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Title: 理論計算模擬有機立體化學反應路徑與選擇性
Theoretical Calculations of Organic Stereochemistry Reaction Pathway and Selectivity
Authors: 蔡明剛
Tsai, Ming-Kang
Chang, Hsiang-Kai
Keywords: Gaussian 09
Gaussian 09
potential energy surface scan
Issue Date: 2020
Abstract: 理論計算用於預測、尋找立體化學的位向選擇已行之有年,透過計算我們對微觀現象有了更進一步的了解。本篇研究以Gaussian 09為計算軟體,分為兩部分探討立體化學的反應位向,第一部份探討質子化反應(protonation)的立體位向選擇,並以不同的酸性分子:4-nitrophenol、acetic acid以及hydronium ion進行質子化反應,分子結構先以B97XD/6-31G初步優化,再以wB97XD/6-311G(d,p)做二次優化,透過分析電荷分布與過去的文獻中1,找到N-oxide與C3是可能被質子化的反應位向,以勢能面掃描與分子結構探討,說明C3的Si-face為本實驗質子化選擇性的依據,並與前人提出Re-face的選擇性進行結構比較2。最後確認反應步驟中的質子化反應與開環反應並非同步,是先進行質子化後再進行開環反應,且速率決定步驟應為開環反應,完成反應路徑分析。 第二部分探討不同配位基的Rh催化劑所產生立體產物的選擇,以及反應路徑的探討,若配位基為COD(1,5-Cyclooctadiene),則立體產物為外消旋;若配位基為4,7,7-trimethyl-2,5-biphenylbicyclo[2.2.1]hepta-2,5-diene,則立體產物為R form,後者為我們主要的分析對象。從結構與產物的關係來判斷中間物與催化劑的可能催化位向,以B3LYP/6-31G*/SDD進行結構優化,發現可能的催化位向為between與conjugate位,再以這兩個位向進行勢能面掃描,找到between位是較為合理的催化路徑,且從掃描的能障與過渡態分子結構,支持實驗所觀察到的R form選擇性。最後再將配位基換成COD進行between位的催化反應,所得的能障也支持實驗所觀察到的外消旋。
Theoretical calculations have been used to predict and determine the possible orientation of organic and inorganic stereochemistry for a long time. It is especially suitable for investigating the transition state structures and reaction mechanism. This thesis study is an example of using Gaussian 09, a conventional computational chemistry package, to study the stereochemistry of an organic and inorganic reaction. In the first part, we discuss the orientation of protonation by using different acids, like 4-nitrophenol, acetic acid, and hydronium ion. All structures in this part are initially optimized at the B97XD/6-31G level, and then re-optimization by using wB97XD/6-311G(d,p). Through analyzing the charge and previous literature1, we find out N-oxide and C3 is the most probable direction for protonation. The potential energy surface scanning and molecular structure are discussed, which shows that Si-face of C3 is the direction for protonation selectivity in the experiment, and compares with the early proposed Re-face structure2. Finally, we have shown that the protonation step and the ring-opening step are asynchronous, and the ring-opening step is carried out after protonation. By using frequency analysis, the rate determining step should be a ring-opening reaction. In the second part, we discuss the selection of stereoisomers generated by Rh catalyst with different ligands. If the ligand is COD(1,5-Cyclooctadiene), the stereo product is racemic. If the ligand is 4,7,7-trimethyl-2,5-biphenylbicyclo[2.2.1]hepta-2,5- diene, the stereo product is R form, which is the main object we discuss. From the structure of intermediate and Rh catalyst, we determine some possible catalytic intermediates. All structures in this part were optimized at B3LYP/6-31G*/ SDD level. And we find out the possible catalytic directions may “between” and “conjugate”. Scanning the potential energy surface with these two directions, we find “between” is a reasonable catalytic path. And from the energy barrier and transition state structure, we support the experimental observation of R form selectivity. Finally, the ligand was replaced by COD to catalyze at the “between” position, the barrier also supports racemic observed in the experiment.
Other Identifiers: G060442034S
Appears in Collections:學位論文

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