Ambident reactivity in the reaction of phenoxide ion with 2-N-(2′,4′-dinitrophenyl)- and 2-N-(4′-nitrophenyl)-4,6-dinitrobenzotriazole 1-oxides, new superelectrophiles

Abstract

Reaction of the novel superelectrophiles 2-N-(2′,4′-dinitrophenyl)- and 2-N-(4′-nitrophenyl)-4,6-dinitrobenzotriazole 1-oxides, 3, and 4, possessing two electrophilic centres, with the ambident nucleophile potassium phenoxide in (CD3)2SO was followed by 400 MHz 1H nuclear magnetic resonance spectroscopy. A dichotomy in the reaction pathways has been observed. With MeO−, attack at C-7 leads to reversible adduct formation, while attack at C-1′ results in irreversible N-2: C-1′ bond scission via the metastable C-1′ adduct. In contrast, the reaction of 3 and 4 with PhO− proceeds by a two-pronged attack: formation of C-7 carbon-bonded phenoxide adducts via the ortho and para carbon sites, and oxygen-based cleavage products by attack at the C-1′ position, accompanied by N-2:C-1′ bond scission, in accord with the ambident reactivity of PhO−. Significantly, in this case reaction of both C-7 and C-1′ is effectively irreversible. Moreover, the reaction of phenoxide with either 3 or 4 shows striking differences compared to the reaction of PhO− with 2-N-(picryl)-4,6-dinitrobenzotriazole 1-oxide, 1. Reaction of PhO− with 1 resulted only in O-attack at C-1′ and N-2:C-1′ bond scission; there was no evidence for C-7 adduct formation via O- or C-attack. This marked difference in behaviour can be attributed to the decreased susceptibility to C-1′ attack exhibited by 3 and 4 as compared to 1 and arises from the successive removal of electron-withdrawing nitro groups from the 2-N′-nitroaryl moiety in the series 1 → 3 → 4. The reactions are discussed on the basis of selectivity considerations and an activation energy/reaction coordinate profile comparing the pathways for both C-attack at C-7 and O-attack at C-l′ as electrophilicity (delocalizability) is progressively modulated in the reaction series.