Carbanions, reactions

Chapters 1 and 2. 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 which can generate the reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde, which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophihc and does not react with benzaldehyde. The much more nucleophilic enolate (carbanion) formed by deprotonation is the reactive nucleophile. 

Amines, thiols, eOH (p. 226), etc., will also add to the 0-carbon atom of 0-unsaturated carbonyl compounds and esters, but the most important reactions of C=C—C=0 systems are in Michael reactions with carbanions reactions in which carbon-carbon bonds are formed. A good example is the synthesis of l,l-dimethylcyclohexan-3,5-dione (dimedone, 100) starting from 2-methylpent-2-ene-4-one (mesityl oxide, 101) and the carbanion 0CH(CO2Et)2 ... 

Although addition of HCN could be looked upon as a carbanion reaction, it is commonly regarded as involving a simple anion. It is of unusual interest in that it was almost certainly the first organic reaction to have its mechanistic pathway established (Lapworth 1903). HCN is not itself a powerful enough nucleophile to attack C=0, and the reaction requires base-catalysis in order to convert HCN into the more nucleophilic CN the reaction then obeys the rate law ... 

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11,125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects, hydrogen, in aromatic substitution reactions, 2,163... 

Nickel halides and nickel complexes resulting from oxidative addition can also give rise to subsequent replacement and insertion reactions. Replacement reactions have been described mainly with arylnickel halide complexes (examples 23, 29, and 31, Table III). Carbanionic species replace halide ions and can undergo coupling or insertion reactions. An example of application of a carbanionic reaction to the synthesis of a natural product is the coupling step between an aromatic iodo-derivative and an active methylene group to form cephalotaxinone (example 23, Table III). 

Condensation reactions are conveniently written as carbanion reactions, and yet it is clear that the metallic cation is important too. For example, sodium and lithium give quite different results in the condensation of acetophenone and tert-butyl acetate.422 The various rate and equilibrium constants depend on the nature of the associated metal. Lithium, zinc, and magnesium, which give the aldol condens-... 

A certain dualism is observable in carbonium ion-carbanion chemistry, a dualism rather like that of lines and points in projective geometry. The reader may recall that interchanging the words "line and "point in a theorem of projective geometry converts it into a statement that is also a theorem, sometimes the same one. For most carbonium ion reactions a corresponding carbanion reaction is known. The dualism can be used as a method for the invention of new, or at least unobserved, carbanion reactions. The carbanionic reaction corresponding to the carbonium ion rearrangement is of course the internal nucleophilic... 

In another study, Streitwieser and Van Sickle (1962) measured the secondary /3-deuterium KIEs for the formation of carbanions from hydrocarbons with lithium cyclohexylamide in cyclohexylamine at 49.9°C. The rate constants needed for determining these KIEs for the formation of the carbanion (reaction (34)) were obtained by analysing the deuterium in the ethylbenzene recovered from the reaction at various times by mass spectrometry. 

Enantiocontrol of carbanion reactions of organolithium reagents has been the subject of a short review. ... 

The production of n-butylbenzene may be attributed to an inherent lack of complete selectivity in carbanion reactions, because the greater stability of an intermediate does not exclude the formation of the less stable product. This stability is only important when the step in forming intermediates is slow or when energy differences are large. On the other hand, the formation of n-butylbenzene from toluene and propylene may be due to a partial radical character of benzyl alkali metals. The latter would not seem to be the case because the potassium compounds should have greater ionic character, but they yield more n-butylbenzene. This agrees with the idea that lack of selectivity may be due to greater rate of reaction of potassium compounds with olefins. 

The alkylation of benzene by alkylpotassium compounds has been reported by Bryce-Smith (S9) and is probably due to the increased base strength of organopotassium compounds over organosodium compounds. The potassium hydride eliminated in the cyclization reaction may add to ethylene to form ethylpotassium, which then may react with the aromatic to yield ethane and a benzylic carbanion [Reactions (16) and (17)]. 

The mechanism of this addition is similar to that for styrenes only an allylic carbanion is formed instead of a benzylic carbanion [Reaction (26)]. 

The primary allylic carbanion apparently predominates and reacts with aromatic to yield the alkenylbenzene and regenerate the benzylic carbanion [Reaction (27)]. 

The relative amounts of these produced at various temperatures are shown in Table VII. The formation of these products may be explained using carbanionic mechanisms. The cyclic material may form by addition of an allylic carbanion to a molecule of the styrene, followed by a cyclization to yield a benzylic carbanion [Reaction (33a, b, c)]. 

Diacetone alcohol is a solvent used in hydraulic fluids and printing inks. Recall that the aldol condensation is an example of a variety of carbanion reactions used to make large molecules from smaller ones. An aldehyde or a ketone with at least one hydrogen on the carbon next to the carbonyl will react to give the aldol condensation. The mechanism is given as follows. 

Deprotonation of 3-picoline is more difficult (the anion cannot achieve stability through resonance, as happens with the others) and a much stronger base, LDA [lithium diisopropylamide (lithium propan-2-ylamide)], is needed. Once achieved, however, the conjugate anion behaves as a nucleophile and undergoes typical carbanion reactions (indeed, it is more reactive than its counterparts, since reactivity is most often the opposite of stability ). 

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