Mechanism of fluorination

Cyclic voltammetry and polarography have been used to study the mechanisms of fluorination and provide an indicator of their relative strengths [92JFC(59)157,92T1595], Two mechanisms have been proposed involving SN2 or electron transfer, although it is likely that which is followed depends on the N—F reagent used. 

So far only a few dozen organofluorine compounds have been isolated from living organisms, for example fluoroacetic acid, 4-fluorothreonine and rw-fluoro-oleic acid [244-246], The reason that nature has not invested in fluorine chemistry could be a combination of low availability of water-dissolved fluoride in the environment due to its tendency to form insoluble fluoride salts, and the low reactivity of water-solvated fluoride ion. However, in 2002, O Hagan and collaborators [247] published the discovery of a biochemical fluorination reaction in a bacterial protein extract from Streptomyces cattleya converting S-adenosyl-L-methionine (SAM) to 5 -fluoro-5 deoxyadenosine (5 -FDA). The same protein extract contained also the necessary enzymatic activity to convert 5 -FDA into fluoroacetic acid. In 2004, the same authors published the crystal structure of the enzyme and demonstrated a nucleophilic mechanism of fluorination [248,249]. 

In principle, two mechanisms involving either one or two-electron transfer from the aromatic substrate to fluorine may occur and, in practice, the mechanism of fluorination of any given aromatic molecule probably lies between these two extremes. The distinction between electron transfer (Path A, Fig. 55) and SN2-type processes (Path B, Fig. 55) in reactions between various electrophilic fluorinating agents and nucleophiles has been addressed by Differding [144, 145] but these principles can also be applied to reactions involving elemental fluorine. 

Thus, it is beyond doubt that the fluorination reaction proceeds via an association between the organic substrate (or its protonated derivative) and the electrochemically generated nickel fluoride(s) on the anode surface. However, since the precise nature of this association, and the exact species involved, are not known, the next stage in the mechanism of fluorine substitution is not apparent and it has generally been the practice to make inferences from studies of product analyses. 

No other saturated acyclic hydrocarbons have been partially fluorinated over cobalt(III) fluoride, etc. It is clear from the results quoted above that partial fluorination is virtually valueless for the preparation of pure partially fluorinated acyclic hydrocarbons unless the need for a specific compound is particularly acute. The work cited is more notable for the light it sheds on the mechanisms of fluorination by cobalt(III) fluoride, etc., and the references should be consulted for extensive discussions on this matter. 

Transport mechanisms of fluorine and water different rates of diffusion and percolation. 

P. Walter, M. Menu, J.C. Dran, Dating of archaeological flints by fluorine depth profiling - new insights into the mechanism of fluorine uptake, Nucl. Instr. Meth. B64 (1992) 494-498. 

The precise mechanism of fluorine transfer from AcOF to the aromatic compound in these reactions is still a controvertial issue, although in all cases the initially formed moiety is probably the same 7i-cation complex. Three reasonable routes leading to this complex should be considered (equation 14) (1) The aromatic n electrons simply act like a base to abstract F+ which leaves its binding electron pair with OAc (as OAc) (2) the... 

The exact mechanism of fluorination is not completely understood, although both radical and ionic mechanisms have been proposed [4]. 

Table 2.1 Dependence on the solvent of the mechanism of fluorination of pivaldehyde with DAST and of the formation of side-products [127],...

Studies on the Mechanism of Fluorine-Induced Adrenergic Selectivities... 

R., and Medraj, M. (2014) In-vitro corrosion inhibition mechanism of fluorine-doped hydroxyapatite and brushite coated Mg-Ca alloys for biomedical applications. Ceram. Int, 40 (6), 7971-7982. 

Little can be said about the mechanisms of fluorination or iodination of alkenes by the molecular halogens. Fluorine reacts violently with alkenes, giving mixtures including products resulting from degradation of the carbon chain. There have been few mechanistic studies of the reaction with iodine. One significant feature of the reaction is its reversibility. Some iodine remains in equilibrium with simple alkenes. 

The mechanism of fluorination effected by SF4 or any of the dialkylaminosulfur trifluorides is illustrated below for a primary alcohol and DAST. The first step involves coordination of the alcohol to the tetravalent sulfur, followed by the departure of a fluoride leaving group ... 

Mechanism of fluorination in this process is In situ reaction with fluorine generation, that is, organic substrate of hydrocarbon reacts with free radical of fluorine and so, this process is classified in direct fluorination. 

List of symbols and abbreviations 180 3.4. Detection mechanism of fluorine ... 

Boltalina, O. V., Darwish, A. D., Street, J. M., Taylor, R., 8c Wei, X. W. (2002). Isolation and characterisation of C60F4, CgoFe, CgoFs, C60F7CF3 and C60F2O, the smallest oxahomofullerene the mechanism of fluorine addition to fullerenes. Journal of the Chemical Society-Perkin Transactions, 2(2), 251-256. 

FIGURE 45.10. Mechanism of fluorination catalyzed by the S. cattleya fluorinase. (Reprinted with permission from Reference 100. 2007 American Chemical Society.)... 

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