Meisenheimer’s complexes

Meisenheimer s complex may be a common first step in the sequence of reactions that may finally lead to the formation of different products (see also Kharasch and Reinmuth s comments [12]). However, the complexity of the following steps in the reactions... 

The kinetics of chlorine displacement is strongly dependent on the position of the activating substituent 4-chloronitrobenzene reacts faster than 2-chloronitro-benzene with potassium fluoride whereas, in 2,4-dichloronitrobenzene, chlorine in the 2-position is exchanged more rapidly than chlorine in the 4-position (refs. 4-8). These observations are consistent with an addition-elimination process involving an anionic adduct (Meisenheimer s complex) which has been observed by h and 19F NMR (ref. 9) or UV spectroscopy (ref. 3) ... 

The formation of Meisenheimer s complexes o leads to interesting applications in quantitative organic analysis. Janovsky in the 1890s showed that m-dinitrobenzene... 

With an excess of the compound possessing an active methylene (Janovsky s conditions), the reaction is fast and stops at the stage-a complex. In Zimmermann s conditions, namely, with an excess of the dinitro derivative, Meisenheimer s complex is irreversibly oxidized to yield the colored anion ... 

When 2,4,6-trinitroanisole is treated witn methoxide in methanol, a red anion having the composition (CbH808N3) is produced. Such anions are called Meisenheimer complexes after the chemist who first suggested the correct structure. What structure do you think he suggested One of Meisenheimer s experiments compared the product of reaction of 2,4,6-trinitroanisole with ethoxide ion with the product of 2,4,6-trinitrophenyl ethyl ether with methoxide ion. What do you think he found ... 

There are several pieces of experimental evidence for the formation of a discrete intermediate in aromatic systems. One of the earliest was Meisenheimer s observation [19] that addition of potassium methoxide to 2,4,6-trinitrophenetole or addition of potassium ethoxide to 2,4,6-trinitroanisole produced identical compounds, (5), which on acidification produced in each case a mixture of methyl and ethyl picrates. Meisenheimer complexes, which are anionic a-adducts, are considered later in this chapter. 

Meerwein s reagent, 680 Meisenheimer, Jacob, 573 Meisenheimer complex, 573 Melmac, structure of, 1223 Melt transition temperature (Tm), 1215... 

In the absence of nucleophile, neither the 412 nm species nor the formation of the radical anion, nor that of the photosubstitution product is found. It is concluded therefore that the 412 nm species results from some kind of interaction between the (excited) aromatic compound and the nucleophilic reagent. The character of this aromatic compound-nucleophile-complex is as yet unknown. However, in our present view, the nature of the complex has to allow for the formation of both the radical anion and the photosubstitution product(s). An attractive possibility for this complex remains the a-complex, in formal analogy with the Meisenheimer complexes in the thermal nucleophilic reactions with aromatic compounds. An exciplex forms another possibility. 

Diamino-4,6-dinitrobenzofuroxan (DADNBF) (82), an impact insensitive high performance explosive (VOD 8050 m/s, d = 1.91 g/cm ), has been prepared in four steps from l,3,5-trichloro-2,4-dinitrobenzene (78), ° and also by treating the Meisenheimer complex (83) with excess hydroxylamine hydrochloride in aqueous base. DADNBF has also been synthesized in five steps starting from 2-nitroaniline. ... 

As early as 1895 Lobry de Bruin [22] described a red solid which he isolated m the reaction of 13>5-trinitrobenzene with potassium hydroxide in methanol. [Ileyer [23] suggested this to be a complex compound. Hantzsch and Kissel ) rationalized that the complex might be a potassium salt of aci-form of one group. Jackson and Gazzolo [25] suggested in 1900 two structural formulae the sodium salt obtained by acting with sodium methoxide on 2,4,6-trinitro-il Fig. 1 Sa. Meisenheimer [26] (probably unaware of Jackson s paper)... 

Pyfe. Malkiewich and Norris [ >41 examined intermediates formed by the action of bases on TNT using high resolution NMR spectroscopy. Wiilmi 0 5 sec of mixing equimolar proportions of TNT and methoxide in l)S() and t lliOII the Jackson Meisenheimer complex is formed. It decayed over S sec and ilie 1 spectrum of anion according to (5) slowly appeared. 

The reaction of l,3,S-trinitroben2ene with diazomethane was carried out as early as 1898-1900 [88, 89), but only recently de Boer rationalized it as a nucleophilic addition leading to seven member ring condensed with two cyclopropane rings when the molar ratio of trinitrobenzene to diazomethane was 1 3. The first step of the reaction was a type of Jackson-Meisenheimer complex (Ai) and the final product was trismethylene-trinitrobenzene (A ) [90—92a] (19). The reaction is temperature dependent at —80 C compound (A ) was formed. 

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