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ステレオセレクティブのRu触媒によるオレフィンメタテシスにおける制御要素としてのH結合
Amir H Hoveyda1, Pamela J Lombardi, Robert V O'Brien
1Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, USA. amir.hoveyda@bc.edu
Journal of the American Chemical Society
|May 30, 2009
まとめ
分子内水素結合は,ルテニウム触媒によるオレフィン転化反応を著しく強化する. このアプローチにより,高度にステレオセレクティブなリング開き/クロスメタテシス反応が可能になり,速度と生産性が向上します.
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
- ステレオセレクティブ合成
背景:
- 水素結合は,立体選択合成のための触媒設計で広く使用されています.
- 金属触媒プロセス,特にオレフィン転化におけるその応用は,まだ十分に研究されていない.
研究 の 目的:
- ルテニウム触媒によるオレフィンメタテシスに対する分子内水素結合の影響を調査する.
- 効率的でステレオ選択的なリング開き/クロスメタテシス (DROCM) 反応を開発する.
主な方法:
- エナティオメリックに濃縮されたアリルアルコールを含むアキラルルテニウム触媒を使用した.
- ステレオ化学を制御するために,分子内水素結合の相互作用を利用した.
- ダイアステロセレクティブのリング開き/クロスメタテシス反応を発達させた.
主要な成果:
- 極めて容易で高度にダイアステロセレクティブなDROCM反応を達成した.
- 反応は急速に進行した (<2モル%の触媒,分) で,高い変換率 (>98%) と出力率 (87%まで) を有した.
- 製品は高いステレオ化学的純度 (98:2 dr と 11:1 E:Z まで) を示した.
結論:
- 分子内水素結合は,Ru-触媒化されたオレフィンメタテシスの速度とステレオコントロールを高める強力な戦略です.
- この方法は,優れたステレオ化学的制御を持つ合成的に価値のある製品へのアクセスを提供します.
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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.
02:17Regioselectivity 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...
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.
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.
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.