Dicarboxylic acid fluorides, reactions

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]. 

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). 

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. 

The reaction tolerates the presence of a phenyl group at the carbon atom that forms the bond with the aryl ring well, and can be stopped before decarboxylation or proceeds further to monocarboxylic acids derivatives of tetralin. As an example, cyclization of malonate 28 at room temperature with the use of anyhydrous hydrogen fluoride gives dicarboxylate 29." The anhydrous pyridinium poly(hydrogen fluoride) is more efficient in the same reaction. Compound 29 is obtained with almost quantitative yield with purity sufficient to use a crude product for the follow-up synthetic transformations, which lead to derivatives of l-phenyl-3-aminotetralins 30 that exhibit potential for the treatment of Parkinson disease. ... 

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