A Mechanistic Characterization of the Spontaneous Ring Opening Process of Epoxides In Aqueous Solution: Kinetic and Product Studies

Article abstract of DOI:10.1021/ja00227a033

A careful determination of the extent of rearrangement during epoxide hydration was carried out in conjunction with nuclear magnetic resonance kinetic studies for both propylene and isobutylene oxides.The complete pH-rate profile was mapped at ionic strength 2.0, and solvent kinetic isotope effects were determined for several limbs.Spectrophotometric analysis of the quantity of aldehyde formed by rearrangement in both the acidic and spontaneous limbs reveals that ionic strength has a profound effect.At ionic strength zero, more rearrangement product is generated in the acidic limb, whereas at ionic strength 2.0, larger amounts of aldehyde are formed in both regions.These results support the conclusion that a different carbocationic intermediate exists in the reaction mechanism for the acidic and spontaneous regions.The experimental observations of the spontaneous region can be explained by the existence of zwitterionic intermediate III.Observed solvent kinetic isotope effects, k_H3O+/k_D3O+, k^H_spon/k^D_spon, of 0.33+/-0.02, 1.78+/-0.25 and 0.35+/-0.03, 1.48+/-0.27 for propylene and isobutylene oxides, respectively, were dissected into components on the basis of classical product studies.At high ionic strength, isolation of compound I, an isobutyraldehyde isobutylene glycol acetal, supports the existence of intermediate III.Apparent partitioning ratios for both the β-hydroxy carbocation, II, and the zwitterion, III, are examined and contrasted.Detailed mechanistic proposals are advanced, and their congruence with a variety of observations described previously in this journal is outlined.