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相关概念视频

Stereoisomerism of Cyclic Compounds02:33

Stereoisomerism of Cyclic Compounds

In this lesson, we delve into the role of ring conformation and its stability, which determines the spatial arrangement and, consequently, the molecular symmetry and stereoisomerism of cyclic compounds. 1,2-Dimethylcyclohexane is used as a case study to evaluate the possible number of stereoisomers. Here, given the multiple (n = 2) chiral centers, there are 2n = 4 possible configurations that lack a plane of symmetry, as the ring skeleton exists in a non-planar chair conformation. In addition,...
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

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.
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

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.
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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
Stability of Conjugated Dienes01:28

Stability of Conjugated Dienes

Introduction
A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.
Cationic Chain-Growth Polymerization: Mechanism00:57

Cationic Chain-Growth Polymerization: Mechanism

The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the generated carbocation,...

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相关实验视频

Updated: Jul 10, 2026

Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level
06:55

Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level

Published on: September 26, 2016

丁二二和二甲基乙烯基碳酸盐的 [8+2] 循环添加的机械扭曲:逐步 [8+2] 与 [4+2] / [1,5] - 乙烯基转移机制相比,通过理论和实验研究揭示了理论和实验研究的转移机制.

Yuanyuan Chen1, Siyu Ye, Lei Jiao

  • 1Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education, College of Chemistry, Peking University, Beijing 100871, People's Republic of China.

Journal of the American Chemical Society
|August 19, 2007
PubMed
概括

这项研究揭示了 [8+2] 循环添加反应的新机制,其中涉及二烯和DMAD. 计算和实验发现澄清了合成较大的碳循环的途径.

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Versatile CO2 Transformations into Complex Products: A One-pot Two-step Strategy
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Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level
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Self-assembling Morphologies Obtained from Helical Polycarbodiimide Copolymers and Their Triazole Derivatives
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科学领域:

  • 有机化学 有机化学
  • 计算化学计算化学

背景情况:

  • 迪尼尔和迪尼利索本佐与DMAD通过 [8+2] 循环添加反应.
  • 了解这些反应是合成10个成员和更大的碳循环的关键.

研究的目的:

  • 阐明二烯和之间 [8+2] 循环添加的详细分子机制.
  • 结合计算和实验方法来实现机械学理解.

主要方法:

  • 密度函数理论 (DFT) 在 (U) B3 LYP/6-31+G (d) 级的计算.
  • 计算预测机制的实验验证.

主要成果:

  • 协调的 [8+2] 路径通常不受欢迎.
  • 对于富含电子的二烯二二酸,预计会有一个带有二中间体的阶段性途径,与迪尔斯-阿尔德 [4+2] 反应相竞争.
  • 对于缺乏电子捐赠群的二烯二烯酸,预测了一种涉及 [4+2] 反应,其后是 [1,5] - 乙烯基转移的新型机制,并得到实验支持.

结论:

  • [8+2]循环添加的机制取决于二甲的电子性质.
  • 发现并验证了一种涉及 [4+2] 反应和随后的乙烯基转移的新型逐步机制.
  • 这项工作为设计新合成路径到复杂的碳循环提供了关键的见解.