Reactions of the ambident super-electrophile series: the 2-(nitroaryl)-4,6-dinitrobenzotriazole 1-oxides with isopropoxide ion. Pathways of decomposition of the anionic σ-adducts

Abstract

To elucidate the reactivity of super-electrophiles such as 4,6-dinitrobenzofuroxan as compared to normal electrophiles such as 1,3,5-trinitrobenzene, reaction of isopropoxide ion (iPrO−) with a series of ambident super-electrophiles was studied by 400 MHz 1H nuclear magnetic resonance spectroscopy. The 2-(nitroaryl)-4,6-dinitrobenzotriazole 1-oxides, 1–3, possess both a super-electrophilic (C-7) site and a normal electrophilic (C-1′) site. Nucleophiles can demonstrate selectivity for attack at C-7, which leads to formation of persistent anionic σ-adducts (Meisenheimer complexes), as compared to C-1′, which leads to N-2:C-1′ bond scission. The most reactive substrate, 2-(2′,4′,6′-trinitrophenyl)-4,6-dinitrobenzotriazole 1-oxide (Pi-DNBT, 1) was found to be the least selective substrate in C-7 adduct formation, while 2-(2′,4′-dinitrophenyl)- and 2-(4′-nitrophenyl)-4,6-dinitrobenzotriazole 1-oxides (DNP-DNBT, 2, and NP-DNBT, 3, respectively) showed increasing selectivity towards iPrO−, in turn. These results are discussed on the basis of overall selectivity for C-7 adduct formation and the relative selectivity of iPrO− as compared to methoxide and tert-butoxide ions. The conclusions are illustrated using comparative energy profiles. In terms of pathways for decomposition of the adducts, the C-7 adducts decompose via dissociation back to substrate and nucleophile and, thence, through C-1′ adduct formation to the scission products. However, for 1, the C-7 adduct 1a has now been found to decompose to 7-isopropyl-2-picryldinitrobenzotriazole, 1c. The possible mechanism of this formal internal redox will be discussed.