A remarkably facile oxygen transfer in a nitrobenzofuroxan structure activated through σ-complex formation

Treatment of 4,6-dinitrobenzofuroxan (DNBF) with the imidazoline 1-NRf is found to afford a zwitterionic nitrogen-bonded complex (2-NRf±) which, in the presence of base (Et3N), undergoes a slow but quantitative transformation to give 7-hydroxy-4,6-dinitrobenzofurazan (5) as the final product. Overall, an oxygen transfer has thus occurred from the N-oxide function to the carbocyclic moiety of DNBF. The key point in this transformation is shown to be a facile abstraction of the sp3 hydrogen bonded at C-7 of 2-NRf±, providing important new evidence that the parent DNBF structure is extremely electron-withdrawing (‘super-electrophile’). The overall conversion is also an unusual case of a catalytic process in which the catalysts (both 1-NRf and Et3N) partake to form covalent reaction intermediates and thereby lower the activation energy, resulting in a facile reaction.