Tertiary carbanion

This reaction is favored because a resonance-stabilized anion (III) is formed, whereas with a simple cyclic monoolefin, such as methylcyclo-hexene, a nonresonance stabilized tertiary carbanion would have to be formed. 

This addition is energetically favored because a more stable primary carb-anion is formed from a less stable tertiary carbanion. The addition step in the side-chain alkylation reaction is probably not energetically favored because a primary carbanion is formed from a resonance-stabilized benzylic carbanion. However, as the rapid and energetically favored transmetalation reaction following it restores the benzylic carbanion, the over-all process takes place readily. An alternative radical combination mechanism has also been proposed by Morton and Ward 39) for the alkylation reaction. 

This indicates that the benzylic carbanion adds to the olefin to yield a primary carbanion rather than a secondaiy or tertiary carbanion. 

The tertiary carbanion perfluoro-1-methyl-1-cyclobutyl is considerably more stable than most carbanions. 

Tertiary carbanions have been isolated, but secondary and primary structures do not exhibit sufficient kinetic stability. The lifetimes of the former is highly dependent on structure, counterion, solvent and other factors. One of the simpler tertiary carbanions is present in (perfluoro-fer/-butyl)cesium obtained as a solid from the reaction of perfluoro(2-methyl-propene) and cesium fluoride in tetraglyme.36 The structure of the salt was analyzed by 19F and l3C NMR spectroscopy. 

All three isomers of butyllithium in the presence of TMEDA give very similar results for the propagation reaction. With n-butyllithium the initiation process proceeds at the same rate as propagation, but with s- and f-butyllithium the initiation is faster than propagation 173 175). In these last two cases, the process of initiation converts the very reactive secondary and tertiary carbanions into the primary ion. A similar phenomenon has been reported by Bartlett et al.176) who found that i-propyllithium in ether solution at —60° adds only a single molecule of ethylene. 

The alkylation of alkyl-substituted aromatic hydrocarbons by propylene and isobutylene described by Pines and Schaap (1960), and the dimerization of olefins with organo-alkali metal compounds, are also found to proceed most easily when primary carbanions are formed since they are more stable than secondary and particularly tertiary carbanions. 

Steric and electronic effects on the rate and regiochemistry of the reaction between p-nitrobenzyl substrates and tertiary carbanions were also studied71. Thus, increasing the size of the alkyl groups attached to the benzylic or anionic carbons of the substrates causes substantial decrease in the proportions of C-alkylation product. In contrast with the previous reaction with nitronate anions, formation of reduction products is observed instead of a significant O-alkylation. 

Ethyl phenylacetate and even the tertiary carbanion methyl diphenylacetate gave 84 and 42%, respectively, of the disubstitution product in their reaction with 2,6-dibromopy-ridine179. However, only monosubstitution product at position 4,134 was obtained in 70% yield in the reaction of 4,7-dichloroquinoline (133) with pinacolone enolate ion (equation 90)179. 

Substituent effects in addition of earbanions to 7r-arenechromium tricarbonyl complexes have been examined. Methyl and chloro substituents give mixtures of ortho- and mefa-substitution, with the amount of mem-isomer increasing with bulk of the anions. Methoxyl and dimethylamino substituents are strongly meta -directing. The more hindered 3-position of 1,2-dimethoxybenzene is substituted even by a tertiary carbanion. Naphthalene shows 99% a -substitution. A trimethylsilyi group is strongly para-directing. 

In the case of unsymmetrically substituted cyclopropanones, one would expect two possibilities for the ring cleavage, reflecting the relative stabilities of the anions formed according to Scheme 14. While this accounts for the facts in most cases, there are examples where the product distribution does not parallel the expected order of carbanion stability. Thus, in the reaction of 2,2-di-t-butylcyclopropanone , the main product is not derived from the primary carbanion but rather from the tertiary carbanion, as shown in Scheme 15. Here, the propensity of the C(l)-C(2) bond to cleave appears to be enhanced by steric considerations involving the presence of two bulky t-butyl substituents at... 

Treatment of Sa-cholestan-3-one tosylhydrazone (115) with excess (8 equiv.) alkyllithium reagent (primary, secondary and tertiary carbanions) provides, after axial protonation, 3P-alkylcholestane products (116) in addition to minor amounts of the anticipated alkene by-products (equation 16). ... 

Sulfones can also be used in Friedel-Crafts-type cyclizations. One example is shown in Scheme 85, where the cyclization in the presence of aluminum chloride of the allylic sulfone occurred in high yield, showing that sulfones can also become electrophiles. Since a tertiary carbanion has about the same stability as a simple allylic cation, tertiary sulfones can also be cyclized (Scheme 85). These last examples illustrated the umpolung provided by the sulfone since carbanion chemistry was used to introduce the 2-methyl groups. 

Unlike the carbanions from 1,3-dithians (see Chapter 3, p. 31 and Chapter 6, p. 90), the sulfinyl carbanions have the ability to undergo Michael additions (conjugate or 1,4-additions) with a,p-unsaturated carbonyl compounds. For instance, the secondary carbanion (44) from the ethyl ethylthiomethyl sulfoxide (45) may be sucessively reacted with ethyl iodide and 3-butene-2-one to give heptan-2,5-dione (46) via the tertiary carbanion (47), as shown in Scheme 23. The carbanion (47) may also be condensed with propyl bromide, and hydrolysis of the product yields ethyl propyl ketone (48) (Scheme 23). 

Acylation of lithiated phosphonate carbanions is observed at low temperature with tertiary carbanions and acyl chlorides or anhydrideBy contrast, acylation of the secondary carbanion is less efflcient ... 

Greengrass, C.W., and Hoople, D.W.T., Reaction of 4-acetoxy-2-azetidinone with tertiary carbanions. Preparation of 4-alkyl- and 4-alkyhdene-2-azetidinones, Tetrahedron Lett., 22, 1161, 1981. 

Tertiary carbanions such as (26) also react with 4-acetoxyazetidinone to give the carbon-substituted /3-lactam (27). The reaction proceeds in high yield, and the product can be oxidized to a sulphoxide and converted into the 4-alkyhdene derivative by elimination of benzenesulphenic acid. Full details of a cyclization reaction of 4-acetoxyazetidin-2-ones have appeared. ... 

Carbanions, on the other hand, are negatively charged, so they are destabilized by alkyl groups. Therefore, methyl anions are the most stable and tertiary carbanions are the least stable. 

Semmelhack and HaU now report that (1) phenylates some carbanions, particularly tertiary carbanions. Thus (1) reacts at 25° with the anion of isobutyronitrile, generated by LDA in THF below 0°. Treatment of the intermediate complex (2) with iodine liberates the free organic ligand (3). 

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