熱力学研究と,ロジウム複合体からBX3化合物へのヒドリド移転反応
Michael T Mock1, Robert G Potter, Donald M Camaioni
1Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA.
Journal of the American Chemical Society
|September 17, 2009
まとめ
移行金属ヒドリド複合体は,ヒドリドの移転によってB-H結合の形成を促進します. この研究は,ヒドリドドナーの能力と親和性を定量化し,ボロン-水素化合物の合成のための好ましい反応の予測を可能にします.
科学分野:
- 無機化学 無機化学とは
- 有機金属化学 有機金属化学
- コンピューティング・ケミストリー
背景:
- 移行金属ヒドリド複合体は,化学合成における重要な中間物質である.
- ハイドリド移転の理解は,新しい合成方法論の開発に不可欠です.
- ボロンハリドおよび関連する化合物 (BX3) は,ボロン-水素結合の多用途な前駆体である.
研究 の 目的:
- 移行金属ヒドリドからBX3化合物への好ましいヒドリド移転を予測するための信頼性の高い方法の確立.
- 特定の移行金属複合体とボラートアニオンのヒドリドドナー能力を定量化するために.
- 様々なBX3基質のヒドリド相性を見極めるために.
主な方法:
- アセトニトリルにおけるHRh (dmpe) 2とHRh (depe) 2のための熱力学水素ドナー能力 (ΔG°(H−)) の実験的決定.
- [HBEt3]−のヒドリドドナー能力を決定するために確立されたスケールを利用しました.
- BX3化合物のヒドリド親和性を評価するために,計算方法 (同位体反応) を採用した.
主要な成果:
- 移行金属ヒドリドのヒドリドドナー能力とBX3化合物のヒドリド相性に関する定量的なスケールが確立されています.
- B-X債券からB-H債券の形成が実現可能であることを実証しました.
- Et3N-BH3を生成するためにHRh(dmpe) 2を使用してB(SPh) 3の減少を示し,B-Xボンドエネルギーの影響を強調しました.
結論:
- この研究は,移行金属ヒドリドおよびBX3化合物を含むヒドリド移転反応の予測的枠組みを提供します.
- この発見は,アンモニア・ボランおよび関連する材料の合成に関連しています.
- ヘテロリティックなB-X結合エネルギーは,還元とB-H結合形成の程度を決定する重要な要因です.
関連する概念動画
Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride
Radical substitution reactions can be used to remove functional groups from molecules. The hydrogenolysis of alkyl halides is one such reaction, where the weak Sn–H bond in tributyltin hydride reacts with alkyl halides to form alkanes. Here, the reagent Bu3SnH yields tributyltin halide as a byproduct.
The bonds formed in this reaction are stronger than the bonds broken, making it energetically favorable. The reaction follows a radical chain mechanism similar to radical halogenation reactions,...
The bonds formed in this reaction are stronger than the bonds broken, making it energetically favorable. The reaction follows a radical chain mechanism similar to radical halogenation reactions,...
Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control
The addition of a hydrogen halide to 1,3-butadiene gives a mixture of 1,2- and 1,4-adducts. Since more substituted alkenes are more stable, the 1,4-adduct is expected to be the major product. However, the product distribution is strongly influenced by temperature; low temperature favors the 1,2-adduct, whereas the 1,4-adduct is predominant at high temperature.
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.
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.
Radical Halogenation: Thermodynamics
The thermodynamic favorability of a reaction is determined by the change in Gibbs free energy (ΔG). ΔG has two components- enthalpy (ΔH) and entropy (ΔS). The entropy component is negligible for alkane halogenation because the number of reactants and product molecules are equal. In this case, the ΔG is governed only by the enthalpy component. The most crucial factor that determines ΔH is the strength of the bonds. ΔH can be determined by comparing the energy between bonds broken and bonds...
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.


