Rh (I) -触媒 (5 + 2) サイクル添加における地域選択性の電子およびステリック制御:実験と理論
Peng Liu1, Lauren E Sirois, Paul Ha-Yeon Cheong
1Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, USA.
Journal of the American Chemical Society
|July 1, 2010
まとめ
ヴィニルcyclopropanes (VCPs) とalkynesの間のRh(I) -触媒 (5 + 2) サイクル添加は,置換物位置に基づいて予測可能な地域選択性を示しています. 移行状態におけるステリック効果と電子効果は,これらの重要な有機反応の結果を支配する.
科学分野:
- 有機化学 オーガニック・ケミストリー
- カタリシス カタリシス カタリシス
- 反応メカニズム 反応機構
背景:
- ビニルサイクロプロパン (VCP) は,有機合成における多用途な構成要素である.
- ロジウムによって触媒化された (5 + 2) サイクロアディションは,複雑なサイクル構造への強力な経路を提供します.
- これらの反応における,特に非対称アルキンの領域選択性は,詳細な調査を必要とする領域であり続けています.
研究 の 目的:
- VCPと非対称アルキンの間のRh (I) -触媒 (5 + 2) サイクロアディションの地域選択性を調査する.
- 地域化学的結果を制御するための予測モデルを確立する.
- 反応の選択性に対する置換剤効果の影響を理解する.
主な方法:
- 様々な置換VCPとアルキンを含む実験研究.
- 機械分析のための密度関数理論 (DFT) 計算.
- 反応産物の分析により,地域選択性比率を決定する.
主要な成果:
- ステリックモデルと一致する,末端アルケンの置換によるVCPでは高い地域選択性 (>20:1) が観察されました.
- 内部アルケンの置換によるVCPでは,変数地域選択性 (>20:1から1:2.3まで) が認められた.
- 観察された傾向を説明するために,アルキン置換剤の電子効果を組み込んだ洗練されたモデルが提案されました.
結論:
- Rh (I) -触媒 (5 + 2) サイクロアディションの地域選択性は,ステリック要因と電子要因の両方によって影響を受けます.
- ターミナルVCP置換は,ステリックによって誘発される予測可能な地域選択性につながる.
- 内部VCP置換と電子取り除くアルキン群は,移行状態安定化によって選択性を変化させることができます.
さらに関連する動画
関連する概念動画
Regioselectivity 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.
Regioselectivity 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.
Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule
If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...
Cycloaddition Reactions: Overview
Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.
Woodward–Hoffmann Selection Rules and Microscopic Reversibility
Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
Electrophilic 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.

![Solid-phase Synthesis of [4.4] Spirocyclic Oximes](/_next/image?url=https%3A%2F%2Fcloudfront.jove.com%2FCDNSource%2Fteasers%2F58508.jpg&w=3840&q=50)
