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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 33, Number 9
BKCSDE 33(9)
September 20, 2012 

 
Title
A Kinetic Study on Aminolysis of t-Butyl 4-Pyridyl Carbonate and Related Compounds: Effect of Leaving and Nonleaving Groups on Reaction Mechanism
Author
Ji-Sun Kang, Jae-In Lee, Ik-Hwan Um*
Keywords
Aminolysis, Mechanism, H-bonding interaction, Steric acceleration, Steric inhibition
Abstract
Second-order rate constants kN have been measured spectrophotometrically for nucleophilic substitution reactions of t-butyl 4-pyridyl carbonate 8 with a series of alicyclic secondary amines in H2O at 25.0 ± 0.1 oC. The Brønsted-type plot for the reactions of 8 is linear with βnuc = 0.84. The βnuc 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., βnuc = 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±, in which expulsion of the leaving-group from T± 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± and its deprotonated form T– on the basis of the fact that the plots of pseudo-first-order rate constant kobsd 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.
Page
2971 - 2975
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