Sensing fluoride ions

Figure 15-18 Calibration curve for fluoride ion-selective electrode. [Data from M. S. Front and J. W. Ross, Jr., Electrode tor Sensing Fluoride Ion Activity in Solution," Science 1966, 154, 1553.]...

The functionalized boronic acids (15 and 16) are useful for sensing fluoride ion in aqueous solution. Electrochemical redox is used for the detection of fluoride ion with ferrocenyl-boronic acid (15) [8] and for fluorescence detection with aminoboronic acid (16) [9]. Molecule (16) can effectively detect concentration of fluoride ion in the range of 5-30 mM, where the fluoride adduct is stabilized by the additional hydrogen bonding with protonated amine at pH 5.5 as shown in structure 17 (Scheme 3.35). 

Frant MS, Ross JW (1966) Electrode for sensing fluoride ion activity in solution. Science 154 1553... 

The anomalous iodoacetamide-fluoride reaction violates this rule, in that a less stable -halonium complex (18) must be involved, which then opens to (19) in the Markownikoff sense. This has been rationalized in the following way estimates of nonbonded destabilizing interactions in the possible products suggest that the actual product (16) is more stable than the alternative 6)5-fluoro-5a-iodo compound, so the reaction may be subject to a measure of thermodynamic control in the final attack of fluoride ion on the iodonium intermediate. To permit this, the a- and -iodonium complexes would have to exist in equilibrium with the original olefin, product formation being determined by a relatively high rate of attack upon the minor proportion of the less stable )9-iodonium ion. 

Tan W, Zhang D, Zhu D (2007) 4-N-Methyl-N-(2-dihydroxyboryl-benzyl) amino benzoni-trile and its boronate analogue sensing saccharides and fluoride ion. Bioorg Med Chem Lett 17(9) 2629-2633... 

In addition to metal ions, detection of halides such as fluoride is important, as the latter is often present in nerve gases and nuclear weapons manufacture. Our group utilized the unique reactivities of F with Si to specifically sense for fluoride ions [30]. PArE 35 was constructed with a masked precursor to a flu-... 

This procedure describes an example of the "aldehyde route". The addition of 2-(trimethylsilyl)thiazole (2-TST) to aldehydes occurs readily and does not require the presence of a fluoride ion source.10 The resulting secondary alcohol is as a rule isolated in very good yield. The sense of the diastereofacial selectivity of the addition reaction to chiral a-amino aldehydes can be controlled by differential protection of the... 

Yamaguchi S, Akiyama S, Tamao K. Colorimetric fluoride ion sensing by boron-containing -electron systems. J Am Chem Soc 2001 123 11372-5. 

The observed activation of allyltrihalosilanes with fluoride ion and DMF and the proposition that these agents are bound to the silicon in the stereochemistry-determining transition structures clearly suggested the use of chiral Lewis bases for asymmetric catalysis. The use of chiral Lewis bases as promoters for the asymmetric allylation and 2-butenylation of aldehydes was first demonstrated by Denmark in 1994 (Scheme 10-31) [55]. In these reactions, the use of a chiral phos-phoramide promoter 74 provides the homoallylic alcohols in high yield, albeit modest enantioselectivity. For example, the ( )-71 and benzaldehyde affords the anti homoallylic alcohol 75 (98/2 antUsyn) in 66% ee. The sense of relative stereoinduction clearly supports the intermediacy of a hexacoordinate silicon species. The stereochemical outcome at the hydroxy center is also consistent with a cyclic transition structure. 

More recently, it was demonstrated that the thermistor approach could be used to monitor specific interactions of fluoride ions with silica-packed columns in the flow injection mode. A thermometric method for detection of fluoride [56] was developed that relies on the specific interaction of fluoride with hydroxyapatite. The detection principle is based on the measurement of the enthalpy change upon adsorption of fluoride onto ceramic hydroxyapatite, by temperature monitoring with a thermistor-based flow injection calorimeter. The detection limit for fluoride was 0.1 ppm, which is in the same range as that of a commercial ion-selective electrode. The method could be applied to fluoride in aqueous solution as well as in cosmetic preparations. The system yielded highly reproducible results over at least 6 months, without the need of replacing or regenerating the ceramic hydroxyapatite column. The ease of operation of thermal sensing and the ability to couple the system to flow injection analysis provided a versatile, low-cost, and rapid detection method for fluoride. 

Strong and weak, like many other adjectives, are used in a relative sense. We do not mean to imply that the fluoride ion, F, is a strong base compared with species such as the hydroxide ion, OH. We mean that relative to the anions of strong acids, which are very weak bases, F is a much stronger base. 

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