通过伊米多基复合体进行烯立体反转的机制
Forrest E Michael1, Andrew P Duncan, Zachary K Sweeney
1Department of Chemistry and Center for New Directions in Organic Synthesis, University of California at Berkeley, Berkeley, California 94720, USA.
Journal of the American Chemical Society
|June 12, 2003
概括
这项研究探讨了使用金属循环在艾伦中介导的立体反转. 研究人员观察到这些有机金属化合物的中间体,并提出了这些有机金属化合物中立体化学转换的机制.
科学领域:
- 有机金属化学 有机金属化学
- 立体化学是一种立体化学.
- 化学化学 化学
背景情况:
- 艾伦是具有独特反应性的多功能有机化合物.
- 有机合成中的立体化学控制对于开发新分子至关重要.
- conocene复合物是有机转化中的有价值的试剂.
研究的目的:
- 为了研究由艾伦基基衍生的介导金属循环的立体化学行为.
- 阐明这些有机金属中间体中的立体反转机制.
- 了解不同替代剂对立体化学结果的影响.
主要方法:
- [2 + 2] 丰富的艾伦和imidozirconocene复合体之间的循环添加反应.
- 使用循环二极化谱法测量金属循环的种族化速率.
- 通过核磁共振 (NMR) 光谱学观察反应中间体.
主要成果:
- 成功测量了金属循环的相对种族化率.
- 鉴定了二烯基的立体逆转中的中间体.
- 建立了用于dialkylallene和diarylallene金属循环种族化的明确的拟议机制.
结论:
- 艾伦的立体反转可以通过复合物有效调解.
- 从dialkylallenes衍生出的金属循环可能通过β-化物消除进行种族化.
- 利拉烯衍生的金属循环的立体反转速度较慢,可能涉及eta4-azatrimethylenemethane过渡状态.
相关概念视频
02:43E1 Reaction: Stereochemistry and Regiochemistry
One of the critical aspects of the E1 reaction mechanism, as also observed in E2, is the regiochemistry, with multiple regioisomers obtained as products. In the example discussed, the presence of water as a weak base favors elimination over substitution to generate two alkenes. Given that alkenes’ stability increases with the number of alkyl groups across the double bond, typically, E1 reactions lead to the Zaitsev product, for this is more substituted and stable than the Hofmann product.
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.
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.
01:28Diels–Alder Reaction Forming Cyclic Products: Stereochemistry
The Diels–Alder reaction is one of the robust methods for synthesizing unsaturated six-membered rings. The reaction involves a concerted cyclic movement of six π electrons: four π electrons from the diene and two π electrons from the dienophile.
01:23Aldehydes and Ketones with Amines: Imine Formation Mechanism
Imine formation involves the addition of carbonyl compounds to a primary amine. It begins with the generation of carbinolamine through a series of steps involving an initial nucleophilic attack and then several proton transfer reactions. The second part includes the elimination of water, as a leaving group, to give the imine.
Imines are formed under mildly acidic conditions. A pH of 4.5 is ideal for the reaction.
If the pH is low or the solution is too acidic, the reaction slows down in the...
Imines are formed under mildly acidic conditions. A pH of 4.5 is ideal for the reaction.
If the pH is low or the solution is too acidic, the reaction slows down in the...
01:12Stereochemical Effects of Enolization
The chiral α-carbon of the carbonyl compound is the stereocenter of the molecule. As shown in the figure below, when such a carbonyl compound undergoes racemization under an acidic or basic condition, an achiral enol is formed.