Reactivity-Selectivity Correlations. 4. The α Effect in SN2 Reactions at sp3 Carbon. The Reactions of Hydrogen Peroxide Anion with Methyl Phenyl Sufates

Article abstract of DOI:10.1021/ja00382a025

A kinetic study is reported of the reactions of substituted phenyl methyl sulfates with hydrogen peroxide and methoxide ion in methanol solvent at 25 deg C.Dissection of the rate data for the H2O2/MeONa system allows the specific coefficients k_HOO- and k_MeO- to be evaluated.Hydroperoxide anion is found to be more reactive than methoxide ion (k_HOO-/k_MeO- varies from 6 to 11 depending on the sustrate), indicating than an α effect is operating for this nucleophilic attack at saturated carbon.This is in agreement with earlier results on the enhanced reactivity of hydrazine relative to glycine ethyl ester.The results further show that the Hammett p value for the α nucleophiles is smaller than for the normal nucleophiles; however an inverse order is found for the respective Bronsted β values.Analysis of these contrasting relationships in terms of reaction coordinate diagrams leads to the conclusion that the ordinary two-dimensional reaction coordinate is not applicable to the α nucleophiles and that significant anti-Hammond effects arise in these cases.The shift in the transition state perpendicular to the diagonal is discussed in terms of destabilization of the α nucleophile or via a tight transition-state structure as previously suggested for transmethylation.It is concluded that the Hammett p value is a valid criterion of the reactivity-selectivity principle (RSP) for the normal nucleophiles which conform to the two-dimensional representation but not for the α nucleophiles.This leads to a limitation on the applicability of the RCP as a measure of transition-state structure.