Aminolysis of Y-substituted-phenyl 2-methoxybenzoates in acetonitrile: Effect of the o-methoxy group on reactivity and reaction mechanism

Abstract

Second-order rate constants (k N) were measured for aminolyses of Y-substituted-phenyl 2-methoxybenzoates 2a-i and 4-nitrophenyl X-substituted-benzoates 3a-j in MeCN at 25.0 °C. The Brønsted-type plot for the reactions of 2a-i with piperidine curves downward, indicating that a change in rate-determining step (RDS) occurs. The Hammett plot for the reactions of 3a-j with piperidine consists of two intersecting straight lines, which might be taken as evidence for a change in RDS. However, the nonlinear Hammett plot has been suggested not to be due to a change in RDS but rather to the stabilization of the ground state of substrates possessing an electron-donating group (EDG) (e.g., 3a-c) through a resonance interaction, since the corresponding Yukawa-Tsuno plot exhibits an excellent linear correlation with ρ = 0.54 and r = 1.54. The ρ value found for the reactions of 3a-j in MeCN is much smaller than that reported previously for the corresponding reactions in H 2O (i.e., ρ = 0.75). It is proposed that the reactions of 3a-j in MeCN proceed through a forced concerted mechanism due to instability of T ± in the aprotic solvent, while the reactions of 2a-i proceed through a stepwise pathway with a stabilized T ± through an intramolecular H-bonding interaction. © 2011 American Chemical Society.