REGIOSELECTIVITY AND STEREOELECTRONIC FACTORS IN THE REACTIONS OF ARYLOXIDE NUCLEOPHILES WITH 4-NITROBENZOFUROXAN

The reactivity of 4-nitrobenzofuroxan (NBF, 1) with several aryloxide nucleophiles has been monitored by 400 MHz H-1 NMR spectroscopy in ace tonitrile-glyme (CD3CN-[H-2(10)]glyme, 1:1, v/v) at reduced temperatur es and in dimethyl sulfoxide ([H-2(6)]DMSO) at ambient temperature. Wi th the ambident (O- and C-) nucleophile phenoxide ion, formation of a C-7 O-adduct (3b) is observed in CD3CN-[H-2(10)]glyme at -40-degrees-C . As the temperature is raised this adduct gives way to 7-hydroxy-4-ni trobenzofurazan (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 prod ucts are the sole species observed when the reaction is carried out in [H-2(6)]DMSO. Thus in contrast to the reaction of 1 with methoxide io n, where the C-5 adduct (2a) is kinetically preferred while the C-7 ad duct (3a) is thermodynamically more stable, no C-5 O-adduct of phenoxi de is observed in the present system. In addition, C-adducts that coul d potentially arise from the ambident nature of phenoxide were not for med. A similar reaction sequence was found for the reaction of 1 with 3,5-di-tert-butylphenoxide ion and 2,4,6-trimethylphenoxide ion. Howev er, C-attachment was observed in the reaction of 1 with 2,6-di-tert-bu tylphenoxide ion where O-attack is sterically blocked and, again, pref erence for C-7 attachment was found. Comparison is drawn between the p resent results concerning the reactivity of 1 with aryloxide nucleophi les and the reactivity displayed by methoxide ion. Stereoelectronic fa ctors in the C-7 aryloxide O-adducts are also discussed.