Mayr electrophilicity predicts the dual Diels-Alder and σ-adduct formation behaviour of heteroaromatic super-electrophiles

We report on the dual reactivity, i.e. anionic Meisenheimer sigma adduct formation and Diels-Alder adduct formation, of a series of heteroaromatic super-electrophiles, including 4,6-dinitro-benzofuroxan, -N-arylbenzotriazoles (4), -benzothiadiazole and -benzoselenadiazole. Measured pK_a H ₂O values for sigma adduct formation provide a quantitative measure of super-electrophilic reactivity with a satisfactory correlation between the Mayr E electrophilicity parameter and pK_aH₂O:E = -0.662 pK_aH₂O (or pK_R+) -3.20 (r² = 0.987)The most highly electrophilic, pre-eminent super-electrophile is 4,6-dinitrotetrazolopyridine (E = -4.67, pK_aH₂O = 0.4), which supercedes the reference Meisenheimer super-electrophile, 4,6-dinitrobenzofuroxan (E = -5.06, pK_a = 3.75), having itself an E value superior by 8 orders of magnitude compared to 1,3,5-trinitrobenzene as the benchmark normal Meisenheimer electrophile (E = -13.19, pK_aH ₂O = 13.43). (For relevant kinetic parameters as well as E and pK_a values, see Table 1.) In a parallel study we have investigated Diels-Alder (normal and inverse electron demand) reactivity of this series of heteroaromatic electrophiles and have shown that Mayr E values are valid predictors of whether DA adducts will form and how rapidly. The observed order of pericyclic reactivity corresponds to E = -8.5 as the demarcation E value, in close agreement with sigma complexation; thus pointing to a common origin for the two processes, i.e. an inverse relationship between the degree of aromaticity of the carbocyclic ring and ease of sigma complexation, or DA reactivity, respectively.