Direct fluorinations, limitations

Direct Fluorination of Aliphatics and Non-C Moieties The basic limitations of direct fluorinations are similar for aliphatic compounds as discussed already using the example of aromatic derivatives and so is the potential of microreactors. The fluorination of ethyl acetoacetate was carried out in an annular-flow microreactor. 

Earlier syntheses of 8-fluoropurines are limited to a few reports involving nucleophilic displacement, Schiemann reactions, halogen-exchange reactions, and electrochemical oxidations. Recently, the first direct fluorination with elemental fluorine has been reported for the synthesis of 8-fluoroguanines, e.g. 

One way of process simplification is to make molecular complex compounds out of much simpler building blocks (e.g., by multi-component one-pot syntheses like the Ugi reaction), at best directly out of the elements. Especially in the latter case, this is often quoted as a dream reaction [14]. Typically, such routes have been realized so far with hazardous elements, easily undergoing reaction, but lacking selectivity. One example is direct fluorination starting with elemental fluorine, which has been performed both with aromatics and aliphatics. Since the heat release cannot be controlled with conventional reactors, the process is deliberately slowed down. While, for this reason, direct fluorination needs hours in a laboratory bubble column it is completed within seconds or even milliseconds when using a miniature bubble column operating close to the kinetic limit. Also, conversions with the volatile and explosive diazomethane, commonly used for methylation, have been conducted safely with microreactors in a continuous mode [14]. 

Different from PIPE and ETFE, cost reduction of the PSVE polymer is one of the problems for prevailing perfluorinated membranes. Recently, a new direct fluorination process with elementary fluorine was reported to produce perfluorinated fluorosulfonyl polymers, as shown in Fig. 8 [ 19). This new process consists of the following features and is free from explosion in the gas phase and the difficulty in finding solvents with stability to fluorine gas and solubihty for hydrocarbon compoimds, these problems having limited the application of the conventional direct fluorination methods ... 

Fluorinated thiophene derivatives have found a broad application as biologically intriguing molecules and especially as modem organic materials. However, methods for their synthesis are still limited. The direct fluorination or trifluoromethylation of thiophene is either not selective or proceeds in low yields. The most convenient approach to fluorothiophenes and their benzoanalogues involves Uthiation-fluorination reactions. Other common methods are based on heterocycUzations with participation of methyl thioglycolate, or cycloaddition reactions. The scarce methods for fluorinated thiophenes synthesis give a chance for synthetic chemists to elaborate new, better pathways to these intriguing and useful compounds. 

The direct fluorination of quinolines has a limited use since a low selectivity of the reaction, and also due to technological and ecological difficulties. However, there are several examples of selective syntheses of monofluorinated quinolines. For instance, 2-fluoroquinolines 72 were obtained by interacting quinoline 71 with elementary fluorine in the presence of I2 [46], yields proved to be in the range of 54-93 %, ratio la-quinoline 71 was 1 1, and ratio Fa-quinoline 71 was 2 1 (Scheme 28). To obtain 2-fluoro-4-chloroquinoline and 2-fluoro-4,7-dichloroquino-line the reaction was carried out in the presence of triethylamine. 

At the same time both the thermal stability and the chemical resistance to acids and alkalis are improved. Huorinated PE sheets exhibit a lowered coefficient of friction and acceptance of printing inks is improved. The finishing of plastic surfaces by direct fluorination is not limited to polyethylene. Principally, any kind of plastic or rubber surface could be fluorinated. 

Preparation. In the laboratory, sulfur tetrafluoride is made by combining SCI2 and NaF suspended in acetonitrile at ca 77°C (106). For commercial production, SF is made by direct combination of sulfur with elemental fluorine (107). Commercial appHcations of SF are limited. It is available from Air Products and Chemicals. 

Table 8.39 shows the main features of EDXRF. EDXRF is not able to detect the fine structure of the K, L, M, etc. lines. EDXRF is used for applications which require measurement of a limited number of elements, and where the resolution and ultralow detection limits of wavelength-dispersive systems are not necessary. For example, EDXRF has been used as a rapid screening technique for the determination of Br and Sb in plastic recyclate at a LOD of 5 ppm [230] the method was validated by means of NAA [231]. Conventional EDXRF systems and benchtop units have a limited detection capability for low-Z-elements and cannot directly measure fluorine in processing aids. 

This review is limited to compounds containing both oxygen and fluorine atoms bonded directly to a common chlorine central atom. Therefore, compounds, such as fluorine perchlorate (O3CIOF), have not been included. Data on O3CIOF were summarized in a recent review on inorganic hypofluorites 180). 

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