相关概念视频
02:35Regioselectivity of Electrophilic Additions-Peroxide Effect
In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.
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.
01:17Thermal 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
01:13Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene
Bromination and chlorination of aromatic rings by electrophilic aromatic substitution reactions are easily achieved, but fluorination and iodination are difficult to achieve. Fluorine is so reactive that its reaction with benzene is difficult to control, resulting in poor yields of monofluoroaromatic products. To address this, Selectfluor reagent is used as a fluorine source in which a fluorine atom is bonded to a positively charged nitrogen.
01:14Photoluminescence: Applications
Photoluminescence offers a wide range of applications due to its inherent sensitivity and selectivity. This technique allows for both direct and indirect analyses of the analyte. Direct quantitative analysis is possible when the analyte exhibits a favorable quantum yield for fluorescence or phosphorescence. However, an indirect analysis may be feasible if the analyte is not fluorescent or phosphorescent, or if the quantum yield is unfavorable. Indirect methods include reacting the analyte with...
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具有高度区域选择性的3-styrylpyridines的无氧光环化.
F D Lewis1, R S Kalgutkar, J S Yang
1Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA. lewis@chem.northwestern.edu
Journal of the American Chemical Society
|July 18, 2001
概括
styrylpyridines 和 aminostyrylpyridines 的光化学反应产生阿扎芬烯. 在无氧条件下,3- styrylpyridine衍生物通过稳定的二皮里丁中间体选择性地形成2-azaphenanthrenes.
科学领域:
- 有机光化学 有机光化学
- 异环化学 异环化学
- 反应机制 反应机制
背景情况:
- styrylpyridines 和 aminostyrylpyridines 是重要的异环化合物.
- 了解它们的光化学转化对于合成应用至关重要.
研究的目的:
- 为了研究cis-styrylpyridines和aminostyrylpyridines的光化学行为.
- 阐明阿扎芬烯形成的反应机制和区域选择性.
主要方法:
- 在有氧和无氧条件下的光化学辐射.
- 介质的光谱表征 (1H NMR,电子吸收) 的介质.
- 实验和计算频谱的比较.
主要成果:
- 在无氧条件下,从3-styrylpyridine及其3'-amino衍生物中区域选择性形成2-azaphenanthrenes.
- 在氧气的存在下形成混合的2-和4-阿扎芬烯产物.
- 通过1,7-转移形成的一种中等稳定的1,4-二皮里丁中间体的鉴定.
结论:
- 光循环的区域选择性受环开放率,西格马特罗普重新排列和氧气火率的影响.
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