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Regioselectivity of Electrophilic Additions-Peroxide Effect02:35

Regioselectivity of Electrophilic Additions-Peroxide Effect

10.6K
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.
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Turnover Number and Catalytic Efficiency01:19

Turnover Number and Catalytic Efficiency

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The turnover number of an enzyme is the maximum number of substrate molecules it can transform per unit time. Turnover numbers for most enzymes range from 1 to 1000 molecules per second. Catalase has the known highest turnover number, capable of converting up to 2.8×106 molecules of hydrogen peroxide into water and oxygen per second. Lysozyme has the lowest known turnover number of half a molecule per second.
Chymotrypsin is a pancreatic enzyme that breaks down proteins during digestion....
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Regioselectivity and Stereochemistry of Hydroboration02:36

Regioselectivity and Stereochemistry of Hydroboration

9.5K
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.
9.5K
Catalytically Perfect Enzymes01:07

Catalytically Perfect Enzymes

5.1K
The theory of catalytically perfect enzymes was first proposed by W.J. Albery and J. R. Knowles in 1976. These enzymes catalyze biochemical reactions at high-speed. Their catalytic efficiency values range from 108-109 M-1s-1. These enzymes are also called 'diffusion-controlled' as the only rate-limiting step in the catalysis is that of the substrate diffusion into the active site. Examples include triose phosphate isomerase, fumarase, and superoxide dismutase.
 
Most enzymes...
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Regioselective Formation of Enolates01:33

Regioselective Formation of Enolates

3.5K
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.
3.5K
Reduction of Alkenes: Catalytic Hydrogenation02:13

Reduction of Alkenes: Catalytic Hydrogenation

14.0K
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...
14.0K

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関連する実験動画

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Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions
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カタリティック・ヒドロチオレーション:対照制御された地域選択性

Xiao-Hui Yang1, Ryan T Davison1, Shao-Zhen Nie1,2

  • 1Department of Chemistry , University of California , Irvine , California 92697 , United States.

Journal of the American Chemical Society
|February 9, 2019
PubMed
まとめ

この研究では,ロジウムのカウンターイオンによる地域選択性を制御するダイエンの触媒性水溶解が詳細に示されています. この方法により,β-ファルネシンから (-) -アゲラシジンAを初めてエナンチオセレクティブに合成することができる.

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Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions
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Fabrication and Testing of Catalytic Aerogels Prepared Via Rapid Supercritical Extraction
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科学分野:

  • 有機金属化学
  • カタリシス
  • 有機合成

背景:

  • 1,3-ダイエンの触媒性水酸化は,アリル硫化物とホモアリル硫化物の合成に不可欠である.
  • これらの反応における地域選択性を制御することは依然として大きな課題です.

研究 の 目的:

  • 1,3-ダイエンの触媒性水溶解を拡大する.
  • アリル酸またはホモアリル酸硫化物の形成において高い地域制御を達成する.
  • (-) - アゲラシジンAの第1回エナント選択合成を可能にします.

主な方法:

  • 異なるカウンターイオン (例えば,SbF6−,Cl−) を含むロジウム (Rh) 複合体を利用した.
  • 異なるダイネの調整モード (η4対η2) を含むメカニズム的経路が調査された.
  • エッセンシャルオイルの成分であるβ-ファルネセンのヒドロチオル化.

主要な成果:

  • 地域選択性はRhカウンターによって決定され,非調整カウンターはアリル硫化物 (η4調整) を好み,調整カウンターはホモアリル硫化物 (η2調整) を好む.
  • マルコビニコフ経路と反マルコビニコフ経路の分数および逆のチオール依存性を合理化するメカニズムを提案した.
  • (-) アゲラシジンAの合成が成功しました.

結論:

  • カウンテリオンの選択は,Rh触媒による1,3-ダイエンの水酸化における地域選択性の重要な決定因子である.
  • 異なるヒドロチオレーション経路におけるチオール依存のメカニズム的理解を開発した.
  • (-) - アゲラシジンAのエナチオ選択的合成のための新しい経路を確立した.