关于放弃电循环反应的"允许"和"禁止"分类的论据
1School of Chemistry, Cardiff University CF10 3AT UK carpenterb1@cardiff.ac.uk.
Chemical science
|February 21, 2025
概括
电循环反应并不总是"允许"或"禁止". 计算研究表明",禁止"的途径可以在能量方面获得,挑战传统的合成策略.
科学领域:
- 有机化学 有机化学
- 计算化学的计算化学
- 反应机制 反应机制
背景情况:
- 根据伍德沃德-霍夫曼规则,电循环反应被归类为"允许"或"禁止".
- 这种分类意味着"禁止"反应在能量上是不可接受的,只有当"允许"的途径被阻塞时才会发生.
研究的目的:
- 通过计算来研究"禁止"的电循环反应的能量可行性.
- 挑战这样的假设,即"禁止"的电循环反应总是从能量上无法获得的.
主要方法:
- 使用了第二阶段的N电子价值扰动理论 (NEVPT2) 计算.
- 使用密度函数理论 (DFT) 的计算.
- 检查了循环布丁环开口和 (Z) -1,3,5-hexatriene环闭口,包括素化变体.
主要成果:
- "禁止"和"允许"过渡状态之间的能量差异大大不同.
- 一些"禁止"路径的能量障碍明显低于预期 (例如,不到一半的乙内部旋转障碍).
- 替代效应 (听觉效应和电子效应) 很容易取代"禁止"机制的电子惩罚.
结论:
- "允许"和"禁止"电循环反应的严格二分法是一个过度简单化.
- 综合规划不应该仅仅依赖于"允许"路径假设.
- 超出电子控制的因素,如替代剂效应,在决定反应结果方面发挥着至关重要的作用.
更多相关视频
09:17Reductive Electropolymerization of a Vinyl-containing Poly-pyridyl Complex on Glassy Carbon and Fluorine-doped Tin Oxide Electrodes
Published on: January 30, 2015
11.8K
10:44Translating Extracellular Electron Transfer Activities with Organic Electrochemical Transistors
Published on: January 31, 2025
500
相关概念视频
01:26Thermal and Photochemical Electrocyclic Reactions: Overview
2.3K
Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
2.3K
01:34Woodward–Hoffmann Selection Rules and Microscopic Reversibility
3.0K
Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
3.0K
01:17Thermal Electrocyclic Reactions: Stereochemistry
2.0K
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
2.0K
01:17Pericyclic Reactions: Introduction
8.2K
Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic...
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic...
8.2K
01:26Photochemical Electrocyclic Reactions: Stereochemistry
1.8K
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
1.8K
01:26Limitations of Friedel–Crafts Reactions
5.2K
Several restrictions limit the use of Friedel–Crafts reactions. First, the halogen in the alkyl halide must be attached to an sp3-hybridized carbon for the Friedel–Crafts reactions to occur. Vinyl or aryl halides do not react since the carbocations formed are unstable under the reaction conditions. Second, Friedel–Crafts alkylation is susceptible to carbocation rearrangement, and the major products obtained have a rearranged carbon skeleton. In contrast, the acylium ion is...
5.2K