Meisenheimer adducts characterization

Because the Meisenheimer adducts are usually characterized by intense UV-visible absorption bands, the equilibrium constants for their formation (K) can be determined spectrophotometrically by standard methods, whether complete conversion to the adduct can be attained39 or not,60 depending on the equilibrium constant value (K 5 10). 

A number of Meisenheimer adducts from the reaction of selenophene derivatives 141, which are analogs of series 121, with methoxide ion have been investigated and characterized.I<9- 81 Their formation is generally even easier than that of the related sulfur compounds. 

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. 

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. ... 

A formally antiaromatic 1,4-dihydropyrazinothiadiazole has been prepared and characterized by single crystal X-ray spectroscopy. The antiaromatic character of which has been supported computationally using NICS measurements <20070L1073>. CHIH-DFT computational studies on acenaphtho[l,2-f]-l,2,5-thiadiazole 1,1-dioxide led to simulations of its infrared (IR) and ultraviolate (LJV) spectra, the dipole moment and polarizability <2007JMT373>. 4,6-Dinitrobenzothiadiazole was determined to have an electrophilic reactivity of —8.40 which corresponds to a pK z° of 7.86 for Meisenheimer complexation with water and is close to the demarcation boundary (E = —8.5) between super-and normal-electrophiles and between reactive dienophiles and inert partners in Diels-Alder adduct formation <20070BC1744>. 

The first 5-membered Meisenheimer-type adduct was obtained from 2-methoxy-3,5-dinitrothiophene (121 X = OMe, Y = Z = NOz) by reaction with methoxide ion in methanol.16,55 The sodium (or potassium) salt can be isolated as a deep purple solid whose elemental analysis is consistent with the formation of a 1 1 adduct. The IR spectrum is characterized by the presence of strong bands in the 1000-1250 cm"1 region, where geminal dialkoxy adducts23 usually exhibit strong absorption. The electronic spectrum shows intense absorption at 312 and 532 nm, which accounts for the red color of the methanol solution. 

---