Ambident reactivity of aryloxide ions towards the super-electrophile, 4,6-dinitrobenzofuroxan. Kinetics, thermodynamics and stereoelectronic factors on regioselectivity

Abstract

Reactions of the aryloxide ions, phenoxide (PhO^-) and 3,5-di-tert-butylphenoxide (3,5-DTBPhO^-), with the super-electrophilic heteroaromatic substrate, 4,6-dinitrobenzofuroxan (DNBF, 1), have been examined by 400 MHz ¹H NMR spectroscopy in acetronitrile–dimethoxyethane ([²H₃]MeCN∶[²H₁₀]DME 1∶1, v/v) as a function of varying temperature (-40 to 23 °C) and in dimethyl sulfoxide ([²H₆]DMSO) at room temperature. We herein report the first observation and full characterization of the O-bonded σ-adduct (DNBF·OPh^-, 3a) formed by attack of PhO^-, acting as an O-nucleophile, at the C-7 super-electrophilic site of 1. No C-5 adduct was seen in the initial spectrum (-40 °C, [²H₃]MeCN∶[²H₁₀]DME) or in subsequent monitoring of the reaction. These results suggest that PhO^- displays K7T7 regioselectivity towards DNBF wherein attack at the C-7 site is favoured by both kinetics and thermodynamics, comparable to the behaviour shown by PhO^- towards 2,4,6-trinitroanisole where the C-1 adduct is the product of both kinetic and thermodynamic control (i.e. K1T1 regioselectivity). Upon warming the reaction mixture to ambient, the C-7 O-adduct, DNBF·OPh^-, 3a, gives way to the more stable C-7 C-bonded σ-adducts (DNBF·ortho-PhOH^- adduct, 4, and DNBF·para-PhOH^- adduct, 5, in a ratio of ca. 1∶6). The C-7 hydroxide adduct, DNBF·OH^-, 2a, and phenol are detected at this temperature. The C-adducts, 4 and 5, are the sole PhO^- adducts previously observed in the DNBF–PhO^- reaction system (in [²H₆]DMSO at room temperature). When C-attachment is precluded by steric hindrance, as in the reaction of 1 with 3,5-DTBPhO^-, the C-7 DNBF·OPhDTB^- adduct, 3b, is observed by ¹H NMR spectroscopy even in [²H₆]DMSO under ambient conditions. The results of the kinetics and thermodynamics of aryloxide adduct formation with DNBF, including the ambident reactivity found, are discussed with regard to stereoelectronic stabilization in the adducts and with comparison to relevant 4-nitrobenzofuroxan (NBF), 2-(nitroaryl)-4,6-dinitrobenzotriazole 1-oxides (2-Ar-4,6-DNBT) systems and to the normal electrophile, 1,3,5-trinitrobenzene (TNB).

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