Ambident anion, behavior

As discussed in Section 3.3.1.6.1, pyrroles are weak acids. The resulting ions react exceedingly readily, even with weak electrophilic reagents at either carbon (30) or nitrogen (31) this behavior is similar to that of the ambident anion from acetoacetic ester which shows alternative reactions (32, 33). 

Another ambident anion species which shows cation and solvent dependent alkylation behavior is pyridone. Although N-alkylation is preferred thermodynamically, and benzylation of the sodium salt yields predominantly the N-alkylated product, 0-alkylation of the anion is favored by dipolar aprotic solvents and soft cations [58]. In pentane or benzene, the silver salt of pyridone anion is 0-benzylated. Likewise, under phase transfer conditions, where the anion is presumably paired with a quater-... 

Ambident reactivity occurs for any substance displaying protomeric behavior. In basic medium anions in which negative charge is delocalized are formed this may be represented by the resonance formulas in Scheme 2. Each charged atom may react with an electrophilic center. The... 

The same behavior has been observed in the attack of electrophiles on the ambident enolate anions, of which many reactions are closely related to those of enamines [Eq. (2)] ... 

The /-butylthiolate anion exhibits ambident behavior, giving both types of product, complexes 56 and 57 ... 

As already discussed in Section 4.19.2.2.1 hydroxyl derivatives of thiazole are tautomeric, according to Scheme 127, the oxo forms usually being the most stable. The interconversion of the hydroxyl and carbonyl forms proceeds under acid or base catalysis. This protomeric behavior of the thiazolinones corresponds to an ambident nucleophilic reactivity which is most marked for the conjugate anion arising in basic medium. 

Gompared to C-C bond forming reaction, the G-N bond formation is still immature. Moreover, new amination methodologies will have a direct impact on pharmaceutical and fine chemical industries for the synthesis of a variety of commercially interesting compounds. Since the pyrrolyl anion exhibits ambident behavior as a nucleophile, alkylation can occur at carbon as well as at nitrogen (Scheme 20). 

This study reports on the reactions of ambident nucleophiles with electron-deficient nitroaromatic and heteroaromatic substrates anionic complex formation or nucleophilic substitution result. Ambident behavior is observed in the case of phenoxide ion (O versus C attack) and aniline (N versus C attack). O or N attack is generally kinetically preferred, but C attack gives rise to stable thermodynamic control. Normal electrophiles such as 1,3,5-trinitrobenzene or picryl chloride are contrasted with superelectrophiles such as 4,6-dinitrobenzofuroxan or 4,6-dinitro-2-(2,4,6-trinitrophenyl)benzotriazole 1-oxide (PiDNBT), which give rise to exceptionally stable a complexes. Further interesting information was derived from the presence in PiDNBT of two electrophilic centers (C-7 and C-l ) susceptible to attack by the ambident nucleophilic reagent. The superelectrophiles are found to exhibit lesser selectivity toward different nucleophilic centers of ambident nucleophiles compared with normal electrophiles. 

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