Conjugation usually stabilising

In general these reactions are base-catalysed in that it is necessary to remove a proton from HCXYZ in order to generate the carbanion, eCXYZ, the effective nucleophile one or more of X, Y and Z are usually electron-withdrawing in order to stabilise it. The initial adduct (84) acquires a proton from the solvent (often H20 or ROH) to yield the simple addition product (85). Whether or not this undergoes subsequent dehydration (86) depends on the availability of an H atom, either on an a-carbon or where X, Y or Z = H, and also on whether the C=C so introduced would, or would not, be conjugated with other C=C or C=0 linkages in the product ... 

In chapter 21 we mentioned nitro compounds as promoters of conjugate addition they also stabilise anions strongly but do not usually act as electrophiles so that self-condensation is not found with nitro compounds. The nitro group is more than twice as good as a carbonyl group at stabilising an enolate anion. Nitromethane (p/ a 10) 1 has a lower pKa than malonates 4 (pKa 13). In fact it dissolves in aqueous NaOH as the enolate anion 3 formed in a way 2 that looks like enolate anion formation. 

Methylbenzoic acid 424 can be laterally lithiated with two equivalents of lithium amide base (LDA387 or LiTMP388) or alkyllithium,389 390 provided the temperature is kept low to avoid addition to the carbonyl group. It is usually preferable to carry out the lithiation using alkyllithiums,389 since with lithium amides the subsequent reaction of 425 with electrophiles is disrupted by the presence of the amine by-product (diisopropylamine, for example).389 391 The dilithio species 425 is stable in THF even at room temperature, and (as with the amide 404) since LDA will also dilithiate 426 stabilisation presumably comes principally from conjugation with the carboxylate. 

The two orbitals that do interact, tp2 and n, which have the same symmetry, create the usual pair of new orbitals, one raised and one lowered. Since there are only two electrons to go into the new orbitals, the overall energy of the conjugated system is lowered. The effect, AES, is small, both because of the poor overlap, and because the two orbitals interacting are far apart in energy, which we shall see later is an important factor. Nevertheless, it is a general conclusion that if the total number of n electrons is a (4 2) number, the spiro system is stabilised, leading to the concept of spiroaromaticity. 

---