Dicarboxylic acid fluorides

Monocationic acyl ions are readily prepared as persistent species in solutions of low nucleophile strength.68 These acyl ions have been thoroughly characterized by IR and NMR spectroscopy, and several acyl ion salts have been characterized by X-ray crystallography. The monocationic acyl ions are often prepared in situ from carboxylic acids, esters, or anhydrides, by the action of a strong Brpnsted acid, or the ions can be prepared from ionization of an appropriate acid halide with a strong Lewis acid. Both methods have been used to prepare acyl-centered dications, some of which can be considered distonic superelectrophiles. As described previously, dicarboxylic acids cleave to the bis-acyl ions in superacid (FSChH-SbFs) provided that the acyl cations are separated by at least three methylene units (eq 54).55 The first bis-acyl dications were reported by Olah and Comisarow, being prepared by the reactions of dicarboxylic acid fluorides with superacidic SbFs (eq 72).69... 

Electrochemical fluorination of dicarboxylic acid fluorides also gives a higher yield than fluorination of the parent acids. The lower yield given by the parent carboxylic acids has been attributed to decarboxylation, similar to the Kolbe reaction [31]. 

Fluorination of thiophene-2,5-dicarboxylic acitl with a sulfur tetrafluoritie-hydrogen fluoride mixture provides 2,5-bis(trifluoromethyl)thiophene in a 69% yield no fluorine addition to the thiophene ring occurs [229J. Also, imidazole mono- and dicarboxylic acids yield only the respective trifluo-romethyliinidazoles [230],... 

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

More drastic conditions of temperature, pressure and time gave carbon as the product from both aromatic and aliphatic compounds. This carbon is an activated char with decolorizing properties. Tars, tarry substances, the oxides of carbon, and the usual products of the oxidation of aromatic compounds (dicarboxylic acids) were all conspicuous by their absence. Benzotrifluoride oxidized to benzoyl fluoride. The aliphatic compounds cyclohexane, methycyclohexane, n-heptane, etc., could be oxidized to carbon and water. 

Treatment of furan-2.4- and furan-2.5-dicarboxylic acid with excess sulfur tetrafluoridc and hydrogen fluoride under forcing conditions results in simultaneous conversion of the carboxylic acid groups into trifluoromethyl groups and addition of two fluorine atoms at C 2 and C5 of the furan ring to give highly fluorinated derivatives of 2.5-dihydrofuran, e.g. formation of... 

Addition of fluorine to the thiophene ring does not occur under any conditions fluorination of thiophene-2,5-dicarboxylic acid with the sulfur tetrafluoride/hydrogen fluoride reagent yields 2.5-bis(trifluoromethyl)thiophene (16) as the sole product.157... 

Sulfur tetrafluoride fluorination of the methyl group has been reported.253 Thus, the reaction with 2,5-dimethylfuran-3,4-dicarboxylic acid gives, besides the expected acid fluoride 16 and 2,5-dimethyl-3,4-bis(trifluoromethyl)furan (17a), higher fluorinated compounds, 2-(difluoro-methyl)-5-methyl-3,4-bis(trifluoromethyl)furan (17b) and 2-methyl-3,4,5-tris(trifluoromethyl)-furan (17c). Low yields are due to considerable tar formation. 

Very little work has been done on fluoro derivatives of thiophenes. 2-Fluorothiophene was obtained in low yield from treatment of 2-iodothiophene with arsenic trifluoride. The action of fluoroboric acid on thiophenediazonium salts was unsuccessful. It may be useful for the preparation of 4-, 5-, 6- or 7-fluorobenzo[6]thiophenes from the appropriate amines. However, these are more conveniently prepared from fluorine-substituted benzenethiols by ring-closure reactions. For example 4,5,6,7-tetrafluorobenzo[6 Jthiophene was obtained by decarboxylation of the corresponding 2,3-dicarboxylic acid (equation 99) prepared by condensation of pentafluorobenzenethiol with diethyl acetylenedicarboxylate (Section 3.15.3.4.1). 2-Fluorothiophene has been prepared from 2-thienyllithium using perchloryl fluoride, and 2-fluorobenzo[ Jthiophene from the 2-lithio derivative in a similar manner (Section 3.14.3.9.1). 

Pyrazinecarboxylic acid has been obtained by selenious acid oxidation in pyridine of methylpyrazine or aqueous permanganate oxidation of ethylpyrazine, in yields of 64 and 48%, respectively.171,218 It has also been obtained in 70% yield by partial decarboxylation of pyrazine-2,3-dicarboxylic acid on heating in vacuo at 210°.219 Aqueous permanganate oxidation of 2,5-distyrylpyrazine gives the 2,5-dicarboxylic acid.220 Pyrazine-2,5-dicarboxylic acid has also been prepared in 45% yield by direct carboxylation of pyrazine with carbon dioxide at 50 atm pressure at 250° for 3 hours in the presence of a potassium carbonate and calcium fluoride catalyst.221 Pyrazine-tricarboxylic acid (57), obtainable in only very poor yields by oxidation of 2,5-dimethyl-3-ethylpyrazine, is prepared in 87% yield by alkaline permanganate oxidation of 2-(D-arabo)tetrahydroxybutyl-quinoxaline (56).222 Decarboxylation of the tricarboxylic acid by... 

Several macrocyclic dilactones such as 80 were prepared by reaction of dicarboxylic acid difluorides (e.g. 78) with triphenyltin derivatives of diols (e.g. 79) in excellent yields [63]. The required acid fluorides can be synthesized by reaction of the acids with 2-fluoro-iV-methylpyridmiumtosylate (81) and addition of triethylamine. The triphenyltin derivatives of the diols were prepared in situ from PhsSn—O—SnPh3 and the diols. 

Ether formation can become predominant in the reaction of 1,2-dicarboxylic acids with sulfur tetrafluoride, hydrogen fluoride (see Table 6). The synthesis of bis(l,l-difluoroalkyl) ethers is reviewed in Houbcn-Weyl Vol. E5/1, plO. High reaction temperatures and the presence of hydrogen fluoride favor the formation of highly fluorinated products, cf. the formation of 4, - 5, and 8." ... 

The reaction of furancarboxylic acids with sulfur tetrafluoride leads to the corresponding trifluoromethyl-substituted furans in good yields (see examples in Table 6). A number of furancarboxylic acids, mainly furan-2-carboxylic acids, and furan-2,5- and -3,4-dicarboxylic acids, show a different behavior. Here highly fluorinated dihydrofurans, e g. 25, are formed. This class of compounds is also obtained with the reagent system sulfur tetrafluoride/hydrogen fluoridc/chlorine or sulfur tetrafluoride/hydrogen fluoride/sulfuryl chloride. In some cases chlorine-containing products are found. 

A conjugated chlorofluorination of furan-2,5-dicarboxylic acid (1) occurs on heating with the sulfur tetrafluoride/hydrogen fluoride/chlorine system. 

There has been dicarboxylic acids 2-alkylimidazoles) cadmium fluoride, bicarbonate are in occurs, but in the... 

Perfluoroalkylene telomer diiodide intermediates were purchased from the Thiokol Corp. Perfluoroalkylene ether dicarboxylic acid intermediates were purchased as a mixture of the acid fluorides from PCR, Inc. under an agreement with Dupont. Properties of distilled fractions are indicated in Table II. The discrepancy in observed and calculated equivalent weights results from the presence of close boiling mono acid fluoride by-products, C3F70(CFCF20)nCFCF3C0F. 

Hydrolysis of each acid fluoride fraction and redistillation of the carboxylic acids allowed separation of mono- from dicarboxylic acids (Table III). Thus, the 1 1 dicarboxylic acid and the monocarboxylic acid formed from three molecules of hexafluoropropylene epoxide were isolated from the 1 1 oligomer acid fluoride fraction shown in Table II. The acids were converted to acid chlorides in high yields (about 90%) by prolonged refluxing with thionyl chloride (Table IV). A trace of pyridine was added as a catalyst. 

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