β-hydrogen secondary isotope effect in acyl transfer reactions. Origins, temperature dependence, and utility as a probe of transition-state structure

The anomalous temperature dependence of the β-deuterium (β-D) secondary isotope effect reported by Halevi and Margolin for the hydroxide-promoted hydrolysis of ethyl acetate has been observed also with methyl acetate [CH3CO2CH3 vs. CD3CO2CH3:k/sub 3H/k/sub 3D/ = 1.01 +- 0.02 (00C), 0.90 +- 0.02 (250C), 1.03 +- 0.10 (500C)]. In contrast, the β-D effects for the rates of acidic hydrolysis of methyl acetate (0.93 +- 0.02, 0 to 420C), basic hydrolysis of phenyl acetate (0.98 +- 0.02, 5 to 450C), basic methanolysis of phenyl acetate (0.98 +- 0.02, 5 to 400C) basic methanolysis of p-methoxyphenyl acetate (0.96 +- 0.02, 5 to 450C), and the equilibrium hydration of 1,3-dichloroacetone (ClCH2COCH2Cl vs. ClCD2COCD2Cl:K/sub 4H/K/sub 4D/ = 0.83 +- 0.02, 15 to 460C) over the indicated temperature ranges were statistically indistinguishable from the mean values cited. This constitutes regular, expected behavior. Both kinds of cases are consistent with a cascade model for acyl-transfer reactions in which solvent-reorganization and heavy-atom-reorganization transition states along parallel reaction paths alternate in dominating rate limitation in the 0 to 700C temperature range. Such a model also explains anomalous temperature dependences of solvent isotope effects for anhydride hydrolyses, reported by Rossall and Robertson. Observed transition-state properties would refer to a virtual, weighted-average, transition-state structure, according to the cascade model. 3 figures, 7 tables