Regioselectivity and stereoelectronic factors in the reactions of aryloxide nucleophiles with 4-nitrobenzofuroxan

Abstract

The reactivity of 4-nitrobenzofuroxan (NBF, 1) with several aryloxide nucleophiles has been monitored by 400 MHz ¹H NMR spectroscopy in acetonitrile–glyme (CD₃CN–[²H₁₀] glyme. 1 : 1, v/v) at reduced temperatures and in dimethyl sulfoxide ([²H₆] DMSO) at ambient temperature. With the ambident (O- and C-) nucleophile phenoxide ion, formation of a C-7 O-adduct (3b) is observed in CD₃CN–[²H₁₀]glyme at –40 °C. As the temperature is raised this adduct gives way to 7-hydroxy-4-nitrobenzofurazan (as the anion, 5) and a 7-phenoxynitronate derivative (4b) that arises from transfer of the sp³-bound proton (7-H) of the C-7 phenoxide O-adduct to the N-oxide function. These decomposition products are the sole species observed when the reaction is carried out in [²H₆] DMSO. Thus in contrast to the reaction of 1 with methoxide ion, where the C-5 adduct (2a) is kinetically preferred while the C-7 adduct (3a) is thermodynamically more stable, no C-5 O-adduct of phenoxide is observed in the present system. In addition, C-adducts that could potentially arise from the ambident nature of phenoxide were not formed. A similar reaction sequence was found for the reaction of 1 with 3,5-di-tert-butylphenoxide ion and 2,4,6-trimethylphenoxide ion. However, C-attachment was observed in the reaction of 1 with 2,6-di-tert-butylphenoxide ion where O-attack is sterically blocked and, again, preference for C-7 attachment was found. Comparison is drawn between the present results concerning the reactivity of 1 with aryloxide nucleophiles and the reactivity displayed by methoxide ion. Stereoelectronic factors in the C-7 aryloxide O-adducts are also discussed.