Reactions of oximate alpha-nucleophiles with esters: evidence from solvation effects for substantial decoupling of desolvation and bond formation.

This communication is concerned with deviations from linearity in Brønsted-type nucleophilicity plots as indicators of changes in transition-state structures, in particular as related to solvational effects. Increasing dimethyl sulfoxide (DMSO) content of DMSO-water mixtures removes the leveling off observed in Brønsted type plots of the reaction of p-nitrophenylacetate (PNPA) with oximate alpha-nucleophiles. This is interpreted as due to virtually complete decoupling between desolvation of the oxyanion nucleophile and bond formation in the rate-determining transition state (solvational imbalance).