• Bulletin of the Korean Chemical Society
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• v.34 no.8
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• pp.2325-2329
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• 2013

Second-order rate constants $k_N$ have been measured for reactions of benzyl 2-pyridyl thionocarbonate (4b) and t-butyl 2-pyridyl thionocarbonate (5b) with a series of cyclic secondary amines in MeCN at $25.0{\pm}0.1^{\circ}C$. The $k_N$ values for the reactions of 4b and 5b have been compared with those reported previously for the corresponding reactions of benzyl 2-pyridyl carbonate (4a) and t-butyl 2-pyridyl carbonate (5a) to investigate the effect of changing the electrophilic center from C=O to C=S on reactivity and reaction mechanism. The thiono compound 4b is more reactive than its oxygen analogue 4a. The Br${\o}$nsted-type plots for the reactions of 4a and 4b are linear with ${\beta}_{nuc}=0.57$ and 0.37, respectively. The reactions of 4a were previously reported to proceed through a concerted mechanism, while those of 4b in this study have been concluded to proceed through a stepwise mechanism with formation of an intermediate being the rate-determining step on the basis of the ${\beta}_{nuc}$ value of 0.37. Enhanced polarizability upon changing the C=O in 4a by C=S has been suggested to be responsible for the reactivity order and the contrasting reaction mechanisms. In contrast, the reactivity of 5a and 5b is similar, but they are much less reactive than 4a and 4b. Furthermore, the reactions of 5a and 5b have been concluded to proceed through the same mechanism (i.e., a concerted mechanism) on the basis of linear Bronsted-type plots with ${\beta}_{nuc}=0.45$ or 0.47. It has been concluded that the strong steric hindrance exerted by the t-Bu in 5a and 5b causes a decrease in their reactivity and forces the reactions to proceed through a concerted mechanism.

• Bulletin of the Korean Chemical Society
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• v.33 no.9
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• pp.2971-2975
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• 2012

Second-order rate constants $k_N$ have been measured spectrophotometrically for nucleophilic substitution reactions of t-butyl 4-pyridyl carbonate 8 with a series of alicyclic secondary amines in $H_2O$ at $25.0{\pm}0.1^{\circ}C$. The Br${\emptyset}$nsted-type plot for the reactions of 8 is linear with ${\beta}_{nuc}$ = 0.84. The ${\beta}_{nuc}$ value obtained for the reactions of 8 is much larger than that reported for the corresponding reactions of t-butyl 2-pyridyl carbonate 6 (i.e., ${\beta}_{nuc}$ = 0.44), which was proposed to proceed through a forced concerted mechanism. Thus, the aminolysis of 8 has been concluded to proceed through a stepwise mechanism with a zwitterionic tetrahedral intermediate $T^{\pm}$, in which expulsion of the leaving-group from $T^{\pm}$ occurs at the rate-determining step (RDS). In contrast, aminolysis of benzyl 4-pyridyl carbonate 7 has been reported to proceed through two intermediates, $T^{\pm}$ and its deprotonated form $T^-$ on the basis of the fact that the plots of pseudo-first-order rate constant $k_{obsd}$ vs. amine concentration curve upward. The current study has demonstrated convincingly that the nature of the leaving and nonleaving groups governs the reaction mechanism. The contrasting reaction mechanisms have been rationalized in terms of an intramolecular H-bonding interaction, steric acceleration, and steric inhibition.