Reactions involving carbanions

A summary of the in situ use of the azobenzene probases is given in Table 2. Apart from the generation of ylid, referred to above, the main applications have been for N- and C-alkylation of weak nitrogen and carbon acids, for the promotion of condensation and substitution reactions involving carbanions such as the cyano-methyl anion, for an interesting carboxylation reaction (entries 4 and 17), and for base-promoted cyclisations (entries 5 and 6). 

Not only can the a-methylene carbanion be produced and stabilized, but it can also undergo base-catalyzed aldol-type reactions without decomposition of the chelate ring. The most-studied reactions involve carbanion additions to acetaldehyde to produce threonine and allothreonine. This can be achieved for bis(glycinato)copper(II),48,49 tris(glycinato)cobalt(III)50,51 or glycinato-bis(l,2-diaminoethane)cobalt(III),52 with the best yield being obtained in the last case, where the... 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e. g., malonic and acetoacetic ester syntheses, is one of the most well documented important methods in organic synthesis. Ketone enolates and protected ones such as vinyl silyl ethers are also versatile nucleophiles for the reaction with various electrophiles including alkyl halides. On the other hand, for the reaction of aryl halides with such nucleophiles to proceed, photostimulation or addition of transition metal catalysts or promoters is usually required, unless the halides are activated by strong electron-withdrawing substituents [7]. Of the metal species, palladium has proved to be especially useful, while copper may also be used in some reactions [81. Thus, aryl halides can react with a variety of substrates having acidic C-H bonds under palladium catalysis. 

ESR) spectroscopy, as was seen with the previously mentioned reaction. The conclusion was that processes involving one-electron transfer must be important in many reactions involving carbanions. ... 

REACTIONS INVOLVING CARBANIONS 1. Halogenation of ketones. Discussed in Secs. 21.3-21.4. 

Electron transfer from carbanions to organic acceptors has been established by e.s.r., and it has been concluded that one-electron transfer must be important in many reactions involving carbanions (Russell et al., 1964). An investigation by Kerber et al. (1965) has shown that carbon-alkylation of a nitroparaffin anion by p-nitrobenzyl chloride occurs at least in part via electron transfer ... 

The resin was then filtered off and washed thoroughly with DMF, MeOH, and GH2GI2, and the product resin was used directly for the various reactions involving carbanions. 

Many important organic reactions involve carbanions as nucleophiles. The properties of carbanions were introduced in Section 3.4.2, and their reactivity is discussed in more detail in Chapter 6. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base that can generate the reactive carbanion intermediate. Base-catalyzed addition reactions of carbonyl compounds provide many examples of this type of reaction. The reaction... 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e.g., malonic and acetoacetic ester... 

Streitwieser and co-workers reported extensive measurements of pKa values (designated as pKcsCHA values) of weak acids in cyclohexylamine (CHA) solution using the cesium salt of cyclohexylamine as the base. They also established a pKa scale involving Li and Cs " counterions in tetrahydrofuran (THF) solution. These pKg values are useful because THF is often used for synthetic reactions involving carbanions. The results suggested that lithium salts behave as solvent-separated ion pairs, while the cesium salts appeared to be contact ion pairs. The pKcs/xHP values of p-methylbiphenyl, fluorene, and 9-biphenylfluorene were found to be 38.73,22.90, and 17.72, respectively. ... 

The softness of a carbanion is related to the hybridization of the carbon. Higher p character increases the softness sp > sp > sp) and configurational instability of the carbanion. Reactions involving carbanions formally derived from alkanes often lead to racemization, whereas configuration is retained at the alkenic carbanion center. The fact that cyclopropyl carbanions maintain their original stereochemistry (14) concurs with the above notion, as the s char-... 

Aldol-type condensation of an aromatic aldehyde with activated methylarene or phenylacetic acid is a useful reaction for preparing stilbene derivatives. Starting from para-substituted toluenes or para-substituted aromatic aldehydes, one can obtain 4,4 -disubstituted stilbenes. This reaction is relatively simple but has low yield. As an example, condensation of 2,4-dinitrotoluene and 4-nitrophenylacetic acid with aromatic aldehyde was studied [26]. The reaction involves carbanion addition to the carbonyl group. The carbanion is formed by the extraction of proton from the active methylene group of 2,4-dinitrotoluene by the base (usually, piperidine). The carbanion then adds to carbon atoms of the carbonyl group of the aldehyde. The reaction will therefore be facilitated by the ease of both the formation of the... 

This simple solid liquid system can be efficiently applied in many important reactions involving carbanions and other organic anions. It is particularly useful for alkylation of ethyl malonate, cyanoacetate and acetylacetate(lO) e.g. 

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