Carbanions aggregation state

The solid-state structures of several benzylic carbanion salts have been elucidated by X-ray analysis9 depending on the nature of the benzylic part, the cation, and the additives, the structures range from er-bonded organometallic compounds to delocalized ion pairs, from monomeric to dimeric and polymeric aggregates. Some compounds are listed together with leading references ... 

It is important to keep the position of the equilibria in mind as we consider reactions of carbanions. The base and solvent used determine the extent of deprotonation. Another important physical characteristic that has to be kept in mind is the degree of aggregation of the carbanion. Both the solvent and the cation influence the state of aggregation. This topic is discussed further in Section 1.1.3. 

Direct observation of complex in the a-deprotonation of N,N-dimethylbenzamide and kinetic studies are interpreted in terms of a tetrameric cube-like transition state structure typical of solid-state RLi structures in which ligands bound to the Li centers facilitate the release of the a-carbanion species. The continuing existence of the RLi tetramer on addition of TMEDA contrasts with the dogma that this additive breaks up aggregates and is rationalized by TMEDA advancing the formation of the a-carbanion in the transition state analogous to the effect of the R carbanion character in the tetramer. 

For comparison purposes some substitution reactions were tried under different conditions. While the use of DMSO and sodium hydride or alkali metal hydroxides allowed substitution products to be formed in yields comparable to those employing TEBA, other solvent-base systems failed. These include amide ion in ammonia and sodium hydride in tetrahydrofuran. If any substitution product were isolated using these other methods, the desired product was accompanied by much side-product. Perhaps the reason for successful substitution by methods employing phase transfer catalysis and DMSO solvent lies in the state of aggregation of the carbanion. Free ions and ion pairs are present under the successful conditions but higher aggregates are likely when failure results.39 ... 

The intra- and intermolecular rearrangements of C2h- and D2-74 clearly indicate the importance of rearrangements hitherto not normally covered in a chapter on carbanion rearrangements. Organometallic chemists, however, are learning more and more about the complex structure(s) of such compounds in solution and in the solid state, as well as their rearrangements, e.g. within aggregates. It is thus predictable that the near future will provide us with more examples of this sort. 

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