Browsing by Author "Lee, Shyi-Long"

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Quantum mechanistic insights on aryl propargyl ether Claisen rearrangement
(2014)
Srinivasadesikan, Venkatesan
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Dai, Jiun-Kuang
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Lee, Shyi-Long
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應用化學系
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Department of Applied Chemistry
The mechanism of aryl propargyl ether Claisen rearrangement in gas and solvent phase was investigated using DFT methods. Solvent phase calculations are carried out using N,N-diethylaniline as a solvent in the PCM model. The most favorable pathways involve a [3,3]-sigmatropic reaction followed by proton transfer in the first two steps and then deprotonation or [1,5]-sigmatropic reaction. Finally, cyclization yields benzopyran or benzofuran derivatives. The [3,3]-sigmatropic reaction is the rate-determining step for benzopyran and benzofuran with Delta G(double dagger) value of 38.4 and 37.9 kcal mol(-1) at M06/6-31+G**//B3LYP/6-31+G* level in gas and solvent phase, respectively. The computed results are in good agreement with the experimental results. Moreover, it is found that the derivatives of aryl propargyl ether proceeded Claisen rearrangement and the rate-determining step may be shifted from the [3,3]-sigmatropic reaction to the tautomerization step. The NBO analysis revealed that substitution of the methyl groups on the aliphatic segment has decreased the stabilization energy E(2) and favors the aryl propargyl ether Claisen rearrangement.
The role of cesium fluoride in aryl propargyl ether Claisen rearrangement and its mechanistic elucidation: a theoretical study
(2016-10)
Menkir, Mengistu Gemech
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Srinivasadesikan, Venkatesan
;
Lee, Shyi-Long
;
應用化學系
;
Department of Applied Chemistry
The role of cesium fluoride (CsF) in aryl propargyl ether Claisen rearrangement and its mechanistic pathway have been investigated in gas and solvent phase using the density functional theory implemented in Gaussian 09. Our results indicate that the [3,3]-sigmatropic rearrangement is the rate-limiting step with Delta G (aEuro) value of 37.1 kcal/mol in solvent phase. Furthermore, the results show that the enolization of alpha-allenylketone intermediate (Int1-CsF) has a higher free energy barrier, which implies that the formation of benzopyran is not favored in the presence of CsF. However, the abstraction of the alpha-hydrogen atom in Int1-CsF with CsF shows a very low free energy barrier and is the most favored pathway for aryl propargyl ether Claisen rearrangement in the presence of CsF to form benzofuran. In the case of substituted aryl propargyl ethers, a methoxy group on the benzene ring lowers the activation barrier. The HOMO-LUMO, conformational and NBO analysis indicate that increasing methyl substitution on the propargyl residue enhances the rearrangement reaction.