Nucleophilic aromatic substitution nitro-group activated

Photoexcited aromatic compounds undergo substitution reactions with (non-excited) nucleophiles. The rules governing these reactions are characteristically different and often opposite to those prevailing in aromatic ground state chemistry 501a,b>, in contrast to the well known ortho/para activation in thermal aromatic substitutions, nitro groups activate the meta position in the photochemical substitution, as shown in (5.1) 502). 

A nitro group is a strongly activating substituent in nucleophilic aromatic substitution where it stabilizes the key cyclohexadienyl anion intermediate... 

Antidepressant activity is retained when the two carbon bridge in imipramine is replaced by a larger, more complex, function. Nucleophilic aromatic substitution on chloropyridine 31 by means of p-aminobenzophenone (32) gives the bicyclic intermediate 33. Reduction of the nitro group (34), followed by intramolecular Schiff base formation gives the required heterocyclic ring system 35. Alkylation of the anion from 35 with l-dimethylamino-3-chloropropane leads to tampramine 36 [8]. 

In the literature, there are numerous reports regarding the interactions between amines and both electron and proton acceptors132, but less attention has been devoted to interactions between amines and aromatic electron acceptors, in particular when the substrate/amine system is a reacting system, as in the case of nucleophilic aromatic substitution (SjvAr) reactions between amines and substrates activated by nitro or by other electron-withdrawing groups. 

Diazophenol formation is most competitive when a nitramine substrate contains an electron-withdrawing nitro group ortho to the nitro group being displaced and hence meta to the nitramine functionality, assumedly because that site is then activated towards nucleophilic aromatic substitution. Heating nitramines in inert chlorinated solvents also favours diazophenol formation but this is suppressed by using urea or sulfamic acid as additives. 

Nucleophilic aromatic substitutions 1,3-azoles are more reactive than pyrrole, furan or thiaphene towards nucleophilic attack. Some examples of nucleophilic aromatic substitutions of oxazole, imidazole and thiazoles and their derivatives are given below. In the reaction with imidazole, the presence of a nitro-group in the reactant can activate the reaction because the nitro-group can act as an electron acceptor. 

Grignard reagent.80 Competition by nucleophilic aromatic substitution was not observed unless the only active position(s) was (were) substituted with a leaving group, as in the reaction of l-methoxy-2-nitro-naphthalene which gave 1 -alkyl-2-nitronaphthalenes in 73-95% yields.81 The use of Me3SiCH2MgCl (Peterson reagent) provides an entry to nitro-substituted benzyl anion intermediates, as shown in the example of Scheme 9.82... 

Nucleophilic substitution reactions The presence of nitro-groups activates the aromatic ring to nucleo-... 

Note the differences between electrophilic and nucleophilic aromatic substitutions Electrophilic substitutions are favored by electron-donating substituents, which stabilize the carbocation intermediate, while nucleophilic substitutions are favored by electron-withdrawing substituents, which stabilize a carbanion intermediate. The electron-withdrawing group that deactivate rings for electrophilic substitution (nitro, carbonyl, cyano. and so on) activate them for nucleophilic substitution. What s more, these groups are meta directors in electrophilic substitution, but are ortho-para directors in nucleophilic substitution. 

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