アルケンの水酸化におけるダイナミクスと移行状態理論の失敗
Yatsandra Oyola1, Daniel A Singleton
1Department of Chemistry, Texas A&M University, PO Box 30012, College Station, Texas 77842, USA.
Journal of the American Chemical Society
|February 14, 2009
まとめ
移行状態理論は,ヒドロボレーションの選択性を不正確に予測する. 軌道の研究は,ボーラン (BH3) とのアルケンの反応をよりよく説明し,選択性は単なる移行状態ではなく,過剰なエネルギーから生じることを示しています.
科学分野:
- コンピューティング・ケミストリー
- 有機化学 オーガニック・ケミストリー
- 反応ダイナミクス 反応ダイナミクス
背景:
- ボラン (BH3) でアルケンの水酸化は,基本的な有機反応である.
- トランジション状態理論 (TST) は,反応選択性を予測するために一般的に使用されます.
- 水力採掘における観測された地域選択性は,TSTの予測と一致していません.
研究 の 目的:
- 水力採掘の選択性を予測する移行状態理論の失敗を調査する.
- アルケンの水酸化による地域選択性を支配する要因を解明する.
- TSTの予測を軌道研究と実験データから得られた結果と比較する.
主な方法:
- 移行構造の高レベルの初期計算.
- 計算エラー,溶媒効果,トンネリング,エントロピーの分析.
- 水酸化反応をシミュレートする分子動力学軌道の研究.
主要な成果:
- Ab initio計算は,実験的な地域選択性を説明するには大きすぎるエネルギー差を予測した.
- 軌道の研究は,観察されたアンチ・マルコヴニコフ/マルコヴニコフ・アドゥクト比率を正確に再現した.
- BH3-アルケンの相互作用で発生する余分なエネルギーは,重要な要因として特定されました.
結論:
- トランジション状態理論は,このシステムにおける水力掘削の選択性を正確に予測するには不十分である.
- 反応選択性は,直接軌道,RRKM理論,および熱均衡の組み合わせによる結果である.
- 静的移行状態を超えた反応の動態を理解することは,化学的選択性を予測するために不可欠です.
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.
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.
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.
01:17Radical 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...
02:24Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation
Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
02:13Reduction of Alkenes: Catalytic Hydrogenation
Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the surface of...
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the surface of...