Structure-Reactivity Correlations in Nucleophilic Displacement Reactions of Y-Substituted-Phenyl X-Substituted-Cinnamates with Z-Substituted-Phenoxides

Second-order rate constants (k(N)) have been measured spectrophotometrically for the nucleophilic displacement reactions of 4-nitrophenyl X-substituted-cinnamates (4a-4e) and Y-substituted-phenyl cinnamates (5a-5e) with Z-substituted-phenoxide anions in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Hammett plot for the reactions of 4a-4e with 4-chlorophenoxide (4-C1PhO(-)) consists of two intersecting straight lines, which might be taken as a change in the rate-determining step (RDS). However, it has been concluded that the nonlinear Hammett plot is not due to a change in the RDS but is caused by stabilization of the ground state of substrates possessing an electron-withdrawing group in the cinnamoyl moiety through resonance interactions, since the Yukawa-Tsuno plot exhibits an excellent linear correlation with rho x = 0.89 and r = 0.58. The Bronsted-type plot for the reactions of 4-nitrophenyl cinnamate (4c) with Z-substituted-phenoxides is linear with beta(nuc) = 0.76. The Bronsted-type plot for the reactions of Y-substituted-phenyl cinnamates (5a-5d) with 4-chlorophenoxides (4-ClPhO-) is also linear with beta(lg) = 0.72. The Hammett plot correlated with sigma(-) constants for the reactions of 5a-5d results in a much better linear correlation than that correlated with sigma(o) constants, indicating that a partial negative charge develops on the O atom of the leaving aryloxide. Thus, the reactions have been concluded to proceed through a concerted mechanism.