title: Mayr electrophilicity predicts the dual Diels-Alder and sigma-adduct formation behaviour of heteroaromatic super-electrophiles.
abstract: 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(2)O) 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(a)(H(2)O): E = -0.662 pK(a)(H(2)O) (or pK(R+) -3.20 (r(2) = 0.987). The most highly electrophilic, pre-eminent super-electrophile is 4,6-dinitrotetrazolopyridine (E = -4.67, pK(a)(H(2)O) = 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(a)(H(2)O) = 13.43). (For relevant kinetic parameters as well as E and pK(a) values, see .) 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.
authors.affiliation: Institut Lavoisier de Versailles, UMR CNRS 8180, Université de Versailles, 45 Avenue des Etats-Unis, 78035-Versailles Cedex, France.
authors.forename: Sami
authors.initials: S
authors.lastname: Lakhdar
authors.forename: Régis
authors.initials: R
authors.lastname: Goumont
authors.forename: François
authors.initials: F
authors.lastname: Terrier
authors.forename: Taoufik
authors.initials: T
authors.lastname: Boubaker
authors.forename: Julian M
authors.initials: JM
authors.lastname: Dust
authors.forename: Erwin
authors.initials: E
authors.lastname: Buncel