Hydrofluoride synthesis

Hydrofluoride synthesis is based on the simultaneous fluorination by ammonium hydrofluoride of niobium or tantalum oxides with other metals compounds (oxides, halides, carbonates etc.) [105]. Table 13 presents some properties of ammonium hydrofluoride, NH4HF2 [51, 71]. Ammonium hydrofluoride is similar to anhydrous HF in its reactivity, but possesses some indisputable advantages. The cost of ammonium hydrofluoride is relatively low, it can be dried and handled easily, recycled from gaseous components, and its processing requires no special equipment. 

Since hydrofluoride synthesis is based on thermal treatment at relatively high temperatures, the possibility of obtaining certain fluorotantalates can be predicted according to thermal stability of the compounds. In the case of compounds whose crystal structure is made up of an octahedral complex of ions, the most important parameter is the anion-cation ratio. Therefore, it is very important to take in to account the ionic radius of the second cation in relation to the ionic radius of tantalum. Large cations, are not included in the... 

IR absorption spectra of several fluorotantalates of alkali and alkali earth metals prepared by hydrofluoride synthesis are presented in Fig. 17. 

Thus, the sequence of phases that are formed in hydrofluoride synthesis of lithium-niobium fluoride compounds is (NH4)3NbOF6 - (NH NbOFs -LiNbOF4-Li2NbOF5. 

If the quality of the product is not so critical or if further purification steps are planned, hydrofluoride synthesis can be performed using a regular black steel container. 

Hydrofluoride decomposition, 263-266 Hydrofluoride synthesis fluorotantalates, 41-47 general information, 38-41 oxyfluoroniobates, 49-54 performance, 46-47, 57 Hydrolysis, 7, 16,317-318 Hydrolysis of... 

The synthesis of tantalum and niobium fluoride compounds is, above all, related to the fluorination of metals or oxides. Table 3 presents a thermodynamic analysis of fluorination processes at ambient temperature as performed by Rakov [51, 52]. It is obvious that the fluorination of both metals and oxides of niobium and tantalum can take place even at low temperatures, whereas fluorination using ammonium fluoride and ammonium hydrofluoride can be performed only at higher temperatures. 

Oxides, hydroxides, chlorides or carbonates of the second metal can be used for synthesis instead of fluorides, but the necessary amount of ammonium hydrofluoride must be recalculated and added in order to assure complete fluorination of the second compound. Some excess of ammonium hydrofluoride is recommended to avoid insufficient fluorination. Remainders of NH4HF2 decompose and separate out from the system upon reaching the final temperature. 

The hydrofluoride method can be used successfully both for the preparation of complex fluoride compounds and of complex oxides. The main advantage is that the synthesis is performed at relatively lower temperatures. In addition, the complex oxide material is formed through its respective fluoride compound and the product obtained is therefore more consistent. For instance, Co4Nb209 can be prepared using the hydrofluoride method at 900-1100°C, whereas the regular synthesis, based on the interaction of simple oxides, requires extended treatment at about 1400°C. 

Table 16. Conditions for the synthesis of niobium and tantalum fluoride compounds using the hydrofluoride method.

Silyl ethers that have been used in solid-phase synthesis include TES, TIPS, TBS, and TBDPS ethers. Polystyrene-bound phenols can be converted into TIPS ethers by treatment with TIPS-OTf/imidazole in DMF for 5 min [105], These silyl ethers are stable towards bases and weak acids, but can be selectively removed by treatment with TBAF (Entries 6 and 8, Table 7.8) or pyridinium hydrofluoride (THF, 25 °C, 15 h [24,75,106]). 

The first direct synthesis of 5-fluorocytosine (15) was reported by Robins and Naik,21 in which cytosine (20) was reacted with trifluoromethyl hypofluorite followed by decomposition of the resulting adduct to provide 5-fluorocytosine in 85% yield. Some years later, Takahara disclosed that 15 could be prepared as its hydrofluoride salt in high purity when cytosine was reacted with fluorine gas in the presence of hydrogen fluoride, followed by treatment with methanol.22... 

KF is much more soluble in aHF than is LiF, and it may simply be the higher hydrofluoride ion concentration that can be attained with the former that makes the KAgp4 synthesis easier to achieve in high yield than that of the LiAgF4 relative. Nevertheless, the relatively low solubility of LiAgp4 in aHF... 

Alvemhe G, Haufe G, Lawrent A. 1987. Triethylamine tris-hydrofluoride [(C2H5)3 N.3HF] a highly versatile source of fluoride-ion for the halofluorination of alkenes. Synthesis 6 562-564. 

One of the syntheses of 198 is based on pyrazine 200 (Scheme 47) [165, 166]. Compound 200 was transformed to methoxy derivative 201 via diazotization step 201 was then subjected to Buchwald - Hartwig amination to give 202. Ester 202 was transformed to amide 203 diazotization of 203 in the presence of pyridine hydrofluoride led to the formation of fluoro derivative 204. The last step of the synthesis included deprotection of the methyl ether to give 198. 

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