Organic halides tetrafluoroborate

My first contact with Meerwein was in 1954 when I received, while still working in isolated Hungary, a letter from him. He had read a paper of ours, and offered useful comments, even pointing out that we had missed a relevant reference. We subsequently kept up correspondence. He continued to be interested in our early efforts to obtain long lived carbocations from alkyl (and acyl) fluorides in liquid boron trifluoride (and other Lewis acid halides) at low temperature, as well as via metathetic reactions of organic halides with silver tetrafluoroborate (a reagent we introduced in 1955 for the ionization of bromocyclohexadienes to form benzenium ions and which Meerwein developed to a wide utility akylation method). 

Abstract This chapter discusses the potential usefulness of ionic liquids with respect to biocatalysis by illustrating the stability and activity of enzymes in ionic liquids in the presence or absence of water. Ionic liquids are a class of coulombic fluids composed of organic cations like alkyl-substituted imidazolium, pyrrolidin-ium, and tetraalkylammonium ions and anions such as halides, tetrafluoroborates, hexafluorophosphates, tosylates, etc. The possibility of tunable solvent properties by alternation of cations and anions has made ionic liquids attractive to study biocatalysis which warrants an understanding of enzyme stability and activity in ionic liquids. This chapter systematically outlines the recent studies on the stability of enzymes and their reactivity toward a wide range of catalytic reactions. A careful approach has been taken toward analysis of relationship between stabil-ity/activity of enzymes versus chaotropic/kosmotropic nature of cations and anions of ionic liquids. 

On the basis of these redox potentials it seems likely that direct electron release to the benzenediazonium ion takes place only with iodide. This corresponds well with experience in organic synthesis iodo-de-diazoniations are possible without catalysts, light, or other special procedures (Sec. 10.6). For bromo- and chloro-de-di-azoniations, catalysis by cuprous salts (Sandmeyer reaction, Sec. 10.5) is necessary. For fluorination the Balz-Schiemann reaction of arenediazonium tetrafluoroborates in the solid state (thermolysis) or in special solvents must be chosen (see Sec. 10.4). With astatide (211At-), the heaviest of the halide ions, Meyer et al. (1979) found higher yields for astato-de-diazoniation than for iodo-de-diazoniation, a result consistent with the position of At in the Periodic System. It has to be emphasized, however, that in investigations based on measuring yields of final products (Ar-Hal), the possibility that part of the yield may be due to heterolytic dediazoniation is very difficult to quantify. 

Diaryliodonium salts are the most stable and well-investigated class of iodonium salts. The first example of these compounds, (4-iodophenyl)phenyliodonium bisulfate, was prepared by Hartmann and Meyer in 1894 from iodosylbenzene and sulfuric acid [368]. Diaryliodonium salts Ar2l+ X are air- and moisture-stable compounds, whose physical properties are strongly affected by the nature of the anionic part of the molecule. In particular, diaryliodonium salts with halide anions are generally sparingly soluble in many organic solvents, whereas triflate and tetrafluoroborate salts have a better solubility. The chemistry of aryl-and heteroaryliodonium salts has been extensively covered in several reviews [361,369,370]. 

A -Arylation. Tii-tert-butylphosphonium tetrafluoroborate is frequently used as a ligand in the iV-arylation reaction of heterocycles containing N-H bonds. The transformation is usually catalyzed by palladium conqilexes and performed in the presence of an organic base such as sodium or potassium tert-butoxide. High temperatures and long reaction times are often required to reach good conversions. Aryl halides are the most frequent electrophiles for this type of arylation. 

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