Structure-Reactivity Correlations for Reactions of Substituted Phenolate Anions with Acetate and Formate Esters

Abstract

The reactions of substituted phenolate anions with m-nitrophenyl, p-nitrophenyl, and 3,4-dinitrophenyl formates follow nonlinear Brønsted-type correlations that might be taken as evidence for a change in the rate-limiting step of a reaction that proceeds through a tetrahedral addition intermediate. However, the correlation actually represents two different Brønsted lines that are defined by meta- and para-substituted phenolate anions and by meta- and para-substituted o-chlorophenolate anions. A concerted mechanism for both acetyl- and formyl-transfer reactions is supported by the absence of a detectable change in the Brønsted slope at ΔρK = 0 for the attacking and leaving phenolate anions within each class of Brønsted correlations. Regular increases in the dependence of log k on the pKa of the nucleophile with increasing pKa of the leaving group correspond to a positive interaction coefficient pxy = ∂β1g/∂(pKnuc) = ∂βnuc/∂(pk1g). The observation of two different Brønsted lines for the reactions of substituted phenolate anions with phenyl acetates is attributed to a steric effect that decreases the rate of reaction of substituted o-chlorophenolate anions by 25–50%. The reactions of meta- and para-substituted phenolate and o-chlorophenolate anions with substituted phenyl acetate esters follow values of βnuc = 0.53–0.66 and −β1g = 0.50–0.63. The reactions of meta- and para-substituted phenolate anions with formate esters are ∼103 times faster and follow smaller values of βnuc= 0.43–0.64 and −β1g = 0.31–0.48. However, the reactions of meta- and para-substituted o-chlorophenolate anions with the same formate esters follow larger values of βnuc = 0.63–0.90 and −β1g = 0.46–0.90. The large values of βnuc and –β1g for the reactions of substituted o-chlorophenolate anions with formate esters may arise from destabilization by the o-chloro group of a stacking interaction that is present in the transition state for reactions of formate esters, but not acetate esters. © 1993, American Chemical Society. All rights reserved.