在7-Azahepta-1,2,4,6-tetraeneA CASSCF分子轨道研究中的电循环环闭合中的二次轨道效应
James A Duncan1, David E G Calkins, Mariya Chavarha
1Department of Chemistry, Lewis & Clark College, Portland, Oregon 97219-7899, USA. duncan@lclark.edu
Journal of the American Chemical Society
|May 7, 2008
概括
计算研究表明,E-7-azahepta-1,2,4,6-tetraene的电循环环闭合既不是纯粹的环周,也不是伪环周. 对分子轨道的分析表明,在这种反应机制中起着特殊的作用.
科学领域:
- 计算化学的计算化学
- 有机反应机制 有机反应机制
- 量子化学 是一个量子化学.
背景情况:
- E-7-azahepta-1,2,4,6-tetraene到1-aza-6-methylidenecyclohexa-2,4-diene的电循环闭合是关于其反应机制的争论的主题.
- 之前使用密度函数理论 (DFT) 的计算研究已经得出了相互矛盾的结论,即反应是否通过环周或伪环周路径进行.
- 了解精确的机制对于预测反应性和设计涉及aza结合系统的新型合成策略至关重要.
研究的目的:
- 通过计算来研究E-7-azahepta-1,2,4,6-tetraene的电循环环闭合的潜在能量表面.
- 将反应机制与相关系统进行比较,包括碳化合物类似物和其他以aza替代的聚烯.
- 阐明电子结构,特别是单对在确定反应路径中的作用.
主要方法:
- 使用 (10,9) CASSCF/6-31G*进行高层次的初始计算.
- 使用B3LYP/6-31G*函数的密度函数理论 (DFT) 计算.
- 分析正常模式向量,并使用局部完整活性空间 (CAS) 分子轨道 (MO) 构建轨道相关图.
主要成果:
- 在 (10,9) CASSCF/6-31G* 级别的计算表明,N1-H 组的旋转比B3LYP/6-31G* 计算对3 --> 4重排的过渡状态 (TS) 更大.
- 轨道相关图表明,aza-tetraene的最高占位的非局部化pi分子轨道 (MO) 主要负责西格玛键的形成,而不是终端allenylpi键MO.
- 确定了涉及单对的特殊二次轨道效应,这表明反应既不是纯粹的环周,也不是伪环周.
结论:
- E-7-azahepta-1,2,4,6-tetraene的电循环环闭合通过一个由单双的二次轨道相互作用影响的机制进行.
- 这一发现解决了之前关于反应的环周性与伪环周性之间的分歧.
- 这项研究强调了考虑特定电子特性的重要性,例如异构原子单对对,以了解复杂的有机反应机制.
相关概念视频
01:17Thermal 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.
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.
01:26Thermal 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.
01:18Structure of Benzene: Molecular Orbital Model
According to the molecular orbital (MO) model, benzene has a planar structure with a regular hexagon of six sp2 hybridized carbons. As shown in Figure 1, each carbon is bonded to three other atoms with C–C–C and H–C–C bond angles of 120°. The C–H bond length is 109 pm, and the C–C bond length is 139 pm which is midway between the single bond length of sp3 hybridized carbons (154 pm) and sp2 hybridized carbons (133 pm).
01:18Aromatic Hydrocarbon Cations: Structural Overview
Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group with both...
Removing one hydrogen from the intervening CH2 group with both...
01:26Photochemical 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
Selection Rules: Photochemical Activation
01:18Frost Circles for Different Conjugated Systems
The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.