Anionic Reactivity

In media such as water and alcohols fluoride ion is strongly solvated by hydro gen bonding and is neither very basic nor very nucleophilic On the other hand the poorly solvated or naked fluoride 10ns that are present when potassium fluoride dis solves m benzene m the presence of a crown ether are better able to express their anionic reactivity Thus alkyl halides react with potassium fluoride m benzene containing 18 crown 6 thereby providing a method for the preparation of otherwise difficultly acces sible alkyl fluorides... 

Fig. 12. The relationship between acidity and anion reactivity for naphthol couplers differing in the 2-position ballast where A is ballast 1 B, ballast 2 C,...

The relative reactivities of anions and of leaving groups have been studied by Liotta and co-workers (Table 31). The anion reactivity follows the same order... 

The conversion of benzaldoxime [157] into benzonitrile and benzamide by catalytic amounts of potassium halides and cyanide has also been attributed to proton abstraction by the anion (Rasmussen, 1977). Nucleophilic addition was thought to be unlikely, since the anion reactivities in (26) in acetonitrile with... 

The unusual rate enhancement of nucleophiles in micelles is a function of two interdependent effects, the enhanced nucleophilicity of the bound anion and the concentration of the reactants. In bimolecular reactions, it is not always easy to estimate the true reactivity of the bound anion separately. Unimolecular reactions would be better probes of the environmental effect on the anionic reactivity than bimolecular reactions, since one need not take the proximity term into account. The decarboxylation of carboxylic acids would meet this requirement, for it is unimolecular, almost free from acid and base catalysis, and the rate constants are extremely solvent dependent (Straub and Bender, 1972). 

Both macrocyclic and macrobicyclic ligands allow preparation of alkali metal solutions, but the former yield mainly M-, whereas solvated electrons are obtained with the [2]-cryptates (162,163). The enhancement of anion reactivity should also useful for activating anionic polymeriza-... 

Propagation proceeds in a manner similar to that described for the traditional Ziegler-Natta initiators. The transition metal has two active sites—the polymer chain is held at one site (the one occupied by a methyl group in XXVII) and monomer at the other site (shown as the vacancy ). The reactivity of the active sites is high because the counteranion, which is either (ClMAO) or (OR MAO) or a mixture of the two, is a weakly coordinating anion. Reactivity is decreased when the counterion is strongly coordinating. 

In macrobicyclic cryptate complexes where the cation is more efficiently encapsulated by the organic ligand these ion pair interactions are diminished and the reactivity of the anion is enhanced. This effect is seen in the higher dissociation constant, by a factor of 104, of Bu OK in Bu OH when K+ is complexed by [2.2.2]cryptand (12) compared to dibenzo[18]crown-6 (2). The enhanced anion reactivity is illustrated by the reaction of the hindered ester methyl mesitoate with powdered potassium hydroxide suspended in benzene. 

Loose ion pairs of such charge-localized oxyanion salts as potassium t-butoxlde may be difficult to form. This alkoxide is a tetrameric aggregate in THF (20). and crown addition breaks it down to the more reactive monomeric form. It is unlikely that with benzo-15-crown-5 a 2 1 crown-K loose ion pair can be formed similar to that found with potassium picrate or potassium fluorenyl. However, external complexation itself will slightly stretch the . bond, and this can have a profound effect on the anion reactivity (21). 

Tris(dimethylam1no)sulfonium difluorotrimethylsilicate is a source of soluble organic fluoride ion of high anionic reactivity. Fluoride ion from this salt and other tris(dialkylamino)sulfonium difluorotrimethyl si 11 cates has... 

The difference in anionic reactivity of styrene and MMA is greatly reduced, being due to complex formation with solvents and electrolyte. 

As has been clearly demonstrated by X-ray studies (Section III,B), complexation of a salt by a cryptand results in cation-anion separation or dissociation. This enhances anionic reactivity and may effect solubilization of the salt in organic solvents. For example the highly hindered ester methyl mesitoate, 13, may be smoothly hydrolyzed by powdered potassium hydroxide in dry benzene in the presence of [2.2.2] (98). Indeed cryptands have proved particularly useful in phase-trans-... 

It is clear that the ability of alcohols to protonate anionic reactive intermediates and their ability to coordinate to Sml2 (thus increasing the effective molarity and acidity of the proton donor) are factors that should be considered when planning reaction sequences using the reductant. 

According to Coulomb s law [cf. Eq. (2-17) in Section 2.6], the interaction between cation and anion can be minimized by increasing either the relative permittivity of the medium or the interionic distance. There are several ways of separating anions from cations and thus increasing the anionic reactivity [643, 842] ... 

Another effect, called the cesium effect [843, 844], is also connected with the observation that salts of anions with large counterions are highly dissociated in dipolar aprotic solvents, and consequently more anion-reactive. For example, the higher solubility of cesium carbonate in dipolar aprotic solvents and the fact that this salt is far more dissociated than the corresponding Li+, Na" ", or K" " salts, makes this carbonate a superior base in organic synthesis. Amongst the alkali metal cations, the ionic radius of Cs+ r = 334 pm) is more than twice that of Li+ (r = 152 pm). 

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