Cesium nucleophilic fluoride source

More general procedures for additions of halogen fluorides to highly fluori-nated olefins involve reactions with a source of nucleophilic fluoride ion, such as an alkali metal fluoride, in the presence of aposttive halogen donor [62 107, lOff, 109, 110, 111] (equations 11 and 12) These processes are likely to occur by the generation and capture of perfluorocarbamonic intermediates Tertiary fluormated carbanions can be isolated as cesium [112], silver [113], or tns(dimethylamino)sul-... 

Scheme 27. Addition of cesium fluoride, as the most suitable fluoride source, was necessary to rich the high yields. The fluoride ion reacts with aryltrialkylstannane to form a hypervalent, pentacoordinative, organotin species typically more nucleophilic than its tetravalent precursor. In this manner, the fluoride ion does co-eatalyse the Stille reaction.

Generation of Nitrosoalkenes. Treatment of silyloximes of a-halocarbonyl compounds with fluoride ion generates nitrosoalkenes without extraneous nucleophilic species present. Nitrosoalkenes generated from (6) have been used in intramolecular [4 + 2] cycloadditions (eq 1). Electron-rich alkenes are required for concerted cycloadditions as this is an inverse electron-demand cycloaddition. The choice of fluoride source is critical for success in that the highest yields are obtained when the ni-trosoaUcene is slowly generated via use of sparingly soluble metal fluorides such as cesium fluoride or potassium fluoride. 

Obviously, the fluoride ion cannot be totally naked in a real chemical environment however, successes such as those mentioned above can be attributed totally to more basic or nucleophilic sources of fluoride such as cesium fluoride, tetramethylammonium fluoride, and the phosphazenium fluoride [ (CH3)2N 3P=N=P N(CH3)2 3] F (46). In each case, a bulky cation is used in order to maximize the cation-anion distance and thus minimize their interaction. In addition, it is important to have a cation that is as stable as possible towards oxidation. Furthermore, it is necessary for the fluoride source to be anhydrous. It is with these reasons in mind, and the well-known stabilization of a complex anion by the use of a bulky cation (47,48), that Thrasher and co-workers prepared a potentially "naked" SF5 salt using Cs ([18]crown-6)2 as the cation. Although the closest cation-anion contacts in this salt were over 7 A, the SF5 anion in this salt failed to act as a nucleophile when reacted with methyl iodide (49). Seppelt and co-workers have recently published the structures of an SeFs" salt and several TeFs" salts in which they describe weak interactions between the chalcogen atom of one anion with the fluorine atom of a neighboring anion (50). 

Halogen exchange reactions, too, are valuable sources of fluorinated pyrimidines, especially 2-, 4-, and 6-fluoro derivatives. Sources of fluoride ion have included hydrogen, sodium, potassium, cesium, antimony, silver fluorides, and sulfur tetrafluoride. All of the reactions required heating. Prepared from nucleophilic fluorination processes have been... 

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