ルイス酸がオレフィンのロジウム触媒による水酸化に及ぼす劇的な効果
Christopher J Lata1, Cathleen M Crudden
1Department of Chemistry, Queen's University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada.
Journal of the American Chemical Society
|December 9, 2009
まとめ
トリスペンタフローロボロンなどのルイス酸の添加は,Rh触媒による水酸化反応を著しく強化する. このコカタリストは,アリファティックオレフィンとアロマティックオレフィンの両方の反応を加速し,効率と選択性を向上させます.
科学分野:
- 有機金属化学 有機金属化学
- カタリシス カタリシス カタリシス
- オーガニック・シンセシス オーガニック・シンセシス
背景:
- オレフィンのロジウム触媒による水酸化は,重要な合成変換である.
- 溶媒の非調整と基質の範囲の制限により,以前は特定のオレフィンとの反応が妨げられていた.
研究 の 目的:
- ルイス酸コカタライストがRh触媒による水酸化に及ぼす効果を調査する.
- ピナコールボランを用いたオレフィン水酸化の効率と選択性を向上させるため.
主な方法:
- ルイス酸と併用したロジウム触媒,特にトリス・ペンタフローロフェニル・ボラン (B・C6F5) 3を使用した.
- アリファティックオレフィンとアロマティックオレフィンの両方のピナコールボラン (HBPin) による水酸化を調査した.
- ルイス酸の役割を明らかにするために,予備的なメカニズム研究を行いました.
主要な成果:
- トリスペンタフルオロボロン (B(C6F5)3) は,ピナコールボラン (HBPin) でオレフィンのRh触媒化水酸化を劇的に加速します.
- アリファティックオレフィンは,配合しない溶媒では反応しないが,B ((C6F5) 3.3) の存在で迅速に反応した.
- アロマティックオレフィン・ヒドロボレーションは,B ((C6F5) 3.3) を加えることで,より速く,より選択的になりました.
結論:
- トリスペンタフルオロボロンは重要なコカタリストとして作用し,反応の開始だけでなく,反応全体に不可欠です.
- ルイス酸は,THFと共に,HBPin.のB-H結合のヘテロリチス分裂に関与しています.
- この発見は,オレフィン水酸化のためのより効率的で選択的な方法を提供します.
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.
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.
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.
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...