Kinetic and mechanistic studies on quinuclidinolysis of Y-substituted-phenyl picolinates: Effect of amine nature on reactivity and transition-state structure

Second-order rate constants (k _N) have been measured spectrophotometrically for reactions of Y-substituted-phenyl picolinates (7a–7i) with a series of quinuclidines in 80 mol% H₂O/20 mol% DMSO at 25.0 ± 0.1 °C. The Brønsted-type plot for the reactions of 7a–7i with quinuclidine is linear with β _lg = −0.80. The Yukawa-Tsuno plot exhibits an excellent linear correlation with ρY = 2.37 and r = 0.52, indicating that a negative charge develops partially on the O atom of the leaving group in the rate-determining transition state (TS). The Brønsted-type plot for the reactions of 2-chloro-4-nitrophenyl picolinate (7a) with a series of quinuclidines is also linear with β _nuc = 0.83. Thus, it was concluded that the reactions proceed through a stepwise mechanism, in which expulsion of the leaving group occurs in the rate-determining step. Comparison of the current kinetic data with those reported previously for the corresponding reactions with piperidine revealed that quinuclidine is ca. 10²-fold less reactive than piperidine. This is in contrast to the reports that quinuclidines are more reactive than isobasic secondary amines toward diaryl carbonates and related esters. Effects of amine nature on reactivity and TS structures are discussed