Journal of the Korean Chemical Society (대한화학회지)

Korean Chemical Society (대한화학회)
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Theoretical Studies of Diels-Alder Reaction (Part II). A New United Ionic-Radical Mechanism of Diels-Alder Reaction

Diels-Alder 反應에 對한 理論的 硏究 (第2報). 新 United Ionic-Radical Mechanism

  • 박병각 (慶北大學校 工科大學 工業敎育學科)
  • Published : 1973.02.28
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Abstract

The purpose of this paper is to investigate the mechanism of Diels-Alder reaction by assuming pseudo molecular complex (PMC) which has characters both of ionic and radical bonds. We treated this complex quantum-chemically as an intermediate between the configuration without charge transfer (radical bond character) and the configuration corresponding to the charge transfer from Diene (R) to Dienophile (S) (ionic bond character). The wave function for the complex could be expressed as: ${\psi}_{complex} = {\psi}(R,S) +{ \rho}{\psi}(R^+,S^-)$ where ${\rho}$ is the extent of charge transfer which is a constant to measure the ionic character of PMC. It has been noticed that${\rho}$is related to the difference between Fr + Fr' and Fs + Fs' in free valence (F) when R is united to S through atom r in R to atom s in S and atom r' in R to atom s' in S, That is, ${\rho}{\alpha}$ ${\Delta}F = (Fr + Fr') - (Fs + Fs')$. We have calculated ${\Delta}F$values for more than forty Diels-Alder reactions. The calculated values of ${\Delta}F$ is reversely proportional to the values of Brown's paralocalization energy (Lp) as well as Dewar's differences of delocalization energy$({\Delta}Edeloc.)$ with good linearity. This approach also presents a way of predicting the possibility and the easiness of diene synthesis between any two conjugate compounds. According to the considerations, it could be concluded that Diels-Alder reaction takes place through the united ionic-radical mechanism rather than the separated ionic or radical mechanism.

區區한 Diels-Alder 反應의 구조를 규명하기 위하여 Diels-Alder 反應 基 自體의 本質에 가장 가깝다고 생각한 새로운 擬似分子化合物이라는 遷移狀態의 模型을 假定하여 Mulliken의 分子化合物의 量子力學을 適用하였다. 이 擬似分子化合物은 이온성과 Radical성을 지닌 混成體이다. 이 混成體의 波動函數는 다음 식으로 표현된다. ${\psi}_{complex} = {\psi}(R,S) + {\rho}{\psi}(R^+,S^-)$ 여기 ${\rho}$는 擬似分子化合物의 極性程度를 나타내는 것이고 이 ${\rho}$가 Diene과 Dienpohile의 反應中心原子들의 自由原子價의 차 $({\Delta}F)$에 關係함을 밝혔다. 아울러 이 ${\Delta}F$量이 Brown氏의 Lp量 및 Dewar氏의 ${\Delta}E_{deloc}$量과 直線성이 있음을 알았다. Diels-Alder反應의 可能性與否를 24組의 反應組에 對하여 豫言하였다. 따라서 우리가 假定한 遷移狀態의 模型이 眞實에 가깝다고 볼 수 있으며 결국 Diels-Alder 反應은 同時附加로 융합된 ionic-Radical mechanism으로 反應이 進行한다고 볼 수 있다.

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