在移植的Ti-calixarenes中氧气协调环境的结构评估和催化后果
Justin M Notestein1, Leandro R Andrini, Vitaly I Kalchenko
1Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720, USA.
Journal of the American Chemical Society
|February 1, 2007
概括
在SiO2上植入卡利沙-复合物,可以创建可访问的中心以进行催化. 不同的calixarene结构影响的协调和反应性,为易斯酸强度和氧化催化提供了洞察力.
科学领域:
- 不同质的催化剂.
- 材料科学 材料科学 材料科学
- 有机金属化学 有机金属化学
背景情况:
- -二氧化物材料是氧化反应的关键催化剂.
- 调整中心的协调环境会影响催化活性.
- 卡利沙连接物为金属中心提供了多功能协调可能性.
研究的目的:
- 在SiO2.2上移植的-素复合物的合成和特征.
- 为了研究协调,电子结构和环氧化反应性之间的关系.
- 系统地研究连接物结构如何影响支的催化剂的性能.
主要方法:
- 将Ti-calixarene复合物植入到二氧化支上.
- 使用Ti K边缘X射线吸收光谱学 (XAS) 和NMR光谱学进行表征.
- 在烯环氧化反应中对催化活性的评估.
主要成果:
- 接种的Ti-tert-butylcalix[4]arene复合体表现出具有特定光谱特征的孤立,可访问的Ti中心.
- -homooxacalix[3]arene复合物显示了变化的光谱特征和显著较低的环氧化率由于连接体降解.
- 两种类型的复合物的化结果具有相似的催化性能,表明结构转换为均的Ti中心.
结论:
- 素联体的结构和协调显著影响支的Ti中心的电子特性和催化活性.
- Ti K-edge XAS是一种敏感的探测器,用于评估异质催化剂中的Ti协调和电子结构.
- 这些系统设计的材料提供了一个平台,用于将易斯酸强度与氧化反应性相关联.
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.
01:22Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction
The radical dimerization of ketones or aldehydes gives vicinal diols through a pinacol coupling reaction. However, the behavior of titanium metals used for the reaction as a source of electrons is unusual. When the reaction is carried out in the presence of titanium, diols can be isolated at low temperatures. Else titanium further reacts with diols, forming alkenes through the McMurry reaction.
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.
02:44Oxidation of Alkenes: Syn Dihydroxylation with Osmium Tetraoxide
Alkenes are converted to 1,2-diols or glycols through a process called dihydroxylation. It involves the addition of two hydroxyl groups across the double bond with two different stereochemical approaches, namely anti and syn. Dihydroxylation using osmium tetroxide progresses with syn stereochemistry.
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
01:46Oxidative Cleavage of Alkenes: Ozonolysis
In ozonolysis, ozone is used to cleave a carbon–carbon double bond to form aldehydes and ketones, or carboxylic acids, depending on the work-up.
Ozone is a symmetrical bent molecule stabilized by a resonance structure.
Ozone is a symmetrical bent molecule stabilized by a resonance structure.