在催化 cis-trans 碳-碳双键同质化中的选择性
1Department of Chemistry, University of California, Riverside, California 92521, USA.
Journal of the American Chemical Society
|September 1, 2005
概括
与热力学预测相反,在白金表面上,转化-2-乙烯为cis-2-butene更容易. 这是由于吸附时cis异构体的稳定性,这表明催化表面控制了选择性.
科学领域:
- 表面化学 表面化学
- 催化剂是一种催化剂.
- 有机化学 有机化学
背景情况:
- 在化学合成中,氨酸的异构化是至关重要的.
- 了解金属表面的反应动力学是催化剂设计的关键.
- 热力学稳定性并不总是预测动力学有利性.
研究的目的:
- 研究Pt{111}表面上的2-丁烯异构的动力学和热力学方面.
- 为了阐明控制cis-2-butene比trans-2-butene的优先形成的因素.
- 通过表面工程来探索控制异构体选择性的潜力.
主要方法:
- 吸附能量的计算建模.
- 吸附剂和催化剂表面之间的固体相互作用的分析.
- 转-2-烯到cis-2-烯同质化的动力学研究.
主要成果:
- 转-2-丁烯的异构化到cis-2-丁烯在Pt上动力学上是受青的.
- 吸附增强了cis异构体的稳定性,而不是与trans异构体相比.
- 固体相互作用显著影响吸附能量和反应通路.
结论:
- 动力控制,而不是热力学控制,决定了这个反应中的产品分布.
- 表面结构和吸附剂-表面相互作用对于在烯酸异构化中实现选择性至关重要.
- 催化剂设计可以通过考虑固态效应来优化,以操纵 cis / trans 选择性.
相关概念视频
02:50Catalysis
The presence of a catalyst affects the rate of a chemical reaction. A catalyst is a substance that can increase the reaction rate without being consumed during the process. A basic comprehension of a catalysts’ role during chemical reactions can be understood from the concept of reaction mechanisms and energy diagrams.
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.
01:17Thermal 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.
01:26Photochemical 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
01:27Catalysis
Catalysis influences the rate of chemical reactions by providing an alternative reaction pathway with lower activation energy. A catalyst speeds up a reaction, but it is not consumed during the process. The fundamental principle of catalysis is the ability of a catalyst to alter the reaction mechanism, often introducing a more efficient pathway than the uncatalyzed process.In a catalyzed reaction, the catalyst participates directly in the reaction mechanism. It interacts with reactants to form...
01:22Heterogeneous Catalysis
Heterogeneous catalysis involves a catalyst in a different phase from the reactants. It is a process where the catalyst and the reactants are in distinct phases, typically solid and gas or liquid.Most heterogeneous catalysts are metals, metal oxides, or acids. The list includes transition metals like iron (Fe), cobalt (Co), nickel (Ni), palladium (Pd), platinum (Pt), chromium (Cr), manganese (Mn), tungsten (W), silver (Ag), and copper (Cu). These metals possess partially vacant d orbitals that...