Carbanions, resonance hybrid

CjHsOOC)—CH—COOCjHj (I) + HOC Hj The carbanion (I) is a resonance hybrid (mesomeric anion) to which there are contributions carrying the negative charge on either carbon or oxygen ... 

The mechanisms of the condensation and many other reactions of nitroalkanes are formulated nowadays in terms of carbanions, as in the case of reactions involving CH activated by other groups6. The ion generated from a nitroalkane by the action of base is regarded as a resonance hybrid e.g. for nitromethane ... 

Three base-catalyzed reactions of such a H s are shown in Fig. 17-1. Since the reactions have the same rate expression, they have the same rate-determining step the removal of an a H to form a stabilized carbanion-enolate anion. The name of this anion indicates that the resonance hybrid has negative charge on C (carbanion) and on O (enolate). 

The apparent fickleness of the acyl-pyrroles and -indoles in their reaction with carbanions to form new C—C bonds arises from the contribution made by the zwitterionic structure, e.g. (410b), to the resonance hybrid and the choice of the reaction conditions is critical for a successful nucleophilic reaction. Thus, formyl-pyrroles and -indoles do not normally undergo the Cannizzaro reaction nor do they form stable cyanohydrins or undergo benzoin-type reactions. However, surprisingly, 2-formylpyrrole reacts with arylaldehydes in the presence of potassium cyanide to yield (428), which is easily oxidized to (429) (B-77MI30505). It is noteworthy that the presence of an ester substituent adjacent to the formyl group modifies the mesomeric interaction to such an extent to allow the formation of (430) in low yield, as a result of an initial benzoin-type self-condensation (Scheme 76) (68BSF637). 

The classical example of a stabilized carbanion is the enolate anion, formed from carbonyl compounds upon treatment with a base. Such an ion can be represented as a resonance hybrid of two canonical structures, namely 29, with the charge on carbon, and 30, with the charge on oxygen (Scheme 2.15). The structure is perhaps more properly described by the hybrid structure 31, where the dashed line and the charge sign indicate that the electron pair is spread over... 

Note that 111 is a resonance hybrid that is, we can write the two additional canonical forms shown. The question therefore arises Why does the carbanion pick up a proton at the 6 position to give the 1,4-diene Why not at the 2 position... 

As resonance hybrids of two nonequivalent forms, enolate ions can be looked at either as a-keto carbanions ("C-C=0) or as vinylic alkoxides (C=C-0 ). Thus, enolate ions can react with electrophiles either on carbon or on oxygen. Reaction on carbon yields an 01-substituted carbonyl compound, while reaction on oxygen yields an enol derivative (Figure 22.6). Both kinds of reactivity are known, but reaction on carbon is more common. 

Finally it should be pointed out that carbanion formation is particularly easy when the carbon-hydrogen bond is weakened by two activating groups as it is in malonic ester, cyanoacctic ester, 0-ketoesters, 0-diketones, etc. In these compounds there is a doubled permanent polarization (XVIII), and the carbanion is a resonance hybrid of three forms ... 

The contribution of the resonance forms XXI, XXII, XXIII, and XXIV to the structure of the anions is frequently overlooked, yet many base-catalyzed condensation reactions of phenol and pyrrole undoubtedly proceed through these resonance structures at the moment reaction occurs. The condensation of phenol with aqueous formaldehyde, the Kolbc synthesis (p. 197), and the Reimer-Tiemann reaction (p. 202) are striking examples of reactions which occur through the seemingly less important carbanion structure of the resonance hybrid. (See p. 133.)... 

It is interesting and characteristic of ions which are resonance hybrids of structures such aa XXV and XXVI that alkylation usually occurs almost exclusively through the carbanion (XXV) rather than the al-coholate form (XXVI). This fact is somewhat surprising, since the structure XXVI (in which the negative charge resides on the more electronegative atom) would be expected to make the larger contribution to the structure of the ion. 

A base extracts an a-hydrogen atom from a molecule of the carbonyl compound to form the resonance hybrid (I). This anion adds through its carbanion form to a second molecule of the carbonyl compound to give the alkoxide intermediate (II). Since this intermediate is a relatively strong base, there will be competition between it and the original base for the proton which was extracted initially from the carbonyl compound to form the carbanion. For the formation of aldol, in which the base is usually sodium or potassium hydroxide, we have ... 

If it is essentially ionic the carbanion would be a resonance hybrid of the structures XXIX and XXX. 

In a basic solution, hydroxide ion removes a proton from the a-carbon of the keto tautomer. The anion that is formed has two resonance contributors a carbanion and an enolate ion. The enolate ion contributes more to the resonance hybrid because the negative charge is better accommodated by oxygen than by carbon. Protonation on oxygen forms the enol tautomer, whereas protonation on the a-carbon reforms the keto tautomer. 

We note that two of the resonance contributing structures give particular stability to the resonance hybrid in that the double bonds present in them are conjugated. Thus, because of stability of the subsequently formed carbanion, the reactant 2-methyl-2-butenal, will be deuterated at all positions y to the carbonyl group. 

The a-hydrogen of the cyanide 97 is acidic, and the carbanion produced is a resonance hybrid, to which a structure similar to 98 contributes substantially. 

Ylides can be described as carbanions that are stabilized by attached positive groups, and hence they are dipolar. Most commonly, the phos-phonium ion is used as the stabilizer and the usual expression of a Wittig reagent is shown as structure 4.47 in Scheme 4.48. The literature sometimes shows this stmcture as a member of a resonance hybrid... 

Carbanions in solution can be considered as resonance hybrids of two contributing structures, one purely covalent and one purely ionic (Eq. (3.1)) ... 

The product might be a resonance hybrid of the two structures where the metal is associated with the carbanion in one and with the oxygen in the other [157]. 

The thiazolium ion has an unusual feature a relatively acidic proton located between the two heteroatoms (at C2). The corresponding carbanion is stabilized inductively by the adjacent positive charge that is distributed over both heteroatoms by resonance. The charge-neutral carbene form (Section 12-9) is a minor contributor to the resonance hybrid. 

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