The interplay of electronic, steric and stereoelectronic effects in hydrogen-atom abstraction reactions of SO4–˙, revealed by EPR spectroscopy

Abstract

Attack by SO₄^–˙ (generated by photolysis and one-electron transfer) on a number of carbohydrates and some model compounds has been followed directly by EPR spectroscopy.

SO₄^–˙ is a reactive, electrophilic species but is more selective (less reactive) than HO˙. We present evidence for the importance of polar effects and radical stabilisation, especially in determining trends in selectivity. Examples include the preference for C–H abstraction α to a ring oxygen atom in a five-membered ring (e.g. THF, sucrose, β-D-fructofuranose) and abstraction of the axial C¹ hydrogen atom from β-D-glucose.

The reaction of SO₄^–˙ with α-D-glucose is selective towards the C², C⁵ and C⁶ positions. This reflects the activating effect of a β-oxygen substituent where the radical orbital can eclipse the β-C–O bond providing a SOMO–σ* interaction which stabilises the developing radical centre. For myo-inositol, abstraction is also observed from axial C–H bonds for which the resulting radicals are stabilised by an axial β-OH substituent but, due to the absence of a ring oxygen atom, more rapid abstraction occurs from the equatorial C–H bond.