相关概念视频
03:08Hydroboration-Oxidation of Alkenes
In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.
02:36Regioselectivity and Stereochemistry of Hydroboration
A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
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
01:28Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation
Unlike the easy catalytic hydrogenation of an alkene double bond, hydrogenation of a benzene double bond under similar reaction conditions does not take place easily. For example, in the reduction of stilbene, the benzene ring remains unaffected while the alkene bond gets reduced. Hydrogenation of an alkene double bond is exothermic and a favorable process. In contrast, to hydrogenate the first unsaturated bond of benzene, an energy input is needed; that is, the process is endothermic. This is...
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自组装的bisurea宏循环被用作有机热石,用于对2-cyclohexenone进行高度立体选择性的光二分化.
Jun Yang1, Mahender B Dewal, Linda S Shimizu
1Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, USA.
Journal of the American Chemical Society
|June 22, 2006
概括
自组装的bisurea宏循环使2-cyclohexenone具有高度选择性的2 + 2循环添加,产生头到尾的光电聚合物. 可重复使用的催化剂促进了高效的产品提取和回收,类似于热带石材料.
科学领域:
- 有机化学 有机化学
- 超分子化学 超分子化学
- 摄影化学的使用.
背景情况:
- 循环添加反应是有机合成的基础.
- 摄影重组可以使循环加法产物形成复杂化.
- 宏环化合物可以作为化学反应中的催化剂或模板.
研究的目的:
- 开发一种高度选择性的方法,用于2+2循环添加2-环色.
- 研究使用自组 bisurea 宏循环来促进这种反应.
- 评估催化系统的效率和可重复使用性.
主要方法:
- 使用自组装的bisurea宏循环作为超分子催化剂.
- 在光化学条件下进行2+2循环添加反应的2-cyclohexenone.
- 分析反应混合物的产物形成和选择性.
- 研究产品提取和催化剂回收的方法.
主要成果:
- 实现了高度选择性的头到尾光二聚二的光二聚形成.
- 观察到高反应转换率.
- 证明了二次光学重排的发生率降低.
- 成功提取了产品并回收了 bisurea 宏循环催化剂以重复使用.
结论:
- 自组装的bisurea宏循环有效催化2-cyclohexenone的选择性光二分化.
- 该系统提供高效率,选择性和催化剂可回收性.
- 这种方法提供了一个可重复使用和可回收的催化系统,类似于热石.