Sterically crowded five-membered heterocyclic systems. Part 3. Unexpected formation of stable flexible pyrrolidinoxyl biradicals via nitrone aldol dimers: a spectroscopic and mechanistic study

Abstract

Addition of methylmagnesium iodide to various substituted cyclic nitrones (1) and subsequent aerial oxidation gave new pyrrolidinoxyl radicals (4) and the corresponding crystalline biradical species, to which the structure (5) was assigned unequivocally from spectroscopic (e.s.r., u.v.-visible, mass, and 1H and 13C n.m.r. spectra) and mechanistic considerations. Base-catalysed self-condensation of the nitrones (1), to give the nitrone hydroxylamines (18), is followed by Grignard addition to give the crowded bis-N-hydroxy compounds (3); these are then oxidised to the biradicals (5). The possibility of intramolecular charge transfer interaction between the two aminoxyl groups of the biradicals (5)(in order to explain the observed u.v. absorption at ca. 328 nm), and also the dimerisation of 1-pyrroline 1-oxides (particularly those that are sterically crowded), are discussed briefly.