Solvent-Independent Transition-State Structure for Acyl-Transfer Reactions. A Novel Strategy for Construction of a Bronsted Correlation

The effect of changing solvent (DMSO-water) composition on the rate of the acyl-transfer reaction, for substituted phenoxides reacting with p-nitrophenyl acetate and p-chlorophenoxide reacting with substituted phenyl acetates, has been studied. A correlation of log k with a⁰in preference to σ⁻, for leaving group substituent changes, supports a reaction pathway involving a tetrahedral intermediate, contrary to recent proposals of a concerted mechanism. The results are further analyzed by means of two kinds of Bronsted type correlations: the traditional method (A), where the pK_aof the nucleophile is changed via substituent changes, and a novel method (B), where the change in pK_ais brought about by changes in the reaction medium, in this case by changing the DMSO-water composition. Method B was found to be applicable only over the range of 40–90 mol % DMSO in water where the physical properties of the media do not show any abrupt changes. It is shown that application of method A, which leads to the conclusion (via β_nucvalues) that the transition-state structure changes with changing solvent composition (in accord with the RSP and Hammond's postulate), is inconsistent with the conclusion of a constant transition-state structure based on application of method B. The latter is shown to be the more reliable by independent criteria. It is proposed that this novel strategy for construction of Bronsted type plots could be applied to other reactions and different solvent systems.