Nucleophilic substitution process Meisenheimer complex

If the thiophene ring bears one or more N02 groups, it becomes susceptible to nucleophilic attack by alkoxide ion an anionic cr-complex is thus produced which can be isolated in some cases. This is called a Meisenheimer adduct, and corresponds to the first step in many nucleophilic substitution reactions on activated thiophene substrates. The similarity between these adducts and heterocyclic pseudobases has been pointed out (79AHC(25)l). Kinetic data lead to similar rate equations for both processes both are characterized by negative entropies of activation of similar magnitude. 

Nucleophilic substitution most readily occurs at the 2- and 4-position of the more electron-deficient heterocyclic ring of quinolines. However, SNAr reactions at the carbocyclic ring can occur, mainly at positions 5 and 7. 5,7-Dibromo-8-hydroxyquinoline, 5-bromo-8-hydroxyquinoline, and 7-bromo-8-hydroxy-5-methylquinoline undergo conversion to the corresponding chloroquinolines on treatment with neat pyridine hydrochloride at 220 °C in a process that is postulated to proceed via the formation of stabilized Meisenheimer complexes <1996TL6695> (Equations 20 and 21). 

The outcome of the regioconservative reactions is vaguely attributed to a "direct displacement" process or to a "chelation-driven nucleophilic aromatic ipso substitution". Presumably both times the authors refer to the well-known two-step addition/elimination mechanism of nucleophilic aromatic substitution. However, to make this option practically feasible, the haloarene has to be activated by powerful electron-acceptors, in particular, nitro groups (as in 2- or 4-halonitroarenes) or by ring-incorporated nitrogen (as in 2- or 4-halopyridines). Without that, the crucial Meisenheimer complex is energetically out of reach. 

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