Rate-Controlling Step of Oxazolinone Formation. Secondary and Solvent Kinetic Isotope Effects

Article abstract of DOI:10.1021/ja00307a034

The β-deuterium secondary kinetic isotope effect for the formation of 2-phenyloxazolin-5-one in the alkaline hydrolysis of p-nitrophenyl n-benzoylglycinate (p-NO2C6H4O2CCL2NHOCC6H5, L = H or D) was determined to be k_H/k_D = 1.03 +/- 0.0.2 at temperatures between 10.5 and 40.0 deg C; activation parameters for the protium ester are ΔH(excit.) = 21.7 +/- 0.4 kcal mol^-1 and ΔS(excit.)₂₉₈ = 31.9 +/- 0.6 cal deg^-1mol^-1.These results, combined with those of previous studies, suggest that leaving-group expulsion is the only step contributing to rate limitation for conversion of the glycinate to the oxazolinone.The small, normal sec ondary isotope effect probably has complex origins, which may include relief of steric strain upon cyclization and hyperconjugative stabilization of the rate-controlling transition state.Apparent second-order rate constants for the formation of 2-phenyloxazolin-5-one in the alkaline hydrolysis of the isotopically unsubstituted ester were also obtained in aqueous solvent containing various mole fraction n of D2O (k_n).The kinetic solvent effects (KSIE's) are inverse at all values of n with k_1.0/k₀ = 1.75.Analysis of the bowl-shaped plot of k_1.0/k₀ vs. n indicates that fractionation of the reactant lyoxide ion in the mixed isotopic waters predominates the KSIE.A small, normal transition-state effect of about 1.3 makes the KSIE less inverse than would be expected if the entire effect originated in lyoxide fractionation.