Free-radical reactions of carbohydrate moieties in macromolecular structures. EPR evidence for the importance of steric and stereoelectronic effects and for the influence of inclusion in cyclodextrins

Abstract

EPR experiments involving the attack of oxygen-centred free radicals on polysaccharides in aqueous solution reveal two sources of regioselectivity in C–H abstraction. For SO₄⁻˙ (derived from one-electron reduction of S₂O₈²⁻), hydrogen-atom abstraction is encouraged by SOMO–σ* interactions involving β-C–O bonds which eclipse the orbital of the unpaired electron at the developing radical centre. In addition, for a series of cyclodextrins, the reactivity observed with SO₄⁻˙ points to the selective reaction of the latter within the cyclodextrin cavity; these findings are in contrast to those obtained with ˙OH (from H₂O₂).