Carbanions aromatic substitution

Nucleophilic Aromatic Substitution. An activated aromatic haUde is heated at 100—200°C with the alkaU metal salt of the nucleophile (ArOy ROy Ey CN /Cu", carbanion, etc) and the catalyst in an inert solvent (toluene, chiorohen2ene) for a few minutes to a day (21—24). 

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11,125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects, hydrogen, in aromatic substitution reactions, 2,163... 

There are not many successful examples of arylation of carbanions by nucleophilic aromatic substitution. A major limitation is the fact that aromatic nitro compounds often react with carbanions by electron-transfer processes.111 However, such substitution can be carried out under the conditions of the SRN1 reaction (see Section 11.4). 

Nucleophilic Aromatic Substitution. A natural extension of alkene addition processes is aromatic nucleophilic substimtion. Again, the ease of the process is highly dependent on the stability of the intermediate carbanion and strong EWGs are needed to facilitate these reactions in solution. The classic example is the... 

At least two other special mechanisms exist that are not considered in this chapter. The first is electrophilic aromatic substitution via a carbanion. This pathway is sometimes followed if a strong base is present or if the substrate is a metal-substituted aromatic. For example, Mach and Bunnett have found that the presence of i-BuOK, i-BuOBr brominates 1,3,5-tribromobenzene by the mechanism shown below ... 

The regioselective functionalization of nitrobenzene and benzonitrile derivatives has been performed via nucleophilic aromatic substitution of hydrogen by phosphorus-stabilized carbanions.41 Lithium phosphazenes have been found to be the most suitable nucleophiles for the substitution of hydrogen in nitrobenzene. This method represents a convenient alternative to the vicarious nucleophilic substitution for the synthesis of benzylic phosphorus derivatives using phosphorus-stabilized anions that do not bear a leaving group at the carbanionic centre. 

What is the nature of this intermediate We can best answer that by comparing the l3C NMR spectra of three species benzene itself the simplest version of our carbanion intermediate (that is, with no substituents) and the simplest version of the cationic intermediate in electrophilic aromatic substitution. Direct protonation of benzene gives this last compound. 

Notice the symmetry in this mechanism. Benzyne is formed from an ortho carbanion and it gives an ortho carbanion when it reacts with nucleophiles. The whole mechanism from bromobenzene to aniline involves an elimination to give benzyne followed by an addition of the nucleophile to the triple bond of benzyne. In many ways, this mechanism is the reverse of the normal addition-elimination mechanism for nucleophilic aromatic substitution and it is sometimes called the elimination-addition mechanism, the elimination step... 

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