Tertiary carbanions, formation

Here too, a second alkylation can be made to take place yielding RC=CR or R C=CR. It should, however, be remembered that the above carbanions—particularly the acetylide anion (57)—are the anions of very weak acids, and are thus themselves strong bases, as well as powerful nucleophiles. They can thus induce elimination (p. 260) as well as displacement, and reaction with tertiary halides is often found to result in alkene formation to the exclusion of alkylation. 

Carbanions derived from side chain tertiary amides have also been cyclized to provide isoquinolones and isoindoles (equation 36).125 126 While benzyne intermediacy in the formation of the former is likely, the latter seems to arise through a SrnI reaction pathway. Synthesis of indole from the meta bromo compound (87), on the other hand, clearly involves an aryne cyclization. 27 A more versatile route to indoles is based on intramolecular addition of aminyl anions to arynes (equation 38).128 A somewhat similar dihydroindole preparation constitutes the first step in a synthesis of lycoranes (equation 39).129 The synthesis of (88) also falls in the same category of reactions, but it is noteworthy because only a few examples of ring closure of heteroarynes are mentioned in literature.27 28... 

It is claimed that addition of catalytic quantities of a quaternary ammonium salt greatly enhances both reactivity and yield [48] in the formation and subsequent reactions of (58). Tertiary fluorinated carbanions, generated from corresponding unsaturated precursors, react with a variety of electrophiles [46, 49,50] (Scheme 25), the most surprising being with fluorocarbon iodides, to give remarkable fluoride-bridged products, e. g. (60) [47],... 

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. 

A plethora of examples can be found in the carbohydrate literature in which a sugar lactone is opposed to a suitable carbanion to create a carbon-carbon bond. One advantage of this approach is the strong tendency of the hemiacetal formed in the first condensation to retain its cyclic form, thus preventing overcondensation, which would result in the formation of a tertiary alcohol. 

The ligand effects in the formation of tertiary carbanions from substituted tertiary aromatic amides have been investigated." Reaction of 2-isopropyl-(iVA -diisopropyl)-benzamide (10) with f-BuLi in ether resulted in ortho deprotonation and the formation of a hemisolvate based on a tetranuclear dimer of (10-Li )2 Et2O (Scheme 7). Reaction of (10) with f-BuLi in the presence of the tridentate Lewis base NjNjN pentamethyldiethylenetriamine (PMDTA) or tridentate diglyme (DOME) takes a different course (Scheme 7). The soUd-state structures revealed a dimer core in which the amide oxygen atoms fail to stabilize the metal ions for the former case and a remarkable benzylic deprotonation giving the tertiary benzyllithium (lO-Li )-PMDTA for the latter. 

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