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[摘要]：A kinetic study on hydrogen abstraction from strong hydrogen bond acceptors such as DMSO, HMPA, and tributylphosphine oxide (TBPO) by the cumyloxyl (CumO(center dot)) and benzyloxyl (BnO center dot) radicals was carried out in acetonitrile. The reactions with CumO(center dot) were described in terms of a direct hydrogen abstraction mechanism, in line with the kinetic deuterium isotope effects, k(H)/k(D), of 2.0 and 3.1 measured for reaction of this radical with DMSO/DMSO-d(6) and HMPA/HMPA-d(18). Very large increases in reactivity were observed on going from CumO(center dot) to BnO center dot, as evidenced by k(H)(BnO center dot)/k(H)(CumO(center dot)) ratios of 86, 4.8 X 10(3), and 1.6 X 10(4) for the reactions with HMPA, TBPO, and DMSO, respectively. The k(H)/k(D) of 0.91 and 1.0 measured for the reactions of BnO center dot with DMSO/DMSO-d(6) and HMPA/HMPA-d(18), together with the k(H)(BnO center dot)/k(H)(CumO(center dot)) ratios, were explained on the basis of the formation of a hydrogen bonded prereaction complex between the benzyloxyl alpha-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. |
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