Carboxylic acids reactions with xenon difluoride

Carboxylic acids react with xenon difluoride to produce unstable xenon esters The esters decarboxylate to produce free radical intermediates, which undergo fluonnation or reaction with the solvent system Thus aliphatic acids decarboxylate to produce mainly fluoroalkanes or products from abstraction of hydrogen from the solvent Perfluoro acids decarboxylate in the presence of aromatic substrates to give perfluoroalkyl aromatics Aromatic and vinylic acids do not decarboxylate [91] (equation 51)... 

The dimerization process is observed in the reaction of 3-phenylbicyclo[l. 1. l]pentan-l-oicacid, and free radical intermediates were suggested36,37 however, only a reduction process was found by Della and Headland Michl and coworkers39. Reaction of various bridged carboxylic acids with xenon difluoride gives, beside fluoro-substituted products, also reduced products, while the product distribution depends on the structure of the alkane37,38 (Scheme 10). 

Decarboxylative fluorination, presumably via intermediate acyl hypofluorites, has been achieved in low yield by passing a dilute stream of fluorine in nitrogen into aqueous solutions of sodium carboxylates. A somewhat more promising method, tolerant of aryl groups, ketones and ethers, involves reaction of a dichloromethane solution of the acid with xenon difluoride and hydrogen fluoride (equation 23). ... 

Alkyl fluorides have been generated from the reaction of carboxylic acids with one equivalent of xenon difluoride in dichloromethane or chloroform solution. The process has been named fluorodecarboxylation", the fluorine analog of the Hunsdieckerand Kochi reaction.8384 Both primary and tertiary acids react very well, but secondary acids react less readily. A possible scheme involves a free-radical mechanism including an unstable fluoroxenon ester of an appropriate acid.84... 

Xenon difluoride reacted with various carboxylic acids, and the type of transformation depends on the structure of the organic molecules35-39. The reaction with primary carboxylic acids involves free-radical intermediates. 6-Hexenoic acid was used as a free-radical clock device in which a A abs of 1.1 x 106 M-1s-1 at 25 °C was determined, while the alkyl radical was also spin-trapped to give an ESR signal37. The primary free radical was trapped by internal cyclization, and (fluoromethyl) cyclopentane in 25% yield was formed, while 6-fluoro-l-hexene could be formed from a radical or ionic intermediate, but 1-fluo-rocycloclohexane was not observed as a product (Scheme 42). 

Radical-cations may also be involved in the formation of phenyl esters from aromatic carboxylic acids and benzene in the presence of xenon difluoride, and can apparently be detected by e.s.r. spectroscopy during the reaction of iViV-dimethylaniline with perfluoropiperidine. The last reagent gives a low yield of o- and p-fluorophenol in the ratio 3.5 1 in its reaction with sodium phenoxide, and the predominance of ortho substitution is ascribed to a cage mechanism of the type shown in (3). However, towards 2,4,6-tri-t-butylphenoxyl radicals the piperidine acts as a source of fluorine atoms to give the dienone (4). ... 

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