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Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal 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.
2.0K
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal 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
Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

Woodward–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
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical 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
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Cycloaddition Reactions: Overview01:16

Cycloaddition Reactions: Overview

2.5K
Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.
2.5K
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction01:16

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

10.1K
The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.
10.1K

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关于二甲基乙烯电循环的预测指南.

Clàudia Climent1, Zhen Xu2, Michael O Wolf2

  • 1Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.

The journal of physical chemistry letters
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概括

设计用于光化学电循环的有机分子通过从边界分子轨道定位和对称性预测光循环能力来简化. 这指导着新的分子光开关的开发.

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科学领域:

  • 有机化学 有机化学
  • 摄影化学的使用.
  • 计算化学计算化学

背景情况:

  • 分子光开关在暴露于光线时可逆地改变结构.
  • 环开式和环闭式形式是常见的相互转换.
  • 滴乙烯 (DTE) 化合物是广泛研究的光开关.

研究的目的:

  • 为经历光化学电循环的分子制定设计准则.
  • 预测有机化合物的光环化能力.
  • 扩展伍德沃德-霍夫曼规则以获得更广泛的染色体应用.

主要方法:

  • 在基于DTE的化合物上使用电子结构计算.
  • 分析了边境分子轨道的定位和对称性.
  • 经过实验验证的指导方针与基于素的新型DTE.

主要成果:

  • 光环化能力与边境分子轨道属性相关.
  • 建立了分子设计的简单,可预测的指南.
  • 证明了计算方法的实验验证.

结论:

  • 轨道局部化和对称性是光化学电循环化的关键预测因素.
  • 制定的指导方针适用于DTE以外的其他日用乙烯.
  • 这项工作促进了新型分子光开关的合理设计.