Kinetics and mechanisms of the reactions of 3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitrophenyl thionocarbonates with alicyclic amines

The reactions of 3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitro phenyl thionocarbonates (1, 2, and 3, respectively) with a series of second ary alicyclic amines are studied kinetically in 44 wt % ethanol-water at 25 .0 degreesC and an ionic strength of 0.2 M (KCl). Pseudo-first-order rate c oefficients (k(obsd)) are obtained for all reactions (amine excess was used ). The reactions of compound 1 with piperidine, piperazine, and 1-(2-hydrox yethyl)piperazine and of compounds 2 and 3 with these amines and morpholine exhibit linear k(obsd) versus amine concentration plots with slopes (k(1)) independent of pH. In contrast, the plots are nonlinear upward for the rea ctions of substrate I with morpholine, 1-formylpiperazine, and piperazinium ion and of substrates 2 and 3 with the two latter amines. For all these re actions, a reaction scheme is proposed with a zwitterionic tetrahedral inte rmediate (T +/-), which can be deprotonated by an amine to yield an anionic intermediate (T-). When the nonlinear plots are fit through an equation de rived from the scheme, rate and equilibrium microcoefficients are obtained. The Bronsted-type plots for k(1) are linear with slopes of beta (1) = 0.22 , 0.20, and 0.24 for the aminolysis of 1, 2, and 3, respectively, indicatin g that the formation of T+/- (k(1) step) is rate-determining. The k(1) valu es for these reactions follow the sequence 3 > 2 > 1, which can be explaine d by the sequence of the electron-withdrawing effects from the substituents on the nonleaving group of the substrates.