Adducts from Picryl Ethers

Recently crystal structure determinations of the complexes derived from 2,4,6-trinitroanisole and methoxide (Ueda et al., 1967) and 2,4,6-trinitrophenetole and ethoxide (Destro et al., 1967) have shown that the two alkoxy groups are indeed identical in the complex and that the plane defined by their oxygen atoms and Cl is orthogonal to the ring. The C6-C1-C2 bond angle is 109°, very close to the tetrahedral angle required for an sp3-hybridized carbon. 

There is then no doubt that the crystalline solids result from addition of alkoxide at Cl, and the NMR spectra indicate that no change in structure occurs on solution. However, Servis (1965) made the important observation that, if concentrated sodium methoxide solution was added to a solution of 2,4,6-trinitroanisole in dimethyl sulphoxide, the NMR spectrum initially produced was that of the adduct 9. The spectrum showed two pairs of doublets (J= 1-2 Hz) due to the spin coupled ring protons with shifts of — 6 17 and — 8-42 p.p.m. With time the spectrum gradually changed to that of the thermodynamically more stable Cl adduct (3, R=R =Me) (see Fig. 1). 

The conversion of the C3 adduct to the Cl adduct was found to be catalysed by methanol. These conclusions have been confirmed by Crampton and Gold (1966b), who found that the conversion is also catalysed by methoxide ions, suggesting an inter- rather than intramolecular process. In solutions rich in methanol the rate of conversion is fast so that the NMR spectra obtained initially give no evidence of the C3 adduct. Similar spectra observed by Foster and Fyfe (1965) 

These results throw light on the experiments of Ainscough and Caldin (1966), who determined the rates at low temperatures of two reactions 

Azide ions react with 2,4,6-trinitroanisole at temperatures below —10° in aprotic solvents to give an addition complex (Caveng and Zollinger, 1967). The NMR spectrum indicates addition at Cl. Similarly it has been found from NMR evidence (Servis, 1967) that diethyl-amine and triethylamine give adducts at Cl with 2,4,6-trinitroanisole in dimethyl sulphoxide. The latter adduct is unusual in that it is apparently the zwitterion 11. However, addition of triethylamine to 

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The spectral data for a variety of 1 1 and 1 2 adducts are collected in Table 2. As with the adducts from picryl ethers the visible spectra of 1 1 adducts from trinitrobenzene show two visible maxima whereas the 1 2 adducts have a single rather broad visible absorption. The NMR spectra of the 1 1 adducts show, typically, two bands due to ring protons, often exhibiting spin-spin coupling, at c. —8 4 (relative intensity 2) and — 6-0 p.p.m. (relative intensity 1). The position of the low-field band shows little dependence on the nature of the added group,... 

Picramide and its N-substituted derivatives introduce the added complication that proton loss may occur from the amino-group to give the Bronsted bases 23 (R = H, alkyl, phenyl). Green and Rowe (1913) found support for this formula from the alkali metal analyses of the solids formed from many polynitroanilines with bases. In fact picramide itself has been frequently used as an indicator for establishing H-acidity scales in basic media on the assumption that its indicator behaviour is due to proton loss (Schaal, 1955 Stewart and O Donnell, 1962 Stewart et al., 1962). The most likely other alternatives for the products of 1 1 interaction of picramides with bases are 24 and 25 (R = H, alkyl, phenyl R =R = H, alkyl). Formula 24, the analogue of Meisenheimer s formula for the adducts of picryl ethers, was suggested by Busch and... 

Whereas in the reaction of TNB with phenoxide ion the adduct from C attack is obtained, picryl chloride and other nitroaryl halides react with phenoxide ion to give only 14, the product of O attack. This is in fact the normal route for the formation of picryl phenyl ethers, and no instance of a picryl halide... 

Although this type of structure was first reported by Jackson et al. in 1900 for the reaction between picryl ether and potassium alkoxide, it was Meisenheimer who actually isolated and characterized the structure of such molecule for the first time in 1902. Since then, the stable or transient anionic a complexes formed from electron-deficient aromatic compounds and a variety of organic and inorganic bases" are generally called the Meisenheimer complexes. Occasionally, this type of molecule is also referred to as the Meisenheimer adducts, Meisenheimer compounds, Meisenheimer cr-adducts, Meisenheimer a complexes, Jackson-Meisenheimer adducts, or Jackson-Meisenheimer complexes. ... 

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