Kinetic Study on Nucleophilic Displacement Reactions of Phenyl Y-Substituted Phenyl Carbonates with 1,8-Diazabicyclo[5.4.0]undec-7-ene: Effects of Amine Nature on Reaction Mechanism

Abstract

Second-order rate constants (k(N)) for nucleophilic displacement reactions of phenyl Y-substituted phenyl carbonates (7a-7l) with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C have been measured spectrophotometrically. The Bronsted-type plot for the reactions of 7a-7l with DBU is linear with beta(lg) = -0.48, indicating that the reactions proceed through a concerted mechanism, which is in contrast to the stepwise mechanism reported previously for the corresponding reactions with ethylamine (a primary amine) and piperidine (a secondary amine). The Hammett plots correlated with sigma(-) and sigma(o) constants exhibit many scattered points. In contrast, the Yukawa-Tsuno plot results in an excellent linear correlation with rho(Y) = 1.27 and r = 0.57, implying that a negative charge develops partially on the O atom of the leaving group in the transition state. The bulky DBU is less reactive than the primary and secondary amines toward substrates possessing a weakly basic leaving group. It has been concluded that steric hindrance exerted by DBU in the plausible intermediate (T-+/-) forces the reactions to proceed through a concerted mechanism because expulsion of the leaving group from T-+/- could reduce the steric hindrance.