Other carbanions

The use of sodium ethoxide for a Claisen condensation-type reaction. 

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Other carbanionic groups, such as acetylide ions, and ions derived from a-methylpyridines have also been used as nucleophiles. A particularly useful nucleophile is the methylsulfinyl carbanion (CH3SOCHJ), the conjugate base of DMSO, since the P-keto sulfoxide produced can easily be reduced to a methyl ketone (p. 549). The methylsulfonyl carbanion (CH3SO2CH2 ), the conjugate base of dimethyl sulfone, behaves similarly, and the product can be similarly reduced. Certain carboxylic esters, acyl halides, and DMF acylate 1,3-dithianes (see 10-10. )2008 Qxj(jatjye hydrolysis with NBS or NCS, a-keto aldehydes or a-... 

In the reaction of the phenylacetonitrile carbanion with thiocyanates, a major side reaction leads to the formation of the dialkyl disulphides, as a result of the base-catalysed decomposition of the thiocyanate. This side reaction is reported to be insignificant in the reactions of the other carbanions. Phenylacetonitrile reacts with 1,2-ethanyl bisthiocyanate to produce 2-cyano-2-phenyl-1,3-thiolanes [52] under conditions analogous to those used for the synthesis of the thioethers (Scheme 4.12). 

Some carbyne complexes, in particular cationic ones with good Ji-accepting ligands, can react with nucleophiles to give carbene complexes [187,521]. Several reductions of carbyne complexes to carbene complexes by treatment with metal hydrides have been reported. Similarly, organolithium or other carbanionic reagents can react with electrophilic carbyne complexes to yield carbene complexes. Illustrative examples of both reactions are sketched in Figure 3.23. 

The term condensation refers to the joining of two molecules with the splitting out of a smaller molecule. The Claisen condensation is used extensively in the synthesis of dicarbonyl compounds. In biochemistry it is used to build fatty acids in the body. The Dieckmann condensation, the crossed Claisen condensation, and others (with other carbanions) cire variations of the Claisen condensation. In this section we briefly look at these variations. 

The carbanion, though relatively stable, compared to other carbanions, is still reactive enough to pick up a proton from water to give CHBr3 ... 

TABLE 25. Proton affinity (PA) and relative stability with regard to the CH3 anion (AE of equation 20) given for cyclopropane and some other carbanions"... 

Methylsulfinyl Carbanion. Strong bases, eg., sodium hydride or sodium amide, react with DMSO producing solutions of methylsulfinyl carbanion, known as the dimsyl ton, which arc synthetically useful. The solutions also provide a strongly basic reagent for generating other carbanions. 

The great synthetic utility of the reaction of alkyllithium and Grignard reagents with ketonic functions has been well documented.105 These reactions take place via the intermediacy of alkoxy derivatives formed by addition of the M—C bond across the C=0 function. Hence ketones, aldehydes and formaldehyde will lead to tertiary, secondary and primary alkoxides, respectively. This type of reactivity is known for a number of other carbanionic metal alkyl derivatives, both main group and transition metals, although the synthetic utility of the reactivity has in most cases not been well documented. 

Addition of Other Carbanionic Species Bearing Leaving Groups... 

Cyclopropanations are known for several other carbanionic intermediates of the general type (7), in which the substituent G is ultimately lost as an anionic leaving group in the last step of the ring-forming pathway (see Scheme 3 above). The substituent G is most often a functional group based upon sulfur, selenium or nitrogen. Halide-substituted derivatives probably react via the a-elimination pathway in most cases (see Section 4.6.3.1), but in some reactions with electron deficient alkenes as substrates, the normal order of steps may be altered (e.g. Table 10, ref. 162). 

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. 

Resonance stabilization of the enolate anion makes it a weaker base than other carbanions, so it is easier to generate by acid-base reactions. 

Because the cyclopentadienyl anion (six pi electrons) is aromatic, it is unusually stable compared with other carbanions. It can be formed by abstracting a proton from cyclo-pentadiene, which is unusually acidic for an alkene. Cyclopentadiene has a pKa of 16, compared with a pKa of 46 for cyclohexene. In fact, cyclopentadiene is nearly as acidic as water and more acidic than many alcohols. It is entirely ionized by potassium terf-butoxide ... 

This type of reaction is not observed with polybutadienyllithium as expected from this mechanism, but this and other carbanions react slowly with tetrahydrofuran to form colorless species which are inactive in polymerization. 

Methoxypyrazine (142) gave 2-(a-cyanobenzyl)pyrazine (143) (PhCH2CN, NaH, THF, reflux, 30 min then substrate j, reflux, N2, TLC monitored 46%).309 other carbanions seem to have been less successful.309... 

The solvent effects on the absorption spectra of ion pairs were studied by many authors and the direction of the observed shift depends on the change (increase or decrease) of dipole moment upon the electronic transition [25]. Generally a bathochromic shift is observed with an increase of solvent polarity. When going from a polar solvent to a less polar one, the association in the ground state increases more strongly than in the excited state this may be understood if the ion pair switches progressively from SSIP to CIP status. Observations of this type were often made, together with cation effects, as for instance in the case of alkali phenolates and enolates [7], fluorenyl and other carbanion salts [22] or even for aromatic radical anions [26, 27],... 

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