Defluorination

Minerals. Supplementation of macrominerals to mminants is sometimes necessary. Calcium and phosphoms are the minerals most often supplemented in mminant diets. One or both may be deficient, and the level of one affects the utilization of the other. Limestone, 36% calcium, is commonly used as a source of supplemental calcium. Dolomite, 22% calcium oyster sheUs, 35% calcium and gypsum, 29% calcium, are sources of calcium. Bone meal, 29% calcium, 14% phosphoms dicalcium phosphate, 25—28% calcium, 18—21% phosphoms and defluorinated rock phosphate, 32% calcium, 18% phosphoms, are sources of both calcium and phosphoms. Diammonium phosphate, 25% phosphoms phosphoric acid, 32% phosphoms sodium phosphate, 22% phosphoms and sodium tripolyphosphate, 31% phosphoms, are additional sources of phosphoms (5). 

Octafluoronaphthalene [313-72-4] is prepared in 53% yield by defluorination of perfluorodecahydronaphthalene [306-94-5] over iron or nickel at 500°C. Exchange fluorination of octachloronaphthalene with KE in sulfolane (235°C) gave 60% yield of octafluoronaphthalene. This product exhibits good StabiHty to ionizing radiation (274). 

Defluorination occurs even with sodium fluoride at 530 °C when tetra-fluorothiolene is converted to 2,5-difluQrothiophene [63], Dehydrofluonnation would be expected at such high temperature, but defluonnation is favored. The product composition also excludes a disproportionation reaction mechanism (equation 32). 

Irradiation of lomefloxacin 271 in dilute neutral aqueous solution (in which it exists as a zwitter ion) in Pyrex-filtered 500 W medium pressure mercury (Helios Italquartz) at 17°C gave pyrrolo[3,2,l-(/ ]quinoline 272 (99JOC5388). Under this condition, reductive defluorination via a radical anion took place. This study is important because of the phototoxicity of the fluorinated compounds which could be used as antibacterials (Scheme 49). 

The defluorination of the complex acids is a key step in the production of tantalum and niobium oxides as it defines the quality of the products and durability of the production equipment. 

The resulting product depends on the precipitation conditions, and in particular, on the over-saturation level of the solution. Formation of ammonium oxyfluorometalate crystalline compounds occurs at a relatively low pH of the solution. From the standpoint of the interactions described in Equation (143), this means that the interaction between NH4F and Me205 (denoted as interaction 1) is stronger than the interaction denoted as interaction 2. In this case, subsequent processes of the hydroxide treatment lead to some defluorination of the product, but the performance of such processes is usually very problematic. Precipitation at high pH values leads to a strong oversaturation of niobium- or tantalum-containing compounds, which in turn... 

Ammonium hydrofluoride is relatively stable, even in the molten state. In addition to being in contact with tantalum or niobium oxide, the compound will initiate the fluorination process yielding complex tantalum or niobium fluoride compounds. There is no doubt that thermal treatment of the hydroxides at high temperatures and/or at a high temperature rate leads to the enhancement of the defluorination processes, which in turn results in an increase in fluorine content of the final oxides. 

Aryl halides can be dehalogenated by Friedel-Crafts catalysts. Iodine is the most easily cleaved. Dechlorination is seldom performed and defluorination apparently never. The reaction is most successful when a reducing agent, say, Br or 1 is present to combine with the I" or Br coming off." Except for deiodination, the reaction is seldom used for preparative purposes. Migration of halogen is also found," both intramolecular and intermolecular." The mechanism is probably the reverse of that of 11-11." ... 

Renganathan V (1989) Possible involvement of toluene-2,3-dioxygenase in defluorination of 3-fluoro-sub-stituted benzenes by toluene-degrading Pseudomonas sp strain T-12. Appl Environ Microbiol 55 330-334. 

Goldman P (1966) Carbon-fluorine bond cleavage II Studies on the mechanism of the defluorination of fluoroacetate. J Biol Chem 241 5557-5559. 

Key BD, RD Howell, CS Criddle (1998) Defluorination of organofluorine sulfur compounds by Pseudomonas sp. strain D2. Environ Sci Technol 32 2283-2287. 

A NMR study used several species of Rhodococcus and a range of mono-, di-, and trifluorophenols. Both hydroxylation and hydrolytic defluorination were observed (Figure 9.36) (Bondar etal. 1998). 

Brief notes are added on phosphorofluoridates even though their destruction by microbial activity— though clearly possible—is limited by their toxicity to the requisite microorganisms. One of the motivations for their inclusion is the fact that the hydrolytic enzyme(s) responsible for defluorination—organophosphorus acid anhydrase (OPA)—is widespread, and is found in a number of bacteria (Landis and DeFrank 1990). The microbial hydrolysis of organophosphorus pesticides and cholinesterase inhibitors is accomplished by several distinct enzymes, which are collectively termed organophosphorus acid anhydrases (OPAs). These have been reviewed (DeFrank 1991), so that only a few additional comments are necessary. 

The reductive defluorination of perfluorocarbons is an example where the above model helped to resolve a rather complicated mechanism and to rationalize the reactivity pattern.82 The reductive elimination of the three fluorines occurs successively according to the following reaction sequence ... 

Carbenes do not transpire along the reaction pathway. Each defluorination reaction follows a stepwise mechanism ... 

Ward, P.F.V. and N.S. Huskisson. 1969. The metabolism of fluoroacetate by plants. Biochem. Jour. 113 9P. Wong, D.H., W.E. Kirkpatrick, D.R. King, and J.E. Kinnear. 1992a. Defluorination of sodium monofluoroacetate (1080) by microorganisms isolated from western Australian soils. Soil Biol. Biochem. 24 833-838. Wong, D.H., W.E. Kirkpatrick, D.R. King, and J.E. Kinnear. 1992b. Environmental factors and microbial inoculum size, and their effect on biodefluorination of sodium monofluoroacetate (1080). Soil Biol. Biochem. 9 838-843. 

Wong, D.H.,W.E. Kirkpatrick, J.E. Kinnear, andD. R King. 1991. Defluorination of sodium monofluoroacetate (1080) by microorganisms found in bait materials. Austral. Wildl. Res. 18 539-549. 

The order of reactivity of the C-X bond (generally I > Br > Cl > F) is consistent with its strength. For instance, the experimentally found dissociation energies for phenyl halides (DPh x) are 528, 402, 339, and 272 kj mol-1 at 298 K for X=F, Cl, Br, and I, respectively [2]. Consequently, catalytic defluorination in the literature is comparatively rare. The different reactivity of the C-X bonds renders possible the selective dehalogenation of compounds containing two dissimilar halides, leaving intact the stronger C-X bond. 

---