通过催化控制乙烯和基C-X键激活中的化学选择性:基于pK的电子开关
Michael G Organ1, Elena A Arvanitis, Craig E Dixon
1Contribution from the Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario, Canada M3J 1P3.
Journal of the American Chemical Society
|February 14, 2002
概括
一个单一的催化剂可以通过调整离开组的酸度,在多功能烯酸上进行选择性烯酸替代或烯酸交叉合. 这为复杂分子结构提供可互换的合成序列.
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:
- 催化反应在有机合成中至关重要.
- 在多功能基板中控制选择性仍然是一个挑战.
- 不同的反应机制往往需要不同的催化系统.
研究的目的:
- 为了证明单个催化剂在不同的反应路径中可互换的使用.
- 探索烯构件上的选择性烯替代和烯交叉合.
- 为了研究离开组酸度对反应选择性的影响.
主要方法:
- 使用带有多功能烯构建块的ladium催化剂.
- 调节基离子组的结合酸的pK (a).
- 采用核友和木反应元件.
- 通过加热触发的连续反应.
主要成果:
- 在2,3-dibromo-1-propene上使用一个malonate核,甚至在Suzuki合试剂的存在下实现了选择性基替代.
- 选择性乙烯交叉合在2 - - 1 - 4 - 乙基氧 - 2 - 基上进行,而没有激活基组.
- 同一个催化剂促进了两个连续反应,加热促进了第二步.
结论:
- 一个单一的催化剂可以通过调整离开组的电子特性,选择性地引导到基替代或基交叉合.
- 这种方法允许在复杂的烯基底物上进行可互换和顺序的催化反应.
- 这些发现为构建多功能分子提供了一种多功能策略.
相关概念视频
01:23Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control
The addition of a hydrogen halide to 1,3-butadiene gives a mixture of 1,2- and 1,4-adducts. Since more substituted alkenes are more stable, the 1,4-adduct is expected to be the major product. However, the product distribution is strongly influenced by temperature; low temperature favors the 1,2-adduct, whereas the 1,4-adduct is predominant at high temperature.
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
01:17Pericyclic Reactions: Introduction
Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic rearrangements are...
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic rearrangements are...
01:27Catalysis
Catalysis influences the rate of chemical reactions by providing an alternative reaction pathway with lower activation energy. A catalyst speeds up a reaction, but it is not consumed during the process. The fundamental principle of catalysis is the ability of a catalyst to alter the reaction mechanism, often introducing a more efficient pathway than the uncatalyzed process.In a catalyzed reaction, the catalyst participates directly in the reaction mechanism. It interacts with reactants to form...