非対称ロジウム誘導のアンチマルコフニコフ・レジオセレクティブボラサイクロペンタンヌレーション
Momar Toure1, Olivier Chuzel, Jean-Luc Parrain
1Aix Marseille Université, CNRS, ISM2 UMR 7313, case 532, 13397, Marseille cedex 20, France.
Journal of the American Chemical Society
|October 20, 2012
まとめ
研究者らは,分子内水酸化のためのロジウム活性化N-ヘテロサイクルカルベン-ボラン (NHC-ボラン) を使用した新しい方法を開発しました. この突破は,以前の反応性の制限を克服し,キラルサイクルボランの効率的な合成を可能にします.
科学分野:
- 有機金属化学 有機金属化学
- 合成有機化学 合成有機化学について
背景:
- N-ヘテロサイクリックカルベン-ボラン (NHC-ボラン) は,高い安定性があるため,アルケーンに対して一般的に反応しない.
- 以前の方法では,NHC-ボランを有効な水酸化反応剤として利用することが限られていた.
研究 の 目的:
- NHC-ボランの新たな活性化戦略を策定する.
- NHC-ボランを使用した前例のない分子内水酸化反応を達成するために.
- エナンチオ濃縮サイクルボランを合成する.
主な方法:
- NHC-ボランにおけるB-H結合のシモイ型活性化で,ディホスファン結合のカチオン型Rh複合体を用いる.
- この活性化の応用は,単純なオレフィンの分子内水酸化である.
主要な成果:
- NHC-ボランのRh活性化が成功し,その固有の非反応性を回避した.
- 非対称なRh誘導抗マルコフニコフボラサイクロペンタヌレーションにより,サイクルボランのライブラリが生まれた.
- 高い収穫 (94%まで),地域選択性 (100%まで),およびエナチオ選択性 (99.2:0.8まで) が達成されました.
結論:
- NHC-ボランの新たな活性化モードが実証されました.
- この方法は,キラルサイクルNHC-ボランへのアクセスに効率的な経路を提供します.
- 有機金属化学と非対称合成におけるNHC-ボランの潜在力を強調する.
さらに関連する動画
関連する概念動画
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.
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.
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.
02:17Regioselectivity 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...
03:08Hydroboration-Oxidation of Alkenes
In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.
02:47Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation
Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.