相关概念视频
03:08Rate-Determining Steps
Relating Reaction Mechanisms
In a multistep reaction mechanism, one of the elementary steps progresses significantly slower than the others. This slowest step is called the rate-limiting step (or rate-determining step). A reaction cannot proceed faster than its slowest step, and hence, the rate-determining step limits the overall reaction rate.
The concept of rate-determining step can be understood from the analogy of a 4-lane freeway with a short-stretch of traffic-bottleneck caused due to...
In a multistep reaction mechanism, one of the elementary steps progresses significantly slower than the others. This slowest step is called the rate-limiting step (or rate-determining step). A reaction cannot proceed faster than its slowest step, and hence, the rate-determining step limits the overall reaction rate.
The concept of rate-determining step can be understood from the analogy of a 4-lane freeway with a short-stretch of traffic-bottleneck caused due to...
02:23SN2 Reaction: Stereochemistry
In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.
02:46Halogenation of Alkenes
Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
01:21Acid-Catalyzed α-Halogenation of Aldehydes and Ketones
By replacing an α-hydrogen with a halogen, acid-catalyzed α-halogenation of aldehydes or ketones yields a monohalogenated product
In the first step of the mechanism, the acid protonates the carbonyl oxygen resulting in a resonance-stabilized cation, which subsequently loses an α-hydrogen to form an enol tautomer. The C=C bond in an enol is highly nucleophilic because of the electron-donating nature of the –OH group. Consequently, the double bond attacks an electrophilic halogen to form a...
In the first step of the mechanism, the acid protonates the carbonyl oxygen resulting in a resonance-stabilized cation, which subsequently loses an α-hydrogen to form an enol tautomer. The C=C bond in an enol is highly nucleophilic because of the electron-donating nature of the –OH group. Consequently, the double bond attacks an electrophilic halogen to form a...
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
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选择性催化转化因因与二醇一起转化为基双环[3.1.0]六.
Florian Monnier1, Chloé Vovard-Le Bray, Dante Castillo
1Laboratoire Catalyse et Organométalliques, Institut Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, and Groupe Matière Condensée et Matériaux, UMR 6626 CNRS-Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France.
Journal of the American Chemical Society
|April 14, 2007
概括
催化使得从1,6-enynes和diazo化合物中立体选择性合成双环[3.1.0]hexanes. 这种方法为复杂的循环结构提供了一条新的途径,具有受控的立体化学.
科学领域:
- 有机金属化学 有机金属化学
- 合成有机化学 合成有机化学
- 催化剂是一种催化剂.
背景情况:
- 1,6-类是有机合成中的多功能基质.
- 催化剂以调解各种转换而闻名.
- 复杂的碳循环框架的立体选择性合成仍然是一个挑战.
研究的目的:
- 为了研究1,6-enynes与由鲁前体催化的二化合物的反应.
- 探索所观察到的转变的立体化学结果和机械路径.
- 扩大反应的范围,包括相关的基质和试剂.
主要方法:
- 各种1,6-氨酸与三甲基西二亚甲 (N2CHSiMe3) 的催化反应使用RuCl(COD) Cp*.
- 用X射线结晶学对单环产品进行立体化学测定.
- 密度函数理论 (DFT) 计算以阐明反应机制.
主要成果:
- 具有高cis-立体选择性的Z-基双环[3.1.0]六的立体选择性形成.
- 取代剂依赖的结果:循环化与庞大的群体,异环形成与不太庞大的群体.
- 对乙烯基双环[3.1.0]和基双环[4.1.0]的成功应用.
- 催化也有效使用N2CHCO2Et和N2CHPh.
结论:
- 催化反应提供了一个有效的路径,使bicyclo[3.1.0]hexanes.
- 该机制涉及初始的 [2+2] 循环添加和rutenacyclobutane中间体的形成,有利于循环化而不是转化.
- 这项研究强调了催化在构建具有精确立体化学控制的复杂碳循环结构中的实用性.