Application of EPR spectroscopy to the determination of the rates of reaction and selectivity of attack of the tert-butoxyl radical on esters: the interplay of electronic, polar, steric and stereoelectronic factors

Abstract

Rate constants have been determined by steady-state EPR spectroscopy experiments for hydrogen-atom abstraction by Bu^tO˙ from a variety of esters. The values, which range over two orders of magnitude ( 5 × 10³–5 × 10⁵ dm³ mol^–1 s^–1 at ca. 300 K) are significantly lower than those for related ketones and ethers and reflect, at least in part, the importance of the appropriate C–H bond dissociation energies and the stabilization of carbonyl-conjugated radicals. The relative lack of reactivity of C–H bonds in unsubstituted five-membered rings is believed to reflect ring-strain in the derived radicals; possible stereoelectronic effects are discussed.