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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 30, Number 12
BKCSDE 30(12)
December 20, 2009 

Kinetics and Mechanism of Michael-type Reactions of Ethyl Propiolate with Alicyclic Secondary Amines in H2O and MeCN: Solvent Effect on Reactivity and Transition-State Structure
Song I Kim, Hye Won Baek, Ik Hwan Um*
Ethyl propiolate, Michael-type reaction, Kinetic isotope effect, Rate-determining step, Brønsted-type plot
The amines studied in this study are less reactive toward ethyl propiolate (3) in MeCN than in H2O although they are 7 to 9 pKa units more basic in the aprotic solvent. The reactivity of morpholine and deuterated morpholine toward 3 is found to be identical, indicating that proton transfer occurs after rate-determining step (RDS). The fact that kinetic isotope effect is absent excludes a stepwise mechanism in which proton transfer occurs in RDS as well as a concerted mechanism in which nucleophilic attack and proton transfer occur concertedly through a 4-membered cyclic transition state (TS). Thus, the reactions have been concluded to proceed through a stepwise mechanism in which proton transfer occurs after RDS. Brønsted-type plots are linear with small βnuc values, i.e., βnuc = 0.29 in H2O and βnuc = 0.51 in MeCN, indicating that bond formation is not advanced significantly in RDS. The small βnuc value also supports the conclusion drawn from the study of kinetic isotope effect.
2909 - 2912
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