Radical-nucleophilic substitution (SRN1) reactions: electron spin resonance studies of electron-capture processes. Part 4. α-Substituted 2-methyl-5-nitrofurans

Abstract

We have probed two steps postulated for the radical-nucleophilic substitution (S_RN1) mechanism for α-substituted 2-methyl-5-nitrofurans (O₂NFurCH₂X) by detecting the intermediates involved using e.s.r. spectroscopy. These two steps are the electron-capture by O₂NFurCH₂X to form radical-anions, and their dissociation to yield the 5-nitrofurfuryl radical (O₂NFurCH₂) and anion. A range of radical-anions (O₂NFurCH₂X)^–˙ with X = I, Br, SCN, SO₂Ph, and H have been unambiguously identified by e.s.r. spectroscopy and shown to be infinitely long-lived at low temperature. The results are compared with those obtained for intermediates in the S_RN1 reactions. Their relevance to the S_ET2 mechanism is also discussed.