Radical–nucleophilic substitution (SRN1) reactions: electron spin resonance studies of electron capture processes

Abstract

We have probed two of the steps postulated for the radical–nucleophilic substitution (S_RN1) mechanism for α-substituted aliphatic nitro-compounds [Me₂C(X)NO₂] by detecting the intermediates involved by e.s.r. spectroscopy. These two steps are the electron capture by Me₂C(X)NO₂ to form radical-anions, and their break-down to yield radicals and anions [equations (1) and (A2) or (B2) in Scheme 1]. A range of radical-anions, [Me₂C(X)NO₂]^–˙ with X = Br, Cl, SCN, NO₂, CN, PO₃Et₂, CO₂Et, COMe, SO₂Me, SO₂Ar, and Me have been unambiguously identified by e.s.r. spectroscopy and shown to be infinitely long lived at low temperature. Our results indicate that one or more of at least three pathways are followed on the reaction of Me₂C(X)NO₂ with electrons: (a) electron-capture to yield a stable radical-anion, [Me₂C(X)NO₂]^–˙, (b) dissociative electron capture to yield Me₂ĊNO₂ and X^–(for X = Br, Cl, and SCN); (c) dissociative electron-capture to yield Me₂ĊX and NO₂^–(for X = CN, NO₂, PO₃Et₂, and CO₂Et). The effect of solvent on these processes, and the structure of the radical-anions and the radicals Me₂ĊNO₂, Me₂ĊPO₃Et₂, and Me₂ĊCN are discussed.