通过终端模板协助远程元CH债券的激活
Dasheng Leow1, Gang Li, Tian-Sheng Mei
1Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA.
Nature
|June 29, 2012
概括
研究人员开发了用于选择性碳- (C-H) 键功能化的新烯模板. 这些模板可以激活远距离的超C-H键,克服传统正确选择方法的局限性.
科学领域:
- 有机化学 有机化学
- 催化剂是一种催化剂.
- 合成方法论 合成方法论
背景情况:
- 功能化未激活的碳- (C-H) 键是有效分子合成的关键.
- 在C-H激活中实现选择性是具有多个不等价的C-H键的挑战.
- 使用定向组的现有方法通常通过刚性循环过渡状态产生正向选择性,限制远程C-H键激活.
研究的目的:
- 开发一种用于选择性激活远端元C-H键的新策略.
- 为了克服传统的C-H激活方法的近距离驱动的反应能力限制.
- 引入可轻松移除的模板,使得元选择性C-H功能化.
主要方法:
- 使用含有酸盐的模板来直接激活C-H.
- 研究了meta-C-H激活的机制,专注于和金属中心之间的"终端"相互作用.
- 测试了模板在烯基基底物 (如烯衍生物和酸酸) 上的有效性.
主要成果:
- 通过使用烯模板,证明成功激活远端元-C-H键 (距离10个以上的键).
- 提出一个弱的"终端"烯酸金属相互作用促进了紧张的旋式过渡前状态,使得元选择性.
- 表明化模板取代了内在的电子,硬质和正位导向效应.
结论:
- 一个新的含有亚的模板类别可以选择性激活远程的C-H甲基键.
- "终端"协调模式对于克服近距离限制和实现元选择性至关重要.
- 这种方法为功能化领域提供了一种通用方法,扩大了合成可能性.
相关概念视频
01:13meta-Directing Deactivators: –NO2, –CN, –CHO, –CO2R, –COR, –CO2H
All meta-directing substituents are deactivating groups. These substituents withdraw electrons from the aromatic ring, making the ring less reactive toward electrophilic substitution. For example, the nitration of nitrobenzene is 100,000 times slower than that of benzene because of the deactivating effect of the nitro group. The first step in an electrophilic aromatic substitution is the addition of an electrophile to form a resonance-stabilized carbocation. The energy diagrams for the...
01:11ortho–para-Directing Activators: –CH3, –OH, –NH2, –OCH3
All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
02:34Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration
The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.
01:24ortho–para-Directing Deactivators: Halogens
Halogens are ortho–para directors. They are more electronegative than carbon. Therefore, as ring substituents, they can withdraw electrons through the inductive effect and deactivate the aromatic ring towards electrophilic substitution. Halogens also have an electron-donating resonance effect on the ring, which influences the orientation of the incoming electrophile. If an electrophile attacks at the ortho or the para position, the halogen donates electrons and stabilizes the intermediate...
01:09Directing Effect of Substituents: meta-Directing Groups
Substituents on the benzene ring that direct an incoming electrophile to undergo substitution at the meta position are called meta directors. All meta directors either have a positive charge on the atom directly bonded to the ring or a partial positive charge. These groups function by withdrawing electrons from the ring through inductive and resonance effects. Consider the carbocation intermediates formed upon the addition of an electrophile on nitrobenzene at the ortho, meta, and para...
01:16Cycloaddition Reactions: MO Requirements for Thermal Activation
Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.