Delocalisation carbanions

One significant difference from the simple aldol reaction, however, is that the original adduct (113) now possesses a good leaving group (OEt) thus instead of adding a proton, as in the aldol reaction proper (p. 224), eOEt is lost to yield a /1-ketoester, ethyl 3-ketobutanoate (ethyl acetoacetate, 114). This is finally converted by base (eOEt) into its stabilised (delocalised) carbanion, (115). 

In the cases of a series of allyl chlorides derived from the antibiotic cephalosporin, reduction leads to a delocalised carbanion, which is protonated on die ester... 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

Classically the base catalyst, eOEt, is introduced by adding just over one mole of sodium (as wire, or in other finely divided form) plus just a little EtOH to generate an initial small concentration of Na eOEt. Further EtOH is generated in step (1), which yields further Na eOEt with sodium, and the concentration of eOEt is thereby maintained. A whole mole is required as it is essential for the / -keto ester (114) to be converted (step 3) into its anion (115)—MeCOCH2-COzEt is more acidic than EtOH (cf. p.272)—if the overall succession of equilibria is to be displaced to the right. This is necessary because the carbanion-formation equilibrium—step (1)—lies even further over to the left than that with, for example, CH3CHO this reflects the less effective stabilisation through delocalisation in the ester carbanion (111) than in that from the aldehyde (116) ... 

With second row elements, as we saw above, any inductive effect they exert may be complemented by delocalisation, through use of their empty d orbitals to accommodate the carbanion carbon atom s lone pair of electrons this can happen with S in, for example, an ArS02 substituent, and also with P in an R3P substituent. 

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

Carbanions which have substituents capable of conjugative delocalisation of the electron pair will perforce be planar (sp2), in order to allow the maximum orbital overlap of the p orbital with those of the substituent, e.g. (4) and (10) ... 

The H atom flanked by the two 0=0 groups in (22) exhibits hardly any more acidic character than the analogous one in the corresponding hydrocarbon. The different behaviour of (22) stems from the fact that after proton removal, the carbanion s lone pair would be in an sp3 orbital more or less at right angles to the p orbitals on each of the adjacent carbonyl carbon atoms (cf. p. 259) no sp3/p overlap could thus take place, consequently there would be no stabilisation of the -ve charge through delocalisation, and the (unstabilised) carbanion does not, therefore, form. 

This all suggests slow, rate-limiting breaking of the C—H bond to form the stabilised carbanion intermediate (54), followed by fast uptake of D from the solvent D20. Loss of optical activity occurs at each C—H bond breakage, as the bonds to the carbanion carbon atom will need to assume a planar configuration if stabilisation by delocalisation over the adjacent C=0 is to occur. Subsequent addition of D is then statistically equally likely to occur from either side. This slow, rate-limiting formation of a carbanion intermediate, followed by rapid electrophilic attack to complete the overall substitution, is formally similar to rate-limiting carbocation formation in the SNi pathway it is therefore referred to as the SE1 pathway. 

Related to these diphosphine dichalcogenides are the triphosphine trisulfides [R2P(S)]3CH (12) which can be prepared from lithiated methylene diphosphine disulfides upon treatment with thiophosphinic chlorides (Equation 9). Deprotonation of 12 with tBuLi gives a resonance-stabilised anion 13 containing a planar central carbanion whose charge is delocalised onto the three neighbouring phosphorus and sulfur atoms.32... 

Bases such as OH and OEt are used to obtain the carbanion, and whether or not the / -hydroxynitro compound (102) undergoes subsequent dehydration to R2C=CHN02 depends on the conditions. Where the carbonyl compound is an aldehyde there is some danger of its undergoing an aldol reaction with itself, but the delocalised... 

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