Nucleophilic aromatic substitution Meisenheimer adducts

Two of three nitrofluorobenzene isomers react with methoxide, but the third is unreactive. Obtain energies of methoxide anion (at left), ortho, meta and para-nitrofluorobenzene, and the corresponding ortho, meta and para-methoxide anion adducts (so-called Meisenheimer complexes). Calculate the energy of methoxide addition to each of the three substrates. Which substrate is probably unreactive What is the apparent directing effect of a nitro group Does a nitro group have the same effect on nucleophilic aromatic substitution that it has on electrophilic aromatic substitution (see Chapter 13, Problem 4) Examine the structures and electrostatic potential maps of the Meisenheimer complexes. Use resonance arguments to rationalize what you observe. 

The above reaction resembles previously mentioned preparation of Meisenheimer adducts by reaction of benzofuroxan with potassium carbonate in aqueous solution. It is interesting that using 7-chlorofuroxan instead of furoxan as a starting material, under otherwise similar conditions, leads to a nucleophilic aromatic substitution and formation of fully aromatic phenol salts. The same is true for furazans (Read, Personal Communication). 

Meisenheimer adduct A cyclohexadienyl derivative formed as Fewis adduct from a nucleophile (Fewis base) and an AROMATIC or heteroaromatic compound, also called Jackson-Meisenheimer adduct. In earlier usage the term Meisenheimer complex was restricted to the typical Meisenheimer alkoxide adducts of nitro-substituted aromatic ethers, for example... 

Aromatic nucleophihc substitution proceeds by an addition-elimination mechanism, where one of the vacant Jt orbitals of aromatic ring is used for bonding interaction with the nucleophile [162]. In this way, the addition of nucleophile to the aromatic ring occurs with the formation of -complexes - Meisenheimer adducts - without displacement of any of the existing substituents (Scheme 100). The leaving group is then expelled in the second step [162]. 

Evidence for the formation of Meisenheimer, or a a-complex, involving aniline as a nucleophile has been presented with 1,3,5-trinitrobenzene (TNB) by Buncel and coworkers107. Reactions of the TNB-methoxide ion adduct with a series of substituted anilines in DMSO solution yield new TNB-aromatic amine o-complexes (Scheme 12). 

Meisenheimer complexes, the stable intermediates in aromatic nucleophilic substitution reactions, are of interest since there is speculation as to whether the attack of an alkoxide ion on nitroaryl compounds results in a covalent species, or alternatively in a charge-transfer complex. The analysis of one such intermediate, the potassium methoxide adduct of 4-methoxy-5,7-dinitrobenzfurazan (89), confirms that in this case the alkoxide moiety... 

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