Mesomeric stabilization

Organic acids convert the blue mesomerically stabilized phenolate anion to the red undissociated acid. Reductones (e.g. ascorbic acid) reduce the reagent to a colorless salt. 

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

The UV spectra of 3,6-di-ter/-butyl-substituted 2H-, 3H- and 4//-azepines in various ethanol solutions have been recorded.30 On the basis of pH-dependent spectra, pATa values of 7.02, 4.54 and 6.81, respectively, have been calculated for the conjugate acids of these azepines. The increase in basic strength (2H > 4H > 3//) can be reconciled with mesomeric stabilization of the respective azepinium cations. 

Benzyl-type carbanions and their metallo compounds, derived from aromatic or hetero-aromatic precursors, bearing carbon- or hetero-substituents, are readily available with variable substitution patterns due to their mesomeric stabilization (see Section 1.3.2.2)2. Even dicarbanions are accessible without difficulty3,4. The equilibrium acidities of many aromatic hydrocarbons have been determined5-7. The acidities of a-hetero-substituted toluenes8 are similar to those of the corresponding allylic compounds and can usually be generated by the same methods. 

For the deprotonation of less acidic precursors, which do not lead to mesomerically stabilized anions, butyllithium/TMEDA in THF or diethyl ether, or the more reactive, but more expensive,. seobutyllithium under these conditions usually are the most promising bases. Het-eroatomic substitution on the allylic substrate, which docs not contribute to the mesomeric or inductive stabilization often facilitates lithiation dramatically 58. In lithiations, in contrast to most other metalations, the kinetic acidity, caused by complexing heteroatom substituents, may override the thermodynamic acidity, which is estimated from the stabilization of the competing anions. These directed lithiations59 should be performed in the least polar solvent possible, e.g.. diethyl ether, toluene, or even hexane. 

Presumably the active chlorine of the chloramines formed by reaction with chlorine gas or hypochlorite reacts with TDM in the presence of acetic acid to yield dark blue, mesomerically stabilized quinoid reaction products that possibly rearrange to yield triphenylmethane dyestuffs. 

The y-lactone ring of the steroid skeleton forms an intermediate cardenohde anion in alkaline medium that nucleophilically adds to the 3,5-dinitrobenzoic acid in the position ortho to the two nitro groups. A mesomerically stabilized red-violet anion is produced (Meisenheimer complex). 

The reaction course has not been elucidated (cf. also sodium hydroxide reagent). Hydrolyzation reactions and aromatizations are probably primarily responsible for the formation of colored and fluorescent derivatives. Substituted nitrophenols - e.g. the thiophosphate insecticides — can probably be hydrolyzed to yellow-colored nitro-phenolate anions by sodium hydroxide or possibly react to yield yellow Meisenheimer complexes. Naphthol derivatives with a tendency to form radicals, e.g. 2-naphthyl benzoate, react with hydrolysis to yield violet-colored mesomerically stabilized 1,2-naph-thalenediol radicals. 

This reaction type has been intensely studied °. The application of highly polar solvents, catalysis with tertiary amines" or with acids mesomeric stabilization of intermediate carbenium ions " (allylic and benzylic systems propargylic systems" ) as well as derivatives of sulfinic acids with increasing acidity - usually indicate an ionic pathway (intra- and/or inter-molecular) ... 

If the mesomeric stabilization is provided by a double bond, the lithiated species is a homoenolate synthon, as shown in Scheme 44a. Reaction with an electrophile typically occurs at the y-position, yielding an enamine, which can then be hydrolyzed to a carbonyl compound. An important application of this approach is to incorporate a chiral auxiliary into the nitrogen substituents so as to effect an asymmetric synthesis. 2-AzaaUyl anions (Scheme 44b), which are generated by tin-lithium exchange, can be useful reagents for inter- and intramolecular cycloaddition reactions. ... 

II. NON-MESOMERICALLY STABILIZED ORGANOLITHIUM COMPOUNDS A. Acidification by Dipole Stabilization... 

Benzylic compounds have—compared to the corresponding methyl derivatives—a higher thermodynamic acidity by 10 to 15 pATa units . Mesomeric stabilization requires a considerable flattening of the carbanionic centre towards sp hybridization (the sum of bond angles is 360° for sp and 328° for sp ). However, we should be aware that even if the carbanionic framework would be completely planar, the ion pair 209 is a planar-chiral species. For epimerization, the cation has to migrate from one face to the other one (equation 48). Due to a more facile flipping of the carbanionic centre and an easier formation of solvent-separated ion pairs, most of chiral benzyUithium compounds 208/ewi-208 racemize with great ease. 

Allylic compounds have acidities similar to those of the corresponding benzylic analogues . Due to the mesomeric stabilization, the carbanionic centre is considerably... 

The effect of the ring heteroatom can be seen in the reactivity of alkylpyridines. Deprotonation from the a-carbon of a substituent at position 4 or 2 is much easier than from a similar substituent at position 3. For example, mesomeric stabilization of the negative charge in (4a) and (5a) will involve contributions from canonical forms (4b) and (5b) in which the charge is carried on ring nitrogen, but a similar contributor cannot be drawn for (6a Scheme 2). 

Pyrid-2- and -4-one are protonated on the oxygen atom, rather than at nitrogen, in view of the mesomeric stabilization this affords, e.g. (104) (76AHC(S1)84). The anion of pyrid-2-one carries the negative charge mainly on the oxygen atom <66JCS(B)996). 

Fluorine in organic compounds is associated with a set of electronic effects inductive and mesomeric, stabilizing and destabilizing, pulling or pushing electrons, which are convincingly... 

The mechanism of the toluene and xylene oxidation bears a close resemblance to the oxidation of propene. Abstraction of a H-atom from the reactive methyl group and formation of a complex between the resulting radical and the catalyst is the first and probably the rate-determining step for both. However, the effect of the mesomeric stabilization of this radical complex is different. While a symmetrical allyl structure is formed from propene, an asymmetrical situation occurs for toluene and xylene, which is illustrated below for the case of toluene, viz. 

Nucleophilic displacement of an a- or y-halogen atom by the classical SAE mechanism of nucleophilic displacement via a Meisenheimer intermediate (e.g. 900) is facilitated by mesomeric stabilization of the transition state. However, the mechanistic balance is fine whereas 4-bromopyridine 1-oxide reacts with potassium amide by the A-E mechanism, the 2-bromo analogue prefers the E-A route. Again, nucleophilic displacement in chloropyrazines can involve the ANRORC mechanism. 

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