サイクルエーテルにおける二次C−H結合のイリジウム触媒によるボリル化
Carl W Liskey1, John F Hartwig
1Department of Chemistry, University of Illinois, Urbana, Illinois 61801, United States.
Journal of the American Chemical Society
|July 19, 2012
まとめ
この研究は,循環性エーサの新しいイリジウム触媒化ボリレーションを報告し,酸素へのC-H結合βを選択的に標的としています. この方法は,C-H活性化によるサイクルエーサーの機能化のためのユニークなアプローチを提供します.
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
- 有機金属化学 有機金属化学
背景:
- C-H結合の機能化は,有機合成において極めて重要です.
- 周期性エーサの選択的ボリレーションは依然として困難です.
- 既存の方法では,ヘテロ原子に隣接するC-H結合の地域選択性が欠けていることが多い.
研究 の 目的:
- 周期性エーテルにおける二次C−H結合の地域選択的ボリレーションのための新しい触媒システムを開発する.
- エーテル酸素に対するアルファとベータの位置でのボリレーションの選択性を調査する.
- 観測された地域選択性の根底にあるメカニズムを解明する.
主な方法:
- テトラメチルフェナントロリンリガンドとイリジウムの前駆体を使用した触媒製剤.
- さまざまなサイクルエーサのボリル化のための反応最適化.
- イソトープの標識と中間捕獲 (該当する場合) を含むメカニズム研究.
主要な成果:
- 周期性エーテルにおける二次C-H結合のボリル化が成功しました.
- アルファ位置よりもベータ位置でのC-H結合のユニークな選択性が観察されました.
- 機械学的調査は,ベータ位置での直接的なC-H結合の割れ方を示唆しています.
結論:
- 新しいイリジウム触媒法により,ベータ位置での周期性エーサの選択的ボリル化が可能になりました.
- この発見は,サイクルエーテルにおけるC-H活性化の地域選択性についての洞察を提供します.
- この研究は,機能化された循環エーテル誘導体の合成のための貴重なツールを提供します.
関連する概念動画
Hydroboration-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.
Alkynes 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.
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.
Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene
Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.
Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene
The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule.
Electrophilic Addition to Alkynes: Halogenation
Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.


