Nucleophilic aromatic substitution SnAt mechanism

FIGURE 8.3 Nucleophilic aromatic substitution (SnAt) mechanism and perfluorocarba-nions observed by NMR. 

There are similarities between nucleophilic aromatic substitution (SnAt) and its more usual counterpart, electrophilic aromatic substitution. Each involves the formation of a resonance-stabilized intermediate, and each involves a temporary loss of aromaticity that is regained in the final step of the reaction. But the similarities are only so deep. The electrophilic reaction involves cationic intermediates the nucleophilic involves anionic intermediates. Use the differing effects of a nitro group, strongly deactivating in the electrophilic substitution and strongly activating in the nucleophilic substitution, to keep the two mechanisms distinct in your mind. 

The most practical method for the preparation of polyfarylcnc ether)s employs nucleophilic aromatic substitution (SnAi). Although nucleophilic substitution can occur via four principal mechanisms,49 the most important mechanism utilized for the synthesis of poly(arylene etlier)s has been SnAt, in which activating groups are present on the aromatic ring (Scheme 6.10). 

The simplest example of an aromatic nucleophilic substitution is the SnAt reaction between l-chloro-2,4-dinitrobenzene and piperidine, the two-step mechanism of which, given in Eq. (5-26), is now fully established [501-503], It involves formation of a Meisenheimer-type zwitterionic intermediate via a dipolar activated complex, followed... 

There has been a major review of substitution by the radical-chain 5rn1 mechanism. It has been shown that reaction by the SrnI pathway of the enolate anions of 2- and 3-acetyl-l-methylpyrrole may yield a-substituted acetylpyrroles. The dichotomy of reactions of halonitrobenzenes with nucleophiles has been nicely summarized major pathways include reduction via radical pathways and. SnAt substitution of halogen. EPR spectroscopy has been used to detect radical species produced in the reactions of some aromatic nitro compounds with nucleophiles however, whether these species are on the substitution pathway is questionable. The reaction of some 4-substimted N,N-dimethylanilines with secondary anilines occurs on activation by thallium triacetate to yield diphenylamine derivatives radical cation intermediates are proposed. ... 

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