Carbanions carbonyl-stabilized

The decarboxylation reaction usually proceeds from the dissociated form of a carboxyl group. As a result, the primary reaction intermediate is more or less a carbanion-like species. In one case, the carbanion is stabilized by the adjacent carbonyl group to form an enolate intermediate as seen in the case of decarboxylation of malonic acid and tropic acid derivatives. In the other case, the anion is stabilized by the aid of the thiazolium ring of TPP. This is the case of transketolases. The formation of carbanion equivalents is essentially important in the synthetic chemistry no matter what methods one takes, i.e., enzymatic or ordinary chemical. They undergo C—C bond-forming reactions with carbonyl compounds as well as a number of reactions with electrophiles, such as protonation, Michael-type addition, substitution with pyrophosphate and halides and so on. In this context,... 

Base-catalyzed transformations can be carried out elsewhere on a complex molecule in the presence of such protected -dicarbonyl magnesium chelate. For example, the chelated magnesium enolate of a /3-ketoester such as 71 prevents the carbonyl keto group becoming an acceptor in aldol condensations. However, in the presence of excess of magnesium methanolate, exchange of the acetyl methyl protons can occur via a carbanion 72 stabilized by delocalization into the adjacent chelate system (equation 99). 

The stabilized ylides have at least one substituent on the ylidic carbon that can either conjugate with the P=C rcbond (16a) or delocalize the carbanion charge (16c). Most of the crystal structures of the stabilized ylides have a carbonyl group attached to the ylidic carbon, although other groups are represented. The dominant resonance structures of the carbonyl stabilized ylides are shown in 16a-c. A summary of the X-ray structural data for the stabilized ylides is given in Table 3. 

These are very facile processes, and here this subject is divided into reaction of halides with (i) organometallic compounds, (ii) reaction with carbonyl stabilized anions and (iii) reaction with other carbanions. 

Dipole-Stabilization is a term coined by Beak to describe the situation that results when a carbanion is stabilized by an adjacent dipole. Such a situation arises when, for example, an amide is deprotonated a to nitrogen. The chemistry of these systems has been reviewed, - so only a few pertinent points will be made here. Firstly, metalation occurs syn to the carbonyl oxygen, and when the system is cyclic, the equatorial proton is removed selectively, and the electrophile attacks equatorially, as shown in equation (5), 24, 25 Thus, in contrast to nitrosamines, amide anions give the less stable equatorial product. 4,i25... 

Furan-2,5-dione, or maleic anhydride, can quite reasonably be regarded as furanoquinone, the reduced form being furan-2,5-diol. The carbonyl groups do have some ketonic activity, and in Scheme 23 the key step of a second route to freelingyne (86 in Vol. 30) makes use of this capacity in a reaction with another phosphorus ylide reagent.96 (cf. Scheme 16). Even ordinary (carbonyl stabilized) carbanions will undergo such aldol con-... 

Perhaps the most useful of nucleophilic carbanion reagents are the carbonyl stabilized carbanions, the enolates. Enolate anions will be discussed in Chapter 9. 

In the formation of the first synthetic intermediate in Sequence D, the very effective Verley-Doebner modification of the fundamental Knoevenagel condensation is used. This modification uses malonic acid in place of the conventional ester to promote enoUzation. In addition, the heterocyclic amine, pyridine, functions as both the base catalyst and the solvent. A cocatalyst, P-alanine (an amino acid), is also introduced. Mechanistically, the reaction closely resembles the aldol condensation in that in both cases a carbanion is generated by abstraction, by base, of a proton alpha to a carbonyl group. The resulting carbanion is stabilized as an enolate anion (see below). 

The stabilization of the C-5 carbanion by both the carbonyl and the sulfur of the ring in 187 affords a very powerful nucleophilic center... 

There are at least two mechanisms available for aziridine cis-trans isomerism. The first is base-catalyzed and proceeds via an intermediate carbanion (235). The second mechanism can be either thermally or photochemically initiated and proceeds by way of an intermediate azomethine ylide. The absence of a catalytic effect and interception of the 1,3-dipole intermediate provide support for this route. A variety of aziridinyl ketones have been found to undergo equilibration when subjected to base-catalyzed conditions (65JA1050). In most of these cases the cis isomer is more stable than the trans. Base-catalyzed isotope exchange has also been observed in at least one molecule which lacks a stabilizing carbonyl group (72TL3591). 

Carbanions derived from carbonyl compoimds are often referred to as etiolates. This name is derived from the enol tautomer of carbonyl compounds. The resonance-stabilized enolate anion is the conjugate base of both the keto and enol forms of carbonyl... 

A hydrogen attached to the a-carbon atom of a p-keto ester is relatively acidic. Typical P-keto esters have values of about 11. Because the a-carbon atom is flanked by two electron-withdrawing carbonyl groups, a carbanion formed at this site is highly stabilized. The electron delocalization in the anion of a p-keto ester is represented by the resonance structures... 

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. 

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