环烯的内反应的过渡结构
1Contribution from the Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, and the Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260.
Journal of the American Chemical Society
|November 19, 2013
概括
与乙烯反应相比,环烯反应的激活障碍明显较低,与实验结果一致. 在环烯中减轻应变,因为粉驱动这些能量差异.
科学领域:
- 计算化学计算化学
- 有机反应机制 有机反应机制
背景情况:
- 反应是一种基本的有机转化,涉及基与基和双或三重键.
- 环烯是一种应力循环烯,由于其高环应力,具有独特的反应性.
研究的目的:
- 为了研究涉及环烯作为酶友的一种反应的过渡结构和能量学.
- 为了比较环烯在某种反应中的反应性与烯的反应性.
主要方法:
- Ab initio分子轨道计算使用6-31G*基础集.
- 用Becke3LYP函数和6-31G*基础集进行密度函数理论 (DFT) 计算.
- 对于选定的过渡结构,完成主动空间自相一致场 (CASSCF) 计算.
- 用于能源评估的第二阶Møller-Plesset (MP2) 扰动理论.
主要成果:
- 环烯反应显示的激活障碍明显低于乙烯反应.
- 过渡结构表现出不同程度的异步性,受激素中间体稳定性的影响.
- 循环二元化过程中的内转基因结构在能量上比外转基因结构优于2.7kcal/mol.
结论:
- 当循环烯作为阴性剂时,减轻其应变是观察到较低激活障碍的主要原因.
- 建议二次轨道重叠来解释在环二元化中偏爱内转变状态的偏好.
- 计算的二基中间体很可能是所采用的理论方法的产物.
相关概念视频
01:26Thermal and Photochemical Electrocyclic Reactions: Overview
2.2K
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.
2.2K
01:17Thermal Electrocyclic Reactions: Stereochemistry
1.6K
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.
1.6K
01:18Reactivity of Enols
3.4K
Enols are a class of compounds where a hydroxyl group is attached to a carbon–carbon double bond, which implies that it is a vinyl alcohol. A carbonyl compound with an α hydrogen undergoes keto–enol tautomerism and remains in equilibrium with its tautomer, the enol form. Usually, the keto tautomer is present in a higher concentration than the enol tautomer due to the higher bond energy of C=O compared to C=C. Moreover, the direction of the keto–enol equilibrium is...
3.4K
01:26Photochemical Electrocyclic Reactions: Stereochemistry
1.4K
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
1.4K
01:16Cycloaddition Reactions: Overview
2.4K
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.
2.4K
03:00Preparation of Epoxides
7.7K
Overview
Epoxides result from alkene oxidation, which can be achieved by a) air, b) peroxy acids, c) hypochlorous acids, and d) halohydrin cyclization.
Epoxidation with Peroxy Acids
Epoxidation of alkenes via oxidation with peroxy acids involves the conversion of a carbon–carbon double bond to an epoxide using the oxidizing agent meta-chloroperoxybenzoic acid, commonly known as MCPBA. Since the O–O bond of peroxy acids is very weak, the addition of electrophilic oxygen of peroxy...
Epoxides result from alkene oxidation, which can be achieved by a) air, b) peroxy acids, c) hypochlorous acids, and d) halohydrin cyclization.
Epoxidation with Peroxy Acids
Epoxidation of alkenes via oxidation with peroxy acids involves the conversion of a carbon–carbon double bond to an epoxide using the oxidizing agent meta-chloroperoxybenzoic acid, commonly known as MCPBA. Since the O–O bond of peroxy acids is very weak, the addition of electrophilic oxygen of peroxy...
7.7K