Reactions of the Cumyloxyl and Benzyloxyl Radicals with Strong Hydrogen Bond Acceptors. Large Enhancements in Hydrogen Abstraction Reactivity Determined by Substrate/Radical Hydrogen Bonding

A kinetic study on hydrogen abstraction from strong hydrogen bond acceptors such as DMSO, HMPA, and tributylphosphine oxide (TBPO) by the cumyloxyl (CumO•) and benzyloxyl (BnO•) radicals was carried out in acetonitrile. The reactions with CumO• were described in terms of a direct hydrogen abstraction mechanism, in line with the kinetic deuterium isotope effects, kH/kD, of 2.0 and 3.1 measured for reaction of this radical with DMSO/DMSO-d6 and HMPA/ HMPA-d18. Very large increases in reactivity were observed on going from CumO• to BnO•, as evidenced by kH(BnO•)/kH(CumO•) ratios of 86, 4.8 × 103, and 1.6 × 104 for the reactions with HMPA, TBPO, and DMSO, respectively. The kH/kD of 0.91 and 1.0 measured for the reactions of BnO• with DMSO/DMSO-d6 and HMPA/HMPA-d18, together with the kH(BnO•)/kH(CumO•) ratios, were explained on the basis of the formation of a hydrogen-bonded prereaction complex between the benzyloxyl α-C−H and the oxygen atom of the substrates followed by hydrogen abstraction. This is supported by theoretical calculations that show the formation of relatively strong prereaction complexes. These observations confirm that in alkoxyl radical reactions specific hydrogen bond interactions can dramatically influence the hydrogen abstraction reactivity, pointing toward the important role played by structural and electronic effects.