The versatile reactivity of 2-aryl-4,6-dinitrobenzotriazole 1-oxides in Diels-Alder type condensations and in σ-complexation - A relationship between superelectrophilicity and pericyclic reactivity

Abstract

A two-pronged study of a series of 2-aryl-4,6-dinitrobenzotriazole 1-oxide substrates (2a–c) is reported: Diels–Alder type pericyclic reactivity and covalent hydration to yield Meisenheimer type hydroxy anionic σ-adducts. The most activated benzotriazole 1-oxide 2a is found to exhibit both dienophilic and heterodienic behaviour on treatment with cyclopentadiene, providing a highly functionalized stereoselective diadduct (7). This diadduct is shown to be the result of two consecutive inverse demand Diels–Alder condensations proceeding through the endo mode with a trans addition of two cyclopentadiene molecules. Kinetic and thermodynamic measurements of the ease of covalent hydration of 2a–c to give σ-adducts 4a–c in aqueous solution indicate that the electrophilic reactivity of the 4,6-dinitrobenzotriazole 1-oxide structure is much closer to that of the superelectrophilic 4,6-dinitrobenzofuroxan (DNBF, 1a) than that of the standard Meisenheimer electrophile 1,3,5-trinitrobenzene (TNB). The pKa values range from 6.70 for the 2-(2',4',6'-trinitrophenyl)-4,6-dinitrobenzotriazole 1-oxide (Pi-DNBT, 2a) to 10.73 for the 2-phenyl-4,6-dinitrobenzotriazole 1-oxide (Ph-DNBT, 2c). The observation that the most activated benzotriazole 1-oxide structure 2a which resembles 4,6-dinitrobenzofuroxan (DNBF) in superelectrophilic behaviour, is the only one that undergoes Diels–Alder reactions, points to a direct relationship between superelectrophilic and pericyclic reactivity.Key words: σ-complexes, arylnitrobenzotriazole 1-oxides, super-electrophiles, Diels–Alder condensations, pericyclic reactions.