Ambident ions

Treatment of an ethylidene malonic ester such as (a) with strong bases results in loss of a proton from the allylic position to produce the ambident ion fb). Alkylation of such carban-ions usually occurs at the carbon bearing the carbonyl groups, resulting in the establishment of a quaternary center and deconjugation of the double bond fc). 

Kornblum et al. (1963) demonstrated that O- vs. C-alkylation (24) of yS-naphthoxide ion (an ambident ion) is markedly solvent dependent. For example, the reaction with benzyl bromide conducted in dimethylformamide gave 97% O-alkylated product, whereas in water 81% C-alkylated product resulted. The difference is attributed to changes in the solvation of ambident anions. The course of the reaction is also influenced by water concentration in... 

Since the carbanion-enolates are ambident ions with two different nucleophilic sites, they can be alkylated at C or at O. 

Ions of this type, which can react at either of two different sites, often are called ambident ions. 

Ambident ions containing a second row element as one of the nucleophilic atoms also give different reaction products depending on the structure of the electrophile as shown by the following examples ... 

In reaction (d) the transition state probably resembles the reactants, and we may assume that the relative reactivity of the two nucleophilic atoms is related to the ratio of their charge densities in the ambident ion (5o/qp). Thus for a displacement reaction in general,... 

We have recently given an M. O. treatment of ambident ions (17). 

A variation of the alkylation of imines has been developed188,189. The imine is first converted to the ambident ion by sodium hydride, sodium amide or lithium diisopropy-lamide and an alkylating agent is then added. 

The high tendency to O-alkylation by triethyloxonium fluoroborate compared with other alkylating agents is observed also in the alkylation of keto-enols,28 the ambident ions of nitroparaffins,29,30 and potassium diethylthiophosphate.31... 

In non-HBD, non-dissociating solvents, a corresponding proposal can be made hard counterions (alkali metal cations) should associate preferably with the hard site, and the substrate RX with the soft site in the activated complex composed of RX and the ambident ion pair [366]. With increasing hardness of the counterion (increasing charge density), the fraction of C-alkylation should increase in non-HBD solvents and decrease on solvent insertion into the ion pair. Indeed, the C-ethylation of M (ethyl acetoacetate) in dimethyl sulfoxide or hexamethylphosphoric triamide increases in the order M = R4N < Cs < K < Na < Li [373]. 

Ambident-ion sphingolipids (sphingomyelins) containing no sugar residues. 

W14. White, E. H., Roswell, D. F., Dupont, A. C., and Wilson, A. A., Chemiluminescence involving acidic and ambident ion light-emitter. The chemiluminescence of the 9-acridine-percarboxy-late anion. J. Am. Chem. Soc. 109, 5189-5196 (1987). 

Other aspects of nucleophilicity toward vinyl cations are the site of capture of ambident ions and the easy intramolecular cyclization by o-methoxy and o-thiomethyl substituents on a P ring. The extensively studied P-aryl rearrangement across the double bond could be regarded as intramolecular substitution by the aryl ring, and data are available on the relative rate of rearrangement and capture by the solvent (69, 70). These topics are not discussed here for lack of space but should be addressed in a more complete discussion of the nucleophilicity. 

Cyanide is a ambident ion. The only reported example of isonitrile formation in cation radical chemistry is in the anodic formation of [29] from 9,10-di-isopropylanthracene (Parker and Eberson, 1972). 

HSAB axiom. However, results contradictory to HSAB prognostication are known in reactions involving ambident ions, e.g.. 

At one time, it was thought that the protonation of ambident ions always leads initially to the less stable isomer. This misconception (which can still be found in some texts) arose from the case of keto-enol tautomerism, where the enol is formed initially and where the ketone is usually the more stable isomer. However, the enol is still the initial product even in cases where it is more stable than ketone. Reaction on carbon is slow because it involves a change in hybridization, not because it leads to a more stable product. 

The final product could be 1,3-cyclohexadiene (49) or 1,4-cyclohexadiene (50). Since the protonation is a typical reaction of an ambident ion and since it is very rapid, one would expect (see p. 251) that attack would take place at the point of maximum formal negative charge. Unfortunately, the PMO method does not help us much here since the first-order charges at the three positions in (48) are equal (51). 

The reaction leads to O-alkylation for the reasons discussed in the treatment of ambident ions (p. 251). The negative charge in (63) is concentrated on oxygen and its bond structure approximates to the ad configuration. Since benzyl chloride is a rather reactive halide, it reacts to give the ad isomer preferentially. 

Phenolate anions are ambident ions and as such may suffer both C- and O-alkylation [26]. Very small changes in free energy usually separate the alternate routes in such reactions and it requires somewhat less than 3 k-calories of free energy to... 

The competition between N- and C-substitution in the alkali metal derivatives of pyrrole is, of course, only one example of the general problem of substitution into ambident ions 225-26 ... 

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