一般ベース触媒エノライゼーションにおけるステリック効果の起源:ソルベーションと静電吸引
Scott O C Mundle1, Graeme W Howe, Ronald Kluger
1Davenport Chemical Laboratories, Department of Chemistry, University of Toronto, Toronto, Canada M5S 3H6.
Journal of the American Chemical Society
|November 30, 2011
まとめ
立体的に阻害された塩基は,イミニウム誘導体とは異なり,溶解の問題のためにアルデヒドとケトンのエノ化を遅らせます. このステリック阻害は静電的要因に関連しており,金属イオンによって減少させることができます.
科学分野:
- 有機化学 オーガニック・ケミストリー
- 物理化学 物理化学
- 反応の動力学
背景:
- ブロンステッドプロットは,一般基底触媒の理解に不可欠です.
- 塩基におけるステリック阻害は反応速度に影響を与える.
- ブロンステッド相関からの偏差は,複雑な反応機構を示します.
研究 の 目的:
- 一般基底触媒エノリゼーションのためのブロンステッドプロットにおける負の偏差の起源を調査する.
- アルデヒド/ケトンとイミニウム誘導体のステリカル阻害塩基の振る舞いを比較する.
- ステリック効果における溶解と静電的要因の役割を明らかにする.
主な方法:
- 陽子伝送率のためのブロンステッドグラフの分析.
- 異なる基質 (アルデヒド/ケトン対イミニウム誘導体) の反応運動の比較.
- 溶解と静電相互作用を含む理論的解釈.
主要な成果:
- 阻害塩基を持つアルデヒドおよびケトンのエノライゼーションにおいて,著しい負の偏差が観察されました.
- 阻害塩基を持つN1'-メチル-2-(1-ヒドロキシベンジル) シアミン (NMHBnT) からプロトン除去の偏差は見つかりませんでした.
- エノラート溶解の破壊に起因するステリック効果は,静電的要件の影響を受けます.
結論:
- これらの反応におけるステリック効果は主に溶解変化によるもので,陽子移転部位での直接的な相互作用によるものではない.
- 全体的なプロセスの静電的な性質は,溶解障害の影響を左右します.
- 金属イオンは,溶解動態を変更することによって,ステリック効果を軽減することができます.
関連する概念動画
01:12Stereochemical Effects of Enolization
The chiral α-carbon of the carbonyl compound is the stereocenter of the molecule. As shown in the figure below, when such a carbonyl compound undergoes racemization under an acidic or basic condition, an achiral enol is formed.
02:12Solvating Effects
An understanding of the solvating effect helps rationalize the relation between solvation and acidity of the compound. In addition, this also explains the relative stability of conjugate bases for compounds with different pKa values. This lesson details, in-depth, the principle of solvating effects. The strength of an acid and the stability of its corresponding conjugate base are determined using pKa values. This observed relationship is a consequence of solvation, which is the interaction...
01:33Regioselective Formation of Enolates
As depicted in the figure below, the unsymmetrical ketones can form two possible enolates: less substituted or more substituted enolates. Usually, the thermodynamic enolates are formed from the more substituted α-carbon atom, while the kinetic enolates are formed faster by deprotonation from the less substituted position. The thermodynamic enolates have lower energy, so they are more stable. But the energy required to form kinetic enolates is less.
01:18Reactivity of Enols
Enols are a class of compounds where a hydroxyl group is attached to a carbon–carbon double bond, which implies that it is a vinyl alcohol. A carbonyl compound with an α hydrogen undergoes keto–enol tautomerism and remains in equilibrium with its tautomer, the enol form. Usually, the keto tautomer is present in a higher concentration than the enol tautomer due to the higher bond energy of C=O compared to C=C. Moreover, the direction of the keto–enol equilibrium is governed by factors like...
01:23Reactivity of Enolate Ions
Enolate ions are formed by the acid–base reaction of a carbonyl compound with a base. This leads to deprotonation of the α hydrogen atom, leading to a resonance-stabilized enolate ion where one of the contributing structures is an oxyanion, which imparts additional stability. Therefore, the proton on the α carbon is more acidic in nature than that of other sp3-hybridized C–H bonds but less acidic than those in O–H bonds where the negative charge in the conjugate base is localized on the oxygen...
01:14Dehydration of Aldols to Enals: Base-Catalyzed Aldol Condensation
This lesson delves into the aldol condensation catalyzed by bases, where aldols undergo dehydration to enals. As shown in Figure 1, the β-hydroxy aldehyde formed in a base-catalyzed aldol addition reaction dehydrates on heating to yield an unsaturated carbonyl product, which is commonly referred to as an enal.