Resonance in some carbanions

Direct C-G bond formation at the 2-position of imidazole and benzimidazole was reported when imidazole was deprotonated with potassium /-butoxide in liquid ammonia via electrochemically induced SrnI. Coupling products 340 and 341 were isolated in a 4 1 ratio when the reaction started from 4-methylimidazole 338. Only one product 343 was obtained when 2-methylimidazole was used. It was proposed that delocalization of the negative charge of 345 resulted in some carbanion character (resonance contributors 346-348), and thus subsequent SrnI reaction with an electron deficient aryl chloride at carbon (Scheme 81) <1995JOC8015>. 

All Pi values obtained for oxygen bases A- are between 0.94 and 1.09 (Table 7), with the exception of two reactions involving nitromethane, with lower p2 values. This indicates that proton transfer between these oxygen bases must be fast as expected. (The best value is probably a little larger than 1.) It follows that the assumption of the constancy of the p, values is essentially correct for these examples. Furthermore, the predicted relationship between p value (of a CH bond) and resonance effect in the carbanion appears to be fulfilled (Table 7). For some reactions of the type RH + A-, the values calculated for n2max = 1 — nmax are of approximately the same magnitude as the experimental Bronsted (5 values [132],... 

Stork enamine synthesis takes advantage of the fact that an aldehyde or ketone reacts with a secondary amine to produce an enamine. Enamines are resonance stabilized (see Figure 15-25) and have multiple applications. In the first resonance structure, the nitrogen is the nucleophile, while in the second resonance structure, the carbanion is the nucleophile. Some commonly used secondary amines, pyrrolidine, piperidine, and morpholine, are shown in Figure 15-26. 

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

The most convincing evidence for this mechanism is that in the reaction between isobutene-1-I4C and carbene the product 2-methyl-1-butene was labeled only in the 1 position.236 This rules out a free radical or other free intermediate such as a carbocation or carbanion. If 28 (or a corresponding ion) were an intermediate, resonance would ensure that some carbene attacked at the 1 position ... 

The ylide is prepared by deprotonating a triphenylalkylphosphonium salt with a strong base, commonly an organometallic base such as butyllithium or phenyllithium. The hydrogens on the carbon that is bonded to the phosphorus of the salt are somewhat acidic because the carbanion of the conjugate base (the ylide) is stabilized by the inductive effect of the positive phosphorus atom. In addition, a resonance structure with five bonds to phosphorus makes a minor contribution to the structure and provides some additional stabilization. The triphenylalkylphosphonium salt can be prepared by an SN2 reaction of triphenylphosphine with the appropriate alkyl halide (see Section 10.9). 

The reason for this - at first glance - unexpected behavior is inversion of the electrostatic partial charges (compared, e. g., with the corresponding iodoalkanes) by the negative inductive effect of the perfluoroalkyl moiety. Nevertheless, in the presence of some classes of nucleophile, for example thiolates, resonance-stabilized carbanions, or enamines, the behavior of perfluoroalkyl halides is sometimes puz-... 

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