Carbanions types

The reaction with hydroxide ion is frequently used as proof for the chemical structure of cyclopropenones and has been examined in some detail with respect to the factors governing ring-cleavage. Thus, methyl cyclopropenone23 and aqueous NaOH react to yield a mixture of methacrylic and crotonic acids in a ratio of 3 1 as expected from the relative stabilities of the two possible intermediate carbanions (type 317) ... 

Natural resonance theory (Section 1.6) provides a quantitative gauge of the contributions of various resonance structures to the total electronic density. The results are shown in Table 4.41 demonstrating the remarkable intermediacy in the nature of metal-alkene interaction relating metallacycle, nonbonded, and carbanion-type resonance forms. 

Ketones can be prepared from special carbanions types by elimination reactions which follow oxygenation. 

The reactions reviewed here can be divided into two classes those involving electron-rich alkenes or alkynes and which are of the radical type, and those involving electron-poor olefins (Michael acceptors) and which are of the radical or carbanion type. 

However, hydrocarbons that have a hydrogen atom at the asymmetric carbon may be racemized if they can be converted either to carbocations or to car-banions. The ease of carbanion-type racemization will depend on the acidity of the attached hydrogen and on the stereochemical stability of the intermediate carbanion that is formed. If the configuration of the carbanion intermediate inverts, racemization will result (also see Section 6-4E) ... 

The counterpart of this reaction type is base catalysis, in which the reaction intermediate would be of the carbanion type. However no such reaction has been described in the literature on organic catalysts. 

Hydrogen exchange reactions of heteroaromatics64 69, 65 carried out in strongly alkaline media, such as potassium amide/liquid ammonia, alcoholic solutions of alkoxides, or solutions of potassium <-butoxide in dimethyl sulfoxide, proceed through an entirely different mechanism (sometimes called protophilic ) involving a carbanion-type intermediate66, 67 they are not electrophilic substitutions as such and will not be treated in this review. 

Base-induced deuterium-protium exchange at the C-5 atom of N-substituted tetrazoles occurs readily via carbanionic type intermediates, e.g. (57). The reaction for 1-substituted tetrazoles had a Hammett p-value of -1-1.3 in piperidine-MeOD-DMF (69TL3377). In the acidic pH range 3-5, ylide t5q3e species (58 R = H) are also considered to be involved in the exchange arising from initial protonation at N-1 or N-2 followed by abstraction. 

It has previously been shown that in the triplet state a nitro group on a benzene ring is highly electron withdrawing at the 3- and 4- positions and induces decarboxylations, retro-Aldol type processes and a novel intramolecular redox-type reaction by way of nitrobenzyl carbanion-type intermediates in all cases. In this context, the irradiation of the three 4-nitrobiphenyl derivatives (217), (218) and (219) has been examined in order to assess the potential of photoexcited nitro groups to induce such processes across the biphenyl system.Indeed analogous reactions are observed in these biphenyls but with enhanced quantum efficiencies to those for the reactions in benzenoid compounds. Thus (217) yields the aldehydes (220) and (221) at pHs of 2 and 7, respectively, and (218) gives (222) and (223) under similar conditions, whereas (223) is also formed from (219) at pH 13 but no reaction is observed in neutral solution. The authors note that these results provide more evidence that the... 

Over the years there have been a number of mechanistic proposals for substrate oxidation by TMADH. An early proposal considered a carbanion mechanism in which an active site base deprotonates a substrate methyl group to form a substrate carbanion [69] reduction of the flavin was then achieved by the formation of a carbanion-flavin N5 adduct, with subsequent formation of the product imine and dihydroflavin. A number of active site residues were identified as potential bases in such a reaction mechanism. Directed mutagenesis and stopped-flow kinetic studies, however, have been used to systematically eliminate the participation of these residues in a carbanion-type mechanism [76-79], thus indicating that a proton abstraction mechanism initiated by an active site residue does not occur in TMADH. Early proposals also invoked the trimethylammonium cation as the reactive species in the enzyme-substrate complex, owing to the high (9.81) of free... 

The [Cr(H20)5CH2l] ion is postulated to react via a carbanion-type transition state in which a proton is transferred from the incoming water molecule to the leaving CH2I" group to give Mel and [Cr(H20)50H], which rapidly protonates. 

The carbanion -type reactivity of dienamine activation can be promoted by a silyl-protected diarylprolinol catalyst in the presence of reactive electrophilic species, such as nitroolefins or diarylmethanols (Scheme 2.10) [20]. Although both direct addition (a-addition) and vinylogous addition (y-addition) are viable reaction pathways, the a-reactivity is the more typical one and usually provides better selectivities due to more effective shielding of the a-position by the aminocatalyst. Interestingly, the a-/y-selectivity of dienamine-mediated carbanion -type addition reactions seems to be strongly influenced by the substitution pattern of the enal reactant For example, it was demonstrated that y,y-disubstituted enals favor the a-addition product while y-functionaUzation is the predominant reaction pathway for y-monosubstituted enals. 

Scheme 2.10 Representative examples of carbanion -type reactivity via dienamine activation.

For selected examples of carbanion -type dienamine activation promoted by silyl-protected diarylprolinol catalysts, see ... 

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