Phosphazenium fluoride

The displacement of cyclooctene or C2H4 from an iridium(I) centre by a variety of chiral phosphines (L) leads to the formation of [ (L)IrCl 2] which, in conjunction with a source of F (phosphazenium fluoride), has been used for catalytic hydroamination of olefins. This combination leads to a 6.5 fold increase in the activity of the system and a total reversal in the enantioselectivity compared to that of the chloride analogue. There is no direct evidence of formation of a metal fluoride complex, but it is proposed that it may well form in situ and that this might explain these interesting results [75]. 

As a Carbon Nucleophile in Lewis Base-catalyzed Reactions. Allylation of alkyl iodides with allyltrimethylsilane proceeds in the presence of phosphazenium fluoride. Tetra-butylammonium triphenyldifluorosilicate (TBAT) is useful for allylation of aldehydes, ketones, imines, and alkyl halides with allyltrimethylsilane (eq 63). 55 Similarly, TBAHF2 is an effective catalyst for allylation of aldehydes. The homoallylamines are synthesized from allyltrimethylsilane and imines with a catalytic amount of TBAF (eq 64). The reactions of thioketones as well as sulfines with allyltrimethylsilane can be mediated by TBAF to give allylic sulfides and allyl sulfoxides, respectively. Besides fluoride ion, 2,8,9-triisopropyl-2,5,8,9-tetra-aza-1-phosphabi-cyclo[3.3.3]undecane promotes the allylation of aldehydes with allyltrimethylsilane as a Lewis base catalyst (eq 65). ... 

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). 

When activated by anionic catalysts [potassium fluoride, cesium fluoride, tctraalkylammonium fluorides, tris(dimethylamino)sulfonium difluorotrimethylsilicate, phosphazenium, hexa-methylpiperidinium and cobaltoccnium fluorides, tetrabuiylammonium difluoro(triphenyl-silyl)silicatc. the complex tetrakis(dimethylamino)ethene/pcrfluoropropene, ammonium (and phosphonium) periluorocyclobuianc ylides], trimethyl(perfluoroalkyl)silanes will generate C-Rp bonds from carbon-halogen bonds. 

A very effective means of increasing the nucleophilicity of naked fluorides is efficient charge distribution over a large organic cation, for example tetrakis(dialky-lamino)phosphonium or phosphazenium ions (Scheme 2.12). 

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