Anion acceptor

Perfluoro- rt,4 -phenylene mercury, as anion acceptor, 2, 451 Perfluoro- rt,4 -phenylenes, with mercury, 2, 456-457 Perfluorophenyl groups, in a-organonickel complexes, 8, 33 Perfluorophenyl ligation, and monomeric Cr(III) derivatives,... 

This reaction may be considered analogous to those of Section A,2, in which the acid is the anion acceptor, this time for the hydride rather than the halide anion. 

Fig. 2.1.2. Chiral artificial receptors for the recognition of carbohydrates via ionic hydrogen bonds involving anionic acceptors [1].

Usanovic acid-base theory — A general theory of acids and bases taking into consideration electron exchange processes (redox reactions) was proposed in 1939 by Usanovic. His definition is a symmetrical one and includes all concepts discussed above, i.e., an acid is defined as a substance which is able to liberate protons or other cations (cation donator) or to take up anions or electrons (anion acceptor, electron acceptor). A base is defined as a substance which is able to release anions or electrons (anion donator, electron donator) or to take up protons or other cations (cation acceptor). According to that theory, all chemical reactions (excluding reactions between radicals resulting in covalent bindings) can be considered as acid-base reactions. 

An interesting application for imidazolium salts has been the use as anion acceptors employing the C -ff group as a hydrogen bond donor [167,168,178]. It was claimed by the authors that one molecule of the Fc-modified imidazolium salt can bind either two hahde anions or one nitrate or bisulfate anion [167],... 

Ammonio- and phosphoniohexafluorocyclobutanide ylides 11, prepared from pcrfluorocy-clobutene and tertiary amines or phosphanes, are soluble in a wide range of (even nonpolar) organic solvents and transfer fluoride, and therefore can be used as an easily soluble source of fluoride anions. Although they contain no ionic fluoride they react with fluoride anion acceptors such as silicon derivatives with elimination of thermodynamically favored fluoro-trimethylsilane to produce carbanions which can be quenched with suitable electrophiles. The reaction proceeds upon slightly heating the reactants in tetrahydrofuran or dimethylform-amide solution. 

According to well-established facts the hydride anion can only be shifted or transferred to the carbenium ion the onium ions themselves are no hydride anion acceptors. Nevertheless, some of the onium ions exist in equilibrium with the correspondii carbenium ions (cf. Sect. 3.1) and in this isomeric form can abstract hydride anions. The classical experiments were performed by Jaacks in the polymerization of 1,3-diox-olane initiated by methoxymethylium perchlorate ... 

Donor anion Acceptor cation Solvent diss (m) Ref. 

There is also an active search for selective anion acceptors.89 The cryptate (7.21) favors fluoride over chloride by a factor of 40 million.90... 

Donor Radical Anion Acceptor Molecule Rate Constant CM 1 sec. 1)... 

Rather interesting is the possibility of transition from benzyl-type cations to ben-zenium ions. The benzyl cations can, just as other carbenium ions, act as hydride-anion acceptors turning, in so doing, into an aromatic hydrocarbons which, in a strong acid, are protonated to form the benzenium ions. Similar transformations occurred in the case of the 2,3,4,5,6-pentamethylbenzyl cation, the hydride-anion donors being the aliphatic hydrocarbons, 9,10-dihydroanthracene and even the molecular hydrogen ... 

In binary liquid mixtures of the so-called aprotic acids (Lewis acids or L-acids) formed by halogenides of metals of III-IV groups of the periodic system, there are often cases when one of the components is an anion-donor (base) and the second component is an anion-acceptor (acid) ... 

Donor-acceptor interactions in ionic media are often deseribed as acid-base interactions according to Lewis and Lux-Flood " definitions. The elassie variant of the former definition considers acids as acceptors of electron pairs and bases as their donors. In modem variant of this definition, acids are the electron pair (or anion) acceptors or cation (proton) donors, bases are the electron pair (anion) donors or cation (proton) acceptors. 

Certain halogen fluorides also act as fluoride anion acceptors ... 

The reactants in the key step of an aldol reaction are an enolate anion and an enolate anion acceptor. In self-reactions, both roles are played by one kind of molecule. Crossed aldol reactions are also possible, such as the crossed aldol reaction between acetone and formaldehyde. Because it has no a-hydrogen, formaldehyde cannot form an enolate anion. It is, however, a particularly good enolate anion acceptor because its carbonyl group is unhindered. Acetone forms an enolate anion, but its carbonyl group, which is bonded to two alkyl groups, is less reactive than that of formaldehyde. Consequently, the crossed aldol reaction between acetone and formaldehyde gives 4-hydroxy 2-butanone ... 

In a crossed Claisen condensation (a Claisen condensation between two different esters, each with its own a-hydrogens), a mixture of four j8-ketoesters is possible therefore, crossed Claisen condensations of this type are generally not synthetically useful. Such condensations are useful, however, if appreciable differences in reactivity exist between the two esters, as, for example, when one of the esters has no a-hydrogens and can function only as an enolate anion acceptor. These esters have no a-hydrogens ... 

A crossed Claisen condensation is most effective when one of the carbonyl compounds can only function as an enolate anion acceptor. (15.3)... 

Crossed Claisen condensations are useful only when an appreciable difference exists in the reactivity between the two esters. Such is the case when an ester that has no a-hydrogens can function only as an enolate anion acceptor ... 

When the attempt is made to reach definitions independent of a solvent they become similar to those given by Ussanovich s according to which an acid is either a cation donor or an anion acceptor, and a base either an anion donor or a cation acceptor. In this way certain redox reactions are also considered as acid-base reactions. 

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