Fluoborate ions

The controlled thermal decomposition of dry aromatic diazonium fluoborates to yield an aromatic fluoride, boron trifluoride and nitrogen is known as the Schiemann reaction. Most diazonium fluoborates have definite decomposition temperatures and the rates of decomposition, with few exceptions, are easily controlled. Another procedure for preparing the diazonium fluoborate is to diazotise in the presence of the fluoborate ion. Fluoboric acid may be the only acid present, thus acting as acid and source of fluoborate ion. The insoluble fluoborate separates as it is formed side reactions, such as phenol formation and coupling, are held at a minimum temperature control is not usually critical and the temperature may rise to about 20° without ill effect efficient stirring is, however, necessary since a continuously thickening precipitate is formed as the reaction proceeds. The modified procedure is illustrated by the preparation of -fluoroanisole ... 

Cleaning Up Solutions that contain the fluoborate ion (the neutralized filtrate, the nmr sample, the filtrate from the iodide preparation) should be treated with aqueous CaClj to precipitate insoluble Cap2, which is removed by filtration and placed in the nonhazardous solid waste container. The filtrate can be flushed down the drain. Allow the ether to evaporate from the sodium sulfate, and then place it in the nonhazardous solid waste container. Recrystallization solvent goes in the organic solvents container. 

Tropylium salts are starting materials for the preparation of a wide range of substituted tropilidenes. The fluoborate is the salt of choice for work involving the tropylium ion because it is indefinitely stable, non-hygroscopic, and, unlike the perchlorate, non-explosive. Its preparation by this method avoids the use of... 

Fluoboric acid is also an efficacious promoter of cyclic oxo-carbenium ions (Scheme 4.24) bearing an activated double bond which, in the presence of open-chain and cyclic dienes, rapidly undergo a Diels-Alder reaction [91]. Chiral a, -unsaturated ketones bearing a -hydroxy substituents, protected as acetals, react with various dienes in the presence of HBF4, affording Diels-Alder adducts that were isolated as alcohols by hydrolysis of the acetal group by TsOH. Some examples of reactions with isoprene are reported in Table 4.23. The enantios-electivity of the reaction is dependent on the size of the substituent R on the of-carbon high levels of asymmetric induction were observed with R = z-Pr (90 1) and R = t-Bu (150 1) and low levels with R = Me (2.7 1) and R = Ph (3.0 1). Scheme 4.24 shows the postulated reaction mechanism. 

The acetylium ion is the strongest acid species in the acetic anhydride system, and acetyl fluoborate, which is a good conductor in liquid sulfur dioxide, is a strong acid in acetic anhydride. Acetyl fluoborate, chloride, thiocyanate, and benzene sulfonate may all be titrated as acids in acetic anhydride using sodium acetate, a strong base in that system. 08 The neutralization reaction is ... 

Abstraction of hydride ions from tropylidenes by nitrosyl or trityl tetra-fluoborates leads to 2-imino-3-arylthiazolotropylium salts like 640a,b [94H(38)2691] and to cycloheptaazaazulenium salt 643 or (unstable) 14-77-electronic pyrrolotropylium salt 647 [Scheme 173 94JCS(P1)2579]. 

It is recommended that the total hydrogen-ion concentration should not be greater than one mole per liter during the precipitation of the diazonium fluoborate. U. S. pat. 1,9x6,327 [C. A. 27, 4539 (1933)]. 

The fluoborate group can be displaced by the iodide ion to prepare tropilium iodide. You can see that the iodide ion is ionic by watching the reaction of the aqueous solution of this ion with silver ion. 

The synthesis of azidocycloimonium fluoborates, compounds which possess a quasi-aromatic heterocyclic nucleus, has been reported by BalU and Kersting . Halogen atoms adjacent to quaternary nitrogens in heteroaromatic salts such as 154 157 undergo replacement by azide ion at low temperatures to produce the resonance stabilized azidinium salts (158). On the basis of infrared studies and reactivity towards nucleophiles, Balli has suggested that these salts are best considered as iV-diazonium compounds... 

Another reactive system is based on the diepoxides. In the case of l,2 3,4-dianhydroerythritol, it has been shown that the rate of the alkali-catalyzed reaction at room temperature is proportional to the product of the concentrations of the hydroxyl ion, the diepoxide, and the cellulose. Catalysis by hydrogen ion is very slow and, for efficient reaction, zinc fluoborate at elevated temperatures has been used. 

The purpose of carrying out the electrolysis in the FLUBOR divided diaphragm cell is to produce Pb cathodes in the cathodic compartment and to oxidise the ferrous ion to ferric ion in the anodic compartment, thereby regenerating the ferric fluoborate leaching solution. The electrolysis reactions are the following ... 

PROBLEM 6.18 The stability of oxonium ions depends on the nature of the negatively charged counterion. Fluoborate ( BF4) is an especially favorable counterion. Why Hint-. LeAvis bases, Bt can transfer methyl (and other) groups through a process called the Sn2 reaction. You will learn all about it in Chapter 7, but you might want to use it here in this problem. 

The classification in terms of electron-donor or -acceptor species actually allows us to consider many pluriatomic compounds as being complexes, as we just saw. Some pluriatomic inorganic ions may be in this group, including fluoboric BF4, fluosilicicSiF6 , nitrate N03, sulfate S04 , and phosphate P04 . Sulfate ions, for example, may be considered as resulting from the reaction of oxide ion, ... 

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