Carbanions nucleophilic addition/electrophilic coupling

Nucleophilic Addition-Electrophilic Coupling with a Carbanion Intermediate... 

Hulce, M. Nucleophilic Addition-Electrophilic Coupling with a Carbanion Intermediate, in Comp. Org. Synth, (eds. Trost, B. M.,Eleming, I.),... 

When analyzing plausible mechanisms of the VNS reactions of nitroarenes with a-chlorocarbanions, one should clarify a few key questions how to proceed the addition and subsequent conversion of adducts and how other substituents may affect both of these steps - rate and orientation of the addition, rate of the elimination, etc. It is well known that nitroarenes are active electron acceptors, whereas carbanions are good electron donors thus, these reactants can enter a single-electron transfer (SET) to form anion radicals of nitroarenes and radicals from carbanions [21, 22]. Further coupling of these electrophilic radicals with nucleophilic anion-radical species could give adducts. This SET pathway, alternative to the direct addition, is often favored by authors and the concept is sometimes abused, see [23] and rebuttal [24]. Nevertheless, numerous observations contradict participation of the SET mechanism in the VNS reactions ... 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

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