Carbanions as nucleophiles

Carbanions are very useful intermediates in the formation of carbon-carbon bonds. This is true both for unstabilized structures found in organometallic reagents and stabilized structures such as enolates. Carbanions can participate as nucleophiles both in addition and in substitution reactions. At this point, we will discuss aspects of the reactions of carbanions as nucleophiles in reactions that proceed by the 8 2 mechanism. Other synthetic aj lications of carbanions will be discussed more completely in Part B. 

In addition to their reactions with alkenes and carbanions as nucleophiles benzhydryl cations react with hydride donors.282 284 These hydride transfer reactions show the same linear dependence of log k upon E as the reactions with alkenes and the same constant relative selectivity, that is with slopes of plots close to 1.0, for structures ranging from cycloheptatriene to the... 

A relatively general procedure for the preparation of dialkyl 2-oxoalkylphosphonates by direct acylation of dialkyl 1-lithioalkylphosphonates has been introduced by Corey and Kwiatkowski in 1966. The use of phosphonate carbanions as nucleophiles in reaction with carboxylic esters avoids the problems associated with the Michaelis-Arbuzov reaction. The reaction sequence is initiated by the addition at low temperature of dimethyl 1-lithiomethylphosphonate (2 eq and frequently more) to a carboxylic ester (1 eq) to give the transient lithium phosphonoenolate. The dimethyl methylphosphonate, being readily available and easy to eliminate, is the most frequently used phosphonate, but other phosphonates such as diethyl and diisopropyl methylphosphonates can be used. When the resulting enolate is treated with acid, dimethyl 2-oxoalkylphosphonate is produced in moderate to good yields (45-95%, Scheme 7.20). The reaction has been achieved with... 

The palladium-mediated substitution of allylic substrates proceeds in two independent steps. For stabilized carbanions both oxidative addition and the nucleophilic displacement occur with inversion of configuration. Thus, overall retention results, in contrast to the corresponding reactions of nonstabilized carbanions as nucleophiles (see Section D.l. 5.6.3.). The steric course of the reaction is proved by the absence of racemization in Lhe conversion of chiral substrates into chiral alkylated products. Furthermore, chiral n-allylpalladium complexes formed with inversion from stoichiometric reactions of palladium(O) with allyl substrates have been isolated. Coupling of these stereodefined complexes with soft carbanions yields the chiral alkylated products, again with inversion of configuration. 

Many important organic reactions involve carbanions as nucleophiles. The properties of carbanions were introduced in Section 3.4.2, and their reactivity is discussed in more detail in Chapter 6. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base that can generate the reactive carbanion intermediate. Base-catalyzed addition reactions of carbonyl compounds provide many examples of this type of reaction. The reaction... 

The reaction can also be carried out stereoselectively employing enantiomerically pure planar chiral ri -allyl(dicarbonyl)(nitrosyl)iron complexes. The required complexes are obtained from TBAFe and allyl bromides bearing a chiral auxiliary adjacent to the allyl complex in the form of chiral ester or amide groups (Scheme 4-80). Amination of enantiomerically pure rj -allyliron complexes proceeds with excellent stereoselectivity (de > 98%). Carbanions as nucleophiles give lower de values of 79-86%. The nucleophiles enter preferably anti to the iron complex unit with excellent regiocontrol in... 

The mechanisms given simply use a carbanion as nucleophile in terms of identifying the product, this will serve. 

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