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Kinetic Study on Aminolysis of Phenyl 2-Pyridyl Carbonate in Acetonitrile: Effect of Intramolecular H-bonding Interaction on Reactivity and Reaction Mechanism
- Kinetic Study on Aminolysis of Phenyl 2-Pyridyl Carbonate in Acetonitrile: Effect of Intramolecular H-bonding Interaction on Reactivity and Reaction Mechanism
- Song, Ji-Hyun; Lee, Jae-In; Um, Ik-Hwan
- Ewha Authors
- SCOPUS Author ID
- Issue Date
- Journal Title
- BULLETIN OF THE KOREAN CHEMICAL SOCIETY
- 0253-2964; 1229-5949
- vol. 35, no. 7, pp. 2081 - 2085
- Aminolysis; Steric hindrance; Inductive effect; Resonance contribution; H-bonding interaction
- KOREAN CHEMICAL SOC
- SCI; SCIE; SCOPUS; KCI
- Second-order rate constants (k(N)) have been measured spectrophotometrically for the reactions of phenyl 2-pyridyl carbonate(6) with a series of cyclic secondary amines in MeCN at 25.0 +/- 0.1 degrees C. The Bronsted-type plot for the reaction of 6 is linear with beta(nuc) = 0.54, which is typical for reactions reported previously to proceed through a concerted mechanism. Substrate 6 is over 10(3) times more reactive than 2-pyridyl benzoate (5), although the reactions of 6 and 5 proceed through the same mechanism. A combination of steric hindrance, inductive effect and resonance contribution is responsible for the kinetic results. The reactions of 6 and 5 proceed through a cyclic transition state (TS) in which H-bonding interactions increase the nucleofugality of the leaving group (i.e., 2-pyridiniumoxide). The enhanced nucleofugality forces the reactions of 6 and 5 to proceed through a concerted mechanism. In contrast, the corresponding reaction of 4-nitrophenyl 2-pyridyl carbonate (7) proceeds through a stepwise mechanism with quantitative liberation of 4-nitrophenoxide ion as the leaving group, indicating that replacement of the 4-nitrophenoxy group in 7 by the PhO group in 6 changes the reaction mechanism (i.e., from a stepwise mechanism to a concerted pathway) as well as the leaving group (i.e., from 4-nitrophenoxide to 2-pyridiniumoxide). The strong electron-withdrawing ability of the 4-nitrophenoxy group in 7 inhibits formation of a H-bonded cyclic TS. The presence or absence of a H-bonded cyclic TS governs the reaction mechanism (i.e., a concerted or stepwise mechanism) as well as the leaving group (i.e., 2-pyridiniumoxide or 4-nitrophenoxide).
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