Pyrrole, carbanion formation from

Some examples of ring opening reactions with carbanions leading to five-membered heterocyclic ring formation are shown in Scheme 85. Pyrrole syntheses from functionally substituted oxiranes and amines are often described and typical examples are shown in Scheme 86. 

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). 

Evidence for the formation of carbanions from ketoximes and DMSO in the presence of large amounts of KOH is provided by partial deuterium exchange between DMSO-D6 and the a-position of ketoximes (75MI2 76S281) in the synthesis of pyrroles, along with partial deuteration of hydroxyl. 

This intermediate, however, must lead to l-vinyl-2-methyl-3-ethoxy-pyrrole, which is not the case. Otherwise, of two possible carbanions (170 and 171), the latter seems to be more stable since it is free from destabilizing interaction of the negative charge with the lone electron pairs of the neighboring oxygen atom, which does occur in the carbanion 170. This is likely to be responsible for the regiospecific formation of l-vinyl-2-ethoxymethylpyrrole (54). 

We have presented evidence that pyrrole-2-carboxylic acid decarboxylates in acid via the addition of water to the carboxyl group, rather than by direct formation of C02.73 This leads to the formation of the conjugate acid of carbonic acid, C(OH)3+, which rapidly dissociates into protonated water and carbon dioxide (Scheme 9). The pKA for protonation of the a-carbon acid of pyrrole is —3.8.74 Although this mechanism of decarboxylation is more complex than the typical dissociative mechanism generating carbon dioxide, the weak carbanion formed will be a poor nucleophile and will not be subject to internal return. However, this leads to a point of interest, in that an enzyme catalyzes the decarboxylation and carboxylation of pyrrole-2-carboxylic acid and pyrrole respectively.75 In the decarboxylation reaction, unlike the case of 2-ketoacids, the enzyme cannot access the potential catalysis available from preventing the internal return from a highly basic carbanion, which could be the reason that the rates of decarboxylation are more comparable to those in solution. Therefore, the enzyme cannot achieve further acceleration of decarboxylation. In the carboxylation of pyrrole, the absence of a reactive carbanion will also make the reaction more difficult however, in this case it occurs more readily than with other aromatic acid decarboxylases. 

Carbanions from hydrocarbons, nitriles, ketones, esters, TV./V-dialkyl acetamides and thioamides, and mono and dianions from (3-dicarbonyl compounds are some of the most common nucleophiles through which a new C-C bond can be formed. This C-C bond formation is also achieved by reaction with aromatic alkoxides. Among the nitrogen nucleophiles known to react are amide ions to form anilines however, the anions from aromatic amines, pyrroles, diazoles and triazoles, react with aromatic substrates to afford C-arylation. 

Michael addition has been applied to the formation of cyclopropanic systems. Thus, the addition of phosphonate carbanions generated by LDA in THF, by NaH in THF, by thallium(I) ethoxide in refluxing THF, or by electrochemical technique to oc,P-unsaturated esters provides a preparation of substituted 2-(alkoxycarbonyl)cyclopropylphosphonates in moderate to good yields via a tandem Michael addition-cyclization sequence (Scheme 8.47). The cyclopropanation has also been achieved via oxidation with iodine in the presence of KF-AI2O3. Nitrile ylides prepared from acyl chlorides and diethyl isocyanomethylphosphonate in the presence of Et3N react with methyl acrylates by a 1,3-dipolar cycloaddition to give phosphoryl pyrrolines or pyrroles. ... 

Thus, the stereoselective methylthiovinylation of pyrroles, formed from ketoximes and acetylene in the KOH/DMSO system, represents a rare example of interaction of the vinyl carbanion with sulfoxide function. Despite the negligible yield of methylthiovinylpyrroles, the fact of their formation sheds additional light on peculiarities of the reaction of ketoximes with acetylene in the systems MOH/DMSO. 

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