is fluorinated65 over potassium tetrafluorocobaltate(III), the major products at 330-340cC are notable for being unsaturated and for containing a difluoromethyl group they are also similar in type to those produced in the potassium tetrafluorocobaltate(III)/ben-zene reaction. Over cesium tetrafluorocobaltate(III), toluene and -xylene give66 about 25-30 % of aromatic compounds. 

Many of these compounds have also been fluorinated over potassium tetrafluorocobalt-ate(III) when compounds with double bonds result. Naphthalene and 1,2,3,4-tetrahydronaphthalene at 300°C give72 very similar product mixtures, the main components of which were the monoenes 17 (58%) and 18 (33%). 

Several partially and fully fluorinated aromatics have been further fluorinated. Fluoro- and 1,4-difluorobenzene give13 the same products as benzene over potassium tetrafluorocobalt ate(lll), in keeping with the current theory5 of fluorination. 

Potassium tetrafluorocobaltate(III) at 200°C reacts with tetrahydrofuran to give82 unsaturated products the major ones are 5 and 6, although the overall yield is poor (< 30%). Furan itself gives no products at all over cobalt(IIl) fluoride it presumably polymerizes. This does not, however, rule furan out as an intermediate in the tetrahydrofuran fluorinations (it could form by desaturation, as does benzene in the fluorination of cyclohexane, vide supra). 2-Methyl-and 2,5-dimethyltetrahydrofuran83 have also been fluorinated with similar results to tetrahydrofuran. 

Dioxane is partially fluorinated84 over cobalt(lll) fluoride and potassium tetra-fluorocobaltate(III). With cobalt(III) fluoride at 100°C yields are poor (ca. 15%), and the major products are 7-10 (as with the oxolanes, equilibrium occurs between the stereoisomers during the fluorination). With potassium tetrafluorocobaltate(III) at 220°C, compound 11 comprises 61 % of the product mixture in an overall yield of about 40%. 

Oxathiane has only been fluorinated84 over potassium tetrafluorocobaltate(III) (vide infra). and this gives products very similar in structure to those formed in the dioxane/cobalt(Hl) fluoride fluorination. 

Fluorination of phenylacetic acid over potassium tetrafluorocobaltate(lll) at 360 "C gives65 a very low yield of methyl perfluoro(cyclohexylacetate) (workup by methanol treatment). 

Methylpropanoyl fluoride gives88 a very low yield of the a-fluoro derivative on fluorination with potassium tetrafluorocobaltate(III) at 250 C, while 2,2-dimethylpropanoyl fluoride gives no carbonyl-containing products. 

The methyl esters of dicarboxylic acids88 give no dicarboxylic products only mono-carboxylic acid derivatives are formed. Dimethyl succinate, dimethyl maleate and dimethyl acetylenedicarboxylate over potassium tetrafluorocobaltate(III) at 270-350"C all give mixtures of (crude products are converted into ethyl esters) ethyl pentafluoropropanoate and ethyl... 

Propanoyl chloride is unusual in that over potassium tetrafluorocobaltate(III)somechloro-fluoro derivatives are formed,88 e.g. ethyl 2-chloro-2-fluoropropanoate. Chlorination during fluorination is known with other substrates (Section 25.1.1.6.). 

Methyl-1 //-pyrrole gives93 a very similar product mix Over cobalt(III) fluoride at 140 C to 1-methylpyrrolidine,92 suggesting that the latter is converted to the former by some sort of desaturation reaction. Fluorination over potassium tetrafluorocobaltate(III) at 220 C gives a similar result.93... 

Using cobalt(III) fluoride at 150 , pyridine gives94 mainly perfluoro(l-methylpyrrolidine) and its 3H derivative, together with a small quantity of an open-chain compound. With potassium tetrafluorocobaltate(III),94 95 open-chain compounds predominate, the main ones being perfluoro(A-methylbutan-l-imine) and 2//,3//-heptafluoro(Af-methylbut-2-en-l-imine). Also formed are smaller amounts of polyfluorinated 1-methylpyrrolidines (ca. 5 % of the product mixture) and polyfluoropyridines (ca. 10%). 4-Methylpyridine gives95 only open-chain products, akin to those formed from pyridine, over potassium tetrafluorocobaltate(III) at 200-220 CC. 

Acetonitrile gives99 very similar reaction mixtures over cobalt(III) fluoride (145 C) and potassium tetrafluorocobaltate(III) (400°C). The main product is fluoroacctonitrile (ca. 20% true yield) followed by 1,1,1,2-tetrafluoroethane (ca. 10%). Rhombohedral nickel(III) fluoride and potassium hexafluoronickelate(IV) in anhydrous hydrogen fluoride solution at — 20 to — 25 =C convert58 acetonitrile into trifluoroacetonitrile and 7V,7V-difluoropentafluoroethyl-amine, in a reaction reminiscent of the Simons electrochemical process. 

The fluorination of propanonitrile" was very reminiscent of methyl propanoate and pro-panoyl fluoride (Section 25.1.1.3.), in that replacement of the cc-fluorines is favored over the fi. Again cobalt(III) fluoride (205°C) and potassium tetrafluorocobaltate(III) (335°C) give similar product mixtures albeit at different temperatures. Yields are better over potassium tetra-fluorocobaltate(III) (> 40%) and the major product is 2-fluoropropanonitrile followed by 2,2-di-, 2,2,3-tri- and 2,2,3,3-tetrafluoropropanonitrile. 

Succinonitrile behaves very oddly" over potassium tetrafluorocobaltate(III) at 250°C. A mixture of comparable quantities of difluoromaleo- and -fumaronitriles results, in ca. 40% true yield. Maleonitrile itself gives a very similar mixture of products. As was previously stated, the fluorination of the analogous carboxylic compounds results in the loss of one of the carboxy moieties. 

Malononitrile over potassium tetrafluorocobaltate(III) gives a mixture of at least 35 products and the benzonitrile product is almost as complex." However, over cesium tetra-fluorocobaltate(III) at 300°C, benzonitrile gives100 a much simpler mixture and perfluorocyclo-hexanecarbonitrile (ca. 30%) is the major product, with perfluorocyclohexane and per-fluorobenzonitrile as minor ones. Perfluorobenzonitrile is converted, in over 50% yield, by both cobalt(III) fluoride (165-170°C) and potassium tetrafluorocobaltate(III) (210°C), into mainly the nonconjugated monoene perfluoro(cyclohex-3-enecarbonitrile). 

The fluorination of oxathiane is described in Section 25.1.1.3.84 1,4-Dithiane reacts quite differently107 from dioxane and oxathiane over potassium tetrafluorocobaltate(III). The major products (ca. 60 % of the product mixture, itself in ca. 50 % yield) are the result of rearrangement to the 2-methyl-l, 3-dithiolane skeleton, e.g. 1-3. Fluorinated 1,4-dithiancs comprised about 30% of the product the major product is 4. The extent of fluorination, that is the number of fluorine atoms introduced, is much greater than that of dioxane under comparable conditions (tetrahydrothiophene and tetrahydrofuran show a similar contrast). 

The product from the fluorination of thiophene over potassium tetrafluorocobaltate(III) varied, not surprisingly, with temperature.108 At 120 C, virtually the sole product was the dihydrothiophene 5, albeit in low yield (14%) this compound is obviously analogous to 2,2.5,5-tetra-fluoro-2,5-dihydrofuran from tetrahydrofuran and potassium tetrafluorocobaltate(IIl). At 350-370 °C, 5 is still a major product, but now seven others are also present, the main ones being 6-8 and the sulfur extruded product perfluorobutane. 

Tetrahydrothiophene gives108 much the same product mixture as thiophene, lending further support to the theory that saturated compounds are desaturated before fluorination in this case this theory is strengthened by the conversion, in 10% yield, of tetrahydrothiophene into thiophene by potassium trifluorocobaltate(II) (KCoF,) containing a small amount of potassium tetrafluorocobaltate(III) (at 290 °C). 

Manganese(III) fluoride gives108 much the same product mixture from thiophene at 300-310°C as did potassium tetrafluorocobaltate(III) at 350°C. 

Methylthiophene is remarkable in that, over potassium tetrafluorocobaltate(III) at 200 CC, it gives109 the dihydrothiophene 9 in 87 % yield as virtually the sole product. 

Trichloroethene and cobalt(III) fluoride react at 120°C to give111 l,l,2-trichloro-l,2,2-trifluoroethane (26%) and l,l,2,2-tetrachloro-l,2-difluoroethane (10%) as major products, and the simple fluorine adduct, l,l,2-trichloro-l,2-difluoroethane (5%), as a minor product (seven products are identified). With manganese(III) fluoride at 220"C, the simple adduct111 is the major product (40 %), as it is112 (70 %) with potassium tetrafluorocobaltate(III) at 250 CC. 

Tetrachloroethene over potassium tetrafluorocobaltate(III) at 180°C gives112 mainly the fluorine addition compound l,l,2,2-tetrachloro-l,2-difluoroethane (41 %). 

Hexachloroacetone is fluorinated87 over potassium tetrafluorocobaltate(III) at 230°C. As with nonchlorinated ketones (Section 25.1.1.3.), no ketonic products are detected, and low yields of trichlorofluoromethane, methyl dichlorofluoroacetate (the reaction mixture is worked up by methanol treatment) and methyl trichloroacetate are isolated. 

Tetrachloro-l-methylpyrrole is fluorinated92 with potassium tetrafluorocobalt-ate(III) at 180°C to give a mixture (ca. 60% yield) of 3,4-dichloro-2,2,5,5-tetrafluoro-l-methyl-2,5-dihydropyrrole (1) and 3,4-dichloro-2,2,5,5-tetrafluoro-l-(fluoromethyl)-2,5-dihydropyr-role (2). 

Complex fluorides (LiBiF6, and KBiF6) are known1 and they are weaker oxidizing agents than bismuth(V) fluoride itself. If parallels with potassium tetrafluorocobaltate(III) and co-balt(III) fluoride are valid (see Section 25.1.), then they will also be weaker fluorinating agents. 

Fluorination of tetrachlorothiophene with potassium tetrafluorocobaltate(III) gave 3,4-dichlorotetrafluoro-3-thiolene (156). Similar fluorination of thiophene yielded the 3-thiolene (157) and the thiolan (158) <71JCS(C)346, 72T43). It has been suggested that the fluorination proceeds by initial oxidation of thiophene to the cation-radical by the metal ion, and subsequent quenching of this by atomic fluorine (Scheme 30). 

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