Carbanion—halide exchange reactions

Unlike the nucleophilic substitution reactions which generate stable onium halide after the reaction, nucleophilic additions to electrophilic C=X double bonds (X=C, N, O) provide rather basic onium anion species as an initial product. If the anion is sufficiently stable under the reaction conditions, onium anion will then exchange the counter ion for the other metal carbanion at the interface to regenerate the reactive onium carbanion Q+R. In another scenario, the basic onium anion may abstract the acidic hydrogen atom of the other substrate to provide Q 1 R directly. Such a reaction system ideally requires only a catalytic amount of the base although, in general, a substoichiometric or excess amount of the base is used to lead the reaction to completion. An additional feature of this system is the substantial possibility of a retro-process at the crucial asymmetric induction step, which might be problematic in some cases. 

The production of the (Z)-haloalkenes is thought to proceed via initial exchange of the tetrafluoroborate and halide ions and collapse of the resulting vinyliodonium halides by the addition-elimination (Ad-E) mechanism (equations 203 and 204)84. As with Ad-E reactions of moderately activated vinyl halides (X = Cl, Br), which typically occur with configurational retention (> 95%)143 145, the intermediate carbanions apparently prefer a least motion rotation of 60° prior to the expulsion of iodobenzene. It has been demonstrated by an NMR study that anion exchange between (Z)-(2)-phenylsulfonyl-l-decenyl)-phenyliodonium tetrafluoroborate and tetrabutylammonium chloride occurs instantaneously in deuteriochloroform84. Furthermore, when authentic halide salts of the... 

Now, in these elimination reactions, the reactivity of alkyl halides follows the same sequence as for substitution—and with element effects of just about the same size. Clearly, the rate of breaking the carbon halogen bond does affect the overall rate of reaction. On this evidence, if carbanions were formed, they carbanion formation has been ruled out by the absence of isotopic exchange. 

The mechanism of the Wurtz coupling is not well understood, and the currently accepted mechanism involves two steps 1) formation of a carbanionic organosodium compound via metal-halogen exchange and 2) the displacement of the halide ion by the organosodium species in an Sn2 reaction. Alternatively, a radical process can also be envisioned, although to date there has been no experimental evidence to support this assumption. 

A halogen atom, especially bromine, gives sufficient stabilization to an attached car-banion to allow preparation by metal halogen exchange from 1,1-dihalo compounds [39, 40]. These a-halocarbanions decompose rapidly via a carbene intermediate, since the halide is a good leaving group. The preparatively useful reactions involve in situ reactions of the transient carbanions in the presence of aldehydes or ketones [41]. 

Disconnections (a), (b), and (d) in Table 10.1 all require reagents for the carbanion synthon R. Simple carbanions are almost never formed in reactions so we shall need reagents in which carbon is joined to a more electropositive atom such as a metal. The most popular are Li and Mg. Bnty lithium (BuLi) is commercially available and other alkyl lithiums can be made from it by exchange (i). Otignard reagents (2) arc usually made directly from alkyl halides and magnesium metal (iii)—a method also available for RIJ (ii). These methods are available for aryl compounds too. Transformation of RHal into RIa or RNfgBr involves a formal inversion of polarity. 

Direct reaction between sodium or potassium and alkyl and aryl halides is complicated by exchange and coupling reactions, which can lead to mixtures of products. These complications can be reduced by rapid stirring and the use of finely divided metal or amalgams. Phenylsodium can be made in this way PhCl -h Na->PhNa-h NaCl. Acidic hydrocarbons react with alkali metals in ether solvents. Cyclopentadiene, for example, affords sodium cyclopentadienide in tetrahydrofuran (p. 279). Triphenylmethylpotassium is obtained as a deep red solution from triphenylmethane and potassium in 1,2-dimethoxyethane. These carbanions, in which the negative charge is delocalized over several carbon atoms, do not attack ethers, in contrast to the simple alkyl or aryl carbanions present in methylsodium or phenylpotassium. 

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