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

張翔凱(Chang Hsiang-Kai)

指導教授 ： 蔡明剛

國立臺灣師範大學/理學院/化學系/碩士(2020年)

https://doi.org/10.6345/NTNU202000206

摘要

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。

關鍵字

Gaussian 09 ； 立體化學 ； 質子化反應 ； 勢能面掃描

並列摘要

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.

並列關鍵字

Gaussian 09 ； stereochemistry ； protonation ； potential energy surface scan

參考文獻

1. S. Gokarneswar, R. Hasibur, M. Ádám, P. Imre, M. Mikko, V. Arto and P. P. M., Angewandte Chemie International Edition, 2012, 51, 13144-13148.

2. S. Dieter, S. Xiaoyu, S. Christof, E. Marc-Olivier, S. W. Bernd and B. A. K., Helvetica Chimica Acta, 2012, 95, 1064-1078.

3. J. Brecher, Pure and Applied Chemistry, 2009, 78, 1897-1970.

4. C. R. S., I. Christopher and P. V., Angewandte Chemie International Edition in English, 1966, 5, 385-415.

5. M. T. Miller, Transactions of the American Ophthalmological Society, 1991, 89, 623-674.