Fluoride displacement

With certain exceptions, primary fluorination of carbohydrate derivatives is very facile. Fluoride displacement of sulfonyloxy groups by potassium, or cesium, fluoride in 1,2-ethanediol and a variety of other solvents has frequently been used. Tetrabutylammonium (sometimes tetraethylammonium) fluoride in dipolar, aprotic solvents (mainly ace-... 

Secondly, the factors controlling nucleophilic displacements of sulfonic esters had already been determined to a considerable extent, and have been described in reviews in this Series103,104 and, in addition, the steric and polar factors governing such displacements have been summarized qualitatively,101 and will therefore not be discussed here in any detail. This accumulated knowledge made possible the prediction of the course of such fluoride displacements. 

Aromatic substituent effects due to phosphorus groups have been studied for a number of reactions.47 Thus ester hydrolysis and fluoride-displacement rates, for (56) and (57) respectively, are enhanced by phosphorus substituents (X = O or ), while the rate of hydrolysis of the halide (58) is enhanced for X = , but slowed for X = O.47 A perturbation M.O. analysis of these observations has been presented.48... 

Fluoro-2-phenylquinazoline has been prepared by fluoride displacement of chlorine from 4-chloro-2-phenylqui-nazoline <2002JOC8991>. 

The electrochemical oxidation of bromoalkanes on platinum in anhydrous hydrogen fluoride displaces bromine by fluorine.27... 

Anhydrous potassium fluoride displaces both the halogen atoms in 3-bromo-3,3-difluoro-propanoyl chloride (5) with fluorines at room temperature to give the acid fluoride 6 in 66% yield.30... 

Formamidc has been found to be a very suitable solvent for fluoride displacement reactions on optically active substrates leading to more reduced racemization that is in comparison to other aprotic solvents like tetramethylene sulfone, l,3-dimethyl-3,4,5,6-tetrahydropyrimidin-2(l/7)-one etc.146 Formamide has a high polarizability favoring SN2 reactions and a high polarity147 rendering potassium fluoride sufficiently soluble in the reaction mixture. Although the reaction rate is reduced, the less polar solvents A-methylformamide, acetamide or A-methyl-acetamide can also be used as solvents for the reaction. 

Cesium fluoride displaces the nitro group in the activated l,l,2,2,3,3-hexafluoro-4-nitroindanc (17) with fluorine to give 1,1,2,2,3,3,4-heptafluoroindane (18).171... 

The complexation of a calixpyrrole dimer with the p-nitrophenolate anion [49] has been used as a colorimeter sensor by displacing the chromogen anion upon the addition of targeted anions. In this case anions, such as fluoride, displace the p-nitrophenolate anion from the complex thus enhancing the absorbance of the p-nitrophenolate anion. This was observed as a colourless to yellow colour change. 

Scheme 40 exemplifies a common situation in these systems which display competing reaction channels, i.e. fluoride displacement (a) as well as a rather unusual reaction (b and c) thought to occur via attack of the nascent F onto the neutral product276. 

F]DGal, 119, a useful tracer to measure galactose metabolism in the liver with PET162b, has been synthesized by 18F ion displacement from a triflate163-165. The protected carbohydrate 120 has been oxidized163 and then reduced by LAH to give the talose derivative 121 which, in turn with triflic anhydride and pyridine, furnished the triflate intermediate 122. [18F]fluoride displacement on the latter and deprotection by acid hydrolysis yielded 119, in 30% r.y. in 90 minutes. 

Cyclization can also occur through halide displacement after initial metalation. For example, acylation of o-lithiofluorobenzenetricarbonylchromium with y-butyrolactone at 25 °C for 24 h is followed by spontaneous fluoride displacement to give complex 36. Oxidation with excess iodine liberates the lactone in 48 % overall yield (Scheme 15) [20]. 

In a synthesis of (-)-reserpine, Stork was unable to effect selective protiodesilylation, using HBF4, of a PhMe2Si group in the presence of a benzyl ether. However, alternative use of a 2-futyldimethylsilyl group obviated diis difficulty, due to the enhanced reactivity of the fiiran ring towards fluoride displacement under neutral conditions. 

We have also seen that ort/w-lithiation can be used in conjunction with other reactions - such as fluoride displacement or an anionic Fries rearrangement - that make ort/w-lithiation a particularly versatile strategy. 

Fluoride ruins the MI enhancement resulting from the two-step activation of Cr/silica-titania catalysts. This tendency probably indicates that fluoride binds to surface titania to displace chromium on the more reactive sites that produce low-MW polymer. An example is shown in Table 42. A silica-titania (5 wt% Ti) was calcined at 820 °C, impregnated with 0.5 wt% Cr as bis(f-butyl) chromate in hexane, and then activated in air at 315 °C. It produced polymer with a MI of 77. It was then dry mixed with 1 wt% ammonium hexafluorosilicate and calcined again at 315 °C. When retested, it produced polymer having a MI of only 0.5, which is comparable to that of Cr/silica activated at 820 °C. This comparison suggests that fluoride displaced chromium from the titania, leaving a catalyst comparable to Cr/silica. 

This tendency is seemingly an advantage to polyethylene manufacturers. However, the titania tends to absorb the fluoride, perhaps selectively, to form Ti-F surface groups. Consequently, the fluoride displaces chromium from the titania. It converts the Cr/silica-titania catalyst back to one resembling Cr/silica, which is known to concentrate the branching mostly in the low-MW side of the distribution (see Figure 103). 

LC-MS studies indicate that tyrosinase oxidation of 4-fluorocatechol gives several products including 2,3-dihydroxy-6-fluorodioxin, formed via fluoride displacement from 4-fluoro-l,2-benzoquinone and cyclisa-tion of the resulting 4-(2-hydroxyaryloxy)-l,2-benzoquinone (08MI1). 

Two papers describe the reaction of 6,3-lactone derivatives with fluorinating agents. In one, the 5-triflate (22) was subjected to fluoride displacement and the product concerted to 5-deoxy-5-fluoro-D-glucofuranose as depicted in Scheme 6. In the second, 1,2-0-... 

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