Elimination/addition reactions Meisenheimer complex

The product of this reaction as its sodium salt is called a Meisenheimer complex after the Ger man chemist Jacob Meisenheimer who reported on their formation and reactions in 1902 A Meisenheimer complex corresponds to the product of the nucleophilic addition stage in the addition-elimination mechanism for nucleophilic aromatic substitution... 

The formation of the C-X bond in hetero-cross coupling reactions is thought to proceed via a migration of the hetero atom to the aryl group, which develops a negative charge, which is Jt-stabilised by mesomeric interaction with acceptor substituents. Both for this reductive elimination and its reverse (oxidative addition) resonance-stabilised Meisenheimer complexes have been proposed [42,49,50,51], This stabilised structure is depicted in Figure 12.18. 

The second example of a cine-substitution of a thiophene involves the reaction of arylthiolates with 3,4-dinitrothiophene (331) or 3-nitro-4-phenylsul-fonylthiophene (332) to give the 2,4-substituted products 333. Both an elimination-addition mechanism via the aryne (334) or an abnormal addition-elimination (AEa) mechanism (Section II.2.A.e) via the Meisenheimer complex (335) have been considered for these reactions. The former is unlikely for several reasons including the lack of precedence for aryne formation from aryl nitro compounds (Section II. 1) under these reaction conditions and the fact that addition of the nucleophile ArS" to the aryne (334) would have to proceed via the 3-thienyl anion (336) rather than via a more stable 2-thienyl anion such as 320 as would be expected. Contrariwise, cine-substitution by the AEa mechanism is favored by the ability of the complex (335) to stabilize the negative charge by delocalization to both the NO2 group and the a position of the thiophene ring." As in the pyrrole series (Section III.2.B) the actual mechanism appears to be more complex, however, involving several addition and elimination steps via 337, which was recently isolated from the reaction (X = NO2) and shown to go to the product 333 under the reaction conditions. It therefore appears that neither of the cine-substitutions of thiophene described in this Section proceeds via an aryne intermediate. 

To rationalize these results, geometry optimizations for the coupling of 57c with 58b wctc carried out computationally with the RHF level of theory and 3-2 lG(d) basis sets for all atoms [49]. The reaction presumably proceeds via the generally accepted addition-elimination mechanism involving a Meisenheimer complex intermediate (Schane 8.14). In the addition step, to avoid stoic repulsion, the 2-methylindole approaches the arenechromium complex from the less stoically... 

The nucleophilic displacement of a halogen from an activated aryl halide system occurs in a two step addition-elimination reaction (S Ar). The nucleophile adds to the electron-deficient aryl halide, forming a negatively charged Meisenheimer complex from which the... 

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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