通过结促进的氧化添加和olefins与化的区域选择性化
Jiwu Ruan1, Jonathan A Iggo, Neil G Berry
1Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK.
Journal of the American Chemical Society
|October 30, 2010
概括
一个新的催化系统有效地将化与烯酸结合起来. 乙烯基醇加速反应,并通过帮助氧化添加和解离在催化过程中改善区域选择性.
科学领域:
- 有机化学 有机化学
- 催化剂是一种催化剂.
- 有机金属化学 有机金属化学
背景情况:
- 赫克化反应是C-C键形成的关键方法.
- 结合化,特别是未激活的化,仍然具有挑战性,因为氧化添加的难度很高.
研究的目的:
- 开发一种通用且高效的催化系统,用于激活和未激活的化与烯酸的区域选择性赫克化.
- 阐明溶剂和催化剂在控制反应效率和选择性方面的作用.
主要方法:
- 开发一种以催化的赫克化系统,利用一种富含电子的二氨酸连接物 (4-MeO-dppp).
- 对各种溶剂的研究,重点是乙烯基醇,以获得最佳的反应性能.
- 机理学研究,包括DFT计算,以了解乙烯基醇在催化循环中的作用.
主要成果:
- 建立了一个高效的催化系统,用于将广泛的化与烯酸进行区域选择性合.
- 乙烯糖醇被确定为一种关键溶剂,与DMF或DMSO相比,显著加快反应并改善区域选择性.
- 机械学研究表明,乙烯基醇通过结促进了通过结限制速度的氧化添加步骤,并促进了化物解离.
结论:
- 开发的催化系统提供了一种一般且高效的方法,用于使用化的Heck arylation.
- 乙烯基醇具有双重作用,增强催化剂活性,并通过催化循环中的特定相互作用指导区域选择性.
- 这些发现为催化赫克反应的机制和溶剂效应的影响提供了宝贵的见解.
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01:25Radical Anti-Markovnikov Addition to Alkenes: Overview
The addition of hydrogen bromide to alkenes in the presence of hydroperoxides or peroxides proceeds via an anti-Markovnikov pathway and yields alkyl bromides.
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.
02:04Oxidation of Alkenes: Anti Dihydroxylation with Peroxy Acids
Diols are compounds with two hydroxyl groups. In addition to syn dihydroxylation, diols can also be synthesized through the process of anti dihydroxylation. The process involves treating an alkene with a peroxycarboxylic acid to form an epoxide. Epoxides are highly strained three-membered rings with oxygen and two carbons occupying the corners of an equilateral triangle. This step is followed by ring-opening of the epoxide in the presence of an aqueous acid to give a trans diol.
02:35Electrophilic Addition to Alkynes: Hydrohalogenation
Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.
01:17Radical Anti-Markovnikov Addition to Alkenes: Mechanism
The reaction of hydrogen bromide with alkenes in the presence of hydroperoxides or peroxides proceeds via anti-Markovnikov addition. The radical chain reaction comprises initiation, propagation, and termination steps.
The mechanism starts with chain initiation, which involves two steps. In the first chain initiation step, a weak peroxide bond is homolytically cleaved upon mild heating to form two alkoxy radicals. In the second initiation step, a hydrogen atom is abstracted by the alkoxy radical...
The mechanism starts with chain initiation, which involves two steps. In the first chain initiation step, a weak peroxide bond is homolytically cleaved upon mild heating to form two alkoxy radicals. In the second initiation step, a hydrogen atom is abstracted by the alkoxy radical...
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.