Fluorination of aromatic rings

Fluorination of aromatic rings (other than via carbanions)... 

Fluorination of aromatic rings (other than those via carbanions described above) consists principally of fluorodehydrogenation and fluorodemetallation reactions, both ofwhich are thought to proceed via an arenium ion mechanism (Scheme 25) [85]. 

Scheme 25. The arenium ion mechanism of electrophilic fluorination of aromatic rings via fluorodehydrogenation or fluorodemetallation reactions.

Another example of a direct electrophilic fluorination of aromatic rings is the synthesis of the purine derivatives 8-[ F]fluoroganciclovir, 8-[ F]fluoropenciclo-vir and 8-[ F]fluoroacyclovir (Scheme 27). The radiochemical yields obtained for these compounds are low, about 1%, but no protecting group is required, allowing a one-step radiosynthesis [107]. 

Direct fluorination of aromatic rings is so exothermic that a tarry mixture of products is obtained. Reaction of benzene with the xenon fluorides, XeF2 or XeF4, does give fluorobenzene, but the mechanism is probably free radical rather than polar.137... 

Fluorine. Direct fluorination of aromatic rings with F2 is not feasible at room temperature, because of the extreme reactivity of F2. It has been accomplished at low temperatures (e.g., —70 to —20°C, depending on the... 

Fluorination. This new reagent, which that needs no special caution or glassware in handling, can be used for fluorination of aromatic rings, alkenes, enol ethers, and dienol acetates. In the presence of ZnCl2, either mono- or difluoro derivatives of active methylene compounds can be isolated from its reactions. 

Several flaoraza reagents shown in Tables 3a and 3b (B, C, E, F, J, and K) are reactive enough to fluorinate an aromatic ring (Table 1). The ortho isomer predominates in the o/mlp mixture Reagent K has been used to prepare fluorinated derivatives of tyrosine and estradiol [77 (equation 35) (Table 1, entry 10)... 

The most important application for bismaleimide resin is multilayer boards. The development in this area requires resins with low dielectric constants. It is well documented in the literature that fluorine containing linear polyimides show lower dielectric constants vis a vis their non-fluorinated counterparts. Recently, Hitachi Research Laboratory, Japan, reported the thermal and dielectric behaviour of fluorine-containing bismaleimides (29). The chemical structures of the fluorinated BMIs investigated are provided in Fig. 6. The non-fluorinated four aromatic rings containing BMI, 4,4 -bis(p-maleimidophenoxyphenyl) propane, was tested in comparison. 

Another typical example concerns the introduction of aromatic rings between the fluorinated chain and the chemical function in order to improve the thermal properties. The same behavior occurs for the fluorinated chains/aromatic nucleus links... 

At the first glance, treatment with antimony pentafluoride should be expected to cause addition of fluorine to aromatic rings, or to replace other halogens with fluorine. Under very energetic conditions, bromine... 

Acetyl hypofluorite will only fluorinate activated aromatic compounds and fluorinations arc generally performed at low temperatures. In general a mixture of products is formed, for example, the fluorination of anisole with acetyl hypofluorite results in a mixture of 2-fliioroanisole and 4-fluoroanisole (ratio 9.6 1) in 85% yield (Table 11). This preferential formation of the ortho-Komcr is explained by an addition-elimination mechanism with an intermediate 15. This mechanism is support by the fact that the addition product 16 from acetyl hypofluorite and pipcronal is isolated in 55 % yield. Selective fluorination is achieved when the para position is blocked by a second substituent (Table 12). This approach has been used to selectively fluorinate an aromatic ring in peptide 17. " ... 

Introduction of fluorine into aromatic rings is carried out using isolated diazonium tetrafluorobofate salts. These compounds are then thermally decomposed, giving the aryl fluoride. This process is known as the Schiemann reaction. 

Direct fluorination or iodination of aromatic rings is also possible but requires special methods. 

The Introduction of Fluorine Into Aromatic Rings.— The direct fluorination (F2-N2) of nitrobenzene, benzotrifluoride, benzoic acid, and toluene, usually in trifluoroacetic acid at 5 C, has been described by Russian workers. Where direct comparisons can be made, the isomer distributions of the monofluoroarenes obtained are qualitatively similar to those formed in acetonitrile during concurrent American work, sununarized in Vol. 1 (p. 189). However, the Russians favour a homolytic mechanism, the preference for attack at the /wc/o-position in nitrobenzene being attributed to the electrophilic character of the attacking fluorine atom. Electrophilic displacement of a proton by F, via the usual addition-elimination mechanism, was preferred in the American paper, and this view has been emphasized in a recent review of the field by the same author. ... 

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