Structure-reactivity correlations for reactions of substituted phenolate anions with acetate and formate esters

Article abstract of DOI:10.1021/ja00058a006

The reactions of substituted phenolate anions with m-nitrophenyl, p-nitrophenyl, and 3,4-dinitrophenyl formates follow nonlinear Br?nsted-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 Br?nsted 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 Br?nsted slope at ΔpK = 0 for the attacking and leaving phenolate anions within each class of Br?nsted correlations. Regular increases in the dependence of log k on the pK_a of the nucleophile with increasing pK_a of the leaving group correspond to a positive interaction coefficient p_xy = ?β_1g/?(pK_nuc) = ?β_nuc/?(pK_1g). The observation of two different Br?nsted 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 β_nuc = 0.53-0.66 and -β_1g = 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 β_nuc = 0.43-0.64 and -β_1g = 0.31-0.48. However, the reactions of meta- and para-substituted o-chlorophenolate anions with the same formate esters follow larger values of β_nuc = 0.63-0.90 and -β_1g = 0.46-0.90. The large values of β_nuc and -β_1g 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.