Fluorides, acyl sulfonyl

Two other functional groups that contain a single fluorine substituent are acyl fluorides and sulfonyl fluorides. Such fluorines are among the rare few that absorb in the highly deshielded region downfield of CFC13. 

Sulfonylation. Under Friedel-Crafts reaction conditions, sulfonyl haUdes and sulfonic acid anhydrides sulfonylate aromatics (139), a reaction that can be considered the analogue of the related acylation with acyl haUdes and anhydrides. The products are sulfones. Sulfonyl chlorides are the most frequently used reagents, although the bromides and fluorides also react ... 

Although the chemical shifts of most commonly encountered orga-nofluorine compounds are upheld of CFC13 and thus have negative values, there are a number of structural situations for fluorine that lead to positive chemical shifts (downfield from CFC13). These include acyl and sulfonyl fluorides as well as the fluorines of SF5 substituents. 

As indicated in Chapter 2, the single fluorine substituent has an extremely broad range of observed chemical shifts, which include sulfonyl fluorides and acyl fluorides absorbing downfield in the region of +40 and +25 ppm, respectively, all the way up to fluoromethyltrimethylsilane, with its signal far upheld at -277 ppm. 

Furthermore, the cleavage of organic sulfites and sulfates by hydrogen fluoride gives the corresponding alkyl or acyl fluorides in fair to good yield,287 e.g. formation of acetyl fluoride from the mixed anhydride287 or sulfonyl fluorides from sulfonic acid anhydrides.287... 

A very reactive halogen atom, such as that of mi acyl or sulfonyl halide, is replaced by fluorine by the action of almost any inorganic fluoride. The most convenient method consists in heating gently a mixture of an acyl or sulfonyl chloride with zinc or antimony fluoride in an apparatus which permits the acyl fluoride to distil as it is formed. The acyl fluoride usually boils about 40° lower than the chloride, and its removal from the reaction mixture results in quantitative yields. Com- plete interchange also can be effected with hydrogen fluoride, but more elaborate equipment is required. Good results have been reported for the synthesis of formyl and acetyl fluorides from mixtures of fonnic or... 

Lithiation of compound 560 with s-BuLi-TMEDA in THF at —78 °C following an inverse addition protocol provided the anion 561. It reacts with primary alkyl iodides and triflates, silyl chlorides, diphenyl disulfide, epoxides, aldehydes, ketones, imines, acyl chlorides, isocyanates and sulfonyl fluorides to yield the expected compounds 562 (Scheme 152). The transmetallation of compound 561 with ZnBr2 allowed the palladium-catalyzed cross-coupling reaction with aryl and vinyl bromides837. When the reaction was quenched with 1,2-dibromotetrafluoroethane, the corresponding bromide 562 (X = Br) is obtained838. 

Before workup of the reaction of the dibutylstannylene acetal of a diol with an electrophile such as an acyl, alkyl, or sulfonyl halide, the product present in nonpolar solvents has a halodibutylstannyl group attached to the nonreacted oxygen atom. This organotin derivative can be cleaved with water or mild acid, but chromatography on silica gel is usually sufficient to remove it. Some research groups have made use of the strong Sn-F bond by washing with fluoride ions. 

Kinetic studies of the acylation of imidazoles with benzoyl halides and sulfonyl halides have been carried out. In acetonitrile, benzoyl fluoride reacts with imidazole in a mixed third- and fourth-order reaction i.e., the rate-determining transition state contains the acid fluoride and two or three molecules of imidazole. With benzoyl chloride in benzene a second-order equation is followed similar to the aroylation of anilines ... 

Kinetic studies of the acylation of imidazoles with benzoyl halides and sulfonyl halides have been carried out. In acetonitrile, benzoyl fluoride reacts with imidazole in a mixed third- and fourth-order reaction ... 

---