Fluoride ion activator

The lanthanum fluoride crystal is a conductor for fluoride ions which being small can move through the crystal from one lattice defect to another, and equilibrium is established between the crystal face inside the electrode and the internal solution. Likewise, when the electrode is placed in a solution containing fluoride ions, equilibrium is established at the external surface of the crystal. In general, the fluoride ion activities at the two faces of the crystal are different and so a potential is established, and since the conditions at the internal face are constant, the resultant potential is proportional to the fluoride ion activity of the test solution. 

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.]...

Equation 76) <1993OM3019>, which react as ester and ketone enolate equivalents, respectively. The latter reaction requires the use of fluoride ion activation (tetrabutylammonium fluoride, TBAF) to actuate the addition. Central carbon alkylation is less common for allylpalladium reactions despite this, nucleophilic alkylation of TMEDA-stabilized 1,3-diphenylallyl palladium complexes proceeds selectively to the central carbon (Equation 77) <1995AGE100>. 

Various fluorides may be precipitated from aqueous solution for use as constituent powders in solid state reactions. Co-precipitation offers very elegant access to intimate mixtures, but the actual products are strongly dependent on the fluoride ion activity within the solution but also on the stability constants of the respective metal complexes. Accordingly, not only anhydrous fluorides are obtained, but also hydrated fluorides or hydroxide fluorides, which may be very difficult to convert to pure fluorides. As noted already [3], reactive compounds, e.g. carbonates, acetates, oxalates, hydroxides etc., which quite easily dissolve in acidic HF solutions, are the preferred starting materials for fluoride syntheses. In contrast, many oxides which have been heated to rather high temperature are frequently unreactive and may not dissolve at all. To enhance reactivity but also improve crystallinity of the product, it has proved useful to perform reactions above the boiling point of water in adapting the hydrothermal method, which has already been shown to be useful in the recrystallisation of materials which are more or less insoluble at ambient temperatures and pressures. Up to about 240°C even PTFE vessels may be used. A number of selected examples with respective reaction conditions are listed in Table 3. 

The fluoride ion activator can be replaced by a hydroxide ion, and the fluoride ligand on silicon by chlorine [15]. Thus, the palladium-catalyzed coupling of alkenylchlorosilanes with aryl halides was accomplished in the presence of NaOH under mild conditions (Eq. 12). It is worthy to note that the system can be... 

The kind of a fluoride ion activator and the leaving group in electrophiles affects the stereochemistry in the cross-coupling reaction of allylsilanes as exemplified with 2-cyclohexenyl(difluoro)phenylsilane (Eq. 31) 135]. 

In practice, a single crystal of europium-doped lanthanum fluoride (K,p = 2 X 10" ) responds in a matter of seconds according to (13-4) to fluoride ion activity in solutions containing fluoride from about 1 Af to 10" M. Europium doping increases fluoride mobility by introducing lattice disorder. Hydroxide ion, beginning at about 10" Af, is the principal interference, probably the result of competition due to the low solubility of lanthanum hydroxide. 

For the specific case of a standard for fluoride ion activity KF rather than NaF has been suggested. KF is a better choice because ion pairing is much less. Further, the average hydration number of the fluoride ion is almost the same as that for potassium ion, so that activity coefficients of the two ions are similar. Suggested reference activity values (pM or pAJ for use in the operational definitions for ion-activity measurements [Equations (13-26) or (13-27)] are shown in Table 13-2. For the case of fluoride ion, measurements of its activity in NaF-NaCl mixtures up to 1 m and KF-KX mixtures up to 4 m yielded the same values as pure NaF or KF at the same ionic strength. 

Nordstrom (2001) demonstrated that if one compares computed free-fluoride ion activities or concentrations (in this example, with WATEQ4F) with measured values obtained with a fluoride ion-selective electrode, the results are in good agreement down to 10m. This agreement between measurement and calculation corroborates the fluoride speciation by the lA method. 

Nordstrom D. K. (2001) A test of aqueous speciation measured vs. calculated free fluoride ion activity. In Proc. 10th Int. Symp. Water-Rock Interaction, WRI-10, Villasimius, Sardinia, July 10-15, 2001 (ed. R. Cidu), pp. 317-320. 

In addition to alkenyl fluorosilanes, alkenyl(alkoxy)silanes also undergo the crosscoupling reaction with the aid of a palladium catalyst and fluoride ion activator [Eq.(ll)] [9]. 

Fluoride ion activator can be replaced by hydroxide ion. To avoid hydrolysis of chlorosilanes, a heterogeneous system consisting of powdered sodium hydroxide in benzene was found to be particularly effective [Eq.(21)]. [20]. 

A crystalline electrode for fluoride ion is available from commercial sources. The membrane consists of a slice of a single crystal of lanthanum fluoride that has been doped with europium(II) fluoride to improve its conductivity. The membrane, supported between a reference solution and the solution to be measured, shows a theoretical response to changes in fluoride ion activity from 10 to 10 M. The electrode is selective for fluoride ion over other common anions by several orders of magnitude only hydroxide ion appears to offer serious interference. 

Syntheses and transformations of heterocycles mediated by fluoride ion-activated organosilicon compounds 02H(57)361. 

Fluoride ion activation of aryltrimethylstannane is important for the enantioselective catalytic arylation of aldimine (16) (Scheme 3.43) [6]. Titanium enolates (20) of trifluo-romethyl ketones (19) are stable and can be alkylated anfi-selectively at the a-carbon in... 

Mechanistically, hydrophosphonylation and hydrothiophosphonylation takes place via pentacoordinate state that could be achieved only toward fluoride ion activation F in ionic liquid medium. Other anions, e.g. Cl , Br , I or HP04 did not work because they are not strong enough to lead to hypervalency. If other sterically hindered and less reactive cyclic phosphites (47a) and (47b) were treated with equimolar quantity of TBAF, the P-F bonded intermediate (48a) or (48b) was formed as confirmed by NMR analysis. These intermediates were then reacted with allyl bromide H2C = C(C02Me)CH2Br, to give phosphonate (49) as a sole product (Figure 4). 

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

Dimethyl(phenyl)silane reduces aldehyde and ketone carbonyls with the aid of fluoride ion or acid. a-Acylpropionamides, 1-aminoethyl ketones, and 1-alkoxyethyl ketones are readily converted into the corresponding -hydroxy amides, o -amino alcohols, and a-alkoxy alcohols, respectively. The stereoselectivity is complementary and generally high erythro (or syn) isomers are obtained with trifluoroacetic acid (TFA), whereas threo (or anti) isomers are obtained with fluoride ion activator (eq 1). The erythro selectivity in the acid-promoted carbonyl reduction is ascribed to a proton-bridged Cram s cyclic transition state. On the other hand, the threo selectivity in the fluoride-mediated reduction is explained in terms of the Felkin-Anh t) e model, wherein a penta-or hexacoordinated fluorosilicate is involved. No epimerization at the chiral center is observed during the reaction. 

In a final example, Denmark has described a detailed study on a tandem RCM/Si-assisted Pd cross-coupling reaction sequence that led to unsaturated macrolactones 128 (Scheme 11.15) through the intermediacy of siloxane 127 233 typically difficult 11- to 14-membered ring sizes could be accessed under the optimized conditions, which included a hydrated fluoride ion activator. High dilution conditions were not required and this process was adapted to provide benzo-fused products as well. 

The EMF of a cell containing fluoride ion solution, fluoride electrode and a suitable reference electrode follows the fluoride ion activity between 1 and 10" M according to the Nemst equation ... 

As Fig. 27 shows, the working pH range for the fluoride electrode lies between 5.5 and 6.5. This is easily attained with an acetate or citrate buffer [80]. At this pH value the free fluoride ion activity can be determined between 1 and < 10 M. 

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